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 3 of 5)
Beaird agrees. “The tachometer indicated 3,000, but I’m not sure that was prop rpm. The tip speed was about 1.2 Mach, so what Leonard calculated is correct,” he says.
The airplane was not popular at Edwards and is to this day rumored to have caused several miscarriages. “It’s hard working on a project like that when you know everybody’s against it,” von Wolffersdorff says. “Nobody wanted the damn thing. First the Navy backed out and then the Air Force canceled the project. A lot of people thought we were trying to go supersonic with a prop, but that wasn’t true at all.”
The Navy had gotten wind of the -84H and initially wanted in on the project, so Republic planned to build three—two for the Air Force and one for the Navy. But because the Navy canceled its order, only the first two made it out the door. The Navy originally liked the fast-turboprop concept because pure jets caused problems aboard carriers. The early catapults had a hard time accelerating fighters to takeoff speed, and even today on landing, standard procedure is to go to full power right at touchdown in case the tailhook misses the arresting wires and the aircraft has to go around. Jets are slow to spin up after a “bolter,” as such misses are called.
Three manufacturers were asked to provide experimental props for the -84H: Aeroproducts, Curtiss-Wright, and Hamilton Standard. In the end, only Aeroproducts stepped up to the plate, providing a stubby three-blade paddle prop, each blade about four times as long as it was wide. “It was a funny-looking propeller,” Beaird recalls. “I think it was just one they happened to have available.”
The Thunderscreech’s Allison T40 engine was, even in the words of the company’s own authorized history, Power of Excellence, “a monstrosity, a mechanical nightmare…. Allison was in the throes of developing the turboprop concept, and began probably 20 years ahead of where it should have been.” The T40 was a pair of 2,750-shaft-horsepower T38s inside a common engine case. It was mounted behind the cockpit, where the F-84’s Allison J35 turbojet had originally lived. Although the -84H’s swept wings and main landing gear were straight off the RF-84F, its fuselage was almost entirely new, substantially modified to fit the big T40 engine. In fact, the airplane was so different from the F-84 that it was originally to be called the XF-106, a designation that eventually was given to the Convair Delta Dart.
During the mid-1950s, the T40 was the most powerful aircraft engine on the planet, putting out between 5,850 and 7,400 shp, depending on the model. Each of its T38s turned an 18-foot driveshaft that led to a big gearbox in the XF-84H’s nose. Though the pilot couldn’t see them, the shafts were spinning at stunning speed on either side of the cockpit, just under the floorboards. To stiffen the relatively flexible shafts, Republic installed numerous bearings along their entire lengths. One of the company’s major concerns was that the driveshafts would overheat the bearings, so each one had temperature and vibration sensors, with meters and warning-light readouts on the glareshield directly in front of the pilot.
“We looked at the damn gearbox and thought Jeez, that’s gonna be a bear,” Ed von Wolffersdorff recalls. “And those shafts that ran past the cockpit on each side, boy, that made you pucker up just to think about it. We were expecting the worst, but they never gave us a bit of trouble.
“We did have some problems with the gearbox, but it was operator error,” he adds. “You’d get the left engine going first, then you’d engage its clutch and get the gearbox turning, drive the righthand engine back through the gearbox and get it going…. I was checking out another crew chief and told him to be careful, but he forgot to get the coolant oil flowing, and man, it just cooked one clutch.”