The experimental Republic XF-84H (s/n 51-17060) in flight in 1955/56. Two F-84Fs were converted into mixed powerplant experimental aircraft and only two pilots ever flew them, the first one quitting after just one flight. Of the twelve test flights flown, ten developed into full-blown emergencies. Each aircraft had been fitted with an Allison XT40-A-1 turboprop engine of 5,850 shaft horsepower (4,365 kW) driving a paddle-bladed prop in an ultimately ill-conceived attempt to see whether a prop could go close to supersonic and thus help solve some of the problems asscociated with early jets, principally their slow-to-accelerate requirement for very long runways. Ground crews dubbed the XF-84H the "Thunderscreech" due to the horrendous noise levels generated by prop blades spinning at Mach 1.71. Often described as the fastest prop aircraft of all time, this is manifestly incorrect since the Russian Tu-95 Bear turboprop bomber is much faster ...although probably not quieter.