The aviation industry’s transition from propellers to jet engines saw the emergence of a new kind of engine called the turboprop. A turboprop engine is a turbine engine but instead of generating thrust from exhaust, the engine drives a propeller.
In 1955, the US Air Force developed an experimental aircraft called the XF-84H, manufactured by Republic Aviation. The purpose of the XF-84H was to determine whether it was possible for a fighter airplane to ditch the catapult and takeoff from a carrier on its own accord. A turboprop engine was chosen to power the aircraft, because such an engine uses big fans to move large volumes of air, which enables the aircraft to produce greater thrust at lower speeds. Bigger thrust means faster acceleration, which translates to shorter takeoffs.
The XF-84H was based on the well-known swept-winged F-84 Thunderstreak, but instead of a jet engine, the XF-84H was fitted with 5,850 hp Allison XT40-A-1 turboprop engine that turned three steel blades. With a sweep of 12 feet, the blades were so long that even at idling thrust the tip of the blades moved at supersonic speeds producing a continuous visible sonic boom that could be heard from 40 km away. The shockwaves were so powerful it could knock a full grown man down.
From Air&Space Magazine:
“One day, the crew took it out to an isolated test area [at Edwards Air Force Base in California] to run it up,” recalls Henry Beaird, a Republic test pilot at the time and one of only two men ever to fly the -84H. “They tied it down on a taxiway next to what they assumed was an empty C-47, but that airplane’s crew chief was inside, sweeping it out. Well, they cranked that -84H up, made about a 30-minute run, and shut it down. As they were getting ready to tow it back to the ramp, they heard this banging in the back of the C-47.” It was the crew chief, Beaird relates, knocked silly by the high-intensity noise and on his back on the floor of the –47, flailing his limbs. “He eventually came out of it,” Beaird recalls.
“As long as you stood ahead of or behind the airplane,” says Beaird, now 78 and flying Learjets, “it really wasn’t so bad, but if you got in the plane of the prop, it’d knock you down.” Really? “Really.”
The XF-84H’s horrendous noise earned the aircraft the nickname “Thunderscreech”.
The Thunderscreech’s engine ran at full speed all the time, and the propeller rotated at 2,100 rpm from startup until shutdown. Thrust was obtained and adjusted by changing the pitch of the blades. The response from the propellers was instantaneous. But the noise was terrible.
Edward von Wolffersdorff, Beaird’s crew chief, recalls:
I remember making my first ground runs with the thing, down on the main base, and I was wondering Why are they flashing that red light at me over on the control tower? It turned out they couldn’t hear a damn thing over their radios, so they kicked us out and sent us over to the north base.
Edwards feared that the shockwave from the propellers would break the windows in the control tower, located about a mile away from the runway. To prevent injury from a blown in window, whenever the XF-84H was flying, the traffic controllers would get under their desks with their radios and cover themselves with blankets.
“Nobody ever actually recorded the decibels,” recalls Beaird. “I think they were afraid the measuring device might get broken.”
Photo: US Air Force
The XF-84H was never well-liked at the Edwards Air Force Base, and it wasn’t solely for the noise. The XF-84H was an impractical machine that took half an hour just to warm up and be cleared for takeoff which made it clearly unsuited for combat. More than noise or delays, it was mechanical problems that led to its undoing.
The T40 turboprop engine was—in the words of the company’s own authorized history, Power of Excellence by William A. Schoneberger and Paul Sonnenburg—“a monstrosity, a mechanical nightmare.” The XF-84H suffered from vibrations that originated from the supersonic propellers and the powerful torque the engine produced. Lin Hendrix, one of the Republic test pilots assigned to the program, flew the aircraft once and refused to ever fly it again. “You aren’t big enough and there aren’t enough of you to get me in that thing again,” he told the formidable Republic project engineer.
The fearless Hank Beaird flew eleven times in that machine, and ten of those flights ended in forced landing. “By jingo, that airplane is going to hurt somebody!” Beaird once said after an emergency landing.
In the end, nobody wanted anything to do with the aircraft. First the Navy backed out and then the Air Force canceled the project after only two XF-84Hs was built with a total flight time of less than 10 hours between them.
The XF-84H was widely believed to be the fastest propeller-driven aircraft ever built, with an official top speed of 670 mph, as predicted by Republic, although neither of the two planes ever made it past 450 mph.