The Republic XF-84H – nicknamed Thunderscreech – was a tremendously flawed prototype aircraft based on a Republic F-84F Thunderstreak. The aircraft was originally designed to fulfil a US Navy brief which called for a carrier fighter which was able to take off from the deck without the assistance of a catapult. However the Navy lost interest in the aircraft early on in the project and left Republic with the two finished prototypes. After the Navy dropped out of the aircraft’s development, Republic took the prototypes to Edwards Air Force Base to be used as pure research aircraft where they were used to test supersonic propellers and study propeller responsiveness at jet speeds.
Unlike the jet-powered Thunderstreak on which it was based, the Thunderscreech was powered by a massive three-bladed propeller. The powerplant was a 5,850 hp Allison XT40-A-1 turboprop engine which was centrally located behind the cockpit. A long extension shaft connected it to the nose-mounted propeller. The turbine engine also provided thrust through its exhaust, and an afterburner which could further increase power to 7,230 hp was installed but never used. Thrust was adjusted by changing the blade pitch of the 12 ft (3.7 m)-diameter propeller, consisting of three steel, square-tipped blades turning at a constant speed, with the tips traveling at approximately Mach 1.18.
The XF-84H Thunderscreech was destabilized by the powerful torque from the propeller, as well as inherent problems with supersonic propeller blades. In an attempt to counteract this, the aircraft was fitted with a T-tail to reduce turbulent airflow over the horizontal stabilizer and elevator surfaces. It also had independently controlled left and right flaps, and the left leading edge intake was mounted 12 inches (30 cm) further forward than the right.
Right from the start the Republic XF-84H Thunderscreech was beset with problems. The engine was extremely temperamental, although this wasn’t really Republic’s fault – the Allison T40 engine proved to be very problematic in other completely unrelated prototype aircraft of the time. However even when the engine was working it created its own problems…
The aircraft got its nickname for good reason, it was possibly the loudest aircraft ever built! 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 boom that radiated laterally from the propellers for hundreds of yards. During on the ground “run ups” it was reportedly heard up to 25 miles (40 kms) away. The noise was also hazardous for ground crew. An unfortunate crew chief who was inside a nearby C-47 cargo plane was severely incapacitated during a 30-minute ground run. While in a separate incident, a Republic engineer suffered a seizure after close range exposure to the shock waves of a nearby powered-up XF-84H. In fact the aircraft was so loud that it disrupted operations in the control tower, and after several complaints Republic were ordered to tow the aircraft out onto Rogers Dry Lake before powering up the engine.
In the air the XF-84H Thunderscreech also proved to be a complete pig. Lin Hendrix, one of the Republic test pilots assigned to the program, flew the aircraft once and refused to ever fly it again. The other test pilot for the aircraft, Hank Beaird, flew it 11 times, 10 of these ended with a forced landing. Problems encountered included engine failures, hydraulic and nose gear problems and vibration issues.
Despite these insurmountable problems, the aircraft did have one major success. It is listed by the Guinness book of records as the fastest propeller-driven aircraft ever built, with design top speed of 670 mph. However this figure is disputed, with the National Museum of the United States Air Force claiming that the Thunderscreech’s top speed was significantly lower at just 520 mph. Even so, it would still hold the title of second fastest propeller driven aircraft.
But speed alone wasn’t enough to save such a problematic prototype. And in 1956, just a year after it first flew, the project was cancelled. Of the two prototypes constructed, the first aircraft constructed (serial no. FS-059), still exists. It currently resides in Research and Development Gallery at the National Museum of the United States Air Force in Dayton, Ohio.