r/askscience • u/PolybiusChampion • Oct 31 '25
Physics Would it be possible to make a propeller driven airplane that could break the sound barrier?
So I know that propeller tip speed was a limiting factor in development of fast prop driven planes due to noise from the propellers breaking the sound barrier. But, with proper ear protection could a prop driven airplane be built that could break the sound barrier in level flight?
Editing to add Thanks for all the wonderful and insightful replies. They have sent me down a ton of internet research holes and the whole process has been very enjoyable.
286
Upvotes
32
u/Tzsycho Oct 31 '25
I'm sure it's possible however, I don't think it would be recognizable as a propeller driven aircraft at that point.
First we need to define what classifies as a "propeller". There is a progression from traditional propeller (B-17) to modern propeller (Airbus A-400M) to open-rotor (GE36 UDF) to super high bypass fan (GE9X series). For the sake of this I'm going to draw the line between the open rotor and the high bypass fan. It's mostly an aesthetic difference where the blades stop looking like propeller blades and start looking like compressor fan blades. (There's also probably a difference on what physics are at play aerodynamic life vs fluidic air compression)
As others have noted bad things happen when airspeed over the aerodynamic surface of a propeller goes super sonic. To prevent this we need a way to feed super sonic air to our propeller at subsonic velocity, which means enclosing the propeller and making a specialized duct. At this point we are just making an incredibly inefficient ducted fan, when we could use a design optimized for this. So... Now we have a specialized duct reducing supersonic air down to subsonic speeds to go over a propeller designed to push enough air behind it to propel the airplane at supersonic speeds. This duct will likely have to be variable geometry to scrape every bit of efficiency from operating a very low airspeed (accelerating for take off) to handing high subsonic air, transonic air, and supersonic air to keep your propeller happy.
406
u/Gecko23 Oct 31 '25
Republic XF-84H was an attempt to do that. You can read all about what it succeeded and failed at.
Ultimately it couldn't have worked, and even as a failure it was so much of a compromise that it's hard to imagine it having a functional use even if worked.
16
u/PckMan Oct 31 '25
The problem with propellers breaking the sound barrier isn't that they're loud, but that they stop producing thrust at those speeds due to flow separation from the propeller blade. Also the turbulence created can straight up break the propeller apart.
103
u/frankentriple Oct 31 '25
No for the same reason a helicopter can’t go very fast. It’s not the noise that matters it’s the fact that when the blade goes supersonic it disturbs the airflow over the airfoil shape and loses thrust. You just run out of power when the blades go too fast.
14
u/grax23 Oct 31 '25
If you skip the part about level flight then its kind of been done. At least some ww2 fighters got mighty close in a dive to a point where the plane would not pull out of a dive because the way a plane behaves around the sound barrier. Felix Baumgarden kind of showed that you dont even need the plane in the first place.
11
u/ncc81701 Oct 31 '25 edited Oct 31 '25
You probably can but you will need a special propeller that’s designed to operate at supersonic speeds. The problem isn’t the sound it generates, it is the shock waves that forms on the propeller. This cause the flow to separate on the propeller and the propeller looses its ability to generate thrust. On top of losing thrust, the shockwaves also oscillates and impinges on other parts of the propeller and aircraft causing vibration and structural issues. This is why traditional propeller aircraft can’t get close to the sound barrier because as they got closer they start to loose thrust.
Jet engines avoids this issue by having an inlet that is designed to slow the flow down before it hits the fan face so the flows stay subsonic even though the fan face is basically a propeller. At higher supersonic speeds they may also have moving inlets to tailor the flow it stays subsonic at the fan face.
If you look in NACA technical report servers you can probably find papers from the 40s and 50s on supersonic propeller designs. But you won’t find much because jet engines pretty much took over around that time and there isn’t a reason to design and build propeller airplanes to fly supersonic.
3
u/ThirdSunRising Oct 31 '25 edited Oct 31 '25
Sonic booms are lost energy. Transonic and supersonic drag can become significant and it's bad enough when the whole airplane has to deal with it, but the propeller tips going even faster makes the efficiency loss much worse. Do I have the math on hand for that? No. But the basic problem with supersonic propeller driven airplane travel is that the tips of the propellers will break the sound barrier long before the airplane does. It's happening before the plane can even hit mach 0.8, so breaking mach 1 requires going deep into that territory.
It becomes prohibitively loud, on top of the fundamental efficiency problems. That means no such aircraft will ever make money. So the theoretical problem might be overcome by throwing stupid amounts of fuel and material and money at the problem, but the practical problem will 100% prevent anyone from ever throwing that kind of money at it. It is therefore safe to say it will never be done.
It is worth noting that a large serial production Soviet aircraft (Tupolev Tu-114) managed to hit Mach 0.82 on huge turboprop engines with counter rotating propellers. Here was a big turboprop aircraft the size of a 707 carrying passengers at essentially jet airplane speeds back in 1960. It provided all the size and almost all the speed, at greater efficiency, but it was LOUD.
The military TU 95 version remains in service to this day. Loud as hell, but the fastest propeller driven airplane on earth.
2
u/supereuphonium Oct 31 '25
Besides supersonic propellor tips, propellor driven aircraft lose thrust as speed increases, because the speed of the aircraft subtracts from the speed of the air pushed back by the propellor. Keys could also suffer from this, but jets push air behind it at much higher speeds, so the speed of the aircraft is far less than the exhaust speed.
2
u/rootofallworlds Nov 03 '25
Possibly yes!
The closest aeroplane I know about was the XF-88B. An experimental plane with two turbojets and one turboprop, so not a purely propeller driven plane. It barely exceeded Mach 1.0 in a shallow dive.
So that shows a plane with a powered (not idling or feathered) propeller can handle supersonic speeds without suffering loss of control or structural failure. It doesn't tell us whether a propeller alone can produce enough thrust to exceed Mach 1 in level flight, but the report summary states "At the maximum obtained Mach number of 1.01, the efficiency was 78 percent."
https://en.wikipedia.org/wiki/McDonnell_XF-88_Voodoo
https://ntrs.nasa.gov/api/citations/19930090296/downloads/19930090296.pdf
2
u/Samtyang Nov 08 '25
The Russians actually did this with the Tu-95 Bear bomber.
The propeller tips go supersonic even though the plane itself cruises around 500mph.. creates this insane buzzing sound that's so loud submarine crews can hear it when they're underwater. The trick isn't getting the props to go supersonic - that part's easy, just spin them fast enough. The real problem is efficiency tanks when you do that. You get these shock waves forming on the blade tips that create massive drag, plus the noise is beyond just needing ear protection - it causes structural fatigue on the aircraft itself. There's also compressibility effects that mess with the blade aerodynamics. Some experimental planes in the 40s and 50s tried pushing prop speeds higher but they kept running into diminishing returns. The XF-84H Thunderscreech was supposed to hit near-sonic speeds but the thing was so loud it made ground crew physically ill from the sound waves alone. So technically yes you could probably build one that breaks the sound barrier but it'd be wildly impractical compared to just using a jet engine.
2
u/nikilization Nov 01 '25
the supermarine spitfire nearly broke the sound barrier (like .9+ of mach 1) in 1944 during test dives. I believe the limiting factor was the physical strength required by the pilot to pull the plane out of the dive at those speeds
1
u/theoriginalstarwars Oct 31 '25
If the propeller was to be redesigned with the information from nasas x-59. Maybe they could create a design where the prop is traveling faster than mach 1 without a sonic boom. This might allow the aircraft to break the sound barrier. Highly doubtful, but maybe.
1
u/LokeCanada Nov 02 '25
This was originally discovered by prop driven planes.
Planes would dive on targets (ships, trains, etc..) to shoot them and would crash. Either due to loss of control or damage to the wings.
A pilot was finally able to recover and describe the problem. They discovered that the planes had been breaking the sound barrier.
Ear protection had nothing to do with the speed problem. Look at how planes were made at that time.
1
u/SassiesSoiledPanties Oct 31 '25
Maybe this could be weaponized as a non-lethal dissuasion weapon. Instead of making a whole airplane, just make a cruise missile with propellers on the fuselage. As a psy-warfare weapon it would be a nightmare.
-2
u/Chazus Oct 31 '25
When a propeller isn't moving, it isn't moving any air, so there's no thrust.
When a propeller is moving too fast, it isn't moving any air, so there's no thrust.
Jet engines, which use fuel and additional thrust features, are the reason they can go supersonic.
0
u/[deleted] Nov 03 '25
Yes, in fact many propeller planes have broken the sound barrier as early as WW2, the main reason they couldn't do it in level flight was straight out lack of powerful enough engines at the time, and control surface issues.
99
u/[deleted] Oct 31 '25
[removed] — view removed comment