Aerodynamics

[beansbaxter]

If an airplane goes too slow, it stalls. So why do helicopters stall by going too fast??

 

[Gibsons]

Originally posted by: beansbaxter
If an airplane goes too slow, it stalls. So why do helicopters stall by going too fast??

One side of the rotor is moving backwards relative to the direction of travel.

 

[murban135]

I believe it is because of the forward moving rotor blade. That blade is moving through the air at the speed of the aircraft plus its rotational speed. When this speed becomes fast enough, the airfoil reaches "critical mach" or the speed at which some localized airflow over a part of the airfoil becomes supersonic. This produces a shock wave that causes the airflow to separate from the airfoil.

 

[MetalStorm]

Originally posted by: murban135
I believe it is because of the forward moving rotor blade. That blade is moving through the air at the speed of the aircraft plus its rotational speed. When this speed becomes fast enough, the airfoil reaches "critical mach" or the speed at which some localized airflow over a part of the airfoil becomes supersonic. This produces a shock wave that causes the airflow to separate from the airfoil.

Indeed.

Also it is worth noting that aircraft stall is different to aerofoil stall. Aerofoils stall when the boundary layer seperates and as a result reduces lift due to an excessive angle of attack. That however does not mean the aircraft has stalled. The aerofoil still produces lift but as the angle of attack increases the lift reduces.
The aircraft stalls when it is no longer able to provide enough lift to hold its self in the air and will essentially fall to the earth.

 

[dkozloski]

The retreating blade stalls because as the forward speed of the machine increases, the relative speed of the retreating blade approaches zero. The pitch of the retreating blade is increased as it slows to keep the lift equal to the advancing blade until the point at which the lift decreases even as the pitch increases. A classic stalled airfoil.