# Rotorheads and Freedom of Movement

The purpose of the rotorhead is to rotate the blades and control the tilting direction of the rotordisc, which is the way in which the direction of thrust is managed. It turns out that, to function properly, the rotorhead must cope with rotorblades that want to move and vibrate, meaning that it has to support blade movements in several different directions! For this reason, understanding the design of a rotorhead begins with an understanding of what kind of blade movements must be supported. This section will summarize these for you. Note that these explanations are largely contained in the section: Aerodynamic Rotor Theory.

• The blades must rotate around a shaft (applying torque which leads to a circular motion).

• Blades have to feather (pitch) around their feathering shaft, in order to implement cyclic and collective control

• When the rotordisc tilts (as a result of cyclic control), the blades have to flap up and down.

• The asymmetry of the lift of a rotordisc in forward flight requires blades that can flap up and down.

• By applying cyclic pitch, a tilted rotordisc, in combination with blade coning, leads to the Hookes' joint effect: the blades must lead and lag in the plane of rotation.

• The flapping movement of the blades creates a change in their Centre of Gravity (C of G). The law of the Preservation of Angular Momentum dictates that the blade must lead and lag (the second reason for the lead / lag of rotor blades). An equivalent explanation is provided by looking at the Coliorus forces at work in a rotating frame of reference.

These are the most important reasons why blades must both flap up and down and lead and lag (also called blade dragging). As might be expected, there are several ways of designing a rotorhead. The required freedom of blade movement is realized by using hinges, and /or by applying flexible materials at strategic positions in the rotorhead. Moreover, the blade can be designed to provide some flexibility in itself (for example, in the root section).

A rotorhead which doesn’t support the required blade movements has to cope with the huge forces that work upon it, and the results thereof would be catastrophic mechanical failure. Hinges and flexible materials not only give the blades their required freedom of movement, but also prevent the blade forces (more accurately, moments) from being transmitted to the rotorhead.

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