The Art of the Helicopter John Watkinson - Karatunov.net

Introduction to helicopter dynamics 113
Fig. 3.42 In an active anti-vibration system, the transmission is mounted on actuators that are driven by a
suitable forcing function to cancel the vibration.
require increased static pressure in the actuators to prevent them extending. In practice
a feedback mechanism measures the average extension of the actuator and centralizes
each one with a relatively long time constant.
No practical hull can be rigid, and so there will be various modes of vibration excited
in it. One of the sources of vibration will be due to the non-uniform downwash impinging
on the hull surface. Conventional isolators between hull and transmission can do
nothing about this because they only oppose structure-borne vibration. However, an
active system can use the transmission as a reaction mass to create forces opposing
vibration from any source. Unfortunately the amplitude of vibration in the transmission
will be increased. Better results may be obtained by applying active control to a
raft carrying the engines, etc. as described above.
3.28 Harmonic pitch control
It was seen in Figure 3.30 that the use of sinusoidal feathering in a cyclic pitch control is
incorrect. The airspeed seen by a blade element changes sinusoidally in forward flight,
but the lift is proportional to the square of the airspeed. This non-linear function generates
flapping harmonics that cannot be cancelled by sinusoidal feathering. It follows
immediately that if there are to be no flapping harmonics, the feathering function must
contain harmonics.
A harmonic cyclic pitch waveform is approximately a square root function of a raised
sine wave although some modification will be necessary because of inflow effects. When
multiplied by the sine-squared function due to the speed, the product, namely the
lift, is constant. The pitch function above will need further refinement to account for
upwash at the leading edge of the disc. Figure 3.43 shows how an actuator between the
swashplate and the blade control arm could add the harmonic control.
In practice perfect constant lift can only be achieved at one radius, but it should be
possible to find a pitch function that makes the average blade lift moment essentially
constant. A more advanced system would have means to control the angle of attack
of different parts of the blade independently. This may be possible using, for example,
piezo-electric actuators. It is impossible to obtain constant lift over the whole blade
because of the reverse flow area, and so some harmonic flapping will always remain.
Whilst theoretically obvious, harmonic blade control, also known as higher harmonic
control, is rarely found. One of the difficulties is that the cyclic control becomes a