The Art of the Helicopter John Watkinson - Karatunov.net

126 The Art of the Helicopter
Fig. 4.8 (a) The application of cyclic pitch in translational flight results in reduced induced drag on the
advancing blade, but increased profile drag. (b) The root of the retreating blade experiences down thrust due to
reverse flow.
called a Coriolis force, has to be imagined to act. If the head itself is completely rigid,
the in-plane blade movement due to conservation of momentum results in bending in
the blade. The resultant forces are often called Coriolis forces. In fact the forces arise
because the rigid head tends to prevent the blade conserving its momentum whereas
drag hinges allow momentum to be conserved.
In translational flight there is considerable asymmetry in the conditions experienced
by the advancing and retreating blades. The most obvious consequence of this asymmetry
is the application of cyclic pitch in order to produce equal lift moments on each
side of the rotor. This will result in the induced drag and the profile drag being functions
of blade phase angle and radius. Note that these two functions are quite different.
Figure 4.8 shows that the induced drag falls on the advancing blade because the angle
of attack is reduced, whereas the profile drag increases because the velocity of the RAF
is higher. Figure 4.8 shows that, at an inboard radius, reverse flow causes the root end
of the blade to develop an undesirable down force.
The result of these induced and profile drag variations will be in-plane bending
moments within the blade as well as overall in-plane moments at the blade root. Dragging
hinges are intended to relieve in-plane root moments. However, the presence of
dragging hinges allows in-plane blade motion and it is important that this motion is
stable. Unlike flapping, the restoring force when the blade drags away from its neutral
position is quite small and so the resonant frequency is much lower than the rotational
frequency. For in-plane motion, the degree of aerodynamic damping is also very small.
It was seen in Chapter 1that additional damping is often necessary on the dragging
axis to prevent ground resonance. This phenomenon will be discussed in sections 4.16
and 4.17.
4.7 Order of hinges
The order in which the flapping, dragging and feathering hinges are disposed is subject
to a certain amount of variation from one design authority to the next. The use of
coincident flapping and dragging axes is common because it allows a compact bearing