The Art of the Helicopter John Watkinson - Karatunov.net

152 The Art of the Helicopter
Fig. 4.32 A supercritical rotor has such a high dragging frequency that the rotor can never reverse it. No
damping is then needed.
becomes progressive again. The frequency then rises to become asymptotic to the
value determined by centrifugal stiffening. Point D in Figure 4.31corresponds to a
coincidence between a hull frequency and a progressive backwards whirling frequency,
at which ground resonance can occur. Consequently soft-in-plane rotors still require
damping.
A ‘stiff-in-plane’ or ‘supercritical’ rotor has a powerful dragging restoring force due to
its construction and so has a significant dragging resonant frequency, even at rest. When
the rotor is turning, the dragging frequency increases even further due to centrifugal
stiffening. Figure 4.32 shows that, by definition, in the case of a ‘stiff-in-plane’ rotor,
the whirling frequency is always higher than the rotor frequency. The behaviour of the
forward whirling is benign as before. However, as the backwards whirling frequency is
so high, the rotor frequency can never reverse it and so backwards whirling is always
regressive in the case of a stiff-in-plane rotor. In that case the unstable combination of
progressive backwards whirling can never occur and such a rotor can function with no
damping at all. Two-bladed teetering rotors take advantage of this concept. They are
built without dragging hinges to obtain supercritical dragging behaviour.
There is a small possibility of a damper failure and so it is as well to know how to
handle it. There are two ways to recover from incipient ground resonance, depending
on the rotor speed at the time it happens. If sufficient revs are available to permit flight,
the answer is to take-off immediately. With no ground reaction against the undercarriage,
the hull rocking resonance cannot occur and the shaking stops immediately. An
attempt at a gentle landing may succeed, but if not then a landing should be attempted
on a soft or high friction surface which is more likely to absorb energy than tarmac.
It is important that there is no groundspeed whatsoever at the moment of touchdown.
If insufficient rotor revs exist for flight, the solution is to change the frequency of the
excitation away from the hull/undercarriage resonance. This may be done by reducing
rotor speed by cutting the throttle and using the rotor brake if fitted.
4.18 Air resonance
Air resonance is a condition primarily relevant to helicopters having hingeless rotors.
In articulated rotors the natural frequency of the blade about the lagging hinge is low