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Subject: Republic XF-84H
Date: 4 Mar 2018 07:58:05 -0800
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https://en.wikipedia.org/wiki/Republic_XF-84H
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 teething aerodynamic deficiencies, resulting in the program's
cancellation.
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
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 5,850 hp
(4,360 kW) 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 7,230 hp (5,390 kW), was installed but
never used. 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 flow 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 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 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.
Role
Experimental fighter
Manufacturer
Republic Aviation
First flight
22 July 1955
Primary user
United States Air Force
Number built
2
Developed from
Republic F-84F Thunderstreak
The XF-84H was quite possibly the loudest aircraft ever built (rivaled only by
the Russian Tupolev Tu-95 "Bear" bomber), earning the nickname "Thunderscreech"
as well as the "Mighty Ear Banger". On the ground "run ups", the prototypes
could reportedly be heard 25 miles (40 km) away. Unlike standard propellers that
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.
Specifications
General characteristics
Crew: 1
Length: 51 ft 5 in (15.67 m)
Wingspan: 33 ft 5 in (10.18 m)
Height: 15 ft 4 in (4.67 m)
Wing area: 30.75 m ()
Empty weight: 17,892 lb (8,132 kg)
Loaded weight: 27,046 lb (12,293 kg)
Performance
Maximum speed: 520 mph (837 km/h)
Range: >2,000 mi (3,200 km)
Service ceiling: >40,000 ft (14,600 m)
Rate of climb: 5,000 ft/min (1,520 m/min)
Thrust/weight: 0.66
*
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