Twitter
Facebook
While “annoying” is not a term that automatically springs to mind when discussing aircraft, the Republic XF-84H experimental “turbo-fighter” surely meets that description.
The XF-84H was conceived in 1951 to combine two programs – a test vehicle for supersonic propeller designs and as a high performance, long-range strike fighter for the U.S. Air Force and the U.S. Navy. A turbo-prop fighter seemed to solve all the problems of both the reciprocating engine and the jet engine in one swoop.
***image1***It would have a (theoretical) top speed close to Mach 1 – the speed of sound – and, while cruising, one of the jet engines could be shut down to increase endurance. Since its turboprop engine ran at a constant 3,000 rpm, thrust could be quickly generated by simply changing the propeller pitch, thus giving the aircraft the acceleration needed for “waved off” carrier landings.
Originally designated XF-106, the new aircraft was renamed XF-84H because it was generally based on the Republic F-84F Thunderjet. The F-84F fuselage was lengthened and widened to hold the Allison XT40-A-2 engine, which was actually two jet engines mounted side-by-side, driving two 18-foot-long drive shafts that went into a single gearbox. The shaft from the gearbox drove a 12-foot diameter propeller with three stubby, square-tipped blades only about four times longer than they were wide.
The powerful propeller’s torque required more structural changes. The aircraft jet engine intakes were lopsided to offset the effects and a “T” tail was used to avoid the turbulent airflow over the horizontal stabilizer/elevator surfaces from
propeller wash.
When the aircraft was complete and the T-40 engine began its ground runs, huge problems appeared. The propeller’s blades went supersonic even while the airplane was running on the ground in idle and each blade created a shock wave that spiraled outward. And the problem wasn’t just the volume.
The supersonic shock wave noise didn’t even qualify as “sound,” which is a series of smooth waves of variable frequency and amplitude, like a sine wave. The XF-84H shock waves had a completely different profile than sound waves and provoked a nearly instantaneous pressure increase. On an oscilloscope, the pressure spikes from the XF-84H propeller appeared more like saw teeth than a sine wave and they traveled faster than sound waves.
On the ground, the noise was reportedly audible 25 miles away, and anyone around the XF-84H when the engine was running was subjected to the rapid fire shock waves. These acted directly on the large intestine, causing nausea – and worse. Hearing protection made no difference. The Air Force Flight Test Center quickly directed Republic to tow the XF-84H out on Rogers Dry Lake, far from the flight line, before running up its engine, and it was quickly dubbed the “Thunderscreech.”
The propeller was not only excruciatingly noisy – which in itself was enough to disqualify it as an operational aircraft – but the T-40 engine and the driveshaft gave additional problems. It took 30 minutes just to get all the hydraulic pressures and electrical circuits correct before one engine could be started, then the clutch had to be engaged to start the gearbox spinning before the other engine could be started.
The long drive shafts had to be tightly supported along their entire length by numerous bearings. Since the failure of a single bearing might be disastrous, each bearing had to be monitored for temperature and vibration and each bearing needed its own oil supply.
Not surprisingly, the XF-84H only flew a total of 12 test flights. Eleven of these flights ended in premature or emergency landings because of problems with the drive shafts’ vibrations or with the propeller controller. The XF-84H program was dropped and one prototype became a “gate guard” at an airport in California, but was rescued, refurbished and is now at the Air Force Museum.
The XF-84H did make one positive contribution to aviation. It was the first aircraft to carry a ram air turbine – a small engine with a propeller that would automatically swing out into the airstream to provide hydraulic and electrical power for aircraft with engine problems. The RAT proved especially useful in combat (including for the author) for battle damaged aircraft.
