The XF-84H Thunderscreech is widely recognized as the loudest plane in history, ever. But just how loud was it? For those accustomed to the noise levels of modern commercial aircraft like the Airbus, the Thunderscreech’s sound is on an entirely different level. In fact, it would make the Rolls Royce Spey engines on BAC One-Elevens seem like a gentle purring kitten by comparison. The extreme noise levels of the Thunderscreech have even caused seizures in people nearby.
In the past, speed was prioritized over efficiency, making the Thunderscreech an ideal example of this philosophy. Originally intended for speed, this aircraft never went into mass production and remained a prototype. Instead of achieving the expected speed, it produced such a deafening noise that it could be heard from miles away.
XF-84H Thunderscreech Propeller Speeds
What caused this extraordinary noise? The Republic XF-84H Thunderscreech was an experimental aircraft developed in the 1950s, designed to merge the speed of jet propulsion with the reliability of turboprop engines. Based on the F-84F Thunderstreak, the XF-84H underwent significant modifications, including the installation of a powerful Allison T40-A-1 turboprop engine. This engine drove a supersonic propeller, with its tips traveling faster than the speed of sound, resulting in extreme noise levels. The aircraft earned the nickname “Thunderscreech” due to its deafening sound, which was reportedly audible up to 25 miles away.
The propeller tips moved at a speed of Mach 1.18, generating more than 100 sonic booms every second. This resulted in noise levels exceeding 200 decibels, making it the loudest plane ever built. To put this in perspective, the human ear can typically withstand up to 70 decibels, and exposure to sounds at 85 decibels or higher can lead to hearing loss. The XF-84H produced noise three times the limit, with proximity to the aircraft during a run-up potentially causing permanent hearing loss or even seizures.
During its early testing phases, air traffic controllers would flash red lights at the test pilots during engine run-ups of the XF-84H because the aircraft was so loud that controllers couldn’t hear radio communications. Various bases even requested the relocation of the Thunderscreech’s engine starts due to its excessive noise. Ground crew members had to communicate using signal flags and lights to keep themselves away from the deafening sound produced by the aircraft.
One infamous incident involved the sound knocking out a C-47 crew chief. The XF-84H performed a 30-minute engine start and run-up next to an apparently empty C-47. After the run-up, while the plane was being towed to the hangar, the crew heard banging from inside the C-47, only to discover that the crew chief had been inside, performing cleaning duties. The Thunderscreech’s noise had knocked him out.
In addition to its extreme noise, the XF-84H faced numerous technical challenges. The powerful engine caused intense vibrations that affected both the aircraft’s structure and the pilot’s comfort. Furthermore, the torque generated by the supersonic propeller made the aircraft extremely difficult to control, leading to stability issues.
The XF-84H Never Went into Full Production
While the XF-84H was expected to reach speeds of up to 650 mph, it only managed to achieve around 450 mph due to unresolved aerodynamic and mechanical problems. The aircraft required a long takeoff roll and was prone to unexpected system failures, including frequent gearbox and propeller malfunctions.
Only two prototypes of the XF-84H Thunderscreech were built, and it never achieved operational status. Just 12 test flights were conducted, with 10 resulting in emergency landings. Test pilots struggled to manage the aircraft’s instability and overwhelming noise. The challenges faced during testing made it clear that the XF-84H was not a viable solution for military service.
The project was ultimately canceled in 1956, and the aircraft was retired before entering production. Although it never saw combat or frontline service, the XF-84H Thunderscreech remains one of the most unique and ambitious experimental aircraft of its time and will always be remembered as the loudest plane in history.
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