The Art of the Helicopter John Watkinson - Karatunov.net

112 The Art of the Helicopter
Fig. 3.41 (a) In a dynamic anti-vibration isolator, a lever is used to amplify the motion of a weight thus giving
it enormously increased effective mass. (b) The nodal beam system. A suitably tuned flexing beam will have
nodes where the hull is attached.
the lever causes any relative motion to be amplified. The lever changes the mechanical
impedance of the bob weight by a factor equal to the square of the mechanical advantage.
Thus a 1 kg weight on a 10 : 1 lever would appear to weigh 100 kg. Mounting
the transmission on, for example, four such mounts would raise the effective weight
by 400 kg. Given the increase in apparent weight of the transmission, the supporting
spring can be made much stiffer than would be possible with a conventional resilient
mount, without increasing the resonant frequency.
Figure 3.41(b) shows the nodal beam system developed by Bell. At one time most
Bell helicopters had two blades and suffered 2P hop. The nodal beam was developed
to isolate the hop. Between the hull and the transmission is a flexible beam made from
a sandwich of elastomer and steel. The transmission mounts are closer together on the
beam than the hull mounts. As relative movement takes place between the hull and
the transmission, the beam must bend. The proximity of the hull and transmission
mounts makes a virtual lever system resulting in larger amplitude of movement at the
centre of the beam. The effective mass of the beam is amplified by the mechanical
advantage of the virtual levers, making the system mathematically equivalent to a pair
of DAVI isolators in parallel. The geometry of the system is tailored so that at the
bending resonant frequency of the beam the hull attachment points are at nodes and
so do not move.
In active vibration cancelling systems, Figure 3.42, a number of high speed actuators,
typically hydraulically operated, are used to attach the hull to the transmission. The
actuators are controlled by signals from processors and accelerometers in the hull. The
signal processors are needed to adapt the waveforms fed to the actuators to changing
conditions such as all up weight and airspeed. An increase in all up weight would