One would think the “fastest propeller driven aircraft ever built” would be some kind of phenomenon- widely recognized by those interested in aviation. It might be a warbird mated to a specially prepared engine or a super-sleek late-war design that never went into production, right? Nope. According the Republic Aviation, who firmly believed the claim (at the time), the fastest propeller driven aircraft ever built was going to be the Republic XF-84H Thunderscreech. Why did Republic build such a beast? Air Force research at first, but the Navy Bureau of Aeronautics (BuAer) wanted a fighter capable of operating from carrier decks without the need to be catapulted from it, so they requested a prototype.
Republic came up with the XF-84H by taking the wings from an RF-84F Thunderflash and modifying the fuselage to park a 5,850 horsepower Allison XT40-A-1 turboprop engine where the turbojet engine would normally be. The T40 engine, housed behind the cockpit and driving a three-bladed 12 foot diameter Aeroproducts propeller via extension shafts, produced thrust from its exhaust as well. An afterburner was installed but was never used in flight. The propeller was turned at a constant (high) speed and thrust was controlled via blade pitch. Those blades were turning so fast that the tips were reaching Mach 1.18. That’s 905.4 miles per hour to you and me.
As one might expect, the propeller created massive amounts of torque. The empennage was changed to a T tail configuration to keep the horizontal stabilizers and elevators out of the roiled air created by the propeller. A dorsal yaw vane was also added. Other attempts to control torque included mounting the port side leading edge engine air intake a foot further forward than the starboard intake and selectable asymmetrical wing flap operation. The XF-84H was the first aircraft equipped with a retractable/extendable ram air turbine (RAT). Which came in handy given all the engine problems these aircraft experienced.
Ah, the engines. Other T40 powered aircraft, most notably the North American XA2J Super Savage carrier based attack bomber and the Douglas XA2D Skyshark– itself a development of the venerable Douglas AD Skyraider, experienced crippling engine problems. The XT-40 was tremendously complicated powerplant. Essentially a pair of T38 engines driving a linked gearbox, the engine made sense for a contra-rotating propeller but for a single propeller design? Not so much. Then there was the propeller. Blade materials and compositions wore out entire cases of slide rules. But the proof was in the flight testing. The Navy decided to back out when they realized a flight deck bolter situation would be disastrous with the XF-84H, so only two prototypes were built.
After they were built at the Republic plant in Farmingdale, Long Island the two XF-84H prototypes were disassembled and shipped via rail to Edwards AFB for flight testing. First flown (and heard) on July 22nd 1955, the two aircraft only flew a total of 12 test flights totaling a little more than six hours of Republic pilot-only flight time over the high California desert. Out of those 12 flights, ten ended in forced landings. The aircraft’s propeller drive system gearing required 30 minutes of warm up before it could be flown. Propeller pitch gearing failures and vibration from the drive shafts and the propeller itself plagued the test flights. The XF-84H just wasn’t a practical aircraft- in large part because of its sound. Or noise. It sure looked cool though.
Oh, the noise! The XF-84H was probably the loudest aircraft ever built. Dubbed Thunderscreech or the Mighty Ear Banger by those who heard it, ground engine run ups could be heard 25 miles away. The outer foot to foot and a half of the XF-84H prop blades were moving at supersonic speed even with the engine throttled back to idle. That in turn created a continuous visible sonic boom that radiated laterally from the propellers for hundreds of yards. It was said that the shock wave was actually powerful enough to knock a man down. One anecdote says a C-47 crew chief who was inside his Skytrain while an XF-84H ground engine run was done nearby was severely incapacitated by just the sound of the Thunderscreech. Another story says that a Republic engineer suffered a seizure after close range exposure to the shock waves emanating from an XF-84H turning up on the ground. Coupled with the already considerable noise from the subsonic portion of the propeller and the T40’s dual turbine sections, the aircraft was notorious for inducing severe nausea and headaches among ground crews. You can’t make this stuff up! But wait…there’s more!
The noise got so bad that the glass in the Edwards tower was being damaged by vibration from the constant high energy sound coming from the XF-84H. Finally the Air Force Flight Test Center made Republic tow the aircraft out to Rogers Dry Lake for engine run ups. The XF-84H test program went nowhere after the Republic Phase I proving flights, so no USAF or Navy test pilots ever flew the XF-84H. For a time the Air Force used the XF-84H prototypes at the US Air Force Propeller Laboratory at Wright-Patterson Air Force Base (AFB) to test supersonic propellers and to explore propeller responsiveness at jet speeds. The test program was cancelled for good in September of 1956- without the prototypes ever having flown faster than 450 miles per hour. One prototype, 51-17059, resides at the National Museum of the US Air Force in Dayton, Ohio.