How Do Helicopters Fly?
Lift is Generated by Rotor Blades to Allow a Helicopter to Take Off
Aug 20, 2008
It has been said that a helicopter is like a bumble bee – it shouldn’t theoretically be able to fly, but no-one told the bumble bee. But this is not quite true. The same principles of flight as for aeroplanes enable rotary winged aircraft to become airborne. The process is just a little more complicated when the aircraft has rotating wings…
The Turning of the Rotor Blades Provides Lift.
In order for an aircraft to fly, its wings need to act as an aerofoil. This means that when air is blown over the wings, the pressure difference between the upper and lower surfaces allow the aircraft to rise into the air. For more details of this process, see How Do Airplanes Fly?
In the case of an airplane, air is blown over the wings when the aircraft moves forward. But for helicopters, this is done by rotating the wings. The rotor blades then act as an aerofoil, in the same way as for any other aircraft.
Altering the Pitch of the Rotor Blades.
The helicopter engine is therefore started, and this makes the blades rotate. The pilot then needs to alter the pitch of the blades, ie the angle between them and the airflow. This is done by raising the collective lever on the pilot’s left side. This increases the pitch of all the blades together, causing the required pressure difference, and the helicopter will then lift off the ground into a hover.
Moving the Helicopter Forward, Backwards, and Sideways.
What does the pilot do if he or she wants to move forward? He needs to change the direction of the lift force. This is done by altering the pitch of the rotor blades separately, using the helicopter’s cyclic control, so that the lift force has a horizontal component. Another way of putting this is that the cyclic is used to tilt the rotor disc, which is the hypothetical disc the rotors make when they are turning. The rotors then move in the direction of tilt, and take the helicopter with them.
The pilot therefore moves the cyclic in the direction in which he wants to go. He also speeds the helicopter up, slows it down, or makes it turn, in the same manner.
The Tail Rotor and the Helicopter’s Yaw Pedals.
Rotating the rotor blades would cause the helicopter fuselage to rotate in the opposite direction, if nothing were done to prevent it doing so. This is because of Newton’s Third Law, which states that every action has an equal and opposite reaction. To prevent this happening, a small rotor is placed sideways on the helicopter’s tail, and this pushes the fuselage back when it starts turning. The yaw pedals control the pitch of the tail rotor blades, and this also gives the pilot a means of turning the helicopter, by varying this pitch angle. Therefore the pedals are used to turn the helicopter on the spot.
Thus the pilot can lift the helicopter into a hover, move it in any direction he wishes, or turn on the spot. But is this all there is to it? No, but this includes all the basic principles of rotary flight. Future articles will go into this in more detail…
Reference
Helicopter Trial Flight Guide, Ruth Downey, Pooley’s Flight Equipment Ltd 2007
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