The Art of the Helicopter John Watkinson - Karatunov.net

4.1 Introduction
4
Rotors in practice
The loads on the rotor blades are large and time variant and the articulated head was
developed to minimize them. Articulation also reduces the rate at which the aircraft
responds to the controls. In this chapter the dynamics and control of the articulated
rotor will be introduced, along with later developments such as hingeless rotors that
became possible with the development of modern materials.
The essential functions of the rotor head are to transfer shaft power to the blades, to
transfer the resulting rotor thrust to the mast, to resist the blade tension and yet to allow
the blades to move on their feathering axes so that cyclic and collective pitch control
is possible. This simple and elegant arrangement was initially not possible because it
requires extremely strong materials. Thus practical rotor heads are often much more
complex.
Figure 4.1demonstrates that the rotor blade of a typical helicopter pulls outwards
with a force roughly equal to the weight of a bus. It’s no wonder those floppy blades
straighten out. It requires little imagination to predict what would happen if a blade
attachment failed. Clearly the rotor head has to be extremely strong and reliable and
as a result it is one of the heaviest individual components in the machine.
Fig. 4.1 Given the coning angle and the weight of the helicopter, the blade root tension follows. Here it will
be seen that the same coning angle could be obtained by suspending the helicopter on cables with a bus on the
other end. Blade attachments have to withstand tremendous loads yet allow the blades to feather.