The Thunderscreech: The Propeller Aircraft That Tried to Outrun the Jet Age

By Garth Calitz

The Republic XF-84H was an exceptional aviation experiment: a turboprop fighter featuring a supersonic propeller, producing an astounding level of noise and earning a reputation that has endured long after the aircraft vanished from the skies.

There are aircraft that are remembered because they were successful. Others are remembered because they changed aviation. And then there are aircraft remembered simply because engineers were brave enough, or perhaps optimistic enough, to build them. The Republic XF-84H belongs firmly in the third category.

Nicknamed the “Thunderscreech”, this extraordinary American experimental aircraft was an attempt to combine the speed and performance of a jet with some of the advantages of a propeller aircraft. Instead, it produced one of the strangest aircraft of the 1950s: a turboprop fighter that was spectacularly noisy, mechanically troublesome and ultimately unable to deliver the performance its designers had hoped for. Yet more than 70 years later, the XF-84H remains one of the most fascinating aircraft ever built. A fighter caught between two eras, the XF-84H emerged at a pivotal moment in aviation history.

The jet age was rapidly taking over military aviation, but turboprop technology still offered attractive advantages. Turbine engines could provide enormous power while retaining the fuel economy and potentially lower-speed operating characteristics of a propeller-driven aircraft. The US Navy and US Air Force were interested in an aircraft that could combine these characteristics with the performance of a fighter. Republic Aviation therefore took the basic structure of its F-84F and created something radically different.

The resulting aircraft retained the swept-wing fighter appearance of the Thunderstreak, but underneath the skin was an entirely different propulsion system; only two XF-84H prototypes were built. The first aircraft flew on 22 July 1955, beginning a test programme that would prove just how difficult it was to make a very large propeller work at very high speed.

The heart of the XF-84H was the Allison XT40-A-1 turboprop, producing approximately 5,850 shaft horsepower. That was an enormous amount of power for a single-engine aircraft. But the engine was not driving a conventional propeller.

Mounted on the nose was a huge three-bladed, approximately 12-foot-diameter propeller designed to operate at extremely high rotational speeds. The propeller tips could reach supersonic speeds, creating shock waves that produced the aircraft's extraordinary acoustic signature.

The XF-84H was essentially trying to do something that conventional propeller aircraft had generally been designed to avoid: push the propeller into the transonic and supersonic regime and physics was not impressed. At high propeller rotational speeds, shock waves formed around the blades. Instead of producing simply the familiar roar of a powerful aircraft engine, the XF-84H generated a continuous, violent acoustic disturbance.

The XF-84H developed a reputation at Edwards Air Force Base that went well beyond simply being “noisy”. Smithsonian's Air & Space Magazine described the aircraft as having the unhappy distinction of being the loudest aircraft of its era. Its propeller tips were travelling faster than sound, producing a continuous shock-wave effect. Ground crews reportedly suffered headaches, nausea and other physical effects during engine operation.

The noise was also a practical problem for the test programme. The aircraft's powerful acoustic and vibration effects were sufficiently disruptive that operations around it became difficult. The XF-84H eventually had to be moved away from the normal flight-line environment for engine testing. In other words, this was an aircraft that could cause problems without even leaving the ground.

The propeller was not simply there to generate thrust. It was effectively the aircraft's defining aerodynamic experiment. At conventional aircraft speeds, propellers work efficiently because the blades remain largely within a subsonic flow regime. As the aircraft gets faster, however, the outer sections of the blades encounter increasingly high local airflow speeds. Once parts of the blade become supersonic, shock waves form; the XF-84H deliberately operated in this difficult regime.

The result was enormous aerodynamic and structural complexity. The aircraft experienced severe vibration and stability problems, while the propeller reduction gearbox and pitch-control system proved troublesome. The very technology intended to make the aircraft exceptionally fast was becoming one of the reasons it could not operate effectively.

Fast – but not as fast as the legend suggests; this is where the XF-84H's reputation becomes complicated.

The aircraft is frequently described as the fastest propeller-driven aircraft ever built, with figures of around 623 mph (1,003 km/h) often quoted. That figure has appeared in historical record claims, but it should be treated with caution. The National Museum of the United States Air Force lists the XF-84H's maximum speed as 520 mph (837 km/h) and explicitly states that the aircraft never approached supersonic speed.

That does not make the aircraft any less remarkable. Even approximately 520 mph was an extraordinary target for a propeller-driven fighter in the mid-1950s.

The problem was not just the noise; the XF-84H's biggest problem was that the aircraft was extremely difficult to operate. Between July 1955 and October 1956, the two prototypes accumulated just 12 test flights. Of those, 11 ended in emergency landings, according to the US Air Force museum. That statistic tells the story better than almost any performance figure.

The aircraft suffered from mechanical problems, vibration and aerodynamic difficulties. At high speeds, the propeller and propulsion system generated severe forces that made the aircraft difficult to control. The propeller's enormous torque also created challenges for the airframe. A conventional piston or turboprop aircraft already has to deal with engine torque and propeller effects. The XF-84H took those problems to another level.

The designers incorporated a T-tail and a triangular vertical fin behind the cockpit, partly to deal with the effects generated by the powerful propeller system. But the modifications could not overcome the fundamental problems.

Perhaps the most revealing assessment came from Republic test pilot Lin Hendrix. After flying the aircraft, Hendrix reportedly refused to fly it again. His colourful response to Republic's chief engineer has become part of aviation folklore: “You aren't big enough, and there aren't enough of you to get me in that thing again.”

Smithsonian's account of the programme records Hendrix's concerns about the aircraft's behaviour at higher speeds, including violent rolling associated with propeller-governor problems. For a period when test pilots routinely climbed into experimental aircraft that could quite literally kill them, such a reaction carried considerable weight.

By the mid-1950s, the writing was already on the wall. Pure jet propulsion was developing rapidly. Aircraft such as the F-86 Sabre and MiG-15 had demonstrated the enormous potential of swept-wing jets, while increasingly powerful jet engines were making the hybrid turboprop fighter concept less attractive.

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The XF-84H therefore found itself caught between technologies. It was trying to achieve jet-like speeds with a propeller at precisely the moment that jet engines were becoming capable of delivering those speeds without the enormous aerodynamic complications of a supersonic propeller. The US Air Force eventually abandoned the programme. The two prototypes had accumulated only a small amount of flight time, and the XF-84H never entered production or operational service.

The only surviving aircraft is displayed at the National Museum of the United States Air Force in Dayton, Ohio. It is the initial prototype and completed eight of the programme's 12 test flights. The second prototype was eventually scrapped.

For visitors, it is difficult to appreciate from a static museum display just how extraordinary this aircraft must have been when its enormous propeller was turning at full power. With the engine silent, the Thunderscreech looks almost elegant – a sleek 1950s swept-wing fighter with a propeller on its nose. Start imagining the engine running, however, and the name suddenly makes perfect sense.

The XF-84H was a failure in operational terms. It never became a fighter. It did not achieve the performance originally envisaged. It suffered from mechanical reliability problems and proved extremely difficult to fly. But experimental aircraft are not necessarily failures simply because they do not enter production. The Thunderscreech demonstrated the enormous aerodynamic and mechanical difficulties involved in pushing conventional propeller technology into the transonic regime.

It also arrived at exactly the wrong moment; the jet engine was evolving so rapidly that there was increasingly little reason to accept the disadvantages of a high-speed propeller. The XF-84H therefore became a technological dead end – but a spectacular one.

The XF-84H was built at a time when aviation engineers were exploring the boundaries of what was possible. It combined a jet-age fighter airframe with a huge turboprop engine and a propeller deliberately pushed into a regime where shock waves became part of the propulsion system.

But more than seven decades later, the Republic XF-84H Thunderscreech remains one of aviation's great “what if?” aircraft – a magnificent example of what happens when engineers decide that the laws of physics are merely suggestions.