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GenericUsername2056 t1_izeifbn wrote

Regular, subsonic propellers or rotors going supersonic are incredibly inefficient, with efficiencies close to zero, or the propellers/rotors even producing a net drag. It's not a question of material strength. The book 'Introduction to Flight' by Anderson has a nice exercise (6.24 for edition 5) in which it is shown that claims by WWII fighter pilots of breaking the sound barrier in vertical, power-on dives are theoretically impossible. Rotors naturally suffer from the same problems as propellers.

The only propeller aircraft I know of which broke the speed of sound is the modified McDonnell XF-88B fitted with a turboshaft engine. It did not use the propeller as a primary source of thrust, but it did achieve a supersonic dive using only the propeller. I assume this was using basically flat, angled plates as propellers, which would be better suited for generating lift (which in this case would be thrust) in a supersonic airflow than regular, subsonic propellers.

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RufusSwink OP t1_izf1mfx wrote

Why does the efficiency drop so low with a rotor but not with wings on a supersonic airplane?

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GenericUsername2056 t1_izf3xsr wrote

It does, which is why fighter jets for instance have very flat airfoils which are not very efficient for generating lift in subsonic flight. The cross-section of the wings of the famous SR-72 Blackbird reveals a simple ellipse. They require a tremendous amount of thrust to generate enough lift when flying at supersonic speeds, hence why their jet engines have afterburners. When flying at supersonic speeds, airfoil shape is of lesser importance to generating lift.

Aircraft designers are very keen to prevent (local) shockwaves forming on their wings. If you look at a passenger jet, you will notice it has swept wings. These swept wings delay the formation of shockwaves on the wing when flying at transonic speeds, thus increasing efficiency whilst still enabling the aircraft to fly at these speeds.

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RufusSwink OP t1_izgmau4 wrote

That is very interesting, thank you so much for the detailed answer.

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