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.”
Beaird says the starting procedure consumed half an hour: building up hydraulic pressures, establishing nominal electric power levels, and getting the proper green lights.
The driveshaft had high levels of vibration in flight, Beaird says. “It was very sensitive. If it got to where the vibration was so bad that I thought it was going to cause damage, they just left it up to me to decide whether to get out of the airplane”—he means eject, which he never did, but 10 of his 11 flights ended in premature or emergency landings due to vibration or prop-controller problems. “The only time it became a handful was when you got it out around 400 knots,” he says. “The propeller governor [which controlled rotational speed] would start surging, and the airplane would roll rather violently.” The entire airframe was trying to rotate around the propshaft, torquing like a big flywheel with wings.
The late Lin Hendrix, a Republic test pilot who made a single Thunderscreech flight and was the only pilot to fly the second of the two airplanes, wrote in the August 1977 issue of the British magazine Aeroplane Monthly that Beaird, “who never swore, once said after an emergency landing, ‘By jingo, that airplane is going to hurt somebody!’ ” Hendrix himself declined further opportunities to fly the ’Screech, telling Republic’s chief engineer, Jim Rust, a muscular six-foot-four and 235 pounds, “You aren’t big enough and there aren’t enough of you to get me in that thing again.”
Only a single XF-84H survives, the number-two airplane having been junked. The original test bed spent several decades at the entrance to Meadows Field, the Bakersfield, California municipal airport, where an electric motor in the spinner turned the prop at a stately 10 rpm, hardly hinting at the ’Screech in full song. In 1992, the old gate guardian got hangar space at the U.S. Air Force Museum in Dayton, Ohio. It has since been restored to display condition, and about a year ago was finally put on exhibit in the museum’s experimental-aircraft hangar.
Robert Schneider and Darrell Larkin had both flown F-84s in the Ohio Air National Guard, and they assembled a team of volunteers who spent a total of 3,710 hours on the restoration. “You know you’re in trouble when you have to have pilots working on an airplane,” Schneider says, laughing, “but Darrell and I found a lot of retired chief master sergeants who’d been sheet metal guys and had other specialties. They’re the ones who really did the work.”
Aside from the supersonic-prop experiment, F-84s served as test bed aircraft for a considerable variety of other oddball projects. Schneider lists some of the reasons why: “It had a roomier cockpit than the F-86, and there were a lot of them made,” he says. “It was a good-flying aircraft—a little underpowered but extremely strong. I had a midair collision once with another F-84, and we both kept flying and landed safely.” A careless ground controller vectored Schneider and a flight of three other F-84s into a thunderstorm, and in the murk, the -84 to his right slammed into Schneider’s airplane, its stabilator shearing off the front of his wing tank and then whacking the fuselage.