Republic XF-84H Thunderscreech


The Republic XF-84H "Thunderscreech" was an 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.

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, 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 cancelled 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.
The XF-84H was created by modifying a F-84F airframe, installing a Allison XT40-A-1 turboprop engine in a centrally-located housing behind the cockpit with a long extension shaft to the nose-mounted propeller. The turbine engine also provided thrust through its exhaust; an afterburner which could further increase power to was installed but never used. Thrust was adjusted by changing the blade pitch of the diameter Aeroproducts propeller, consisting of three steel, square-tipped blades turning at a constant speed, with the tips traveling at approximately. 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.
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 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 XA2D Skyshark and North American A2J 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.

Testing

After manufacture at Republic's Farmingdale, Long Island, plant, the two XF-84Hs were disassembled and shipped via rail to Edwards Air Force Base for flight testing. First flown on July 22, 1955, the XF-84F had incredible acceleration but soon its impracticality was discovered. It was unsuited to combat due to the engine's 30 minute warm up time but the most serious concerns were vibration generated from the 12-foot propeller diameter and mechanical failures of the prop pitch gearing. The prototypes flew a total of 12 test flights from Edwards, accumulating only 6 hours and 40 minutes of flight time. Lin Hendrix, one of the Republic test pilots assigned to the program, flew the aircraft once and refused to ever fly it again, claiming "it never flew over indicated, since at that speed, it developed an unhappy practice of 'snaking', apparently losing longitudinal stability". Hendrix also told the formidable Republic project engineer, "You aren't big enough and there aren't enough of you to get me in that thing again". The other test flights were fraught with engine failures, and persistent hydraulic, nose gear, and vibration problems. Test pilot Hank Beaird took the XF-84H up 11 times, with 10 of these flights ending in forced landings.

Noise

The XF-84H was almost certainly the loudest aircraft ever built, earning the nickname "Thunderscreech" as well as the "Mighty Ear Banger". On the ground "run ups", the prototypes could reportedly be heard away. Unlike standard propellers that turn at subsonic speeds, the outer of the blades on the XF-84H's propeller traveled faster than the speed of sound even at idle thrust, producing a continuous visible sonic boom that radiated laterally from the propellers for hundreds of yards. The shock wave was actually powerful enough to knock a man down; an unfortunate crew chief who was inside a nearby C-47 was severely incapacitated during a 30-minute ground run. Coupled with the already considerable noise from the subsonic aspect of the propeller and the T40's dual turbine sections, the aircraft was notorious for inducing severe nausea and headaches among ground crews. In one report, a Republic engineer suffered a seizure after close range exposure to the shock waves emanating from a powered-up XF-84H.
The pervasive noise also severely disrupted operations in the Edwards AFB control tower by risking vibration damage to sensitive components and forcing air traffic personnel to communicate with the XF-84H's crew on the flight line by light signals. After numerous complaints, the Air Force Flight Test Center directed Republic to tow the aircraft out on Rogers Dry Lake, far from the flight line, before running up its engine. The test program did not proceed further than the manufacturer's Phase I proving flights; consequently, no USAF test pilots flew the XF-84H. With the likelihood that the engine and equipment failures coupled with the inability to reach design speeds and subsequent instability experienced were insurmountable problems, the USAF cancelled the program in September 1956.

Historical significance

Although The Guinness Book of Records recorded the XF-84H as the fastest propeller-driven aircraft ever built, with a design top speed of and during tests, this claim has been disputed. The unofficial record speed is also inconsistent with data from the National Museum of the United States Air Force, which gives a top speed of , nonetheless, making the XF-84H the fastest single-engine propeller-driven aircraft until 1989 when "Rare Bear", a highly modified Grumman F8F Bearcat reached .

Operators

Aircraft disposition

Two prototypes were built, with buzz numbers FS-059 and FS-060.