Chicagoland's Only Aviation Museum

Principles of Helicopter Flight
(Single Main Rotor)

The Main Rotor generates lift in the same way as a propeller on a fixed wing aircraft.

Pitch of the Blades is altered, causing them to pivot together (collectively) to bite into the air more effectively, and produce more lift. When the lift attained is greater than the helicopters weight, it will rise.

As the blades' angle of attack is increased, they not only provide more lift but also provide greater airflow resistance as they rotate, demanding extra power to maintain blade revolution speed. This is supplied by a mechanical linkage of, in turbine powered helicopters, by the fuel system.

Collective pitch variation is responsible for controlling vertical lift, in which the rotor spins horizontally on its axis. The collective pitch lever is located at the pilot side (right side of the aircraft).

Directional Control - forward / backwards / sideways - is achieved by tilting the rotor disc to give a horizontal component to the vertical lift being created. The resultant force is a combination of vertical / horizontal components. If vertical lift remains equal to the helicopter's weight, the directional control will move the helicopter in the direction desired. The rotor disc is tilted via the cyclic pitch control, usually located like a conventional control column, between the pilot's knees.

Cyclic pitch also has the function of equalizing lift within the rotor disc during horizontal flight. While lift provided by each blade is constant in purely vertical flight, the blades' speed will vary when the helicopter is in horizontal motion.

When a blade advances, its airspeed relative to airflow increases, as does its lift. The reverse is true for the retreating blade. The blades "flap up and down", tending to flap up when advancing (more lift) and flap down (less lift) when retreating. But the advancing blade will lose lift because its angle of attack relative to airflow will be diminished, while the retreating blade will find its angle of attack - and therefore it lift - increasing. Gains and losses in lift cancel each other out. But the flapping motion of the blades continues, causing the rotor disc to tilt backwards. The pilot corrects this tendency to climb by pushing the cyclic pitch control forward, altering the pitch of the individual blades (advancing blade, more reduced pitch angle; retreating blade greater pitch angle). This restores horizontal stability by tilting the rotor disc forward once more.

The rotor head is the helicopter's Achilles heel. The shaft connecting rotor to can't be moved horizontally, It's rotating at high RPM. Directional control (cyclic pitch) is usually accomplished via a two piece "swash plate": two similarly sized circular plates connected by a bearing. The upper plate tilts and rotates with the rotor blades, to which it is attached by rods. The lower plate, to which the control transmission is linked, tilts only.

Cyclic pitch changes are attained by tilting the lower swash (changes in direction). This imparts the same change to the angle of the upper component via push / pull rods. The results - cyclic pitch changes as the rotor blades move around the rotor disc.

Collective pitch changes are obtained by moving both portions of the swash plate up and down the rotor shaft, affecting both rotor blades equally.

Inter-relation of Helicopter Controls

Change from hovering to forward flight requires:

1) immediate increase in collective pitch, because a portion of vertical lift is being translated into horizontal propulsion.

2) Increase in collective pitch may be accompanied by an overall power increase, which in turn will increase the torque developed by the main rotor.

3) The tail rotor, whose function is to offset main rotor torque, will requires an increase in collective pitch (via rudder pedals) to keep the helicopter on a constant heading.

A helicopter can be defined as: having a powered main rotor(s), creating a downward airflow through which the pilot exercises directional control in all planes of flight.

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Air Classics Museum of Aviation
43W624 US Route 30
Sugar Grove, IL 60554
Phone: (630) 466-0888


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