Question for Jay or other engineers

[Obviousman]

I was having a discussion the other week about the reasons for aircraft having multiple blades when a friend said that the Tu-95 had props that went supersonic. I said this was a very undesirable result, but they insisted that the props went supersonic and that we (western agencies) still didn't know how they had achieved it.

This person holds a senior position in the technology division of an intelligence service, so I gave them due regard.

What are peoples thoughts on this?

[colinr]

The Tu-95's blade tips are rotating at approx Mach 1.7-1.8 depending on air density - making it the noisiest and indeed fastest propeller driven aircraft in the world .

[Obviousman]

But doesn't that cause mechanical stresses on the blades? Such that they should fail?

[jaydeehess]

Very much so but not , IMH(non-engineer)O , a situation that cannot be engineered to withstand.

However, I could see that this bit of info could be construed by some as meaning that the aircraft itself went above mach 1. That is a situation that would be untenable for a propeller driven a/c would it not?

[Obviousman]

Certainly not the whole aircraft - not with turboprops.

My understanding is that as engine power increased, you could fit bigger blades which would give you more thrust. The drawback is that at a certain blade length and RPM, the tips go supersonic. Not only is it very noisy, there is an associated loss of effciency of the blades at supersonic speeds. It also creates stresses on the blade and the shockwaves damage the engine mount / aircraft.

So if you increase engine power, you put more but shorter blades on it. This develops more thrust but avoids the problem of the blade tip going supersonic.

I found a site which talks about the Bear, and it appears the blades actually do go supersonic - or perhaps transonic.

aeroweb.lucia.it/rap/RAFAQ/Tu-95.html

[PhantomWolf]

The major problem with the Bear was that it was so noisy that NATO could pick it up on it SONAR!!! network.