1. The minimum Helicopter Theory

A helicopter can be viewed as being built up of 4 main parts: the main rotor, the engine, the tail rotor, and the fuselage. We will, briefly, look at each of them in turn. For an in depth treatment of helicopter theory, please visit the Engineering Section of the HeliStart website (Main menu -> Engineering).

 

2. Main rotor

The main rotor provides the thrust needed to move the helicopter. The area (plane) in which the rotors rotate is called the rotor disc. The attitude of the rotor disc can be changed under pilot control. It can be moved forwards to get a thrust vector which will move the helicopter forward, and in the same manner, the helicopter can be moved backwards and sideways. An important feature of the rotor system is that its rotational speed is (and has to be) constant. To change the amount of thrust needed, the pitch angle of the rotorblades is changed. The pitch of a rotor blade is defined as the angle of the chord to the plane of rotation.

When operating near to the earth’s surface (maximum distance being no greater than the rotor disc diameter), the so-called 'ground effect' comes into play. This means that the rotor system is producing more thrust than is the case when there is no ground surface nearby. The ground effect exists because of the higher air pressure produced by the rotor wake.

Rotor thrust is also dependent on the airspeed that is parallel to the rotor disc. When this airspeed is high (e.g. when the helicopter is moving quickly, or is encountering a strong wind) the rotor system will produce substantially more lift. This effect begins from relative airspeeds of 15 to 20 knots and upwards.

3. Engine

The engine provides the torque with which to make the main rotor and tail rotor rotate. As the rotor encounters torque applied by the engine, the engine encounters an equally reactive torque in the opposite direction. This results in the fuselage starting to rotate in the opposite direction to the main rotor. 

4. Tail rotor

As explained above, the helicopter will rotate as a result of the reactive torque encountered by the engine when driving the rotor. The tail rotor's main job is to counteract this effect by providing thrust in the opposite direction.

The tailrotor is also used to provide yaw control by controlling the amount of thrust generated by the tail rotor (by changing the blade pitch).

5. Fuselage

The fuselage is the 'enclosure' of the helicopter, and provides its necessary aerodynamic characteristics. At higher speeds, the fuselage stabilizes the helicopter's flight path.

6. The controls

A helicopter is flown by the use of 3 controls: the cyclic, the collective, and the pedals.

6.1 Cyclic

The cyclic controls the attitude of the rotor disc. By moving the cyclic in a forward direction, the rotor disc will tilt towards the nose of the helicopter. This leads to a forward thrust and movement. By moving the cyclic backwards, the helicopter will do so too. The same holds for sideways movements. The tilting of the rotor disc is initiated by changing the pitch of each rotor blade in such a way that the rotor disc tilts to the target direction. The rotor blade's pitch is cyclic, that is, it depends on the rotor blades’ angular position within the rotor disc, hence the name, cyclic.

6.2 Collective

The collective controls the amount of lift the helicopter produces. It therefore directly influences its altitude. The collective works by manipulating the pitch of all the rotorblades, independent of their angular position within the rotor disc, hence the name, collective.

6.3 Throttle

The throttle controls the engine power. In a helicopter there is always a (forced) relationship between the collective control and the throttle, because a high collective needs a large amount of power (large pitch angle, large amount of lift, large amount of rotor drag that must be overcome by engine power). Depending on the type of helicopter, the pilot has to manipulate the throttle in order to maintain a constant and valid rotor speed (rpm). Often, the terms collective and power setting are used interchangeably.

6.4 Pedals

The pedals control the yaw of the helicopter by changing the amount of tail rotor thrust. The tail rotor thrust is changed in exactly the same manner as the collective changes the thrust generated by the main rotor system.

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