The Art of the Helicopter John Watkinson - Karatunov.net

Introduction to rotorcraft 19
wings is that the former are in comparison very thin and flexible, and the forces acting
upon them are greater and more rapidly varying. The rotors have more to do than an
aeroplane wing, because they are also the control system. Chapter 3 explains how the
rotors produce lift and introduces the control of the machine.
1.12 The control system
The control system cannot be treated in isolation, but must be integrated into the design
of a machine from the outset. In the pure helicopter, control of the machine is achieved
entirely by changing the pitch of the main and tail rotor blades in various ways. This will
then determine the amount of engine power needed. The rotors are generally designed
to turn at constant speed and the throttle setting will have to be modified whenever the
rotor power demand changes so that the speed does not change. Chapter 6 considers
engines and power control.
There are two main forces acting on a helicopter, the force due to gravity, which is
always downwards, and the rotor thrust vector, which is always at right angles to the
tip path plane, otherwise called the rotor disc. Chapter 2 explains how the result of
forces acting in various ways can be predicted and Chapter 3 shows how rotors develop
thrust. The pilot can control the magnitude of the rotor thrust with the collective pitch
lever held in his left hand, and the direction of the rotor thrust with the cyclic stick
held in his right hand. The cyclic stick works in two dimensions: if the stick is pushed
in any direction, the rotor thrust tilts the same way. These two fundamental controls
are illustrated in Figure 1.21.
The blade movements necessary to produce lift and to achieve control will be outlined
in Chapter 3, whereas Chapter 4 treats the construction and dynamics of the blades
Fig. 1.21 The fundamental rotor controls.The cyclic stick tilts the rotor in the direction it is moved, whereas
the collective lever changes the magnitude of the thrust.