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How Does A Helicopter Work?

A helicopter has one or more motor-driven rotors instead of fixed wings. It can take off and land vertically, move in any direction on a lateral plane, or hover in one place. The lift developed by a fixed-wing aircraft's wing depends on two things: the angle of attack of the wing and the velocity of the air in relation to the wing.

To get the necessary lift for flight in a conventional aircraft, the aircraft must have a forward movement. In the case of the helicopter, that forward movement is replicated by the rotation of the rotor blades; when the angle of attack gets to that magic number, the lift overcomes the weight of the helicopter. The aircraft then takes off - straight up! To move forward, backward, or sideways, the pilot tilts the rotor in the direction he wants to go. This is done by what is known as cycle pitch change - changing the pitch of each blade once per revolution. More particularly, the angle of attach of each blade is increased each and every time it is pointing in the opposite direction that the pilot wants to go, temporarily creating more thrust than the other blades. The thrust developed by the rotor can actually be figured into a vertical component (the force that keeps the helicopter in the air) and a lateral component (the force that moves the helicopter forward, backward, or sideways).

Each blade can swivel about its longitudinal axis and its pitch is changed cyclically, through a linkage system, by a swash-plate which rotates around the shaft and allows the blades to swivel as the rotor turns.

The blade has hinges called lag hinges. If there were no hinges, tilting of the plane of rotation of the rotor blades relative to the helicopter causes a lot of change in the speed of the blades. This would produce extreme stresses in the blades - these stresses are relieved and cancelled by the lag hinge. Motion about the hinge enables the blade to rotate at constant speed regardless of how much the rotor is tilted.

In forward movement of the helicopter, the velocity from blade rotation and velocity from overall forward motion are added together on the advancing side of the rotor; on the retreating side they are subtracted from each other. This means that as the rotor turns, one blade is moving significantly faster than the other (in relation to the air around them). If the rotor blades were rigidly fixed to the shaft, the lift would vary cyclically and cause the helicopter to roll. This is prevented by the lag hinges which allow the rotor blades to 'flap' cyclically as they rotate.

The rotation of the rotor tends to cause the fuselage of the aircraft to rotate in the opposite direction (on the principle that any action calls forth a reaction). To prevent this, the single-rotor helicopter is provided at its tail with a small propeller producing a counteracting sideways thrust. Alternatively, the helicopter may have two rotors which revolve in opposite directions and thus counterbalance each other.



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