Category Archives: World War II


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


Kawasaki Ki-64 Experimental Fighter

By William Pearce

In the late 1930s, designers at Arsenal de l’Aéronautique in France began working on a new fighter powered by two engines installed in tandem. One engine was positioned in front of the cockpit, and the other engine was behind the cockpit. Each engine drove half of a contra-rotating propeller. This design eventually evolved into the Arsenal VB 10. Takeo Doi was a Japanese designer at Kawasaki and was aware of Arsenal’s tandem-engine design.


The Kawasaki Ki-64 fighter undergoing gear retraction tests in a hangar in Gifu. Note the exhaust stacks for the front engine and the dorsal air intake scoop for the rear engine.

Doi was also aware of the evaporative cooling system used on the German Heinkel He 100. Japan had sent a delegation to Germany in December 1938 that successfully negotiated the purchase of three He 100 and two He 119 aircraft. The He 100s were delivered to Japan in the summer of 1940.

In 1939, Doi began to contemplate a high-speed fighter for the Imperial Japanese Army Air Force that used tandem engines and evaporative cooling. At the time, the Japanese aircraft industry was more focused on conventional aircraft, and Kawasaki and Doi were busy with designing the Ki-60 and Ki-61 Hien (Tony) fighters. In October 1940, Kawasaki and Doi received support for the tandem-engine fighter project, which was then designated Ki-64 (Rob). The aircraft’s design was refined, and a single Ki-64 prototype was ordered on 23 January 1941.

The Kawasaki Ki-64 looked very much like a continuation of the Ki-61 design, and while some of its features were inspired by other aircraft, the Ki-64 was an entirely independent design. The single-seat aircraft had a taildragger configuration and was of all-metal construction. Although designed as a fighter, the Ki-64 was primarily a research aircraft intended to test its unique engine installation and evaporative cooling system. Proposed armament included one 20 mm cannon installed in each wing and two 12.7 mm machine guns or 20 mm cannons installed in the upper fuselage in front of the cockpit. The armament was never fitted to the prototype.


The Ki-64 appears to be preparing for an early test flight. The front engine’s intake scoop can be seen just above the exhaust stacks. Note the exhaust stains from the front engine and that the lightning bolt has not yet been painted on the fuselage.

The Ki-64 was powered by a Kawasaki Ha-201 engine that was comprised of two Kawasaki Ha-40 inverted V-12 engines coupled to a contra-rotating propeller. The Ha-40 was a licensed-built Daimler-Benz 601A engine and had a 5.91 in (150 mm) bore, a 6.30 in (160 mm) stroke, and a displacement of 2,070 cu in (33.9 L). As installed in the Ki-64, the shaft for the rear engine extended under the pilot’s seat and through the Vee of the front engine to the propeller gearbox. The rear engine drove the front adjustable-pitch propeller of the contra-rotating unit. The front engine drove the rear fixed-pitch propeller. Each set of propellers had three blades that were 9 ft 10 in (3.0 m) in diameter. The Ha-201 displaced a total of 4,141 cu in (67.9 L) and produced 2,350 hp (1,752 kW) at 2,500 rpm for takeoff and 2,200 hp (1,641 kW) at 2,400 rpm at 12,795 ft (3,900 m). Each engine section could operate independently of the other.

The engine sections had separate evaporative cooling systems. Heated water from the engine at 45 psia (3.1 bar) was pumped to a steam separator, where the water pressure dropped to 25 psia (1.7 bar), and about 2% of the water flashed to steam. The steam was then ducted at 16 psia (1.1 bar) through panels in the wings, where it was cooled and condensed back into water. The water then flowed back into the engine. The evaporative cooling system eliminated the drag of a radiator, and this enabled the aircraft to achieve higher speeds. It was believed that battle damage would not be much of a problem for the cooling system. The low pressure of the steam combined with steam’s low density meant that the amount of coolant lost through a puncture would be minimal, and the separate engines and cooling systems helped minimize the risk of a forced landing if damage did render one system ineffective.

The evaporative cooling system for the front engine was housed in the left wing, and the rear engine’s system was housed in the right wing. Each system consisted of two steam separators, an 18.5-gallon (70 L) tank in the wing’s leading edge near the fuselage, four upper and four lower wing condenser panels, an upper and lower condenser section in the outer flap, and a water tank in the fuselage. Sources disagree regarding the size of each fuselage tank, but combined, the tanks held around 52.8 gallons (200 L). Suspended below the right wing was a scoop that held oil coolers for the engines.


Another image of the Ki-64 doing a ground run. Note the aircraft’s resemblance to a Ki-61 Hein. Exhaust for the rear engine was collected in a manifold that exited the fuselage just above where the trailing edge of the wing joined the fuselage. That exhaust exit can just barely be discerned in this image.

The Ki-64 had a 44 ft 3 in (13.50 m) wingspan and was 26 ft 2 in (11.03 m) long. The aircraft had a top speed of 435 mph (700 km/h) at 13,123 ft (4,000 m) and 429 mph (690 km/h) at 16,404 ft (5,000 m). The Ki-64 could climb to 16,404 ft (5,000 m) in 5.5 minutes and had a service ceiling of 39,370 ft (12,000 m). Since the wings housed the cooling system, little room was left for fuel tanks. Each wing had a 22-gallon (85 L) fuel tank, and an 82-gallon (310 L) tank was housed in the fuselage; this gave the Ki-64 a 621 mile (1,000 km) range. The aircraft weighed 8,929 lb (4,050 kg) empty and 11,244 lb (5,100 kg) loaded.

While the Ki-64 was being built, a Ki-61 was modified to test the evaporative cooling system. With its radiator removed and evaporative panels added to its wings, the modified Ki-61 first flew in October 1942. Around 35 flights were made before the end of 1943, and they served to develop and refine the cooling system. The aircraft proved the validity of the evaporative cooling system and achieved a speed 25 mph (40 km/h) in excess of a standard Ki-61. However, the evaporative cooling system did require much more maintenance than a conventional system.

The Ki-64 was completed at Kawasaki’s plant at Gifu Air Field in November 1943. The aircraft underwent ground tests that revealed a number of issues. By December, the issues were resolved enough for flight testing to commence. The aircraft made four successful flights, but the rear engine caught fire on the fifth flight. The pilot was able to make an emergency landing at Kakamigahara, but the rear engine and parts of the rear fuselage and cooling system had been damaged. The Ha-201 engine was sent to Kawasaki’s engine plant in Akashi for overhaul, and the Ki-64 airframe was sent back to Gifu for repairs.


A poor image, but perhaps the only one, showing the Ki-64 in flight. The lightning bolt has been painted on the fuselage.

The short flying career of the Ki-64 had shown that its cooling system was insufficient. The system worked well for level flight, but it was inadequate for ground running, takeoff, and climb. When the system was overloaded, steam was not condensed back to water and was subsequently vented overboard via a 16 psi (1.1 bar) relief valve. The cooling system lost about 12 gallons (45 L) of water during a rapid climb from takeoff to 18,000 ft (5,500 m). Water freezing within the system, either while in flight or on the ground during cold temperatures, was another concern. Adding an alcohol mixture to the water coolant was a possible solution, but the Ki-64 never underwent any cold weather testing.

While undergoing repairs, the Ki-64 was to be modified and redesignated Ki-64 Kai. The existing propellers would be replaced with fully adjustable and feathering contra-rotating propellers, which would make it easier for one engine to be shut down in flight. The engines were to be replaced with more powerful Ha-140s that provided an output of 2,800 hp (2,088 kW) for the coupled unit. With the changes, it was estimated that the Ki-64 Kai would have a top speed of 497 mph (800 km/h). However, the propeller and engines were delayed with more pressing war-time work, and the Ki-64 program was cancelled in mid-1944.

The Ki-64 airframe remained at Gifu where it was captured by American forces in 1945. Various parts of the cooling system were removed from the aircraft and shipped to Wright Field in Dayton, Ohio for further analysis and testing. The remainder of the Ki-64 was eventually scrapped.


The K-64 as discovered by American forces at the end of World War II. The engines had been removed, and the aircraft was in a rather poor state. Note the canopy frame sitting on the wing.

Japanese Army Fighters Part 1 by William Green and Gordon Swanborough (1977)
Japanese KI-64 Single Fighter with Two Engines in Tandem and Vapor-Phase Cooling, Air Technical Intelligence Review Report No. F-IR-100-RE by Petaja and Gilmore (31 July 1946)
Japanese Secret Projects by Edwin M. Dyer III (2009)
Japanese Aircraft of the Pacific War by René J. Francillon (1979/2000)
Encyclopedia of Japanese Aircraft 1900–1945 Vol. 4: Kawasaki by Tadashi Nozawa (1966)
The Xplanes of Imperial Japanese Army & Navy 1924–45 by Shigeru Nohara (1999)
Heinkel He 100 by Erwin Hood (2007)


FIAT CR.42 DB Fighter

By William Pearce

In late 1930s, FIAT developed the CR.42 Falco (Falcon), one of the last biplane fighter aircraft. The CR.42 was powered by an 840 hp (626 kW) FIAT A 74 RC38 radial engine. With good performance and excellent maneuverability, the CR.42 was one of the best biplane fighters ever built. However, frontline fighters had adopted new tactics in which speed controlled the fight, so the maneuverability of the biplane was traded for the speed of a monoplane. Looking to maximize a combination of speed and maneuverability, the Italian Air Ministry asked FIAT to re-engine the CR.42 with a 1,000 hp (746 kW) Daimler-Benz DB 601A engine. The resulting aircraft was designated CR.42 DB.


The FIAT CR.42 DB undergoing an engine run. Its Daimler-Benz DB 601 engine made the aircraft the fastest biplane ever built. However, its performance could not match contemporary monoplane fighters.

Some sources incorrectly list the DB 601-powered aircraft as the CR.42 B, which was a trainer built from a standard CR.42 by moving the engine forward, elongating the fuselage, and adding a second cockpit. Additionally, some sources claim the CR.42 DB’s engine was an Alfa Romeo RA 1000 RC41, which was a DB 601A built under license in Italy. However, the Alfa Romeo RA 1000 engine had not proceeded beyond initial testing by late 1941, after the CR.42 DB had already flown. It is unlikely that an untried RA 1000 test engine was installed in the CR.42 DB.

The FIAT CR.42 DB project was underway by early 1941. The aircraft was assigned serial number MM 469. In the span of a few weeks, a standard CR.42 was re-engined with the DB 601 power plant. Switching from a large, air-cooled, 14-cylnder radial engine to a long, liquid-cooled, V-12 engine necessitated many changes to the aircraft.

Like all CR.42s, the CR.42 DB consisted of a welded steel tube and alloy airframe. The fuselage was skinned in aluminum with the exception of the rear fuselage’s sides and bottom, which were covered with fabric. The wings and tail had a duralumin frame. The wings’ leading and trailing edges were aluminum, and fabric covered the rest of the surface. The horizontal and vertical stabilizers were aluminum-skinned. All control surfaces were had a duralumin frame and were covered in fabric.

FIAT CR42 DB right

The CR.42 DB with its lower wing removed. The removed bottom panel exposes some of the aircraft’s structure.

The entire front of the CR.42 DB was redesigned to accommodate the DB 601A engine and its radiator. The DB 601A was encased in a close-fitting, streamlined cowling. Positioned on the left side of the cowling was the engine’s air intake. Faired into the cowling’s upper deck were the blast tubes for the aircraft’s two 12.7 mm guns—each had 400 rounds of ammunition. A housing for the radiator was located under the engine. Scoops for oil coolers were placed in the wing roots of the lower wing (in the same location as a standard CR.42).

The CR.42 DB had the same 31.8 ft (9.70 m) upper and 21.3 ft (6.50m) lower wingspans as the standard CR.42, but those were the only specifications the two aircraft shared. The CR.42 DB was 1.8 ft (.54 m) longer at 28.9 ft (8.80 m). The aircraft was 507 lb (230 kg) heavier at an empty weight of 4,299 lb (1,950 kg). The CR.42 DB’s performance improved substantially over the standard CR.42. The CR.42 DB had a top speed of 323 mph (520 km/h) at 17,388 ft (5,300 m) and could climb to 16,404 ft (5,000 m) in 5:40. The aircraft had a ceiling of 34,777 ft (10,600 m) and a range of 715 mi (1,150 km). The standard CR.42 was 56 mph (90 km/h) slower, took an additional 1:40 to reach 16,404 ft (5,000 m), and had a 1,312 ft (400 m) lower ceiling.

FIAT CR42 DB color

This image shows the wing root scoop for the oil cooler and the induction scoop for the DB 601 engine. The CR.42 DB is shown at Caselle airfield in May 1941.

The CR.42 DB’s first flight was in March 1941, piloted by Commander Valentino Cus. The aircraft was delivered to the Centro Sperimentale (Experimental Center) at Guidonia Airfield (near Rome) for military tests in the summer of 1941. The CR.42 DB proved to be an exceptional aircraft; it was (and still is) the world’s fastest biplane. While not much slower than monoplane fighters then in service, the CR.42 DB’s speed could not be improved, whereas the speed of monoplane fighters would continue to increase as advancements were made.

Although an order for 150 aircraft was placed on 10 April 1941, series production was never started. The short supply of DB 601 engines available to Italy and the engine’s priority use in the more advanced Macchi MC.202 Folgore (Lightening) and Re.2001 Falco II (Falcon II) monoplane fighters left no DB 601s available for the CR.42 DB. Only one CR.42 DB was built. Some consideration was given to lengthening the CR.42 DB to 30.8 ft (9.38 m) and modifying it into a two-place training or reconnaissance aircraft. However, this project never proceeded beyond the initial design phase. Although the FIAT CR.42 DB was the pinnacle of biplane fighter performance, it was outclassed by frontline monoplane fighters as the era of biplane fighters came to an end.


The two-place, DB 601-powered CR.42. Some sources refer to the aircraft as the CR.42 R. However, the drawing appears to be labeled “R.42 P”. The “CR” stood for Caccia (Fighter) Rosatelli. Rosatelli was the aircraft’s designer, Celestino Rosatelli. Since the two-place aircraft was not a fighter, it makes sense that the “Caccia” designation would not be used.

The FIAT Fighters 1930–1945 by Piero Vergnano (1969)
Italian Civil and Military Aircraft 1930–1945 by Jonathan W. Thompson (1963)
Aeronuatica Militare Museo Storico Catalogo Motori by Oscar Marchi (1980)
Tutti gli aerie del Re by Max Vinerba (2011)
“Fantasmi di aerie e motori Fiat dal 1935 al 1945 (prime parte)” by Giovanni Masino Ali Antiche 106 (2011)
Fiat CR.42 Falco by Przemyslaw Skulski (2007)

Martin-Baker MB3 runup

Martin-Baker MB3 Fighter

By William Pearce

By 1939, it was clear that the British Air Ministry would not order the Martin-Baker MB2 into production. James Martin (main designer) and Captain Valentine H. Baker had already been at work designing a new fighter aircraft—the MB3. Since the MB2 had proved to be a well-designed fighter, the British Air Ministry ordered three prototypes of the MB3 fighter on 16 June 1939. The new aircraft would be built under Specification F.18/39, issued to Martin-Baker in May 1939. The minimum requirements of Specification F.18/39 were a speed of 400 mph (644 km/h) at 15,000 ft (4,572 m), a ceiling of 35,000 ft (10,668 m), an endurance of 2.5 hours, and an armament of four 20 mm cannons. With the contract issued, Martin worked to finalize the MB3’s design.

Martin-Baker MB3 Denham guns

The nearly complete Martin-Baker MB3 in the summer of 1942 at Martin-Baker’s factory in Denham. The aircraft is not painted, and its six 20 mm cannons are installed. The cannons were removed before flight testing.

The timetable for completing the aircraft was rather optimistic for the relatively small Martin-Baker company. The original contract stated the first MB3 prototype was to be ready by 15 December 1939, with the two remaining aircraft completed by 15 February 1940. At this early stage, the aircraft was to be powered by a Rolls-Royce Griffon engine. By September 1939, it was apparent that the Griffon engine would not be available to Martin-Baker for some time. At the insistence of the Air Ministry, the Napier Sabre replaced the Griffon, and the entire aircraft was redesigned for the new engine. This resulted in a new contract that was somewhat delayed but ultimately signed on 11 August 1940. Britain was now fully involved in World War II, and Martin-Baker was inundated with other work of a higher priority. Therefore, completing the first MB3 took longer than anticipated. By the end of 1941, Martin-Baker was informed that there would be no production orders for the MB3, but the first prototype was so far along that it made sense to finish it.

Construction of the Martin-Baker MB3 followed the established company practice of using a tubular steel frame to make up the fuselage structure. The main wing spar was made of laminated steel, with the number of laminations decreasing near the wingtips. The rest of the wing structure formed a torsion box of extreme rigidity. The entire aircraft was covered with stressed aluminum skin, but many panels could be opened or removed for quick access to equipment and armament. The rudder was fabric-covered, but the rest of the control surfaces were skinned with aluminum.

Martin-Baker MB3 left

The MB3 during its brief flight testing career at RAF Wing. Note the retractable stirrup and fold-down door for cockpit entry.

The aircraft used pneumatically controlled split flaps and had spring loaded aileron gap seals to increase its roll rate and improve aerodynamics. The elevator also had gap seals. Fuel was carried in a fuselage tank in front of the cockpit. The aircraft’s fully retractable main landing gear had a wide track of 15 ft 5 in (4.7 m). The tailwheel retracted into an open well under the tail. The landing gear was lowered by gravity and raised by a pneumatic system, which was separate from the system that controlled the flaps.

Each wing housed three 20 mm cannons with 200 rpg, all installed outside of the aircraft’s main gear. The ammunition belts were installed parallel to the cannons; each bullet had to turn 90 degrees before being fed into the breach. This “flat-feed” ammunition system was patented by Martin. The cannon and ammunition arrangement made for a compact package that could be easily accessed and quickly serviced. With its six 20 mm cannons, the MB3 was one of the most heavily armed fighters of World War II.

Martin-Baker MB3 runup

This image of the MB3 running up gives a good view of the aircraft’s wide-track landing gear and the close-fitting cowling that covered the Sabre engine. Also visible are the under-wing scoops for the radiator and oil cooler.

The Rolls-Royce Vulture X-24 engine was also considered to power the MB3. The V-12 Griffon was initially selected because it was a far less complex power plant than the Vulture or Sabre. However, because the Sabre was more readily available than the Griffon and was favored by the Air Ministry, it was ultimately selected to power the MB3. The 2,020 hp (1,506 kW) Sabre II engine had 24 cylinders arranged in a horizontal H configuration and used sleeve valves. The engine drove a three-blade de Havilland propeller that was 14 ft (4.27 m) in diameter. Engine cooling was provided by a radiator installed in the right wing and an oil cooler installed in the left wing. The radiator ran from the wing root to the main gear, and the oil cooler was about half the size of the radiator. The scoops for the radiator and oil cooler extended about 5 in (127 mm) under the wings and were positioned between the gear wells and the flaps.

The MB3 had a 35 ft (10.7 m) wingspan and was 35 ft 4 in (10.8 m) long. The aircraft had a gross weight of 11,497 lb (5,215 kg). The MB3 had a top speed of 418 mph (673 km/h) at 20,000 ft (6,096 m). However, Martin claimed that Captain Baker had achieved 430 mph (632 km/h) at the same altitude, albeit without the drag that the six cannons would produce. At sea level, the aircraft was capable of 372 mph (599 km/h), and maximum cruising speed was 370 mph (595 km/h) at 15,000 ft (4,572 m). The MB3’s landing speed was 88 mph (142 km/h). The aircraft had a service ceiling of 35,000 ft (10,668 m) and a range of approximately 420 miles (676 km).

Martin-Baker MB3 rear

This rear view of the MB3 illustrates the aircraft’s fine fit and finish. The aileron and elevator gap seals can just be seen.

The first MB3 was given the serial number R2492. The aircraft was expected in March 1942 but was not completed until early August. The aircraft was trucked to Royal Air Force Station Wing (RAF Wing) in Buckinghamshire for flight testing. Surrounded by small fields and many trees, the small airbase of RAF Wing was not an ideal location for flight testing. Martin had objected to using RAF Wing, but the Air Ministry insisted.

Captain Baker was at the controls when the MB3 flew for the first time on 31 August 1942. The six wing cannons had been installed when the aircraft was built at Denham (near London) but were removed before the aircraft flew and were never reinstalled. Ballast had been added to simulate the weight of the cannons and their ammunition. Flight testing revealed that the aircraft had excellent maneuverability and handling characteristics. However, difficulty was experienced with the Sabre engine, and engine overheating issues troubled the MB3.

Martin-Baker MB3 right rear

Many sources claimed that the MB3 was fitted with a bubble canopy after its first flight. This belief stems from a doctored image of the MB3 with a bubble canopy meant to illustrate what the production version of the aircraft would look like. A bubble canopy was never installed on the MB3.

On 12 September 1942, the aircraft made its 10th flight. Captain Baker had just taken off when the engine seized, a result of a sleeve drive crank failure. Low to the ground and without any options, Captain Baker put the MB3 down in one of the many small fields lined with hedgerows and other obstacles surrounding RAF Wing. The aircraft clipped a pile of straw and crashed through a hedgerow at high speed. The MB3 cartwheeled, broke apart, and caught fire. Captain Baker was killed instantly.

The death of Captain Baker was a bitter blow for the Martin-Baker company. Martin took it especially hard; he had lost his friend in an aircraft powered by an engine he did not want to use and at a test site that he thought was inadequate. It was not long before Martin and the Martin-Baker company began work to improve aircrew safety and developed a series of ejection seats, which the company still manufactures today.

Martin-Baker MB3 with Captain V H Baker

Captain Valentine H. Baker posses with the MB3 shortly before a test flight. The engine seized on the MB3’s 10th flight, and Captain Baker was killed during the subsequent crash landing.

With the first MB3 prototype destroyed, Martin’s attention turned to the partially completed second prototype (R2496). Construction of the third prototype (R2499, or possibly R2500) was probably never started. Martin had already designed the MB3A, which was the production version of the MB3. The MB3A had a bubble canopy (that was never fitted to the prototype), and its cockpit was moved slightly forward to improve the pilot’s view over the wing. The MB4 had also been designed; it used a Bristol Centaurs engine in the same basic MB3 airframe. However, since the Air Ministry was finally willing to provide Martin-Baker with a Griffon engine and with the MB3’s performance now on par with existing aircraft, Martin sought to redesign the entire aircraft as the improved MB5 fighter. The Air Ministry was agreeable, and serial R2496 was reallocated to the MB5 aircraft in late 1943. The MB5 flew in 1944 and was another outstanding aircraft. However, the MB5 never went into production, and it was the last aircraft built by Martin-Baker.

“Martin-Baker Fighters,” by Bill Gunston, Wings of Fame Volume 9 (1997)
British Experimental Combat Aircraft of World War II by Tony Buttler (2012)
RAF Fighters Part 2 by William Green and Gordon Swanborough (1979)
The British Fighter since 1912 by Francis K. Mason (1992)
Interceptor Fighters of the Royal Air Force 1935–45 by Michael J. F. Bowyer (1984)

Republic XP-69 side

Republic XP-69 Fighter

By William Pearce

In February 1940, the United States Army Air Corps (AAC) issued Request for Data R40-C to various engine and aircraft manufacturers. R40-C encouraged aircraft manufacturers to propose unorthodox aircraft capable of at least 450 mph (724 km/h), but preferably 525 mph (845 km/h), and to meet other requirements outlined in Type Specification XC-622. R40-C also asked aircraft engine manufacturers to develop new power plants. Initially, a total of 26 aircraft designs were submitted by six selected aircraft companies and included a mix of eight different engines from four engine companies. Republic Aviation’s entry carried the company designation AP-12.

Republic AP-12 Rocket

The AP-12 Rocket was Republic’s entry into the R40-C fighter competition. Note the mid-fuselage-mounted Wright R-2160 Tornado engine.

Like almost all of the other R40-C entries, the Republic AP-12 ‘Rocket’ was not a conventional aircraft. The AP-12 had a streamlined, cigar-shaped fuselage and utilized a tricycle undercarriage. The aircraft’s Wright R-2160 Tornado engine was placed behind the pilot. The engine’s extension shaft ran under the cockpit and drove a six-blade, contra-rotating airscrew at the front of the aircraft. Four machine guns were installed in the AP-12’s nose and fired through the propellers, and an additional machine gun was installed in each wing, outside of the propeller arc. A 20 mm cannon was installed in the nose of the aircraft and fired through the propeller hub.

After the AP-12 placed 13th out of the R40-C entries, Republic literally went back to the drawing board and created a new design, designated AP-18. The AP-18 possessed some of the same lines and used the same engine as the AP-12; however, the R-2160 engine was now installed in the nose of the aircraft. Republic submitted its AP-18 design to the AAC in July 1941 and was awarded a contract in December 1941 to produce two prototypes of the aircraft, designated XP-69 (it also carried the experimental project designation MX-162).

Republic XP-69 15-Sept-1941 inboard drawing

This XP-69 drawing dated 15 September 1941 clearly shows the Wright Tornado installed in the nose of the aircraft, with the turbosupercharger and its ancillary equipment mounted behind the cockpit. While the leading edge is distorted, the trailing edge shows the inner wing section perpendicular to the fuselage, then tapering toward the wing tip. This drawing was discovered in the National Archives by Kimble McCutcheon of the Aircraft Engine Historical Society.

The Republic XP-69 was an all-metal, high-altitude interceptor fighter with a conventional layout. The aircraft was powered by a 42-cylinder R-2160 engine that produced 2,500 hp (1,864 kW) at 4,600 rpm and was installed in a normal manner, without an extension shaft. The engine drove a 13 ft 8 in (4.17 m) diameter, six-blade, contra-rotating propeller built by Hamilton Standard. The turbosupercharger, intercoolers, radiator, and oil coolers were all positioned behind the cockpit. The scoop mounted under the cockpit brought in air for the radiator, oil coolers, intercoolers, and turbosupercharger via a complex series of ducts. The scoop also incorporated a boundary layer air bleed duct. Initially, four air exit doors were located under the fuselage, but the exits were later relocated, with two on each side of the XP-69 (the oil cooler was the lower exit and the intercooler the upper). However, radiator and boundary layer air as well as exhaust from the turbosupercharger exited from the bottom of the aircraft.

Most sources contend that the R-2160 engine was installed behind the XP-69’s cockpit. However, all of the equipment and associated ducting that was installed behind the cockpit left no room for anything else. In addition, a drawing dated 15 September 1941 found in the U.S. National Archives by Kimble McCutcheon clearly shows the Wright Tornado installed in the nose of the aircraft.

Republic XP-69 side

The Republic XP-69 model undergoing wind tunnel tests. Note the revised belly scoop and the air exits on the rear fuselage. The man pictured at the bottom of the photo gives some scale to the large size of the model, which was 3/4-scale. (image via Langley Memorial Aeronautical Laboratory / NASA)

The XP-69 utilized a pressurized cockpit in a fairly narrow fuselage, and its standard taildragger landing gear was fully retractable. The aircraft’s armament consisted of two .50 cal machine guns and one 37 mm cannon installed in each wing, outboard of the main landing gear. The machine guns had 320 rpg, and the cannons had 40 rpg. Some sources state an alternative armament installation consisted of six .50 cal machine guns in the wings and no cannons. Initially, the leading and trailing edges of the inboard wing sections were exactly perpendicular to the fuselage. This was later revised so that the wing’s taper was unchanged throughout its leading and trailing edges. Slotted flaps extended across about 50 percent of the wing’s trailing edge to help lower the heavy aircraft’s landing speed.

The XP-69 was a large aircraft with a wingspan of 52 ft (15.85 m), a length of 51 ft 8 in (15.75 m), and a height of 17 ft 3 in (5.26 m). The aircraft had a top speed of 450 mph (724 km/h) at 35,000 ft (10,668 m), an initial climb rate of 2,750 fpm (13.97 m/s), and a ceiling of 48,900 ft (14,905 m). Eight wing fuel tanks provided a total capacity of 386 gal (1,461 L), and a 114 gal (432 L) fuselage tank brought the aircraft’s total fuel capacity to 500 gal (1,893 L), which provided a maximum range of 1,800 miles (2,897 km). Wind tunnel tests were conducted with a 75 gal (284 L) drop tank under each wing of the aircraft. The XP-69 had an empty weight of 15,595 lb (7,074 kg), a gross weight of 18,655 lb (8,462 kg), and a maximum weight of 26,164 lb (11,868 kg).

Republic XP-69 top

Top view of the XP-69 model illustrates the aircraft’s relatively narrow fuselage and that its wings had a continuous taper. Note the 75 gallon drop tank mockups on the left of the image and the Douglas XB-19 model on the right. (image via Langley Memorial Aeronautical Laboratory / NASA)

A 1/20-scale XP-69 model was used for spin recovery tests, the results of which were generally satisfactory—although, recovery was problematic at 30,000 ft (9,144 m). A 3/4-scale model of the XP-69 was completed around June 1942 and began wind tunnel tests in August. The extensive tests were to analyze and evaluate the aircraft’s stability, controls, and cooling system and included fitting the model with 10 ft (3.0 m) diameter, contra-rotating propellers driven by two 25 hp (19 kW) electric motors in the fuselage. The tests indicated some longitudinal instability; the forecasted rate of roll was inadequate, and the estimated control forces for full aileron deflection were excessive. The XP-69 would utilize a control yoke, which would provide a certain degree of mechanical advantage over a control stick. Tests also revealed that the cooling system was not as efficient as expected and required some revision.

Construction of the first prototype began in November 1942 and incorporated changes shown necessary from the various wind tunnel experiments. While development of the XP-69 continued, the R-2160 engine was delayed with design issues that, in turn, would delay the aircraft. Also, a miscommunication had occurred: Republic thought the first engines would be capable of 2,500 hp (1,864 kW) at 4,600 rpm. In reality, the R-2160 would produce only 2,350 hp (1,752 kW) at 4,150 rpm; 2,500 hp (1,864 kW) was the engine’s developmental goal. The reduced power would inhibit the XP-69’s performance, and its 450 (724 km/h) mph top speed was already seen as optimistic.

Republic XP-69 flaps

The XP-69 model with its flaps fully deployed at 40 degrees. The slotted flaps extended aft and down. Note the air exits on the side of the fuselage. (image via Langley Memorial Aeronautical Laboratory / NASA)

Republic wanted to end work on the XP-69 and focus their resources on an alternative project. The company believed their AP-19 design (in a way, a Pratt & Whitney R-4360-powered P-47) held more potential and could fly sooner than the XP-69. The AP-19 (designated XP-72) was designed for and proposed to the AAC at the same time as the AP-18/XP-69. Since the AAC wanted an R-2160-powered fighter as soon as possible, Republic’s AP-18/XP-69 design was contracted, as it was the most appealing candidate. But now, with the engine issues affecting the XP-69, the XP-72 could no longer be overlooked as the superior aircraft. The XP-69 was cancelled on 11 May 1943, and two prototypes of Republic’s XP-72 were ordered on 18 June 1943. The Wright R-2160 Tornado was cancelled on 12 February 1944.

Note: Most sources list the XP-69’s wingspan as 51 ft 8 in (15.75 m) and its length as 51 ft 6 in (15.70 m). The dimensions given in this article, a 52 ft (15.85 m) wingspan and a 51 ft 8 in (15.75 m) length, come from two NACA reports from the 1940s.

Republic XP-69 nose

This image of the XP-69’s nose displays the propellers that were powered by two 25 hp motors for the wind tunnel tests. Also note the complex segmentation of the belly scoop inlet. (image via Langley Memorial Aeronautical Laboratory / NASA)

Tornado: Wright Aero’s Last Liquid-Cooled Piston Engine by Kimble D. McCutcheon (2001)
U.S. Experimental & Prototype Aircraft Projects: Fighters by Bill Norton (2008)
American Secret Projects 1937–1945 by Tony Buttler and Alan Griffith (2015)
American Secret Pusher Fighters of World War II by Gerald H. Balzer (2008)
Stability and Control Tests of a 3/4-Scale Model of the XP-69 Airplane in the NACA Full-Scale Tunnel by Harold H. Sweberg (7 January 1943)
Compilation of Test Data on 111 Free-Spinning Airplane Models Tested in the Langley 15-Foot and 20-Foot Free-Spinning Tunnels by Malvestuto, Gale, and Wood (1947)

CAC CA-14A front

Commonwealth Aircraft Corporation CA-14/A Fighter

By William Pearce

In late 1941, the Australian aviation industry took stock of its resources and worked to create an indigenous fighter aircraft to defend against the Japanese. The result of this effort was the Commonwealth Aircraft Corporation (CAC) CA-12, CA-13, and CA-19 Boomerang fighters. In many respects, the Boomerang was an outgrowth of the CAC Wirraway general use aircraft. The Wirraway itself was a modified, licensed production version of the North American NA-16 (also referred to as NA-33) trainer. With a low top speed and poor altitude performance, the very maneuverable and rugged Boomerang found itself excelling in the ground attack role. In late 1942, The Australian War Cabinet and CAC sought to improve the Boomerang’s altitude performance by adding a turbosupercharger. This new aircraft was designated CA-14.

CAC CA-14 front

At first glance, the CAC CA-14 looks like a standard Boomerang fighter, but the aircraft’s unique turbosupercharger scoop can be seen on the side of the fuselage. Less noticeable modifications from a standard Boomerang include a new wing root fairing and a slightly enlarged tail.

The CA-14 was a standard CA-13 Boomerang that had been heavily modified to accommodate a turbosupercharger. Like all CA-13 Boomerangs, the CA-14 had a 1,200 hp (895 kW) Pratt & Whitney (P&W) R-1830 engine. The fuselage was built with a steel tube frame, and the wings and tail were built up from aluminum components. The wings housed four .303 machine guns and two 20 mm cannons. The tail, cowling, lower part of the fuselage, and in front of the cockpit were skinned with aluminum. All tail control surfaces were fabric-covered, and the ailerons were aluminum-skinned.

Unlike a normal Boomerang, the CA-14 had a new cowling that omitted the air intake scoop positioned above the engine on a standard Boomerang. A large scoop was added on the left side of the fuselage, next to the cockpit, and provided intake air for the engine and air for the turbosupercharger’s intercooler. Air exited the intercooler via an adjustable flap located on the right side of the upper fuselage, just behind the cockpit. The engine’s exhaust pipe was extended back along the right side of the fuselage to the turbosupercharger installed behind the cockpit. The General Electric (GE) B-2 turbosupercharger was from a Consolidated B-24 Liberator, and the Harrison intercooler was from a Boeing B-17 Flying Fortress; these parts were chosen because they were available, not because they were ideal. The fuselage was skinned with aluminum to just behind the turbosupercharger. Farther aft, the fuselage was wood-covered. A new, more streamlined fairing was installed on the wing’s leading edge. The fairing ran from the wing root to the fuselage, over the main gear wheel bays. The CA-14’s vertical stabilizer was slightly enlarged, and it used an 11 ft (3.35 m), three-blade, Curtiss propeller.

CAC CA-14 left side

The CA-14’s large scoop can be seen in this view. The scoop created turbulence that interfered with the aircraft’s tail. Pilot visibility was improved over the standard Boomerang by removing the engine intake scoop on the upper cowling.

The CA-14 was assigned serial number A46-1001 and first flew on 13 January 1943 piloted by Flt. Lt. John Holden. Its performance was on par with a standard Boomerang below 10,000 ft (3,048 m) but was superior above that altitude. At 28,000 ft (8,534 m), the CA-14 had a top speed of 354 mph (570 km/h) and a 1,400 fpm (7.1 m/s) rate of climb, while the standard Boomerang was 76 mph (122 km/h) slower at 278 mph (447 km/h) and could only climb at 450 fpm (2.3 m/s). The CA-14 had a 2,150 fpm (10.9 m/s) initial rate of climb and a ceiling of 36,000 ft (10,973 m), which was 2,000 ft (610 m) higher than a standard Boomerang’s ceiling. The CA-14 had the same 36 ft (10.97 m) wingspan and 25.5 ft (7.77 m) length as the Boomerang; however, it was some 400 lb (180 kg) heavier, at 8,095 lb (3,672 kg). The aircraft had a range of 930 miles (1,497 km).

Flight testing revealed directional instability and cooling issues with engine and turbosupercharger. The large scoop mounted on the side of the fuselage created turbulent air which interfered with the aircraft’s tail and caused some instability and buffeting. Starting in May 1943, the CA-14 was reworked to solve its issues and was redesignated CA-14A. Changes included adding a new, larger vertical stabilizer with an aluminum-skinned rudder and deleting the scoop from the aircraft’s fuselage. The engine cowling was reworked to provide better cooling, and a geared (3 to 1), 10-blade cooling fan was added behind the spinner. Air for the engine and intercooler was taken from the high-pressure area behind the cooling fan and internally ducted in the left side of the fuselage back to the turbosupercharger. A GE B-13 turbosupercharger and an AiResearch intercooler replaced the original units. The CA-14A was fitted with a three-blade Hamilton Standard or de Havilland propeller (sources disagree on which, but perhaps both propellers were tested), and its guns were removed. First flown around 26 July 1943, the CA-14A most likely achieved better performance than the CA-14; however, specifics have not been found. Sources indicate the CA-14A’s ceiling was in excess of 40,000 ft (12,192 m).

CAC CA-14 and CA-14A

A comparison of the CA-14 (top) and CA-14A (bottom), with its revised tail and cowling. The exit flap for the intercooler can bee seen in the upper fuselage, just behind the cockpit. The installation of the supercharger and its required accessories in the Boomerang’s small airframe was an impressive feat of engineering.

The ultimate goal of improving the Boomerang was to install a 1,450 hp (1,081 kW) P&W R-2000 engine and GE B-9 turbosupercharger in the aircraft. Originally, these changes were to be incorporated when the aircraft was rebuilt as the CA-14A. However, the United States was very reluctant to provide a license for supercharger production, and CAC’s production of licensed R-2000 engines encountered technical setbacks. The estimated speed of an R-2000-powered Boomerang was 286 mph (460 km/h) at sea level and 372 mph (599 km/h) at 27,000 ft (8,230 m). The aircraft’s rate of climb at sea level was 2,100 fpm (10.7 m/s) and 1,770 fpm (9.0 m/s) at 30,000 ft (9,144 m).

Based on the known performance of the CA-14 and the estimated performance of the R-2000-powered Boomerang, the Minister for Aircraft Production recommended that 120 R-2000-powered fighters be ordered. However, the Australian War Cabinet approved only 50 aircraft. With such a short production run, it was not worth the inevitable delays and required resources to upgrade Boomerang production to a new standard, especially with better performing fighters from the United States and Britain already arriving in Australia. As a result, the 50 aircraft were completed as CA-19 Boomerangs, which differed little from the CA-13s and CA-12s.

CAC CA-14A front

This view of the CA-14A displays its 10-blade engine cooling fan as well as its lack of armament. Undoubtedly, the aircraft’s performance was much improved, but its usefulness was in question since superior British and American aircraft were available in Australia. Note the Republic P-47 Thunderbolts in the background.

While the A46-1001 airframe was being designed and tested with its turbosupercharger, CAC looked to take the next step to enhance performance by fitting a 1,700 hp (1,268 kW) Wright R-2600 engine to an even more modified Boomerang. However, the availability of R-2600 engines to Australia was in question, and modifications to the Boomerang airframe would be substantial. It was deemed more practical to start development of a new aircraft with a 2,000 hp (1,491 kW) P&W R-2800 engine. Designated CA-15, this new aircraft would eventually fly, but with a Rolls-Royce Griffon V-12 engine and little resemblance to its initial design heritage.

The obsolete CA-14A continued to undergo flight testing and was used for high altitude weather observations, regularly flying at 40,000 ft (12,192 m). It was removed from service in 1946 and scrapped in 1947 (some sources say March 1949).

Note: The B-13 turbosupercharger was interchangeable with the B-2. Several sources state that CAC intended to install a B-9 turbosupercharger in the CA-14/A aircraft, but no GE references to a B-9 turbosupercharger have been found. Perhaps “B-9” was a typo or was a designation given to a licensed production or export model (like “B-10” for turbosuperchargers supplied to Britain).

CAC CA-14A left side

For the CA-14A, the large fuselage scoop was removed, and air to the turbosupercharger was delivered via an internal duct. The location of that duct can be discerned by the bulge running along the side of the fuselage

Wirraway, Boomerang & CA-15 in Australian Service by Stewart Wilson (1991)
Wirraway to Hornet by Brian L Hill (1998)
Australia’s Lost Fighter: The CA-15 and its Demise by David Clark (2010)