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.
- By Stephan Wilkinson
- Air & Space magazine, July 2003
Edwards Air Force Base History Office
(Page 2 of 5)
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.
Propellers were different. On a powerful fighter like the P-51, you had to feed in power judiciously, because if you firewalled the throttle, the entire airplane tried to counterrotate against the prop’s torque. With a tractor propeller spinning clockwise (as seen from the cockpit), the airplane would turn hard left and plow straight off the runway. But compared to jets, propellers provided power right now.
The XF-84H was built for the Air Force’s Propeller Laboratory at Wright-Patterson Air Force Base in Dayton, Ohio. Engineers there wanted to test supersonic propellers to see if they could get the best of both worlds—jet speeds and propeller responsiveness. “That didn’t mean the airplane will run supersonic,” Beaird cautions, “because with that big a prop disc up front, it’s like a big speed brake. It meant that on the -84H, the outer 12 to 18 inches of the propeller were supersonic all the time.”
That, of course, was the source of the horrendous noise. The Thunderscreech’s engine ran at full speed all the time, and the propeller rotated at 2,100 rpm from startup until shutdown. “All you had to do was move the propeller pitch control to get power and you got it pretty instantaneously,” Beaird explains. He thinks it might have gotten even louder with power, because he remembers he could hear it better where he lived, 22 miles away from the base, when the crew ran up the engine to full power.
“Edwards was worried that the noise of the airplane would break the windows in the control tower,” he remembers. “The runway’s about a mile from the tower, but they’d put blankets over the top of the shelf where the radios were, and they’d get up under their desks, under the blankets. Nobody ever actually recorded the decibels. I think they were afraid the measuring device might get broken.”
“Oh, man, that noise was terrible,” recalls Edward von Wolffersdorff, Beaird’s crew chief. “You can’t imagine,” he adds with a groan. “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.”
Most accounts of the XF-84H program specify that the propeller spun at 3,000 rpm, which would have resulted in the prop tips traveling at an incredible Mach 1.71. Extensive research and computation by John M. Leonard of the Rolls-Royce Heritage Trust (Rolls Aerospace currently owns Allison) indicate that an engine turning at 14,300 rpm driving a 6.8:1 gearbox, as the T40 did, would push the tips of a 12-foot-diameter propeller to a far more logical Mach 1.18.