Allison V-3420-A front

Allison V-3420 24-Cylinder Aircraft Engine

By William Pearce

In the mid-1930s, the United States Army Air Corps (AAC) was interested in a long-range bomber. Boeing won a contract to build the aircraft, which was originally designated XBLR-1 (eXperimental Bomber Long Range-1), but ultimately became the XB-15. By 1935, the AAC realized that current engines, and those under development, lacked the power needed for such a large aircraft. At the time, the AAC was pursuing its next experimental long-range bomber, the Douglas XBLR-2. The AAC requested the Allison Engineering Company build a 1,600 hp (1,193 kW) engine for the XBLR-2, which later became the XB-19.

Allison V-3420-A front

The Allison V-3420 was much more than two V-1710 engines coupled together. However, as many V-1710 components were used as possible, resulting in only 340 new parts. This is a V-3420-A engine with an attached single-rotation gear reduction.

In 1935, Allison was in the middle of developing its 1,000 hp (746 kW) V-1710 engine. The AAC requested that the new 1,600 hp (1,193 kW) engine have a single crankshaft and use as many V-1710 components as possible to keep development time to a minimum. After evaluating a few different configurations, Allison decided to double the V-1710 to create a 24-cylinder engine in an X configuration. This engine became the X-3420.

The X-3420 would have an entirely new crankcase, crankshaft, gear reduction, supercharger, and accessory section, but it would keep the basic V-1710 cylinder and head. The X-3420 had a flattened X arrangement with a left and right cylinder bank angle of 60 degrees, an upper cylinder bank angle of 90 degrees, and a lower cylinder bank angle of 150 degrees. The fuel-injected engine would produce 1,600 hp (1,193 kW) at 2,400 rpm for takeoff and 1,000 hp (746 kW) at 1,800 rpm for economical cruise. The engine would have an 8.5 to 1 compression ratio and weigh 2,160 lb (980 kg).

While using as many V-1710 components as possible made Allison’s job easier, the X-3420’s single crankshaft and its master and articulating rods required much design work, as did its fuel-injection system. Very quickly, Allison realized it did not have the resources to develop the X-3420 and needed to focus on the V-1710, which was encountering technical issues. Development of the X-3420 was effectively abandoned in 1936. As an alternative, Ron Hazen, Allison’s Chief Engineer, proposed a new 2,000 hp (1,491 kW) engine that had two crankshafts and was more closely based on the V-1710. The engine would produce more power than the X-3420 and be developed in less time. The AAC approved of Hazen’s proposed engine, which became the V-3420. The engine was often referred to as a W-24 or double Vee (DV) and was occasionally called the DV-3420.

Allison V-3420-A rear

Rear view of the V-3420-A shows the supercharger mounted behind the right engine section and various accessories mounted behind the left engine section. The V-3420’s design enabled the engine to produce more power than its X-3420 progenitor.

The Allison V-3420 design was more complex than just coupling two V-1710 engines together. As with the proposed X-3420, a new crankcase, gear reduction, supercharger, and accessory section were at the center of the engine, but the V-3420 would utilize many V-1710 components. The use of two V-1710 crankshafts along with their connecting rods made the V-3420’s design and development much more manageable for Allison. The engine consisted of two 60 degree V-12 engine sections mounted on a common crankcase and separated by 90 degrees, which gave the inner cylinder banks 30 degrees of separation.

As V-1710 development progressed, Allison was able to offer the V-3420 with 2,300 hp (1,715 kW) for takeoff. At 2,300 lb (1,043 kg), the engine would only weigh 140 lb (64 kg) more than the single crankshaft X-3420, but it would produce an additional 700 hp (522 kW). In May 1937, the AAC contracted Allison to build the V-3420 engine prototype.

A large aluminum crankcase sat at the center of the 24-cylinder V-3420 engine. Attached to the crankcase were four cylinder banks. Each cylinder bank consisted of six steel cylinder barrels shrink fitted to a one-piece aluminum cylinder head. Each cylinder barrel was surrounded by an aluminum water jacket. A single overhead camshaft actuated two intake and two exhaust valves for each cylinder. Each cylinder had a 5.5 in (140 mm) bore and a 6.0 in (152 mm) stroke. The engine displaced 3,421 cu in (56.1 L) and had a compression ratio of 6.65 to 1. At the rear of the engine was a supercharger driven by the right crankshaft, and all accessories were driven by the left crankshaft. The engine was also intended to be used with a General Electric turbosupercharger.

Allison V-3420-B NMUSAF rear

This V-3420-B was the type installed in the Fisher XP-75. About 15 ft (4.6 m) of shafting separated the engine from the gear reduction. Note the much larger supercharger compared to the image of the V-3420-A engine. The V-3420-B used a two-stage supercharger and no turbosupercharger. (Gary Brossett image via the Aircraft Engine Historical Society)

There were only 340 parts unique to the V-3420 engine, and those accounted for 930 pieces of the 11,630 that made up the engine. Initially, the V-3420 had a takeoff rating of 2,300 hp (1,715 kW) at 3,000 rpm, a maximum rating of 2,000 hp (1,491 kW) at 2,600 rpm, and a cruise rating of 1,500 hp (1,119 kW) at 2,280 rpm. The basic 24-cylinder engine was 97.7 in (2.48 m) long, 60.0 in (1.52 m) wide, and 38.7 in (.98 m) tall. The engine weighed 2,665 lb (1,209 kg)—365 lb (166 kg) more than the original estimate.

In January 1938, Allison was authorized to release V-3420 engine specifications to aircraft manufacturers and airlines. This resulted in a number of aircraft designs incorporating the engine; however, only four V-3420-powered aircraft types were actually flown. The V-3420 engine was first run in April 1938, followed by an AAC order for six engines in June 1938. An engine was also displayed in the 1939 World’s Fair in New York.

The US Navy was aware of the V-3420 engine and asked Allison if it could be converted for marine use. Allison responded with the appropriate designs. In December 1939, the Navy ordered two V-3420 marine engines for installation in a new, aluminum-hulled Patrol Torpedo boat designated PT-8. The two V-3420 marine engines were delivered to the Navy, and the PT-8 boat started trials in November 1940. The PT-8 was tested through 1941, but no further boats or V-3420 marine engines were ordered. The sole PT-8 was later re-engined and still exists as of 2017.

Allison V-3420-B NMUSAF

On the V-3420-B engine, an idler gear kept the crankshafts in sync. The engine’s large crankcase can be seen in this image. The large aluminum casting had front and rear covers and a magnesium oil pan. (Gary Brossett image via the Aircraft Engine Historical Society)

For aircraft use, the V-3420 required further development, which was slow due to Allison’s ongoing commitments to the V-1710 engine as well as the AAC’s preoccupation with vastly expanding its resources for the coming war. In late 1940, Allison focused on two major models of the V-3420 engine: -A and -B. The V-3420-A had crankshafts that rotated the same direction—either clockwise or counterclockwise, depending on the desired rotation of the propeller. The -A engine used a single-rotation propeller with either an attached or remote gear reduction, but most commonly with an attached gear reduction. The V-3420-B had crankshafts that rotated in opposite directions and was used with contra-rotating propellers. Different versions of the -B engine could accommodate either an attached or remote gear reduction, which allowed a number of propeller shaft configurations, including right-angle drives. The -B engine almost always had a remote gear reduction. The two crankshafts of the V-3420-B were kept in sync by idler gears at the front of the engine. The idler gears also balanced power loads from the crankshafts to the contra-rotating propeller shafts.

In September 1940, Allison’s V-1710 commitments became overwhelming, and development of the V-3420 engine was put on hold. As a result, the XB-19 had four 2,000 hp (1,491 kW) Wright R-3350 18-cylinder radial engines installed in place of the V-3420s. However, the R-3350 was encountering its own extensive developmental issues that put its use in the Boeing B-29 Superfortress in question. In February 1941, the AAC requested that Allison restart development of the V-3420-A with an output of 3,000 hp (2,237 kW) as a possible replacement for the Wright R-3350. The B-29 bomber was too important for its fate to be tied to one engine.

Allison V-3420-B right-angle drive

One V-3420-B engine was built to be mounted in an aircraft’s fuselage with extension shafts leading through the wings to right angle drives that would connect to the propellers. This type of engine configuration would have been used in the McDonnell Model 1. Only one engine was built with this configuration.

A V-3420 engine was delivered to Wright Field in October 1941, but with the bombing of Pearl Harbor in December, the V-3420 program was again put on hold so that Allison could focus on the V-1710 engine. History repeated itself in mid-1942 when the suitability of the R-3350 engine was again in question. Allison was instructed by the Army Air Force (AAF—the AAC was renamed in June 1941) to prepare the V-3420 for installation in a B-29, which was redesignated XB-39. Nine engines were built and delivered by October 1942. On 1 October 1942, the AAF ordered two Fisher XP-75 Eagle fighter prototypes that were powered by the V-3420-B engine. This was followed by an order placed on 28 October for 500 V-3420-A engines for installation in 100 production B-39 aircraft.

As the aircraft projects were underway, continued development of the V-3420 engine increased its output to a takeoff rating of 2,600 hp (1,939 kW) at 3,000 rpm with 8 psi (.55 bar) of boost, a normal rating of 2,100 hp (1,566 kW) at 2,600 rpm at 25,000 ft (7,620 m), and a cruise rating of 1,575 hp (1,175 kW) at 2,300 rpm at 25,000 ft (7,620 m). However, the engine could be overboosted in emergency situations to 3,000 hp (2,237 kW) at 3,000 rpm with 10.2 psi of boost (.70 bar).

Fisher P-75A Eagle

The Fisher P-75A was the end of a very tumultuous fighter program. The original design consisted of various parts from other aircraft that, when combined, would somehow make an aircraft superior to all others. The reality was that the combined parts created an aircraft that was downright dangerous and needed to be redesigned. A partial redesign did not completely cure the problems, and problems still existed after a subsequent complete redesigned. Still, 2,500 aircraft were ordered before better judgment prevailed and the program was cancelled. The P-75 was the only aircraft flown with V-3420-B engines.

The first aircraft to fly with the V-3420 was the Fisher XP-75. Developed by the Fisher Body Division of General Motors, the XP-75 was a long-range escort fighter. Through 1943, the AAF felt a desperate need for such an aircraft and ordered six additional XP-75 prototypes, bringing the total to eight. In addition, the AAF expressed its intent to purchase 2,500 P-75s if the prototypes met their performance estimates. The V-3420-B engine for the P-75 had a two-stage, variable speed supercharger (and no turbosupercharger) that was hydraulically coupled to the right crankshaft. The engine alone weighed 2,750 lb (1,247 kg), and its weight increased to 3,275 lb (1,486 kg) with its 3.5 in (89 mm) diameter extension shafts and remote gear reduction.

The XP-75 first flew on 17 November 1943, and the aircraft almost immediately ran into issues. Its V-3420-B engine was not entirely trouble free either; unequal fuel distribution was a continuing problem for the V-3420. The issue was mostly solved by having each alternate engine section fire every 30 degrees of rotation, rather than both engine sections firing every 60 degrees of rotation. The aircraft was redesigned to correct its deficiencies and was given the new designation of P-75A. The AAF ordered 2,500 P-75As on 7 June 1944, and production started immediately. However, the entire P-75 program was cancelled four months later, in October 1944. The P-75A did not live up to expectations, it was outmatched by aircraft already in service, and the end of the war was in sight. Eight XP-75 and six P-75A aircraft were built, but three of the aircraft crashed during testing. One P-75A was preserved and is on display in the National Museum of the US Air Force. The rest of the surviving aircraft were scrapped.

Douglas XB-19A

With V-3420-A engines installed, the Douglass XB-19A realized a boost in its performance. While the engines proved reliable, it was very time-consuming for Fisher to design and fabricate the new nacelles to house the V-3420. The same basic nacelle was also used on the XB-39.

Actual work to install V-3420-A engines in the XB-19 started in November 1942 at Fisher. The aircraft was redesignated XB-19A and flew for the first time with its V-3420 engines in January 1944. The V-3420 installation served as a test for the engine’s use in the XB-39. With the exception of range, the XB-19A’s performance increased across the board: maximum speed increased by 40 mph (64 km/h); cruising speed increased by 50 mph (80 km/h); service ceiling increased by 16,000 ft (4,877 m), but normal range decreased by 1,000 miles (1,609 km). The XB-19A was strictly an experimental aircraft and was never intended to enter production.

In February 1943, V-3420-A engines were selected to power the Lockheed XP-58 Chain Lightning. The V-3420 was not Lockheed’s first choice, or second, or third. The XP-58 heavy fighter program was initiated in 1940 but was beset with constant design and role changes, which were made worse by developmental issues of the aircraft’s previously selected engines. By the time it was completed, the XP-58 was oversized, overweight, underpowered, and not needed. First flown on 6 June 1944, the aircraft’s lackluster performance matched Lockheed and the AAF’s enthusiasm for the project. Only one prototype was built, and the XP-58 program was cancelled in May 1945.

Allison V-3420 XB-19A nacelle

The men working on the V-3420 installed in the XB-19A give some perspective as to the engine’s size and the size of the aircraft. The V-3420’s radiator, oil cooler, turbosupercharger, and intercooler were all mounted in the nacelle, under the engine. This configuration prevented the need for heavily modifying the aircraft.

Even though it helped spur the V-3420 engine program, the V-3420-powered B-29 was the last aircraft to take flight with the engine. A B-29 (actually a YB-29, the first pre-production aircraft) was delivered to Fisher for conversion to an XB-39 with V-3420-A engines. Work on the XB-39 was slow because Fisher’s main focus was the XP-75. The XB-39 finally flew on 9 December 1944. Performance of the XB-39 was superior to that of the B-29: its top speed was 50 mph (80 km/h) faster, and it had a 3,000 ft (914 m) higher service ceiling. However, standard B-29s were proving to be more than adequate, and it was not worth the time or trouble to convert any other airframes to V-3420-power.

To meet the power needs for extremely large aircraft designs during World War II, Allison proposed the DV-6840. The DV-6840 consisted of two V-3420s driving a common remote gearbox for contra-rotating propellers. A gearbox for the DV-6840 was completed in 1946, but no information has been found regarding it being tested. Allison had also planned a further development of the V-3420. This fuel-injected V-3420-C engine had a forecasted emergency output of 4,800 hp (3,579 kW) and a takeoff/military rating of 4,000 hp (2,983 kW)—both ratings at 3,200 rpm with water injection. However, the V-3420-C was never built.

Lockheed XP-58 Chain Lightning

The Lockheed XP-58 was another program than inexplicably pressed on despite the many signs that it was heading nowhere. Somewhere between three to seven engines were selected before the V-3420-A was finally chosen to power the aircraft. It was not Lockheed’s fault; they had no control over which experimental engines would actually be produced. Lockheed also had no control over the constantly changing roles the AAF asked the XP-58 to fulfill.

The Allison V-3420 was not a trouble-free engine, but it did work well in its few applications once initial issues were resolved. The engine held a lot of potential, but that potential faded as its development languished. At the start of 1944, only 33 V-3420 engines had been delivered, and two of those were marine engines. Had the AAC committed to the engine in 1936 and provided Allison with the resources needed to develop the engine, the V-3420 very well could have powered the B-29 and various post-war aircraft. The four aircraft projects that used the V-3420 did not fail because of the engine. By the time the V-3420 program was in order in 1944, other engines were adequately fulfilling the 3,000 hp (2,237 kW) role.

Allison built a total of 157 V-3420 engines: 37 -A engines (including the two marine engines) and 120 -B engines. A number of V-3420s were sold as surplus after the war. Some eventually made their way into museums, while other engines were used in a hydroplane (Henry J. Kaiser’s Scooter Too driven by Jack Regas) and a tractor puller (E. J. Potter’s Double Ugly). However, none of the V-3420 engines took flight again.

Fisher XB-39

The Boeing / Fisher XB-39 program is what put the V-3420 engine back on track to production. It was the most promising aircraft out of the four powered by the V-3420. Delayed by Fisher’s work on the XP-75, there was little point to the aircraft when it took to the air in December 1944. The image above shows the V-3420 engines being installed at the Fisher plant in Cleveland, Ohio. Fisher was producing various subassemblies for the B-29, which can be seen in the background. On the right side of the image, just behind the XB-39’s wing, is the fuselage of a P-75A.

Vees For Victory!: The Story of the Allison V-1710 Aircraft Engine 1929-1948 by Dan Whitney (1998)
The Allison Engine Catalog 1915-2007 by John M. Leonard (2008)
Jim Allison’s Machine Shop: The First 30 Years by John M. Leonard (2016)
Aircraft Engines of the World 1946 by Paul H. Wilkinson (1946)
Allied Aircraft Piston Engines of World War II by Graham White (1995)
US Army Air Force Fighters Part 2 by William Green and Gordon Swanborough (1978)
McDonnell Douglas Aircraft since 1920: Volume I by Rene J. Francillon (1988)
Lockheed Aircraft since 1913 by Rene J. Francillon (1982/1987)
Boeing Aircraft since 1916 by Peter M. Bowers (1966/1989)

Fokker Dekker CI front

Dekker-Fokker C.I Rotary Propellers

By William Pearce

In the 1920s, Adriaan Jan Dekker helped redesign windmill sails in the Netherlands to improve their efficiency. His modified sails were streamlined and acted more as airfoils than the traditional sails in use. Dekker’s first sail was tested briefly in 1927, with more expansive tests in 1928. By 1930, 31 windmills were using Dekker’s sails, and the number increased to 75 by 1935.

Dekker patent rotary propellers

Drawings from Adriaan Dekker’s rotary propellers patent (US 2,186,064). The direction of rotation was actually opposite of the unit that was built and installed on a Fokker C.I. Note the airfoil sections of the blades.

In the 1930s, Dekker began to focus on improving aircraft propellers. In 1934, Dekker filed for a patent on a new type of turbine rotor blade for aircraft use. British patent 450,990 was awarded on 27 July 1936, and it outlined the use of a single rotation, four-blade rotary propeller. However, Dekker found that a single set of rotors caused a divergent airflow that virtually bypassed an aircraft’s tail. This caused control issues because it decreased airflow over the aircraft’s rudder and elevator.

Dekker continued to evolve his design and applied for another patent in June 1936, before the first patent was awarded. The new British patent (476,226) was awarded on 3 December 1937 and outlined the use of contra-rotating rotors. Strangely, the gearing for the propellers was not included in the British patent but was included in the US (and French) patent filed on 19 May 1937 and granted patent 2,186,064 on 9 January 1940.

Dekker propeller construction

Construction images of the Dekker rotary propeller. The images are mainly the hub and blades of the front set of rotors. ( image)

Almost all of the information contained in the British patent was also in the US patent. However, the US patent was more detailed and included additional information. The patents illustrate a large, streamlined hub from which two sets of four-blade rotors protrude. The original patent stated that the ideal blade length was one third of the hub diameter. The fixed-pitch blades were highly curved airfoils of a complex shape. The angle of the blade decreased from 40 degrees at the root to 5 degrees at the tip. In addition, the blade’s cord (length from leading edge to trailing edge) steadily increased from its root to its tip.

The two sets of blades were contra-rotating. The rear set of blades served to straighten the airflow from the front set, providing additional thrust and increasing efficiency. The contra-rotation of the blades also helped eliminate torque reactions. Through a gear reduction, the rear set of blades only turned at two-thirds the speed of the front set of blades. Dekker also noted that the rotary blades would be quieter than conventional propellers.

Fokker Dekker CI front

Dekker’s finished C.I with its large rotary propellers. Note the complex airfoil shape of the blades.

The drive for the rotors consisted of a sun gear mounted on the engine’s crankshaft that turned planetary gears against a fixed, internally-toothed ring gear. The planetary gears were mounted in a carrier from which a shaft extended to power the front set of blades. These blades rotated in the same direction as the engine and at an unspecified reduction. Attached to the shaft powering the front set of blades was another sun gear. This sun gear turned three idler gears that turned three planetary gears against another fixed, internally-toothed ring gear. This gear train reduced the rotation speed by 66% from the sun gear (and front set of blades). A hollow shaft extended from the planetary gear carrier to power the rear set of blades. Inside the hollow shaft was the propeller shaft for the front set of blades. The rear set of blades rotated the opposite direction of the engine.

To turn theory to reality, Dekker formed a company, Syndicaat Dekker Octrooien (Dekker Patents Syndicate), and acquired a Fokker C.I trainer aircraft around 28 March 1936. The C.I was a late World War I era biplane reconnaissance aircraft powered by a 185 hp BMW IIIa engine. As the aircraft’s design aged, transport and trainer versions were built. Dekker’s C.I was registered PH-APL on 15 April 1937.

Fokker Dekker CI taxi

Registered PH-APL, Dekker’s heavily modified Fokker C.I bears little resemblance to a standard C.I; the wings and tail are about all the aircraft have in common. Note how the fuselage shape tapers the diameter of the large propeller hub back to the tail. With its contra-rotating rotary propellers spinning, the aircraft is shown before taxi tests at Ypenburg airfield.

To accommodate the rotary propellers, Dekker’s aircraft was so heavily modified that it was nearly unrecognizable as a C.I. The aircraft retained the BMW engine but had the contra-rotating rotary propellers mounted to its front. The fuselage of the aircraft was modified and tapered from the very large propeller hub back to the tail. The fuselage was metal-covered immediately behind the propellers, but the rest of the fuselage was covered with fabric.

The rotary propellers differed from those illustrated in the patents in that six blades made up the front set of rotors, and seven blades made up the rear set. Construction of the individual blades was similar to that of a wing. The blades were made of a shaped aluminum sleeve fitted around three spars. The spars passed into and were connected to the hub. The roots of the blades were also attached to the hub. The hub was formed of an aluminum frame and covered with aluminum sheeting. Video indicates that the rear set of blades had roughly a 66% speed reduction compared to the front set—which matches what was stated in the patent.

Fokker Dekker CI captured Germans

Two views of Dekker’s C.I after it was captured by German forces. The right image clearly shows six blades on the front rotor and seven blades on the rear rotor.

The aircraft’s completion date is unknown, but Dekker’s C.I underwent taxi tests at Ypenburg airfield, near The Hauge, Netherlands. The aircraft reportedly made a few hops into the air, but no true flight was achieved. It is not clear if there was an issue with the rotary propellers (such as insufficient thrust or excessive vibrations) or if the project simply ran out of time. Dekker’s C.I was moved to Waalhaven Airport, where it was captured by German forces on 18 May 1940, eight days after the Germans started their invasion of the Netherlands at the start of World War II. Reportedly, the aircraft was taken to Johannisthal airfield near Berlin, Germany for testing. Some sources state the aircraft crashed on its first test flight and that its remains were later destroyed as Russian troops advanced late in the war. However, exactly what happened to Dekker’s C.I and its rotary propellers is not known.

Below is video uploaded to YouTube of the Fokker Dekker C.I undergoing taxi tests. Note the stroboscopic effect of the rotors turning at different speeds. Adriaan Dekker is shown at the end of the video. It is interesting to contemplate how much weight the rotary propellers added to the nose of the aircraft. Unfortunately, the date of the tests is not known.

“Screw Propeller, Turbine Rotor, and Like Device” US patent 2,068,792 by Adriaan Jan Dekker (granted 26 January 1937)
“Rotary Propeller and the Like Device” US patent 2,186,064 by Adriaan Jan Dekker (granted 9 January 1940)
Power from Wind: A History of Windmill Technology by Richard L. Hills (1996)

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

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

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