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There's a big difference between a propeller with supersonic tip speeds and a supersonic propeller. A supersonic propeller is not strictly impossible, but even

by jjk166 6y ago

There's a big difference between a propeller with supersonic tip speeds and a supersonic propeller. A supersonic propeller is not strictly impossible, but even after decades of active research remains elusive. You can't simply handwave that you'll use a 3 meter propeller spinning at mach 3 just because it's technically below the limit where it would tear itself apart from centrifugal forces. It doesn't matter what battery energy density you can achieve if the propellers you're using to drive them don't work.

FlyMoreRockets 6y ago
You should check out this: https://en.wikipedia.org/wiki/Republic_XF-84H_Thunderscreech https://en.wikipedia.org/wiki/Republic_XF-84H_Thunderscreech Yes, an aircraft employing a supersonic propeller first flew in the 1950's. It wasn't terribly efficient, but it was terribly loud .
- 0_____0 6y ago
  > Coupled with the already considerable noise from the subsonic aspect of the propeller and the T40's dual turbine sections, the aircraft was notorious for inducing severe nausea and headaches among ground crews.[11] In one report, a Republic engineer suffered a seizure after close range exposure to the shock waves emanating from a powered-up XF-84H.[18] Holy hell
  - zaroth 6y ago
    Per TFA; > Yes, it would be very noisy.
- jjk166 6y ago
  That's not a supersonic propeller, that's a propeller with supersonic tip speeds. A supersonic propeller is a propeller that can work in a supersonic airstream. The XF-84H had a top theoretical speed of Mach 0.9, though in testing it was evaluated to be more like Mach 0.83, and even that's disputed, with another estimate being Mach 0.7. It never achieved these speeds. The only plane to fly with a propeller at supersonic speeds was the XF-88B, though that was actually a jet aircraft that just had a propeller on the front for testing purposes - the vast majority of the thrust was still coming from the jet engines.
mrfusion 6y ago
Would a ducted fan work better? How about heating the air and pushing it out?
- redis_mlc 6y ago
  > Would a ducted fan work better? This comment is ironic because it initially sounds naive, but is probably the most insightful comment. You could make a supersonic propeller-driven airplane by mounting the propeller(s) in a shroud, using a powerful engine (possibly geared), and slowing down the airstream to subsonic speed in the shroud (the same way a jet engine is designed.) The result would be a propeller system that outwardly looks like a jet engine, but internally still has propeller(s) on a shaft(s). The advantage would be better efficiency at low altitudes if a piston engine was used, since piston engines burn about half as much fuel as jet engines. Nobody will build this unless high-altitude flight is banned, and thus need the efficiency of piston engines. Rotax uses essentially a geared lawnmower engine in aviation piston applications that is half the weight of a Lycoming or Continental (because of the gearing.) So hey ... there is room for innovation. You're not going to get a supersonic helicopter (that looks like a conventional helicopter) without retracting the blades for several reasons, starting with the advancing blade being higher speed than the retreating blade, which makes transsonic and supersonic flight with horizontal blades impractical and risky. Current fast helicopters have stubby wings, so behave like an airplane, which is kind of cheating. To create a supersonic helicopter would mean either ducted fans instead of blades, or an entirely new technology. Source: I study aerodynamics.
- dghughes 6y ago
  Heating it would require an enormous amount of power.
  - Groxx 6y ago
    A heat-producing fuel of some sort would make that more efficient. As a bonus, the aircraft would get lighter and lighter as it flew. Imagine the electricity savings!
- kikokikokiko 6y ago

by jjk166 6y ago

FlyMoreRockets 6y ago

You should check out this: https://en.wikipedia.org/wiki/Republic_XF-84H_Thunderscreech https://en.wikipedia.org/wiki/Republic_XF-84H_Thunderscreech Yes, an aircraft employing a supersonic propeller first flew in the 1950's. It wasn't terribly efficient, but it was terribly loud .

0_____0 6y ago
> Coupled with the already considerable noise from the subsonic aspect of the propeller and the T40's dual turbine sections, the aircraft was notorious for inducing severe nausea and headaches among ground crews.[11] In one report, a Republic engineer suffered a seizure after close range exposure to the shock waves emanating from a powered-up XF-84H.[18] Holy hell
- zaroth 6y ago
  Per TFA; > Yes, it would be very noisy.
jjk166 6y ago
That's not a supersonic propeller, that's a propeller with supersonic tip speeds. A supersonic propeller is a propeller that can work in a supersonic airstream. The XF-84H had a top theoretical speed of Mach 0.9, though in testing it was evaluated to be more like Mach 0.83, and even that's disputed, with another estimate being Mach 0.7. It never achieved these speeds. The only plane to fly with a propeller at supersonic speeds was the XF-88B, though that was actually a jet aircraft that just had a propeller on the front for testing purposes - the vast majority of the thrust was still coming from the jet engines.

mrfusion 6y ago

Would a ducted fan work better? How about heating the air and pushing it out?

redis_mlc 6y ago
> Would a ducted fan work better? This comment is ironic because it initially sounds naive, but is probably the most insightful comment. You could make a supersonic propeller-driven airplane by mounting the propeller(s) in a shroud, using a powerful engine (possibly geared), and slowing down the airstream to subsonic speed in the shroud (the same way a jet engine is designed.) The result would be a propeller system that outwardly looks like a jet engine, but internally still has propeller(s) on a shaft(s). The advantage would be better efficiency at low altitudes if a piston engine was used, since piston engines burn about half as much fuel as jet engines. Nobody will build this unless high-altitude flight is banned, and thus need the efficiency of piston engines. Rotax uses essentially a geared lawnmower engine in aviation piston applications that is half the weight of a Lycoming or Continental (because of the gearing.) So hey ... there is room for innovation. You're not going to get a supersonic helicopter (that looks like a conventional helicopter) without retracting the blades for several reasons, starting with the advancing blade being higher speed than the retreating blade, which makes transsonic and supersonic flight with horizontal blades impractical and risky. Current fast helicopters have stubby wings, so behave like an airplane, which is kind of cheating. To create a supersonic helicopter would mean either ducted fans instead of blades, or an entirely new technology. Source: I study aerodynamics.
dghughes 6y ago
Heating it would require an enormous amount of power.
- Groxx 6y ago
  A heat-producing fuel of some sort would make that more efficient. As a bonus, the aircraft would get lighter and lighter as it flew. Imagine the electricity savings!
kikokikokiko 6y ago