The Republic XF-84H is probably one of the most disliked aircraft the United States has ever put in the air. It was born in the 1950s, an era where if it had an wings and was dangerous, the US would test it. This aircraft however, was too much for even the most daring test pilots. It was based on the F-84F, and has the unenviable record as the loudest aircraft in history, earning it the nickname ‘Thunderscreech’.
Creation of the XF-84H
Like many other strange airborne concepts, this aircraft began in the 1950s. The ‘jet age’ was here, and the newfangled jet engine had almost completely replaced the propeller engine in fighter aircraft.
Jet engines considerably improved the top speed an aircraft can reach. Without a physical propeller to turn, the sky was quite literally the limit in terms of speed. Jets may have been the engineers’ newest toy, but the old propeller still had a major advantage over jet engines.
Early turbojet fighters suffered with poor acceleration. This could cause pilots trouble on landing, which is essentially a tightrope balance of slowing down enough, while not falling out the sky. If a pilot needed to suddenly increase speed while coming into land, you could make a cup of tea before the engine would respond. Take-offs were harder too, as longer acceleration needs a longer runway.
A propeller however, provides immediate power. The XF-84H was created to combine the best of both worlds; high speed, and fast acceleration.
The engine chosen was the Allison T40 turboprop engine, which comprised of two T38 turboprop engines mounted side by side. The two engines met at a common gearbox that rotated the propeller. In total, the output of the T40 was 5,850 hp, making it one of the most powerful engines in the world at the time.
This is where the problems of the XF-84H start.
The XF-84H’s Noise
Only two XF-84H’s were built.
Both aircraft were sent to Edwards Air Force Base (Edwards AFB) for testing. During the tests, many issues were observed.
Aircraft speed was controlled by rotating the propeller blades themselves, as the XF-84H’s propeller rotated at a constant speed.
Unfortunately, the constant speed the propeller turned at – was supersonic.
The blades span so quick, that the outer 60 cm of the blades were travelling faster than sound, at about Mach 1.18 or 900 mph. As the propeller speed doesn’t change, this meant a sonic shock wave continuously emanated out laterally from the propeller for hundreds of metres.
Reportedly, this deafening noise could be heard over 25 miles away. It was so loud, it could physically knock a man down if he walked through the shock waves.
In once instance, a test run of the XF-84H’s engine took place while a serviceman, unbeknown to the test crew, was inside a nearby C-47. It was discovered after the 30 minute test he had been severely incapacitated by the noise.
Another report states that a Republic engineer experienced a seizure after exposure to the XF-84H, and ground crews would become nauseous from being near it.
Personnel at Edwards AFB had to communicate with visual signals during tests. The control tower’s sensitive instruments were also at risk. Eventually, the air base became so tired of the XF-84H, they made Republic tow it out to Rogers Dry Lake for future tests.
The actual decibels reached by the aircraft were never recorded (probably because everyone wanted this thing gone as quick as possible). Looking at the affects it had on the personnel in the vicinity of the XF-84H, it is most likely the loudest aircraft in history. The only potential rivals being rocket powered aircraft like X-15.
The XF-84H did have incredible acceleration, but that was about it.
Its problems, like the noise, originated from the engine and propeller system.
The T40 engine proved to be a mechanical nightmare. Each engine had an 18 ft shaft to the gear box. These shafts suffered extreme vibrations at certain speeds, and the mechanism that controlled the stubby propeller’s pitch was prone to failure.
The rotating propeller was so powerful that the aircraft’s handling was severely affected. The powerful torque caused the aircraft to want to spin around the propeller, like a helicopter without a tail rotor. This made the aircraft a pig to fly for the test pilots who had to constantly compensate for this.
Two Republic test pilots flew the aircraft, with a total of only 12 flights. One pilot, Lin Hendrix, flew it once and vowed to never fly it again. The second pilot, Hank Beaird, flew it for the remaining 11 flights. 10 of these required premature or emergency landings.
Republic generously estimated the aircraft could reach over 650 mph, which would have made it the fastest propeller aircraft in history. But, once over 450 mph, the vibrations in the XF-84H would reach dangerous amounts.
The XF-84H flew for less than 10 hours, but has so many extreme reports, statistics and records.
Of the two models built, only one survives after being a gate guardian for decades. It was restored in the 1990s, and is now displayed at the National Museum of the United States Air Force.
The aircraft was awful, in almost every single way, although it did provide useful research.
It featured the first ram air turbine. This emergency turbine deployed into the oncoming air after engine failure to maintain power for vital on board systems. Unsurprisingly, the XF-84H’s turbine was used more than a pilot would like. Ram air turbines are used on almost every aircraft today, proving invaluable in potentially disastrous situations.
It also featured air brakes near the end of the fuselage that would be found in the same place on a later Republic aircraft, the F-105.
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If its issues were resolved, the aircraft would have most likely been the fastest propeller driven aircraft even to this day. Instead, its remembered for being the loudest aircraft in history, and irritating an entire airbase.