Notes

How Helicopters Work

This chapter may be skipped if you are flying already – it's here for people who buy the book before they get their licence, or who feel they need a refresher.
The helicopter is just a flying machine that has its wings going round instead of remaining still, cynically referred to by some as 50,000 rivets in loose formation – this means that the lift-producing surfaces (i.e. rotors) are separate from the body, unlike an aeroplane.

Another difference is that an aeroplane engine is directly related to the forward movement of the aircraft, whereas the engine on a helicopter isn't – its function is to drive the rotor system which is really what makes the machine move through the air.
In fact, the rotors provide lift, thrust and directional control in one go – all three are separate functions on an aeroplane. In some ways, this is good, since loss of an engine on a twin does not require the strength of a gorilla to keep it straight, but it certainly makes the flying more demanding. In a helicopter, lift and thrust together (or the sum of the lift of all blades) are often referred to as Total Rotor Thrust: The Tip Path Plane is the path described by the tips of the rotor blades, viewed horizontally. The Coning Angle is the angle between the blade and the Tip Path Plane. The Rotor Disc is the area formed by the rotation of the blades, inside the blade tips. As the blades start coning, it will change its size slightly (the word rotor includes the blades, the hub and the shaft). The smaller the disk gets, the less area there is to generate lift – a situation that would arise if the RPM were too low, causing the coning angle to increase and centrifugal force to decrease. Coning is the resultant of centrifugal and lift forces – as the blades produce lift, the coning angle increases, but it decreases as RPM increases -the blades need centrifugal force for stiffness, and their ability to support the machine in flight. It is measured in tons, against pounds for lift, thrust weight or drag, which will give you an idea of its importance.