Question

Which of these conditions is essential for a rotor airfoil when the helicopter has high forward speed?

(a) Low critical Mach number

(b) Low drag divergence

(c) High critical Mach number

(d) High angle of attack

The question was asked in an online interview.

This intriguing question comes from Blades and Propellers in portion Helicopter Dynamics of Aerodynamics

Answer 1

Right choice is (c) High critical Mach number

The explanation: When the helicopter moves at high forward speeds, the front tip of the rotor blade usually has a very high Mach number. For this reason, it is essential for the rotor blade airfoil to have a high critical Mach number so that it doesn’t reach stall conditions very fast.