The Art of the Helicopter John Watkinson - Karatunov.net

Rotors in practice 147
Fig. 4.27 Effects of dragging blade motion. (a) A sudden application of drive torque may result in all three
blades dragging in the same phase. (b) When the dragging of each blade is at 120 ◦ phase to the next, the hub
moves in a circle. (c) If the phase relationship of (b) is changed, the direction of whirling is reversed. (d) Where
two blades drag in anti-phase and one blade is stationary, the hub is driven in a line. Note that all of these
examples are in rotating co-ordinates.
In the absence of preventive measures, if a resonant frequency in the hull is the same
as that of a whirling frequency, one of two things can happen. Either the hull will act as
a vibration absorber as was described in section 3.27, and the whirling will be opposed,
or the rocking will become increasingly violent until the machine either comes apart or
turns on its side. The phenomenon was first observed when a taxiing autogyro struck
a rock with a wheel and literally disintegrated.
Needless to say this Jekyll and Hyde behaviour in seemingly similar circumstances
means that the mechanism involved must be extremely subtle. Whilst the results of
ground resonance are painfully obvious, the mechanism causing it is not obvious at
all. Until a mathematical basis for the phenomenon was found, designers proceeded
empirically. For example, Frank Piasecki obtained an improvement on his first
machine by filling the tyres with cork. It was concluded early that ground resonance
is purely a mechanical phenomenon and that aerodynamic forces are not significant.
There are some parallels with whirling phenomena observed in other disciplines such
as steam turbines.