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.

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 “Materiel, Experimental” project designation MX-162).

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.

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).

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.

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.

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)

Sources:
– 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)
– http://www.weakforcepress.com/tornado_errata.shtml
– http://www.weakforcepress.com/XP-69/index.html
– http://crgis.ndc.nasa.gov/historic/Test_139:_XP-69_3/4%E2%80%93scale_Model_%28Stability_and_Cooling%29

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.

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.

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).

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.

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).

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

Sources:
– 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)
– http://www.australianflying.com.au/news/warbirds-the-turbo-interceptor-boomerang

By William Pearce

With an interest in aviation and a large fortune, Howard Hughes founded the Hughes Aircraft Company (HAC) in 1934. HAC was a division of the Hughes Tool Company. The H-1 Racer of 1935 was the first aircraft that HAC built; Hughes flew the racer to set a number of records. Before the H-1 was completed, Hughes submitted a fighter version of the racer to the United States Army Air Corps (AAC) for a design competition (Specification X-603). Hughes had designated the aircraft XP-2, but the AAC turned down the design, selecting the fighter version of the Wedell-Williams Model 45 racer (as the XP-34) instead.

The Hughes D-2 under construction at the Hughes Airport in Culver City, California. Hughes can be seen looking over the engine installation as he hands his jacket to Glenn Odekirk, Hughes’ long-time mechanic, engineer, and assistant. Note the aircraft’s airframe and smooth Duramold skin. The large housing on the bench in front of the engine appears to be an exhaust manifold to expel gases from the turbosupercharger and its wastegate.

The next HAC aircraft Hughes envisioned was a twin-engine fighter. HAC had entered discussions regarding the aircraft with the AAC in 1936. In 1937, HAC submitted its proposal to the AAC for a twin-engine interceptor design competition (Specification X-608). The specifics and configuration of HAC’s aircraft are not available, but Hughes later contended that Lockheed had copied the basics of his design and used it for their proposal. Lockheed’s proposal won the competition and was ultimately produced as the P-38 Lightning.

In July 1938, Hughes and crew established a new record for an around the world flight in a modified Lockheed 14 Super Electra. The flight took 91 hours, 14 minutes, and 10 seconds. Hughes knew that a specially designed aircraft could be used to establish an even better time. Hughes also knew that the US and various European nations were purchasing all manner of aircraft because of the unrest the German government was creating in Europe. Hughes directed HAC to design a new aircraft with performance so outstanding that the AAC would be unable to turn it down. This new aircraft was designated the D-2. The designations D-2A, DX-2, DX-2A, XD-2, and D-3 were also applied at various times.

HAC artwork depicting numerous D-2s (A-37s) in combat over a German factory. Note the bomb bay in the rear of the fuselage.

Stanley Bell began the initial D-2 design work in the summer of 1939. Some believe the D-2 was designed to set a new world record flight, and since such a flight could not be accomplished with the world at war, Hughes proposed a number of modifications and configurations to appeal to the AAC. However, others believe that Hughes intended the aircraft solely for AAC use. Virginius Clark, an HAC representative, visited the AAC in late 1939 to obtain a better understanding of what the AAC wanted in an interceptor-type aircraft. After his casual meetings, Clark suggested to Bell that a fast and maneuverable light bomber could act as a fighter. The belief was that such an aircraft would wreak havoc on enemy installations, forcing enemy aircraft to engage. Its maneuverability, speed, and powerful defensive armament would allow it to counter enemy fighters. Hughes had the D-2 built around this concept, not from an official request outlined by the AAC.

In December 1939, HAC offered to provide the government a performance report of the D-2 once it was completed. The D-2 was described as a pursuit-type aircraft, and the report was priced at $50. While $50 would do absolutely nothing to recover the funds Hughes was spending on the D-2, a government contract associated with the aircraft would technically allow the release of war material (such as engines) to HAC. The AAC felt they had little to lose and were interested in the performance of the aircraft and its Duramold skin construction. Hughes was very focused on creating a streamlined aircraft, and had purchased the rights to using the Duramold Process. Duramold is resin-impregnated layers of wood molded to shape under pressure and heat; it replaced riveted aluminum aircraft skin. Duramold provided a surface free of joints and imperfections that was also quicker to construct. At the time, there were dire predictions of an aluminum shortage, and Duramold was seen as a possible substitution. The AAC issued a contract to HAC for the D-2 performance reports on 22 May 1940. By this time, the aircraft was described as a bomber capable of over 300 mph (483 km/h) and possessed a 4,000 lb (1,814 kg) bomb load.

As the project progressed, wind tunnel models were used to evaluate the aircraft’s configuration. However, Hughes was so secretive that the models were not actually representative of the D-2’s configuration. Only the specific part being tested was accurate; the rest of the model’s configuration was inaccurate to prevent anyone from knowing the D-2’s true form.

A Hughes D-2 model preserved as part of the Howard Hughes Personal Aviation Collection at the Florida Air Museum in Lakeland. Note how the wings and forward fuselage are the only accurate parts of the model. (Robert Beechy image via SecretProjects.co.uk)

By March 1941, the D-2 had reverted back to a fighter role for bomber convoy protection. The D-2 was forecasted to have a 2,600 mile (4,284 km) range, a 450 mph (724 km/h) top speed, and would be armed with seven .50-cal machine guns. By July 1941, the D-2 and most of the HAC operation was moved from the airport in Burbank, California to a the new Hughes Airport, where a 9,500 ft (2,896 m) runway sat on 380 acres (1.54 sq km) of land in Culver City, California.

In November 1941, the Army Air Force (AAF—the AAC was renamed on 20 June 1941) officially turned down the D-2 as a prospective fighter aircraft. The Duramold skin was seen as insufficient for a high performance aircraft, and the D-2 was not stressed to fighter aircraft load factors nor did it have any armor protection. Undeterred, Hughes moved forward with the construction of the aircraft while HAC engineers investigated ways to meet military specifications. The AAF’s position was reversed in early 1942 when they decided the D-2 prototype and its Duramold construction offered sufficient promise of aerodynamic improvement to warrant its procurement. Hughes had invested $3,000,000 in the D-2, but the AAF only wanted to spend $500,000. Hughes turned down the offer; he would wait until after the D-2 was flown, in the hope that its unparalleled performance would demand a higher price from the AAF.

His jacket now hung on the engine, Hughes continues to look over its installation as Glenn Odekirk, Stanley Bell, and Kenneth Riley congregate on the other side. Perhaps they are discussing the proposed turbosupercharger installation. Note the construction of the D-2’s pressurized cockpit.

The Hughes D-2 was a twin-boom aircraft with a sleek central nacelle. The D-2’s initial design featured a conventional, taildragger gear layout, but this was discarded in favor of a tricycle gear arrangement. The main wheels retracted back into the booms, while the nose gear rotated 90 degrees and retracted back into the central nacelle. Control of the ailerons, elevator, and rudder were all hydraulically boosted.

The aircraft was constructed from both wood and metal. The central part of the D-2 was made up of a tubular frame to which formers and longerons were attached. The entire aircraft was covered with the Duramold plywood skin. The two person crew sat tandem in a pressurized cockpit. The crew consisted of a pilot and a navigator/bomber/gunner. Originally, two 42-cylinder, 2,350 hp (1,753 kW) Wright R-2160 Tornado engines were planned for the D-2. However, the AAF later promised these engines to the Lockheed XP-58 Chain Lightning program and provided HAC three (one as a spare) 2,000 hp (1,491 kW) Pratt & Whitney R-2800-49 engines. HAC was to develop its own twin-turbosupercharger installation for each engine, but neither this nor the cockpit pressurization equipment was ever fitted to the D-2.

In late 1941, the D-2 design was unarmed and powered by R-2160 engines. The aircraft had a 60.5 ft (18.4 m) wingspan, was 34.25 ft (10.4 m) long, and had a gross weight of 26,400 lb (11,975 kg). The aircraft had a maximum speed of 451 (726 km/h) mph at 25,000 ft (7,620 m) and a cruise speed of 270 mph (435 km/h). The D-2’s climb rate was 3,620 fpm (18.4 m/s), and its service ceiling was 41,000 ft (12,497 m).

A three-view drawing of the proposed Hughes XA-37, with six machine guns in the nose and four in the rear fuselage.

By mid-1943, the D-2 had been redesigned with R-2800 engines. The aircraft had a 60 ft (18.3 m) wingspan and was 57.8 ft (17.6 m) long. The D-2’s gross weight had increased to 31,672 lb (14,366 kg). Part of this increase was from the aircraft’s weapon load of 2,200 lb (998 kg) of bombs and ten .50-cal machine guns. Four of the guns were positioned one above the other in a turret of sorts at the rear of the central nacelle. The twin booms and horizontal stabilizer would severely restrict the rear machine guns’ field of fire. The six nose guns were hard-mounted, with three on each side of the fuselage. The R-2800-powered D-2 had a maximum speed of 433 mph (697 km/h) at 25,000 ft (7,620 m) and a cruising speed of 274 mph (441 km/h). The aircraft’s climb rate was 2,620 fpm (13.3 m/s), and the aircraft had a service celling of 36,000 ft (10,973 m).

By June 1942, the AAF was interested in a high altitude reconnaissance aircraft and thought the D-2 might be a good fit. At the same time, the D-2’s intended role was defined as a fighter aircraft, and the AAF considered designating it the XP-73. In July 1942, the D-2 was envisioned in more of an attack role, and the designation XA-37 was recommended. However, the D-2 was never officially designated XP-73 or XA-37.

The completed D-2 at what is believed to be Harper Dry Lake. The building behind the aircraft was the air conditioned hangar that would later burn with the D-2 inside. Note the aircraft’s sleek fuselage.

In early 1943, and still lacking a definite role, the nearly finished D-2 was moved for final assembly to a facility Hughes had constructed at the remote Harper Dry Lake near Barstow, California. Ground runs and numerous high-speed taxi tests accompanied by brief hops into the air revealed friction in the control system and that the controls were insufficient when the boost system was off. The first flight was delayed until the D-2 had been modified with larger ailerons to improve control. With Hughes in the cockpit, the D-2 flew for the first time on 20 June 1943. The first flight was 15 minutes long, and a flight of 35 minutes followed after some adjustments. Subsequent test flights demonstrated that the aileron control forces were high and the aircraft had a tendency to roll. In addition, some aileron control reversal was experienced. The D-2’s wing tips were extended, and its ailerons were further modified in an attempt to fix the issues.

The D-2 accumulated at least nine hours of flight time, but the control issues persisted. The aircraft needed major modifications, including a new wing, before it would be an acceptable aircraft. The fuselage also needed modifications to enlarge its bomb bay. Incorporating these changes into the D-2’s design resulted in a new HAC designation: D-5.

Side view of the D-2 at Harper Dry Lake. While the turbosuperchargers were never fitted, the engine’s exhaust can be seen just below the wing. Note the aircraft’s long nose and tailstrike bumper.

Many in the AAF were still not interested in the D-2 or the D-5. Part of the problem was that Hughes acted erratically and was difficult to work with. They also felt HAC did not have the capacity to enter series aircraft production. In late August 1943, the AAF officially directed no further action be taken with the D-2 or D-5. However, Hughes had already taken steps to sufficiently impress those in power and secure a contract for the D-2/D-5.

In early August 1943, Col. Elliot Roosevelt, President Franklin Roosevelt’s son, was in the Los Angeles area looking in on various aircraft manufacturers to find a reconnaissance aircraft. Col. Roosevelt, who had previously commanded a reconnaissance unit, was wined and dined by Hughes and taken to Harper Dry Lake for a personal tour of the D-2. At the time, the aircraft was undergoing modification to become the D-5 and was not available for flight, but Col. Roosevelt was sufficiently impressed. This led the AAF to issue a letter of intent on 6 October 1943 for the purchase of 100 D-5 aircraft. An official contract was issued in January 1944, and the D-5 was designated F-11 by the AAF.

A Hughes D-5 drawing dated 17 June 1943. The aircraft is very similar to the D-2, but with a new wing. The D-5 was of Duramold construction and powered by R-2800 engines. Its specifications included a wingspan of 92 ft (28.0 m), a length of 58 ft (17.7 m), a top speed of 488 mph (785 km/h) at 30,000 ft (9,144 m), a service ceiling of 37,000 ft (11,278 m), and a gross weight of 36,400 lb (16,511 kg). Note the bombs in the internal bay and rear guns.

In November 1944, an unexplained fire broke out in the Harper Dry Lake hangar that housed the D-2. Both the plane and the hangar were completely destroyed. This mattered little to Hughes; he had moved on to building the XF-11 prototypes, which had changed considerably from the original D-5 design. Hughes claimed that he should be reimbursed for the $3.65 million he had spent on the D-2. He felt the D-2 served as the prototype for the F-11—although they only shared the same basic configuration. The AAF countered that the aircraft were significantly different. Ultimately, Hughes and the AAF came to an agreement in which HAC recovered $1,906,826.13 for the cost of the D-2. The F-11 contract was later cut to just two prototypes. Hughes was nearly killed in the crash of the first XF-11 prototype during its first flight on 7 July 1946.

The D-2’s ultimate design configuration and purpose seemed to change at the whim of Hughes. The fact that the military applied so many designations and roles indicates that they struggled to find a niche for the aircraft. Since it was designed to Hughes’ own specifications and not to the needs and wants of the military, the D-2 was of little use to the AAF unless it underwent extensive redesign. Rather than being the high-performance aircraft Hughes had envisioned, the D-2 exhibited unsuitable control issues. Some believe Hughes had the hangar and the D-2 burned to rid himself of the failed aircraft and avoid having to admit it was a failure.

The Hughes XF-11 was of all-metal construction and much larger than the D-2 and the D-5 design. The aircraft was powered by Pratt & Whitney R-4360 engines, had a wingspan of 101 ft 4 in (30.9 m), a length of 65 ft 5 in (19.9 m), a top speed of 450 mph (725 km/h) at 33,000 ft (10,058 m), a service ceiling of 42,000 ft (12,802 m), and a gross weight of 58,315 lb (26,451 kg). The first XF-11 prototype can be identified by its contra-rotating propellers. It was a reversal of the right rear propeller that caused Hughes to crash the aircraft. However, if Hughes had adhered to the AAF’s test flight guidelines, the accident would not have occurred.

Sources:
– McDonnell Douglas Aircraft since 1920: Volume II by René J. Francillon (1990)
– “A Visionary Ahead of His Time: Howard Hughes and the U.S. Air Force—Part I” by Thomas Wildenberg, Air Power History (Fall 2007)
– “A Visionary Ahead of His Time: Howard Hughes and the U.S. Air Force—Part II” by Thomas Wildenberg, Air Power History (Spring 2008)
– Tornado: Wright Aero’s Last Liquid-Cooled Piston Engine by Kimble D. McCutcheon (2001)
– U.S. Experimental & Prototype Aircraft Projects: Fighters 1939–1945 by Bill Norton (2008)
– Howard Hughes: An Airman, His Aircraft, and His Great Flights by Thomas Wildenberg and R.E.G. Davies (2006)
– World’s Fastest Four-Engined Piston-Powered Aircraft by Mike Machat (2011)
– American Combat Planes of the 20th Century by Ray Wagner (2004)
– http://www.secretprojects.co.uk/forum/index.php/topic,5974.0.html
– http://www.joebaugher.com/usaf_fighters/p73.html

By William Pearce

In 1944, Yakovlev sought to achieve higher performance from its Yak-3 fighter by installing a Klimov VK-108* engine. The standard Yak-3 was originally designated Yak-1M and designed in 1942 as a lightweight Yak-1. When this new aircraft entered production in 1943, it was redesignated Yak-3, taking the designation used for an earlier fighter prototype (the I-30) that did not enter production. Thus, Yak-3 production followed that of the Yak-7 and Yak-9 fighters.

The Yak-3 VK-108 built in the closing days of World War II was the fastest Soviet piston-powered aircraft. Note the heat-resistant panels behind the exhaust stacks and the inlet for the supercharger under the engine. The upper row of exhaust stacks can just be seen.

The Yak-3 was a maneuverable fighter that incorporated everything the Yakovlev team had learned by producing its previous fighter aircraft. The fuselage was of metal construction and covered by duralumin from the cockpit forward, with plywood covering the rear fuselage. The aircraft’s wings had duralumin spars and wooden ribs and stringers. The wings were skinned with plywood that was covered with doped fabric. The Yak-3’s control surfaces consisted of a duralumin frame covered with fabric. The standard Yak 3 was powered by a VK-105PF2 engine producing 1,290 hp (962 kW) for takeoff and 1,240 hp (925 kW) at 6,890 ft (2,100 m) altitude.

The VK-105 engine can trace its origin back to the 750 hp (559 kW) M-100 engine of 1935, which was a licensed-built Hispano-Suiza 12Ybrs. However, many changes had been implemented by the time VK-105 production began in 1939. For example, the M-100 had a single-stage, single-speed supercharger, a 5.91 in (150 mm) bore, and two valves per cylinder. The VK-105 had a single-stage, two-speed supercharger, a 5.83 in (148 mm) bore, and three valves per cylinder.

Constructed under the supervision of lead designer Vladimir Klimov, the VK-105 was a liquid-cooled, V-12 engine with provisions for firing a cannon through the propeller hub. With its 5.83 in (148 mm) bore and 6.69 in (170 mm) stroke, the engine had a total displacement of 2,142 cu in (35.1 L). Each cylinder bank had a single overhead camshaft that actuated the two intake valves and single exhaust valve. The engine’s intake and exhaust ports were located on the outer sides of the engine. The intake manifold for each cylinder bank incorporated three carburetors. The VK-105 had a compression ratio of 7.1 to 1.

This three-view drawing of the Yak-3 VK-108 shows that even with the cockpit moved aft 15.75 in (.40 m), the aircraft was very similar to a standard VK-105-powered Yak-3.

The ideal standard production Yak-3 had a top speed of 404 mph (650 km/h) at 14,108 ft (4,300 m) and 354 mph (570 km/h) at sea level. The aircraft could climb to 16,404 ft (5,000 m) in 4.2 minutes—averaging 3,906 fpm (19.8 m/s). The Yak-3 had an empty weight of 4,641 lb (2,105 kg) and a loaded weight of 5,864 lb (2,660 kg). The aircraft had a 20 mm cannon that fired through the propeller hub and two 12.7 mm machine guns mounted above the engine (the post-war Yak-3P was fitted with three 20 mm cannons).

It was on the standard Yak-3 platform that a VK-108 engine was substituted to create an aircraft of much higher performance. The VK-108 engine was a further evolution of the basic M-100 design, but by this time in its design history, the VK-108 had little in common with the original M-100 engine. The VK-108 was closely related to the VK-107 engine from which it was directly derived.

With the exception of the VK-107 engine, the VK-108 differed from previous Klimov engines by having new induction and exhaust systems, a new valve train with two intake and two exhaust valves per cylinder, strengthened components for increased rpm and power, an improved gear reduction, and increased boost via a redesigned supercharger drive. The VK-108 retained the 5.83 in (148 mm) bore, 6.69 in (170 mm) stroke, and 2,142 cu in (35.1 L) total displacement of previous Klimov engines.

Pictures of a Klimov VK-108 engine are hard to come by. Seen here is a VK-107A engine which was very similar to the VK-108. Note the silver induction manifold along the outer side of the engine. The yellow linkages are for the three carburetors. The remaining four intake runners provide air only into the cylinders. The exhaust manifolds in the Vee of the engine are missing; the VK-107 used six-into-one manifolds while the VK-108 used individual exhaust stacks. (Mike1979 Russia image via Wikimedia Commons)

The most unusual features of the VK-108 engine were its intake and exhaust systems and the function of its four valves per cylinder. Pressurized air from the single-stage, two-speed supercharger flowed through an intake manifold located on the outer side of each cylinder bank. Each intake manifold had seven intake runners that led to the cylinder head. Four of the runners provided air only; the remaining three runners had individual carburetors to supply the air/fuel mixture to the cylinders. The first runner supplied just air to the first cylinder. The second runner provided the air/fuel mixture to the first and second cylinders. The third runner supplied just air to the second and third cylinders. The fourth runner provided the air/fuel mixture to the third and fourth cylinders. The fifth runner supplied just air to the fourth and fifth cylinders. The sixth runner provided the air/fuel mixture to the fifth and sixth cylinders. The seventh runner supplied just air to the sixth cylinder. So each carburetor supplied air/fuel for two cylinders; the engine had a total of six carburetors.

The two intake valves were positioned in tandem on the cylinder’s centerline. One intake valve in each cylinder opened to let supercharged air into the cylinder while the other intake valve opened to bring in the air/fuel mixture from a carburetor. The air intake valve opened 65 degrees before and closed after the air/fuel intake valve. This allowed the supercharged air to scavenge the cylinder and also aid in its cooling.

One exhaust valve was positioned on the outer side of the engine, and the other exhaust valve was on the Vee side. This configuration meant that there were separate exhaust ports on each side of the cylinder head. The VK-108 engine had one row of six exhaust stacks on the outer side of the cylinder bank and one row of exhaust stacks on the Vee side of the cylinder bank. The complete engine had four rows of six exhaust stacks.

Basic drawing of the Klimov VK-108 valve arrangement. A single overhead camshaft acted directly on the intake valves and actuated the exhaust valves via a follower.

A single overhead camshaft was used with three lobes for each cylinder. The center lobe acted on a follower that actuated both exhaust valves. One of the other lobes actuated the fresh air valve, and the last lobe actuated the air/fuel mixture valve. This arrangement allowed for the completely different valve timing and duration of the two intake valves.

The VK-108 was cleared for up to 8.5 psi (0.6 bar) of boost and had a compression ratio of 6.75 to 1. The engine produced 1,850 hp (1,380 kW) at 3,200 rpm for takeoff, 1,650 hp (1,230 kW) at 4,921 ft (1,500 m), and 1500 hp (1,119 kW) at 14,764 ft (4,500 m). The VK-108 weighed 1,731 lb (785 kg).

The installation of the VK-108 engine necessitated some changes of the Yak-3 airframe. Built under the supervision of A. N. Kanookov, the Yak-3 VK-108 had a new radiator, oil cooler, and propeller installed. A new cowling was constructed to accommodate the two additional rows of exhaust stacks. Heat-resistant panels were added behind each of the four rows of exhaust stacks. The cowling also omitted the ports for the two machine guns which were deleted from the Yak-3 VK-108. The supercharger air inlet was relocated under the engine, and the aircraft’s ailerons were skinned in duralumin rather than fabric. Because of the heavier engine, the cockpit was moved 15.75 in (.40 m) aft to keep the aircraft’s center of gravity within limits.

The VK-108-powered Yak-3’s first flight was on 19 December 1944 with Viktor L. Rastorgooyev at the controls. The aircraft had a wingspan of 30 ft 2 in (9.2 m), a length of 28 ft 1 in (8.55 m), an empty weight of 5,251 lb (2,382 kg), and a loaded weight of 6,385 lb (2,896 kg). With no armament and a light fuel load, the Yak-3 VK-108 achieved a top speed of 463 mph (745 km/h) at 20,636 ft (6,290 m), making it the fastest piston-powered Soviet aircraft. The aircraft also exhibited a phenomenal climb rate, reaching 16,404 ft (5,000 m) in 3.5 minutes—averaging 4,687 fpm (23.8 m/s). The Yak-3 VK-108 had a service ceiling of 34,121 ft (10,400 m).

The upper exhaust stacks are well illustrated in this rear view of the Yak-3 VK-108. The aircraft had excellent performance, but the engine was not reliable.

Although the Yak-3 VK-108’s performance was very good, fight testing the aircraft was difficult because of engine issues. The VK-108’s high rpm and boost resulted in constant overheating problems. Vibration issues and excessive smoke were also encountered. The problems were so severe that flight testing was halted on 8 March 1945, with the aircraft only accumulating 1 hour and 17 minutes of flight time.

A second VK-108-powered Yak-3 was built in late 1945 under the supervision of V. G. Grigor’yev. This aircraft was reportedly armed with a 20 mm cannon that fired through the propeller hub and an additional 20 mm cannon mounted above the engine and offset to the left—each gun had 120 rounds of ammunition. A new radiator with additional surface area was installed to prevent overheating issues. However, engine trouble persisted. Despite its excellent performance, the Yak-3 VK-108 project was abandoned in favor of more reliable piston aircraft and jets.

*The Soviet Union changed some aircraft engine designations from starting with an “M” to starting with the designer’s initials. In 1944, the M-105 engine became the VK-105; the M-107 became the VK-107; and the M-108 became the VK-108—VK standing for Vladimir Klimov, the engine’s lead designer. The VK designation was used throughout this article for simplicity.

By 1945, it was clear that future fighter aircraft would be jet-powered, and there was no need to continue the development of the VK-108 engine.

Sources:
– Yakovlev Fighters of World War II by Yefim Gordon, Sergey and Dmitriy Komissarov (2015)
– Russian Piston Aero Engines by Vladimir Kotelnikov (2005)
– Hispano Suiza in Aeronautics by Manuel Lage (2004)
– Yakovlev Aircraft since 1924 by Bill Gunston and Yefim Gordon (1997)
– Yakovlev Piston-Engined Fighters by Yefim Gordon and Dmitriy Khazanov (2002)
– http://www.airpages.ru/mt/m107_klimov.shtml

By William Pearce

Although not readily apparent at the time, Curtiss-Wright’s Airplane Division (Curtiss) was already in a state of decline at the start of World War II. The company’s final two truly successful aircraft, the P-40 Warhawk fighter and C-46 Commando transport, had already flown. While the Curtiss SB2C Helldiver carrier-based dive bomber would achieve some success toward the end of the war, its development was prolonged and plagued with issues, and the aircraft was never liked by its pilots and crews. Throughout the war years, Curtiss continually strove to develop world-beating aircraft but only managed to build one dead-end prototype after another. A brief glimmer of hope lay in the last model of the P-40, the P-40Q (Curtiss model 87X).

The Curtiss XP-40Q-1 (42-9987) with its standard canopy and sleek nose. Note the scoop for the engine air intake above the cowling.

XP-40Q development was initiated by 1943. The goal was to improve the P-40 to equal or surpass the performance of newer fighter aircraft. It was thought that the improved performance of the P-40Q would justify the aircraft entering production, and its similarities with P-40s then being produced would minimize tooling and production delays. In addition, there would be some part interchangeability with older P-40 aircraft, and current P-40 pilots and crews would be familiar with the new aircraft and its systems.

Three XP-40Q prototypes were built; their origins and histories have always been a point of disagreement between sources. All XP-40Qs were built up from other P-40 airframes. They all had only four .50-cal machine guns with 235 rpg. All of the XP-40Q aircraft were powered by two-stage supercharged Allison V-1710 engines and a four-blade propeller.

Another view of the XP-40Q-1. Note the radiators and oil coolers mounted in the wing center section.

The XP-40Q-1 was the first aircraft, and it was built in 1943 from a P-40K-10 (serial 42-9987) that had been damaged in a landing accident on 27 January 1943. The Q-1 was painted olive drab and had the standard P-40 wing and canopy. The nose of the aircraft was lengthened to accommodate the V-1710-101 (F27R) engine. At 3,200 rpm, the -101 engine produced 1,500 hp (1,119 kW) at 6,000 ft (1,829 m) and 1,325 hp (988 kW) for takeoff. The Q-1’s engine air intake was positioned above the cowling. The radiator and oil cooler were moved from the P-40’s iconic chin location to the wing center section, just below the fuselage (similar to the XP-40K). The XP-40Q-1 had a 37 ft 4 in (11.4 m) wingspan and was 35 ft 4 in long (10.8 m)—about 2 ft (.6 m) longer than a standard P-40.

The Q-1’s first flight reportedly occurred on 13 June 1943 from the Curtiss plant in Buffalo, New York. It is not clear if the aircraft suffered another accident, or if Curtiss was unhappy with its configuration and decided to modify it further. Regardless, by November 1943, the Q-1 had been modified and redesignated XP-40Q-2. The aircraft’s rear fuselage was cut down and a bubble canopy installed. Engine coolant radiators were positioned in the wings just outboard of the main gear. The oil cooler and engine air intake were relocated to the classic P-40 chin position, but the scoop was shallower and more elegant. The Q-2 retained the olive drab paint.

The Curtiss XP-40Q-2 (still 42-9987) after modification with a bubble canopy. The oil cooler and engine air intake have been relocated to the scoop under the engine. The coolant radiators have been moved outside of the main gear. The wings are still the standard P-40 wings, but they were later clipped by about one foot.

Still utilizing the -101 engine, the Q-2 was noted for having excellent visibility and handling. The aircraft had balanced controls and was very maneuverable, with a tight turn radius. Capt. Gustav Lundquist had evaluated the Q-2 and judged it to be the best P-40 he had flown; he recommended that further flight testing should be conducted. In December 1943, the Air Materiel Command recognized the XP-40Q-2’s performance and recommended that two additional prototypes be constructed.

Reportedly, the Q-2 was delivered to Eglin Field, Florida for testing in January 1944, but it was back at the Curtiss plant in Buffalo, New York in March for a series of flight tests. By this time, the Q-2 had its wingtips clipped about one foot each, and a V-1710-121 (F28R) engine was installed. The -121 produced 1,800 hp (1,342 kW) with water injection at 3,200 rpm up to 20,000 ft (6,096 m) and 1,425 hp (1,062 kW) for takeoff.

The XP-40Q-2A (42-45722) looking very much like the XP-40Q-2 but with clipped wings. This aircraft would change little throughout its existence.

A flight evaluation from April 1944 again noted the XP-40Q-2 as superior to all other P-40s and a very good aircraft overall. The XP-40Q-2 had a 35 ft 3 in (10.7 m) wingspan and was 35 ft 4 in (10.8 m) long. With full engine power at 3,000 rpm and water injection, the aircraft achieved 420 mph (676 km/h) at 15,000 ft (4,572 m) and had a maximum climb rate of 4,410 fpm (22.4 m/s) at 5,000 ft (1,524 m). At 3,200 rpm and with water injection, maximum speed was 422 mph (679 km/h) at 20,500 ft (6,248 m), and the climb rate increased by as much as 530 fpm (2.7 m/s) depending on altitude. However, the 3,200 rpm engine speed was only shown to offer an advantage between 12,000 and 33,000 ft (3,658 and 10,058 m). With just military power, the Q-2 recorded a speed of 407 mph (655 km/h) at 24,000 ft (7,315 m) and a climb rate of 3,210 fpm (16.3 m/s) at sea level. The aircraft could climb from sea level to 20,000 ft (6,096 m) in 4.8 minutes, 30,000 ft (9,144 m) in 8.9 minutes, and 39,000 ft (11,887 m) in 26.1 minutes. The Q-2’s service ceiling was 39,000 ft (11,887 m), and it had a gross weight of 9,000 lb (4,082 kg). The aircraft’s range was 700 miles (1,127 km).

The Q-2 was damaged when it nosed over after a test flight on 24 March 1944. The aircraft was repaired and then sent to Wright Field, Ohio in mid-1944. The aircraft was damaged again when it ground looped while landing on 31 July 1944. It is not clear if the aircraft was repaired or if the damage was too severe.

This image of the XP-40Q-2A illustrates the clipped wings. Note the size of the bubble canopy and how to could be a bit smaller. The four .50-cal wing guns are easily seen. The XP-40Q was definitely a nice looking aircraft.

The next aircraft was the XP-40Q-2A. It was built from the initial P-40K-1 (serial 42-45722) that had been converted to the (unofficial) XP-40N. During the XP-40N conversion, the aircraft had a bubble canopy installed. This modification predated and served as the template for the bubble canopy that was installed on the Q-2.

The Q-2A was very similar to the final configuration of the Q-2—with a bubble canopy, clipped wings, and -121 engine. However, some modifications to the cockpit and canopy were made, and automatic radiator and oil cooler shutters were added. The Q-2A had a natural metal finish.

The Q-2A’s first flight occurred prior to the end of March 1944. The aircraft was plagued with engine trouble that resulted in a number of forced landings. The Q-2A spent most of its test time down for repairs. As a result, the Army Air Force (AAF) focused on the next aircraft, the Q-3, and loaned the Q-2A to Allison for engine tests. The Q-2A most likely had the same specifications and performance as the -121-powered Q-2.

The XP-40Q-3 was the last aircraft in the series. The Q-3 was built in early 1944 from a P-40N-25 (serial 43-24571) and was the only XP-40Q actually classified as such by the AAF. The aircraft was very similar to the XP-40Q-2A except for some refinements to the canopy and windscreen. The canopy was a bit smaller, and the flat windscreen was longer and more angled than the windscreen used on the preceding aircraft. Overall, the changes improved pilot visibility. The Q-3 had a -121 engine and a natural metal finish.

The last of the Curtiss P-40Qs: the XP-40Q-3 (43-24571). This aircraft later had anti-glare paint applied to the upper cowling, its serial number painted on the the tail, and “12” painted on the chin scoop. Note the radiator air inlets in the wings.

Delivered to AAF in April 1944, the Q-3 suffered an engine failure during an early test flight. The aircraft was moderately damaged in the subsequent forced landing. At this time, other aircraft with superior performance were available, and there was no AAF interest in repairing the Q-3 because there was no need for a P-40Q. It is doubtful that much performance testing was conducted on the Q-3, but the results should have been similar to those of the Q-2.

In March 1946, Allison still had the XP-40Q-2A (the second XP-40Q) when the AAF declared the aircraft as surplus. It is not clear if Allison purchased the aircraft and then later resold it or if it was sold as surplus directly from the AAF. Regardless, Joe Ziegler acquired the aircraft, and it was registered as NX300B. Given race number 82, the Q-2A was entered in the 1947 Thompson Trophy Race (run on 1 September 1947), but it did not qualify. Ziegler started the race anyway and was running in fourth place when the engine caught fire after just completing the 13th lap. Ziegler pulled up and off the course and bailed out of the Q-2A. Zeigler suffered a broken leg, and the Q-2A was destroyed.

This view of the XP-40Q-3 illustrates the revised canopy compared to the XP-40Q-2A. Note the oil cooler exit doors on the cowling just in front of the wing.

The story of the XP-40Q aircraft is a confusing one involving only three airframes but somewhere around eight designations and a number of different configurations. The P-40Q was one of the finest fighters Curtiss ever built, but the aircraft was two years or so too late. Its performance and capabilities were matched or exceeded by other aircraft already in service. Even if the P-40Q airframe had been ready two years earlier, the two-stage Allison engines would not have been ready, as they were still having developmental trouble in 1944. Sadly, the XP-40Q scenario was played out again and again as Curtiss tried to create another successful aircraft but only managed to produce aircraft that were ill-timed and outclassed.

Note: There is no indication that any of the XP-40Q aircraft used any type of a laminar flow wing. There is also no indication that any XP-40Q information was passed from Curtiss to North American Aviation (NAA) during the NA-73X’s (P-51’s) development. Not only are the two aircraft different in almost every way, there is no part of their separate developmental timelines that coincide. NAA did purchase some information from Curtiss at the request of the British government, but that information pertained to the XP-46 and arrived after the NA-73X was already designed.

The XP-40Q-2A seen at Cleveland, Ohio for the Thompson Trophy Race in 1947. Other than some paint, including its registration and race number, the aircraft had changed little since its AAF days. It is truly unfortunate that the aircraft would soon be destroyed as a result of an engine fire.

Sources:
– Curtiss Fighter Aircraft by Francis H. Dean and Dan Hagedorn (2007)
– Vee is for Victory by Dan D. Whitney (1998)
– U.S. Experimental & Prototype Aircraft Projects: Fighters 1939-1945 by Bill Norton (2008)
– Memorandum Report on P-40Q Airplane, AAF No. 42-9987 by Capt. Gustav E. Lundquist (2 November 1943) via www.wwiiaircraftperformance.org (1.1 MB)
– Flight Tests on the Curtiss XP-40Q-2, AAF No. 42-9987 by Lt Norman A. Krause (5 April 1944) via www.wwiiaircraftperformance.org (10.9 MB)