Sonic booms can be loud. Really loud. Commercial aviation was restricted from breaking the sound barrier after a handful of tests in the 1960s showed just how harmful going supersonic could be to those still on the ground. That hasn't stopped us from getting reminders, as a few weeks back in Chicago the Thunderbirds made some window-shattering damage.
Even just one or two sonic booms can be incredibly loud, but what if there was a plane that made one that was constant? What kind of carnage could that create? Against the grace of God and man, the U.S. Air Force and Navy accidentally found the answer in 1955.
Meet the Republic XF-84H "Thunderscreech," a plane that, despite setting the record for propeller-driven speed, would leave a legacy baked in infamy. Its massive turboprop motor made the plane want to constantly barrel-roll and vibrated the plane into an emergency landing nearly every time it flew. But it was able to push its propellers to speeds past the sound barrier, and become a sonic boom machine, giving it the unofficial record for the loudest plane ever built.
This thing sucked. Let's talk about it.
U.S. Air Force/Wikimedia Commons
In the mid-1950s, jet engines were the hot commodity, with early jet fighters like the p-80 and F-84 easily able to outrun prop-driven fighters. But these new jets sucked through fuel, and unlike the snappy and responsive props, needed time and space to fully spool up and get to speed.
For the Navy, that was a problem. Aircraft carriers lacked the real estate for a jet to take off without some help from a catapult. But the tech was still in its infancy, and early launch systems struggled to get the heavy fighters up to full song. What the Navy needed was an interceptor that could fly off the deck unassisted, but still have the performance of a jet fighter. The Air Force liked the concept as well, looking for an escort for long-range bombers that could be far easier on fuel than its jet counterparts.
What Republic came up with was a modified F-84F Thunderstreak, ripping out its jet engine and replacing it with an Allison XT40 turboprop engine, cranking out an unreal 5,850 horsepower. Unlike World War II-era fighters, that engine had a fixed speed, moving the propeller at 2,100 rpm from startup to shutdown, with pilots adjusting the propellers' pitch to control thrust. That helped the plane with its giddy-up, but the engine's unreal torque was enough for the entire airframe to try rotating around the propshaft, essentially wanting to do a nonstop barrel roll. And there were other, far more noticeable issues.
U.S. Air Force/Wikimedia Commons
That Allison motor was rotating fast enough to push the tips of the plane's three propellers to supersonic speeds, essentially creating a constant sonic boom. On the ground, it could be heard over 20 miles away. In the air, it sounded something like this:
That still can't do the plane's volume justice, as commercial regulations keep speakers and headphones from being loud enough to make you sick. That keeps you from having a seizure, unlike a crew chief working on a C-47 who collapsed listening to the engine during a warm-up. Other ground crews reported nausea and headaches as a result of working near the plane. And it wasn't much better in the cockpit. After the XF-84H had its maiden flight, says Smithsonian Magazine, test pilot Lin Hendrix hopped out and walked over to Republic's chief engineer Jim Rust, saying, "You aren't big enough and there aren't enough of you to get me in that thing again."
Henry Beaird flew the remainder of the tests, but the drama was far from over. Ten of his 11 flights had such bad vibration from the driveshaft that they had to be cut short.
Shockingly, not a ton of people were amped for a deafening plane that shook itself apart and constantly rolled, and the project closed after the final test flight in October of 1956. The program ended with the XF-84H prototypes reaching a top speed of 520 mph — nowhere near the projected limit of 670 mph. It was eventually outpaced by the current quickest prop-driven plane, the Tupolev TU-95. One of the planes Russia lost in Ukraine's June 2025 drone strike, that bomber has four engines that propel it to 575 mph — while not making its crew sick.
