How Do Helicopters Fly?


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For many people, helicopter flight is a mystery. Photo by Helen Krasner

Many people think it is a complete mystery that helicopters manage to fly at all.

They reckon that a helicopter is like a bumblebee – it shouldn’t be able to fly, but nobody told the bumblebee.

However, helicopters fly by the same aerodynamic principles as any other aircraft, and the basic principles are really not all that complicated.

Lift, Drag, Thrust and Weight

The force needed to keep an aircraft in the air is called ‘Lift.’  Lift is produced by air flowing over the wings.

The shape of the wings is designed so that the air flowing over the top surface has to travel further, and it therefore travels faster, than the air under the wing. This causes the pressure of the air on the top of the wing to be lower, and so effectively the wing – and therefore the aircraft – is sucked up by this lower pressure.

In fact, a certain amount of lift is produced by almost any object in a stream of air.  You can experience the process for yourself if you put you arm out of the window of a moving car.  You will feel your arm start to rise upwards, and this is due to lift.  But of course a correctly-shaped wing produces far more lift than this.

Lift is produced independently of power from the engine. However, the engine produces ‘Thrust,’ which gets the aircraft moving along the runway, so that air flows over the wings. Thrust also overcomes ‘Drag,’ which is the force which tries to oppose the motion of the aircraft through the air. The other force involved is ’Weight.’ and the weight of the aircraft determines how much lift is required to get it into the air.

As long as there is enough lift to overcome the aircraft’s weight, and enough thrust from the engine to overcome the drag, then the aircraft will fly.

What About Helicopters?

While a helicopter is a far more complex machine than an aeroplane, the fundamental principles of flight are the same.  The rotor blades of a helicopter are identical to the wings of an aeroplane -when air is blown over them, lift is produced.  The crucial difference is that the flow of air is produced by rotating the wings – or rotor blades – rather than by moving the whole aircraft.  When the rotor blades start to spin, the air flowing over them produces lift, and this can cause the helicopter to rise into the air.  So, the engine is used to turn the blades, and the turning blades produce the required lift. Very simple!

Controlling the Helicopter

However, there is rather more to it than that.  You need to be able to lift your helicopter into the air when you want to, rather than just have it rise up as soon as you start the engine.  In the case of an aeroplane, flight can start as soon as the aircraft is moving fast enough.  But in a helicopter, for all sorts of reasons, the blades need to turn at the same speed all the time.  So you need a different way to control the amount of lift produced.

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