Category Archives: Aircraft

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)


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 gear boxes. 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


Pander S.4 Postjager Trimotor Mailplane

By William Pearce

In the early 1930s, Dutch pilot Dirk Asjes was disappointed with the slow development of Dutch airmail flights and Fokker aircraft. Asjes sketched out an aircraft design and asked the aircraft manufacturer Pander to build a special mailplane to compete with KLM (Koninklijke Luchtvaart Maatschappij or Royal Dutch Airlines) mail and passenger service. Officially, Pander was called the Nederlandse Fabriek van Vliegtuigen H. Pander & Zonen (H. Pander & Son Dutch Aircraft Company). Pander was a furniture company that had expanded to aircraft construction in 1924 when its owner, Harmen Pander, purchased the bankrupt VIH (Vliegtuig Industrie Holland or Holland Aircraft Industry).


The Pander S.4 Postjager displays its clean lines. The trimotor aircraft was purpose-built as a mail carrier to fly from Amsterdam to Batavia.

Airmail service to the Dutch East Indies involved using the relatively slow Fokker F.XVIII, which had a top speed of 149 mph (240 km/h). To improve service, KLM ordered the Fokker F.XX Zilvermeeuw, which had a top speed of 190 mph (305 km/h). While the F.XX was being built, Pander took up the challenge to build a faster aircraft solely to transport mail. Pander’s new design was the S.4 Postjager, and financial support came from a few Dutch shipping companies who hoped to break KLM’s monopoly on air transport to the East Indies.

The Pander S.4 Postjager was designed by Theodorus (Theo) Slot, who was originally with VIH. The aircraft was a low-wing trimotor with retractable main gear. The S.4 was made almost entirely of wood. The aircraft was powered by three 420 hp (313 kW) Wright Whirlwind R-975 engines. The aircraft’s interior was divided into three compartments: cockpit, radio room, and mail cargo hold.


On paper, the S.4 appeared to be an impressive, purpose-built aircraft that could improve airmail service for the Netherlands. In practice, the aircraft never had an opportunity to fully demonstrate its capabilities without outside difficulties hindering its performance.

The S.4 used external ailerons that mounted above the wings’ trailing edge. Sometimes called “park bench” ailerons because of their appearance, they are often mistaken for Flettner tabs. A Flettner tab is a supplementary control surface that attaches to and assists the primary control surface. By contrast, a “park bench” aileron is the primary control surface, and there is no other control surface integral with the wing. External ailerons operated in the undisturbed airflow apart from the wing and were more responsive during minor control inputs or during slow flight. In addition, external ailerons allowed the use of full-span flaps to give the aircraft a low landing speed. However, external ailerons had a tendency to flutter at higher speeds, potentially causing catastrophic damage to the aircraft (but flutter was not well understood in the 1930s). On the S.4, the flaps extended from the engine nacelles to near the wingtips.

The S.4 had a wingspan of 54 ft 6 in (16.6 m) and was 41 ft (12.5 m) long. The aircraft had a maximum speed of 224 mph (360 km/h), a cruising speed of 186 mph (300 km/h), and a landing speed of 60 mph (97 km/h). The S.4 was designed to carry 1,102 lb (500 kg) of mail. It had an empty weight of around 6,669 lb (3,025 kg) and a loaded weight of around 12,125 lb (5,200 kg). Six fuel tanks, three in each wing, carried a total of 555 gallons (2,100 L). The aircraft had a range of 1,510 miles (2,430 km) and a ceiling of 17,717 ft (5,400 m).


This underside view of the S.4 shows its PH-OST registration. Also visible are the external ailerons attached to the wings’ upper surfaces. The aircraft’s slot flaps (not visible) extended from the engine nacelle to near the wingtip.

Cleverly registered as PH-OST, the completed S.4 mailplane made its public debut on 23 September 1933. The Fokker F.XX also made its debut at the event, which was attended by Prince Henry of the Netherlands. The S.4 flew the following month, when Gerrit Geijsendorffer and Funker van Straaten made the maiden flight on 6 October 1933. Flight testing went well, and on 9 December 1933, the S.4 departed on an 8,700-mile (14,000-km) flight from Amsterdam to Batavia (now Jakarta, Indonesia). Flown by Geijsendorffer, Asjes, and van Straaten, this flight was a special run to demonstrate the aircraft’s speed and range and also to deliver 596 lb (270 kg) of Christmas mail (made up of some 51,000 letters and postcards) to the Dutch colony. At the time, the Fokker F.XX was being prepared for the same flight.

The S.4 had made a scheduled stopover in Rome, Italy and was proceeding to Athens, Greece when the right engine lost oil pressure. The aircraft made an emergency landing in Grottaglie, Italy, and inspection revealed that the right engine needed to be replaced. With no engines available anywhere in Europe, one was shipped from the United States and set to arrive on 22 December. This setback put the Christmas mail service in jeopardy. To make sure the mail was delivered, arrangements were made for the F.XX to pick up the S.4’s mail and continue to Batavia. But, the F.XX had its own engine issues before it even took off. This left the Fokker F.XVIII, the aircraft the S.4 and F.XX were meant to replace, as the only alternative. A F.XVIII picked up the mail and continued to Batavia with enough time for Christmas delivery. The failed Christmas flight was a huge embarrassment for both the S.4 and F.XX programs.


This side view of the S.4, now named Panderjager, shows the aircraft as it appeared in the MacRobertson Race. Note the “park bench” aileron extending above the wing.

The repaired S.4 set out for Batavia on 27 December and arrived on 31 December. It made the return flight, leaving Batavia on 5 January 1934 and arriving in Amsterdam on 11 January. Although the S.4 averaged 181 mph (291 km/h) on the flight from Batavia, the aircraft’s mail flight failed to impress, and the S,4 was not put into service. Pander decided to prepare the aircraft for the MacRobertson Trophy Air Race flown from London to Melbourne, Australia.

The MacRobertson Race started on 20 October 1934 and covered some 11,300 miles (18,200 km). For the race, the S.4 was flown by Geijsendorffer, Asjes, and Pieter Pronk and carried race number 6. The aircraft had been renamed Panderjager, but some referred to it as the Pechjager (“pech” meaning “bad luck” and “breakdown”). After leaving Mildenhall airfield in England, the S.4 arrived in Bagdad, Iraq in third place at the end of the first day of the race. The next day, the aircraft proceeded to Allahabad, India, still in third place. Upon touchdown in Allahabad, the left gear collapsed, resulting in bent left and front propellers and a damaged left cowling and main gear.


This rear view of the S.4 shows the external brace on the horizontal stabilizer and the elevators’ trim tabs. The image also provides a good view of the “park bench” ailerons.

Allahabad did not have the facilities to repair the S.4. Geijsendorffer took the propellers and traveled by train to the KLM depot in Calcutta (now Kolkata), India to make the needed repairs. This delay took the S.4 out of competition, but the decision was made to finish the race. Repairs were completed, and the S.4 was ready to fly on the evening of 26 October 1934. A service vehicle towing a light was positioned across the field from the S.4 to illuminate its path. The S.4’s crew found the light distracting and asked for it to be shut off, as the aircraft could provide its own lighting.

Once the service vehicle’s light was shut off, the S.4 prepared for takeoff. Unfortunately, the crew of the service vehicle misunderstood the instructions. They thought they were to proceed to the S.4 and illuminate the aircraft from behind. As they made their way toward the S.4 in darkness, the aircraft began its takeoff run. At about 99 mph (160 km/h), the S.4’s right wing struck the service vehicle. Fuel spilled from the ruptured wing and quickly ignited as the S.4 skidded 427 ft (130 m) to a stop. Pronk was uninjured, and Geijsendorffer and Asjes escaped with minor burns, but the S.4 was completely destroyed by the fire. The two operators of the service vehicle were severely injured.

Pander planned to convert the S.4 to a scout or bomber after the race and sell it to the military. With the loss of the S.4, there was no aircraft to sell, and Pander was not able to recover its expenses. The company went out of business a short time later.

The S.4 sits at Allahabad, India with bent propellers on its front and left engines. The de Havilland DH 88 Comet “Black Magic” suffered engine trouble, and work to repair its engine was underway as it sat next to the S.4. The S.4 never left Allahabad.

The S.4 sits at Allahabad, India with bent propellers on its front and left engines. The de Havilland DH 88 Comet “Black Magic” suffered engine trouble, and work to repair its engine was underway as it sat next to the S.4. The S.4 never left Allahabad.

Nederlandse Vliegtuigen Deel 2 by Theo Wesselink (2014)
Jane’s All the World’s Aircraft 1934 by G. G. Grey (1934)
Blue Wings Orange Skies by Ryan K. Noppen (2016)
“High-Speed Mail Machine” Flight (7 September 1933)
“The Aerial Phost” Flight (5 October 1933)
“Opening of Amsterdam Aero Club’s New Clubhouse” Flight (28 September 1933)
“The Pander Postjager Pauses” Flight (14 December 1933)


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)


Savoia-Marchetti S.65 Schneider Racer

By William Pearce

After the Italian team was defeated on its home turf at Venice, Italy in the 1927 Schneider Trophy Race, the Italian Ministero dell’Aeronautica (Air Ministry) sought to ensure victory for the 1929 race. The Ministero dell’Aeronautica instituted programs to enhance aircraft, engines, and pilot training leading up to the 1929 Schneider race. Early in 1929, the Ministero dell’Aeronautica requested racing aircraft designs from major manufacturers and encouraged unorthodox configurations.


The Savoia-Marchetti S.65 in its original configuration. Note the single strut extending from each float to the tail, the short tail and rudder, and the short windscreen.

Alessandro Marchetti was the chief designer for Savoia-Marchetti and was preoccupied with the design of the long-range S.64 aircraft. Originally, he did not submit a Schneider racer design, but the Ministero dell’Aeronautica encouraged him to reconsider. Soon after, Marchetti submitted the rather unorthodox S.65 design. On 24 March 1928, the Ministero dell’Aeronautica ordered two S.65 aircraft and allocated them the serial numbers MM 101 and MM 102.

The Savoia-Marchetti S.65 was a low-wing, tandem-engine, twin-boom monoplane that utilized two long, narrow floats. The aircraft was designed to incorporate the largest amount of power in the smallest package. The S.65’s tension rod and wire-braced wings were made of wood and almost completely covered with copper surface radiators. The floats were made of wood (some say aluminum), had a relatively flat bottom, and housed the S.65’s fuel tanks. The floats were around 28 ft 8 in (8.75 m) long and were mounted on struts. Originally, one strut extended from the rear of each float to the tail, but a second strut was later added.


The S.65 has been modified with an additional strut extending from each float to the tail. The tail and rudder have also been extended below the horizontal stabilizer. Note that the windscreen has not changed, that the rudder has a rather square lower trailing edge, and that there are no handholds in the wingtips.

A narrow boom extended behind each wing to support the tail. The boom was hollow and had flight cables running through its interior. Sources disagree on whether the booms were made of metal or wood. The horizontal stabilizer was mounted between the ends of the booms. The vertical stabilizer was positioned in the center of the horizontal stabilizer. Originally, the rudder and tail extended only above the horizontal stabilizer, and the rudder was notched to clear the elevator. Later, the tail and rudder were enlarged and extended below the horizontal stabilizer, and the elevator was notched to clear the rudder. The tail and all control surfaces were made of wood and were fabric-covered.

Attached to the wing was a small fuselage nacelle that housed two Isotta Fraschini Asso 1-500 engines. The engines were mounted in a push-pull configuration with one engine in front of the cockpit and the other behind. The nacelle was made of a tubular steel frame and covered with aluminum panels. Oil coolers were mounted on both sides of the cockpit between the engines. Two windows to improve the pilot’s lateral visibility were positioned above each oil cooler. Just behind the front engine was a windscreen for the cockpit. Initially, a short windscreen was installed, but this was later replaced by a longer, more streamlined unit. The fuselage nacelle was around 18 ft (5.48 m) long, including the propeller spinners.


The 1,050 hp (783 kW) Isotta Fraschini Asso 1-500 engine. It is unclear how much this engine differed internally from a standard Asso 500 engine. The three cantilever mounts and the nearly-flush rear of the engine can clearly be seen. The exhaust ports have been relocated from the outer side of the cylinder head to the Vee side. A water pump and magneto are just visible on the extended gear reduction case. The vertical ribbing on the lower crankcase served to increase its strength.

The S.65’s Asso 1-500 V-12 engines were based on the Asso 500 Ri engine and were heavily modified by Giustino Cattaneo, head engineer at Isotta Fraschini. The engine’s crankcase was ribbed and strengthened to become a structural member of the S.65’s fuselage nacelle. Each engine mounted directly to a steel bulkhead on the end of the cockpit via three cantilever supports. The rear of the engine sat flush with the bulkhead. At the front of the engine was an extended gear reduction case which allowed for a streamlined cowling. Engine accessories, such as the two water pumps and two magnetos, were mounted to the gear case. Each Asso 1-500 engine produced 1,050 hp (783 kW) at 3,000 rpm.

At the bottom of each side of the cowling were two inlets. Air flowed from each inlet into a carburetor and then into three cylinders of the engine. Exhaust ports were located on the Vee side of the engine, and the exhaust gases were expelled up though the top of the cowling. Both engines turned counter-clockwise. Since the rear engine was installed backward, the propellers of each engine turned in opposite directions relative to one another. This installation effectively cancelled out the propeller torque that had been an issue for a number of Schneider racers. The metal, two-blade, fixed pitch propellers had a diameter of approximately 7 ft 5 in (2.26 m). The rear propeller’s spinner was about one-third longer than the front spinner.


The S.65 as seen at Calshot, England. The long windscreen has now been installed. The lower trailing edge of the rudder is now rounded, and the wingtips now have handholds. This image gives a good view of the surface radiators that cover nearly all of the wings. Also visible is the rectangular cover of the exhaust ports between the cylinder banks.

Italian sources and drawings from Savoia-Marchetti list the S.65 as having a wingspan of 31 ft 2 in (9.5 m) and a length of 35 ft 1 in (10.7 m). However, other sources often cite a wingspan of 33 ft (10.05 m) and a length of 29 ft (8.83 m). It is not entirely clear which figures are correct. The weight of the aircraft was approximately 5,071 lb (2,300 kg) empty and 6,173 lb (2,800 kg) loaded. The top speed of the S.65 was estimated between 375 and 400 mph (600 and 645 km/h).

In mid-1929, Alessandro Passaleva, one of Savoia-Marchetti’s pilots, tested the first S.65 (MM 101) on Lake Maggiore, near the company’s factory in Sesto Calende, Italy. Although the aircraft was not flown, Passaleva recommended a number of changes to stiffen and improve the S.65’s tail. The second S.65 (MM 102) was modified with the additional tail brace and extended rudder and tail. It is doubtful that MM 101 was ever flown or that MM 102 was flown on Lake Maggiore. MM 102 was delivered to the Reparto Alta Velocità (High Speed Unit) at Desenzano on Lake Garda in July 1929.

Initial flight tests of the S.65 were conducted by Tommaso Dal Molin and began in late July 1929. This is most likely the first time an S.65 was flown. Dal Molin was an experienced pilot and also small enough to fit inside the S.65’s very cramped cockpit. Some accounts state that Dal Molin did not bother with a parachute because the cockpit was so small, and the rear propeller made bailing out nearly impossible. A number of issues were encountered with the aircraft’s engines and cooling system. In addition, exhaust fumes constantly entered the cockpit.


This image shows the S.65’s rear engine being run-up at Calshot. The oil radiator is clearly seen between the two engines, and it gives some perspective as to the small size of the cockpit. Note the various engine accessories mounted to the extended gear reduction case.

It was soon obvious that the S.65 would not be ready in time for the Schneider Trophy Race held on 6–7 September 1929 in Calshot, England. However, the Italians decided to send the aircraft anyway, to give the British team something to consider. Before the S.65 arrived at Calshot, the lower rudder extension was rounded; the longer windscreen was installed, and handholds were added to the wingtips. During the races, the S.65 MM 102 was displayed, and its rear engine was run-up on at least one occasion. Some saw the S.65 as a sign of future high-speed aircraft to come.

Italy had developed four new aircraft for the 1929 Schneider Trophy Race: Macchi M.67, FIAT C.29, Savoia-Marchetti S.65, and Piaggio P.7. The end result was that Italian resources were spread too thin, and none of their aircraft were developed to the point of offering serious competition to the British effort, which was victorious. Once back in Italy, the head of the Reparto Alta Velocità, Mario Bernasconi, decided to recover some pride by making an attempt on the world speed record. Britain had just set a new record on 12 September 1929 at 357.7 mph (575.7 km/h) in its Schneider race-winning Supermarine S6 (N247) piloted by Augustus Orelbar.


Tommaso Dal Molin poses in front of the S.65. Note the longer windscreen and the side windows just above the oil cooler. Each rectangular port on the cowling leads to a carburetor. Also visible are the louvers that cover the cowling.

The S.65 underwent further refinements in late 1929, and it was believed that the aircraft could exceed the S6’s speed by a reasonable margin. It appears the aircraft was fitted with new aluminum (duralumin), V-bottom floats. In addition, the engine cowling had what appear to be six exhaust ports positioned on each side. Exhaust fumes entering the cockpit was an issue due to the central exhaust location, and relocating the ports to the engine sides (their original location in the Asso 500 engine) would help solve the issue. The carburetor intakes were not changed.

Dal Molin took the S.65 on a test flight from Lake Garda on 17 January 1930 to prepare for his speed record attempt the following day. On 18 January, Dal Molin made three takoff attempts, which were all aborted due to excessive yaw. On the fourth attempt, the S.65 became airborne and then pitched up at an extreme angle. The aircraft stalled some 80 to 165 ft (25 to 50 m) above the water and crashed into the lake. Rescue vessels arrived quickly, but the S.65 with Dal Molin still aboard had quickly sunk 330 ft (100 m) to the bottom of the lake. It was Tommaso Dal Molin’s 28th birthday. A special recovery vessel called the Artigilo retrieved the S.65 on 29 January. Dal Molin’s body was recovered on 30 January. While the exact cause of the crash was never determined, many believe the elevator jammed, resulting in the abrupt pitch up and subsequent stall.

Note: As mentioned above, many sources disagree on various aspects of the S.65. For example, sources (some of which were not used in this article) list the wing spars as being made from four different materials: duralumin, walnut, mahogany, and spruce. While images were closely scrutinized to give an accurate account of the S.65 in this article, only so much can be determined from analyzing a grainy, 85-year-old image. In addition, some sources claim that only one S.65 was built (MM 102). Others say construction of MM 101 was started but never completed, and still others contend that MM 101 was completed and stored at the Reparto Alta Velocità at Lake Garda until 1939.


The remains of the S.65 after it was recovered from Lake Garda and placed onboard the Artigilo. The rear engine is in the foreground. Note what appear to be exhaust ports along the sides of the cowling. The aircraft’s fuselage seems to be rather undamaged. Reportedly, the S.65 sank quickly, and some sources claim that Dal Molin could not swim.

Schneider Trophy Seaplanes and Flying Boats by Ralph Pegram (2012)
Aeroplani S.I.A.I. 1915–1945 by Giorgio Bignozzi and Roberto Gentilli (1920)
Schneider Trophy Aircraft 1913–1931 by Derek N. James (1981)
MC 72 & Coppa Schneider by Igino Coggi (1984)
L’epopea del reparto alta velocità by Manlio Bendoni (1971)

Koolhoven FK55 mockup front

Koolhoven FK.55 Fighter

By William Pearce

In November 1936, the Dutch aircraft manufacturer Koolhoven surprised many by bringing a very advanced fighter aircraft mockup to the Paris Salon de l’Aviation (Air Show). Mounted on stands to make it appear suspended in flight, the Koolhoven FK.55 mockup caught everyone’s attention. The impressive mockup was so detailed that anyone who did not study it for a period of time would think that it was a real aircraft. But converting the unique ideas showcased in the mockup into a workable aircraft would pose serious problems for Koolhoven.

Koolhoven FK55 mockup front

The sleek lines of the Koolhoven FK.55 can be seen in this image of the mockup at the 1938 Paris Salon de l’Aviation. Note the machine guns mounted in the wings and the radiators in the aircraft’s nose. The outline of the aircraft’s main gear is just visible under the wings.

The FK.55 was designed by company founder Frederick (Frits) Koolhoven. The mockup was of all wooden construction and featured an aerodynamic fuselage with a somewhat triangular cross section. One corner of the triangle formed the lower part of the fuselage, and the wings extended from the other two (upper) corners. The shoulder-mounted wings were well blended into the fuselage and located just behind the cockpit. The wing center section was built integral with the fuselage.

The FK.55 mockup did not include ailerons. Roll control was to be achieved by slot-spoilers in the outer wing sections. While the “slots” did exist, the “spoilers” were never installed on the mockup, and the slots were covered by aluminum panels. The pivot point of the retractable main landing gear was just off the aircraft’s center line. The legs of the main gear had a bend that allowed them to retract flush into the sides of the fuselage and underside of the wings. At the rear of the aircraft was a non-retractable tail skid.

Koolhoven FK55 mockup gear

The elaborate FK.55 mockup being built at the Koolhoven factory. The very long main gear posed problems when adapted to the prototype.

It is not clear whether or not an engine was actually installed in the mockup. If an engine was installed, it was a Lorraine Pétrel water-cooled V-12 engine installed behind the cockpit and at the aircraft’s center of gravity. A shaft extended from the engine, ran under the pilot’s seat, and connected to a propeller gear reduction unit in the nose of the aircraft. The gear reduction unit enabled the use of contra-rotating propellers. Metal, fixed-pitch propellers were fitted to the prototype.

A cannon could be positioned behind the gear reduction unit and fire through the propeller hub. Each wing had a machine gun installed outside of the propeller arc. Radiators were located on each side of the mockup’s cockpit, between the nose and the wings. Two scoops under the mockup’s fuselage provided air to the engine. The position of the cockpit, forward of the wings and at the very front of the aircraft, provided the pilot an excellent view.

The FK.55 mockup had a 29.5 ft (9.0 m) wingspan and was 27.6 ft (8.4 m) long. The complete aircraft was forecasted to weigh 2,425 lb (1,100 kg) empty and 3,638 lb (1,650 kg) loaded. Estimated performance for the FK.55 included a top speed of 323 mph (520 km/h) at 13,123 ft (4,000 m) and a cruising speed of 280 mph (450 km/h) at the same altitude. The aircraft had an initial rate of climb of 2,983 fpm (15.2 m/s), a service ceiling of 31,496 ft (9,600 m), and a range of 559 mi (900 km).

Koolhoven FK55 mockup

Suspended on stands, the FK.55 mockup was an impressive sight. Note the tail skid and the aluminum covers over the openings for the slot-spoilers.

Back in their factory at Waalhaven Airport in Rotterdam, Netherlands, the Koolhoven team went to work building a flying FK.55 prototype. The aircraft grew wider, longer, heavier, and slower than the original estimates. Each change necessitated another change as the FK.55 prototype came together, and what was once the sleek airframe of the FK.55 mockup eventually resembled a “pregnant duck” (in the words of one Dutch pilot).

The triangular cross section of the mockup’s fuselage had been replaced by a larger, mostly circular form. The wings had lost their blended look and now appeared tacked onto the fuselage. Strength issues with the long and complex landing gear necessitated using fixed gear temporarily attached to the fuselage until the retractable gear issues could be resolved. The front and middle sections of the fuselage were made from welded steel tubing, while the rear section and tail were made from wood. The wings were also made of wood and had split flaps and ailerons. The FK.55 maintained provisions for a 20 mm or 37 mm cannon to fire though the propeller hub, and each wing now housed two machine guns with 500 rpg. However, no armament was installed in the prototype.

Lorraine Petrel and Sterna engine CR props

The Lorraine Pétrel engine (top) and the Sterna (bottom). Note how the front propeller rotates clockwise on the Pétrel but counterclockwise on the Sterna. Roughly translated, the sign under the Sterna reads, “The engine Lorraine Sterna 900 hp; Has offset reducer and double propellers; Dutch Koolhoven FK.55 in flight since 1938.”

There is some disagreement about which engine powered the FK.55 prototype. Most sources state an 860 hp (641 kW) Lorraine Pétrel 12Hars was used, but the 12Hars typically produced around 700 hp (522 kW). Some sources claim a 1,000 hp (746 kW) Lorraine Sterna was used. A 900 hp (671 kW) Sterna with an extension shaft and propeller gear reduction unit was displayed at the Paris Salon de l’Aviation in November 1938. A sign under the engine indicated that it was intended for the FK.55, but it is doubtful that the engine was ever installed in the aircraft, as the FK.55 flew before the 1938 Salon. In images of the FK.55 prototype, the gear reduction unit appears to be the one used with the Pétrel engine. In addition, the front propeller of the Pétrel engine rotated clockwise. The front propeller on both the FK.55 mockup and prototype also rotated clockwise; however, the front propeller of the Sterna engine rotated counterclockwise. Therefore, the Pétrel engine was most likely used, with the Sterna intended to replace it in the near future.

Two two-blade, adjustable-pitch, metal Ratier propellers were installed. The engine’s induction scoops had grown in size and were now positioned on the lower sides of the prototype. The radiators were retained in their original position but had also grown in size, again spoiling the aircraft’s aerodynamics.

The FK.55 prototype had a 31.5 ft (9.60 m) wingspan and was 30.3 ft (9.25 m) long. The complete aircraft weighed 3,527 lb (1,600 kg) empty and 5,027 lb (2,280 kg) loaded. The performance estimates for the FK.55 had been reduced to a top speed of 317 mph (510 km/h) at 11,811 ft (3,600 m) and a cruising speed of 280 mph (450 km/h) at the same altitude. The aircraft had an initial rate of climb of 1,367 fpm (6.9 m/s), a service ceiling of 33,136 ft (10,100 m), and a range of 528 mi (850 km).

Koolhoven FK55 prototype front

The FK.55 prototype was an odd and awkward aircraft, especially when compared to the mockup. Note the fixed landing gear and that the front propeller turned clockwise (when viewed from the rear).

In June 1938, the FK.55 was trucked to Welschap Airfield, a more secluded location for flight testing. The prototype was given the serial number 5501 and had been registered as PH-APB, but the registration was never applied to the aircraft. On the morning of 30 June 1938, Koolhoven pilot Thomas Coppers conducted high-speed taxi tests and hopped the FK.55 into the air on three separate occasions. Later that afternoon, Coppers took the FK.55 into the air for its first flight. Shortly after takeoff, Coppers made a 180 degree turn and quickly landed with the wind. Frits Koolhoven approached the aircraft, where he and Coppers engaged in an animated discussion regarding the FK.55. Some sources state that Coppers had found the cockpit unbearably hot. The taxi test should have given some indication of the heat experienced in the cockpit. Whatever the reason, the FK.55 never flew again.

The FK.55 mockup appeared to be a maneuverable fighter aircraft that afforded the pilot an excellent view, and its contra-rotating propellers eliminated engine torque, making the aircraft manageable for inexperienced pilots. The FK.55 prototype was an odd, ungainly aircraft that was underpowered and incomplete. The Koolhoven team endeavored to rework the FK.55’s design, changing to low wings and a Lorraine Sterna engine of at least 1,100 hp (820 kW), but Frits Koolhoven himself wanted nothing more to do with the aircraft. On 10 May 1940, a German bombing raid struck the Waalhaven Airport. The FK.55 mockup and prototype were destroyed, along with the entire Koolhoven factory, affectively putting an end to the company.

Koolhoven FK55 prototype engine run

When viewed from the side, the FK.55 prototype had a rather “pregnant” appearance. This image illustrates how the pilot was positioned between several heat sources.

Jane’s All the World’s Aircraft 1936 by C. G. Grey and Leonard Bridgham (1936)
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The Complete Book of Fighters by William Green and Gordon Swanborough (1994)
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