The Art of the Helicopter John Watkinson - Karatunov.net

102 The Art of the Helicopter
Fig. 3.33 A rotor is not an ideal actuator disc. The downwash velocity is not constant but increases as each
blade passes and falls in between. A fixed point on a blade will cross its own path and the paths of other blades
in forward flight.
the point on the blade at which the lift dips is constantly changing in radius. However,
it can be seen from Figure 3.33 that at about 60 ◦ of azimuth the blade is nearly parallel
to the cycloid, meaning that a significant length of the blade is experiencing the effect
at the same time.
The non-uniformity of the downwash will also directly excite the hull and is commonly
observed as flexing of the canopy at the blade passing frequency. In the absence
of vibration structurally transmitted from the rotor, this mechanism will still vibrate
the hull.
In addition to the vibration sources noted above, in some flight regimes the rotor
may ingest its own tip vortices. These are mostly carried away by downwash in powered
flight or upwash in autorotation, but in some conditions, such as the end of a landing
approach or a roll manoeuvre, the vortex from one blade may hit the next blade headon.
Figure 3.34(a) shows that the effect of a vortex approaching a blade is a local
modulation of the RAFdirection causing a dip in the angle of attack. Figure 3.34(c)
shows that as the vortex leaves the blade the angle of attack is increased. The time taken
for the blade to transit the vortex is only a few milliseconds and so the resulting pressure
dipulse shown at (c) produces audio frequencies. This is the origin of blade slap.
In order to simplify study and to allow comparison between rotors turning at different
speeds, the frequencies of vibration are always divided by the rotor frequency. Thus the
units of vibration are not cycles per second but cycles per revolution. An imbalanced
rotor will cause a lateral vibration at the rotor frequency having a vibration frequency
of one cycle per rev. This is usually abbreviated to ‘one-per’.
In establishing the effect on the hull of vibrations created in the rotor, it is vital
to appreciate that there are two completely different mechanisms at work. Vibrations
in the direction of the mast axis and torsional vibrations about the mast axis are
directly transmitted and will be felt in the hull at the same frequency. However, the
rotor is turning with a frequency of its own and so vibrations in the plane of the rotor
are taking place in a rotating frame of reference. When these vibrations reach the hull,
which is in a stationary frame of reference, the same frequency will not be felt. Instead,