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Republic XF-84H in flight. (USAF Museum)

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That's the sound of the prop-driven XF-84H, and it brought grown men to their knees. It didn't fly all that great either

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IT WAS THE ERA OF SOUNDED-LIKE-A-GOOD-IDEA-AT-THE-TIME DESIGNS. Airplanes that took off straight up, hanging from enormous contra-rotating props or climbing a beanstalk of jet thrust. Jets launched from flatbed trucks, flung into the air by rockets. Inflatable airplanes. Flying wings. Tail-less deltas. Jet seaplanes. Jet seaplane fighters. So there was nothing unusual about taking an early jet fighter, the Republic F-84 Thunderjet, and putting a propeller on it.

From This Story

But wasn’t aviation trying to get rid of propellers?

Never mind, we’re going to drive this propeller with an enormous turbine engine—two engines, in fact, coupled through a common gearbox—and we’ll spin it so fast that the prop tips will be traveling at 901 mph—Mach 1.18. At least the prop will be supersonic.

The result was the Republic XF-84H, a swept-wing, single-seat, T-tail turboprop that, at the time of its rollout in 1955, had the unhappy distinction of being the loudest airplane ever built.

The –84H had an otherwise honorable pedigree. The original straight-wing F-84 was named the Thunderjet to remind everyone that it was part of the Republic family that had begun with the World War II P-47 Thunderbolt. Among U.S. fighters, the F-84 was a first: Its slim, bud vase of a fuselage was wrapped around a slender axial-flow engine, in which the air’s path is a straight line from front to back. (The earlier—and chubbier—centrifugal-flow engines compressed the air by whirling it outward.) The F-84’s swept-wing follow-on, the F-84F, was tagged the Thunderstreak, which was followed by a reconnaissance version, the RF-84F, called the Thunderflash.

The XF-84H, however, was given an inglorious nickname by one of its test pilots: Thunderscreech.

“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.”

But there was a good reason to test the propeller: Early jets—the P-80, the F-84, even the vaunted F-86—were like overgeared vintage Ferraris. Put the thing in top gear and step on it and you may eventually do 150, but you’d be forever getting there. The jets accelerated with aching slowness, so when they were loaded for bear—a fighter’s natural state—they needed long runways. Short on concrete? Better leave some fuel and weapons home.

On landing, a turbojet pilot had to be very careful about speed control: Get a little too low and slow on final approach, cob the power to correct, and you might hit the ground before the engine wakes up and puts out enough thrust to accelerate.

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