| XF-84H "Thunderscreech" | |
|---|---|
 XF-84H serial number51-17060 in flight, with theram air turbine extended | |
| General information | |
| Type | Experimental fighter |
| National origin | United States |
| Manufacturer | Republic Aviation |
| Primary user | United States Air Force |
| Number built | 2 |
| History | |
| First flight | 22 July 1955 |
| Last flight | 9 October 1956 |
| Developed from | Republic F-84F Thunderstreak |
TheRepublic XF-84H "Thunderscreech" is an American experimentalturboprop aircraft derived from theF-84F Thunderstreak. Powered by aturbine engine that was mated to asupersonic propeller, the XF-84H had the potential of setting the unofficialair speed record forpropeller-driven aircraft, but was unable to overcome aerodynamic deficiencies and engine reliability problems, resulting in the program's cancellation.[1] Its name, Thunderscreech, is a reference to its extremely loud supersonic propeller.
Although the USAFWright Air Development Center was the key sponsor of the Republic Project 3347 turboprop fighter, the initial inception came from aU.S. Navy requirement for a carrier fighter not requiring catapult assistance.[2] Originally known asXF-106 (a designation later reused for theConvair F-106),[3] the project and its resultant prototype aircraft were redesignated XF-84H,[4] closely identifying the program as an F-84 variant, rather than an entirely new type.[5] 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 atWright-Patterson AFB to test supersonic propellers in exploring the combination of propeller responsiveness at jet speeds.[6]
The XF-84H was created by modifying aF-84F Thunderstreak airframe, installing a 5,850 hp (4,360 kW)XT40-A-1turboprop engine[7] in a centrally located housing behind the cockpit, driving an 18 ft (5.5 m) extension shaft to the nose-mounted propeller.[8] The turbine engine also provided thrust through its exhaust; anafterburner was installed, but it was only lit on a test stand and was never used in flight.[9] Thrust was adjusted by changing theblade pitch of the 12 ft (3.7 m) diameterAeroproducts 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.[10] The tail was changed to aT-tail to avoid turbulent airflow over the horizontal stabilizer/elevator surfaces from propeller wash.[11]
The XF-84H was destabilized by the powerfultorque from the propeller, as well as inherent problems with supersonic propeller blades.[12] A number of exotic blade configurations were tested before settling on a final design.[10] 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.[8] The two prototypes were equally plagued with engine-related problems affecting other aircraft fitted with T40 engines, such as theDouglas A2D Skyshark andNorth 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/extendableram 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.[10]
After manufacture at Republic's plant in Farmingdale, Long Island, the two XF-84Hs were disassembled and shipped via rail toEdwards Air Force Base for flight testing.[2] First flown on July 22, 1955, the XF-84H 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 diameter propeller and mechanical failures of the prop pitch gearing.[13] 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 450 knots (830 km/h) indicated, since at that speed, it developed an unhappy practice of 'snaking', apparently losing longitudinal stability".[14] 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".[13] The other test flights were fraught with engine failures, and persistent hydraulic, nose gear, and vibration problems.[2] Test pilot Hank Beaird took the XF-84H up 11 times, with 10 of these flights ending inforced landings.[15]
The XF-84H was almost certainly the loudest aircraft ever built, earning the nickname "Thunderscreech" as well as the "Mighty Ear Banger".[16] On the ground "run ups", the prototypes could reportedly be heard 25 miles (40 km) away.[17] Unlike standard propellers that turn at subsonic speeds, the outer 24–30 inches (61–76 cm) of the blades on the XF-84H's propeller traveled faster than the speed of sound even at idle thrust, producing a continuous visiblesonic boom that radiated laterally from the propellers for hundreds of yards. Theshock 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.[17] 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.[11] In one report, a Republic engineer suffered a seizure after close range exposure to the shock waves emanating from a powered-up XF-84H.[18]
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 bylight signals. After numerous complaints, theAir Force Flight Test Center directed Republic to tow the aircraft out onRogers Dry Lake, far from the flight line, before running up its engine.[14] 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 canceled the program in September 1956.[19]
AlthoughThe Guinness Book of World Records recorded the XF-84H as the fastest propeller-driven aircraft ever built,[20][failed verification] with a design top speed of 670 mph (1,080 km/h) (Mach 0.9) and 623 mph (1,003 km/h) (Mach 0.83) during tests, this claim has been disputed.[14]
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 520 mph (840 km/h) (Mach 0.70), nonetheless, making the XF-84H the fastestsingle-engine propeller-driven aircraft[11] until 1989 when "Rare Bear", a highly modifiedGrumman F8F Bearcat, reached 528 mph (850 km/h) (Mach 0.71).[21]
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Two prototypes were built (51-17059 and 51-17060), with buzz numbersFS-059 andFS-060.[22]
General characteristics
Performance
Related development
Aircraft of comparable role, configuration, and era
Related lists
