The magnitude of this lift is actually changed by altering the angle at which the rotor blades meet the air blowing over them.
This is known as increasing or decreasing the pitch angle of the rotor blades, and this alters the amount of lift produced. The pilot does this by using a control called the ‘collective,’ which is on his left side in the cockpit.
This control ‘collectively’ alters the pitch of the blades, and means the pilot can lift the helicopter into the hover when he decides to, by increasing the pitch on the rotor blades.
However, increasing the lift also means that there is more drag. So when the collective is raised, the pilot needs to open the throttle to produce more engine power in order to prevent the rotor blades slowing down.
There is a twist-grip throttle on the end of the collective for this purpose, but in most modern helicopters it is operated automatically, and the helicopter can be safely lifted into the hover using the collective alone.
Forwards, Backwards, and Sideways
The pilot now needs to be able to move the helicopter in different directions. This is done primarily by means of another control, the ‘cyclic,’ which the pilot holds in his right hand. The cyclic also changes the pitch angle of the rotor blades, but it alters each blade individually and by a different amount. The helicopter is designed so that the net result of this is that when the cyclic is moved forward, the helicopter moves forwards, when it is moved aft the helicopter moves backwards, and similarly to move sideways – something aeroplanes cannot do!
Turning the Helicopter
The fourth helicopter control is the yaw pedals. These alter the pitch angle of the tail rotor – the small rotor at the end of the helicopter. Doing this enables the pilot to turn the helicopter either left or right.
Coordination of Helicopter Controls
In practice, the three controls all affect each other to a certain extent, and have to be coordinated carefully in order for controlled flight to happen. It is this fact which makes helicopter flying rather difficult in the beginning. Also, various other aerodynamic factors complicate matters. For example, the turning of the rotor blades causes differing amounts of air to flow over each blade, and if this is not corrected for, a helicopter could easily turn over as soon as it started to move! So, there is actually a lot more to understanding the principles of helicopter flight than the above. We will look at some of the other things involved in later articles.
Decoding Science. One article at a time.