CTA - ITA Heliconair Convertiplano drawing

CTA / ITA Heliconair HC-I Convertiplano

By William Pearce

In 1923, Henrich Focke partnered with Georg Wulf to create Focke-Wulf Flugzeugbau (Aircraft Company) in Bremen, Germany. Focke became fascinated with helicopters and other rotorcraft in the 1930s. This interest led to what is considered the first practical helicopter, the Focke-Wulf Fw 61, which first flew in 1936. That same year, Focke was ousted from Focke-Wulf due to internal disagreements about allocating company resources. In 1937, Focke partnered with Gerd Achgelis, the Fw 61’s lead designer, to create Focke-Achgelis & Co in Hoykenkamp, Germany. The new company would focus on helicopter and rotorcraft designs.

CTA - ITA Convertiplano side

The Heliconair HC-Ib Convertiplano sits nearly finished in a hangar. The slit behind the cockpit was the intake for air used to cool the fuselage-mounted R-3350 engine. The scoop on the upper fuselage brought air to the engine’s carburetor. Note the Spitfire wings and main gear.

In 1941, the RLM (Reichsluftfahrtministerium or Germany Air Ministry) requested that Focke-Achgelis design a fighter capable of vertical takeoff and landing (VTOL). Focke-Achgelis responded with the Fa 269 design, which was a tiltrotor convertiplane. The Fa 269 had two rotors—one placed near the tip of each wing in a pusher configuration. The rotors were powered by an engine housed in the aircraft’s fuselage via extension shafts and gearboxes. The rotors and extension shafts leading from the right-angle gearboxes mounted in the aircraft’s wings rotated down to “push” the Fa 269 into the air, achieving vertical flight. Once airborne, the rotors and shafts would slowly translate back into the wing to propel the aircraft forward, allowing the aircraft’s wings to provide lift. The project moved forward until 1944, when much of the developmental work, including models, a mock-up, and gearboxes, was destroyed in an Allied bombing raid.

CTA - ITA Heliconair Convertiplano

Drawings of how the completed HC-Ib was anticipated to look reveal a pretty compact aircraft, considering the engine installation and associated shafting. The R-3350 engine took up the space intended for a passenger compartment in the Double Mamba-powered HC-I. The Double Mamba would have been installed aft of the passenger compartment.

Immediately following World War II, Germany was prohibited from designing and manufacturing aircraft. Post war, Focke assisted with helicopter development in France and worked for a car company in Germany. He also spent some time in the Netherlands, where he began to design a VTOL aircraft that was capable of relatively high speeds. In 1952, Focke was recruited by the CTA (Centro Técnico de Aeronáutica or Technical Center of Aeronautics) to work in the recently established ITA (Instituto Técnico de Aeronáutica or Technical Institute of Aeronautics). The ITA was the first of four institutes formed by the CTA, all of which were located in São José dos Campos, Brazil. Brazil was working on building an aeronautics and aerospace industry and was actively recruiting German engineers. In addition to Focke, many of his associates and former co-workers were also recruited.

The CTA was impressed with Focke’s VTOL aircraft design and approved its construction. The CTA believed that the aircraft’s capabilities would allow it to reach remote parts of Brazil. Focke set to work on the aircraft—a tiltrotor convertiplane design that was partially inspired by the Fa 269. The aircraft was known as the Heliconair HC-I Convertiplano. Its fuselage and wings were fairly conventional for an aircraft, but it had of two sets of rotors. One pair of rotors was placed near the nose of the aircraft, and the other pair was placed between the wings and tail. All of the rotors were of a tractor configuration and rotated up for vertical flight. The HC-I accommodated two pilots in the cockpit and four passengers in the fuselage. The aircraft’s estimated performance included a top speed of 311 mph (500 km/h) and a range of 943 miles (1,517 km).

CTA - ITA Convertiplano engine test rig

The test rig for the engine, transmission, gearboxes, shafts, right-angle drives, and rotors illustrates the complexity of the HC-Ib’s power system. The R-3350 engine did not have any Power Recovery Turbines, which means it was not a Turbo Compound engine.

To save time and money, the decision was made to build the HC-I using the wings and the horizontal stabilizer from a Supermarine Spitfire. A Spitfire XIVe (RM874) was purchased without its Rolls-Royce Griffon 65 engine from Britain by the Brazilian Air Attaché on 19 December 1952. A new fuselage was built to house a 3,000 hp (2,237 kW) Armstrong Siddeley Double Mamba turboprop engine behind the passenger compartment. However, Armstrong Siddeley and the British did not want one of their new, advanced engines being used in such a radical project and declined selling a Double Mamba engine to Brazil.

Focke and the Convertiplano team changed the HC-I’s design to accommodate a 2,200 hp (1,641 kW) Wright R-3350 radial engine and redesignated the aircraft HC-Ib. The R-3350 was larger and heavier than the Double Mamba, and it produced less power. Some sources state a Turbo Compound R-3350-DA3 (3,250 hp / 2,424 kW) was used, but images show that there are no Power Recovery Turbines on the engine installed in a test rig. Extensive modifications to the aircraft’s fuselage were required to accommodate the air-cooled engine. The passenger compartment was omitted, and the R-3350 was installed in the middle of the fuselage. An annular slit behind the cockpit was added to bring in cooling air for the engine. After passing through the engine’s cylinders, the air exited via a jet-like duct at the rear of the aircraft. The Spitfire’s landing gear was strengthened to compensate for the R-3350’s weight.

CTA - ITA Convertiplano components

The HC-Ib sits in the background with the front and rear gearboxes and rotor drives in the foreground. The rotor blades, the only surviving component of the Convertiplano project, are not seen in the image. Note the opening at the rear of the fuselage, which was the exit for engine cooling air.

A gearbox transmission mounted to the front of the R-3350 split the engine’s power to two shafts. The front shaft extended from the engine to the front gearbox. The front gearbox had shafts that extended to the left and right. These shafts led to right-angle gearboxes that powered the front rotors. Power delivery for the rear rotors was more complex. A shaft extended vertically from the transmission on the front of the engine and met a right-angle gearbox positioned directly above the engine. From the right-angle gearbox, a shaft extended back to the rear gearbox. The rear gearbox had the same shafts and right-angle drives for the rear rotors as the front gearbox. The transmission and gearboxes were designed by Willi Bussmann and built by BMW in Germany. Bussmann was a former BMW employee and had worked with Focke on several Focke-Achgelis projects.

Each rotor consisted of three blades. The blades were built in Sweden and made of a steel frame that was covered with wood. The blades’ pitch automatically adjusted and had collective and cyclic control. The rotors were counter-rotating, with the right rotors turning counterclockwise and the left rotors turning clockwise. The HC-Ib had a 37 ft 6 in (11.42 m) wingspan and was 35 ft 3 in (10.74 m) long.

CTA - ITA Convertiplano engine hoist

Given the state of the aircraft and the surrounding unchecked growth of vegetation, it can be assumed this image is of the R-3350 engine being removed sometime after the HC-Ib project was cancelled. The image does give proof that the engine was installed in the airframe at one point.

A rig was built, and tests of the engine, gearboxes, shafts, right-angle drives, and rotors began in late 1953. However, vibrations from the radial engine caused some issues that took time to resolve. The HC-Ib airframe was almost completely constructed and had its engine installed when the project was cancelled in 1955. The aircraft was more expensive than anticipated, and interest in the HC-1b had steadily declined after the switch to the R-3350 engine. To make matters worse, many of the Germans returned to Europe or went to the United States as their contracts with the CTA expired. Some Germans did stay and ultimately became part of Embraer. After the project was cancelled, the HC-Ib Convertiplano was left to rot in outside storage for some time and was eventually scrapped in the 1970s. There are some reports that the rotor blades are the only part of the aircraft that survived.

A follow up Convertiplano project was considered. Designated HC-II, the aircraft would be powered by four 1,400 hp General Electric T58 turboshaft engines and reincorporate a four to six passenger cabin. The HC-II never progressed beyond the initial design phase.

CTA - ITA Convertiplano HC-II

The C-II Convertiplano had a GE T58 engine mounted directly to each of its four rotors. Otherwise, it retained the configuration of the original HC-I.

Axis Aircraft in Latin America by Amaru Tincopa and Santiago Rivas (2016)
“Uma Breve História das Atividades do Prof. Focke no Brasil” by Joseph Kovacs, ABCM Engenharia Volume 9 Número 2 (April–September 2003)


Mercedes-Benz 500 Series Diesel Marine Engines

By William Pearce

Daimler-Benz was formed in 1926 with the merger of Daimler Motoren Gesellschaft and Benz & Cie. Prior to their merger, both companies produced aircraft engines under the respective names Mercedes and Benz. After the merger, the Daimler-Benz name was used mostly for aircraft engines, and the Mercedes-Benz name was used mostly for automobile production. However, both names were regularly applied to marine engines. For clarity in this article, the name Daimler-Benz will refer to aircraft engines, and the name Mercedes-Benz will refer to marine engines.


Two V-12 Mercedes-Benz diesel engines, most likely MB 500s. The MB 500 was the foundation for the post-war MB 820.

As Germany began its rearmament campaign in the 1930s, high-performance marine diesel engines were needed to power various motorboats. The Kriegsmarine (German Navy) turned to Mercedes-Benz to supply a series of high-speed diesel engines. These engines were part of the MB 500 series of engines that were based on the Daimler-Benz DB 602 (LOF-2) engine developed to power the LZ 129 Hindenburg and LZ 130 Graf Zeppelin II airships. The 500 series diesel engines were four-stroke, water-cooled, and utilized a “V” cylinder arrangement.

The first engine in the 500 series was the MB 500 V-12. The engine’s two cylinder banks were separated by 60 degrees. The MB 500 used individual steel cylinders that were attached to an aluminum alloy crankcase. About a third of the cylinder was above the crankcase, and the remaining two-thirds protruded into the crankcase. This arrangement helped eliminate lateral movement of the cylinders and decreased vibrations. The crankcase was made of two pieces and split horizontally through the crankshaft plane. The lower part of the crankcase was finned to increase its rigidity and help cool the engine oil.


The MB 501 shows the close family resemblance to the DB 602, but the engines had Vees of different angles and completely different valve trains. The tubes for the push rods can be seen on the outer side of the cylinders. Note the two water pumps on the rear sides of the engine.

Each cylinder had two intake and two exhaust valves. The camshaft had two sets of intake and exhaust lobes per cylinder. One set was for normal operation, and the other set was for running the engine in reverse. The fore and aft movement of the camshaft to engage and disengage reverse operation was pneumatically controlled. Bosch fuel injection pumps were located at the rear of the engine and were geared to the camshaft. Each injection pump provided fuel to the cylinders at 1,600 psi (110.3 bar). Fuel was injected into the center of the pre-combustion chamber, which was in the center of the cylinder head and between the four valves. For low-speed operation, fuel was cut from one bank of cylinders.

The MB 500 had a compression ratio of 16.0 to 1. The engine used fork-and-blade connecting rods that rode on roller bearings fitted to the crankshaft. The camshaft also used roller bearings, but the crankshaft was supported by plain bearings. Speed reduction of the engine’s output shaft was achieved through the use of bevel planetary gears. Two water pumps mounted to the rear sides of the engine circulated water through the cylinder banks. Each pump provided cooling water to one cylinder bank. The pumps were driven by a cross shaft at the rear of the engine. The engine was started with compressed air.


The crankcases of the wrecked MB 501 engines on Crackington Haven Beach have completely dissolved over the years from constant exposure to salt water. Only the engine’s steel components remain. Note the fork-and-blade connecting rods. The engine’s gear reduction can be seen on the left side of the image. (gsexr image via 350z-uk.com)

The MB 500 had a 6.89 in (175 mm) bore and a 9.06 in (230 mm) stroke. This cylinder size directly corresponded to the cylinder size used on the DB 602. The MB 500’s displacement was 4,051 cu in (66.39 L). The engine had a continuous output of 700 hp (522 kW) at 1,460 rpm and a maximum output of 950 hp (708 kW) at 1,630 rpm. Fuel consumption was .397 lb/hp/hr (241 g/kW/hr). The MB 500 was 9.6 ft (2.93 m) long, 3.2 ft (.98 m) wide, and 5.7 ft (1.73 m) tall. The engine weighed around 4,784 lb (2,170 kg). MB 500 engines were installed in Schnellboote that Germany built for Bulgaria. A Schnellboot, or S-boot, was a fast attack boat and was referred to as an E-boat (Enemy boat) by the Allies. The MB 500 engine design served as a basis for the post-war MB 820 industrial engine that was used in the V 200 Class locomotives and various ships.

For more power, the MB 501 was built with two rows of ten cylinders, creating a V-20 engine. The MB 501 was similar to the MB 500, but it also had a number of differences. A 40 degree angle separated the cylinder banks, and the engine used two camshafts positioned in the upper crankcase, one on each side of the engine. Rollers on the lower end of the pushrods rode on the camshaft. Two pushrods for each cylinder extended up along the outer side of the cylinder bank to operate a set of duplex rocker arms for the two intake and two exhaust valves. The fork-and-blade connecting rods were attached to the crankshaft with plain bearings.


With the exception of the different intake manifolds, the MB 502 was nearly identical to the DB 602. Note the Mercedes-Benz emblem on the rear of the V-16 engine.

The MB 501’s bore and stroke were increased over the MB 500’s to 7.28 in (185 mm) and 9.84 in (250 mm) respectively. The engine displaced 8,202 cu in (134.40 L). The MB 501 had a continuous output of 1,500 hp (1,119 kW) at 1,480 rpm and a maximum output of 2,000 hp (1,491 kW) at 1,630 rpm. Fuel consumption was .397 lb/hp/hr (241 g/kW/hr). The engine was 12.7 ft (3.88 m) long, 5.2 ft (1.58 m) wide, 5.6 ft (1.71 m) tall, and had a weight of 9,303 lb (4,220 kg). Three MB 501 engines were installed in each 1937 class Schnellboot. Six engines were installed in each of the U-180 and U-190 submarines. However, the MB 501 engines proved unsuitable in the submarines, and they were soon replaced by MAN diesels. The remains of three MB 501 engines can be found on Crackington Haven Beach in southeast Britain. The engines belonged to Schnellboot S-89, which was surrendered to the British after World War II. S-89 slipped its tow on 5 October 1946 and was wrecked upon the shore.

The MB 502 was essentially a Daimler-Benz DB 602, except it had water jacketed intake manifolds that protruded above the engine’s Vee. The rest of the MB 502’s specifics mirrored those of the DB 602. The MB 502 was a 50 degree V-16 with a single camshaft located in the Vee of the engine. The engine had a 6.89 in (175 mm) bore and a 9.06 in (230 mm) stroke. The MB 502 displaced 5,401 cu in (88.51 L) and had a continuous output of 900 hp (671 kW) at 1,500 rpm and a maximum output of 1,320 hp (984 kW) at 1,650 rpm. The engine was 9.9 ft (3.02 m) long, 4.0 ft (1.22 m) wide, and 6.2 ft (1.90 m) tall. The MB 502 weighed 5,952 lb (2,700 kg) and had a fuel consumption at cruising power of 0.37 lb/hp/hr (225 g/kW/hr). Three MB 502 engines were installed in each 1939 class Schnellboot.


The MB 507 was based on the DB 603 inverted V-12 aircraft engine. Although the engine’s architecture was similar, the MB 507 had a completely different crankcase and reduction gear than the DB 603, and it was not supercharged.

The MB 507 was based on the Daimler-Benz DB 603 inverted V-12 aircraft engine, but some features from the DB 602 were incorporated. The normally aspirated MB 507 was an upright V-12 diesel engine that used monobloc cylinders and had a compression ratio of 17 to 1. A new finned crankcase was fitted that was similar to those used on other MB 500 series diesel engines. For the initial MB 507 engines, the bore was decreased from the 6.38 in (162 mm) used on the DB 603 to 6.22 in (158 mm). The stroke was unchanged at 7.09 in (180 mm). This gave the MB 507 a displacement of 2,584 cu in (42.35 L). The DB507 weighed 1,834 lb (850 kg). The engine had a continuous output of 700 hp (522 kW) and a maximum output of 850 hp (634 kW) at 2,300 rpm. An updated version of the engine, the MB 507 C, reverted back to the 6.38 in (162 mm) bore, which increased its displacement to 2,717 cu in (44.52 L). The MB 507 C produced 750 hp at 1,950 rpm and 1,000 hp at 2,400 rpm. The engine was 6.0 ft (1.83 m) long, 2.6 ft (.79 m) wide, 3.5 ft (1.06 m) tall, and had a weight of 1,742 lb (790 kg). Two MB 507 engines were used in a few LS boats (Leicht Schnellboot or Light Fast boat), and the engine was also installed in some land vehicles, such as the Karl-Gerät self-propelled mortar.


The MB 511 engine on display in the Aeronauticum museum in Germany. Note the finning on the lower half of the crankcase. On the front of the engine (left side of image) is the gear reduction with the supercharger above. The square connection above the engine is for the induction pipe. (Teta pk image via Wikimedia Commons)

The MB 511 was a supercharged version of the MB 501 V-20 engine. The bore, stroke, and displacement were unchanged, but the compression ratio was decreased to 14 to 1. The supercharger was positioned at the front of the engine, above the gear reduction. With the supercharger, output increased to 1,875 hp (1,398 kW) at 1,480 rpm for continuous power and 2,500 hp (1,864 kW) at 1,630 rpm for maximum power. The MB 511 was 13.1 ft (4.00 m) long, 5.2 ft (1.58 m) wide, and 7.6 ft (2.33 m) tall. The engine weighed 10,406 lb (4,720 kg). Three MB 511 engines were installed in each 1939/1940 class Schnellboot. An MB 511 engine is on display in the Aeronauticum maritime aircraft museum in Nordholz (Wurster Nordseeküste), Germany. Also, the MB 511 engine was built by VEB Motorenwerk Ludwigsfelde as the 20 KVD 25 in East Germany in the 1950s. Two 20 KVD 25 engines were installed in an experimental torpedo boat.


The sectional and cylinder drawing are for the MB 518 but were basically the same for the MB 501 and MB 511—all were 40 degree V-20 engines. Note the pre-combustion chamber, valve train, and two camshafts.

The MB 512 was a supercharged version of the MB 502. Its compression was decreased to 14 to 1, but its output increased to 900 hp (1,398 kW) at 1,500 rpm for continuous power and 1,600 hp (1,864 kW) at 1,650 rpm for maximum power. The MB 512 was 10.0 ft (3.05 m) long, 4.2 ft (1.28 m) wide, and 6.3 ft (1.92 m) tall. The engine weighed 6,834 lb (3,100 kg). MB 512 engines replaced MB 502s in some Schnellboot installations.

The MB 517 diesel engine was a supercharged version of the MB 507. Returning to its DB 603 roots, the engine was inverted, but it retained the 6.22 in (158 mm) bore and 7.09 in (180 mm) stroke of the early MB 507. The supercharger boosted power from the 2,584 cu in (42.35 L) engine to 1,200 hp (895 kW) at 2,400 rpm. The MB 517 was installed in the Panzer VIII Maus V2 tank prototype.


The MB 518 was the last development of the V-20 engines. This image shows the large intercooler installed on the engine’s induction system.

The MB 518 was a continuation of the MB 511 and featured an intercooler. The large intercooler was positioned in the intake duct, above the engine and between the supercharger at the front of the engine and the intake manifolds in the engine’s Vee. The first MB 518s had a continuous output of 2,000 hp (1,696 kW) at 1,500 rpm and a maximum output of 3,000 hp (2,237 kW) at 1,720 rpm. After World War II, updated versions of the engine went into production starting in 1951. The MB 518 B had a continuous output of 2,275 hp (1,696 kW) and a maximum output of 3,000 hp (2,237 kW). The MB 518 C had a continuous output of 2,500 hp (1,864 kW) and a maximum output of 3,000 hp (2,237 kW). A turbocharger was added to create the MB 518 D. It had a continuous output of 2,900 hp (2,163 kW) and a maximum output of 3,500 hp (2,610 kW). The MB 518 engine was 14.8 ft (4.52 m) long, 5.2 ft (1.58 m) wide, and 8.0 ft (2.44 m) tall. The engine weighed around 11,332 lb (5,140 kg). MB 518 engines were used to power several different vessels for the German Navy and were also exported to 35 countries. Some of the engines are still in use today.

Schnellboot S-130, the only remaining German S-boot from World War II, was originally powered by three MB 511 engines. After the war, S-130 was reengined with two MB 518s, and one MB 511 was retained. S-130 is currently part of the Wheatcroft Collection and undergoing restoration. Four MB 518 C engines for the restoration were obtained from the Arthur of San Lorenzo, formerly known as the S39 Puma and originally built as a German Zobel Class fast patrol boat in the early 1960s.


A number of MB 518 engines under construction show many different details. The lower crankcase half is on the floor, while the upper half is in the engine cradle; note the two camshaft tunnels. The crankshaft and its fork-and-blade connecting rods can be seen. Farther down the line is an engine with cylinder studs installed, and farther still is an engine with studs and pushrod tubes installed.



Daimler-Benz DB 602 (LOF-6) V-16 Diesel Airship Engine

By William Pearce

Around 1930, Daimler-Benz* developed the F-2 engine, initially intended for aviation use. The F-2 was a 60 degree, supercharged, V-12 engine with individual cylinders and overhead camshafts. The engine had a 6.50 in (165 mm) bore and an 8.27 in (210 mm) stroke. The F-2’s total displacement was 3,288 cu in (53.88 L), and it had a compression ratio of 6.0 to 1. The engine produced 800 hp (597 kW) at 1,500 rpm and 1,000 hp (746 kW) at 1,700 rpm. The engine was available with either direct drive or a .51 gear reduction, and weighed around 1,725 lb (782 kg). It is unlikely that the Daimler-Benz F-2 powered any aircraft, but it was used in a few speed boats.

The Daimler-Benz OF-2 diesel engine was very similar to the spark ignition F-2. Note the dual overhead camshafts in the Elektron housing above the individual cylinders. This was one of the OF-2’s features that was not incorporated into the LOF-6.

The Daimler-Benz OF-2 diesel engine was very similar to the spark ignition F-2. Note the dual overhead camshafts in the Elektron housing above the individual cylinders. This was one of the OF-2’s features that was not incorporated into the LOF-6.

In the early 1930s, Daimler-Benz used the F-2 to develop a diesel engine for airships. This diesel engine was designated OF-2, and it maintained the same basic V-12 configuration as the F-2. The individual cylinders were mounted on an Elektron (magnesium alloy) crankcase. Each cylinder had four valves that were actuated by dual overhead camshafts. The OF-2 had the same bore, stroke, and displacement as the F-2, but the OF-2’s compression ratio was increased to 15 to 1.

Fuel was injected into the cylinders at 1,330 psi (91.7 bar) via two, six-plunger injection pumps built by Bosch. The fuel was injected into a pre-combustion chamber located between the four valves in the cylinder head. This design had been used in automotive diesels built by Mercedes-Benz. Sources disagree on the gear reduction ratio, and it is possible that more than one ratio was offered. Listed ratios include .83, .67, and .58.

The Daimler-Benz OF-2 engine had a normal output of 700 hp (522 kW) at 1,675 rpm, a maximum output of 750 hp (559 kW) at 1,720 rpm, and it was capable of 800 hp (597 kW) at 1,790 rpm for very short periods of time. Fuel consumption at normal power was .392 lb/hp/hr (238 g/kW/hr). The engine was 74.0 in (1.88 m) long, 38.6 in (.98 m) wide, and 42.5 in (1.08 m) tall. The OF-2 weighed 2,061 lb (935 kg).


This view of a display-quality DB 602 engine shows the four Bosch fuel injection pumps at the rear of the engine. The individual valve covers for each cylinder can also be seen.

The OF-2 passed its type test in 1932. At the time, Germany was developing its latest line of airships, the LZ 129 Hindenburg and LZ 130 Graf Zeppelin II. These airships were larger than any previously built, and four OF-2 engines would not be able to provide sufficient power for either airship. As a result, Daimler-Benz began developing a new engine to power the airships in 1933. Daimler-Benz designated the new diesel engine LOF-6, but it was soon given the RLM (Reichsluftfahrtministerium or Germany Air Ministry) designation DB 602.

Designed by Arthur Berger, the Daimler-Benz DB 602 was built upon lessons learned from the OF-2, but it was a completely new engine. The simplest way to build a more powerful engine based on the OF-2 design was by adding two additional cylinders to each cylinder bank, which made the DB 602 a V-16 engine. The two banks of eight cylinders were positioned at 50 degrees. The 50 degree angle was selected over the 45 degree angle typically used for a V-16 engine. This gave the DB 602 an uneven firing order which helped avoid periodic vibrations.

The individual steel cylinders were mounted to the aluminum alloy crankcase. About a third of the cylinder was above the crankcase, and the remaining two-thirds protruded into the crankcase. This arrangement helped eliminate lateral movement of the cylinders and decreased vibrations. The crankcase was made of two pieces and split horizontally through the crankshaft plane. The lower part of the crankcase was finned to increase its rigidity and help cool the engine oil.


Originally called the LOF-6, the Daimler-Benz DB 602 was a large 16-cylinder diesel engine built to power the largest German airships. Note the three-pointed star emblems on the front valve covers. Propeller gear reduction was achieved through bevel planetary gears.

A single camshaft was located in the Vee of the engine. The camshaft had two sets of intake and exhaust lobes per cylinder. One set was for normal operation, and the other set was for running the engine in reverse. The fore and aft movement of the camshaft to engage and disengage reverse operation was pneumatically controlled. Separate pushrods for the intake and exhaust valves rode on the camshaft and acted on duplex rocker arms that actuated the valves. Each cylinder had two intake and two exhaust valves. Four Bosch fuel injection pumps were located at the rear of the engine and were geared to the camshaft. Each injection pump provided fuel at 1,600 psi (110.3 bar) to four cylinders. Fuel was injected into the center of the pre-combustion chamber, which was situated between the four valves. For slow idle (as low as 300 rpm), fuel was cut from one cylinder bank.

The DB 602 engine was not supercharged and had a .50 propeller gear reduction that used bevel planetary gears. The engine used fork-and-blade connecting rods that rode on roller bearings fitted to the crankshaft. The camshaft also used roller bearings, but the crankshaft was supported by plain bearings. Two water pumps were driven by a cross shaft at the rear of the engine. Each pump provided cooling water to one cylinder bank. The engine’s compression ratio was 16.0 to 1, and it was started with compressed air.

The DB 602 had a 6.89 in (175 mm) bore and a 9.06 in (230 mm) stroke, both larger than those of the OF-2. The engine displaced 5,401 cu in (88.51 L). Its maximum continuous output was 900 hp (671 kW) at 1,480 rpm, and it could produce 1,320 hp (984 kW) at 1,650 rpm for 5 minutes. The DB 602 was 105.9 in (2.69 m) long, 40.0 in (1.02 m) wide, and 53.0 in (1.35 m) tall. The engine weighed 4,409 lb (2,000 kg). Fuel consumption at cruising power was 0.37 lb/hp/hr (225 g/kW/hr).


The ill-fated LZ 129 Hindenburg on a flight in 1936. The airship used four DB 602 engines housed in separate cars in a pusher configuration. Note the Olympic rings painted on the airship to celebrate the summer games that were held in Berlin.

Development of the DB 602 progressed well, and it completed two non-stop 150-hour endurance test runs. The runs proved the engine could operate for long periods at 900 hp (671 kW). Four engines were installed in both the LZ 129 Hindenburg and the LZ 130 Graf Zeppelin II. Each engine powered a two-stage compressor. Each compressor filled a 3,051 cu in (50 L) air tank to 850 psi (59 bar) that was used to start the engine and to manipulate the camshaft for engine reversing.

Plans for a water vapor recovery system that used the engines’ exhaust were never implemented, because the airships used hydrogen instead of the more expensive helium. The recovery system would have condensed vapor into water, and the collected water would have been used as ballast to help maintain the airship’s weight and enable the retention of helium. Without the system in place, expensive helium would have been vented to compensate for the airship steadily getting lighter as diesel fuel was consumed. With the United States unwilling to provide helium because of Germany’s aggression, the airships used inexpensive and volatile hydrogen, as it was readily available. The Hindenburg was launched on 4 March 1936, and the Graf Zeppelin II was launched on 14 September 1938.

Engines for the Hindenburg were mounted in a pusher configuration. In April 1936, the Hindenburg’s DB 602 engines experienced some mechanical issues on its first commercial passenger flight, which was to Rio de Janeiro, Brazil. The engines were rebuilt following the airship’s return to Germany, and no further issues were encountered. The Hindenburg tragically and famously burst into flames on 6 May 1937 while landing at Lakehurst, New Jersey.


Front view of the DB 602 engine in the Musée de l’Air et de l’Espace, in Le Bourget, France. Above the engine are the cooling water outlet pipes. In the Vee of the engine is the induction manifold, and the pushrod tubes for the front cylinders can be seen. Note the finning on the bottom half of the crankcase. (Stephen Shakland image via flickr.com)

The Graf Zeppelin II was still being built when the Hindenburg disaster occurred. Design changes were made to the Graf Zeppelin II that included mounting the DB 602 engines in a tractor configuration. The inability of Germany to obtain helium, the start of World War II, and the end of the airship era meant the Graf Zeppelin II would not be used for commercial travel. The airship was broken up in April 1940.

The DB 602 engine proved to be an outstanding and reliable power plant. However, its capabilities will forever be overshadowed by the Hindenburg disaster. Two DB 602 engines still exist and are on display; one is in the Zeppelin Museum in Friedrichshafen, Germany, and the other is in the Musée de l’Air et de l’Espace, in Le Bourget, France. Although the DB 602 was not used on a wide scale, it did serve as the basis for the Mercedes-Benz 500 series marine engines that powered a variety of fast attack boats (Schnellboot) during World War II.

*Daimler-Benz was formed in 1926 with the merger of Daimler Motoren Gesellschaft and Benz & Cie. Prior to their merger, both companies produced aircraft engines under the respective names Mercedes and Benz. After the merger, the Daimler-Benz name was used mostly for aircraft engines, and the Mercedes-Benz name was used mostly for automobiles. However, both names were occasionally applied to aircraft engines in the 1930s.


Rear view of the DB 602 engine on display in the Zeppelin Museum in Friedrichshafen, Germany. A water pump on each side of the engine provided cooling water to a bank of cylinders. (Stahlkocher image via Wikimedia Commons)

Aircraft Diesels by Paul H Wilkinson (1940)
Aerosphere 1939 by Glenn D. Angle (1940)
Diesel Engines by B. J. von Bongart (1938)
High Speed Diesel Engines by Arthur W. Judge (1941)
Diesel Aviation Engines by Paul H Wilkinson (1942)
“The Hindenburg’s New Diesels” Flight (26 March 1936)
“The L.Z.129’s Power Units” Flight (2 January 1936)


Vought XF5U Flying Flapjack

By William Pearce

Following the successful wind tunnel tests of the Vought V-173 low-aspect ratio, flying wing aircraft in late 1941, the US Navy asked Vought to propose a fighter built along similar lines. Charles H. Zimmerman had been working on such a design as early as 1940. He and his team at Vought quickly finalized their fighter design for the Navy as VS-315. On 17 September 1942, before the V-173 had flown, the Navy issued a letter of intent for two VS-315 fighters, designated XF5U-1. One aircraft was a static test airframe, and the other aircraft was a flight test article.


Charles Zimmerman’s fighter aircraft from a patent application submitted in 1940. Although the drawing shows fixed horizontal stabilizers (45/50) and skewed ailerons (34/36), the patent also covered the configuration used on the Vought XF5U. Note the prone position of the pilot, and the guns around the cockpit.

The Vought XF5U was comprised of a rigid aluminum airframe covered with Metalite. Metalite was light and strong and formed by a layer of balsa wood bonded between two thin layers of aluminum. The XF5U had the same basic configuration as the V-173 but was much heavier and more complex.

The XF5U’s entire disk-shaped fuselage provided lift. The aircraft had a short wingspan, and large counter-rotating propellers were placed at the wingtips. At the rear of the aircraft were two vertical tails, and between them were two stabilizing flaps. When the aircraft was near the ground, air loads acted on spring-loaded struts to automatically deflect the stabilizing flaps up and allow air to escape from under the aircraft. The stabilizing flaps enhanced aircraft control during landing. On the sides of the XF5U were hydraulically-boosted, all-moving ailavators (combination ailerons and elevators). The ailavators had a straight leading edge, rather than the swept leading edge used on the V-173’s ailavators. Two large balance weights projected forward of each ailavator’s leading edge.


The XF5U mockup was finished in June 1943. Note the gun ports by the cockpit. The mockup had three-blade propellers and single main gear doors, items that differed from what was ultimately used on the prototype. The acrylic panel under the nose was most likely to improve ground visibility, like the glazing on the V-173. However, test pilots reported that the glazing was not useful.

Zimmerman originally proposed a prone position for the pilot, but a conventional seating position was chosen. The pilot was situated just in front of the leading edge and enclosed in a bubble canopy. Some sources state that an ejection seat was to be used, but no mention of one has been found in Vought documents, and an ejection seat does not appear to have been installed in the XF5U-1 prototype. The cockpit was accessed via a series of recessed steps that led up the back of the aircraft. The acrylic nose of the XF5U housed the gun camera and had provisions for landing and approach lights.

The aircraft’s landing gear was fully retractable, including the double-wheeled tailwheel. The main gear had a track of 15 ft 11.5 in (4.9 m). A small hump in the outer gear doors covered the outboard double main gear wheel. The long gear gave the aircraft an 18.7 degree ground angle. A catapult bridle could be attached to the aircraft’s main gear to facilitate catapult-assisted launches from aircraft carriers. For carrier landings, an arresting hook deployed from the XF5U’s upper surface and hung over the rear of the aircraft. Armament for the XF5U consisted of six .50-cal machine guns—three guns stacked on each side of the cockpit—with 400 rpg. The lower four guns were interchangeable with 20 mm cannons, but the proposed rpg for the cannons has not been found. Two hardpoints under the aircraft could each accommodate a 1,000 lb (454 kg) bomb. No armament was installed on the prototype.


The two XF5Us under construction. The left airframe was used for static testing, and the right airframe was the test flight aircraft. The engine cooling fans and oil tanks can be seen on the right airframe.

Originally, the XF5U was to be powered by two 14-cylinder, 1,600 hp (1,193 kW) Pratt & Whitney (P&W) R-2000-2 engines. It appears P&W stopped development of the -2 engine, and the 1,350 hp (1,007 kW) R-2000-7 was substituted sometime in 1945. The engines were buried in the aircraft’s fuselage, and engine-driven cooling fans brought in air through intakes in the aircraft’s leading edge. Cooling air exit flaps were located on the engine nacelles on both the upper and lower fuselage. An exit flap for intercooler air was located farther back on the top side of each nacelle.

Engine power was delivered to the propellers via a complex set of shafts and right angle gear drives. A two-speed gear reduction provided a .403 speed reduction for takeoff and a .177 reduction for cruising and high-speed flight. With the engines operating at 2,700 rpm (1,350 hp / 1,077 kW) at maximum takeoff power, the propellers turned at 1,088 rpm. At maximum cruise with the engines at 2,350 rpm (735 hp / 548 kW), the propellers turned at 416 rpm.


The complex power drive of the XF5U was the aircraft’s downfall. The system was unlikely to work flawlessly, and the Navy chose to use its post-war budget on jet aircraft rather than testing the XF5U. The inset drawing is from Zimmerman’s patent outlining the propeller drive.

A power cross shaft was mounted between the gearboxes on the front of the engines. In the event of an engine failure, the dead engine would be automatically declutched, and the cross shaft would distribute power from the functioning engine to both propellers. The two engines were declutched from the propeller drive at startup. The clutches were hydraulically engaged, and a loss of fluid pressure caused the clutch to disengage. The engines were controlled by a single throttle lever and could not be operated independently (except at startup).

By November 1943, the ongoing flight tests of the V-173 indicated that special articulating (or flapping) propellers would be needed on the XF5U. Propeller articulation was incorporated into the hub by positioning one two-blade pair of propellers in front of the second two-blade pair. The extra room provided the space needed for the 10 degrees of articulation and the linkages for propeller control. As one blade of a pair articulated forward, the opposite blade of the pair moved aft. To relieve the load and minimize vibrations, the propeller hub mechanism caused the blade pitch to decrease as the blade articulated forward and to increase as the blade moved aft. The XF5U’s wide-cord propellers were 16 ft (4.9 m) in diameter, made from Pregwood (plastic-impregnated wood), and built by Vought. The propellers were finished with a black cuff, a woodgrain blade, and a yellow tip. The pitch of the propellers was controlled by a single lever and could not be independently controlled; the set pitch of all blades changed simultaneously. If both engines failed, the propellers would feather automatically. Construction of the special propellers was delayed, and propellers from a F4U-4 Corsair were temporarily fitted to enable ground testing to begin.


The completed XF5U ready for primary engine runs with F4U-4 propellers. The aircraft was completed over a year before the articulating propellers were finished. Had the propellers been ready sooner, it is likely the XF5U would have been transported to Edwards Air Force Base for testing in late 1945.

The XF5U had a wingspan of 23 ft 4 in (7.1 m) but was 32 ft 6 in (9.9 m) wide from ailavator to ailavator and 36 ft 5 in (8.1 m) from propeller tip to propeller tip. Each ailavator had a span of about 8 ft 4 in (2.5 m). The aircraft was 28 ft 7.5 in (8.7 m) long and 14 ft 9 in (4.5 m) tall. The XF5U could take off in 710 ft (216 m) with no headwind and in 300 ft (91 m) with a 35 mph (56 km/h) headwind. The aircraft had a top speed of 425 mph (684 km/h) and a slow flight speed of 40 mph (64 km/h). Initial rate of climb was 3,000 fpm (15.2 m/s) at 175 mph (282 km/h), and the XF5U had a ceiling of 32,000 ft (9,754 m). A single tank located in the middle of the aircraft carried 261 gallons (988 L) of fuel. The internal fuel gave the XF5U a range of 597 miles (961 km), but with two 150-gallon (568-L) drop tanks added to the aircraft’s hardpoints, range increased to 1,152 miles (1,854 km). The XF5U had an empty weight of 14,550 lb (6,600), a normal weight of 16,802 lb (7,621 kg), and a maximum weight of 18,917 lb (8,581 kg).


The XF5U with its special, wide-cord, articulating propellers installed. Note the winged Vought logo on the propellers. The purpose of the bottles under the fuselage is not clear. The aircraft used compressed air for emergency extension of the landing gear and tail hook. Perhaps that system was being tested. Note that the inner main gear doors have been removed.

A wooden mockup of the XF5U was inspected by the Navy in June 1943. At this time, the mockup had narrow, three-blade propellers that were very similar to those used on the V-173. The XF5U’s complex systems and unconventional layout delayed its construction, which was further stagnated by higher priority work during World War II. The aircraft was rolled out on 20 August 1945 with the F4U-4 propellers installed. Some ground runs were undertaken, but more serious tests had to wait until Vought finished the special articulating propellers in late 1946.

The aircraft started taxi tests on 3 February 1947, but concerns over the XF5U’s propeller drive quickly surfaced. Vought’s chief test pilot Boone T. Guyton made at least one small hop into the air, but no serious test flights were attempted. The test pilots and Vought felt that the only suitable place for test flying the radical aircraft with its unproven gearboxes and propellers was at Edwards Air Force Base in California. Given the XF5U’s construction, the aircraft could not be disassembled, and it was too large to be transported over roads. The only option was to ship the XF5U to California via the Panama Canal. Faced with the expensive transportation request, no urgent need for the XF5U, questions about propeller drive reliability, and the emergence of jet aircraft, the Navy cancelled all further XF5U project activity on 17 March 1947.


This side view of the XF5U shows how the propeller blades were staggered. Note the balance weights on the ailavator, the hump on the gear door, and the slightly open engine cooling air exit flap on the upper fuselage. Strangely, the tail markings appear to have been removed from the photo.

With the original 1,600 hp (1,193 kW) P&W R-2000-2 engines, the XF5U had a forecasted top speed of 460 mph (740 km/h) and a slow speed of 20 mph (32 km/h). The aircraft had a 3,590 fpm (18.2 m/s) initial rate of climb and a service ceiling of 34,500 ft (10,516 m). With a fuel load listed at 300 gallons (1,136 L), the aircraft would have a 710-mile (1,143-km) range. To increase the XF5U’s performance and try to keep the program alive, Vought proposed a turbine-powered model to the Navy, designated VS-341 (or V-341). While it is not entirely clear which engine was selected, the engine depicted in a technical drawing closely resembles the 2,200 hp (1,641 kW) General Electric T31 (TG-100) turboprop. The estimated performance of the VS-341 was a top speed of 550 mph (885 km/h) and a slow speed of 0 mph (0 km/h)—figures that would allow the VS-341 to achieve Zimmerman’s dream of a high-speed, vertical takeoff and landing (VTOL) aircraft.


Rear view of the XF5U shows padding taped to the aircraft to protect its Metalite surface. The engine cooling air exit flaps are open. The intercooler doors have been removed, which aided engine cooling during ground runs. Note the tail markings on the aircraft.

The XF5U intended for flight testing (BuNo 33958) was smashed by a wrecking ball shortly after the program was cancelled. The XF5U’s rigid airframe withstood the initial blows, but there was no saving the aircraft; its remains were sold for scrap. At the time, the second XF5U (BuNo 33959) had already been destroyed during static tests.

Zimmerman’s aircraft were given several nicknames during their development: Zimmer’s-Skimmer, Flying Flapjack, and Flying Pancake. It is unfortunate that a radical aircraft so close to flight testing was not actually flown. Zimmerman continued to work on VTOL aircraft for the rest of his career.


To bring the XF5U into the jet age, Vought designed the turbine-powered VS-341. The aircraft had the same basic layout as the XF5U. Note the power cross shaft extending from the gearbox toward the other engine.

Chance Vought V-173 and XF5U-1 Flying Pancakes by Art Schoeni and Steve Ginter (1992)
Aeroplanes Vought 1917–1977 by Gerard P. Morgan (1978)
XF5U-1 Preliminary Pilot’s Handbook by Chance Vought Aircraft (30 September 1946)
XF5U-1 Illustrated Assembly Breakdown by Chance Vought Aircraft (1 January 1945)
Langley Full-Scale Tunnel Investigation of a 1/3-scale Model of the Chance Vought XF5U-1 Airplane by Roy H. Lange, Bennie W. Cocke Jr., and Anthony J. Proterra (1946)
“Airplane of Low Aspect Ratio” US patent 2,431,293 by Charles H. Zimmerman (applied 18 December 1940)
“Single or Multiengined Drive for Plural Airscrews” US patent 2,462,824 by Charles H. Zimmerman (applied 3 November 1944)
“The Flying Flapjack” by Gilbert Paust Mechanix Illustrated (May 1947)


Vought V-173 Flying Pancake (Zimmer’s Skimmer)

By William Pearce

In the early 1930s, Charles H. Zimmerman became determined to design a low-aspect ratio, flying wing aircraft with a discoidal planform. The wing would have a short span and make up the aircraft’s fuselage. Zimmerman believed that large, slow-rotating propellers placed at the tips of the aircraft’s wings would cancel out wingtip vortices, provide uniform airflow over the entire aircraft, and effectively increase the aircraft’s span. In addition, the propellers would provide continuous airflow over the aircraft’s control surfaces even at very low forward velocities. The propellers were counter-rotating; viewed from the rear, the left propeller turned counterclockwise and the right propeller turned clockwise. The envisioned aircraft would be able to execute short takeoffs and landings, maintain control at very low speeds, and have a high top speed. Zimmerman’s ultimate goal was a high-speed aircraft that could ascend and descend vertically and could hover.


Drawings from Charles Zimmerman’s 1935 patent showing his low-aspect ratio, flying wing aircraft. Note the three occupants lying in a prone position. The aircraft’s layout was very similar to the Vought V-173. The power transfer shaft (22) can been seen connecting the two propeller shafts.

While working at the National Advisory Committee for Aeronautics (NACA), Zimmerman won a design competition in 1933 for a light, general aviation aircraft. However, his low-aspect ratio design was deemed too radical to be built. Undeterred, Zimmerman designed a three-place aircraft in which the occupants lay in a prone position. Zimmerman called this aircraft the Aeromobile. The aircraft’s propellers were forced to rotate at the same speed via a power cross shaft that linked the engine’s propeller shafts together. Each engine could be disconnected from its respective propeller shaft in the event of an engine failure. The power cross shaft would distribute power from the functioning engine to both propellers.

To test his theories, Zimmerman and some friends built a small proof-of-concept aircraft based on the three-place design. The aircraft had a short 7 ft (2.1 m) wingspan and was powered by two 25 hp (19 kW), horizontal, two-cylinder Cleone engines. Despite several attempts, the aircraft was unable to takeoff. The difficulties were caused by an inability to synchronize the propellers, as the power cross shaft was omitted due to the aircraft’s small size.


The proof-of-concept aircraft built to test Zimmerman’s theories. This image illustrates the aircraft’s 7 ft (2.1 m) wingspan. Due to trouble with synchronizing the engines/propellers, the aircraft was not flown. Charles Zimmerman is on the right side of the image.

Following the unsuccessful trials of small aircraft, Zimmerman took a step back and turned to models. By 1936, he had a rubber band-powered scale model with a 20 in (508 mm) wingspan routinely making successful flights. Others at NACA reviewed Zimmerman’s work and encouraged him to seek financial backing from the aviation industry to further develop his designs—as an individual, his efforts to interest the US Armed Forces had not been successful. Zimmerman found support from Vought Aircraft and was hired on to continue his work in 1937.

Again, the radical nature of Zimmerman’s designs made the establishment question their worth. The US Army Air Corps turned down various proposals, but the US Navy could not overlook the fact that a short wingspan fighter with a short takeoff distance, a very low landing speed, and a high top speed would be ideal for carrier operations. In fact, such an aircraft could operate from just about any large ship. In 1938, the Navy funded the Vought V-162, which was a large model to further test Zimmerman’s ideas. The model was powered by electric motors and took two people to operate. The model sufficiently demonstrated Zimmerman’s design, and the Navy contracted Vought to build a full-size test aircraft on 4 May 1940. The aircraft was designated V-173 by Vought and was given Bureau Number (BuNo) 02978 by the Navy.


The Vought V-173 in the Langley wind tunnel. Note the forward rake on the two-blade propellers. The rake (or cone angle) was adjustable, and three-blade propellers of the same type were soon fitted to the aircraft. (Langley Memorial Aeronautical Laboratory / NASA image)

The airframe of the Vought V-173 was made mostly of wood, but the forward cockpit structure and propeller nacelles were made of aluminum. The front part of the fuselage back to the middle of the cockpit was covered with wood, and the rest of the aircraft was fabric-covered. Originally, the pilot was to lie in a prone position, but this was changed to a more conventional, upright seat. The lower leading edge of the aircraft had glazed panels to improve visibility from the cockpit while the V-173 was on the ground. Cockpit entry was via a hatch under the aircraft, but the canopy also slid back. Housed in the aircraft’s fuselage were two 80 hp (60 kW) Continental C-75 engines. Most sources list the engines as Continental A-80s, but C-75s were actually installed in the aircraft. The 80 hp (60 kW) rating was achieved through the use of fuel injection. The C-75 was a flat, four-cylinder, air-cooled engine that displaced 188 cu in (3.1 L). One engine was on each side of the cockpit. The engines were started by pulling a handle through an access panel under the aircraft. Each engine had a cooling fan attached to its output shaft, and engine cooling air was brought in through inlets in the aircraft’s leading edge. The air exited via flaps in the upper fuselage.

Via shafts and right angle drives, the engines powered two 16 ft 6 in (5.06 m), three-blade, wooden propellers at around .167 times engine speed. The variable-pitch propellers turned around 450 rpm at maximum power (2,700 engine rpm) and around 415 rpm at cruise power (2,500 engine rpm). The individual blades could articulate (flap) automatically to compensate for side gusts and uneven loading. The blades were hinged inside the propeller hub in which hydraulic dampers limited their articulation. The rake (or coning) angle of the blades could be adjusted on the ground. This moved the tips of the blades either forward or aft relative to the propeller hub.


Underside view of the V-173 shows the windows in the aircraft’s leading edge. The hinge line for the control surfaces between the tails can just be seen near the aircraft’s trailing edge. The surfaces were omitted when the aircraft first flew, but stabilizing flaps were later installed in their place. (Langley Memorial Aeronautical Laboratory / NASA image)

A power cross shaft that ran just behind the cockpit connected the engine gearboxes. The cross shaft ensured that power was delivered equally between the two propellers, and it also synchronized propeller rpm. A failed engine would automatically declutch from the propeller drive system, and the remaining engine would power both propellers. The left engine was started first and then clutched to the propeller drive system. The right engine was then started and automatically clutched to the propeller drive system after it came up to speed.

Under the V-173 were two very long fixed main gear legs that supported the aircraft at a 22.25 degree angle while it sat on the ground. At the rear of the aircraft were two vertical stabilizers. Attached to each side of the V-173 was a horizontal stabilizer with a surface that acted as both an aileron and an elevator (ailavator or ailevator). The ailavators were not part of the initial V-173 design (and were not on the V-162 model), but early model tests indicated that the flight controls were needed.


View of the V-173 on an early test flight that shows no stabilizing flaps between the tails. Note the deflection angle of the ailavator; the V-173 always flew at a nose-high angle because it was underpowered.

The V-173 had a wingspan of 23 ft 4 in (7.1 m) but was about 34 ft 9 in (10.6 m) wide from ailavator to ailavator. The aircraft was 26 ft 8 in (8.1 m) long and 12 ft 11 (3.9 m) in tall. The V-173 could take off in 200 ft (61 m) with no headwind, and it could lift right off the ground with virtually no roll in a 30 mph (48 km/h) headwind. The aircraft’s top speed was 138 mph (222 mph), and cruising speed was 75 mph (121 km/h). With normal prevailing winds, the V-173 would routinely take off in 20 ft (6 m) and land at 15 mph (24 km/h). The aircraft had an empty weight of 2,670 lb (1,211 kg) and a normal weight of 3,050 lb (1,383 kg). The V-173 only carried 20 gallons (76 L) of fuel in two 10 gallon (38 L) tanks.

In November and December 1941, the V-173 was tested in NACA’s Langley wind tunnel in Hampton, Virginia. The aircraft had its original two-blade propellers, but these were found to be insufficient and were replaced by three-blade units shortly after the tests. Two small control surfaces that made up the trailing edge of the aircraft were present between the tails. However, these were removed before the V-173’s first flight. The Navy was encouraged enough by the wind tunnel tests that they asked Vought to prepare a proposal for a fighter version of the aircraft, which eventually became the Vought XF5U-1.


The V-173 is shown with redesigned ailavators and the stabilizing flaps installed. The cooling air exit flaps can be seen near the cockpit. The two ports forward of each cooling air exit flap were for engine exhaust.

After an extended period of taxi tests, the V-173’s first flight took place on 23 November 1942 at Bridgeport Airport (now Sikorsky Memorial Airport) in Stratford, Connecticut, with Vought test pilot Boone T. Guyton at the controls. Guyton found the aircraft’s controls extremely heavy and thought that he might need to make a forced landing. Fortunately, He had enough control to make a large circuit and land the aircraft after 13 minutes of flight. Adjustments to the propellers were made, and the ailavators were redesigned as all-moving control surfaces with servo tabs. These changes improved aircraft control, but landing the V-173 was still difficult. As it approached the ground, air would get trapped under the aircraft and force the tail up. Subsequently, the nose of the aircraft would drop, causing the V-173 to rapidly descend the last few feet. The aircraft would hit the runway harder than intended and bounce back into the air. After about 40 flights, the two stabilizing flaps were added between the aircraft’s tails. The flaps were larger than the control surfaces tested in the wind tunnel, and they were separated by the tailwheel. When the aircraft was near the ground, air loads acted on spring-loaded struts to automatically deflect the stabilizing flaps up and allow air to escape from under the aircraft.

A number of different pilots, including Charles Lindberg, flew the V-173. Over its flight career, the aircraft did experience a few difficult landings that resulted in minor damage. The most serious issue occurred on 3 June 1943 when Vought-pilot Richard Burroughs made an emergency landing on Lordship Beach, Connecticut. Vapor lock had caused fuel starvation and subsequent engine failure. Immediately after touchdown, Burroughs flipped the V-173 onto its back to avoid hitting a sunbather. No one was injured, and the aircraft was not seriously damaged.


The V-173 undergoing an engine run. The engine cooling air intakes can be seen in the aircraft’s leading edge. The canopy is open, and the cockpit access hatch on the aircraft’s underside is also open. Note that the stabilizing flaps are deflected up and that streamlined fairings have been fitted to cover the wheels.

Overall, the V-173 flew as expected, but it was not entirely like a conventional aircraft. The V-173 was underpowered, and there were unresolved vibration issues caused by the propeller gearboxes and drive shafts. The aircraft made around 190 flights and accumulated 131 hours of flight time.

The V-173 made its last flight on 31 March 1947. The Navy kept the aircraft in storage at Norfolk Naval Air Station, Virginia for a number of years and gave it to the National Air and Space Museum in September 1960. The V-173 was stored at the Paul E. Garber Facility in Suitland, Maryland until 2003, when it was moved to Vought’s Grand Prairie facility near Dallas, Texas for restoration by the Vought Aircraft Heritage Foundation. Restoration was completed in February 2012, and the aircraft was loaned to Frontiers of Flight Museum in Dallas, where it is currently on display.

Zimmerman’s aircraft were given several nicknames during their development: Zimmer’s Skimmer, Flying Flapjack, and Flying Pancake. Test pilot Guyton said that the V-173 could fly under perfect control while maintaining a 45 degree nose-up angle with full power and full aft stick. During the flight test program, the pilots were not able to make the V-173 stall completely or enter a spin. The aircraft rapidly decelerated in sharp turns, and this could prove advantageous in getting on an opponent’s tail during a dogfight. But if the shot were missed, the aircraft could be at a disadvantage because of its decreased speed. The V-173 proved the viability of Zimmerman’s low-aspect ratio, flying wing aircraft concept, provided much information on how to refine the design, and directly contributed to the Vought XF5U-1.


Painstakingly restored by volunteers, the V-173 is currently on display in the Frontiers of Flight Museum in Dallas, Texas. The aircraft is on loan from the National Air and Space Museum until at least 2022. (Frontiers of Flight Museum image)

Chance Vought V-173 and XF5U-1 Flying Pancakes by Art Schoeni and Steve Ginter (1992)
Aeroplanes Vought 1917–1977 by Gerard P. Morgan (1978)
“Aircraft” US patent 2,108,093 by Charles H. Zimmerman (applied 30 April 1935)
“The Flying Flapjack” by Gilbert Paust Mechanix Illustrated (May 1947)
Correspondence with Bruce Bleakley, Director of the Frontiers of Flight Museum


FIAT A.38, A.40, and A.44 Aircraft Engines

By William Pearce

In the early 1930s, Italy was a world leader in aviation and had developed both liquid-cooled and air-cooled engines. In 1933, the Italian Air Ministry decided to focus on air-cooled radial engines, and the development of liquid-cooled inline engines was essentially abandoned. By 1939, the shortsightedness of this decision became clear as most premiere frontline fighters from Britain, France, Germany, the Soviet Union, and the United States were powered by liquid-cooled engines. As a result, the Ministero dell’Aeronautica (Italian Air Ministry) began to encourage the development of liquid-cooled engines.


The FIAT A.38 RC15-45 was a 2,118 cu in (34.7 L) inverted V-16. The supercharger was mounted between the cylinder banks to decrease the engine’s length. Note the magnetos and contra-rotating propeller shafts.

In 1939, the Italian Air Ministry asked FIAT to design a new aircraft engine to power the next generation of Italian fighter aircraft. FIAT engineers Antonio Fressa and Carlo Bona began designing the new engine, designated A.38. The A.38 was initially an upright V-16 engine closely based on the FIAT AS.8, which was originally designed to set speed records. While the AS.8 had individual cylinders, the A.38 used two cast cylinder blocks.

After the initial upright engine design, the Italian Air Ministry was inspired by the German Daimler-Benz 600 series of inverted V-12s and requested the A.38’s configuration be changed to an inverted engine. Fressa completely redesigned the A.38, leaving very little in common with the AS.8. The AS.8 engine was a 45 degree V-16 with a 5.51 in (140 mm) bore and stroke, and by 1940, the A.38 had become an inverted, 90 degree V-16 with a 5.43 in (138 mm) bore and a 5.71 in (145 mm) stroke.

The A.38’s 16-cylinder arrangement was selected to maximize the engine’s power output while keeping its cylinder size and supercharger boost within known and reliable limits. However, a V-16 engine is very long, and its crankshaft is subject to torsional vibrations. To keep the engine’s length as short as possible, Fressa used a 90 degree cylinder bank arrangement and positioned the supercharger horizontally between the cylinder banks. This resulted in a rather complex supercharger drive.


The AC.38 in a test cell. The supercharger arrangement greatly increased the engine’s otherwise small frontal area. The 1,200 hp (895 kW) engine could have sufficed with a single-rotation propeller, but the contra-rotating unit would eliminate asymmetrical torque.

The A.38 was of all-aluminum construction with two detachable monobloc cylinder blocks. Each cylinder bank had eight cylinders, and each cylinder had two inlet and two exhaust valves. The valves were actuated by dual overhead (underhead in this case) camshafts that were driven by a single vertical shaft from the front of the engine. Two spark plugs were installed in each cylinder, and the spark plugs for each cylinder bank were fired by two magnetos driven at the front of the engine. The A.38 had a compression ratio of 7 to 1.

The engine had contra-rotating propeller shafts that were driven at .514 engine speed. Between the cylinder banks were the carburetor, supercharger, intake manifolds, and water pump. There were plans to use fuel injection, but this was never completed. The single-stage supercharger had two-speeds that gave critical altitudes of 4,931 ft (1,500 m) and 14,764 ft (4,500 m). The supercharger was powered by a shaft driven from the front of the engine and situated in the Vee between the cylinders. This shaft also drove the oil and water pumps. The supercharger’s outlet was at the center of the engine, and the air was fed into four manifolds, each serving four cylinders.

The engine was officially designated A.38 RC15-45: “RC” for Riduttore de giri (gear reduction) and Compressore (supercharged), and 15/45 for the altitudes (in hectometers) at which maximum power was obtained. The A.38 had a 5.43 in (138 mm) bore, a 5.71 in (145 mm) stroke, and a displacement of 2,118 cu in (34.7 L). The engine produced 1,200 hp (895 kW) at 2,800 rpm at 4,931 ft (1,500 m) and 14,764 ft (4,500 m). The 1,200 hp (895 kW) output was not normally enough to justify the use of contra-rotating propellers, but a photo of the engine in a test cell and a drawing of the FIAT G.55 fighter powered by the A.38 show propellers with just two-blades. It would appear that contra-rotating propellers were used more to eliminate asymmetrical torque than to compensate for exceeding the capabilities of a single-rotation propeller. The engine weighed 1,698 lb (770 kg).


The FIAT G.55 fighter was originally designed to use the A.38 engine with contra-rotating propellers (top), but the aircraft was redesigned once the switch to a single-rotation propeller (bottom) was made. Delays with the A.38 led to the Daimler-Benz DB 605 being installed in the G.55.

Three A.38 engines were ordered, but it is not clear if all were built. The A.38 underwent tests in 1941 and was able to achieved 1,300 hp (969 kW), but even more power was desired. Some developmental changes to the engine included switching to a single-rotation propeller shaft. Trouble was experienced with the engine’s crankshaft and supercharger drive, and despite multiple attempts, the engine failed to pass airworthy certification tests. Fressa continued to work on the engine into 1942, but the Italian Air Ministry had already obtained licenses to produce Daimler-Benz engines and was no longer interested in the A.38—FIAT would build the DB 605 as the RA 1050 Tifone (Typhoon). It is interesting to note that the AS.8 had proven itself reliable and probably would have been a faster and better starting point for Fressa than an all-new engine design.

A number of aircraft designs were made to accommodate the A.38 engine. The only design that was actually built was the G.55. The G.55 was originally planned to be powered by the A.38 turning contra-rotating propellers, but the design was later altered for a single-rotation, three-blade propeller. In late 1941, it became obvious that the G.55 airframe would be completed before the A.38 engine was cleared for flight tests. As a result, a change to the DB 605 engine was initiated. First flown on 30 April 1942, the G.55 arguably became the best Italian fighter of World War II. Due to the state of the Italian aircraft industry in wartime, the G.55 was never made in sufficient numbers to have any impact on the conflict.


The FIAT A.40 was a 2,000 hp (1,491 kW) X-24 that had the same bore and stroke as the A.38. Although two A.40 engines were built, they were never tested because of shifting priorities during World War II. Note the cannon installed in the upper Vee on the side view drawing.

In 1940, Fressa tasked Dante Giacosa to create a new engine to compete with the A.38 and produce 2,000 hp (1,491 kW) at 8,202 ft (2,500 m). Instead of the V-16 layout, Giacosa turned to an X-24 configuration with four six-cylinder banks positioned 90 degrees from each other. The X-24 engine was designated A.40 RC20-60, and it used the same 5.43 in (138 mm) bore and 5.71 in (145 mm) stroke as the A.38. The A.40 engine had a single crankshaft and used one master connecting rod with three articulated connecting rods for each row of cylinders. The induction manifold was installed in the Vee between the lower cylinder banks and fed the two-speed supercharger mounted at the rear of the engine. The A.40 used a fuel injection system that Giacosa and his team had designed. The gear reduction unit raised the single-rotation propeller shaft, which enabled a 20 mm or 37 mm cannon to be fitted in the Vee between the upper cylinder banks and to fire through the propeller hub. The A.40 displaced 3,176 cu in (52.1 L), and an output of 2,000 hp (1,491 kW) was expected at 6,562 ft (2,000 m) and 26,247 ft (6,000 m). Reportedly, two A.40 engines were built in 1943, but Italy’s surrender prevented the engines from ever being tested. No information has been found on the disposition of any A.38 or A.40 engines.

While Fressa was working on the A.38, he also designed a more powerful engine. There is some evidence that suggests the engine was originally designated A.42 and used four A.38 cylinder blocks in an H-32 configuration. However, the engine was redesigned and redesignated A.44 RC15-45. The FIAT A.44 was comprised of two V-16 engines stacked together to form an X configuration. The V-16 engine sections were independent of each other, and each section powered half of the A.44’s contra-rotating propeller at a .429 reduction. A.38 cylinder blocks, pistons, and crankshafts were used, but the V-16 engine sections had a wider bank angle of 135 degrees. The X-32 engine displaced 4,235 cu in (69.4 L) and was forecasted to produce 2,400 hp (1,790 kW) at 2,800 rpm and a maximum of 2,800 hp (2,088 kW) at 2,950 rpm. The engine was estimated to weigh 3,307 lb (1,500 kg), and the design progressed through 1942. While FIAT designed a few aircraft to be powered by the A.44, like the CR.44 fighter/bomber and the BR.44 torpedo bomber, the engine failed to gain the support of the Italian Air Ministry and was never built.


The FIAT CR.44 fighter/bomber was planned around the 2,400 hp (1,790 kW) FIAT A.44 engine. The A.44 X-32 engine was essentially two V-16 engines mounted together. The A.44 engine would have shared most parts with the A.38, except the crankcase. Neither the A.44 nor the CR.44 were built.

Aeronuatica Militare Museo Storico Catalogo Motori by Oscar Marchi (1980)
Ali D’Italia Fiat G 55 by Piero Vergnano and Gregory Alegi (1998)
Forty Years of Design with Fiat by Dante Giacosa (1979)
“Fantasmi di aerie e motori Fiat dal 1935 al 1945 (prime parte)” by Giovanni Masino; Ali Antiche 106 (2011)
“Fantasmi di aerie e motori Fiat dal 1935 al 1945 (seconda parte)” by Giovanni Masino; Ali Antiche 108 (2012)


Union Pacific 4-8-8-4 Big Boy Locomotive

By William Pearce

For some time, locomotives of the Union Pacific Railroad (UP) had struggled to climb the Wasatch mountains between Ogden, Utah and Green River, Wyoming. This 176-mile (283-km) stretch of track started out at 4,300 ft (1,310 m) above sea level in Ogden, climbed the Wasatch Range to 7,300 ft (2,225 m) at the Aspen Tunnel, and then dropped to 6,100 ft (1,859 m) at Green River. Occasionally, up to three helper engines were used to assist heavily loaded trains over the Wasatch mountains.


Union Pacific Big Boy 4012 hauling a load of freight through Green River, Wyoming in November 1941. This may have been the recently delivered engine’s first trip west. (Otto Perry image via Denver Public Library)

In 1940, UP was enjoying a period of expansion, and its president, William Jeffers, was interested in a new locomotive that could conquer the Wasatch Range pulling 3,600 tons (3,266 t) unassisted. At the same time, World War II was on the horizon, and the United Sates had begun to increase its production of war material. This put even more traffic on the heavily-traveled Oden-Green River route. Headed by Otto Jabelmann, UP’s Department of Research and Mechanical Standards (DoRMS) in Omaha, Nebraska calculated that 135,000 lb (61,235 kg) of tractive effort was needed for the engine to achieve its design goal. DoRMS quickly designed the new, massive locomotive and worked closely with the American Locomotive Company (ALCO), the company that agreed to build the engine. The engines were assigned numbers in the 4000-class, and there were plans to name the new series “Wasatch.” However, a worker wrote “Big Boy” in chalk on the front of the first engine while it was being built, and the name stuck. With its tender, the Big Boy was one of the largest and heaviest steam locomotives ever built.

The Big Boy’s design was based closely on the UP’s 4-6-6-4 Challenger that went into service in 1936. However, the Big Boy was larger and heavier than the Challenger and necessitated that UP make many changes to the track between Ogden and Green River. Heavier rail was laid in many places, and curves were realigned and adjusted to maintain a constant curvature. At stations, larger turntables were installed to accommodate the Big Boy’s length. The Big Boy was essentially the largest thing that could normally operate on an existing standard gauge railroad.


The crew standing next to newly-completed Big Boy 4002 gives scale to every part of the engine: the cylinders, wheels, boiler, etc. The railing on the front of the -1 class engines was originally coolers for the air pump. The -2 class used a standard Wilson aftercooler, as the custom set up on the Class -1 would often crack. As the coolers failed on the -1 class, they were removed and replaced by Wilson units. (Union Pacific image)

The Big Boy utilized a 4-8-8-4 wheel arrangement and was the only locomotive to do so. At the front of the engine was a four-wheel leading truck that had 36 in (.91 m) wheels. This was followed by eight 68 in (1.73 m) drive wheels, with a single piston driving a set of four wheels on each side of the engine. Another set of eight drive wheels followed that were identical to the first. Finally, under the cab was a four-wheel trailing truck with 42 in (1.07 m) wheels. The leading truck and first eight drive wheels were attached to a separate frame than the second set of drive wheels and trailing truck. Between the two sets of drive wheels was a tongue and groove pivot point that allowed the front frame to articulate independently of the rear frame. Mounted to the rear frame was the boiler, firebox, and cab. The articulated locomotive was pioneered by Swiss engineer Anatole Mallet and could handle tighter curves than a standard ridged locomotive. In the case of a long locomotive like the Big Boy, articulation allowed the engine to operate on tracks with curves as sharp as 20 degrees.

ALCO built the Big Boys in Schenectady, New York, and two versions of the engine were made. Starting in 1941, 20 of the 4-8-8-4-1 class engines were made and numbered 4000–4019. In 1944, five of the 4-8-8-4-2 class engines were made and numbered 4020–4024. The difference between the two versions was mainly a different superheater that necessitated changes to the tubing arrangement in the boiler and increased water storage capacity in the tender. These changes were made for maintenance reasons and also due to material shortages during World War II. The first engine, 4000, was delivered to UP in Omaha on 5 September 1941.


The Big Boy’s firebox (left), boiler (middle), and smokebox (right) were all mounted as a single unit and can been seen here, ready to be lowered onto the engine’s frame. The steel that formed the boiler was 1.375 in (35 mm) thick. The two humps above the boiler are the sandboxes. Between the sandboxes is the steam dome, its exposed studs waiting for the cover plate. Exiting the lower part of the smokebox is a duct to feed steam from the superheater to the cylinders. (ALCO image)

All Big Boys were 132 ft 10 in (40.5 m) long and made up of an 85 ft 9.5 in (26.2 m) long engine and a 47 ft .5 in (14.3 m) long tender that carried the locomotive’s coal and water. The locomotive was 16 ft 2.5 in (4.9 m) tall, and its whistle was mounted horizontally so as to not increase the engine’s height. Various ladders and handholds were recessed into the engine and tender to keep the locomotive’s width at a maximum of 11 ft 6 in (3.5 m). The loaded weight of the -1 class was 762,000 lb (345,638 kg) for the engine and 427,500 lb (193,911 kg) for the tender, which gave a total weight of 1,189,500 lb (539,549 kg). The -2 class was heavier at 772,250 lb (350,276 kg) for the engine, 436,500 lb (197,993 kg) for the tender, and a total weight of 1,208,750 lb (548,280 kg). Each set of eight driving wheels supported 540,000 lb (244,940 kg) on the -1 class and 545,200 lb (247,299 kg) on the -2 class. The maximum weight permitted on each of the engine’s 12 axles was 67,800 lb (30,754 kg).

The centipede-style tender was supported by 14 wheels, each 42 in (1.07 m) tall. The first four wheels made up the leading truck, and the 10 trailing wheels were mounted directly to the tender. The tender originally carried 56,000 lb (25,401 kg) of coal in a front compartment. In the late 1940s, 10 in (254 mm) tall steel sideboards were added to the top of the coal compartment. The sideboards enabled an additional 8,000 lb (3,629 kg) of coal to be loaded, increasing the tender’s capacity to 64,000 lb (29,030 kg). A rear compartment held 24,000 gallons (90,850 L) of water for the -1 class and 25,000 gallons (94,635 L) of water for the -2 class. At full steam, a Big Boy engine would consume the tender’s coal and water supply in two hours, but a proper facility could replenish the coal and water in eight minutes.


This image of engine 4023’s tender helps illustrate why the type is known as a centipede tender. Visible on this side are the five wheels mounted to the tender and the two installed in the leading truck. The diagonal row of rivets indicates the partition between the water tank in the rear of the tender and the coal bunker in the front. Note the recessed ladder on the left and the 10 in (254 mm) sideboards atop the tender on the right. (Larry Pieniazek image via Wikimedia Commons)

A large, mechanical stoker auger transported coal from the supply in the tender to the engine’s firebox; no regular fireman could keep up with the Big Boy’s prodigious need for fuel. The firebox was 235 in (5.97 m) long and 96 in (2.44 m) wide and burned coal at around 2,000 °F (1,093 °C). Heat from the firebox flowed through the boiler via a series of tubes, each 22 ft (6.7 m) long. The -1 class engine had 259 tubes: 75 2.25 in (57.2 mm) tubes and 184 4.0 in (101.6 mm) flues. With its altered boiler, the -2 class engine had 285 tubes: 212 2.25 in (57.2 mm) tubes and 73 5.5 in (139.7 mm) flues. If laid end-to-end, the tubes and flues would stretch 5,698 feet (1,737 m) for the -1 class and 6,270 feet (1,911 m) for the -2 class. After passing through the tubes, the soot, embers, smoke, and heat from the burning coal flowed into a smokebox at the front of the engine and then out into the atmosphere via dual stacks.

The hot tubes, flues, and firebox provided the surface area to turn water in the boiler to steam. The -1 class had 5,889 sq ft (547.1 sq m) of evaporative surface area, and the -2 class had 5,755 sq ft (534.6 sq m). The water in the boiler was heated until 300 psi (20.7 bar) of steam had been generated. With a temperature of over 420 °F (215 °C), the wet, saturated steam was collected in a steam dome positioned above the boiler. The steam flowed from the dome to the saturated steam chamber in the superheater. Small superheater elements (tubes) took the wet steam back into the flues where it was heated well above its saturation value and converted to dry, superheated steam. The superheater elements delivered the dry steam to the superheated steam chamber in the superheater. Combined, the superheater elements stretched for over a mile (1.6 km). The -1 class had a Type E superheater with a surface area of 2,466 sq ft (299.1 sq m). The -2 class had a Type A superheater with a surface area of 2,043 sq ft (189.8 sq m). The Type A required less maintenance than the Type E and provided more than enough steam for the engine, and this is why the older Type A superheater was used. From the superheater, steam was piped to the Big Boy’s two sets of two cylinders.


The smokebox of engine 4014 as it undergoes restoration. The workers inside give some perspective to the immense size of the Big Boy. The large vertical ducts are the engine’s dual stacks. The large pipes behind the stacks and leading down the side of the smokebox take steam from the superheater to the cylinders. The vertical tubes are the superheater elements, and just beyond them are the horizontal tubes and flues that extend through the boiler to the firebox. (Union Pacific image via video screenshot)

The Walschaerts valve gear controlled the flow of steam in and out of the cylinders. A piston valve mounted in a valve chest above each cylinder slid back and forth to allow steam to enter one side of the double-acting cylinder while simultaneously opening the other side to the atmosphere for the previous steam charge to escape. The steam flowed into the front of the cylinder and filled its 14,176 cu in (232 L) volume, pushing the 23.75 in (603.3 mm) diameter piston back 32 in (812.8 mm) to the rear end of the cylinder. The valve then slid rearward to open the front part of the cylinder to the atmosphere and direct steam into the rear part of the cylinder. The second blast of steam pushed the piston back to its original position. Although the cylinder was uniform in size, the cylinder’s return volume was only 13,345 cu in (219 L) on account of the 5.75 in (146 mm) diameter, hollow piston rod taking up some room. The piston rod was attached to the connecting rod via a crosshead. The connecting rod extended back to the third driving wheel in the four-wheel set. Here, the connecting rod was attached to the coupling rod, which was connected to all four driving wheels. To aid traction, sand could be deposited on the rails in front of each drive wheel. The Big Boy had two sandboxes mounted on top of the boiler and each held 4,000 lb (1,814 kg) of sand.

The Big Boy was designed for a top speed of 80 mph (129 km/h), but its highest speed reported was a test at 72 mph (116 km/h). It is unlikely the engine was ever operated in service much beyond 50 mph (80 km/h). Of course, hauling the heaviest loads up the steepest grades reduced the engine’s speed to around 12 mph (19 km/h), the speed at which its tractive effort was at a maximum of some 135,375 lb (61,405 kg). The 80 mph (129 km/h) speed design ensured that parts were built to withstand stresses well beyond what was needed to haul freight at 40 mph (64 km/h).


The front drive wheels on engine 4017. The black box on the right is the cylinder, with the piston rod extending out to the left. A crosshead joins the piston rod with the connecting rod. The connecting rod extends back and attaches to the third drive wheel, and a coupling rod connects all the drive wheels together. (National Railroad Museum image)

At 41 mph (66 km/h), the Big Boy produced some 6,290 hp (4,690 kW) at the drawbar, which would be around 7,157 hp (5,337 kW) produced at the cylinders. Without any slip, each rotation of the drive wheels moved the engine 17.8 ft (5.4 m). At 41 mph (66 km/h), each drive wheel rotated 202 times a minute, and each double-acting piston made 404 strokes. This resulted in roughly 12,869 cu ft (364 cu m) of steam passing through the Big Boy’s cylinders every minute.

Four seats were provided in the Big Boy’s cab, although the engine only required a crew of three: an Engineer, a Fireman, and a Brakeman. If needed, the cab could accommodate six occupants with two additional makeshift seats. Each of the 20 -1 class engines cost $265,174 in 1941, and each of the five -2 class engines cost $319,600 in 1944. The equivalent cost for each engine would be over $4,335,000 in 2016.


Smoke and steam billow out of Big Boy engine 4017 as it starts off from Rawlins, Wyoming. Even though it is a -1 class, the cooler has been removed from the railing on the front of the engine. (Stan Kistler image)

All Big Boy locomotives were pressed into service as soon as they could be delivered. Originally cleared to pull 3,200 tons (2,903 t) up the 1.14% grade between Ogden and Green River, the engines were eventually allowed to haul 4,450 tons (4,037 t) as experience was gained. On a .82% grade, the engines were cleared to haul 5,360 tons (4,863 t). Theoretically, the Big Boy could pull a train 5.5 miles (8.9 km) long on flat ground from a standing start. In practice, the engine routinely pulled over 100 cars.

During World War II, the Big Boys spent most of their time moving freight between Ogden and Green River. On a typical run from Oden to Evanston, Wyoming, with a stop in Echo, Utah, a Big Boy would take about four hours to cover the 76-mile (122-km), uphill route and climb some 2,500 ft (762 m). Engine 4016 made the trip in 3 hours and 50 minutes while hauling 71 cars, for a weight of 3,883 tons (3,523 t). The Big Boy consumed 74,700 lb (33,883 kg) of coal and 34,800 gallons (131,732 L) of water. This averages to 19,487 lb (8,839 kg) of coal and 9,078 gallons (34,364 L) of water used per hour, or 996 lb of coal and 464 gallons of water per mile (280 kg and 1,089 L per km). Under full steam, the Big Boy was said to consume 22,000 lb (9,979 kg) of coal and 12,000 gallons (45,425 L) of water per hour.


To expedite service, especially with heavy trains, even the Big Boy used helper engines or was doubleheaded. Here, engines 4013 and 4004 team up to doublehead a train over Sherman Hill on the way from Laramie to Cheyenne in August 1958. (Otto Perry image via Denver Public Library)

After World War II, Big Boys were occasionally used for trips to southern Utah and did make regular trips into Wyoming, going as far as Cheyenne, 483 miles (777 km) from Ogden. The Cheyenne trips required conquering the 1.55% grade up Sherman Hill and passing through the Hermosa Tunnel at around 8,000 ft (2,438 m). In the 1950s, their service expanded on occasion as far east as North Platte, Nebraska and as far south as Denver, Colorado. Although the engines were cleared for other routes, like Ogden to Los Angles, they never made the journey in regular service. The ever-increasing tonnage needing to move on the rails resulted in even the Big Boys using helper engines to speed up travel over the steep mountain passes. Rarely, two Big Boy engines would be linked to doublehead a train quickly over the mountain.

The Big Boy engines proved very reliable in service, but they did require a significant amount of maintenance. UP considered purchasing additional engines, and other railroads thought about buying Big Boys, but resources were somewhat limited during World War II. After the war, diesel locomotives were proving themselves as the prime mover of the future. Still, Big Boys soldiered on and were one of the last steam locomotives in regular service.


Well-worn engine 4021 hauls freight through Wyoming in June 1956. The Big Boys were one of the last steam engines in regular service. (Chris Zygmunt Collection image)

The last Big Boy was removed from revenue service on 2 July 1959. The engines were kept in storage until August 1961, when the first were retired. The last Big Boy was retired in July 1962. At the time of their retirement, each of the -1 class Big Boys had accumulated over 1,000,000 miles (1,610,000 km)—the equivalent of traveling from the Earth to the Moon and back twice. Engine 4006 had the most miles, at 1,064,625 (1,713,348 km). Each of the -2 class engines had traveled over 800,000 miles (1,290,000 km)—the equivalent of circling the Earth 32 times. At 855,163 miles (1,376,252 km), engine 4021 had the highest mileage of the -2 class. All total, the Big Boys accumulated 25,008,054 miles (40,246,574 km); this is about the distance from Earth to Venus when the planets are at their closest point.

Although the Big Boy was very impressive, there were other locomotives that were larger, heavier, and more powerful, but probably none that were all three. What makes the Big Boy unique is that even with its massive size and colossal power, it was in regular service for nearly 20 years—it was not an experimental train, and it was not limited to a small section of track. The Big Boy was also not a Mallet-type locomotive. Although it was articulated, the Big Boy was not a compound steam engine, which is the second hallmark of a true Mallet.

Seventeen of the Big Boy engines were scrapped, while the remaining eight were put on display in various museums. As of 2016, seven of the Big Boys are still on display. The remaining engine, 4014, was reacquired by UP in 2013 and is undergoing restoration to working order at their facility in Cheyenne, Wyoming. The restoration is planned to be completed by 2019, in time for UP’s 150th anniversary. However, the amount of work needed to return 4014 to working order is substantial. Part of the restoration includes converting the engine from coal fired to oil fired. Regardless, Big Boy 4014 will once again take to the rails, but only for special excursion service; its days as a workhorse ended some 50 years ago.


Big Boy 4014 sits in Cheyenne undergoing restoration. The cab has been removed, and the locomotive has been stripped down to the boiler. (Union Pacific image)

Big Boy by William W. Kratville (1972)
“Big Boy: On the Road to Restoration” Trains Magazine Special (2014)
Last of the Giants (Part 1 and Part 2) by Union Pacific