Development and Purpose
In 1955, the U.S. Air Force commissioned Republic Aviation to develop the XF-84H prototype to test whether fighter jets could eliminate the need for catapults on aircraft carriers. The aircraft was powered by a turboprop engine, which used a large propeller to move a greater volume of air at lower speeds. This increased thrust allowed for faster acceleration and shorter takeoff distances.
The Infamous "Thunderscreech"
The XF-84H was based on the F-84 Thunderstreak swept-wing design, but instead of a jet engine, it featured an Allison XT40-A-1 turboprop with a 5,850-horsepower rating and three steel blades. The propeller, with a diameter of nearly 12 feet, rotated at 2,100 RPM even at idle, creating supersonic shock waves that could be heard up to 25 miles away.
The sheer volume of the sound earned the XF-84H the unflattering nickname "Thunderscreech." The aircraft's engines ran at full throttle throughout its operation, adjusting thrust by altering the propeller's pitch. While the response to propeller adjustments was instantaneous, the noise was unbearable.
Pilot and Air Traffic Control Hell
Test pilots were concerned that the shock waves from the propeller blades could shatter the windows of the control tower, located nearly a mile from the runway. To avoid shattered glass injuries, air traffic controllers would crawl under their desks, cover their radios with blankets, and lie down whenever the XF-84H was flying.
"No one ever actually measured the decibel level," recalled pilot Hank Beaird. "I think they were afraid it would blow the measuring device up."
Mechanical Woes and Unpopularity
Beyond its deafening noise, the XF-84H was a cumbersome aircraft. It took up to half an hour to start and go through pre-flight checks, making it impractical for combat situations. Furthermore, mechanical issues plagued the aircraft due to its unconventional design.
The T40 turboprop engine proved unreliable, plagued by vibrations caused by the supersonic propellers and the engine's immense torque. Republic Aviation test pilot Lin Hendrix flew the aircraft once but refused to fly it again.
Beaird flew the prototype 11 times, but only managed to complete one without an emergency landing. Eventually, no one wanted to be associated with the XF-84H.
Cancellation and Legacy
The U.S. Air Force canceled the project after only two XF-84H aircraft were produced, with a combined flight time of less than 10 hours. Despite being classified as the fastest turboprop-powered aircraft ever built, with a maximum speed of 670 mph, neither of the prototypes ever exceeded 450 mph.
Summary
The XF-84H Thunderscreech remains a unique and infamous chapter in aviation history. Its deafening noise, mechanical issues, and cumbersome operations made it a nightmare for pilots and air traffic controllers alike. While its ambitious goal of eliminating aircraft catapults was not achieved, the Thunderscreech's legacy serves as a reminder of the challenges of developing cutting-edge military technology.