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 5 of 5)
Turboprops typically use their engine’s tailpipe simply as a vent for gases that have already done most of their work, though the exhaust flow usually produces residual thrust as well—almost 1,300 pounds’ worth, in the case of the Thunderscreech. The Navy wanted all the carrier-takeoff thrust it could get, so it had Allison fit the baby ’burner to the T40. The afterburner was lit only on the test stand, never in flight.
The -84H was also the first airplane to carry a “RAT”—a ram air turbine, which automatically deployed from a compartment in the dorsal fin and pinwheeled in the airstream to provide extra electrical and hydraulic power.
“The airplane had full-span ailerons whenever the gear was down, since the flaps became ailerons too,” Hank Beaird says. “It took a lot of [additional hydraulic] power to move those surfaces if you had to move them in a hurry, and the RAT provided that. It would come out whenever the gear was down. That was one of the airplane’s biggest contributions. We put that on other jets as well, particularly the F-105 [Thunderchief, the next in Republic’s series], which also had a full-span aileron system.”
Another XF-84H feature Beaird liked was its speed brakes, located all the way aft alongside the afterburner nozzle and opening to each side like flower petals. “Yeah, we learned a few things with that airplane,” he says. “We put the same speed brakes on the F-105, but bigger—a four-petal arrangement. They made little or no trim change but tremendous drag. On the -105, you could put those things out at 1.8, 1.9 Mach and you’d just be standing on the rudder pedals, it slowed down so fast.”
The two XF-84Hs flew less than 10 hours total. It may be the only U.S. Air Force aircraft that has never been flown by a military pilot. And to this day, nobody knows how fast a production F-84H would have gone. Republic made a wildly optimistic prediction of 670 mph, but neither of the two X-planes ever made it past 450 mph.
Still, at the time this was thought fast enough to make the XF-84H the fastest propeller-driven airplane in the world, a claim that can still occasionally be heard today. But in fact, that speed record was already held by the huge, four-engine, eight-propeller Soviet Tupolev Tu-95 Bear bomber, which, with its high cruise speed of 545 mph, remains by far the world’s fastest propeller-driven aircraft.
The Bear was already in service in 1955, when the XF-84H made its first flight. When the big Soviet bomber first appeared, Western observers pegged its speed at 400 mph, based on what they had observed during the XF-84H project. Tupolev, however, had realized that the key to high prop-driven speed was long, multiple, slow-turning blades, contra-rotating for maximum efficiency, not a screeching little three-blade paddle.