XF-84H’s Propeller Breaks Sound Barrier

The XF-84H aircraft’s propeller blades rotated so fast that they broke the sound barrier, even when the engine was idle and the plane was parked on the ground. This created shock waves audible up to 40 kilometers away, a phenomenon that made ground personnel sick before the aircraft even took off.

Design Goals of the XF-84H

Developed by Republic Aviation, the XF-84H aimed to combine the power of a turboprop engine with the speed of a jet. It was based on the F-84 Thunderstreak and used an Allison XT40-A-1 turboprop engine that delivered 5,850 horsepower to drive a large three-bladed propeller.

Propeller Speed and Sonic Bangs

The issue was not the aircraft’s cruise speed, which was average for the time, but the speed of the propeller’s tip, which reached Mach 1.18 continuously. This generated sonic bangs audible up to 40 kilometers, causing nausea and headaches in mechanics on the ground and earning the aircraft the nickname "Thunderscreech."

Test Flights and Technical Failures

Between July 1955 and October 1956, two prototypes of the XF-84H conducted only twelve test flights, accumulating less than six hours and 40 minutes of flight time. The program failed due to structural vibrations, frequent mechanical issues, and the need for a 30-minute preheating period before each flight.

Abandonment and Legacy

The US Air Force abandoned the project due to the unbearable noise, technical unreliability, and the aircraft’s unsuitability for combat. The XF-84H prototype 51-17059 is now on display at the National Museum of the United States Air Force, serving as a rare example of an aerospace experiment that pushed performance limits too far.