Monthly Archives: March 2018

Nakajima [Ha-54] (Ha-505) 36-Cylinder Aircraft Engine

By William Pearce

On 6 December 1917, Chikuhei Nakajima founded one of the first aircraft manufacturing companies in Japan. Originally known as the Japanese Aeroplane Research Institute (Nihon Hikoki Kenkyūsho), the company went through a few name changes and was reorganized as the Nakajima Aircraft Company, Ltd (Nakajima Hikōki KK) in December 1931, with Chikuhei Nakajima as its chairman. At the onset of World War II, Nakajima felt that the United States had vast industrial resources and that the attack on Pearl Harbor would have dire consequences for Japan. Nakajima believed that the only way to ensure a Japanese victory was for Japan to have a reliable method to consistently attack the US mainland. Such attacks would require the US military to divert offensive forces to defend and protect the US.

The 36-cylinder, 5,000 hp (3,728 kW) Nakajima [Ha-54] was an ambitious engine program. The engine was needed long before it would be ready. The project was cancelled to reallocate resources to more urgent needs.

In 1942, Nakajima made his thoughts known to the Imperial Japanese Army (IJA) and Imperial Japanese Navy (IJN), but neither service gave Nakajima much consideration or any support. Frustrated, Nakajima used his company’s own resources to design a large bomber capable of striking the US mainland from Japan. Nakajima designated his bomber design Project Z (or Z Airplane). After months of research, Nakajima again approached the IJA and IJN in April 1943 with his Project Z proposals. The IJA and IJN were more receptive, but there was no consensus between the services on the aircraft’s design, specifications, or mission. The IJA and IJN submitted an updated proposal to Nakajima in June 1943, and through the remainder of 1943, Nakajima worked to solidify the Project Z aircraft design.

In August 1943, Nakajima distributed his thesis, “Strategy for Ultimate Victory,” to IJA and IJN officials and to prominent politicians. The thesis outlined strategic use of Project Z bombers to attack industrial targets in the US. The bombers could fly from Japan to Germany and strike targets in the western US as well as industrial areas in the mid-western states. Once in Germany, the bombers would be rearmed and refueled to make the return trip to Japan, again striking the US. The attacks, combined with their effect on production, was predicted to halt the US advance against Japan and aid in the defense of Germany.

Sectional drawing of what is believed to be the final configuration of the [Ha-54] engine. A single-rotation propeller is depicted. Note the cooling fan at the front of the engine, the annular intake manifold at the middle of the engine, and the supercharger impeller at the rear of the engine. Induction manifolds are shown at the top of the drawing, and exhaust manifolds are at the bottom.

Although several different designs and configurations were investigated, the Project Z aircraft that entered development was a six-engine, long-range bomber of all metal construction. The proposed aircraft had a wingspan of 213.2 ft (64.98 m), a length of 147.6 ft (44.98 m), and a height of 28.8 ft (8.77 m). Project Z had an empty weight of 143,300 lb (65,000 kg) and a maximum weight of 352,739 lb (160,000 kg). The bomber’s top speed was 422 mph (679 km/h) at 32,808 ft (10,000 m). Project Z had a maximum range of 11,185 miles (18,000 km) and a ceiling of 49,213 ft (15,000 m).

Such a massive aircraft required very powerful engines; however, no engine in Japan had the power that Project Z required. To solve this issue, Nakajima decided to make a power plant that could support Project Z by coupling together two existing [Ha-44] engines. The [Ha-44] was the most powerful engine that Nakajima was developing, and the new combined engine for Project Z was designated [Ha-54].

The heritage of Nakajima’s radial engines lies in licenses the company acquired to produce the Bristol Jupiter, Pratt & Whitney R-1340 Wasp, Wright R-1820 Cyclone, and Gnome-Rhône 14M. Nakajima obtained the Jupiter license in 1925, the Wasp license in 1929, the Cyclone license in 1933, and the 14M license in 1937. While Jupiter engines were produced, the Wasp and Cyclone licenses were obtained to gain knowledge, and Nakajima did not produce those engines. Many Nakajima radial engines used a 146 mm bore, which directly corresponded to the Jupiter’s 5.75 in bore, and several Nakajima personnel spent a few months in the US being instructed by Wright as part of the license agreement. The general construction of Nakajima radials resembled a combination of Bristol, Pratt & Whitney, and Wright engines, and Nakajima engineers consistently incorporated their own thoughts and ideas into their engines.

This model of the [Ha-54] illustrates the significant structure proposed to mount the engine. Such a mounting system would make engine maintenance very difficult, with the entire engine needing to be pulled to change a cylinder on the rear engine section. Note the exhaust collector ring and its outlet, which would lead to a turbosupercharger.

When Nakajima started to develop their own engines in the late 1920s, their designation system consisted of their name and the engine type followed by the engine model letter. As an example, the designation “NAL” represented Nakajima Air-cooled L model. When Nakajima started to develop a new class of high-power engines in the early 1940s, the focus was on air-cooled engines. Subsequently, the letter “N” and the engine type were dropped from the engine designation system. A new letter was used to designate the next generation of engines, followed by the engine model letter. As an example, the designation “BH” stood for B generation H model.

Japanese companies, the IJA, and the IJN all had their own designations for engines. To eliminate confusion (or perhaps add more), a joint designation system was introduced in May 1943. Nakajima [Ha-44] was the joint designation for the engine that would be the basis for Project Z’s engines. The [Ha-44] carried the Nakajima designation BH, the IJA designation Ha-219, and the IJN designation NK11. The [Ha-44] was an experimental, 2,942 cu in (48.2 L), 18-cylinder, air-cooled, radial engine. Development of the [Ha-44] began around 1941, and the engine was based on the 14-cylinder Nakajima [Ha-34] (Nakajima NAL, IJA Ha-41/Ha-109, and IJN NK5). By mid-1943, the [Ha-44] was producing 2,400 hp (1,790 kW) at 2,700 rpm. Two [Ha-44] power sections would be connected to make up the [Ha-54] engine.

This drawing depicts the cooling air flow of the [Ha-54] engine. This was the proposed configuration, but sufficient cooling for the rear cylinders was never achieved. Note the cooling fan at the front of the engine and the intake manifold ring at the center of the engine.

The [Ha-54] engine carried the Nakajima designation D-BH, for Double-BH. It was also designated Nakajima Ha-505 by the IJA but had no known IJN designation. Developed under chief designer Kiyoshi Tanaka, the engine was a 36-cylinder, air-cooled radial with two-stage supercharging. The [Ha-54] was comprised of two [Ha-44] engine sections bolted together via an intermediate section. The two engine sections formed a single four-row radial engine with nine-cylinders in each row. The crankshafts of the two engine sections were directly joined, and neither engine section could operate without the other.

The [Ha-54] used cylinders constructed of an aluminum head that was screwed onto a steel barrel. Each cylinder had two valves that were actuated by pushrods driven by cam rings. Exhaust gases flowed into a collector ring positioned behind the engine. Two ducts led from the collector ring, with each duct delivering the exhaust to a turbosupercharger located farther aft in the engine nacelle. The pressurized air from the turbosupercharger was fed through an intercooler and into a three-speed, mechanically-driven supercharger at the rear of the engine. The supercharger’s impeller was 19.69 in (500 mm) in diameter, and it was driven at 1.00, 4.77, or 5.88 times crankshaft speed. The fully-charged air from the supercharger was directed to an annular manifold at the center of the engine. Induction air for the front and rear engine sections was distributed from this central manifold. Because of this configuration, cylinders on the front engine section had rear-facing intake and exhaust ports, while the cylinders on the rear engine section had front facing intake ports and rear facing exhaust ports. Other methods to distribute air into the cylinders were investigated, including the use of two supercharger impellers at the rear of the engine, with each impeller dedicated to one engine section. In addition, some versions of the engine did not employ a turbosupercharger.

Nakajima tests indicated that cooling the [Ha-54] engine would be difficult. This model shows one of the final cooling configurations and matches the above drawing. Air flowed through the cylinders of the front engine section and exited at the center of the engine. Air that flowed over the front cylinders was directed down to flow through the cylinders of the rear engine section.

Some sources state a low pressure fuel injection system was planned for the [Ha-54] engine. Other Nakajima engines injected the fuel directly before the impeller at 14 psi (.97 bar). However, a different fuel injection point may have been selected to avoid the constant presence of a volatile air/fuel mixture in the annular induction ring at the center of the engine. Anti-detonation injection was also available for takeoff or emergency power.

Cooling the huge engine posed a serious issue, and much research was devoted to finding an adequate solution. Investigations were conducted to employ fan-assisted, forced-air cooling with an engine-driven fan positioned at either the front or rear of the engine. Another plan involved ram-air cooling for the front engine section and reverse cooling for the rear engine section. In this configuration, baffling separated the air flow from the front and rear engine sections. Air to cool the rear engine section was brought in via the wing’s leading edge, made a 180-degree turn, flowed through the cylinders of the rear engine section, and exited cowl flaps positioned at the middle of the engine.

A fully-cowled model of the [Ha-54] with ram-air (no fan) cooling for the front engine section and reverse cooling for the rear engine section. Note how the cooling air for both engine sections exited the cowl flaps at the center of the nacelle. Also, the engine depicted did not use a turbosupercharger and had nine exhaust stacks protruding from the cowling.

The final cooling configuration involved an engine-driven fan at the front of the cowling. Air was ducted through the cooling fins of the front engine section’s cylinders. The air then exited cowl flaps at the center of the engine cowling. Separately, air was ducted over the cylinders of the front engine section and through the cooling fins of the rear engine section’s cylinders. This air then exited via cowl flaps at the rear of the engine cowling.

At the front of the [Ha-54] engine, the crankshaft drove a propeller gear reduction and the cooling fan. The planetary gear reduction turned the propeller at .414 times crankshaft speed. Both single-rotation and contra-rotating propellers were considered, with the final decision in favor of six-blade (or possibly eight-blade) contra-rotating propellers that had a diameter of 14.8 ft (4.5 m). The [Ha-54] engine would be supported by a series of mounting rings that connected to the engine at its center and rear.

The Nakajima [Ha-54] had a 5.75 in bore (146 mm) and a 6.30 in (160 mm) stroke. The 36-cylinder engine had a 7.5 to 1 compression ratio and a total displacement of 5,885 cu in (96.4 L). The [Ha-54] had a maximum output of 5,000 hp (3,728 kW) at 2,800 rpm with 11.6 psi (.80 bar) of boost for takeoff and 4,600 hp (3,430 kW) at 2,800 rpm with 7.73 psi (.53 bar) of boost at 29,528 ft (9,000 m). The engine had a diameter of 61 in (1.55 m) and was 141 in (3.58 m) long. The [Ha-54] weighed approximately 5,400 lb (2,450 kg).

This drawing of the [Ha-54] shows some of the engine mount positions. Note that there are two supercharger impellers depicted. In this version of the engine, each impeller provided induction air to one engine section.

The question of cooling the [Ha-54] was never entirely resolved, and development of the engine was forecasted to take too long. By January 1944, work on the [Ha-54] had stopped, and it is unlikely that a complete engine was ever built. With the the cancellation of the 5,000 hp (3,728 kW) [Ha-54], a scaled-down version of the Project Z aircraft was designed and powered by [Ha-44] engines, which had been developed to 2,500 hp (1,864 kW). The new aircraft, designated Nakajima G10N Fugaku (Mount Fiji) was slightly smaller but still possessed the same, six-engine configuration as Project Z. By the summer of 1944, all of Japan’s resources were needed to protect the homeland, and the prospect of a long-range bomber attacking the US mainland was out of reach. The G10N Fugaku was cancelled, and all work stopped on its engines.

After World War II, the Nakajima Aircraft Company became the Fuji Industrial Company, Ltd. (Fuji Sangyo KK). In July 1953, the company merged with four others to form Fuji Heavy Industries, Ltd. (Fuji Jūkōgyō Kabushiki-gaisha). Fuji Heavy Industries manufactured Subaru automobiles and supplied equipment for the aerospace and transportation industries. In April 2017, Fuji Heavy Industries was renamed the Subaru Corporation.

The final version of the original Nakajima Project Z bomber with its six [Na-54] engines. Development of the [Na-54] was forecasted to take too long, so [Na-44] engines were substituted, and the aircraft was scaled-down as the Nakajima G10N Fugaku. As Japan’s prospects for an offensive victory faded, the project was cancelled, and resources were reallocated to defense.

Sources:
– Japanese Aero-Engines 1910–1945 by Mike Goodwin and Peter Starkings (2017)
– Japanese Secret Projects 1 by Edwin M. Dyer III (2009)
– “Nakajima Engine Design and Development” Air Technical Intelligence Group Report No. 45 by J. H. Morse (5 November 1945)
– “Nakajima Aircraft Company, LTD” The United States Strategic Bombing Survey, Corporation Report No. II (June 1947)
– https://www.secretprojects.co.uk/forum/index.php?topic=24084.0
– http://www.enginehistory.org/Piston/Japanese/japanese.shtml
– http://forum.valka.cz/topic/view/199974/Ha-505-Ha-54-01
– https://ja.wikipedia.org/wiki/%E5%AF%8C%E5%B6%BD

Republic XP-72 Super Thunderbolt / Ultrabolt Fighter

12 Replies

By William Pearce

In 1941, the Republic P-47 Thunderbolt had just entered production, and hundreds of the aircraft had been ordered. However, led by Alexander Kartveli, the design team at Republic stayed at the forefront of fighter development by incorporating new engines into new airframe designs. In July 1941, Republic submitted two new designs to the United States Army Air Force (AAF), the AP-18 and the AP-19. The AP-18 was a unique interceptor fighter powered by the Wright R-2160 Tornado engine. The AP-19 design was more conventional and was powered by the Pratt & Whitney (P&W) Wasp X (R-4360). Both engines were under development, but the R-2160 was anticipated first and received much interest from the AAF. As a result, the AP-18 design was ordered on December 1941 as the Republic XP-69.

The first Republic XP-72 prototype soon after being completed. The 14 ft 2 in (4.23 m) diameter Curtiss propeller was one of the largest used during World War II.

By 1943, the R-2160 engine had encountered major issues, but development of the R-4360 engine was steadily progressing. Republic felt the AP-19 design held more potential and wanted to end work on the XP-69. The AAF agreed, and the XP-69 project was cancelled on 11 May 1943. Two prototypes of the AP-19 design were ordered on 18 June 1943, and the aircraft was designated XP-72 (it also carried the “Materiel, Experimental” project designation MX-189). In addition, Republic felt the XP-72 was superior to the XP-47J, an interceptor derivative of the P-47, and asked that the second XP-47J prototype be cancelled. The AAF approved this request, and Republic focused on the XP-72.

The XP-72 was often called the Super Thunderbolt, or Superbolt, or Ultrabolt, and it benefitted from everything Republic had learned with the P-47 series, including the XP-47J. The XP-72 was essentially the wings, fuselage, and tail of a bubble-canopied P-47D combined with the close-fitting cowling used on the XP-47J. Of course, numerous internal changes made the XP-72’s resemblance to the P-47 a superficial oversimplification of the new aircraft’s design. Under the close-fitting cowling was a 28-cylinder P&W R-4360 engine. The engine drove a fan at the front of the cowling to assist cooling. A small cowl flap was positioned on each side of the cowling. The cowl flaps were automatic but could be manually controlled. At the cowl flaps, air exiting the cowling was combined with exhaust gases being expelled through ejector stacks and provided a small amount of thrust.

The Pratt & Whitney R-4360-13 and -19 engines had a remote, variable-speed, first-stage supercharger. This large supercharger was installed behind the XP-72’s cockpit and was connected to the engine via a fluid coupling.

The XP-72’s R-4360 engine used two-stage supercharging. The first stage was a mechanically-driven, variable-speed, remote supercharger positioned behind the cockpit, where the turbosupercharger was located on the P-47. To power the remote supercharger, a covered shaft extended from the unit, through the lower cockpit, and connected to the engine via a fluid coupling. The remote supercharger’s impeller was around 3 ft (.9 m) in diameter. The second stage was the standard supercharger that was integral with the engine.

A scoop positioned under the fuselage and in line with the wings leading edge split air into three ducts. The left and right ducts delivered air to oil coolers positioned on the bottom sides of the scoop. The outlet for each oil cooler was on the lower side of the scoop and about at the midpoint of its length. The larger, center duct fed air to the intake on the back of the remote supercharger and to the intercooler. The intercooler was positioned behind the remote supercharger. After being compressed in the supercharger, the air exited via two outlets and passed through the intercooler. After leaving the intercooler, the cooled induction air was split into two ducts and delivered to the R-4360’s downdraft intake, which is where the two ducts merged. The air then passed through the engine’s integral supercharger and into the engine’s cylinders. Cooling air that passed through the intercooler was discharged via an outlet in front of the tailwheel. No exhaust-driven turbosupercharger was installed on or planned for the XP-72 prototypes or the P-72 production aircraft.

This side view of the first XP-72 illustrates the aircraft’s resemblance to the P-47 Thunderbolt. The notch just before the tailwheel is the air outlet from the intercooler. The serial number painted on the tail should actually be “336598” to conform to AAF guidelines. Neither XP-72 had the “correct” serial number painted on their tails.

Some sources state the XP-72 had strengthened landing gear compared to the P-47, while other sources say it was the same landing gear used on the P-47. The wings incorporated six .50-cal machine guns (three in each wing) with 267 rpg. However, the gun package could be changed to four 37-mm cannons (two in each wing). A hardpoint under each wing could carry a 150-gallon (568 L) drop tank or up to a 1,000 lb (454 kg) bomb. Just like on the P-47, an inlet for cabin air was located on the leading edge of the right wing. Dive recovery flaps were fitted under the wings, just behind the main gear wells.

The XP-72 was roughly the same size and weight as the P-47D but was more aerodynamic and possessed about 50% more power. The XP-72 aircraft had a 40 ft 11 in (12.47 m) wingspan, was 36 ft 8 in (11.18 m) long, and was 16 ft (4.88 m) tall. The aircraft had an empty weight of 11,375 lb (5,160 kg), a normal weight of 14,760 lb (6,695 kg), and a maximum takeoff weight of 17,492 lb (7,934 kg). The XP-72 had a top speed of 490 mph (789 km/h) at 25,000 ft (7,620 m) and an initial rate of climb of 5,280 fpm (26.8 m/s), decreasing to 3,550 fpm (18.0 m/s) at 25,000 ft (7,620 m). The aircraft could reach 20,000 ft (6,096 m) in under five minutes. The XP-72’s service ceiling was 42,000 ft (12,802 m). With 370 gallons (1,401 L) of internal fuel and two 150-gallon (568 L) drop tanks, the aircraft had a range of 1,200 miles (1,931 km) at a 300 mph (483 km/h) cruise speed.

The XP-72 was a formidable aircraft with amazing performance. The scoop under the fuselage brought air to the oil coolers, intercooler, and supercharger. The duct in the wing was for cabin air. The close-fitting engine cowling was one of the best installations of an R-4360 and used an engine-driven fan to assist cooling.

The first XP-72 prototype (serial number 43-36598) was completed with a single-rotation propeller and a P&W R-4360-13 engine. The Curtiss Electric four-blade propeller was 14 ft 2 in (4.23 m) in diameter, which was one of the largest propellers used during World War II and was probably the largest propeller fitted to a fighter. The propeller left only 5 in (127 mm) of ground clearance, and the pilots employed three-point takeoffs and landings to make sure there were no propeller ground strikes. The R-4360-13 engine could accommodate the remote, variable-speed supercharger, but sources disagree regarding whether or not the remote supercharger was installed in the XP-72. The -13 engine produced 3,450 hp (2,573 kW) with the remote supercharger and 3,000 hp (2,237 kW) without it. The first XP-72 was finished on 29 January 1944. The aircraft’s first flight was made from Republic Field in Farmingdale, New York on 2 February 1944.

The second XP-72 prototype (serial number 43-36599) used a 13 ft 6 in (4.11 m) diameter, six-blade, contra-rotating Aeroproducts propeller. This propeller gave 9 in (229 mm) of ground clearance, and three-point takeoffs and landings were still the standard practice. Sources disagree on which dash number engine was used in the second prototype. Some sources claim a 3,000 hp (2,237 kW) R-4360-3 engine was used on the second XP-72. The -3 had a single-speed, single-stage (non-remote) supercharger and accommodated contra-rotating propellers, but the -3 engine used SAE #60-80 spline shafts. The Aeroproducts propeller used SAE #50-70 spline shafts, so it seems unlikely that the -3 engine was used. Many sources state the second XP-72 used a R-4360-13 engine, the same type fitted to the first prototype. The -13 engine was single-rotation with a SAE #60 spline shaft and could not accommodate contra-rotating propellers. However, it is possible that a contra-rotating gear case from another engine could have been fitted to the -13. The -8 (Douglas XTB2D) and -10 (Boeing XF8B) engines built for the Navy both used SAE #50-70 spline shafts. It is odd that another dash number was not assigned for such a change, but the -13 engine seems like the most likely candidate to have powered the second XP-72. Other sources propose that the engine was a R-4360-19 (see below), but there is no indication that any -19 engines were built.

With its six-blade, contra-rotating propellers, the second XP-72 is an impressive sight. Even with its hollow blades, the propeller still weighed around 765 lb (347 kg). Note the installed underwing pylons.

Regardless of the exact dash number, the second XP-72 was first flown on 26 June 1944. The contra-rotating propellers had a slight destabilizing effect on the aircraft, but the effect was manageable, and the aircraft still exhibited excellent flight characteristics. It is often reported that the second XP-72 was damaged beyond repair because of an emergency landing following an inflight fire. However, Ken Jernstedt, the pilot on that flight, has stated the incident never happened. In Jernstedt’s account, he was making a high-performance takeoff from Caldwell, New Jersey when an oil seal on the supercharger failed and caused a massive oil leak. Hot oil sprayed into the cockpit, on Jernstedt’s legs, and on the outside of the canopy. When Jernstedt brought the aircraft around for a quick landing, a Vought F4U Corsair suffering from an inflight fire crossed his path. Jernstedt had to veer around the Corsair to land the XP-72, which was damaged in the incident. While the aircraft could have been repaired, the XP-72 program ended soon after.

The Republic XP-72 was noted as exceptionally fast with amazing performance and for being a beautifully flying airplane. It is often reported that test pilot Carl Bellinger attained a speed of 480 mph (772 km/h) at sea level, but this speed was most likely recorded at altitude. Almost all sources indicate that both XP-72 prototypes achieved 490 mph (789 km/h) at 25,000 ft (7,620 m).

The second XP-72 shortly after an engine run. Note that the tail of the aircraft is tied down. The air outlet from the oil cooler is visible on the lower fuselage, just under the wing’s trailing edge. Another outlet was positioned in the same spot on the opposite side of the aircraft.

An order for 100 P-72 aircraft was placed in late 1944. Production P-72s were to be powered by the P&W R-4360-19 engine, which used the Aeroproducts contra-rotating propeller and had an engine-driven, variable-speed, remote supercharger similar to the one used on the -13. The -19 engine was planned to provide 3,650 hp (2,722 kW) at sea level and 3,000 hp (2,237 kW) at 25,000 ft (7,620 m), allowing the P-72 to attain an estimated speed of 504 mph (811 km/h) at 25,000 ft (7,620 m). Further engine development resulting in 4,000 hp (2,983 kW) would reportedly enable the P-72 to reach a speed of 540 mph (869 km/h) at 25,000 ft (7,620 m). The 540 mph (869 km/h) speed seems a little optimistic. An upgraded wing, similar to that used on the P-47N, was to be applied to production P-72s. The P-47N wing held more fuel and increased the aircraft’s span by about 2 ft (.61 m). The speeds mentioned above were most likely estimated with the original, smaller wing.

As excellent as the P-72 may have been, the war situation indicated the aircraft was not needed, and the emergence of jet aircraft indicated that the P-72’s speed would soon be outclassed. The order for 100 P-72 aircraft was cancelled on 4 January 1945 so that Republic could focus on the P-84 Thunderjet fighter. On the day the P-72 was cancelled, the AAF ordered 100 P-84 jet aircraft (25 pre-production YP-84As and 75 production P-84Bs). The two XP-72 aircraft survived the war but did not last much longer. One airframe, without its engine, was given to a local (on Long Island, NY) chapter of the Air Scouts in August 1946. The other airframe was eventually scrapped.

With the war winding down and jet aircraft on the horizon, the XP-72 never entered production, despite the aircraft’s impressive performance. Production P-72 aircraft could have been the ultimate piston-engine fighter.

Sources:
– Correspondence with Tom Fey
– U.S. Experimental and Prototype Aircraft Projects: Fighters 1939–1945 by Bill Norton (2008)
– R-4360: Pratt & Whitney’s Major Miracle by Graham White (2006)
– Republic’s P-47 Thunderbolt by Warren M. Bodie (1994)
– American Secret Projects: Fighters, Bombers, and Attack Aircraft, 1937-1945 by Tony Buttler and Alan Griffith (2015)
– US Army Air Force Fighters Part 2 by William Green and Gordon Swanborough (1978)
– http://www.seabee.info/seabee_history_racac.htm
– http://www.joebaugher.com/usaf_fighters/p72.html