Daimler-Benz DB 606, DB 610, and DB 613 Doppelmotoren

By William Pearce

In 1936, Siegfried and Walter Günter began design work on the Heinkel He 119, an experimental, unarmed, high-speed light bomber and reconnaissance aircraft. The engine for the He 119 was buried in the fuselage, and the Günter brothers quickly realized that no engine available was capable of providing the desired power in excess of 2,300 hp (1,691 kW). Heinkel requested proposals from Germany’s leading aircraft engine manufacturers. Daimler-Benz responded with a plan to construct a doppelmotor (double engine) by coupling two DB 601 V-12 engines to create the 24-cylinder DB 606. Combining two engines as a single unit was seen as a quick way to double engine power without spending years to develop a new powerplant.


The Daimler-Benz doppelmotoren (double engines) were quite literally formed by combining two separate engines. The DB 606 was made from two DB 601 engines. The levers attached to the combining gear reduction housing controlled the coupling and decoupling of the separate engine sections.

Development of the DB 601 was started in the mid-1930s and based on the DB 600. The main differences between the engines were that the DB 600 used a carburetor and geared supercharger, whereas the DB 601 used fuel injection and a variable speed supercharger. The DB 601 was an inverted, liquid-cooled engine with two banks of six cylinders. Its single-piece Silumin-Gamma (aluminum alloy) crankcase was closed out by a cover affixed to its top side. The six-throw crankshaft was supported by seven main bearings, and each main bearing was secured by four bolts and one transverse bolt that passed through the crankcase. The crankshaft turned counterclockwise. Fork-and-blade connecting rods were used, with the forked rods serving cylinders on the right side of the engine (when viewed from the rear). The connecting rods ran on roller bearings, but the blade rod had an additional plain bearing between it and the roller bearing.

The two cylinder blocks were made from Silumin (aluminum-silicon alloy) and attached to the bottom of the crankcase at a 60 degree angle. Each cylinder block consisted of six cylinders with integral cylinder heads. The dry cylinder liners (barrels) were made of chrome steel and were screwed and shrunk into the upper cylinder block. Threaded liner skirts protruded into the crankcase toward the crankshaft. A locking ring screwed onto each liner skirt and drew and secured the entire cylinder block to the crankcase. The locking ring had “teeth” around its outer edge and was tightened by a special pinion tool that was held secure in the crankcase and rotated the ring.

Each cylinder had two spark plugs mounted on its outer side and a fuel injector mounted on its inner side. The Bosch fuel injection pumps were mounted in the Vee between the cylinder banks. Two intake valves on the inner side of the cylinder brought in air. The combustion gasses were expelled through two sodium-cooled exhaust valves on the outer side of the cylinder. All four valves per cylinder were actuated via rockers by a single overhead (technically underhead) camshaft, which was driven by a vertical shaft at the rear of the engine.

The DB 601’s propeller shaft was driven clockwise via spur gears through a gear reduction housing mounted to the front of the engine. The gear reduction was made so that a gun or cannon could be mounted behind the engine and fire through the Vee between the cylinder banks and out the propeller’s hub. Mounted to the rear of the engine was an accessory section that provided the drives for the magnetos, generator, starter, fuel and oil pumps, and the transversely mounted supercharger.


Bottom view of a DB 606 illustrates the separate engine sections. Note the rear engine mount which joined the two engine sections. The fuel injection pump for each engine section can be seen in the Vee between the cylinder banks.

The supercharger was mounted on the left side of the engine and driven from the crankshaft via a variable speed fluid coupling. In simple terms, two oil pumps supplied oil that flowed through the supercharger coupling. One pump continuously supplied the amount of oil needed for the supercharger to operate at its lowest (sea level) speed. The second pump was barometrically controlled and gradually supplied more oil as the aircraft’s altitude increased. At the engine’s critical altitude, the second pump was supplying the maximum amount of oil, and the supercharger was at its maximum speed. There was always some degree of slip in the coupling, but it was minimal (a few percent) at full speed. The variable speed of the supercharger created a gradual power curve rather than the saw-tooth power delivery of two- or three-speed superchargers. Air from the supercharger flowed through an intake manifold that looped in the Vee between the cylinder banks.

To form the DB 606, two DB 601 engines were mounted side-by-side at an included angle of 44 degrees and joined by a common propeller gear reduction. In this configuration, the engine banks formed an inverted W, and the inner cylinder banks were only eight degrees from vertical. The right and left engine sections were respectively referred to as the “W-Motor” (or DB 601 W) and the “X-Motor” (or DB 601 X). The exhaust ports for both inner cylinder banks were positioned in the narrow space between the two engine sections. DB 606 differed from the DB 601 by using the new propeller gear reduction and a modified accessory drive. The two engine sections drove a single propeller, and no gun or cannon could be fitted to fire through the propeller hub. Bolted between the two engine sections and near their rear was a mount for suspending the back of the DB 606 to the aircraft. The left and right engine sections remained separate with the exception of the gear reduction and the rear mount.

The new gear reduction housing combined the output from the two engine sections and fed it into a single propeller shaft, which typically had an extension that was approximately 44 in (1.11 m) long. The combining gear allowed the manual decoupling and recoupling of an engine section. Recoupling could only be accomplished when the engine sections were operating at the same RPM. In addition, an engine section would be automatically decoupled if its speed dropped suddenly compared to the other engine section. The coupling of each engine was accomplished by dogs (often referred to as claws in German literature) on a flange splined to the crankshaft that engaged dogs on a coupler that drove a spur gear in the reduction housing. To disengage an engine section, a lever for that engine section on the gear reduction housing had to be pulled forward. This would pull the coupler toward the propeller and disengage it from the crankshaft. The coupler would still be connected to the gears in the reduction housing. The levers on the engine were linked to levers in the cockpit, and the individual engine sections were started one at a time.

Different combining gear reductions enabled the propeller of the DB 606 to turn either clockwise or counterclockwise without changing the counterclockwise rotation of the engines’ crankshafts. The propeller of the DB 606 A turned clockwise. A 33-tooth gear on each of the two crankshafts meshed with an 80-tooth gear on the propeller shaft to create a .4125 reduction. The combining gear on the DB 606 B incorporated idler gears in the lower housing that enabled the propeller to turn counterclockwise. The idler gears increased the engine’s weight by approximately 88 lb (40 kg). For the DB 606 B, a 31-tooth gear on each of the two crankshafts meshed with 39-tooth idler gears, which engaged the 75-tooth gear on the propeller shaft to create a .4133 reduction. With two fewer gears, the combining gear reduction housing on the DB 606 A was initially smaller with angled corners when compared to that of the DB 606 B. However, to simplify production, later DB 606 A engines used the same larger, more rounded housing as the DB 606 B.

Daimler-Benz DB 606 engine rear

Rear view of a DB 606 displays the mirrored accessories on the back of each engine section. The left engine (X-Motor) had the standard accessory housing and supercharger. The accessory section of the right engine (W-Motor) was unique to the doppelmotor. The square mounting pad for the cannon can be seen at the center of each engine section, but this was not used on the doppelmotoren.

The supercharger and accessory section of the right DB 606 (W-Motor) engine section was basically the same as that used on the DB 601. The supercharger and accessory section of the left DB 606 (X-Motor) engine section was a mirror image of the left section so that the supercharger was on the right side of the engine.

The Daimler-Benz DB 606 A/B had a 5.91 in (150 mm) bore, a 6.30 in (160 mm) stroke, and a total displacement of 4,141 cu in (67.86 L). The engine was 6 ft 10 in (2.08 m) long without an extension shaft, 5 ft 4 in (1.63 m) wide, and 3 ft 6 in (1.06 m) tall. The dry weight of the DB 606 A was 3,263 lb (1,480 kg), and the dry weight of the DB 606 B was 3,373 lb (1,530 kg). Initially, 1,175 hp (864 kW) DB 601 Aa engines were used to create the DB 606 A/B. The supercharger on the DB 601 Aa ran full speed at an altitude of 13,123 ft (4,000 m), and the engine had a compression ratio of 6.9 to 1. For takeoff and emergency power at 2,500 rpm and 20.6 psi (1.42 bar) of boost, the early DB 606 A/B V-series (Versuch, experimental) produced 2,350 hp (1,728 kW) at sea level and 2,200 hp (1,618 kW) at 12,139 ft (3,700 m). For climb and combat power at 2,400 rpm and 19.1 psi (1.32 bar) of boost, the engine produced 2,090 hp (1,537 kW) at sea level and 2,100 hp (1,545 kW) at 13,451 ft (4,100 m). For maximum continuous power at 2,300 rpm and 16.9 psi (1.17 bar) of boost, the engine produced 1,900 hp (1,397 kW) at sea level and 1,760 hp (1,294 kW) at 14,764 ft (4,500 m).

Because it was based on an existing engine, the DB 606 was developed quickly. The engine made its first flight in the He 119 in June 1937 with Gerhard Nitschke at the controls. The single DB 606 was installed in the He 119’s fuselage and drove the 14 ft 1 in (4.30 m) diameter, four-blade propeller via a long extension shaft. DB 606 V1 through V4 powered the four He 119 aircraft that were built, and the engine proved to be reliable in that airframe. One He 119 did crash on 16 December 1937 after a faulty fuel transfer valve caused the engine to quit.

Heinkel also selected the DB 606 to power its new long-range heavy bomber design, which was submitted to the RLM (Reichsluftfahrtministerium, or Germany Air Ministry) in response to their Bomber A specification. The RLM ordered construction of a prototype on 2 June 1937, and the aircraft was soon designated as the Heinkel He 177 Greif (Griffon). Like with the He 119, the He 177 was designed by Siegfried and Walter Günter, although Walter was killed in a car accident on 21 September 1937. As changes in the design requirements mounted, particularly with RLM’s insistence that the He 177 be capable of dive bombing, Siegfried was forced to alter the aircraft and make compromises to its design.


The DB 606 was designed for use buried in the fuselage of the Heinkel He 119 and powered the propeller via a long extension shaft. This aircraft (D-AUTE) was lost on 16 December 1937 following an engine failure due to a faulty fuel transfer valve.

Each of the He 177’s wings had one DB 606 engine installed fairly deep and immediately forward of the main landing gear. Each main gear consisted of two legs, with the inboard leg retracting toward the wing root and the outboard leg retracting toward the wing tip. Because of the cramped installation of the engine and landing gear, there was no firewall behind the DB 606. Room was at such a premium that right-angle fittings were used for connections behind the engine. Originally, surface cooling had been planned, but this was switched to annular radiators installed in the engine nacelle just before the engine. The DB 606’s extension shaft led from the engine, through the radiators, and to the He 177’s four-blade propeller, which was 14 ft 9 in (4.5 m) in diameter.

At least 800 He 177 aircraft had been ordered before the prototype made its first flight on 20 November 1939, piloted by Carl Francke. For reference, the He 177 prototype flew with engines V5 and V6, indicating just how few DB 606s had been produced up to that point. In December 1940, DB 606 A/B-1 engines uprated to 2,700 hp (1,986 kW) were installed in He 177 V6. The uprated DB 606 A/B-1 used two 1,350 hp (993 kW) DB 601 E engines. The supercharger of the uprated DB 601 E ran full speed at an altitude of 15,748 ft (4,800 m).

For takeoff and emergency power at 2,700 rpm and 20.9 psi (1.44 bar) of boost, the DB 606 A/B-1 produced 2,700 hp (1,986 kW) at sea level and 2,650 hp (1,949 kW) at 15,748 ft (4,800 m). For climb and combat power at 2,500 rpm and 19.1 psi (1.32 bar) of boost, the engine produced 2,400 hp (1,765 kW) both at sea level and at 16,076 ft (4,900 m). For maximum continuous power at 2,300 rpm and 16.9 psi (1.17 bar) of boost, the engine produced 2,000 hp (1,471 kW) at sea level and 2,075 hp (1,526 kW) at 16,732 ft (5,100 m).

Starting around 1940, Daimler-Benz used a lower compression ratio in the right (non-supercharger side) cylinder bank. This was due to the crankshaft’s rotation flinging extra oil toward the right cylinders. Some of the oil would get past the piston rings and into the combustion chamber. The presence of this oil increased the possibility of detonation (knock) in the cylinder. The compression ratio was decreased slightly to increase the cylinder’s knock resistance. Because the inner cylinder banks of the doppelmotoren were nearly vertical, they captured more oil than the outer cylinder banks. The inner banks also ran hotter because of their tight installation. The extra oil and the heat both increased the possibility of detonation in the inner cylinder banks. As a result, the inner cylinder banks of the doppelmotoren had a slightly lower compression ratio than that of the outer cylinder banks. For the DB 606 A/B-1, the outer (supercharger side) cylinder banks had a compression ratio of 7.2 to 1, and the inner (non-supercharger side) cylinder banks had a compression ratio of 7.0 to 1.


The Heinkel He 177 bomber was designed to take advantage of the reduced drag offered by the DB 606 doppelmotor. However, the engine and its installation proved to be very problematic. The He 177 A-02 pictured above was the tenth He 177 built and second production machine. It was lost in May 1942 during a crash landing after both engines caught fire.

The DB 606 engine and its installation in the He 177 proved to be disastrous. As doppelmotor production picked up, vibration issues were discovered with the two engine sections, and the combining gear required much more development than had been anticipated. There were also issues with failures of the engine couplings. A major DB 606 issue was with its oil circulation at high altitudes. The oil would foam, leading to inadequate lubrication and the subsequent failure of bearings and seizing of pistons. Some of these failures would be catastrophic, with parts (connecting rods) breaking through the crankcase.

But it was the engine installation that caused the biggest issues. The annular radiators provided inadequate cooling, resulting in the engines running hot. The exhaust between the two inner cylinder banks ran so hot that any fuel or oil that dripped down from leaking fittings or during a catastrophic engine failure was ignited. Weeping fittings and seeping seals (partly caused by material shortages and substitutions during the war) were a constant issue, as the leaked fluid would pool and eventually be ignited by the hot exhausts’ radiant heat. Through lack of a firewall, fires in the engine nacelle would spread to the main gear and ignite any leaking hydraulic fluid. In addition, the hot exhaust being expelled just forward of the extended main gear was enough to ignite any hydraulic oil that had leaked.

Any fire in the wing spread quickly and spelled disaster for the aircraft and its crew. With the crew siting well forward of the engines, fires often went unnoticed until severe damage had occurred. Despite the best efforts of maintenance crews, the DB 606 engines needed constant attention and proved very difficult to service. Engine fires occurred with such regularity that crews referred to the He 177 as the Luftwaffenfeuerzeug, or Luftwaffe’s cigarette lighter. To resolve the engine issues, suggestions were made to extend the engine nacelle, install a firewall, reroute lines to prevent the pooling of fluids under the engine, and redesign the exhaust system. Such changes were ignored at first because they would delay He 177 production, which had already been rushed. However, the aircraft was also experiencing a number of structural issues unrelated to the engines that made modifications necessary.

Toward the end of 1942, the He 177 underwent a redesign as the A-3 variant. This aircraft would do away with the troublesome DB 606 engines and replace them with DB 610s. The DB 610 was a doppelmotor consisting of two 1,475 hp (1,085 kW) DB 605 A engines. The DB 605 was a development of the DB 601 that operated at a higher RPM, had an increased bore, and had a higher compression ratio. The DB 605/610 used plain bearings for the connecting rods rather than the roller bearings used on the DB 601/606.


The DB 610 combined two DB 605 engines and was intended to cure the issues with the DB 606. While the DB 610 was more powerful, issues still persisted, and all doppelmotoren proved to be difficult to service and maintain. The propeller extension shaft was typical, being used on the He 177, Ju 288, and NC.3021.

The DB 610 kept the same engine section naming convention as the earlier doppelmotor, with the “W-Motor” (or DB 605 W) as the right section and the “X-Motor” (or DB 605 X) as the left section. The supercharger ran full speed at an altitude of 18,701 ft (5,700 m). The compression ratio of the outer (supercharger side) cylinder banks was 7.5 to 1, and the compression ratio of the inner (non-supercharger side) cylinder banks was 7.3 to 1.

The Daimler-Benz DB 610 A/B had a 6.06 in (154 mm) bore, a 6.30 in (160 mm) stroke, and a total displacement of 4,365 cu in (71.53 L). For takeoff and emergency power at 2,800 rpm and 20.9 psi (1.42 bar) of boost, the engine produced 2,950 hp (2,170 kW) at sea level and 2,700 hp (1,986 kW) at 18,701 ft (5,700 m). For climb and combat power at 2,600 rpm and 19.1 psi (1.32 bar) of boost, the engine produced 2,620 hp (1,927 kW) at sea level and 2,500 hp (1,839 kW) at 19,029 ft (5,800 m). For maximum continuous power at 2,300 rpm and 16.9 psi (1.17 bar) of boost, the engine produced 2,150 hp (1,581 kW) at sea level and 2,160 hp (1,589 kW) at 18,045 ft (5,500 m). The DB 610 was the same size as the DB 606: 6 ft 10 in (2.08 m) long, 5 ft 4 in (1.63 m) wide, and 3 ft 6 in (1.06 m) tall; however, it was around 130 lb (60 kg) heavier. The dry weight of the DB 610 A was 3,395 lb (1,540 kg), and the dry weight of the DB 610 B was 3,483 lb (1,580 kg).

The DB 610 installation on the He 177 A-3 was extended 200 mm (7.9 in) forward, and a firewall was incorporated behind the engine. On 22 March 1943, the DB 610 made its first flight in an He 177 (V19, VF+QA). Although reliability had been improved, engine fires still occurred, and the DB 610 suffered from the same engine coupling failures that had been experienced with the DB 606. In May 1942, Hermann Göring, commander of the Luftwaffe, made the following comment in reference to the He 177 and DB 606: “I have never been so furious as when I saw this engine. …Nobody mentioned this hocus-pocus with two welded-together engines to me at all.” By early 1944, plans were in motion to build He 177 with four separate engines, a suggestion that Heinkel had discussed back in late 1938 and proposed in mid-1939. Further production and development of the He 177 was abandoned on 1 July 1944. Once the Allies had landed on the continent, German aircraft production was focused on defensive fighters and attackers.


Side view of the DB 610 illustrates the relative ease with which the spark plugs on the outer cylinder banks can be accessed. However, one can imagine the extreme difficulty of accessing the spark plugs of the inner cylinder banks. The bolts on the upper side of the crankcase are the transverse bolts that pass through the main bearing caps.

The Daimler-Benz doppelmotoren were also installed in the Junkers Ju 288 bomber. As issues with its intended 24-cylinder Junkers Jumo 222 inline radial engine created a short supply, the DB 606 was substituted in Ju 288 prototypes. A DB 606 engine was installed on each wing in a form-fitting nacelle with an annular radiator at its front. Like with the He 177, the extension shaft connected the engine to the propeller. The DB 606-powered Ju 288 V11 made its first flight in July 1942. Three additional Ju 288s were powered by DB 606 engines. A switch to the DB 610 was made for the Ju 288 V103, which was first flown in the spring of 1943. Five additional Ju 288s were powered with DB 610 engines. The doppelmotor installation in the Ju 288 did not result in the frequent engine fires experienced with the He 177. The DB 610 was planned for later Ju 288 C and D variants, but the aircraft were cancelled.

Post war, a DB 610 was used in the French SNCAC NC.3021 Belphégor high altitude research aircraft. The large single-engine aircraft had an annular radiator positioned in front of the DB 610 engine. The NC.3021 was first flown on 6 June 1946. Issues servicing the DB 610 were encountered, and the aircraft required much maintenance. SNCAC went bankrupt in mid-1949, and no other funds were provided for the aircraft. The NC.3021 was withdrawn from testing in 1950 and scrapped.

Development of the DB 613, a third doppelmotor, had a lower priority than that of the DB 606 and DB 610. With what appeared to be the successful creation of the DB 606, Daimler-Benz decided to apply the same doppelmotor concept to the DB 603 engine. The DB 603 was based on and built like the DB 601, but it had an enlarged bore and an elongated stroke. Compared to the DB 601, the DB 603 had slightly decreased supercharging at takeoff power but an increased compression ratio. The compression ratio of the outer (supercharger side) cylinder banks was 7.3 to 1, and the compression ratio of the inner (non-supercharger side) cylinder banks was 7.5 to 1.


When Junkers was unable to supply the needed numbers of the Jumo 222 engine, the DB 606 and DB 610 were used in its place to power the Junkers Ju 288 bomber. Ju 288 V103 seen above was probably the first Ju 288 to be powered by the DB 610.

Around 1940, the DB 613 was created by combining two 1,750 hp (1,287 kW) DB 603 G engines. The combining gear housing on the DB 613 was different that those used on the DB 606 and DB 610. The DB 613’s housing was asymmetric with an accessory drive from the W-Motor (right engine). The Daimler-Benz DB 613 A/B had a 6.38 in (162 mm) bore, a 7.09 in (180 mm) stroke, and a total displacement of 5,434 cu in (89.04 L). For takeoff and emergency power at 2,700 rpm and 21.5 psi (1.48 bar) of boost, the engine produced 3,600 hp (2,648 kW) at sea level and 3,100 hp (2,280 kW) at 22,966 ft (7,000 m). For climb and combat power at 2,500 rpm and 19.8 psi (1.37 bar) of boost, the engine produced 3,150 hp (2,317 kW) at sea level and 2,860 hp (2,104 kW) at 23,293 ft (7,100 m). For maximum continuous power at 2,300 rpm and 18.4 psi (1.27 bar) of boost, the engine produced 2,790 hp (2,052 kW) at sea level and 2,650 hp (1,949 kW) at 21,982 ft (6,700 m). The DB 613 was 7 ft 3 in (2.22 m) long without its extension shaft, 5 ft 10 in (1.77 m) wide, and 3 ft 9 in (1.14 m) tall. The dry weight of the DB 613 A was 4,321 lb (1,960 kg), and the dry weight of the DB 613 B was 4,409 lb (2,000 kg). The DB 613 was proposed for the Heinkel He 177 A-7 variant, but the aircraft was not produced, and the engine never progressed beyond the prototype stage. It is not believed that the DB 613 was ever flight tested.

C/D variants of each engine were planned, but it is not clear if they were ever built beyond prototype examples. Development of the C/D variants seemed to start toward the end of 1942. In general, the C/D variants produced more power, had increased critical altitudes, and were planned for 100 octane fuel. The DB 606 C/D produced 2,600 hp (1,912 kW) for takeoff and had a critical altitude of 19,029 ft (5,800 m).

The DB 610 C/D was based on the DB 605 D and had a compression ratio of 8.5 to 1 for the outer (supercharger side) cylinder banks and 8.3 to 1 for the inner (non-supercharger side) cylinder banks. For takeoff and emergency power at 2,800 rpm and 20.6 psi (1.42 bar) of boost, the DB 610 C/D produced 2,870 hp (2,111 kW) at sea level and 2,560 hp (1,883 kW) at 24,934 ft (7,600 m). For climb and combat power at 2,600 rpm and 19.1 psi (1.32 bar) of boost, the engine produced 2,550 hp (1,876 kW) at sea level and 2,400 hp (1,765 kW) at 24,278 ft (7,400 m). For maximum continuous power at 2,300 rpm and 17.6 psi (1.22 bar) of boost, the engine produced 2,100 hp (1,545 kW) at sea level and 2,050 hp (1,508 kW) at 22,966 ft (7,000 m). The dry weight of the DB 610 C was 3,461 lb (1,570 kg), and the dry weight of the DB 610 B was 3,538 lb (1,605 kg).


The SNCAC NC.3021 Belphégor was a high-altitude research aircraft that incorporated a pressurized cabin. Powered by a DB 610, the post-war aircraft carried a crew of three plus two researchers. It was the last aircraft design that used a Daimler-Benz doppelmotor.

The DB 613 C/D had the same compression ratio increase as the DB 610 C/D. For takeoff and emergency power at 2,900 rpm and 20.9 psi (1.44 bar) of boost, the DB 613 C/D produced 4,000 hp (2,942 kW) at sea level and 3,600 hp (2,648 kW) at 19,685 ft (6,000 m). For climb and combat power at 2,700 rpm and 19.1 psi (1.32 bar) of boost, the engine produced 3,500 hp (2,574 kW) at sea level and 3,280 hp (2,412 kW) at 19,685 ft (6,000 m). For maximum continuous power at 2,300 rpm and 17.6 psi (1.22 bar) of boost, the DB 613 C/D produced 2,850 hp (2,096 kW) at sea level and 2,860 hp (2,104 kW) at 17,388 ft (5,300 m).

The Daimler-Benz doppelmotoren represented a quick way to double an engine’s output without quite doubling the drag of installation. While the engines worked well in the He 119 and Ju 288, the engine package failed to work reliably in the He 177, which was the main application. A total of approximately 1,916 doppelmotoren were produced: 820 (544 by some sources) DB 606s, 1,070 (1,346 by some sources) DB 610s, and 26 DB 613s. The engines tested in a Junkers Ju 52 transport and powered four He 119s, 915 (1,135 by some sources) He 177s, ten Ju 288s, and one SNCAC NC.3021.

An early DB 606 A is displayed at the Technik Museum in Sinsheim, Germany. DB 610 engines are on display in Germany at the Deutsches Museum in Munich and the Luftfahrttechnisches Museum (Aviation Museum) in Rechlin; and in the United Kingdom at the Royal Air Force Cosford Museum in Shropshire and the Science Museum at Wroughton. Reportedly, a DB 610 is in France, but its location and condition have not been found. The Smithsonian National Air and Space Museum has in storage a DB 610 engine in a complete He 177 nacelle. A DB 610 combining gear reduction housing is on display at the Muzeum Lotnictwa Polskiego (Polish Aviation Museum) in Krakow, Poland. No DB 613 engines are known to have survived.

Note: The figures in this article listed as hp (horsepower) are actually PS (Pferdestärke, metric horsepower). The kW figures are converted from the PS value.


The DB 613 utilized two DB 603 engines. It was the largest, heaviest, and most powerful of the doppelmotoren. The DB 613 had an asymmetric combing gear housing that incorporated an accessory drive. The engine never progressed beyond prototype testing.

Jane’s All the World’s Aircraft 1945-46 by Leonard Bridgman (1946)
Flugmotoren und Strahltriebwerke by Kyrill von Gersdorff, et. al. (2007)
The Secret Horsepower Race by Calum E. Douglas (2020)
Heinkel He 177 Greif by J. Richard Smith and Eddie J. Creek (2008)
Junkers Ju 288/388/488 by Karl-Heinz Regnat (2004)
Major Piston Engines of World War II by Victor Bingham (1998)
DB 606 A-B Baureihe 0 u. 1 Motoren-Handbuch by Technisches Amt (November 1942)
Ersatzteilliste für Mercedes-Benz-Flugmotor Baumust DB 606 A-B by Daimler Benz (December 1941)
DB 610 A-B Baureihe 0 u. 1 Motoren-Handbuch by Technisches Amt (November 1942)
Betriebs und Wartungsvorschrift zum Mercedes-Benz Flugmotor DB 601 A u. B by Daimler Benz (October 1940)
Motorhandbuch zum Mercedes-Benz-Flugmotor DB 603 A Baureihe 0, 1 und 2 by Daimler Benz (November 1942)


Lear Fan Limited LF 2100

By William Pearce

William “Bill” Powell Lear was born on 26 June 1902 in Hannibal, Missouri. From a very young age, Lear had an interest in electronics and an aptitude for design. Starting in the 1920s and continuing through his entire life, Lear developed a number of electronics, devices, and aircraft. Lear was responsible for the development of the car radio in the late 1920s; various radio direction finders, autopilots, and automated landing systems for aircraft in the 1930s and 1940s; the Lear Jet in the early 1960s; and the 8-track in the mid-1960s. He was personally awarded 121 patents and co-authored another seven. Throughout his life, Lear sold off his successful developments to fund his next round of invention and experimentation.


Lear Fan prototype E-001 lands at Stead Airport in Reno, Nevada after a test flight. Despite the nose-up attitude, note the ample clearance between the ventral fin and the runway. The Lear Fan certainly had the appearance of a capable, high-performance aircraft.

In the mid-1970s, and through his LearAvia Corporation located at Stead Airport in Reno, Nevada, Lear worked on a long-range business jet called the LearStar 600. Plans to develop and produce the aircraft were purchased by Canadair in April 1976. Lear and his team worked with Canadair to refine the aircraft, but engineers at Canadair did the same and changed many aspects of the original LearStar 600 design. Around March 1977, the team at LearAvia proposed an updated business jet design called the Allegro, which incorporated many composite components to increase the aircraft’s performance. Canadair was not interested in the Allegro, nor was it interested in Lear’s advice and meddling in the LearStar 600 design, which Canadair eventually developed as the Challenger 600.

Since the 1950s, Lear had contemplated the design of an aircraft utilizing two turboprop engines in the fuselage that powered a single pusher propeller. The benefit of this centerline thrust configuration was that it would provide twin-engine reliability without any yaw effect from asymmetrical thrust in an engine-out situation. The basic design layout was similar to the Douglas XB-42 bomber prototype, which first flew on 6 May 1944, and the Planet Satellite light aircraft, which first flew in mid-1949. In early 1976, Lear discussed the pusher design with Richard Tracy, LearAvia’s chief engineer. Lear sought an aircraft that could carry six to eight passengers from Los Angeles to New York (2,465 miles / 3,967 km) at 400 mph (644 km/h) and at 41,000 ft (12,497 km) with two 500 hp (373 kW) engines. Lear and Tracy intermittently discussed the design for several months.


The second Lear Fan prototype E-003 was the primary aircraft for gathering fight test data. E-003 is seen here with its original N-number and blue paint. The number on the ventral fin signified the flight number. Note the data boom on the nose.

As the lack of progress with the LearStar 600 at Canadair grew frustrating for the LearAvia staff, Tracy reviewed the pusher design with Rodney Schapel, an aerodynamic engineer, and tasked him with making some preliminary drawings. Lear was initially not interested in the project and would chastise Schapel when he saw him working on the pusher design. However, as Canadair took control of the LearStar 600 and rejected the Allegro, Lear became more interested in the pusher aircraft and reviewed the design with Schapel and Tracy. Around April 1977, Lear decided that the pusher aircraft would be the company’s next design. The new aircraft was briefly called the Futura, but it quickly became the Lear Fan 2100.

The Lear Fan 2100 was a twin-engine, low-wing monoplane with tricycle landing gear. Depending on the configuration, the aircraft could accommodate one or two pilots and up to nine passengers in its pressurized cabin. Other configurations were considered, including a cargo version and an air ambulance that could accommodate two stretchers, each with a dedicated attendant. The Lear Fan was a revolutionary design in several regards. In addition to its two engines powering a single pusher propeller, Lear had decided that the entire aircraft would be made of a composite material. When compared to aluminum, the aircraft’s bonded graphite and epoxy composite structure was smoother, stronger, resistant to fatigue, would not corrode, could be molded into complex shapes, and was 40 percent lighter. The airframe was designed for a maximum loading of +6 and -4 Gs.


E-001 (right) and E-003 (left) in flight together. Note the fixed cooling air duct on E-003 between the propeller and ventral fin. E-001 had a different setup with a movable door. The “windows” for both aircraft were at least painted on in the photograph.

The aircraft’s fuselage was formed with close-spaced frames and longerons bonded to the outer skin. The skin was mostly four plies thick, but the thickness increased to eight or ten plies around window and door openings. The fuselage was made in six sections: upper and lower nose, upper and lower cabin, and upper and lower rear fuselage. The sections were bonded in an autoclave to form the entire fuselage structure. The fuselage had a slightly oval shape, and its interior had a maximum height of 4 ft 8 in (1.36 m) and a maximum width of 4 ft 10 in (1.47 m). The cabin was 12 ft 10 in (3.91 m) long and had a 50 cu ft (1.42 m3) baggage compartment that was accessible in flight at its rear.

Cabin access was via a door located on the left side of the fuselage and just forward of the wing. The first prototype had a split upper and lower door, but subsequent examples had a single door that folded down to form stairs for cabin entry. The passenger compartment originally had six windows on its right side and five windows on its left side. However, none of the prototype aircraft had their full allotment of windows, and some of the “windows” were painted on. It seems the window on the door was eventually omitted. Pressurization provided a nominal pressure differential of 8.3 psi (.57 bar), enabling an 8,000 ft (2,438 m) cabin altitude while cruising at a 41,000 ft (12,497 km) flight altitude. The steerable nosewheel retracted forward into the nose of the aircraft.

The single-piece, high-aspect wing had three continuous spars and was mated to the fuselage via six attachment points. Each wing spar was formed by two channel sections joined back-to-back on a honeycomb core. The upper and lower wing skins had 52 plies at their roots, with the thickness decreased to eight plies at the tips. The wing had four degrees of dihedral. The main landing gear had an 11 ft 8 in (3.56 m) track and retracted inward to be fully enclosed within the wing. Fuel tanks were integrated into the wing’s structure, and each wing housed up to 125 US gallons (104 Imp gal / 473 L) of fuel. Flaps extended along approximately 75 percent of the wing’s trailing edge, with ailerons extending almost to the wing tips. The landing gear and the flaps were hydraulically operated.


The underside of the Lear Fan as perhaps its least photogenic side. Even so, the view of E-003 illustrates the aircraft’s clean aerodynamic form, even with what appears to be a hydraulic leak from the right main gear. This was the aircraft’s 50th flight.

At the rear of the Lear Fan was a Y tail. The ventral fin had two spars, and a rudder was attached to its trailing edge. The structure of the fin was stressed for ground impacts to prevent the propeller from contacting the runway in case of an over-rotation during takeoff or a hard landing and incorporated a strike pad. Each of the two “butterfly” horizontal stabilizers had one spar. They had 35 degrees of dihedral, which increased the aircraft’s directional stability. The control surface on the horizontal stabilizer was a standard elevator for pitch control only. All normal flight controls were mechanically operated using cables and pushrods.

Originally, two Lycoming (probably LTS101) turboprop engines were to be used, but these were replaced with Pratt & Whitney Canada PT6B-35F engines early in the design phase. The PT6B-35F engines produced 850 shp (634 kW) but were flat-rated to 650 shp (485 kW) for the Lear Fan. The engines were positioned in the fuselage behind the wing’s trailing edge. A scoop on each side of the aircraft brought in air to the engine and expelled exhaust to the rear. The scoop was integral with a large service panel, the removal of which enabled access to the engine. A special mount held each engine in such a way that when the engine was disconnected from its drive shaft and other restrictions, the engine could be swung out for servicing and inspection. The pivot point was the mount at the front of the engine, and this action enabled access to the inner side of the engine.

A 6 ft (1.83 m) aluminum drive shaft with a graphite fiber cover extended from each engine to a combining gearbox at the rear of the aircraft. The gearbox was designed and built by Western Gear Corporation and was equipped with sprag overrunning clutches. If an engine failed, the good engine would continue to power the propeller. As originally designed, wax contained in the gearbox would melt to provide continuing lubrication in the event of oil loss. This method did not work as well as expected, and a back-up oil system was devised in 1984. Referred to as the “spin jet,” oil from a reserve tank was intermittently sprayed directly into the meshing gears. The gearbox was successfully run for over three hours with its main oil supply exhausted and its only lubrication provided by the “spin jet” system. An oil cooler was located under the gearbox. The gearbox had a .3125 propeller speed reduction, resulting in the propeller turning at 688 rpm when the engine’s drive shaft was rotating at 2,200 rpm. Originally, a 7 ft 6 in (2.29 m) diameter three-blade propeller built by Hartzell was to be used. However, a switch to a four-blade Hartzell propeller of the same diameter was made during the design phase when tests indicated that the four-blade propeller was less prone to vibration issues. The propeller was reversible and had 3 ft 1 in (.94 m) of ground clearance when the aircraft was on its landing gear.


E-001 with its updated paint, which it still wears today. The two ducts under the aircraft were the inlet and exhaust for oil coolers. An open cooling air exit door is seen between the propeller and ventral fin. Subsequent prototypes used a fixed duct. Most images of E-001 in flight are without a spinner.

Although a Lear Fan brochure dating from 1979 lists the aircraft’s length as 38 ft 8 in (11.79 m), as originally built, the aircraft had wingspan of 39 ft 4 in (11.99 m), a length of 39 ft 7 in (12.07 m), and a height of 11 ft 6 in (3.51 m). The Lear Fan’s estimated performance was a top speed of 375 mph (604 km/h) at 39,000 ft (11,887 m), 403 mph (649 km/h) at 31,000 ft (9,449 m), and 414 mph (666 km/h) at 19,000 ft (5,791 m). Stalling speed was 90 mph (145 km/h). The aircraft had an initial climb rate of 3,550 fpm (18.0 m/s), and a ceiling of 41,000 ft (12,497 km). The Lear Fan had an empty weight of 3,650 lb (1,656 kg) and a gross weight of 7,200 lb (3,266 kg). At gross weight, the aircraft had a range of 1,630 miles (2,623 km) at 400 mph (644 km/h) and 2,300 miles (3,704 km) at 350 mph (563 km/h). On a single engine, the Lear Fan could takeoff, climb at 1,900 fpm (9.7 m/s), and execute a go-around. The aircraft’s single engine ceiling was 29,000 ft (8,839 m).

Lear was slowed down by health problems for a few years, but he was back to his old self in late 1977 as he tried to sell the Lear Fan concept to anyone who would listen. Lear made the decision to proceed with production prototypes rather than constructing a proof-of-concept vehicle first. While this decision could lead to cost savings and quicken development if everything went well, it would result in the exact opposite if things did not go well. By this time, Tracy had been replaced as chief engineer by Nicholas Anderson, and Schapel had been fired. Schapel had designed the aircraft’s original Y tail, but Lear wanted an inverted V tail. Schapel was let go over the disagreement. Ultimately, wind tunnel tests indicated that the Y tail was superior, and the Lear Fan reverted back to Schapel’s original tail design.

In early 1978, Lear’s health faltered again. He made arrangements for Lear Fan development to procced even if he were to die, but he desperately wanted to live long enough to see the prototype take to the air. In March, Bill Lear was diagnosed with leukemia, and he passed away on 14 May 1978. Some of his last words were urging that the Lear Fan be finished.


E-003 with its revised green paint and new N-number. The green paint was applied in honor of the Zoysia Corporation, the project’s major financial backer at the time. The number on the ventral fin indicates that this is the aircraft’s 298th flight. A spin chute is installed between the V tail. Although spin testing was never conducted, if needed, a shaped charge would have blown off the propeller before the chute was deployed.

Development of the Lear Fan did continue, and construction of a prototype was started in November 1978. Moya Lear, Bill’s wife, took over as the face of LearAvia. Progress on the aircraft’s untried propulsion system and gearbox, unusual layout, and all-composite structure proved slow and expensive. LearAvia’s financial resources were quickly depleted. In mid-1980, the company was restructured as Lear Fan Limited with the financial backing of investment firms and the British government. The agreement with the British government was that $25 million would go to the project, and another $25 million would be provided for Lear Fan production in Newtonabbey, near Belfast in Northern Ireland. British financial support would end if the prototype did not fly by the end of 1980. At the time, 126 aircraft were on order. Production was expected to start in 1982 and would create at least 1,200 jobs in Newtonabbey. Paramount for Lear Fan production was for the FAA (Federal Aviation Administration) to issue the aircraft a Certificate of Airworthiness. However, the Lear Fan’s all-composite construction was a first for a production aircraft, and certification was going to be a long and costly process.

Under the newly restructured company, the aircraft became the Lear Fan Limited LF 2100, and all prototypes were registered with the FAA as such. Lear Fan E-001 was registered as N626BL, for June 26 (his birthday) Bill Lear. On 31 December 1980, E-001 was rolled out of the hangar at Stead Airport to conduct taxi tests before its first flight. During a high-speed taxi test, the brakes were burned up and needed to be replaced. With 15 minutes of daylight left, the aircraft was preparing for takeoff when the sleeve of a pilot’s flight suit caught on the cockpit fire extinguisher handle, inadvertently activating it and forcing the flight to be scrubbed. The next day, 1 January 1981, the Lear Fan took to the air. The first takeoff was made by Hank Beaird in the left seat, with Dennis Newton in the right seat. The first landing was made by Newton in the left seat, with Beaird in the right seat. It was Beaird’s idea to switch seats so that both pilots had “firsts” during the Lear Fan’s initial flight. While the aircraft’s first flight was one day past the deadline, in the spirit of all that had been accomplished and by a Royal Decree signed by Queen Elizabeth, the British government declared that the Lear Fan made its first flight on 32 December 1980 and was still qualified for funding.

The remainder of 1981 was spent refining E-001 and continuing flight testing, building E-002 for use as a static test airframe, and building E-003. E-003 was registered as N327ML, for March 27 (her birthday) Moya Lear, and the aircraft was planned as the true workhorse for flight testing. With Lear Fan orders reaching 203 by June 1981 and 263 by early 1982, the future looked bright. E-001 had made 53 flights and had accumulated 78 flight hours by the start of 1982.


The third Lear Fan prototype, E-009, seen outside the Lear Fan hanger at the Stead Airport. E-009 appears to have had all of its windows from the start. Although not quite apparent from the image, its colors were dark green and yellow on white.

The second prototype, E-003, had a new fuselage that was 12 in (.30 m) longer than that used on E-001, resulting in a length of 40 ft 7 in (12.37 m). The longer fuselage increased the cabin’s length to 13 ft 4 in (4.06 m) and the baggage compartment’s capacity to 53.7 cu ft (1.52 m3). The aircraft also incorporated some other minor modifications, such as a ventral duct at the extreme rear to bring in cooling air to the gearbox. E-003 made its first flight on 19 June 1982, most likely piloted again by Beaird and Newton. However, Lear Fan Limited had run out of money. The company was reorganized on 15 September 1982 as Fan Holdings, Inc, with the British investing $30 million and with the Zoysia Corporation, a consortium from Saudi Arabia, supplying $60 million. A major player in the Zoysia consortium was Prince Sultan bin Salman bin Abdulaziz Al Saud.

In December 1982, cracks in the wing were detected during static tests. Rather than undergoing a major wing redesign, the existing wing structure was reinforced. These modifications added weight and reduced the fuel load by 10 US gallons (8 Imp gal / 38 L), both of which decreased the aircraft’s range. At the start of 1983, 276 Lear Fans were on order. Flight testing of E-001 and E-003 resumed during the summer of 1983. In mid-July, the lower aft pressure bulkhead of the static test airframe E-002 failed during a pressurization test. On 20 July 1983, E-001 suffered an explosive decompression while at 25,000 ft (7,620 m). With the recent failure of E-002 on their minds, test pilots John Penny and Mark Gamache declared an emergency and brought the aircraft quickly and safely back to Stead Airport. The cause of the decompression could not be found, and the event marked the end of E-001’s flight career.

In December 1983, another test fuselage failed during pressure tests, and Fan Holdings Inc was running short on funds. At the time, Lear Fans had accumulated some 521 total flight hours. In March 1984, E-003 flew with its updated wing and fuselage. In April 1984, more fuselage issues were encountered. In June 1984, the Newtonabbey plant, which had been tooled up for production and had made various test parts, was shut down. Also in June 1984, the registration of E-003 was changed from N327ML to N21LF. Bill Lear’s will had focused on continuing Lear Fan development, but it created some potential conflicts of interest with the aircraft’s management team. Some of the Lear children filed suit in 1978 and 1979. Moya Lear became involved, and everything was settled as far as the courts were concerned in 1984. However, not all parties were appeased, and some consider the N-number change was done to spite Moya. Others feel it was to bring focus to the Lear Fan rather than to people behind the project.


E-001 on display in the Museum of Flight in Seattle, Washington. The aircraft is in good company with the likes of a Douglas DC-3, Boeing 80, Gee Bee Z, and Lockheed M-21/D-21 in the background. (Josh Kaiser image via airliners.net)

Airframes E-004 through E-008 were all test articles for certification, but the continuous issues resulted in there being no end in sight for the certification process. In late 1984, Fan Holdings Inc was attempting to get the Lear Fan certified for unpressurised, VFR (Visual Flight Rules), day flight by January 1985. Certification for pressurized flight up to 25,000 ft (7,620 m) would follow in the spring of 1985, and certification up to 41,000 ft (12,467 m) would follow in mid-1985.

On 15 December 1984, airframe E-009 (N98LF) made it first flight with John Penny and Bob Jacobs at the controls. In April 1985, the aircraft was flown to William P. Hobby Airport in Houston, Texas to give Sultan bin Salman an orientation flight. At the time, Sultan bin Salman was undergoing training for his Space Shuttle flight abord Discovery, scheduled for June 1985. Most likely, it was hoped that the Lear Fan orientation flight would also result in additional financing from the Zoysia Saudi Arabian consortium, but it was not to be. On 25 May 1985, development of the Lear Fan was halted; all employees in Reno and Newtonabbey were let go, and all Fan Holdings Inc facilities were closed.

The Lear Fan’s revolutionary design and construction proved too much to overcome. The decision to develop the aircraft without a proof-of-concept proved costly, as numerous changes needed to be made. Problems had also been encountered with the gearbox, and its excessive wear was cited as the final blow to the program. After 200 hours of inspection, the FAA refused to issue a Certificate of Airworthiness for the Lear Fan. Some contend that the FAA set requirements for the Lear Fan that were two to three times more stringent than those for a comparable aluminum aircraft.


E-003 hangs on display in Frontiers of Flight Museum at Love Field in Dallas, Texas. Black pneumatic de-icing boots covered the Lear Fan’s leading edges. Hot exhaust from the engines would prevent the buildup of ice on the propeller. (Johnny Comstedt image via http://www.aviationmuseum.eu)

The final disclosed specifications for the Lear Fan were a wingspan of 39 ft 4 in (11.99 m), a length of 40 ft 7 in (12.37 m), and a height of 12 ft 2 in (3.71 m). The aircraft had a maximum speed of 414 mph (666 km/h) at 20,000 ft (6,096 m) and a stalling speed of 88 mph (142 km/h). Best economical cruise speed was 322 mph (518 km/h) at 40,000 ft (12,192 m), which gave a maximum range of 2,003 miles (3,224 km). The Lear Fan had an initial climb rate of 4,000 fpm (20.3 m/s) and a ceiling of 41,000 ft (12,497 km). The aircraft had an empty weight of 4,100 lb (1,860 kg) and a gross weight of 7,350 lb (3,334 kg). At gross weight, the Lear fan had a range of 1,782 miles (2,868 km). Single engine performance was a 1,300 fpm (6.6 m/s) climb rate and a 33,000 ft (10,058 m) ceiling.

Compared to the original flight specifications, the aircraft had become 450 lb (204 kg) heavier. While its maximum speed had increased by 14 mph, its maximum range at gross weight decreased by 670 miles (1,078 km), and its economical cruising speed decreased by 28 mph. After a peak of some 280 aircraft on order, most customers requested a refund as development dragged on. The entire Lear Fan project had consumed over $250 million.


E-009 on display at the FAA’s Civil Aerospace Medical Institute in Oklahoma City, Oklahoma. The aircraft was previously in outside storage at the FAA facility and underwent a restoration starting in 2012. The new paint scheme was applied during the restoration. A dedication ceremony for the restored E-009 was held on 29 September 2015.

Years after their development was abandoned, Lear Fan airframes continued to be used to understand composites and develop techniques for their inspection. From November 1993 to October 1994, Northrup Grumman inspected the composite wing structure of E-009. The project was sponsored by US Department of Transportation and NASA to develop inspection techniques for composite aircraft. Although minor defects were detected, they were evaluated as not severe enough to impose a threat to the integrity of the wing structure. The final inspection report advised that composite assembly standards should be established to minimize defects and damage. It was noted that E-009 had about 230 flight hours.

The FAA acquired two Lear Fan test airframes, presumably from the E-004 to E-008 group. The airframes were tested at the Impact Dynamics Research Facility at the NASA Langley Research Center in Hampton, Virginia. The tests involved swinging the airframes into the ground from a 240 ft (73 m) gantry. This produced a 56 mph (90 km/h) forward velocity and an 1,860 fpm (9.4 m/s) descent rate at impact. The first aircraft was unmodified and tested in 1994. The fuselage broke in two above the wing, and the measured impact forces were greater than those recorded with comparable aluminum aircraft. The deformation and crumpling of aluminum absorbed some of the impact energy, while the composite structure of the Lear Fan absorbed less energy. The second airframe was modified with a composite, energy-absorbing subfloor and was tested on 15 October 1999. In addition, a plywood structure was built for the aircraft to collide with after ground impact. The fuselage cracked in a similar manner to the first airframe but the separation was less.

All three completed Lear Fan aircraft survive. E-001 (N626BL) hangs from the ceiling in the Great Gallery at the Museum of Flight on Boeing Field in Seattle, Washington. E-003 (N327ML/N21LF) hangs from the ceiling in the Frontiers of Flight Museum at Love Field in Dallas, Texas. E-009 (N98LF) was purchased by the FAA and is displayed outdoors at the Civil Aerospace Medical Institute, part of the Mike Monroney Aeronautical Center, adjacent to the Will Roger Airport in Oklahoma City, Oklahoma.


The second of two incomplete Lear Fan airframes owned by the FAA. The aircraft is pictured after its impact test on 15 October 1999. Off frame to the right is the concrete surface where the airframe made initial contact. It then slid onto the grass (note the red marker lines) and through the plywood barrier. A dirt berm was built-up on the left side of the plywood. Cracks in the fuselage can be seen near the plywood. The left engine cover with its integral duct have separated from the airframe. (NASA/Langley Research Center image)

– Email correspondence with John Penny
Stormy Genius by Richard Rashke (1985)
Lear Fan (brochure) by LearAvia Corp (1979)
Lear Fan Propulsion System by Daniel E. Cooney (April 1980)
Jane’s All the World’s Aircraft by John WR Taylor (various editions 1979–1985)
– “Lear Fan 2100—first report” by Bill Sweetman, Flight International (10 January 1981)
– “Lear Fan collapses,” Flight International (8 June 1985)
– “Crosswind TakeoffEnterprise (video, 1984)
Structural Integrity Evaluation of the Lear Fan 2100 Aircraft by H. P. Kan and T. A. Dyer (May 1996)
Simulation of an Impact Test of the All-Composite Lear Fan Aircraft by Alan E. Stockwell (October 2002)


Planet Satellite Light Aircraft

By William Pearce

John Nelson Dundas Heenan was born on 4 October 1892 in Altrincham, England. He became an engineer and worked for the family engineering firm Heenan & Froude in Manchester. Heenan left the family firm in 1935 when its parent company went bankrupt, and it was acquired by outside investors. Heenan worked for the British Air Ministry During World War II and cofounded the engineering consulting firm Heenan, Winn, and Steel (HW&S) in early 1946.


The cockpit mockup of the Planet Satellite on display in 1948. The major difference from the prototype is how the window panels above the door hinged up on the mockup, rather than sliding up as seen on the actual aircraft.

Like many others, Heenan believed that there would be a post-war boom in civil aviation with a huge need for light aircraft for private pilots. Working with others at HW&S, he designed an aircraft capable of carrying four to five passengers. Heenan decided that the aircraft should be built using a magnesium alloy with zirconium. However, due to a lack of experience with the metal, HW&S approached Magnesium Elektron Ltd to build the aircraft. Magnesium Elektron was owned by the Distillers Company Ltd, and its business had experienced a drastic contraction after the war. The Distillers Company was willing to consider options to expand Magnesium Elektron’s business and formed a partnership with HW&S to create Planet Aircraft Ltd. Planet Aircraft operated as a subsidiary of the Distillers Company to construct and produce the new aircraft, which was named Satellite. The aircraft was commonly referred to as the Planet Satellite.

The Satellite was a streamlined, low-wing, pusher monoplane with tricycle landing gear. The pusher configuration was chosen to reduce passenger cabin noise by isolating it from the engine and propeller. The two-piece fuselage was of monocoque construction and consisted of forward and rear sections. The magnesium fuselage was riveted together for the prototype aircraft, but production aircraft were to be welded. The fuselage was split just behind the wings for access to the engine, which was located aft of the passenger cabin and above the center wing section. A firewall separated the passenger cabin from the engine compartment.


The Satellite’s forward fuselage section under construction. The firewall around the engine is visible. Baggage compartments that were accessible in flight existed behind the rear bench seat and on each side of the engine. The many rivets of the prototype would have given way to a welded structure on production aircraft.

The forward fuselage section incorporated the passenger cabin and was 4 ft 8 in (1.42 m) in diameter at its widest point. The pilot and copilot/front passenger sat behind an expansive windscreen that extended to the nose of the aircraft. A bench that could accommodate up to three passengers was behind the pilot’s seat. Cabin access was via two doors that folded down, one by the pilot’s seat and one by the copilot’s seat. As the door was opened downward, the armrest folded down to act as a step. The window above each door slid up toward the center of the fuselage.

An inverted, U-shaped magnesium keel reinforcement ran internally along the bottom of the forward fuselage section from the nose of the aircraft to the wing’s leading edge. At the leading edge, the keel became a single plate that extended to the wing’s trailing edge. The wings and main landing gear were attached to the plate. The pneumatically-operated landing gear was fully enclosed, with the nosewheel retracting to the rear into the keel and the main gear legs retracting forward and into the fuselage. A landing light was incorporated into the front of the aircraft, just above the nosewheel.


The Satellite on display at the SBAC Farnborough Show in September 1948. The aircraft was not registered at the time, and was painted blue with a red accent. The main landing gear appears spindly and collapsed after the aircraft’s first hop.

To power the Satellite, buyers could choose between the 250 hp (186 kW) de Havilland Gipsy Queen 31 or the 145 hp (108 kW) de Havilland Gipsy Major 10. While both engines were inverted, inline, air-cooled designs, the six-cylinder Gipsy Queen had a 4.65 in (120 mm) bore, a 5.51 in (150 mm) stroke, a displacement of 621 cu in (10.18 L), and a weight of 510 lb (231 kg). The four-cylinder Gipsy Major had a 4.65 in (118 mm) bore, a 5.51 in (140 mm) stroke, a displacement of 374 cu in (6.12 L), and a weight of 312 lb (142 kg). The selected engine was affixed to a rail mount and could be slid out 18 in (.46 m) from the forward fuselage for maintenance once the rear fuselage was disconnected. A fan driven from the rear of the engine brought in cooling air via a duct atop the fuselage and expelled the heated air out the lower fuselage. Engine exhaust was also expelled in the same manner.

The wing had one main spar at its center and a false spar that supported the flaps and ailerons. The flaps ran along half of the wing’s trailing edge, with ailerons extending to the wingtips. Magnesium sheets 28 in (.71 m) wide were wrapped around the wing’s leading edge and extended to both the upper and lower trailing edges to form the wing skin. The wing had two degrees of dihedral, and each wing accommodated a 34 US gal (28 Imp gal / 127 L) fuel tank, for a total of 67 US gal (56 Imp gal / 255 L). With two additional wing tanks, the fuel capacity could be increased to 109 US gal (91 Imp gal / 414 L) for a long-range flight with a single pilot.


A good view illustrating access to the passenger cabin. Doors on each side of the aircraft folded down, and the armrest on the door became a step. The window panel above the door slid up. Note the long windscreen, and the landing light in the nose.

The forward and rear fuselage sections were joined via a quick-release locking “ring,” which Heenan had patented (GB 620,462: applied on 20 January 1947 and accepted on 24 March 1949). Control cables were automatically connected or disconnected in conjunction with the locking ring. The rear fuselage section incorporated the extension shaft, propeller, and Y tail.

The hollow extension shaft extended approximately 10 ft (3 m) from the engine to drive a two-blade, adjustable-pitch Aeromatic propeller at the extreme rear of the fuselage. The hollow steel shaft acted as an oil reservoir for the bearings that supported it. The propeller was 6 ft 6 in (1.98 m) in diameter. The ventral fin of the Y tail incorporated a rudder and a spring-loaded bumper to protect the propeller from ground impacts. The two “butterfly” horizontal stabilizers had 30 degrees of dihedral, which increased the aircraft’s directional stability. The Satellite’s control surfaces were of all-metal construction. The Planet Satellite had a wingspan of 33 ft 6 in (10.21 m), a length of 26 ft 3 in (8.00 m), and a height of 9 ft 3 in (2.82 m).

With a Gipsy Queen 31 engine, the aircraft had a top speed of 208 mph (335 km/h) at sea level and a stalling speed of 62 mph (100 km/h) at its maximum load. An economical cruise speed of 191 mph (307 km/h) was achieved at 3,500 ft (1,067 m), which resulted in a range of 1,000 miles (1,609 km) with a normal fuel load at maximum weight and 2,450 miles (3,943 km) with the extra fuel tanks and a single pilot. The Satellite had a 1,450 fpm (7.4 m/s) initial rate of climb and a ceiling of 22,000 ft (6,706 m). The aircraft had an empty weight of 1,600 lb (726 kg) and a maximum gross weight of 2,905 lb (1,318 kg). Fully loaded, the Satellite could take off in 570 ft (174 m). The Gipsy Queen-powered Satellite was offered for £3,500.


Rear view of the Satellite illustrates the aircraft’s Y tail. The line where the front and rear fuselage sections joined is visible just behind the wing’s trailing edge. The inlet for engine cooling air can be seen atop the fuselage.

With the significantly less powerful Gipsy Major 10 engine, the Satellite’s performance was reduced. The aircraft had a top speed of 173 mph (278 km/h) at sea level and a stalling speed of 54 mph (87 km/h) at its maximum load. An economical cruise speed of 161 mph (259 km/h) was achieved at 5,000 ft (1,524 m), which resulted in a range of 500 miles (805 km) with a normal fuel load at maximum weight and 2,150 miles (3,460 km) with the extra fuel tanks and a single pilot. The Satellite had a 950 fpm (4.8 m/s) initial rate of climb and a ceiling of 18,000 ft (5,486 m). The aircraft had an empty weight of 1,408 lb (639 kg) and a maximum gross weight of 2,280 lb (1,034 kg). Fully loaded, the Satellite could take off in 840 ft (256 m). The Gipsy Major-powered Satellite was offered for £2,500.

Detail design work on the Satellite started in April 1946. For Satellite construction, neither Planet Aircraft, Magnesium Elektron, or the Distillers Company had facilities to build the prototype aircraft. Magnesium Elektron contracted Redwing Aircraft Ltd to build two Satellite prototypes at their facility in Thornton Heath, near London. A mockup of the cockpit and forward fuselage section was completed in 1947, and the construction of two prototypes soon followed.

The first, nearly-complete Satellite made its public debut at the SBAC (Society of British Aircraft Constructors) Farnborough Show in September 1948. The aircraft was registered as G-ALOI on 26 April 1949. The Satellite was moved to Blackbushe Aerodrome, near Farnborough, for flight trials. Flight testing was to be conducted by Hugh Joseph “Willie” Wilson, who had resigned from the Royal Air Force as a Group Captain to serve as a director with Planet Aircraft. On 7 November 1945, Wilson had established a new World Air Speed Record at 606.262 mph (975.675 km/h) in a Gloster Meteor.


The Satellite sits derelict in a hangar at Redhill. The aircraft wears its G-ALOI registration, and a scoop to augment the intake of cooling air has been installed. The scoop was probably fitted after the first round of ground tests. Note that the gear doors are closed despite the landing gear being deployed. This did not appear to be possible from the Farnborough images. Perhaps the gear doors seen at Farnborough were mockups or a redesign occurred.

Wilson took the Satellite for high-speed taxi tests and did a tentative hop in the aircraft. Upon settling back on the ground, the landing gear promptly collapsed. The Satellite was repaired, and Wilson restarted the test program. Again at Blackbushe Aerodrome, Wilson took the aircraft to about 20 ft (6 m) above the runway. This time the landing was uneventful. However, a crack in the magnesium keel was discovered when the aircraft was inspected after the flight. Analysis of the crack indicated that the Satellite’s magnesium structure was severely understressed and would need an extensive rebuild to bring it into tolerance of its expected flight regime. The British Air Registration Board required that the aircraft be restressed before any further flights were made.

Although Heenan was an engineer, he was not an aeronautical engineer, and the Satellite was his first aircraft design. He once said that only 400 drawings were made during the Satellite’s design phase, compared to the roughly 3,000 drawings that would be expected for a comparable aircraft. With the design now coming up short, another £40,000 would be needed to resolve the Satellite’s deficiencies. The Distillers Company had already invested over £100,000 and withdrew further funding. The Satellite was moved to Redhill Aerodrome south of London, where it sat and slowly deteriorated until 1958, when it was finally scrapped.

The second Satellite prototype was registered as G-ALXP in 1950, but it was never completed. G-ALXP’s mostly-finished fuselage was later used by Firth Helicopters as the basis for the FH.01/4 Atlantic helicopter, a twin-rotor design which was built in 1952. The FH.01/4 Atlantic was also designed by HW&S, but it never flew and was eventually scrapped in the 1960s. Most likely by coincidence, the basic layout of the Planet Satellite would be resurrected in the late 1970s as the Lear Fan 2100, another unconventional aircraft constructed of unconventional materials in hopes of revolutionizing private air travel.


The fuselage of the second Satellite prototype was used for the Firth FH.01/4 helicopter, which never flew. The helicopter was donated to the College of Aeronautics at Cranfield in 1955, which is probably when the image above was taken.

The Planet Satellite by Planet Aircraft Ltd (cira 1948)
Jane’s All the World’s Aircraft 1949–50 by Leonard Bridgman (1949)
– “Heavenly Body” by Don Middleton, Aeroplane Monthly (October 1983)
– “Ones That Got Away: Planet Satellite” by Mike Jerram, Wingspan International (March/April 2001)
Aircraft Engines of the World 1948 by Paul H. Wilkinson (1948)
– “Improvements in and relating to Aeroplanes” by John Nelson Dundas Heenan, GB patent 620,462 (applied 20 January 1947)


Junkers Jumo 222 Aircraft Engine

By William Pearce

Around 1936, the RLM (Reichsluftfahrtministerium, or Germany Air Ministry) sought the design of an 1,800 hp (1,342 kW) engine for the next generation of bomber aircraft. Otto Mader, the head of the Junkers Flugzeug- und Motorenwerke AG (Junkers Aircraft and Motor Works) research institute in Dessau, considered various V and H configurations for such an engine. However, each configuration had various drawbacks. Mader discussed the engine requirements with Ferdinand Brandner and tasked him with the project. Brandner was an experienced engine and railway engineer that had recently joined Junkers.


A Junkers Jumo 222 A/B-1 engine with a short gear reduction housing. First run in 1939, the Jumo 222 represented what was believed to be the next generation of German aircraft engines. Note the coolant pump below the gear reduction housing and the fuel injection pump between the intake manifolds.

Brandner and his team set a goal of 2,000 hp (1,491 kW) and immediately began designing a completely new engine. The engine originally carried the manufacturer’s designation P2001, and Junkers submitted a proposal that outlined the 1,900 hp (1,417 kW) engine to the RLM on 4 December 1936. On 4 May 1937, the RLM placed an order for a prototype P2001 engine, which would officially become the Junkers Jumo 222 on 4 April 1938. Brandner and his team continued to work on the engine design, which was finalized on 4 June 1937. Nearly from the start, the Jumo 222 was intended to power the improved development of the Junkers Ju 88 bomber, which originally carrier the manufacturer’s designation EF 73 (Entwicklungs-Flugzeug 73, Development Aircraft 73). Later, the aircraft would become the Ju 288 bomber.

The Junkers Jumo 222 was a liquid-cooled inline radial. The engine had six cylinder banks placed radially around the crankcase at 60 degree angles, with the left and right banks horizontal. Each of the six cylinder banks had four cylinders, giving the engine a total of 24 cylinders. The outer points of the six cylinder banks formed a hexagon, making the Jumo 222 one of the rare hexagonal engines, like the Curtiss H-1640 Chieftain, the Wright H-2120, the SNCM 137, and the Dobrynin series of aircraft engines.

The two-piece aluminum crankcase was split horizontally below the left and right cylinder banks. The top four cylinder banks were on the upper crankcase half, and the bottom two cylinder banks were on the lower crankcase half. The two crankcase halves were joined by 11 studs on each side and four long studs that extended from the center main bearing. The one-piece balanced crankshaft was supported in the upper crankcase half by five plain main bearings. Each main bearing cap was secured by four vertical studs and one very long stud that passed transversely through the entire crankcase. Mounted on each of the crankshaft’s four throws was a rather typical radial-engine master rod with five articulating rods. The connecting rod was split, with three articulating rods attached to the bottom and the master rod flanked by two articulated rods on the top. The two pieces of the connecting rod were joined by four bolts. Reportedly, the master connecting rods for cylinder rows 1 and 4 (front and rear) were located in bank 5 (7 o’clock position), and cylinder rows 2 and 3 (middle two) were located in bank 6 (5 o’clock position). However, a drawing of the Jumo 222 depicts a master rod in one of the upper cylinder banks. It appears the drawing shows the configuration used in early engines, and the master rods were relocated to the lower cylinder banks in later variants. The flat-top aluminum pistons were rather short with two compression rings and two oil rings, all located above the piston pin. The engine’s compression ratio was 6.5 to 1.


Rear view of the Jumo 222 A/B-1 illustrating the supercharger and its two-sided inlet. Note how the intake manifold branches to serve two adjacent cylinder banks. The engine’s magnetos are mounted to the upper cylinder banks, and the oil pump is mounted under the supercharger.

The steel cylinder barrels were installed through the crankcase and sealed at their lower end by three rubber rings. Near the bottom of each barrel was a flange with four long studs that extended up through the cylinder head. The tightening of these studs drew the barrel up into the cylinder head and sealed it with a tapered aluminum ring to the combustion chamber. The combustion chamber was wedge-shaped with the exhaust valve on the short side. The cylinder head of each bank was a single aluminum casting secured to the crankcase by 10 studs. At the top of each cylinder were two intake valves and one sodium-cooled exhaust valve. A fuel injector was positioned between the two intake valves, and a spark plug was positioned between each of the two intake valves and the common exhaust valve. The valves for each cylinder bank were actuated via rockers by a single overhead camshaft. The rear of each camshaft was driven from the crankshaft by a series of spur gears. The camshafts turned clockwise for cylinder banks 1, 3, and 5 and counterclockwise for cylinder banks 2, 4, and 6.

Attached to the front of the crankcase was a planetary gear reduction with the propeller shaft positioned at the center of the engine. While the crankshaft turned clockwise, different gear reduction housings could be used to turn the propeller in either direction. The propeller of the Jumo 222 A, C, E and G models turned counterclockwise at .368 crankshaft speed. The counterclockwise gear reduction used a fixed planetary carrier with the propeller shaft driven from the free outer ring gear. The propeller of the Jumo 222 B, D, F and H models turned clockwise at .364 crankshaft speed. The propeller shaft of the clockwise gear reduction was driven from the free planetary carrier that rotated against the fixed outer ring gear. The clockwise gear reduction on the Jumo 222 B, D, F and H models weighed approximately 66 lb (30 kg) additional. A short gear reduction housing was available, but the extended version was most common. An inertia starter was mounted to the crankcase above the gear reduction housing, and the coolant pump was mounted below the gear reduction housing.

Attached to the rear of the crankcase was an accessory housing followed by the single-stage, two-speed supercharger. The supercharger impeller was 12.8 in (325 mm) in diameter and turned at 6.70 and 9.16 times crankshaft speed in low and high gears. It provided 8.8 lb (.61 bar) of boost for takeoff. Each of the three outlets from the supercharger fed an intake pipe that branched into two manifolds. These manifolds were positioned between the cylinder Vees at the 4, 8, and 12 o’clock positions, and each fed once cylinder bank. An eight-cylinder fuel injection pump was also positioned between the intake manifolds in each of these cylinder Vees. Individual exhaust stacks were fitted to the cylinder heads between the bank Vees at the 2, 6, and 10 o’clock positions. Engine mounting pads were located on the crankcase between the bank Vees at the 2 and 10 o’clock positions.


Sectional view of the Jumo 222 with a master connecting rod in an upper cylinder bank. This is most likely a Jumo 222 A/B-1 engine, as it appears to have an early H-beam articulated connecting rod design. Later variants had the master connecting rods in the lower banks and I-bean articulated connecting rods. Note the wedge-shaped combustion chambers.

When viewed from the rear, the cylinder banks were numbered counterclockwise starting at the right horizontal bank at the 3 o’clock position, which was bank 1. Bank 2 was at 1 o’clock, bank 3 was at 11 o’clock, and so on. The front cylinder of each bank was No 1, and the rear cylinder was No 4. Each of the upper two cylinder banks had a magneto mounted to its rear. Each magneto fired all the cylinders for three banks with no redundancy. If a magneto failed, one entire side of the engine would not fire. Cylinders in opposite banks fired simultaneously. The firing order changed during the engine’s development. The following firing order is specific to the Jumo 222 E/F but may be applicable to other engine models. The Jumo 222 E/F’s firing order was as follows: Bank 2 Cylinder 1 & Bank 5 Cylinder 2, B1C1 & B4C2, B6C4 & B3C3, B2C3 & B5C4, B1C2 & B4C1, B6C2 & B3C1, B2C4 & B5C3, B1C4 & B4C3, B6C1 & B3C2, B2C2 & B5C1, B1C3 & B4C4, and B6C3 & B3C4.

The Junkers Jumo 222 A/B-1 had a 5.31 in (135 mm) bore and stroke. The engine had a total displacement of 2,830 cu in (46.38 L). The Jumo 222 A/B-1 initially produced 2,000 hp (1,491 kW) at 3,200 rpm. At the expense of reliability, further development eventually pushed its maximum power at 3,200 rpm to 2,500 hp (1,417 kW) for takeoff and 2,200 hp (1,641 kW) at 16,404 ft (5,000 m). Climbing power at 2,900 rpm was 2,260 hp (1,685 kW) at sea level and 2,090 hp (1,559 kW) at 16,404 ft. Cruising power at 2,700 rpm was 1,900 hp (1,617 kW) at sea level and 1,700 hp (1,268 kW) at 17,060 ft (5,200 m). The engine’s fuel consumption at cruise power was .477 lb/hp/hr (290 g/kW/h) at sea level. The Jumo 222 A-1 weighed 2,690 lb (1,220 kg), and the Jumo 222 B-1 weighed 2,745 lb (1,245 kg). The engine had a diameter of 3 ft 10 in (1.16 m) and was 7 ft 5 in (2.25 m) long.

In early 1938, RLM requested that the Jumo 222’s output be increased to 2,000 hp (1,491 kW). Since the engine was designed from the start for 2,000 hp (1,491 kW), this request did not present any issues, but it foreshadowed what was to come. A single-cylinder test engine was first run in March 1938, followed by one complete row of six cylinders in June 1938. On 24 April 1939, a complete Jumo 222 A/B-1 was run for the first time and taken up to 3,000 rpm. The engine was disassembled and inspected after the test and showed no signs of wear or issues.

In May 1939, Junkers submitted to the RLM design proposals for the Jumo 222-powered EF 73 (Ju 88 development) bomber aircraft. Incidentally, EF 74 was the same basic aircraft but powered by Jumo 224 engines. Encouraged by Junkers’ proposal, the RLM issued specifications in July 1939 for a new medium bomber capable of high-speeds. Originally known as Kampfflugzeug B (Warplane B), the aircraft proposal was eventually renamed Bomber B. The Bomber B specification requested an aircraft that could carry a 2,000 kg (4,410 lb) bomb load 3,600 km (2,237 mi) and have a top speed of 600 km/h (373 mph). For alternatives to the Jumo 222, the RLM requested engine designs from BMW and Daimler-Benz. The Junkers Bomber B proposal became the Ju 288, and other entrants included the Arado E.240, Focke-Wulf Fw 191, Dornier Do 317, and later, Henschel Hs 130C. The additional engine proposals were the BMW 802 18-cylinder radial and the Daimler-Benz DB 604 X-24.


A Jumo 222 installed in the nose of a Junkers Ju 52 transport test bed. The engine was equipped with exhaust manifolds to duct the fumes away from the cockpit. Note how the Jumo 222’s engine nacelle appears no larger than those for the Ju 52’s standard 725 hp (541 kW) engines.

The Ju 288 was selected for production, although prototypes of the Fw 191, Do 317, and Hs 130C would also be built. The Ju 288 and the Fw 191 were to be powered by the Jumo 222, which was ultimately selected over the other engines. The Jumo 222 was also planned for a future development of the Do 317. When war officially broke out on 1 September 1939, the Ju 288 was perceived as an aircraft needed for a decisive victory. It and the 2,000 hp (1,491 kW) Jumo 222 A/B-1 were given a high priority. At the time, three complete Jumo 222 A/B-1 engines were running on test stands. During 1939, Junkers had formed the Otto-Mader-Werke at Dessau to focus on engine design and development. This division was run by Mader and worked on the Jumo 222 and Jumo 004 (turbojet) engines.

In March 1940, the Jumo 222 A/B-1 achieved 2,000 hp (1,491 kW) for the first time, but some difficulties were encountered at this higher output with inadequate lubrication and connecting rod issues. Modifications were made to resolve the deficiencies, and the revised engine was running in August 1940. For flight testing, the Jumo 222 was installed in the center position of a Junkers Ju 52 trimotor transport and made its first flight on 3 November 1940. However, the Jumo 222 was not ready to be installed in the Ju 288, and the aircraft made its first flight on 29 November 1940 powered by 14-cylinder BMW 801 radial engines.

In April 1941, the Jumo 222 A/B-1 completed a 100-hour type test at 2,000 hp (1,491 kW), running at 2,860 rpm. Some of the issues during the test included spark plug damage after 60 hours, a leaking injection pump controller at 75 hours, and a camshaft bearing block failure after 88 hours. When the engine was dismantled after the test, coolant and fuel leaks were discovered, but they were not considered serious. Based on the overall positive results of the 100-hour test, the RLM ordered the Jumo 222 into production on 30 April 1941. The engine would be built at the new Flugmotorenwerke Ostmark (Aircraft Engine Factory in annexed Austria) plant under construction in Wiener Neudorf, Austria, with production expected to start on 30 August 1942. A monthly output of 1,000 engines was forecasted.

Starting in mid-1940, the RLM began to alter requirements for the Ju 288. A fourth crew member, additional equipment, and airframe changes resulted in the aircraft’s weight increasing to the point that 2,000 hp (1,491 kW) was no longer sufficient for the Ju 288 to achieve its originally-specified performance. Around mid-1941, the RLM requested that the Jumo 222 produce 2,500 hp (1,864 kW) for the Ju 288. Junkers had foreseen this request and began developing the Jumo 222 A/B-2 in 1940 to reliably produce 2,500 hp (1,864 kW) and resolve issues encountered with the early engines.


A 2,000 hp (1,491 kW) Jumo 222 A/B-1 installed in a Junkers Ju 288 engine nacelle. Note the individual exhaust stacks protruding from the cowling.

The Jumo 222 A/B-2’s cylinder bore was increased .20 in (5 mm) to 5.51 in (140 mm), while its stroke remained unchanged at 5.31 in (135 mm). This change increased the Jumo 222 A/B-2’s displacement by 214 cu in (3.50 L) to 3,044 cu in (49.88 L). The H-beam articulated connecting rods of the early engines were replaced with an I-bean articulated connecting rod design. The engine’s compression ratio may have been raised to 6.735 to 1, and valve diameters may have been altered slightly. The Jumo 222 A/B-2 had a balance pipe between the intake manifolds of adjacent cylinder banks. Engine speed was limited to 2,900 rpm in an attempt to increase its reliability. The Jumo 222 A/B-2’s maximum power at 2,900 rpm was 2,500 hp (1,864 kW) for takeoff and 2,490 hp (1,857 kW) at 16,404 ft (5,000 m). Climbing power at 2,700 rpm was 2,250 hp (1,678 kW) at sea level and 2,050 hp (1,529 kW) at 16,404 ft (5,000 m). Cruising power at 2,500 rpm was 1,900 hp (1,417 kW) at sea level and 1,750 hp (1,305 kW) at 16,404 ft (5,000 m). The engine’s fuel consumption at cruise power was .449 lb/hp/hr (273 g/KW/h) at sea level.

The Jumo 222 A/B-2 was first run in mid-1941 and was taken briefly to 3,000 hp (2,237 kW) by overboosting to 11.5 psi (.79 bar) in October 1941. However, the increased bore size created a harmonic resonance within the engine. With three Ju 52s serving as Jumo 222 test beds and a number of other engines on test stands, the entire project began to encounter significant issues. Connecting rod bearings were still a problem as was corrosion of the engine’s internal components. Despite the issues, the Jumo 222A/B-1-powered Ju 288 V5 made its maiden flight on 8 October 1941. Brandner had managed to talk his way onto the aircraft for the flight, which was completed without issue. For the Ju 288, the Jumo 222 turned a four-blade Junkers VS 7 propeller that was a 13 ft 1 in (4.0 m) in diameter. An annular radiator was positioned in the cowling, and experiments were conducted on Ju 288 V5 using a ducted spinner to deliver cooling air to the radiator.

As the manufacturing plant in Austria neared completion in late October 1941, it was clear that the Jumo 222 was not going to be ready for production. A decision was made to manufacture the Daimler-Benz DB 603 at the plant with production starting in March 1942. On 24 December 1941, the RLM cancelled the Jumo 222 for the Ju 288. The decision was based on the engine’s then-current takeoff rating of only 2,000 hp (1,491 kW), its ongoing issues, and its operational readiness not being sufficient for the Ju 288’s planned production schedule. The Ju 288 would be powered by DB 610 (two coupled DB 605s) engines, and Junkers would focus on developing the Jumo 213 inverted V-12. Work on the Jumo 222 would continue, but the engine was no longer a priority. Brandner stated that, at the time, various Jumo 222 engines had completed 20 100-hour test runs, and many at Junkers felt that the engine was basically ready for production. However, further issues with the connecting rod bearings caused a developmental delay that extended from January to March 1942.

The connecting rod bearing failures took a long time to resolve with experimentation of different bearing materials and lubrication techniques. Ultimately, a new connecting rod design was employed, the antimony alloy bearing material was replaced with a tin alloy, and the synthetic engine oil used was switched to a natural oil with an increased sulfur content. Due to tin shortages, antimony had been substituted early in the engine’s development.


A Jumo 222 A/B-2 or -3 engine with an extended gear reduction housing. Note the revised intake manifolds with a balance pipe joining the two at their center. Two engine mounting pads are visible between the upper cylinder banks.

the Jumo 222 A/B-3 was developed to cure the vibration and harmonic issues of the A/B-2 and with an improved supercharger to maintain power up to 20,997 ft (6,400 m). Along with a revised gear train, the engine incorporated all other revisions to improve reliability. The Jumo 222 A/B-3’s maximum power at 3,000 rpm was 2,500 hp (1,864 kW) for takeoff and 2,410 hp (1,797 kW) at 9,186 ft (2,800 m). Climbing power at 2,700 rpm was 2,250 hp (1,678 kW) at sea level and 1,980 hp (1,476 kW) at 20,997 ft (6,400 m). Cruising power at 2,500 rpm was 1,860 hp (1,387 kW) at sea level and 1,640 hp (1,223 kW) at 20,997 ft (6,400 m). The engine’s fuel consumption at cruise power was .463 lb/hp/hr (282 g/Kw/h) at sea level.

The Jumo 222 A/B-3 was developed quickly, and was first run in late 1941. Like the Jumo 222 A/B-2, it was briefly tested to 3,000 hp (2,237 kW) by overboosting to 11.5 psi (.79 bar) on 26 May 1942. The RLM became interested in the Jumo 222 A/B-3 and ordered it into production on 5 August 1942. Production would be undertaken at a plant in Prague in German-occupied Czechoslovakia. Optimistically, the first Jumo 222 A/B-3s were expected in October 1944 with a peak production of 1,500 engines per month achieved in September 1945. Running at 2,500 hp (1,864 kW), the Jumo 222 A/B-3 completed a 50-hour test on 9 December 1942 and a 100-hour test on 11 March 1943. The engine was tested in Ju 52s and Ju 288 aircraft, but the production plans were never realized.

The Fw 191 made its first flight in early 1942 and used BMW 801s. It was not until December 1942 that the Fw 191 V6 (third aircraft built) flew with Jumo 222 engines. With engine issues and constant changes to the underperforming bomber aircraft, the Bomber B program was cancelled in June 1943. Germany was short on resources, which were better utilized in the production of fighter aircraft rather than building troubled experimental bombers with problematic engines.

The Jumo 222 C/D was conceived in 1941 to produce 2,500–3,000 hp (1,864–2,237 kW) for high-altitude operations. The Jumo 222 C/D was designed and built with its bore increased to 5.71 in (145 mm) and its stroke increased to 5.51 in (140 mm). This gave the Jumo 222 C/D a total displacement of 3,386 cu in (55.48 L). The engine produced 3,000 hp (2,237 kW) at 3,000 rpm and could maintain much of that power up to 32,808 ft (10,000 m) thanks to an improved supercharger. Some reports indicate the Jumo 222 C/D was first run in mid-1942, but it was never given priority or considered for production until 1945, when a 3,000 hp (2,237 kW) engine was desperately needed. Apparently, two Jumo 222 C/D engines were completed, but the deteriorating war conditions shifted priorities and prevented them from being tested.


The Ju 288 V5 (A-series, three-man crew) was the first of the type to fly with Jumo 222 engines. The cowling incorporated an annular radiator that was fed via a ducted spinner. Subsequent prototypes powered by Jumo 222 engines used standard spinners.

In late 1943, the Jumo 222 E/F was developed from the A/B-3 series with a 5.51 in (140 mm) bore, 5.31 in (135 mm) stroke, and 3,044 cu in (49.88 L) displacement. The engine was equipped with a two-stage, two-speed supercharger. While the primary stage of the supercharger was mechanically driven, the auxiliary stage used an infinitely variable fluid coupling. An air-to-water aftercooler was incorporated on each of the three intake pipes between the supercharger and where the pipe branched into the two intake manifolds. Coolant for the aftercooler system was circulated by a separate pump. Reports indicate that the Jumo 222 E/F had sodium-cooled intake valves and a 6.75 to 1 compression ratio.

The Jumo 222 E/F’s maximum power at 3,000 rpm was 2,500 hp (1,864 kW) for takeoff and 1,930 hp (1,439 kW) at 29,528 ft (9,000 m). Climbing power at 2,700 rpm was 2,220 hp (1,655 kW) at sea level and 1,680 hp (1,253 kW) at 36,000 ft. Cruising power at 2,500 rpm was 1,840 hp (1,372 kW) at sea level and 1,400 hp (1,044 kW) at 34,689 ft (11,000 m). The engine’s fuel consumption at cruise power was .454 lb/hp/hr (276 g/kW/h) at sea level. At 42,651 ft (13,000 m), an output of 1,710 hp (1,275 kW) at 2,900 rpm was possible with GM 1 (Göring Mischung 1 / Göring Mixture 1) nitrous oxide injection. The addition of MW 50 (Methanol-Wasser 50), a 50-50 mixture of methanol and water injected into the induction system, further boosted performance by approximately 400 hp (298 kW) up to the engine’s critical altitude. The engine was 8 ft 2 in (2.50 m) long. The Jumo 222 E weighed 3,009 lb (1,365 kg), and the Jumo 222 F weighed 3,075 lb (1,395 kg). First run in 1944, the engine initially received a high priority. However, development and plans for mass production of the Jumo 222 E/F were halted in mid-1944 to focus resources on the Jägernotprogramm (Emergency Fighter Program).

Reports indicate that the Jumo 222 E/F was flown in Ju 288 V9, and presumably it was also tested in a Ju 52. Heinkel He 219 V16 was planned to test Jumo 222 A/B-3 engines, but Jumo E/F engines were used instead when the aircraft made its first flight on 23 July 1944. Estimates indicated that the Jumo E/F-powered He 219 would be capable of 414 mph (666 km/h) at 32,808 ft (10,000 m) and 435 mph (700 km/h) with MW 50 injection at 26,247 ft (8,000 m). With the shift in priorities, He 219 V16 made less than 20 flights, and the project was abandoned by January 1945. Six Jumo 222 E/F engines were finished by war’s end, and another four were partially completed.

In early 1944, the Jumo 222 G/H (sometimes referred to as the Jumo 222 Turbo) was developed from the A/B-3 series with a 5.51 in (140 mm) bore, 5.31 in (135 mm) stroke, and 3,044 cu in (49.88 L) displacement. The engine incorporated a turbocharger and intercoolers. Running at 3,200 rpm, the Jumo G/H produced 2,400 hp (1,790 kW) for takeoff and 2,070 hp (1,544 kW) at 40,354 ft (12,300 m). At 2,900 rpm, the engine produced 1,970 hp (1,469 kW) at 41,339 ft (12,600 m). A single Jumo 222 B-2 was used as the G/H test engine and made 22 runs before the end of the war, but it was not installed in any test aircraft.


The Ju 288 V9 (B-series, four-man crew) with standard spinners in flight. Just visible is the annular radiator mounted inside the cowling. Note the lower rows of exhaust stacks under the cowling.

On 28 April 1944, the Otto-Mader-Werke at Dessau, which was developing the Jumo aircraft engines, was heavily damaged by an Allied bombing raid. As a result, the Jumo 222 program was relocated to Oberursel near Frankfurt. These events caused major delays with all tests and engine work then in progress.

Various versions of the Jumo 222 were flown in approximately 11 aircraft: three Ju 52 test beds, six Ju 288s (V5, V6, V8, V9, V12, and V14), one Fw 191 (V6), and one He 219 (V16). Jumo 222 engines were also planned for the Heinkel He 219B and C and the Hütter Hü 211 heavy fighters. Engines were not ready for the He 219B and C airframes, and the two Hü 211 prototypes were destroyed while under construction during an Allied bombing raid in December 1944. Some sources state that Jumo 222 engines were fitted to a four-engine Heinkel He 177 (V101), as the burned out remains of this aircraft were found at Cheb in Czechoslovakia. However, examination of the aircraft reveals the engine’s exhaust stacks were in the standard four and eight o’clock positions for a Daimler-Benz DB 603 engine rather than the 2, 6, and 10 o’clock positions for the Jumo 222. The Jumo 222 was proposed for numerous other aircraft designs ranging from fighters, like the Focke-Wulf Ta 152, to bombers, like the Heinkel He 177. However, none of these plans came to fruition. A total of 289 Jumo 222 engines were built.

The Jumo 225 was conceived back in 1937 as a development of the basic Jumo 222. The Jumo 225 was a 36-cylinder engine with six banks of six cylinders. With the original 5.31 in (135 mm) bore and stroke, the Jumo 225 displaced 4,245 cu in (69.57 L). The engine was forecasted to produce 3,500–4,000 hp (2,610–2,983 kW) at 3,000 rpm and was 8 ft 10 in (2.69 m) long. The Jumo 225 was never built.

While the Jumo 222 was not trouble-free, its development progressed as well as could be hoped for considering it was a new engine design, the repeated changes to engine requirements and design, and that the ongoing war resulted in material shortages. Some contend that the changing Ju 288 and Jumo 222 requirements were intentionally made to cause the aircraft and engine to fail.


The 2,500 hp (1,864 kW) Jumo 222 E displayed at the Deutsches Museum in Munich. The two-stage supercharger added to the engine’s overall length. Note the revised induction system that incorporated aftercoolers and new intake manifolds. (Deutsches Museum image)

Heinrich Koppenberg was the managing director of Junkers, the only German company producing both aircraft and aircraft engines. Koppenberg had become a powerful man who worked himself into various positions that gave him control over many strategic resources. Erhard Milch was the Air Inspector General of the Luftwaffe and in charge of aircraft production. He had gained increasing control over aircraft procurement in Germany. Milch felt that Koppenberg and Junkers would have an aircraft production monopoly and economically ruin other companies if the current Ju 288, Jumo 222, and other company projects were successful. New large-scale Junkers production orders meant that resources at other companies would be allotted to produce Junkers products under license rather than develop their own. Some contend that Milch began to alter the official requirements just as they were about to be met by Junkers. After Ernst Udet, head of the T-Amt (Technisches Amt, Technical Office of the RLM), committed suicide on 17 November 1941, Milch took his place. Milch now had the power to dictate programs for the Luftwaffe. Acting as the RLM’s authority, Milch continued to change project requirements, which left Junkers to perpetually chase the goal. Koppenberg was imprisoned in April 1942 when Junkers repeatedly failed to achieve what the RLM asked of them. While the above may be true, it is also true that the Jumo 222 had its own design issues. Brandner felt the engine was “developed to death” with its numerous displacement changes and constant design revisions.

In Spring 1944, Japan and Germany entered negotiations for Japan to purchase production rights for the Jumo 222. An agreement was reached in September 1944 that included drawings, sample parts, and the assistance of Brandner in exchange for 10 million Reichsmarks. The trip was to be made via submarine, and the departure date was set for mid-January. However, Brandner was shifted to resolve issues with the Jumo 004 turbojet in December 1944 and never went to Japan. It is not clear if Jumo 222 parts and plans were ever sent.

Brandner was captured by the Soviets at the end of the war and was interned in the USSR until 1953. While there, he worked on the M-222 engine design, which was essentially a reconstruction of the Jumo 222. Although the Soviets had captured five examples of the Jumo 222, the M-222 engine was never built. Among other projects, Brandner led a team that developed the 12,000 hp (8,948 kW) Kuznetsov NK-12 turboprop engine that powered the Tupolev Tu-95 Bear and other aircraft.

In addition to the Soviets, the United States and the British captured a number of Jumo 222 engines at the end of the war. A Jumo 222 E was built up by the United States Army Air Force at Wright Field with the intent to test the engine’s performance. While the engine was mostly complete by the end of 1946, other priorities took precedence, and the captured Jumo 222 E was never tested. Most likely, this engine was returned to Germany in 1978. It is now on display at the Deutsches Museum in Munich and is the only Jumo 222 known to exist.


This Jumo 222 E was captured and sent to the United States for testing. It was most likely the engine that Wright Field planned to test in late 1946. The engine was returned to Germany in 1978. Note the starter mounted above the gear reduction housing. (Deutsches Museum image)

– “The Junkers Jumo 222” by Kimble D. McCutcheon, Torque Meter Volume 6, Number 3 (Summer 2007)
Junkers Flugtriebwerke by Reinhard Müller (2006)
Flugmotoren und Strahltriebwerke by Kyrill von Gersdorff, et. al. (2007)
Ein Leben Zwischen Fronten by Ferdinand Brandner (1973)
Junkers Aircraft & Engines 1913–1945 by Antony Kay (2004)
Jane’s All the World’s Aircraft 1945/46 by Leonard Bridgman (1946)
Junkers Ju 288/388/488 by Karl-Heinz Regnat (2004)
Heinkel He 219 by R. Francis Ferguson (2020)
Dornier Do 217-317-417 An Operational Record by Manfred Griehl (1991)


Delage 12 GVis and 12 CDirs Aircraft Engines

By William Pearce

Louis Delâge was born in Cognac, France on 22 March 1874. He received an engineering degree in 1893 and started a career in the fledgling automobile industry in 1900. In 1903, Delâge joined the Société Renault Frères (Renault Brothers Company). By 1905, Delâge had a good sense of the incredible potential offered by the automotive industry and formed his own automobile company, la Société des Automobiles Delage (the Delage Automobile Company), in Levallois-Perret, just northwest of Paris.


The Delage 12 GVis seen with its Elektron crankcase side covers removed, revealing the magneto and generator. The engine is equipped with double helical propeller reduction gears. The lower engine support can be seen extending from the valve covers to the rear mount.

The Delage automobile was a success, and the company soon also began developing race cars. Delage racers won the 1908 Grand Prix de Dieppe, the 1911 Grand Prix de Boulogne-sur-Mer, the 1913 Grand Prix de France, and the 1914 Indianapolis 500. Racing and the production of passenger cars was halted during World War I, and Delage produced munitions and vehicles for the military. After World War I, Delage returned to the automotive business and began to produce luxury vehicles. In 1921, Albert Lory was hired as a designer, and he was put in charge of the competition department in 1923. That same year, Delage returned to racing. Lory designed the Delage 15S8 Grand Prix racer and its high-revving, straight-eight engine that won the Manufacturers’ Championship in 1927. The company withdrew from competition after this victory.

In 1930, Louis Delâge believed that the lessons learned through the development of the company’s compact and powerful automotive racing engines could be applied to aircraft engines. Lory was tasked with the development of two aircraft engines—the 12 GVis for fighter aircraft and the 12 CDirs for a Coupe Deutsch de la Meurthe racer. The two engines had similar layouts overall and mainly differed in their size. While there were no real restrictions on the fighter engine, the engine for the Coupe Deutsch de la Meurthe race had to be under 488 cu in (8.0 L).


The 12 GVis crankcase as it would be installed with the crankshaft at top: A) gear reduction mounting flange, B) camshaft housing, C) crankshaft mount, D) one of the four bolts extending through the crankcase, E) magneto mount, F) generator mount, G) studs for mounting the cylinder head, H) barely visible hole to receive a cylinder barrel, and I) pass through holes for the valve train’s pushrods.

The 12 GVis and 12 CDirs were water-cooled, inverted V-12 engines equipped with twin superchargers. The engines and their accessories were designed as a compact package with minimal frontal area to encourage better streamlining. Each engine consisted of a cast aluminum crankcase that also formed the lower part of the two cylinder banks, which had an included angle of 60 degrees. As later described, the two engines did have different styles of crankcase designs. Nitrided steel cylinder barrels were bolted via flanges to the two cast aluminum cylinder heads, which were then secured via studs to the crankcase. The cylinder barrels for each bank passed through a large, open water jacket space in the crankcase and were received by openings near the crankshaft. The balanced, one-piece crankshaft spun in roller bearings and was secured to the crankcase by seven main bearings. The crankcase was closed by an Elektron (magnesium alloy) cover. Side-by-side connecting rods with roller bearings were mounted to the crankshaft.

Each cylinder had two spark plugs, two paired intake valves, and two paired exhaust valves. The paired valves for all cylinders were actuated via rockers and pushrods from the engine’s single camshaft located in the Vee between the cylinder banks. A valve spring did not surround each of the valve stems. The spring for each valve pair was mounted adjacent to the valves and applied pressure to the valve pair via a levered arm. As the pushrod acted on the rocker to open the valve pair, the tip of the lever moved down with the valve stems. The opposite end of the lever moved up, further compressing the spring in its mount. The spring exerted tension on the lever to return and hold the valves in the closed position. Delage believed this system reduced the amplitude of the spring’s oscillations, increased the spring’s damping, and allowed for higher engine rpm. A valve rig was reportedly tested to the equivalent of 10,000 engine rpm, which means each valve had 5,000 actuations per minute.


Left, front view of the 12 GVis illustrating the engine compact structure. The barometric valve can be seen on the intake manifold between the cylinder banks. Right, rear view of the 12 GVis displaying the engine’s twin Roots-type supercharger. Note how the rear of the engine bolts to the mount.

Two Roots-type superchargers were mounted to the rear of the engine. These were of a similar design to the superchargers used on Delage automobiles. The superchargers were driven without clutches and directly from the engine at 1.67 (1.5 in some sources) times crankshaft speed. Via twin two-lobe rotors, the superchargers supplied 17.66 cu ft (500 L) of air per second to the intake manifold positioned in the Vee of the engine. The superchargers provided 14.5 psi (1.00 bar) of boost and enabled the engine to maintain its rated power up to 16,404 ft (5,000 m), at which altitude a barometrically-controlled bypass valve was fully closed. This valve prevented over boosting at lower altitudes and sustained a constant intake manifold pressure. The engine’s single carburetor was installed at the Y junction where the two superchargers fed into the intake manifold.

Some sources indicate that the French government ordered a single prototype of the 12 GVis and a single prototype of the 12 CDirs. However, other sources state that no orders for the 12 GVis were ultimately placed, and only a single order for the 12 CDirs was received. Both engines were proposed to power aircraft manufactured by Avions Kellner-Béchereau.


The 12 GVis as displayed at the 1932 Salon de l’Aéronautique. The engine and cowling represented a complete installation package that could be quickly attached to an aircraft. The access panels covering the magento and generator are removed. Note the valve cover protruding from the cowling and the oil cooler mounted above the engine.

The designation of the Delage 12 GVis stood for 12 cylinders, Grand Vitesse (High Speed), inverse (inverted), and suralimenté (supercharged). The engine had a 4.33 in (110 mm) bore and a 4.13 in (105 mm) stroke. Each cylinder displaced 61 cu in (1.0 L), and the engine’s total displacement was 731 cu in (11.97 L). The 12 GVis had a compression ratio of 5.5 (5.8 in some sources) to 1 and initially produced 450 hp (336 kW) at 3,600 rpm. It was believed that the engine’s output could be increased to 550 hp (410 kW) or even 600 hp (447 kW) with further development. The engine weighed 1,014 lb (460 kg). Two propeller gear reductions were offered: a .472 reduction via double helical gears, which was installed on the prototype, and a .528 reduction via Farman-type planetary bevel gears. The propeller turned counterclockwise.

The crankcase of the 12 GVis was cast with compartments on its sides to mount various accessories. A magneto was mounted in the compartment on each side of the engine, and a generator was mounted in the left-side compartment. The compartments were sealed with Elektron covers. The basic form of the engine and its crankcase created an aerodynamic installation that did not need to be covered by a cowling. The back of the 12 GVis was mounted directly to the airframe, and a conventional engine mount was not used. Four long bolts passed through the entire length of the crankcase to secure the engine to its mount. An additional lower support ran from the engine’s Vee to the rear mount. This support bolted to special pads on the inner sides of the valve covers. The engine was further secured by other mounting pads on its rear side.


The Delage 12 CDirs was a direct development from the larger 12 GVis. The engine had a more conventional crankcase without compartments for accessories. The large pipe on the crankcase was the outlet for the cooling water, and another outlet was present on the opposite side.

The 12 GVis was proposed for the Kellner-Béchereau KB-29 fighter, which was based on the KB-28 racer (see below). The 12 GVis was displayed in November 1932 at the Paris Salon de l’Aéronautique. The engine had a cowling covering its lower half, but the upper sides were uncowled, and the crankcase accessory covers were removed. A surface oil cooler was incorporated in a cowing panel mounted above the engine. The 12 GVis may have suffered from reliability issues and failed to complete an acceptance test. Ultimately, the KB-29 fighter was never built, and there were no other known applications for the 12 GVis.

The designation of the Delage 12 CDirs stood for 12 cylinders, Coupe Deutsch, inverse (inverted), réducteur (gear reduction), and suralimenté (supercharged). The engine had a 3.94 in (100 mm) bore and a 3.32 in (84.4 mm) stroke (some sources state 84.5 or 84 mm stroke). Each cylinder displaced 40 cu in (.66 L), and the engine’s total displacement was 485 cu in (7.95 L). The 12 CDirs had a compression ratio of 5.5 (5.2 in some sources) to 1 and initially produced 370 hp (276 kW) at 3,800 rpm. Development of the engine had increased its output to 420 hp (313 kW) at 4,000 rpm, and it was hoped that 450 hp (336 kW) would ultimately be achieved. The engine weighed 816 lb (370 kg). A .487 propeller gear reduction was achieved via double helical gears, and the propeller turned counterclockwise. While still somewhat aerodynamic, the 12 CDirs possessed a conventional crankcase and did not have the compartments that were incorporated into the 12 GVis. Accessories, including two vertical magnetos, were mounted to the rear of the engine. Engine mounting pads were positioned along each side of the crankcase, and the lower support and rear mounts similar to those used on the 12 GVis were employed.


Rear view of the 12 CDirs displaying the two vertical magnetos, two Roots-type superchargers, and the Y intake pipe. The right water pump can be seen under the supercharger. Note the brace extending from the valve covers to the rear of the engine.

The 12 CDirs passed an acceptance test running 53 hours at 4,000 rpm with no reported issues. The engine was installed in the Kellner-Béchereau KB-28 (also known as 28VD) Coupe Deutsch de la Meurthe racer. The aircraft incorporated a surface oil cooler in the front upper cowling, and surface radiators covered the wings. Flown by Maurice Vernhol, the 28VD made its first flight on 12 May 1933. The aircraft needed to qualify for the Coupe Deutsch de la Meurthe by 14 May, so there was little time for development of the airframe or engine. Based on previous tests, Vernhol felt that the ground-adjustable propeller was not utilizing the engine’s full power and requested that it be set to a finer pitch.

In the afternoon on 14 May 1933, Vernhol took off for a qualification flight. As he went to full throttle during his flight, the engine revved to an excess of 4,400 rpm—600 rpm over its intended limit. A coolant hose blew, and Vernhol was sprayed with steam and hot water. Partially blinded, Vernhol attempted an emergency landing, but misjudged the touchdown and hit the ground hard. The landing gear was sheared off, and the aircraft flipped upside down. The engine was torn free, and the fuselage broke behind the cockpit. Vernhol escaped with only minor injuries, but the 28VD was damaged beyond repair. No other aircraft are known to have flown with Delage engines.

Creating powerful and reliable aircraft engines that ran for long periods at high power proved to be more of a challenge than originally anticipated, and Delage abandoned its work on the type in 1934. The company was in a bad financial state and went into bankruptcy in April 1935. That same year, the Delage name and assets were purchased by the Delahaye automobile company.


The Kellner-Béchereau 28VD (KB-28) seen perhaps right before what may have been its last flight. The 28VD was the only aircraft to fly with a Delage engine. Capitaine Maurice Vernhol sits low in the cockpit, illustrating the aircraft’s limited forward visibility. Jacques Kellner is at left, standing next to Louis Delâge. Albert Lory can be seen on the other side of the cockpit. Kellner joined the French Resistance during World War II and was executed by the Nazis on 21 March 1942. Delâge’s automotive company was a victim of the Great Depression and was sold off in April 1935. He died nearly destitute in 1947. Lory went on to design the SNCM 130 and 137 aircraft engines and then worked for Renault after the war.

– “Les Moteurs d’aviation francias en 1935” by Pierre Léglise, L’Aéronautique No 191 (April 1935)
Aerosphere 1939 by Glenn D. Angle (1939)
– “Le Coupe Deutsch de la Meurthe” by L. Hirschauer, L’Aérophile 14 Annee No 6 (June 1933)
– “The 1933 Contest for the Deutsch de la Meurthe Trophy” by Pierre Léglise, L. Hirschauer, and Raymond Saladin, National Advisory Committee for Aeronautics Technical Memorandum No. 724 (October 1933)
Delage, France’s Finest Car by Daniel Cabart, Claude Rouxel, and David Burgess-Wise (2008)
Les Moteurs a Pistons Aeronautiques Francais Tome I by Alfred Bodemer and Robert Laugier (1987)
– “Les moteurs d’aviation Delage” La Vie Automobile (25 November 1932)
Jane’s All the World’s Aircraft 1933 by C. G. Grey (1933)
– “Le Kellner-Béchereau 28V.D.” by Michel Marrand, L’Album du Fanatique de L’Aviation 23 (June 1971)


Kellner-Béchereau 28VD Air Racer

By William Pearce

Société Kellner was a French luxury coachbuilder run by Georges Kellner. During World War I, the company turned to producing SPAD VII, S.XI, and S.XIII fighter aircraft under license. The SPAD (Société Pour L’Aviation et ses Dérivés / Company for Aviation and its Derivatives) aircraft were designed by French aeronautical engineer Louis Béchereau. After World War I, Société Kellner returned to coach making, and SPAD went out of business. Béchereau progressed through a number of companies until 1926, when he founded the Société pour la Réalisation d’Avions Prototypes (SRAP / Prototype Aircraft Company).


The Kellner-Béchereau 28VD under construction. The cowling attached to the very front of the aircraft contained the surface oil cooler. The top of the coolant tank is visible just behind the fairing atop the engine. Note the fuel tanks forward and aft of the cockpit.

Société Kellner was taken over by Jacques Kellner after his father’s passing. Jacques was an aviation enthusiast and wanted to steer the company back to being involved with aviation. In 1931, Jacques Kellner joined forces with Louis Béchereau to form Avions Kellner-Béchereau in Boulogne-Billancourt, France. Kellner-Béchereau immediately began designing aircraft, and one of their first concepts was that of the 28VD (also known as KB-28), an air racer intended for the 1933 Coupe Deutsch de la Meurthe. The Coupe Deutsch de la Meurthe was a race to cover 1,242 miles (2,000 km) with a mandatory 90-minute stop at 621 miles (1,000 km), and aircraft were limited to using a single engine with a displacement no greater than 488 cu in (8.0 L). Additional stops could be made but were not mandatory and would count against the total time to finish the course. Ten laps of the 124-mile (200-km) course would complete the race, and the rhombus-shaped course was laid out with towns of Chartres, Moisy, Orléans, and Étampes at its corners. The Étampes-Mondésir airfield was the start and finish point, and the prize in 1933 was four million Francs.

The Kellner-Béchereau 28VD was a low-wing taildragger made almost entirely of metal, and its design was tested in a wind tunnel. The aircraft’s slim monocoque fuselage was of all-aluminum construction with an open cockpit at its center. A sloped fairing led up to the cockpit, and an extended headrest trailed from it. This resulted in the pilot sitting rather low with little forward visibility, but side visibility was quite good. Fuel tanks were housed in front of and behind the cockpit. The aircraft’s vertical and horizontal stabilizers were made of aluminum, but the rudder and elevators were made of wood. The angle of the horizontal stabilizers was adjustable and could be altered to trim the aircraft while in flight. An aerodynamic fairing partially covered the tailskid.


The 28VD undergoing final touches. This image gives a good view of how the surface radiators wrapped around the wing’s leading edge. Note the large Ratier metal propeller. Intakes to the engine’s superchargers can just been seen on the cowling’s undersides.

The relatively-short, cantilever wings of the 28VD were attached to the fuselage by a main spar at its center and a rear spar. The wings were further supported by false front and rear spars. A large aileron ran almost the entire length of the wing’s trailing edge and was attached to the false rear spar. Wing construction kept its interior mostly open, and three fuel tanks were positioned in each wing. Each of the wing tanks was equipped with a quick-drain dump valve 3.94 in (10 cm) in diameter. For the valve, carbonic acid gas was fed into a space that blew out a lower seal, allowing an upper plug to fall free followed by the contents of the fuel tank. Although not specifically stated, it is presumed that the pilot would control the flow of the carbonic acid gas to initiate the fuel dump. It is not clear if the fuselage tanks were also equipped with a dump valve.

The upper surface of each wing was covered with radiators in five sections. Each surface radiator section consisted of a forward and rear part. The front radiator for each section curved around the front of the wing to form the leading edge. The inner three radiator sections terminated shortly after making the turn to the wing’s underside. The outer two sections continued around the leading edge to cover the front half of the underwing, and additional radiators covered the rear outer surface under the wing. Water from the coolant tank installed above the engine flowed through pipes in the wing’s leading and trailing edges and then into the surface radiators. After passing through the radiator, the cooled water was collected in a tube running along the center spar and returned to the engine. A large fairing connecting the wing’s trailing edge to the fuselage contained a number of louvers to allow heat, vapors, and moisture to escape from the wing.


The newly completed 28VD is rolled out of the hangar for testing. The aircraft’s streamlining and slim fuselage are apparent. This image provides a good view of the landing gear’s arched supports. For retraction, the top of the gear leg slid toward the wingtip, and the lower gear leg pivoted around the arched support.

Mounted under the inboard sections of the wings was the partially-retractable main landing gear, which had a 4 ft 10 in (1.48 m) track. When extended, a fixed ball at the top of each gear leg was locked into place, and the leg itself was supported by an arched member attached to the fuselage. The ball atop the gear leg was mounted in a channel in the wing. To retract the gear, a retraction lever released the downlock and bled pressure in a cylinder, which unlocked a drum and allowed a cable to unwind. As the gear leg pivoted around its arched support, an elastic cable pulled the top of the gear leg toward the wing tip until the gear leg rested against the underside of the wing. An uplock under the inner wing secured the gear leg in the retracted position, and the arched support provided a crude aerodynamic fairing. To extend the gear, an extension lever released the uplock and fed pressurized air into a cylinder. The piston in the cylinder rotated a drum which wound a cable. The cable was attached to the upper gear legs and pulled them inboard against the tension of the elastic cable. Once the cable had pulled the gear to its extended position, the ball atop the gear leg was secured by the downlock.

Housed in a streamlined, close-fitting cowling at the front of the 28VB was the Delage 12 CDirs engine. Built by la Société des Automobiles Delage (the Delage Automobile Company), the engine was specially made for the Coupe Deutsch de la Meurthe race. Its “12 CDirs” designation stood for 12 cylinders, Coupe Deutsch, inverse (inverted), réducteur (gear reduction), and suralimenté (supercharged). The 400 hp (298 kW) engine was a water-cooled V-12 with twin-Roots superchargers. The 12 CDirs had a 3.94 in (100 mm) bore, a 3.31 in (84 mm) stroke, and a displacement of 483 cu in (7.92 L). Intakes in each side of the lower cowling brought in air to the engine’s superchargers. Exhaust was expelled through individual stacks protruding from the cowling. A saddle water tank sat atop the rear part of the engine. A U-shaped oil tank was installed between the engine and the propeller. A surface oil cooler was positioned atop the engine and covered the area between the water tank and the spinner. The engine turned a two-blade, metal, ground-adjustable Ratier propeller that was approximately 7 ft 9 in (2.37 m) in diameter.


Elevated view of the 28VD illustrates the surface radiators covering the upper wings. Note the vents in the wing’s trailing edge fairing. The race number “5” has been applied to the fuselage. This image was most likely taken on 14 May 1933, the day of the accident, as the aircraft is prepared for its qualification flight.

The Kellner-Béchereau 28VD had a wingspan of 21 ft 10 in (6.65 m), a length of 23 ft 6 in (7.16 m), and a height of 8 ft 8 in (2.64 m). The aircraft weighed 2,176 lb (987 kg) empty and 3,527 lb (1,600 kg) fully loaded. The 28VD had an anticipated top speed of 249 mph (400 km/h) and a cruising speed of 214 mph (345 km/h). On 5 May 1933, the aircraft was moved to the Étampes-Mondésir airfield where it would be completed for the Coupe Deutsch de la Meurthe, to be held on 28 May. Qualifying for the race was scheduled 8–14 May, which left very little time for flight testing. The 28VD was given race number 5 and made its first flight on 12 May. Armée de l’Air Capitaine Maurice Vernhol conducted the very brief flight tests, which did not reveal any issues, and would fly the 28VD for the race. Refining and preparing the aircraft used up most of the qualifying time. Based on previous tests, Vernhol felt that the engine’s full power was not being utilized and requested that the propeller be adjusted to a finer pitch.

During an afternoon qualification flight on 14 May 1933, Vernhol added full power, and the engine revved to an excess of 4,400 rpm—over 600 rpm more than its maximum limit. At that moment, a coolant hose blew free from its mount, and Vernhol was enveloped in a shower of steam and hot water. It is not clear if the increased coolant pressure from the engine overspeed caused the hose to blow free, or if it was just bad timing. Regardless, Vernhol was blinded by the spray and attempted an emergency landing near Ville Sauvage, north of the Étampes-Mondésir airfield. In his impaired condition, Vernhol misjudged the landing, and the 28VD hit the ground hard. The extended landing gear broke off, and the aircraft flipped upside down, tearing off the engine and breaking the fuselage behind the cockpit. Amazingly, Vernhol escaped with only minor injuries, but the 28VD was completely destroyed. A Potez 53 flown by Georges Détré went on to win the 1933 Coupe Deutsch de la Meurthe at a speed of 200.58 mph (322.81 km/h).

Kellner-Béchereau also designed a fighter along the same lines as the 28VD / KB-28. Known as the KB-29, the fighter was powered by a 550 hp (410 kW), 731 cu in (11.97 L) Delage 12 GVis inverted V-12 engine. The engine was displayed at the 1932 Paris Salon de l’Aéronautique, but the KB-29 fighter never materialized.


The remains of the 28VD after its forced lading. The landing gear and engine have been ripped away, and the fuselage is broken at a right angle behind the wing. The surface radiators under the outer wing are visible. The circular openings seen in the wing’s underside are the dump valves for two of the three fuel tanks.

– “Les avions de la Coupe Deutsch de la Meurthe 1933” by Pierre Léglise, L’Aéronautique No 171 (August 1933)
– “L’éphémère Kellner-Bechereau KB 28” by Robert J. Roux, Le Fana de l’Aviation No 253 (December 1990)
– “Le Kellner-Béchereau 28V.D.” by Michel Marrand, L’Album du Fanatique de L’Aviation 23 (June 1971)
– “Le Coupe Deutsch de la Meurthe” by L. Hirschauer, L’Aérophile 14 Annee No 6 (June 1933)
– “The 1933 Contest for the Deutsch de la Meurthe Trophy” by Pierre Léglise, L. Hirschauer, and Raymond Saladin, National Advisory Committee for Aeronautics Technical Memorandum No. 724 (October 1933)


Guidobaldi 1939 Tilting-Body Prototype Racer

By William Pearce

Francois Guidobaldi was born in 1888 and lived in Antibes, France, near Nice and the Italian boarder. In the early 1900s, Guidobaldi became a cycling champion and started to focus on mechanics and engineering. He filed around 28 patents over his lifetime, with a carburetor for automobiles being one of the first, awarded in 1912. In the late 1920s and early 1930s, Guidobaldi was a team mechanic for Benoît Falchetto, who drove Bugatti racers. During that same period, Guidobaldi either founded or worked for the Société anonyme, moteurs à explosion pour aviation, marine et automobiles (MEAMA / the Combustion Engines for Aviation, Marine and Automobiles Corporation), at which he designed and built an air-cooled, two-row, 10-cylinder aircraft engine.


Francois Guidobaldi started work on his race car in 1939, and it took two decades to finish. Looking similar to a late 1930s Auto Union Grand Prix racer, the car’s body hides its unusual suspension system that enabled the chassis to lean into turns and its custom two-stroke, air-cooled, eight-cylinder radial engine. (Cyril de Plater image)

After lengthy consideration and planning in the mid-1930s, Guidobaldi began work on a race car. Guidobaldi’s car would incorporate a number of his ideas on how to improve handling—namely, the body and wheels would tilt in toward the curve when the vehicle was in a turn. This lean would help minimize lateral g-forces and improve the vehicle’s stability. Work on Guidobaldi’s engine design and the car’s chassis development started around 1939. The car’s basic mid-engine layout was similar to that of the Auto Union Grand Prix racers that were very successful in the late 1930s. However, the outbreak of World War II and material shortages after the war caused Guidobaldi to make slow progress on his car, which at times was called the “Guidomobile.”

They chassis of Guidobaldi’s car consisted of two large tubular steel side rails connected by various cross members. Triangular structures at the front and rear of the chassis attached it to the vehicle’s independent suspension. The suspension consisted of a series of rubber dampers sandwiched between steel plates. The chassis mounting point near the top of the triangular suspension structure enabled the chassis to hang and swing like a pendulum from the mount. While in a turn, centrifugal force would swing the chassis and prevent the buildup of lateral g-forces. At the same time, the suspension was engineered so that it would articulate with the body and the steering so that the tires leaned into the turn. The net result of the design was that the car took turns somewhat like a motorcycle. Strong springs connecting the upper suspension links to the chassis centered the chassis when the vehicle was traveling in a straight line and helped moderate the centrifugal effect when in a curve.


This picture appeared in a French newspaper in 1951. However, if Francois Guidobaldi is in the driver’s seat, he is much too young for the picture to have been taken in 1951. It is possible that the picture was taken much earlier and near the start of the car’s construction in 1939. Note the angle of the chassis and rear tires as they lean into the turn.

The driver sat in a low cockpit at the center of the car. The gas tank was located behind the cockpit and extended forward along both sides of the driver at the bottom of the cockpit. The engine was positioned between the fuel tank and the rear suspension mount. Mounts for the engine were clamped to the tubular frame rails. The three-speed transaxle with reverse gear was positioned under the rear suspension mount. At all four corners, the car had Bugatti knock-off wheels and Bugatti finned drum brakes.

Perhaps the only thing more unusual than the car’s leaning chassis was its engine. Guidobaldi designed, built, and installed into his car a two-stroke, eight-cylinder, air-cooled radial engine. The engine was placed horizontally (crankshaft was vertical) to get the car’s center of gravity as low as possible. From the drive side of the engine (under the car), a shaft extended forward to power two Roots-type superchargers built by Bugatti, and another shaft extended to the rear to power the transaxle. Two Guidobaldi-designed carburetors added the fuel. From the supercharger, the air and fuel mixture was delivered to the cylinders via intake manifolds under the car.


The “Guidomobile’s” engine package being tested. Laying a radial engine flat provided a low center of gravity, but it also complicated the drive arrangement. The two Bugatti superchargers on the left were driven via a single shaft from the engine. Note the individual exhaust stacks above the frame rail.

Intake and exhaust ports were on the drive side of the engine, facing the ground. An intake valve was positioned in each cylinder head and controlled via a rocker and pushrod by a cam ring atop the engine. The exhaust port was located on the cylinder just below the intake port. The flow of exhaust was controlled by the piston uncovering and covering the port. Exhaust from each cylinder flowed into an exhaust stack, with four stacks exiting horizontally on each side of the car. Two magnetos were mounted horizontally on the top (non-drive) side of the engine. Each magneto fired one of the two spark plugs mounted in the cylinder and flanking the intake valve. The engine was started via a remote starter engaged through the transaxle at the rear of the car.

The engine’s exact displacement is a bit of a mystery. One publication lists the bore and stroke as 2.36 in (60 mm), which would give a displacement of 83 cu in (1.36 L). However, many other publications state the engine’s displacement was either 61 or 67 cu in (1.0 or 1.1 L), which would result in a bore and stroke of approximately 2.17 in (55 mm). The engine was later enlarged to 91 cu in (1.5 L). If the bore and stroke were originally 2.36 in (60 mm), the bore would have been increased to 2.48 in (63 mm) to achieve a total displacement of 91 cu in (1.5 L). However, if the bore and stroke were originally 2.17 in (55 mm), then the bore would need to be increased to 2.60 in (66 mm) to reach a displacement of 91 cu in (1.5 L). It seems more reasonable to increase the bore of the air-cooled cylinders by .12 in (3 mm) than it does by .43 in (11 mm). Regardless of its actual displacement, the engine produced 180 hp (132 kW) at 6,500 hp in its original form. The engine’s power rating at 91 cu in (1.5 L) has not been found, but was presumably around 200 hp (147 kW). At some point after 1960, the two Bugatti superchargers were replaced by a single Roots supercharger of Guidobaldi’s own design.


The bodiless “Guidomobile” on display at the Exposition Automobile de Nice in 1956. An aged Francois Guidobaldi stands at center holding his hat. The triangular suspension structures are apparent, as is the low-mounted engine. The circular mounts near the top of the suspension structures are the pivot points for the chassis, allowing it to swing.

Guidobaldi built the car almost entirely by himself, even making wooden cores to create clay molds for casting parts. The main structure of the chassis was seemingly complete by 1951, and the unbodied car was displayed at the Exposition Automobile de Nice in 1956. Some sources state the Guidobaldi undertook test drives in the late 1950s on the winding roads in the Maritime Alps. There are accounts that the car gave an unsettling ride while traversing curvy roads. The chassis would rise and fall as the suspension leaned back and forth, subjecting driver to movement in all directions. It is not clear when the aluminum body was added to the car, and no pictures of the original body have been found. A new body was constructed in 2010 when the car was restored. The new body reportedly follows the aesthetics of the original body, which had been badly damaged over the years.

The aluminum body built in 2010 covers the entire chassis. Large cutouts are present by each wheel to allow room for the suspension’s travel. A number of inlets provide ample cooling air to the engine. Air from the from grille passes through the entire body. A large scoop on each side of the car, just behind the front wheels, delivers cooling air to the engine. Two smaller scoops atop the engine cowling provide further cooling. A number of louvers behind the engine help the heated air leave the engine compartment. During the 2010 restoration, the exhaust stacks were lengthened and turned up 90 degrees to exit the top of the car’s sides.


Side view of Guidobaldi’s restored car illustrates the cooling air scoops just behind the front wheel and on the side of the engine compartment. Note the numerous louvers and vertical exhaust stacks. The fuel filler cap is just behind the driver’s headrest. (Cyril de Plater image)

Guidobaldi had always planned to build a passenger car using the same leaning chassis concept, but no such car was ever built. In the 1960s, a number of automobile manufacturers had an interest in Guidobaldi’s car. Perhaps the most interested was Mercedes-Benz, but negotiations failed to yield any agreement. Guidobaldi predicted his race car could achieve a top speed of 174 mph (280 km/h) and had eyed Formula 1 as a testing ground, but a lack of funds curtailed his aspirations. The expenses of building the car and filing patents left Guidobaldi with little extra money. On 6 August 1971, Guidobaldi passed away. His car had already been locked away and nearly forgotten.

In 1980, Antoine Raffaelli purchased the car from Guidobaldi’s son, Virgile. The car was then sold to Adrien Maeght in 1984. Maeght displayed the car at the Musée de l’Automobile à Mougins until the museum closed in early 2009. At that time, Guidobaldi’s car was purchased by David Humbert, who ordered a total restoration. Luc Franza completed the restoration to running order in 2010, and the restored “Guidomobile” debuted at the 8th Avignon Motor Festival in March. The car was then displayed at the Monaco Motor Show in June 2010 and in March 2014 at the

Concours d’Elegance du Monaco, where it won first prize in the technology category. Starting in September 2014, the car was displayed at the Cité de l’Automobile – Musée National, Collection Schlumpf in Mulhouse, France. In late 2019, and still in perfect condition, the “Guidomobile” was offered for sale. The vehicle serves as a lasting tribute to all engineers who chase their dreams.


Rear view of the “Guidomobile” shows the car with a slight lean into a right turn. Note the different angles of the left and right rear suspension arms. Just visible at the top of the car is the rubber damper system for the rear suspension. (Cyril de Plater image)

– “Un mécanicien d’Antibes a réalisé cette monoplace de 180 Ch (Formule 1)” Supplement à l’Automobile N°107 (March 1955)
– “La voiture révolutionnaire de F, Guidobaldi” by Sven-Ake Nielsen, l’Automobile N°157 (March 1959)
– “The Leaning Car” by Sven-Ake Nielsen, Sportscar Graphic (April 1960)
– “Un rêve d’inventeur” by Dominique Pascal, Auto Passion N°6 (September 2006)
– “Un chef d’oeuvre ressuscité” by Philippe Loisel, La vie de l’Auto N°1424 (16 September 2010)
– “Road Vehicle Tilting Inwardly in Curves” US patent 2,791,440 by Francois Guidobaldi (granted 7 May 1957)


Curtiss XP-23 / YP-23 Hawk Biplane Fighter

By William Pearce

On 8 July 1931, the United States Army Air Corps (AAC) issued production contract W535-ac-4434 to the Curtiss-Wright Corporation for the production of 46 P-6E Hawk fighter aircraft. The P-6E was one of many variants that had branched from the P-6 line, which originated in 1928. The basic P-6 was a refined P-1 equipped with a Curtiss V-1570 Conqueror engine; however, the 46th aircraft from contract W535-ac-4434 would not be finished as a P-6E. Rather, it would become the Model 63, which carried the AAC designation XP-23. In the early 1930s, the AAC was interested in exploring advancements with turbosuperchargers to create a fighter capable of high speeds at high altitudes, and the XP-23 was an opportunity to create just such an aircraft.


The Curtiss XP-23 Hawk with an unidentified individual (contact us if you can ID). Visible is the large turbosupercharger, its intake, and the two exhaust pipes feeding the turbine. Note the engine coolant radiator between the main gear.

The Curtiss XP-23 was a single engine biplane with conventional fixed taildragger undercarriage. The only components the aircraft had in common with a P-6E were the wings, although some sources state that the wings had a spar and rib frame built of metal rather than wood. Whether it was made of wood or metal, the wing’s frame was covered in fabric. The upper wing had a 1.5-degree dihedral, was mounted 4 in (102 mm) higher than on the P-6E, and was positioned 28.5 in (724 mm) forward of the lower wing. The lower wing had no dihedral and was 5 ft 6 in (1.68 m) shorter in span. Ailerons were located on the upper wing only.

The aircraft’s monocoque fuselage and tail were of all-metal construction. The main and reserve fuel tanks were housed forward of the cockpit and held a total of 78 US gallons (65 Imp gal / 295 L). The oil tank held 11 US gallons (9 Imp gal / 42 L). A .30-cal machine gun was mounted on each side of the aircraft just forward of the cockpit. A long blast tube extended from each gun, through the engine bay under the exhaust, and exited just behind the spinner. Some sources indicate the armament was one .30-cal and one .50-cal machine gun, while other sources state two .30-cal and one .50-cal machine gun. It is not clear where the third gun would have been located, if indeed there was one. Aerodynamic fairings covered the main wheels except for their outer side.


Side view of the XP-23 illustrated the aircraft’s rather smooth, all-metal finish. Note the machine gun port just under the engine’s exhaust and the left-handed (counterclockwise) propeller. The image was dated 12 April 1932, four days before the aircraft was accepted by the AAC.

The XP-23 was powered by a liquid-cooled Curtiss V-1570 Conqueror V-12 engine, which was equipped with a General Electric F-2C turbosupercharger. The turbosupercharger was externally mounted to the left side of the engine. Exhaust from the left cylinder bank was fed directly into the turbosupercharger, and exhaust from the right cylinder bank was ducted through the cowling just behind the engine and to the turbosupercharger. The intake was just forward of the turbosupercharger. The engine did not have a mechanically-driven supercharger or blower.

The turbosupercharger enabled the V-1570 engine to produced 600 hp (447 kW) at 2,400 rpm from sea level to 15,000 ft (4,572 m). The V-1570 had a 6.1 to 1 compression ratio and consumed 60 US gph (50 Imp gph / 227 L/h) at full throttle and 36 US gph (30 Imp gph / 136 L/h) at 2,100 rpm (cruise power / 88% throttle). At a .500 reduction, the engine turned a metal, three-blade, ground-adjustable Hamilton Standard propeller that was 9 ft 6 in (2.90 m) in diameter. Mounted under the engine and between the main gear was the radiator for the engine’s ethylene glycol cooling system.


The drag-inducing installation of the side mounted turbosupercharger is illustrated in this rear view of the XP-23. Note the reduced span of the lower wing.

The XP-23’s upper wing had a span of 31 ft 6 in (9.60 m), and its lower wing had a span of 26 ft (7.92 m). The aircraft had a length of 23 ft 9 in (7.24 m) and a height of 8 ft 9 in (2.67 m). The XP-23’s top speed was 223 mph (359 km/h) at 15,000 ft (4,572 m) and 178 mph (286 km/h) at sea level. The aircraft’s cruising speed was 192 mph (309 km/h) at 15,000 ft (4,572 m), and its stalling speed was 69 mph (111 km/h) at sea level. The XP-23 had an initial climb rate of 1,370 fpm (6.96 m/s), and its service ceiling was 32,000 ft (9,754 m). The aircraft’s range was 292 miles (470 km) at full throttle and 435 miles (700 km) at cruise power. The XP-23 had an empty weight of 3,142 lb (1,425 kg) and a gross weight of 4,032 lb (1,829 kg).

The XP-23 was allotted serial number 32-278 and built at the Curtiss Airplane Division Plant 1 on Kenmore Avenue in Buffalo, New York. The aircraft was accepted by the AAC on 16 April 1932 at a cost of $12,279.36. Although the XP-23’s performance met expectations, there is some indication that the turbosupercharger overheated and was unreliable. Regardless, the age of biplane fighters was at an end, and the XP-23 was the last biplane fighter accepted by the AAC. The Boeing P-26 Peashooter prototype was the AAC’s first monoplane fighter to enter service and made its first flight on 20 March 1932. The P-26 out-performed the XP-23 and showed that the monoplane type was the future.


The YP-23 with the turbosupercharger removed and a two-blade propeller installed. It also appears that either a support was installed between the main wheels or that a fairing was installed over the existing brace wires.

Curtiss had proposed powering the XP-23 with a V-1800 Super Conqueror engine. The V-1800 had a mechanically-driven supercharger that eliminated the bulbous side-mounted turbosupercharger previously used on the XP-23 and resulted in a much cleaner cowling. The engine produced 800 hp (597 kW) at 2,400 rpm and turned a 10 ft (3.05 m) diameter, metal, three-blade, ground-adjustable Hamilton Standard propeller at a .714 reduction. With the V-1800, the XP-23 had an anticipated top speed of 234 mph (377 km/h) at 12,000 ft (3,658 m) and a cruise speed of 199 mph (320 km/h). At 23 ft 11 in (7.29 m) long and 3,227 lb (1,464 kg) empty, the aircraft was 2 in (51 mm) longer and 85 lb (39 kg) heavier than the V-1570-powered variant. Curtiss did note that the wing might need to be moved forward slightly to achieve a proper center of gravity. However, the V-1800 was never installed in the XP-23.

The sole XP-23 was modified by removing the turbosupercharger, but the V-1570 was retained. It is not clear if the modifications were in anticipation of further changes to incorporate the V-1800, or if it was done to compare the turbosupercharger setup to the normally-aspirated V-1570. With the turbosupercharger removed, the aircraft became commonly known as the YP-23. The engine’s air intake was positioned atop the cowling, a two-blade propeller was fitted, and its armament was removed. In this configuration, the YP-23 achieved 200 mph (322 km/h) at 15,000 ft (4,572 m).


A new cowling was made for the YP-23 that did not incorporate gun ports below the engine’s exhaust stacks. Note the intake atop the cowling and the Wright “Arrowhead” painted on the fuselage. The aircraft as pictured is similar in appearance to the proposed V-1800-powered XP-23.

The YP-23 underwent one last round of modifications to explore the effects of radiator drag on high-speed aircraft. The coolant radiator was removed, the V-1570 engine was switched to a total-loss water cooling system, and the aircraft’s main fuel tank was used as a water reservoir. Using fuel from the reserve tank, cooling water flowed through the engine at a reduced rate from the main tank and was then vented overboard. The previous deletion of the turbosupercharger and the removal of the radiator gave the YP-23 an exceptionally clean appearance. Unfortunately, test results of these modifications have not been found. It is possible that thorough testing was never conducted since monoplanes offered higher performance. The YP-23 was disassembled, and its wings were reportedly used on the XF11C-1 Goshawk prototype fighter for the United States Navy.


The YP-23 in its final form with the radiator removed and serving as the AAC’s last biplane fighter design. While the aircraft exhibits an exceptionally clean appearance, its flight endurance was very short with its total-loss cooling system.

Curtiss Fighter Aircraft by Francis H. Dean and Dan Hagedorn (2007)
Curtiss Aircraft 1907–1947 by Peter M. Bowers (1987)
U.S. Fighters 1925 to 1980s by Lloyd S. Jones (1975)
American Combat Planes of the 20th Century by Ray Wagner (2004)


Wright H-2120 Hexagonal Engine

By William Pearce

In April 1926, the Curtiss Aeroplane and Motor Company (Curtiss) initiated the design of a 600 hp (447 kW) air-cooled aircraft engine. The engine was of a “hexagonal” design, with six banks of two cylinders, and had a relatively small diameter. Known was the H-1640 Chieftain, the two-row engine experienced some cooling issues and was abandoned shortly after the merger of Curtiss with Wright Aeronautical (Wright) in July 1929.


The liquid-cooled Wright H-2120 was developed from the air-cooled Curtiss H-1640 Chieftain. The engine was designed when experiments with two-row radials had just begun and concerns existed about air-cooling being sufficient for the rear cylinders.

In 1930, the United States Navy (Navy) initiated a special “high-speed development program” to challenge the success achieved by foreign high-speed aircraft, especially those demonstrated in the 1929 Schneider Trophy contest. Wright resurrected the hexagon engine design to further exploit its relatively small diameter. Using the H-1640 as a foundation, a liquid-cooled engine with an increased bore and stroke was designed by Wright. The new six-bank engine was to ultimately have four cylinders per bank, giving the 24-cylinder engine a displacement of 4,240 cu in (69.5 L) and an output of over 2,000 hp (1,491 kW). However, development was initiated with just two cylinders in each bank, and the 12-cylinder engine was known as the H-2120.

In June 1931, the Navy issued Contract No. 22625 to Wright for the development of two 1,000 hp (746 kW) H-2120 engines. From these developmental engines, a service type was to be derived. The Navy, always with an interest in air-cooled engines, stipulated that an air-cooled version was to be developed as either a companion to or a replacement of the liquid-cooled version. The Navy felt the air-cooled H-2120 could serve as competition and a backup to the 870 hp (649 kW), air-cooled, 14-cylinder Pratt & Whitney R-2270 radial, which was under development.

In a sense, the Wright H-2120 was three V-4 engines on a common crankcase, which created its hexagonal shape when viewed from the front. The two-row engine had an aluminum, three-piece crankcase that was split vertically at the centerline of the cylinders. The crankcase sections were secured together with bolts positioned between the cylinder banks. The single-piece, two-throw, crankshaft was supported by three main bearings. An odd connecting rod arrangement consisted of one blade rod, four articulated rods, and one fork rod. However, the blade and fork rod moved as a unit, as the pins that held the articulated rods passed through both the blade rod and the fork rod. The connecting rod arrangement was referred to as having dual master rods, with both the blade rod and fork rod technically considered master rods.


With six cylinder banks, the front view of the H-2120 illustrates its hexagonal shape. Note the coolant manifolds at the front of the engine.

The cylinder banks were spaced at 60-degree intervals around the crankcase, with the left and right banks perpendicular to the engine. The individual cylinders had a steel barrel surrounded by a steel water jacket. Each cylinder pair that formed a bank had a common cylinder head. Each cylinder had two intake valves and two exhaust valves, all actuated by dual overhead camshafts. The camshafts for each cylinder bank were geared to a vertical shaft driven from the front of the engine. The cylinders had a compression ratio of 6.5 to 1.

Mounted to the front of the engine was a planetary gear reduction that turned the propeller shaft at .6875 times crankshaft speed. At the rear of the engine was a single-speed supercharger that turned at 5.45 times crankshaft speed. Air was drawn through a downdraft carburetor, mixed with fuel, and compressed by the supercharger’s 11 in (279 mm) impeller. The air and fuel mixture was distributed to each of the six cylinder banks by a separate manifold. Each manifold had four short runners to deliver the charge to each cylinder’s two intake ports. The cylinder banks were arranged so that their intake and exhaust sides were mirrored with the adjacent cylinder banks. Each cylinder’s two spark plugs were fired by magnetos positioned at the rear of the engine. Coolant for the top four cylinder banks was circulated up from the base of each cylinder water jacket and through the cylinder head. Coolant for the lower two cylinder banks was the reverse—it flowed through the inverted head and up to the base of the water jacket.

The Wright H-2120 had a 6.125 in (156 mm) bore, a 6.0 in (152 mm) stroke, and a total displacement of 2,121 cu in (34.76 L). The engine had a sea level rating of 1,000 hp (746 kW) at 2,400 rpm with 2.2 psi (.16 bar) of boost, and it had a takeoff rating of 1,100 hp (820 kW). The H-2120 was 49 in (1.24 m) in diameter and was 57 in (1.45 m) long. The engine weighed 1,440 lb (653 kg).


Side view of the first H-2120 illustrates the relatively short length of the engine. Note the supercharger housing and the intake manifolds.

The first H-2120 engine carried the Wright Manufacture’s No. 11691 and the Navy Bureau of Aeronautics No. (BuNo) 0120. The BuNo is often incorrectly recorded as 0210 or 0119 in Wright and Navy documentation. The H-2120 engine encountered issues that delayed its development. The issues were mainly focused on the connecting rod arrangement. Several different connecting rod arrangements were tested and discarded before the dual master rod type was adopted. The engine was first run in late 1933 or early 1934. It failed a 50-hour endurance test conducted by Wright in January 1935, but the cause of the failure has not been found. The test involved 10 cycles of running the engine for 30 min at 1,000 hp (746 kW) and 4.5 hours at 900 hp (671 kW). The endurance test was rerun, and the H-2120 passed on 10 May 1935.

The Army Air Corps (AAC) was seeking an engine capable of 1,250 hp (932 kW) for takeoff and had been following the development of the H-2120. Starting around January 1935, the Navy and Wright began to share information on the engine’s development with the AAC. In August 1935, progress on the engine had again slowed, and the AAC asked the Navy if it could assist with H-2120 testing and development. The Navy had planned to use the first engine for bench testing and the second engine for at least 25 hours of flight tests. By early September, the first engine was in the middle of a 50-hour Navy type test, with other tests yet to be conducted. The Navy had lost interest in the liquid-cooled engine and was planning to convert the second engine to air-cooling after the 25 hours of flight trials. The conversion was expected to involve just new cylinders and valve gear. If all went well, two additional air-cooled engines would be ordered that incorporated whatever changes were deemed desirable from the previous tests. The second engine was Manufacture’s No. 11692 / BuNo 0121, and it was undergoing its initial test runs after assembly at Wright.

In response to the AAC’s request, the Navy proposed that it continue tests with the first engine, and the second engine would be delivered to the AAC for flight tests. If the AAC wanted to test the engine beyond the 25 hours, they were free to do so. If the engine showed promise, the Navy would order a small number of air-cooled versions. The AAC agreed to these terms, provided they could do some preliminary engine tests before the H-2120 was installed in an aircraft.


Rear view of the engine shows the downdraft carburetor, two magnetos, generator, and starter. Water pumps were located at the bottom of the engine.

By the end of September 1935, testing had included 200 hours of single cylinder tests, and the first H-2120 had completed 56 hours at 1,000 hp (746 kW), 44 hours at 900 hp (671 kW), and 140 hours of calibration and miscellaneous tests. A 50-hour Wright endurance test and a 50-hour Navy type test had been completed. During the Navy test, which was completed on 15 September 1935, four leaks had developed in the water jackets, one camshaft broke, and one valve guide had cracked. The Navy wanted to complete a 150-hour test. The two 50-hour tests counted for 100 hours, and the 140 hours of calibration counted for 25 hours. Wright offered to complete at their own expense the final 25 hours of the 150-hour test. This included 15 hours alternating between 1,100 hp (820 kW) takeoff power and idle, and 10 hours at 1,000 hp (746 kW) and 110% maximum engine speed (2,640 rpm).

On 7 November 1935, the AAC received the second H-2120 engine. The AAC had selected a Bellanca C-27A single-engine transport to serve as the H-2120 test bed. The engine’s installation would add 860 lb (390 kg) to the aircraft. After further evaluation, it was determined that the center of gravity would be out of limits, and the C-27A was deemed unsuitable for the engine tests. A Fokker C-14A was substituted, and serial number 34-100 was assigned for the conversion on 15 November.

Testing of the first engine at Wright had run into issues. After 4.5 hours at 2,640 rpm, an intake valve failed, resulting in a severe backfire. During inspection, the blower housing was found to be cracked, the crankcase had been punctured, and several connecting rods were damaged. Some of the damaged connecting rods were a result of improper assembly. The engine was repaired but damaged again on 20 November, when anther intake valve failed after 3.25 hours at 2,640 rpm. Before the failure, the H-2120 was producing 1,168 hp (871 kW) with a coolant and oil outlet temperature of around 255 ℉ (124 ℃). The engine was repaired again and completed its 10 hours at 2,640 rpm on 23 December 1935. The first H-2120 was retained by Wright for further tests.

By the end of December 1935, the AAC had run in the second engine for five hours and up to 2,300 rpm. The fuel pump diaphragm failed four times, necessitating replacement of the pump. After some vibration issues were overcome, calibration tests were started in mid-January 1936. The AAC concluded its tests in April, stating that the second H-2120 ran smoothly. The engine produced 1,000 hp (746 kW) at 2,400 rpm with 1.8 psi (.12 bar) of boost. It also developed 1,139 hp (849 kW) at 2,550 rpm with 3.2 psi (.22 bar) of boost. Installation of the H-2120 in the C-14A was forecasted to add 800 lb (363 kg), and the AAC felt that more information could be gained by continued ground testing rather than flight tests in the C-14A.


The first H-2120, Manufacture’s No. 11691 / BuNo 0120 appears to be complete. It is not known if it was repaired after its rear connecting rod failure. (NASM image)

Meanwhile, testing of the first H-2120 had continued at Wright. On 20 February 1936, the blade connecting rod on the rear crankpin failed during calibration for a 20-hour test at takeoff power (1,100 hp / 820 kW). The failure was the result of fatigue, and the broken rod caused significant damage to all nearby components.

In May 1936, Wright informed the AAC and Navy of a secret air-cooled engine that is had been developing at its own expense. This engine was expected to have an initial sea level rating of 1,200 hp (894 kW) and a takeoff rating of 1,400 hp (1,044 kW). Wright offered the services an experimental version of the engine for $38,750, with delivery expected in early 1937. Wright did not want any details of this engine leaked to its competitors and asked that the AAC and Navy refer to it as the “Aircooled 2120,” even though that was not the engine’s displacement. Wright felt that this new engine, which was the 14-cylinder R-2600 radial, possessed more potential than the H-2120. Wright wanted to drop further H-2120 development to focus on the R-2600. Both the AAC and the Navy agreed, encouraged Wright to continue R-2600 development, and stated their intention of purchasing experimental examples once money for the 1937 budget was available. The Navy had already lost interest in the H-2120, and the AAC stopped further testing in July.

During the fall of 1935, the Boeing Airplane Company, the Curtiss Aeroplane & Motor Company, and the Glenn L. Martin Company all requested data on the H-2120 so that they could potentially incorporate the engine into designs they were working on. Since the H-2120 was a joint project at the time, the service that received the request would check with the other service to see if there were any objections to sharing information. The only company denied data was North American Aviation, which requested information in January 1936. Both the AAC and Navy said they had no projects with the company that required an engine like the H-2120. Despite the interest, no applications for the H-2120 have been found.

Both H-2120 engines survive and are held in storage by the Smithsonian National Air and Space Museum. The first engine, Manufacture’s No. 11691 / BuNo 0120, is complete. It is not known if it was fully repaired after the failure of the rear connecting rod, or just reassembled. The second H-2120, Manufacture’s No. 11692 / BuNo 0121, was sectioned to expose its inner workings. The H-2120 represented the last of the hexagonal engines from the United States. Other hexagonal engines include the Curtiss H-1640, the SNCM 137, the Junkers Jumo 222, and the Dobrynin series of aircraft engines.


The second H-2120, Manufacture’s No. 11692 / BuNo 0121, neatly sectioned and displaying its internals. Note the four valves per cylinder and odd connecting rods. (NASM image)

– Numerous documents held by the U.S. National Archives and Records Administration at College Park, Maryland under Record Group 342 – Air Force Engineering Division RD 1676 and 3285 (scanned by Kim McCutcheon of the Aircraft Engine Historical Society)
Development of Aircraft Engines and Aviation Fuels by Robert Schlaifer and S. D. Heron (1950)


Rail Zeppelin Propeller-Driven Railcar (Schienenzeppelin)

By William Pearce

During World War I, German engineer Otto Steinitz had the idea of testing aircraft engines and propellers on railcars. Carl Geissen designed the engine mount, and testing was carried out on a special track at the German Aviation Research Institute (Deutschen Versuchsanstalt für Luftfahrt, DVL) in Berlin. The test car reached speeds of up to 97 mph (140 km/h). After the war, the propeller-driven railcar concept led Steinitz to design a special two-axle car with a mount for an aircraft engine at each end. An enclosed area between the engines housed the crew, passengers, and equipment. Known as the Dringos-Wagen, the machine made a 25-mile (40-km) test run from Grunewald to Beelitz on 11 May 1919. Loaded with approximately 40 people (possibly 35 passengers and five crew), the Dringos-Wagen experienced slow acceleration and a limited top speed of about 37 mph (60 km/h). Interest in Steinitz’s Dringos-Wagen declined after the test, but Geissen continued to design propeller-driven railcars for passenger service into the early 1920s.


The Dringos-Wagen testing the concept of a propeller-driven railcar in 1919. Note the radiators installed on the deck

Also in the early 1920s, fellow German engineers Kurt Wiesinger and Franz Friedrich Kruckenberg had similar ideas of using propellers to improve and quicken rail traffic. Wiesinger envisioned propelling railcars along the tracks with propellers, while Kruckenberg was considering a streamlined, propeller-driven gondola suspended from a single overhead track as a Zeppelin-on-rails. Kruckenberg’s design was similar to George Bennie’s Railplane of the same period. The pair met in 1923 but soon had a falling out and went their separate ways.

Kruckenberg had studied shipbuilding at the Technical University in Danzig (now Gdańsk University of Technology). One of his professors, Johann Schütte, had partnered with industrialist Karl Lanz to form Luftschiffbau Schütte-Lanz (Airship Construction Schütte-Lanz) in April 1909. After his graduation in August 1909, Kruckenberg joined the firm as a developmental engineer. Kruckenberg was involved with both airship and aircraft constructions while working at Schütte-Lanz, and he was the firm’s chief designer and director of aircraft production during World War I.

After World War I, Kruckenberg left Schütte-Lanz and began to focus on ways to improve rail travel, which is when he met Wiesinger. In July 1924, Kruckenberg partnered with Curt Stedefeld, an associate from university who had also worked for Schütte-Lanz and had founded the Company for Traffic Engineering (Gesellschaft für Verkehrstechnik, GVT) to promote the overhead rail system. Despite a forecasted top speed of 224 mph (360 km/h), the German Ministry of Transportation (Reichsverkehrsministerium) and the German State Railroad Company (Deutschen Reichsbahn-Gesellschaft, DRG) were not willing to offer any financial support. The main objection was the cost of the overhead rail system, which required the support and construction of a completely new infrastructure.


The DVL’s Propellerwagen was strictly a test machine and not intended to transport passengers. However, the Propellerwagen provided important information on suspension and handling that was applied to the Rail Zeppelin.

In April 1928, Kruckenberg and Stedefeld founded the Trajectory Company (Flugbahn-Gesellschaft, FG) in Heidelberg. The purpose of the new company was to build a propeller-driven railcar for experimentation on existing rail lines to validate the concepts of the overhead rail system. Once FG had demonstrated reliable performance on existing rails, it was hoped that the DRG would be willing to support the overhead rail system.

Around the same time, the DVL was interested in constructing a Propellerwagen to revive the testing of engines and propellers on railcars. Both FG and DVL had petitioned the DRG for the use of a straight, 5-mile (8-km) long, unused track between Langenhagen and Celle. The DRG proposed that the FG and the DVL work together to build a test rig that could be used to test engines and propellers and validate the concepts of propeller-driven railcars.

The DVL Propellerwagen test railcar was completely enclosed with an engine and propeller at each end. The narrow machine was tall with flat sides and had two axles. The rear engine drove its propeller directly via a long shaft, while the front engine drove an elevated propeller shaft via a wide belt. Both engines were six-cylinder, inline BMW IVs that produced 250 hp (186 kW) at 1,400 rpm. The test car weighed around 30,865 lb (14,000 kg) and had a top speed of 109 mph (175 km/h). After operating under its own power for the first time in April 1929, the test railcar eventually made 82 runs that totaled approximately 620 miles (1,000 km). While the DVL test machine did not help advance GVT/BG’s study of aerodynamics, it did provide important information about suspension, handling, and the operation of a propeller-driven railcar.


The bodyless Rail Zeppelin on 30 August 1930 illustrating the machine’s intricate frame. Note the numerous lightening holes in the truss frame. The engine-driven centrifugal fan drew in air via the circular opening (one on each side). The air was then forced through the large, square radiator in the lower rear of the railcar.

With information from the RVL tests in hand, Kruckenberg and his team compared diesel-electric drives against propeller drives for their railcar. They found that the diesel-electric would cost about 19 times more than the propeller drive and would weigh around 19,842 lb (9,000 kg), compared to 772 lb (350 kg) for the propeller and engine. In June 1929, the design of a streamlined, propeller-driven Trajectory Express Car (Flugbahn-Schnellwagen) was laid out and designated Propeller Railcar A (Propellertriebwagen A). This machine was undoubtedly inspired to some degree by the earlier designs of Geissen and Wiesinger. Detailed design work was done in October 1929, and wind tunnel models were tested the following month. Due to its design, construction, and appearance, the streamlined, high-speed railcar became commonly known as the Rail Zeppelin (Schienenzeppelin).

The Rail Zeppelin consisted of a steel chassis with an aluminum truss frame. The engine supports and some other components were also made of steel. The aluminum frame was perforated with extensive lightening holes. The machine was supported on two axles and had a wheelbase of 64 ft 4 in (19.60 m). The axles used rubber ball dampeners for their suspension. Each of its four wheels were 39 in (1.0 m) in diameter. The inner flange of the wheels was made taller than normal to help prevent any possible derailments caused by the machine’s anticipated high speeds. Air-powered friction brakes were used to slow the Rail Zeppelin. An electric drive motor powered the front axle for moving the machine in a limited manner up to 12.4 miles (20 km) and at relatively slow speeds.


The completed Rail Zeppelin displaying its streamlined form for press photographers. Note the two exhaust stacks at the rear of the machine and its long wheelbase.

Above the rear axle was a single BMW VI liquid-cooled V-12 engine. The BMW VI had a 6.30 in (160 mm) bore and a 7.48 in (190 mm) stroke. The engine displaced 2,797 cu in (45.84 L) and produced 500 hp (373 kW) at 1,410 rpm and 600 hp (447 kW) at 1,540 rpm. The engine’s exhaust was expelled through two vertical stacks. The drive end of the engine pointed toward the rear of the Rail Zeppelin and was elevated seven degrees. A shaft, which was also angled at seven degrees, extended approximately 7 ft 7 in (2.3 m) back to the rear of the machine and turned a four-blade, fixed-pitch Heine propeller made from ash wood. The seven-degree angle on the propeller applied downward force on the Rail Zeppelin and directed the propwash up and away from people on rail platforms. The propeller was 9 ft 2 in (2.80 m) in diameter and was comprised of two stacked two-blade units.

Also attached to the drive end of the engine was a centrifugal fan that circulated cooling air through the engine compartment. Air was drawn in via vents on each side of the Rail Zeppelin and entered a duct at the center of the machine. The air then passed through radiators and was expelled out from the bottom of the Rail Zeppelin. The engine also powered the compressor for the air brakes and two generators for the electrical system. Storage batteries were located in the train’s nose.

The Rail Zeppelin was covered by a streamlined, aerodynamic body. The front, lower sides, and rear of the machine were covered by aluminum sheeting. Windows extended along the sides of the passenger compartment. Due to the expected speed of the Rail Zeppelin, none of the windows opened, and ventilation was provided by forced air. The top of the railcar was covered with fire-proof canvas.


Rear view of the Rail Zeppelin with its four-blade propeller. The grate on the side was the cooling air intake. The circular housing under the propeller was for lights.

Two drivers sat side-by-side at the front of the train in a raised cockpit, which also had seats for two observers. Passenger compartment access doors were positioned at the front, middle, and rear on each side of the Rail Zeppelin. The 8 ft 2 in (2.5 m) wide and 52 ft 6 in (16 m) long passenger cabin was insulated and had wood paneling. As designed, the passenger compartment consisted of six sections, with each section accommodating four passengers, and a central aisle extended through each section. In addition to the 24-seat configuration, an alternative configuration with bench seating could accommodate 44 passengers. A lavatory was provided at the rear of the cabin. As built, only the forward three compartments were completed, and the rear three compartments held test equipment. The Rail Zeppelin was 84 ft 10 in (25.85 m) long, 8 ft 9 in (2.66 m) wide, and 9 ft 2 in (2.80 m) tall. The railcar weighed 40,962 lb (18,580 kg).

Construction of the Rail Zeppelin started in early 1930 at the DRG repair works in Leinhausen, near Hannover. Without its body, the railcar was mostly complete in August 1930 and moved under its own power with the electric motor. The body was added, and the Rail Zeppelin was completed in September. The first test with propeller power occurred on 25 September 1930. During the first high-speed test, the Rail Zeppelin reached 62 mph (100 km/h) after 66 seconds and 3,232 ft (985 m) of forward travel. The machine hit 93 mph (150 km/h) after two minutes, and the throttle was pulled back just past three minutes at 113 mph (182 km/h).


The Rail Zeppelin with its two-blade propeller sits at Spandau (Berlin) station after its run on 21 June 1931. The two-blade propeller improved the machine’s top speed but slowed acceleration.

The initial testing was done in secret and revealed that braking was an issue. Due to the Rail Zeppelin’s streamlining and relatively light weight, light breaking took a long distance, and heavy breaking had a tendency to lock the rear axle. In one instance, the brakes locked the rear axle at 112 mph (180 km/h), and it took 1.2 miles (2 km) for the railcar to come to a stop. A flat spot on the rear wheels about .14 in (3.5 mm) deep was discovered during a quick inspection, but the Rail Zeppelin was still operated up to 87 mph (140 km/h) on its return trip.

On 18 October 1930, the Rail Zeppelin was debuted to the press. Tests continued, some of which involved DRG officials. To test the concept of using a propeller with adjustable blades, a propeller with reverse pitch was installed (this may have been the normal propeller installed backward), and the Rail Zeppelin was run in reverse at 37 mph (60 km/h). With the propeller back to its normal forward thrust configuration, propeller braking tests were conducted. The electric motor was used to reverse the train at 28 mph (45 km/h). Then the propeller was engaged, and it alone halted the Rail Zeppelin in 20 seconds. These tests indicated that a fully reversible pitch propeller would greatly enhance the Rail Zeppelin’s braking and improve its safety.


This upper view of the Rail Zeppelin in Berlin illustrates the machine’s canvas covering over its upper body. Note the windshield wipers and the two-blade propeller.

Testing on the isolated track continued until May 1931, when the Rail Zeppelin was operated on the main line. However, no German insurance company would cover the propeller-driven train, and arrangements had to be made with Lloyd’s of London for coverage. The main line test was a 12.2-mile (19.7-km) stretch between Plockhorst and Lehrte. The Rail Zeppelin drew quite a crowd wherever it operated, necessitating a police presence to control the spectators. On 10 May, the machine covered the distance in 10 minutes and reached a top speed of 127 mph (205 km/h).

Testing over a longer distance was needed, so the 160-mile (257-km) route between Hamburg and Berlin was selected. The four-blade propeller was switched in favor of a two-blade unit that would provide a higher top speed at the cost of acceleration. The two-blade propeller was of the same construction as the previous propeller—fixed pitch, wood, and 9 ft 2 in (2.80 m) in diameter.

On 21 June 1931, the Rail Zeppelin left the Hamburg-Bergedorf station at 3:27 AM with a number of observers and crew on board. As the train traveled, its speed continued to increase. However, the track speed limit around many of the curves was 62 mph (100 km/h), which caused the Rail Zeppelin to slow often and accelerate on the straight stretches. Over the 7.5 miles (12-km) separating Karstädt and Dergenthin, the Rail Zeppelin averaged 143.0 mph (230.2 km/h)—a new speed record for passenger rail travel that would stand until 1954. The train arrived in Berlin at 5:05 AM with an average speed of 97.7 mph (157.3 km/h). Along the way, the Rail Zeppelin burned only 48.6 US gal (40.5 Imp gal / 184 L) of fuel, which averaged to 3.3 miles per US gal (1.4 km/L).


With its propeller spinning, the Rail Zeppelin awaits departure at a station. Although the propeller did not really extend beyond the railcar’s body, this view illustrates the rather disconcerting proposition of passengers coming into close proximity of the large propeller. Note the open middle access door.

After its record run, the Rail Zeppelin was put on display at the Rennbahn-Stadion (now Olympiastadion) railway station in Berlin until 25 June 1930. After the display, the train embarked on a short tour of Germany. The four-blade propeller was reinstalled for the tour, and the speed was kept down to conform with normal scheduled traffic on the line. Once again, the Rail Zeppelin drew large crowds wherever it went. The machine returned to Hannover on 28 June.

A new electromagnetic braking system was installed on the Rail Zeppelin and was tested in March 1932. The system was able to stop the train from 103 mph (165 km/h) in 2,067 ft (630 m). While this was a definite improvement, the distance was still longer than desired. Although the Rail Zeppelin had achieved some level of success, the practicality of such a machine was in question. The train’s long wheelbase caused issues on tight curves, and its ineffective brakes necessitated long stopping distances. The propeller-driven design did not allow coupling multiple units together, and the machine was unable to easily maneuver forward and back for short distances. The large propeller always presented a level of danger to anyone in close proximity to the Rail Zeppelin, and that included passengers waiting on rail platforms.


Image of the modified Rail Zeppelin with propeller removed and the engine installed in the nose. The nose and cockpit were revised for the installation of the engine and the hydraulic drive. Barely visible is the dual-axle front bogie.

Kruckenberg and his team took another look at the future of rail travel, and the propeller-driven railcar concept was discarded in favor of a diesel-hydraulic drive that was much lighter than diesel-electric. In May 1932, modifications were started on the Rail Zeppelin to convert the machine to the new power system. The BMW engine and propeller were removed from the rear, and the engine was temporarily installed in the nose of the train until the intended Maybach GO 5 was available. The GO 5 was a 2,588 cu in (42.4 L) diesel V-12 that produced 410 hp (305 kW) at 1,400 rpm. The engine’s exhaust was collected in a central duct that split the center of the cockpit’s windscreen. Via a Föttinger fluid coupling, the engine drove a double-axle bogie positioned under the cockpit. The bogie had a wheelbase of 6 ft 7 in (2.0 m). To accommodate the changes, the train’s nose was elongated, and its cockpit was raised. Its length was increased to 95 ft 2 in (29.0 m) and its weight increased to 62,832 lb (28,500 kg).

The revised Rail Zeppelin was completed in November 1932. The train was tested in early 1933 and reached 87 mph (140 km/h) in under two minutes after traveling 1.5 miles (2,426 m). It was also run at least to 99 mph (160 km/h). However, the DRG had become interested in other trains, namely those powered by diesel-electric engines. The Rail Zeppelin continued to be tested through 1934 and accumulated over 1,491 miles (2,400 km) with its new drive system. The GO 5 engine was finally installed in 1934, and the machine was sold to the DRG. It does not appear that much testing was done with the GO 5 engine. While Kruckenberg and his team continued to design more conventional locomotives throughout the 1930s, the Rail Zeppelin was placed into storage. In 1939, the Rail Zeppelin was scrapped so that its metal could be used to rebuild the German armed forces.

The Rail Zeppelin and its diesel-hydraulic drive served as the basis for the Kruckenberg-designed SVT 137 155, which could accommodate 100 passengers. A single example of the SVT 137 155 was completed in 1938, and the three-section express train set a conventional passenger train speed record on 23 June 1939 at 134 mph (215 km/h). The SVT 137 155 never entered regular service, and it was scrapped in 1967.


The SVT 137 155 built upon the Rail Zeppelin’s diesel-hydraulic experiments. Note the exhaust stack splitting the windscreen.

Der Schienenzeppelin by Alfred Gottwaldt (2006)
BMW Aero Engines by Fred Jakobs, Robert Kroschel, and Christian Pierer (2009)