Today in Aviation History: First flight of the Republic XP-72

Adam Estes
Adam Estes
Republic XP-72 43-36598 in front of the Republic Aviation factory in Farmingdale, New York, with a Grumman Widgeon parked behind the XP-72. (USAF Photo)
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VAN Today in Aviation History BannerOn February 2, 1944, a new prototype fighter, the Republic XP-72, made its first flight. Based off the P-47 Thunderbolt, the XP-72 was indeed to be an upgraded version of the Thunderbolt, but the end of WWII and the development of the jet engine would ensure that the Republic XP-72 would end up being a footnote in aviation history. The XP-72 was the result of two parallel programs inaugurated by Republic Aviation (formerly Seversky Aviation). These were the AP-18 and the AP-19. The AP-18 was to be the XP-69, which was intended to be powered by the experimental Wright R-2160 Tornado, a 42-cylinder, 7-cylinder per row, 6-row liquid-cooled inline engine with an output of 2,350 hp. However, it had been intended for the Tornado to have an output of 2,500 hp, and the development issues with the R-2160 slowed down work on the XP-69. Sensing that the XP-69 would be a dead end, Republic chose to cancel the XP-69 in favor of focusing on project AP-19, which would become the XP-72, with the US Army Air Force ordering two prototypes on June 18, 1943.
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3/4 scale mockup of the Republic XP-69, a project developed in parallel with the Republic XP-72, during wind tunnel testing at Langley Research Center, Virginia (NASA photo)
Informally known as the Super Thunderbolt, the Superbolt, or the Ultrabolt, the Republic XP-72 was in essence a P-47 Thunderbolt that had its Pratt & Whitney R-2800 Double Wasp twin row radial engine swapped for a Pratt & Whitney R-4360 Wasp Major four-row radial engine, one of the most powered American radial engines placed into production. The nose-mounted engine was fitted with tight cowlings and a fan to assist in cooling and was connected to a single-stage supercharger mounted behind the cockpit via a fluid coupling that ran under the cockpit. On the bottom of the fuselage, and air duct was fitted for the XP-72’s oil cooler, intercooler, and supercharger, with a duct in the right-wing root for cabin ventilation.
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Republic XP-72 43-36598 being rolled out of the Republic factory, January 29, 1944 (Old Machine Press blog)
The two prototypes were fitted with six Browning M2 .50 caliber machine guns in the wings, but proposals were also made for the XP-72 to be armed with two 37mm M4 cannons and two .50 caliber machine guns or four 37mm M4 cannons.
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Republic Aviation test pilot stands in the cockpit of the Republic XP-72 (Cradle of Aviation Museum)
On February 2, 1944, the first prototype, XP-72 43-36598, made its first flight from Republic Airport in Farmingdale, on Long Island, New York. This prototype was fitted with a four bladed propeller that was 14 feet in diameter. The next prototype, XP-72 43-36599, was first flown four months later, on June 26, 1944. This example was fitted with 13-foot, 6-inch diameter contra-rotating propellers (with three blades for each propeller) built by General Motors’ Aeroproducts Division. Flight testing of the two XP-72s revealed excellent performance, obtaining speeds of 490 mph at 25,000 feet, with a speed of 387 mph obtained at sea level, with a rate of climb of 5,280 ft/min. This led to the Army Air Force submitting an order for 100 production aircraft, which were to be flown with the contra-rotating propeller on the second prototype. However, at the same time the XP-72’s flight trials showed great potential, the Allied air forces had already gained air superiority over the German Luftwaffe in Europe and the Imperial Japanese Army Air Force and Japanese naval air arm in the Pacific using aircraft already in production, such as Republic’s own P-47 Thunderbolt, and so the need for new fighters was somewhat diminished, especially when the fighters that were need to sustain current operations were long range escort fighters to protect Allied bombers, not high-speed interceptors designed to destroy the ever-dwindling supply of Axis bombers. What’s more, the development of jet engines promised even greater performance than piston engines, which had reached the summit of their power output.
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Side profile view of the contra-rotating equipped XP-72, 43-36599, at Republic Airport. (Cradle of Aviation Museum)
The end of WWII would also see the end of the XP-72 program, and both prototypes would eventually be scrapped. Nevertheless, the Republic XP-72 represents the ultimate evolution of the P-47 family of aircraft, and the changing situation of American military aircraft procurement, which were sometimes defeated not by faults in their overall design, but by the changing nature of the air offensive during WWII that outran even the likes of the XP-72. Today in Aviation History is a series highlighting the achievements, innovations, and milestones that have shaped the skies. All the previous anniversaries are available HERE
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Raised in Fullerton, California, Adam has earned a Bachelor's degree in History and is now pursuing a Master's in the same field. Fascinated by aviation history from a young age, he has visited numerous air museums across the United States, including the National Air and Space Museum and the San Diego Air and Space Museum. He volunteers at the Planes of Fame Air Museum in Chino as a docent and researcher, gaining hands-on experience with aircraft maintenance. Known for his encyclopedic knowledge of aviation history, he is particularly interested in the stories of individual aircraft and their postwar journeys. Active in online aviation communities, he shares his work widely and seeks further opportunities in the field.
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