The Republic XF-84H Thunderscreech was supposed to solve one of aviation’s biggest postwar problems by combining the instant throttle response of a propeller aircraft with the speed of a jet fighter. Instead, it became one of the most infamous experimental aircraft ever built.
Powered by an enormous turboprop engine driving a propeller whose tips traveled faster than the speed of sound, the XF-84H generated relentless shockwaves that reportedly made ground crews faint, vomit, and take cover during engine runs, earning a reputation as the loudest aircraft ever flown.
But deafening noise was only part of the story. The aircraft suffered constant vibration, severe instability, repeated mechanical failures, and an astonishing record of emergency landings that convinced even experienced test pilots to refuse to fly it again. As jet technology rapidly improved, the ambitious concept that once promised to revolutionize carrier aviation quickly became obsolete. This is the incredible story of the Republic XF-84H Thunderscreech, the supersonic-prop fighter that tried to bridge the gap between propellers and jets, and the spectacular engineering gamble that became one of aviation’s greatest dead ends.
The Republic XF-84H “Thunderscreech” is an American experimental turboprop aircraft derived from the F-84F Thunderstreak. Powered by a turbine engine that was mated to a supersonic propeller, the XF-84H had the potential of setting the unofficial air speed record for propeller-driven aircraft, but was unable to overcome aerodynamic deficiencies and engine reliability problems, resulting in the program’s cancellation. Its name, Thunderscreech, is a reference to its extremely loud supersonic propeller.
Design and Development
Although the USAF Wright Air Development Center was the key sponsor of the Republic Project 3347 turboprop fighter, the initial inception came from a U.S. Navy requirement for a carrier fighter not requiring catapult assistance. Originally known as XF-106 (a designation later reused for the Convair F-106), the project and its resultant prototype aircraft were redesignated XF-84H, closely identifying the program as an F-84 variant, rather than an entirely new type. With a projected contract for three prototypes, when the US Navy canceled its order, ultimately, the remaining XF-84H prototypes became pure research aircraft built for the Air Force’s Propeller Laboratory at Wright-Patterson AFB to test supersonic propellers in exploring the combination of propeller responsiveness at jet speeds.
DAYTON, Ohio — Republic XF-84H at the National Museum of the United States Air Force. (U.S. Air Force photo)
The XF-84H was created by modifying a F-84F Thunderstreak airframe, installing a 5,850 hp (4,360 kW) XT40-A-1 turboprop engine in a centrally located housing behind the cockpit, driving an 18 ft (5.5 m) extension shaft to the nose-mounted propeller. The turbine engine also provided thrust through its exhaust; an afterburner was installed, but it was only lit on a test stand and was never used in flight.
Thrust was adjusted by changing the blade pitch of the 12 ft (3.7 m) diameter Aeroproducts propeller, consisting of three steel, square-tipped blades turning at a constant speed, with the tips traveling at approximately Mach 1.18 (1,446 km/h). To counter the propeller’s torque and “P-factor”, the XF-84H was fitted with a fixed dorsal yaw vane. The tail was changed to a T-tail to avoid turbulent airflow over the horizontal stabilizer/elevator surfaces from propeller wash.
Prototype 51-17059
The XF-84H was destabilized by the powerful torque from the propeller, as well as inherent problems with supersonic propeller blades. A number of exotic blade configurations were tested before settling on a final design. Various design features were intended to counteract the massive torque, including mounting the left leading edge intake 12 in (30 cm) further forward than the right, and providing left and right flaps with differential operation.
The two prototypes were equally plagued with engine-related problems affecting other aircraft fitted with T40 engines, such as the Douglas A2D Skyshark and North American XA2J Super Savage attack aircraft. A notable feature of the design was that the XF-84H was the first aircraft to carry a retractable/extendable ram air turbine. In the event of engine failure, it would automatically swing out into the airstream to provide hydraulic and electrical power.
Due to frequent engine problems, as a precaution, the unit was often deployed in flight.
Top Photo: The experimental Republic XF-84H (s/n 51-17060) in flight in 1955/56. Two F-84Fs were converted into experimental aircraft. Each was fitted with a Allison XT40-A-1 turboprop engine of 5,850 shaft horsepower (4,365 kW) driving a supersonic propeller. Ground crews dubbed the XF-84H the “Thunderscreech” due to its extreme noise level.
Sources: YouTube; Wikipedia