XF-84H Thunderscreech
Encyclopedia
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 cancellation of the program.
The XF-84H was created by modifying a F-84F
airframe, installing a 5850 hp Allison
XT40-A-1 turboprop
engine in an admidship housing behind the cockpit with a long extension shaft to the nose-mounted propeller. The turbine engine also provided thrust through its exhaust; in addition, its afterburner which could increase power to 7,230 hp (5,391 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. 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 (30.5 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.
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. 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 indicated, since at that speed, it developed an unhappy practice of 'snaking', apparently losing longitudinal stability." 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 landing
s.
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 dual jet turbines, 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.
reached 528 mph (Mach 0.71).
FS-059 was retired and spent many years mounted on a pole outside Meadows Field Airport
, Bakersfield, California
, where its propeller turned by the use of an electric motor. In 1992, the gate guardian was taken to the 178th Fighter Wing of the Ohio
Air National Guard, whose volunteers spent over 3,000 hours returning the Thunderscreech to display condition. It is now on display in the National Museum of the United States Air Force
at Wright-Patterson Air Force Base
, near Dayton, Ohio
.
FS-060 made only four flights, and is assumed to have been scrapped when the project was cancelled in 1956. Its T40 engine was reportedly used to support the Douglas A2D Skyshark flight test program.
Turboprop
A turboprop engine is a type of turbine engine which drives an aircraft propeller using a reduction gear.The gas turbine is designed specifically for this application, with almost all of its output being used to drive the propeller...
aircraft derived from the F-84F Thunderstreak. Powered by a turbine
Turbine
A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and...
engine that was mated to a supersonic
Supersonic
Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...
propeller, the XF-84H had the potential of setting the unofficial air speed record
Air speed record
An air speed record is the highest airspeed attained by an aircraft of a particular class. The rules for all official aviation records are defined by Fédération Aéronautique Internationale , which also ratifies any claims. Speed records are divided into multiple classes with sub-divisions...
for propeller
Propeller (aircraft)
Aircraft propellers or airscrews convert rotary motion from piston engines or turboprops to provide propulsive force. They may be fixed or variable pitch. Early aircraft propellers were carved by hand from solid or laminated wood with later propellers being constructed from metal...
-driven aircraft, but was unable to overcome teething aerodynamic deficiencies, resulting in the cancellation of the program.
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 designated XF-106, the project and its resultant prototype aircraft were renamed 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
F-84F Thunderstreak
The Republic F-84F Thunderstreak was an American-built swept-wing turbojet fighter-bomber. While an evolutionary development of the straight-wing F-84 Thunderjet, the F-84F was a new design. The RF-84F Thunderflash was a photo reconnaissance version....
airframe, installing a 5850 hp Allison
Allison Engine Company
The Allison Engine Company was a U.S. aircraft engine manufacturer. In 1929, shortly after the death of James Allison, the company was purchased by the Fisher brothers. Fisher sold the company to General Motors, who owned it for most of its history...
XT40-A-1 turboprop
Turboprop
A turboprop engine is a type of turbine engine which drives an aircraft propeller using a reduction gear.The gas turbine is designed specifically for this application, with almost all of its output being used to drive the propeller...
engine in an admidship housing behind the cockpit with a long extension shaft to the nose-mounted propeller. The turbine engine also provided thrust through its exhaust; in addition, its afterburner which could increase power to 7,230 hp (5,391 kW), was installed but never used. Thrust was adjusted by changing the blade pitch
Controllable pitch propeller
A controllable pitch propeller or variable pitch propeller is a type of propeller with blades that can be rotated around their long axis to change their 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
Mach number
Mach number is the speed of an object moving through air, or any other fluid substance, divided by the speed of sound as it is in that substance for its particular physical conditions, including those of temperature and pressure...
1.18. To counter the propeller's torque and "P-factor
P-factor
P-factor, also known as asymmetric blade effect and asymmetric disc effect, is an aerodynamic phenomenon experienced by a moving propeller, that is responsible for asymmetrical relocation of the propeller's center of thrust when aircraft is at a high angle of attack.- Causes :When an aircraft is in...
", the XF-84H was fitted with a fixed dorsal yaw vane. The tail was changed to a T-tail
T-tail
thumb|right|Grob motor gliderA T-tail is an aircraft tail stabilizer configuration in which the horizontal surfaces are mounted to the top of the vertical stabilizer. Traditionally, the horizontal control surfaces are mounted to the fuselage at the base of the vertical stabilizer...
to avoid turbulent airflow flow over the horizontal stabilizer/elevator surfaces from propeller wash.
The XF-84H was destabilized by the powerful torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
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 (30.5 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
Ram air turbine
A ram air turbine is a small turbine that is connected to a hydraulic pump, or electrical generator, installed in an aircraft and used as a power source...
. 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
Edwards Air Force Base
Edwards Air Force Base is a United States Air Force base located on the border of Kern County, Los Angeles County, and San Bernardino County, California, in the Antelope Valley. It is southwest of the central business district of North Edwards, California and due east of Rosamond.It is named in...
for flight testing. 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 indicated, since at that speed, it developed an unhappy practice of 'snaking', apparently losing longitudinal stability." 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 landing
Forced landing
A forced landing is a landing by an aircraft made under factors outside the pilot's control, such as the failure of engines, systems, components or weather which makes continued flight impossible. For a full description of these, see article on Emergency landing...
s.
Noise
The XF-84H was quite possibly 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 25 miles (40 km) away. Unlike standard propellers that turn at subsonic speeds, the outer 24–30 inches 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 boomSonic boom
A sonic boom is the sound associated with the shock waves created by an object traveling through the air faster than the speed of sound. Sonic booms generate enormous amounts of sound energy, sounding much like an explosion...
that radiated laterally from the propellers for hundreds of yards. The shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...
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 dual jet turbines, 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
Aviation light signals
In the case of a radio failure or aircraft not equipped with a radio, air traffic control may use a signal lamp to direct the aircraft. The signal lamp has a focused bright beam and is capable of emitting three different colors: red, white and green. These colors may be flashed or steady, and have...
. After numerous complaints, the Air Force Flight Test Center
Air Force Flight Test Center
The Air Force Flight Test Center conducts research, development, test, and evaluation of aerospace systems from concept to deployment. It has test flown every aircraft in the U.S. Air Force's inventory since World War II...
directed Republic to tow the aircraft out on Rogers Dry Lake
Rogers Dry Lake
Rogers Dry Lake is an endorheic desert salt pan in the Mojave Desert of Kern County, California. The lake derives its name from the Anglicization from the Spanish name, Rodriguez Dry Lake. It is the central part of Edwards Air Force Base as its hard surface provides a natural extension to the...
, 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 670 mph (Mach 0.9) and 623 mph (Mach 0.83) 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 520 mph (Mach 0.70), nonetheless, making the XF-84H the fastest single-engine propeller-driven aircraft until 1989 when a highly modified F8F BearcatRare Bear
Rare Bear is a highly-modified Grumman F8F Bearcat that dominated the Reno Air Races for decades.-History:The Bearcat that became Rare Bear was a severely damaged wreck when discovered by Lyle Shelton in 1969. It had been abandoned next to a runway in Valparaiso, Indiana after it crashed there,...
reached 528 mph (Mach 0.71).
Operators
- United States Air ForceUnited States Air ForceThe United States Air Force is the aerial warfare service branch of the United States Armed Forces and one of the American uniformed services. Initially part of the United States Army, the USAF was formed as a separate branch of the military on September 18, 1947 under the National Security Act of...
Survivors
Two prototypes were built (51-17059 and 51-17060), with buzz numbers FS-059 and FS-060.FS-059 was retired and spent many years mounted on a pole outside Meadows Field Airport
Meadows Field Airport
Meadows Field is the primary airport serving the Greater Bakersfield metropolitan area, and one of two international airports in the San Joaquin Valley. Also known as Kern County Airport #1, it is located in Oildale, California, approximately northwest of Downtown Bakersfield. The airport is owned...
, Bakersfield, California
Bakersfield, California
Bakersfield is a city near the southern end of the San Joaquin Valley in Kern County, California. It is roughly equidistant between Fresno and Los Angeles, to the north and south respectively....
, where its propeller turned by the use of an electric motor. In 1992, the gate guardian was taken to the 178th Fighter Wing of the Ohio
Ohio
Ohio is a Midwestern state in the United States. The 34th largest state by area in the U.S.,it is the 7th‑most populous with over 11.5 million residents, containing several major American cities and seven metropolitan areas with populations of 500,000 or more.The state's capital is Columbus...
Air National Guard, whose volunteers spent over 3,000 hours returning the Thunderscreech to display condition. It is now on display in the National Museum of the United States Air Force
National Museum of the United States Air Force
The National Museum of the United States Air Force is the official museum of the United States Air Force located at Wright-Patterson Air Force Base northeast of Dayton, Ohio. The NMUSAF is the world's largest and oldest military aviation museum with more than 360 aircraft and missiles on display...
at Wright-Patterson Air Force Base
Wright-Patterson Air Force Base
Wright-Patterson Air Force Base is a United States Air Force base in Greene and Montgomery counties in the state of Ohio. It includes both Wright and Patterson Fields, which were originally Wilbur Wright Field and Fairfield Aviation General Supply Depot. Patterson Field is located approximately...
, near Dayton, Ohio
Dayton, Ohio
Dayton is the 6th largest city in the U.S. state of Ohio and the county seat of Montgomery County, the fifth most populous county in the state. The population was 141,527 at the 2010 census. The Dayton Metropolitan Statistical Area had a population of 841,502 in the 2010 census...
.
FS-060 made only four flights, and is assumed to have been scrapped when the project was cancelled in 1956. Its T40 engine was reportedly used to support the Douglas A2D Skyshark flight test program.