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The Republic XF-84H, known as the "Thunderscreech" was an experimental version of the F-84F Thunderstreak. It was intended to be a supersonic turboprop fighter. Even at idle, the prop tips rotated at supersonic speeds and the plane is often claimed to be the loudest ever built, supposedly audible 25 miles away.

Most sources describe a continuous sonic boom due to the supersonic prop tips. However, a fairly common claim on the internet is that they caused "900 sonic booms per minute."

Is there any truth to the claim that the plane caused 900 sonic booms per minute?

One's first thought should be, "no": because the prop tips are continuously above the speed of sound, they emit a continuous sonic boom (as most sources claim), not 900 separate ones per minute. However, since the prop tips were supersonic and the roots were not, it follows that a section of the prop's length would be in the transonic range. Any change to the wind or propeller RPM would cause more or less of the propeller to be moving supersonically. I suppose that, say, an appropriate oscillation in the turbine engine could cause the continuous boom to be modulated by some 15Hz (900-per-minute) component.

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I think it is a matter of where you stand (literally).

From the front or the rear the shock waves from the propeller tips would hit your ears continuously, and the result would be constant noise.

When standing laterally, the propeller tips would move towards and away from you and produce a variation of noise intensity of the frequency produced by the product of propeller RPM and blade count.

It seems that the design RPM of the XF-84H propeller was 3000, and since the propeller had three blades, the frequency should have been 9000 per minute, not 900. The airframe could be split ahead of the cockpit to allow the installation of several nose sections with their own gearbox and propeller. Several gearboxes and propellers were prepared with propeller design speeds between 2100 RPM and 3000 RPM, but the test program was ended prematurely, so most of the planned variations were never flown.

I can only judge from the little information available online and count myself lucky not to have witnessed a power-up of the XF-84H (for me personally, the loudest aircraft is still a Sukhoi 22M-4 – you think "man, this afterburner is loud!" and only then the pilot switches the afterburner on), and it seems that the 900 per minute figure must have lost a zero on its way of becoming an internet meme.

Note that I only speak of a variation of noise – as Jon Story writes, a sonic boom is what arrives at your ear, not what is produced by the propeller.

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    $\begingroup$ You get the +1, this was what I was going to say! (I was going to say what Peter said???? Wow, I've learned a lot... ) $\endgroup$ – FreeMan Oct 7 '15 at 16:33
  • $\begingroup$ Are you sure you heard Su-17? AFAIK, Su-17 has never been exported outside of USSR. Its export version was Su-22, and its main difference was a different engine: R-29B (from MiG-23) instead of AL-21F. Only the last version, Su-22M4, had the original AL-21F engine (though it was fairly common). $\endgroup$ – Zeus Jul 2 '18 at 8:46
  • $\begingroup$ @Zeus: At the time, I was told it was a Su-17. I now looked into it and indeed it must have been a Su-22M-4. Still the loudest I ever experienced, though. $\endgroup$ – Peter Kämpf Jul 2 '18 at 18:33
  • $\begingroup$ The 900 probably comes from the idle rpm of the prop. $\endgroup$ – jwenting Jul 3 '18 at 4:51
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No, because that isn't how it works

A plane doesn't 'make' sonic booms, a sonic boom is the result of an aircraft overtaking the sound it is making, meaning that you're hearing the aircraft now, and several seconds ago, all at once.

It's not an actual object, more a kind of fictional line of where the sound has reached. Any aircraft therefore produces one constant sonic boom, or infinite individual sonic booms, depending on your perspective

The 900 per minute is just internet silliness. As opposed to Internet seriousness, which is to be avoided at all costs.

Now in theory as you say, props passing the sound barrier could make lots of individual sound points, IE you hear the blades pass multiple times as they move towards and away from you vertically (around the propellor hub) - this would be significantly higher than 900 per minute, I'd have thought. So from that perspective you could hear the propellor create a staccato of boom-silence-boom.... But it wouldn't be as loud as the plane itself, and would only be heard as part of the plane passing. The propellor are still moving with the plane, remember.

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    $\begingroup$ The prop tips were supersonic even when the plane was idling on the ground so it's not just about the plane moving past the observer. Also, the internet seems to think that the noise of the supersonic prop itself was a significant contribution to the overall noise of the plane. $\endgroup$ – David Richerby Oct 7 '15 at 7:18
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    $\begingroup$ @DavidRicherby the same is true for many high performance helicopters around takeoff. $\endgroup$ – jwenting Oct 7 '15 at 7:42
  • $\begingroup$ I'm not saying that it's a quiet aircraft, or that supersonic prop tips make no difference - just that the '900 sonic booms per minute' makes no real sense as a number to quite. Also prop booms are nowhere near the volume level of a passing supersonic aircraft - because the props are quieter in general, and because they're only heading toward the listener at supersonic speed for a fraction of a second (unlike an aircraft passing which is building up the pressure wave for a moment or two) $\endgroup$ – Jon Story Oct 7 '15 at 7:54
  • $\begingroup$ All aircraft have at least two sonic waves. While some people on the ground may perceive the sound as a single sonic “boom,” many sonic booms produced from NASA’s research flights are easily heard as distinct “double” booms, The Space Shuttle was notorious for its double boom. nasa.gov/centers/armstrong/news/FactSheets/FS-016-DFRC.html $\endgroup$ – jwzumwalt Jul 9 '18 at 17:24
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A sonic boom is technically the same concept as thunder... It's air that moves over the aircraft and crashes together behind it resulting in the sound. I do highly doubt the XF-84H make 900 sonic booms a minute. I have done extensive research on it and one of the big misconceptions is they were trying to make a supersonic propeller driven aircraft when that's not the case. This goes a little off topic but in fact early jets were not that great and they wanted jet performance out of props, which is doable to an extent. Jets take time to accumulate their power where a prop is instantaneous with the use of variable pitch props. The engine on the Thunderscreech was in fact ran at full throttle all the time and the prop was adjusted to generate the needed power which was instant compared to a jet. I'm linking this article so I don't go too much into a huge chat about this, it's my favorite plane of all time... But read this, it's the most informative article I've read and it's got people from the project actually talking about things I've mentioned. https://www.airspacemag.com/how-things-work/zwrrwwwbrzr-4846149/

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    $\begingroup$ "It's air that moves over the aircraft and crashes together behind it resulting in the sound." is incorrect. "they were trying to make a supersonic propeller driven aircraft when that's not the case" whether they were trying or not is irrelevant: they ended up with an aircraft that ran its propeller tips at supersonic speeds. $\endgroup$ – Hobbes Feb 28 at 8:03
  • $\begingroup$ "supersonic propeller aircraft" can be parsed two different ways. I get the feeling that Kat meant it in the "supersonic ... aircraft" sense and Hobbes read it in the "supersonic propeller" sense. $\endgroup$ – BowlOfRed Feb 28 at 17:49

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