How Do Helicopters Fly?

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Both aeroplanes and helicopters are flying machines. But there are some differences between an aeroplane and a helicopter. When it comes to aircraft, the sleek lines and lightning-fast speeds of airplanes can easily amaze people. Bulky, oddly-shaped helicopters rarely incite the same kinds of feelings.

Difference Between An Aeroplane and A Helicopter

The primary difference between a helicopter and an airplane is the way that the mechanics are designed to generate lift. Helicopters use their rapidly spinning rotors to create the momentum required to lift the aircraft. These rotors can be adjusted to perform maneuvers such as turning or hovering. Airplanes, however, are designed with mechanics that require them to be constantly moving through space so that the air flows over their wings. This is why airplanes require a long runway, while helicopters can simply take off from a smaller location.

How Do Helicopters Fly?

Flight of a helicopter is governed by the pitch or angle of its rotor blades as they sweep through the air. For climbing and descending, the pitch of all the blades is changed at the same time and to the same degree. To Climb, the angle or pitch of the blades is increased. To descend, the pitch of the blades is decreased. Because all blades are acting simultaneously, or collectively, this is known as collective pitch .

For forward, backward and sideways flight an additional change of pitch is provided. By this means the pitch of each blade increases at the same selected point in its circular pathway. This is the cyclic pitch. With these two controls in mind let us make an imaginary flight. With the engine warmed up and the rotor blades whirling above us in flat pitch, that is, with no angle or bite in the air, we are ready to start.

How Do Helicopters Fly

We increase the collective pitch. The rotor blades bite into the air, each to the same degree, and lift the helicopter vertically.

Now we decide to fly forward. We still have collective pitch to hold us in the air and we adjust the cyclic pitch so that as each blade passes over the tail of the helicopter, it has more bite on the air than when it passes over the nose. Naturally the helicopter travels forward.

Now we decide to stop and hover motionless so we put the cyclic pitch in neutral, the rotor blades now have the same pitch throughout their cycle, and the collective pitch holds the helicopter suspended in space without moving in any direction.

In short, it is the cyclic and collective pitch which gives the helicopter its unique ability to fly forward, backwards, sideways, rise and descend vertically and hover motionless in the air , making it one of the most versatile vehicles known by man.

How Do Helicopters Fly

Why Do Helicopters Have Two Rotors?

Helicopters have two rotors performing two different functions:

  • The Main Rotor: The lifting force is produced by the rotors. As they spin they cut into the air and produce lift. Each blade produces an equal share of the lifting force. Spinning the rotor against the air causes lift, allowing the helicopter to rise vertically or hover. Tilting the spinning rotor will cause flight in the direction of the tilt.
  • The Tail Rotor: The tail rotor is very important. If you spin a rotor using an engine, the rotor will rotate, but the engine and the helicopter will try to rotate in the opposite direction. The tail rotor is used like a small propeller, to pull against the torque reaction and hold the helicopter straight.

By applying more or less pitch (angle) to the tail rotor blades it can be used to make the helicopter turn left or right, becoming a rudder. The tail rotor is connected to the main rotor through a gearbox. When using the tail rotor trying to compensate for the torque, the result is an excess of force in the direction for which the tail rotor is meant to compensate, which will tend to make the helicopter drift sideways. Pilots tend to compensate by applying a little cyclic pitch, but designers also help the situation by setting up the control rigging to compensate.

The result is that many helicopters tend to lean to one side in the hover and often touch down consistently on one wheel first. On the other hand if you observe a hovering helicopter head-on you will often note that the rotor is slightly tilted. All this is a manifestation of the drift phenomenon. The engine(s) simply drive the rotors and did not assist directly with forward flight (like they would with a plane). There is a very small amount of thrust that comes from engine exhaust, but it is so small that it does not affect flight performance.

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