Why Can’t Helicopters Fly Fast?

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Even large helicopters cannot fly very fast. Image by Helen Krasner

Helicopters cannot fly very fast.

Even the speediest helicopters move slowly compared to their fixed wing equivalents.

At a European air show on August 6th 1986, a slightly modified version of the ZB500 G-Lynx, manufactured by GKN Westland Helicopters, set a world speed record for a helicopter by flying at 217.5 knots, and the theoretical top speed for a rotary winged aircraft is only about 225 knots.

Limits to High Speed Flight – Flapback

The reasons for this are ultimately to do with the fact that a helicopter has rotating wings.  It is the helicopter’s spinning rotor blades which produce lift and enable the helicopter to fly, and these only produce an equal amount of lift in a still air hover.

When there is any wind at all, or the helicopter moves forward, the advancing blade (the forward moving one) has more air blowing over it – i.e., a higher airspeed – than the retreating blade (the backward moving one), and therefore produces more lift.

To counteract this dissymmetry of lift, we allow the blades to flap up and down, and this flapping equalises the lift across the rotor disc as the blades “flap to equality.”  But a side effect of this flapping is that when the cyclic is moved forward to increase speed, the rotor disc tilts forward initially, but then flaps back, and further forward cyclic movement is required in order to continue to accelerate.

This phenomenon, known as “flapback,” occurs throughout the whole speed range of the helicopter. So if we want to increase our speed, the cyclic has to be moved progressively further and further forward.  There will come a point at which the cyclic is on its forward limit, and the helicopter cannot fly any more quickly.

Airflow Reversal

However, in practice there are other factors which are likely to play a part in limiting the helicopter’s forward speed.  As the helicopter flies faster and faster, there is a progressively increasing difference in the relative velocities of the advancing and retreating blades.  To understand this, let’s put in some numbers.  If the helicopter is moving forward at 30 knots, assuming no wind, the difference between the airspeed of the advancing blade and the retreating one is 60 knots.  But if the helicopter is moving at 150 knots, this difference becomes 300 knots.  There comes a point at which the root of the retreating blade, which is the slowest part, has zero airspeed, since the helicopter is moving forward at a faster speed than that section of blade is rotating.  When that occurs, this particular section of rotor blade cannot produce any lift.

At first this only happens over a small area of the blade, but as the helicopter speeds up, this “airflow reversal” takes place over a larger and larger area of the retreating blade.  The only way the rotor system can compensate for this is for the outer part of the retreating blade to work harder.  So the outer section of the blade has to produce more lift, and it does this by operating at a higher and higher angle of attack, which is achieved through more flapping.  Although it doesn’t sound terribly efficient, the helicopter can operate quite happily in this condition, and tests have shown that some helicopters at maximum forward speed have 40% of the retreating blade affected by reverse flow.

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