The Art of the Helicopter John Watkinson - Karatunov.net

Introduction to helicopter dynamics 107
Fig. 3.38 (a) Axial or hopping modes as a function of the number of blades. (b) In-plane modes have different
characteristics to axial modes because frequencies are heterodyned by the rotor frequency.
it has a number of negative effects. Section 3.26 showed some of the many causes of
vibration and it should be clear that many of these may result in severe alternating
stresses in the rotor head, the blade roots, the pitch control mechanism and the transmission.
This has a large impact on the service life of such parts where a failure cannot
be tolerated. Many parts need to have more generous cross-sections, increasing mass
and reducing payload. Periodic checking for cracks adds to maintenance costs.
A key problem due to vibration is that it impairs the pilot’s vision. The human visual
system (HVS) obtains its acuity by averaging visual information over a considerable
length of time. In the presence of vibration, the image will be unstable on the retina and
will thus appear blurred. There have been cases where damage to rotating components
has caused vibration severe enough that the pilot was unable to read the instruments at
all. Vibration contributes to pilot stress and workload. One of the earliest approaches
therefore was to reduce vibration at the pilot’s seat.
Given the low load factor of a helicopter, the hull is not highly stressed and could be
very lightly built were it not for fatigue inducing vibration. Increasingly helicopters