Super Props: The Fastest Piston Fighters Ever Built
1. North American P-51H Mustang
The P-51H looked like a standard Mustang and shared almost nothing with the D model. It had a longer and deeper fuselage, a different tail, smaller landing gear, significant weight reductions throughout the structure, and an entirely new wing with straight leading edges from root to tip.
The upgraded Packard Merlin V1650-9 with water injection produced over 2,200 horsepower. Top speed exceeded 470 mph with North American’s own calculations suggesting 490 mph at low weights. The P-51H entered production before the war ended but arrived too late for combat. It continued in service but was quickly overshadowed by jets.
2. Republic XP-47J Thunderbolt
The XP-47J was the fully optimized Thunderbolt. Republic took the familiar R-2800 engine, pushed it to 2,800 horsepower, lightened the airframe, and cleaned up the aerodynamics. The result became the first piston-engine fighter to exceed 500 mph in level flight, reaching approximately 505 mph at altitude.
The Air Force considered it a genuine performance success. They declined to pursue it anyway. The J model represented the absolute ceiling of R-2800 development with no realistic path to further improvement. The aerodynamic work was not wasted. Much of it transferred directly to the XP-72 program that followed.
3. Republic P-47H
The P-47H was a standard P-47D-15 fitted with an inverted liquid-cooled V16 built by Chrysler. Performance data is scarce and original test data appears to be non-existent. Secondary sources estimate a top speed around 490 mph though that figure is likely calculated rather than measured.
The aircraft flew but apparently never progressed far enough in development to generate meaningful test data, suggesting the engine had serious problems. The engine never entered production in any aircraft.
4. Republic XP-69
Republic unveiled the XP-69 design shortly after the P-47’s first flight in 1941. It was to be powered by a liquid-cooled 42-cylinder turbosupercharged radial engine, an unusual configuration that has almost no parallel in aviation history. Planned armament included four .50 caliber machine guns and twin 37mm cannons. It was designed around a new NACA wing profile and was to feature a bubble canopy.
The program was canceled and Republic’s records were destroyed in the 1980s, leaving almost no documentation of why. The most probable explanation is engine failure. The R-2160 liquid-cooled radial it was designed around never flew in any production aircraft and every other program designed around it also went nowhere.
5. Republic XP-72
The XP-72 was the most capable of the group. Powered by the Pratt and Whitney R-4360 Wasp Major, a four-row 28-cylinder engine displacing nearly 71.5 liters, it produced 3,450 horsepower with significant development potential beyond that figure. Republic borrowed the engine-driven cooling fan from the Focke-Wulf Fw 190 to keep the massive engine tightly cowled. A variable-speed rear-mounted mechanical supercharger similar to the system on the Bf 109 allowed the engine to maintain full manifold pressure across a wide altitude range.
Testing reached 490 mph with a standard four-bladed propeller. The second prototype used counter-rotating props with a projected top speed safely over 500 mph. Climb rate exceeded 5,200 feet per minute. Armament options included six .50 caliber machine guns, four .50s with dual 37mm cannons, or four 37mm cannons alone.
The Army Air Force ordered 100 aircraft after seeing the test flights. That order was canceled when it became clear the war would be won without them and jets were arriving.
