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Principles of Helicopter Aerodynamics

Helicopters combine lift, thrust and torque in a delicate balance to produce controllable flight and hovering. Their capability for vertical, forward and reverse flight, as well as static hovering, has produced a utility that fixed-wing aircraft cannot rival. Helicopters are indispensable to military, industrial, rescue and law enforcement applications. Versatility breeds complexity, however, and getting and keeping a helicopter in the air gracefully involves careful management of competing aerodynamic forces.
  1. Lift

    • The vertical force of lift is produced by the helicopter's rotor blades. Think of them as spinning wings or horizontal propellers, because they are really both. Like the leading edge of an airplane wing, a helicopter's rotor has an airfoil shape. When an airfoil moves through the air, its shape induces a zone of lower air pressure above it and higher pressure beneath it. This disparity produces lift. In a fixed-wing aircraft, the movement of the airfoil through air is provided by the forward motion of the airplane. In a helicopter, it's caused by the rapid spinning of the rotor blades.

    Thrust

    • Thrust produces horizontal flight. In fixed-wing aircraft, thrust generated by propellers or jet engines moves the aircraft forward. Thrust in helicopters is generated by tilting the horizontal spinning rotor blades in the desired direction of flight, which may be forward, backward or to either side.

    Torque

    • For every action there's a reaction. The rapid spinning of the rotor blades generates opposing torque that acts to turn the body of the helicopter in the opposite direction of the blades. This action is opposed by the tail rotor, a vertically mounted propeller with adjustable pitch that counteracts the torque forces. The amount of tail rotor pitch is controlled by the pilot's foot pedals.

    Hover

    • When the main rotors are in the neutral or flat position without any tilt, and the pitch of the blades and amount of engine power produces lift equal to the weight of the helicopter, the helicopter hovers. If the pitch of the blades is increased or decreased while the power setting remains the same, the helicopter will leave hover and vertically rise or descend. If the rotors are tilted, the helicopter will move laterally in the direction of the tilt. During stable hovering, the pitch of the tail rotor must be manipulated with the foot pedals to maintain nose heading and prevent yaw in response to torque changes or gusty winds.


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