The Art of the Helicopter John Watkinson - Karatunov.net

Introduction to helicopter dynamics 111
Fig. 3.40 (a) A bob weight on a leaf spring attached to a bell crank will oppose vibrations in a control run.
(b) The pendular absorber is designed to oppose vibrations in the axial direction. (c) A bifilar pendulum absorber
achieves the effect of very short arms by using rollers as shown here.
softening the mounts, or by increasing the supported mass. Soft mounts may cause
other problems, such as handling difficulties because the hull would tend not to follow
the rotor in a manoeuvre. As a result, the best solution is to increase the mass to be
isolated as much as possible.
This is the principle of the machinery raft, which is a rigid structure carrying the
engines, transmission, control actuators, oil coolers, generators, etc. as a single unit.
This raft is then mounted to the hull by widely spaced resilient supports. The large
mass of the raft opposes the forcing function of the rotor and makes the resonant
frequency as low as possible, as well as reducing the amplitude of vibration seen by the
components on the raft.
The problem with resilient supports is that for good isolation they have to be soft,
and this will always result in a static deflection. Some techniques have been developed
for helicopters to circumvent the problem. Figure 3.41(a) shows a system developed by
Kaman known as DAVI, or dynamic anti-resonant vibration isolator. This works using
a mechanical impedance convertor to raise the moving mass artificially. Between the
transmission and the hull is a lever carrying a bob weight. The mechanical advantage of