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When the shaft of the helicopter rotates, it creates a low pressure. Because of the low pressure, the helicopter lifts. Is my understanding that this is just an application of Bernoulli's theorem?

Danu
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dexterous
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2 Answers2

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The blades of a helicopter are contoured much like the wing of an airplane. The physics behind both are, basically, the same.

Bernoulli's Effect is usually quoted as the reason behind flight in so many physics textbooks. While this isn't wrong, Bernoulli's Effect isn't actually the main reason that blades/wings can cause flight.

If you've noticed, the wing of an airplane is tilted a bit. This is so that the air molecules hit the bottom surface at an angle. If you have a ceiling fan, you can observe the slight angle in their blades too. This air hits the blade and is rebounded downwards. From the wing/blade point of view, it's being pushed upwards. This is what causes lift.

You can try this, by holding out a piece of cardboard while you're traveling in a fast car. Keep it horizontal and you won't experience lift. Tilt it a bit and you'll feel it being pushed up.
The Bernoulli effect simply adds to this.

mikhailcazi
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    +1 for making two significant points: 1) a helicopter blade is just like an airplane wing; and 2) Bernouilli adds to (but is not the major component of) lift - it is the angle of attack of the wing that results in increased pressure from below. In essence - the air "after the wing" is flowing in a different direction; and this change in momentum of the air is the result of force applied to the air (with the reaction force felt on the wing). See for example http://en.wikipedia.org/wiki/Lift_(force) – Floris May 13 '14 at 11:15
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You are basically correct.

The principle making a helicopter fly is basically the same as for a fixed wing plane. The "wings" of a helicopter are the rotor blades. They rotate at high speed and so have apparent velocity relative to the air, just as plane wings do when moving forward.

As the air moves over the blade it generates lift by deflecting the air and by the low pressure on top of the wing due to Bernoulli's principle.

There are some technical complexities to stop the helicopter spinning round or flipping over due to the torque generated but these are not really to do with lift generation.

nivag
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    Bernoulli's effect causes only a small amount of lift when aeroplanes are concerned. Planes have wings which are tilted upwards. When the wing moves through air, the molecules bounce off it, pushing it upwards. This force plays the major role. – mikhailcazi May 13 '14 at 10:15
  • Can you explain me how a low pressure is created when the shaft rotates? – dexterous May 13 '14 at 11:06
  • @SHREYASJOSHI the rotating rotor blade is moving through the air so it acts just as a normal wing. This wikipedia recommended by Floris in the comment to the other answer provides quite a good explanation. There are two factors. 1) Deflection of the air causes a reaction force upwards. 2) The air on top of the wing is constricted, therefore flows faster, resulting in lower pressure on top (Bernoulli's principle). – nivag May 13 '14 at 11:42