Dumb Question

"JohnO" wrote in message
ps.com...
When designing rotor systems, you have to keep the tip speed subsonic
to prevent all sorts of problems and losses that would otherwise occur
at the sound barrier, right?
When props (or rotors I would suppose) exceed the sound barrier at about
1100 fps tip speed, they have a strong tendancy to self distruct. I assume
due to acoustic vibrations at the supersonic tip. I do know they get very,
very loud.


What about large turbofan engines such as on modern airliners? The fan
tips must be travelling at many orders of magnitude faster than the
speed of sound? Why are they not subject to the same limitation?
I think it is the ducting. It greatly limits (or directs) the acoustic
vibrations or waves from the tips. In a jet engine, the blades usually run
very close to the duct, even touching occasionally in some cases. And you
are correct, they often run 2 or 3 times the speed of sound.


Just wondering.