How does a helicopter fly?

Adelaide Helicopters HYAThis is not meant to be a comprehensive description but is designed to give you a basic understanding of what makes a helicopter fly and what the controls do in various phases of flight.

The type of helicopter referred to in this article has one main rotor, turning anti-clockwise as viewed from above, and a conventional tail rotor. This is the most common arrangement in modern helicopters.

The function of the main rotor is to lift, propel and control the helicopter. The main rotor consists of 2 or more rotor blades that, like wings, derive their lift from the speed that they travel through the air (a combination of Rotor RPM and helicopter airspeed) and the angle that they strike the air (Angle of Attack).

This Angle of Attack is an aerodynamic angle that is a combination of the Pitch Angle that the pilot selects, and the direction of the air approaching the rotor blade.

The pilot maintains the Rotor RPM of the Schweizer 300 (a typical light helicopter) between 440 & 460 RPM in flight, and at this speed there is a centrifugal force in excess of 4 tonne trying to wrench each blade out of the rotor hub. With this amount of inertia it is not possible to alter the RPM quickly enough for it to be an effective means of controlling the lift, therefore the only viable means of controlling lift is by altering the pitch angle of the blades.

The pilot controls the helicopter with 4 controls:

The COLLECTIVE lever, which is held in the left hand beside the pilot. When raised or lowered, it increases or decreases the pitch angle of ALL the blades collectively, this in turn, increases or decreases the amount of lift that the rotor produces.

The CYCLIC control, which is held in the right hand between the pilot's knees, can be moved fore & aft or left & right. Moving the cyclic increases the pitch angle of the blades as they reach a pre-determined point around the rotor disc, and decreases the pitch angle of the blades 180 degrees later. This causes one blade to climb and the other to descend, which tilts the entire rotor disc. The lift from the rotor is produced at right angles to the plane of rotation of the rotor disc and therefore, by tilting the disc, you can align the rotor thrust in the direction you want the helicopter to go. The helicopter will then move in the direction that the cyclic is displaced, at a speed that is proportional to the amount of stick displacement.

For every action there is an equal and opposite reaction. This means that the torque of the engine driving the main rotor creates an opposing force in the opposite direction (torque reaction). If this force was not controlled, the helicopter would rotate underneath the rotor in the opposite direction. This controlling force is derived from the Tail Rotor. The engine drives the main rotor gearbox and the main rotor gearbox drives both the main rotor and the tail rotor (hence the tail rotor continues to turn if the engine stops). The pilot uses the TAIL ROTOR PEDALS to alter the pitch angle of the tail rotor blades in order to control the amount of tail rotor thrust. The pedals are interconnected; pushing the left pedal forward brings the right pedal back as it turns the helicopter to the left.

This only leaves the THROTTLE to discuss. This is a twist grip control that forms the handle for the collective lever (which is held in the left hand). There is a cam in the collective control system that opens the butterfly in the carburettor (or fuel injection unit) as the collective is raised, and closes it as the collective is lowered. This cam is the primary means of keeping the RPM in its normal operating range during flight. In flight, the throttle is only used to make minor changes to the engine power to maintain the required RPM.
Individually, the controls are not difficult to understand or use, however the inter-relationship between the controls makes a helicopter more difficult to control than an aeroplane.

For example, when you raise the collective, the load (and drag) increases with the increase in pitch angle, and the cam increases the engine power to maintain the RPM. This increase in engine power causes the nose to turn to the right (torque reaction); at the same time the nose pitches up and rolls to the right, requiring forward cyclic and left cyclic to maintain the required attitude.

If you push the cyclic forward you tilt the main rotor and create more forward thrust but less vertical thrust, therefore the speed will increase but the helicopter will descend. As the speed builds up, the nose pitches up, rolls to the right and turns to the left. This requires more forward cyclic, some left cyclic and some right pedal. The RPM will increase slightly (rotor efficiency increases as the forward speed of the helicopter increases) requiring a decrease in throttle setting. This requires co-ordinated control movements to achieve a smooth flight..

But what happens if the engine stops?

This is a question that we get asked frequently; Flight after the engine stops is called Autorotation. When the engine stops, the nose will turn rapidly to the left because the pilot had left pedal applied to counteract several hundred horsepower that is suddenly lost when the engine fails.

The pilot must push in enough right pedal to keep the nose straight and IMMEDIATELY lower the collective to the bottom stop to reduce the pitch angle of the blades and therefore reduce the drag (and the lift) from the main rotor.

Without the lift from the rotor, the helicopter commences to descend, creating an airflow that is directed up through the rotor. This upward airflow drives the rotor (almost like a windmill) and actually increases the Rotor RPM slightly over what it was in powered flight. From this point on, the helicopter is established in autorotation.

The helicopter descends at around 1,800 feet/min with the pilot controlling the airspeed to around 55 knots (100 kph). At approx 15/20 metres above ground level (AGL) the pilot pulls the nose up (back cyclic) to reduce the forward speed, and then, as the helicopter descends through 1 to 2 metres AGL the collective must be raised to increase the lift and cushion the landing.

This collective pitch pull has to be perfectly timed, as there is no engine power to maintain the Rotor RPM at the increased pitch angles, therefore the RPM decreases RAPIDLY along with the lift that is generated. This sounds like a critical manoeuvre and it is, but helicopter pilots are trained to handle this, and other emergencies during their initial training, and again with constant check and training flights throughout their flying career.

If this sounds interesting, come along and have a
Trial Introductory Flight.

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