In the late 1940s, the U.S. Navy came to Republic Aviation with an unusual request: to build an aircraft that could be launched from an aircraft carrier under its own power, without use of a catapult.
The Navy eventually lost interest, but the Air Force picked up the project, simply because such an aircraft would have to be extremely fast, and the Air Force has always had an interest in having the fastest planes around.
The result was the Republic XF-84H. Starting with the design for their existing jet-powered F-84F Thunderstreak, Republic removed the jet engine and replaced it with a turboprop so powerful it could drive the propeller blades at supersonic speed. This created a propeller plane that could fly at 520 miles per hour, making it one of the fastest propeller planes ever built.
It also created an enormous headache -- literally. The XF-84H's propeller blades weren't just CAPABLE of spinning at supersonic speed; they ALWAYS spun at supersonic speed, even if the engine was set to idle. This meant that the plane emitted a continuous, powerful sonic boom every second it was running, even if it was only sitting on the runway.
Running the engine of the XF-84H created a shock wave powerful enough to knock a man off his feet that could travel hundreds of yards, and made a loud enough noise that it could be heard 25 miles away. Ground crew required to service the aircraft developed nausea and headaches. One had a seizure.
The Air Force tried to mitigate the problem by moving testing of the XF-84H (which crews had now dubbed the "Thunderscreech") from the flight line at Edwards Air Force Base to Rogers Dry Lake, an enormous plot of empty desert nearby. But even there, the incredible noise and vibration the plane generated made it too difficult to work with.
The XF-84H project was terminated in 1956.
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