Why Can’t Helicopters Fly Fast?


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There will probably never be any super-fast helicopters, image by Helen Krasner

Retreating Blade Stall

This process can only continue up to a certain point.

As with any aerofoil, if the angle of attack increases beyond a certain point, the blade will stall.

This phenomenon is known, perhaps unsurprisingly, as “retreating blade stall.”

It starts at the tip of the blade, since this is the area which has the highest angle of attack, and spreads inwards to the root.  And, basically, the retreating blade ceases to produce any more lift, as would occur with any stalled “wing.”

When this occurs, you might expect the helicopter to roll to the retreating blade side.  But in fact, gyroscopic precession means that the effect of the stalled blade occurs 90 degrees later, i.e., at the rear of the rotor disc.  Therefore the helicopter pitches nose up, then rolls to one side or the other, quite arbitrarily.

There are symptoms which occur before it happens.  Retreating blade stall is heralded by vibration and rotor roughness.   But since most pilots have not experienced the phenomenon, the symptoms may go unrecognised.  However, as the retreating blade starts to stall and the helicopter pitches nose up, this in itself will slow the helicopter down, and it may correct the problem before the result becomes catastrophic.

If the pilot does recognise what is happening, he or she should lower the collective to decrease the blade pitch angles.  Aft cyclic, which would raise the nose of the helicopter further, might sound as though it would save the day by slowing down the aircraft, but in fact, it ends up increasing the angle of attack of the blades.  However, once the collective has been lowered, then aft cyclic can be used to slow the helicopter down.

Air Compressibility

Another factor which can slow helicopters down is air compressibility.  At high relative airspeeds approaching the speed of sound, the character of the airflow is changed, and compressibility must be taken into account.  The helicopter may not be moving at anything approaching this sort of speed, but its rotor blades certainly are, and in this case, it is the speed of the advancing blade which can cause problems.  In forward flight at 150 knots, the advancing blade tip of a typical turbine helicopter has a speed of around 295 m per sec, very close to the speed of sound at sea level, which is 340 m per sec.   This means that compressibility is significant, which means that more power is required for the same rotor thrust, and shock waves may be produced with increasing vibration and noise.

So to summarise, if you get beyond a certain forward speed in a helicopter, your retreating blade suffers from the effects of too low airspeed, while the advancing blade has problems because its airspeed is too high.  So it should be quite clear now just why there are no super-fast helicopters, and probably never will be.

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