Bite-sized knowledge for the hungrily curious

Which way do helicopter blades rotate?

Photo Credit: Bradley's Aviation Photography Flickr via Compfight cc

The Answer

French or Russian helicopters tend to rotate in a clockwise direction. Those from the US and elsewhere generally rotate counterclockwise.

A helicopter can be designed to have blades that rotate clockwise or counterclockwise. However, the direction chosen seems to depend on where the helicopter is designed and built.

American helicopters manufactured by companies like Sikorsky, Boeing Vertol, Bell, and MD Helicopters all appear to rotate counter-clockwise [when viewed from above] … The opposite convention is used in all French helicopters built by AĆ©rospatiale … as well as all Russian helicopters built by Mil | Ask Us – Helicopter Rotation Conventions

Helicopters from the UK, Germany, Italy, and Japan follow the US convention.

How does the tail rotor differ according to the direction of rotation?

The rotation of the main rotor of a helicopter exerts a rotational force on the body of the aircraft. Thanks to Newton’s Third Law we know that the body of the aircraft would spin unless that rotational force was counteracted by an equal and opposite force. That’s where the tail rotor comes in.

In American helicopters, the tail rotor will need to exert a force in the clockwise direction, which French copters will need to exert a counterclockwise force. I would have guessed that this would dictate which side the rotor is on, but that doesn’t seem to be the case. Tail rotors can be designed to “push” or to “pull” and can be on either side.

What about helicopters with two rotors?

There are two main types of helicopters with two rotors – tandem rotor helicopters where the rotors spin on different axes, and coaxial rotor helicopters. In both cases, it’s necessary for the two rotors to spin in opposite directions (regardless of the country of origin). This is because the rotational force exerted by one needs to be cancelled out by the other.

Helicopter with Tandem rotor – MichaelFrey

The advantage of a double-rotor helicopter is that the thrust from all of the rotors is used to power the aircraft. In the case of a horizontally oriented tail rotor, some power has to be diverted from the main rotor to create thrust in a direction that doesn’t contribute to lift or forward movement.

If you reversed the rotation on a helicopter would it fly?

Suppose you could reconfigure a helicopter so that the main rotor and the tail rotor (or secondary rotor) spun in the opposite direction. Would this helicopter fly.

The answer to this question lies in the explanation of how helicopter blades create lift in the first place. The primary source of thrust is from the downward deflection of air from the bottom of the angled rotor blade. Rotor blades also typically have an aerofoil shape for a cross-section (like an airplane wing).

If you reverse the direction of movement of the rotor you would continue to have downforce from the deflection of air provided that you continue to angle the leading edge of the blade up. If simply just reversed the direction of the rotors, they would provide downforce instead of upforce, but since you’re already changing the direction of motion, we can presume that you can reverse the angle as well.

In this scenario you would lose some of the lift that you would normally get from the aerofoil because the shape is not as efficient for lift when oriented backward.

So, you would get lift, but it wouldn’t be quite as efficient as the lift you would get if the rotors were oriented in the correct direction.

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