Ask an Explainer

Q:

What is the limiting factor for designing of helicopters in terms of forward speed?

A:

There are a number of limiting factors on helicopter forward speed. The most obvious one is when drag equals thrust. At that point, the forces balance out and the helicopter can't go any faster.

 

Another less obvious factor has to do with dissymmetry of lift. On a helicopter that's moving forward, the forward moving blade is moving faster relative to the air than the backward moving blade. This creates more lift on one side of the helicopter. Helicopters are built to compensate for this by flapping their blades so that the angle of attack changes depending on which way the blade is moving. The big problem comes when either the forward blade hits the speed of sound relative to the air it's moving in, or the backwards moving blade stops moving relative to the air it's moving in. If the first happens, a shock wave is formed. If the second one happens, then no lift at all is produced on that side of the helicopter and there is a stall. Both of these scenarios are bad and a pilot never wants to go fast enough for either one to happen.

Ask an Explainer
Categories: