Expert: Paul Soderman - 9/16/2009

Question
Hello, I wanted to ask a question about helicopter speed.
What is the theoretical speed limit of a helicopter? And what is the fastest speed attained by any type of helicopter in history?
Thank you.

Answer
Helicopters are not built for high speed.  As airspeed increases the advancing rotor blade tip goes supersonic.  Depending on strength of the shocks (small local ones are not bad, strong ones are bad) high drag and flow separation on the rotor occurs.  Meanwhile, the retreating blade's resultant airspeed is going down, so angle of attack must go up.  At some point the retreating blade stalls - a bad thing because drag goes up, lift goes down.  That would cause helicopter roll and possible pitch due to gyroscopic effects. You would not want to fly into this condition.  So, to get the theoretical speed limit would require a detailed analysis requiring blade geometry, blade cyclic pitch, rotor disc attitude, helicopter airspeed, altitude, body drag, etc. - things that are way beyond the limits of this website.  And the answer would only be good for one machine.  It's kind of like asking what is the theoretical limit for the speed of a car - which car ?

I found on the web that the record might be 210 kts for a Chinook or 250 mph for a U.K. Westland Lynx.  I have no personal knowledge of the correct answer.
Paul