The Art of the Helicopter John Watkinson - Karatunov.net

Introduction to helicopter dynamics 99
Fig. 3.30 (a) Instantaneous airspeed experienced by a certain point on a blade is a fixed average added to a
sinusoidal variation. (b) Lift is proportional to airspeed squared. (c) Sinusoidal control of angle of attack due to
application of forward cyclic. (d) Non-constant lift around the disc is a source of vibration.
3.26 Sources of vibration
Whilst vibration in the hover can be minimized by attention to dynamic balance and
tracking, vibration in translational flight is inevitable because of the varying conditions
caused by the alternate advancing and retreating of each blade. This causes harmonic
flapping and dragging of the blades, resulting in forcing functions at the hub which
may be axial, i.e. along the mast, lateral, i.e. tending to cause rocking, or torsional, i.e.
tending to twist the mast.
On the retreating blade, the lift on the blade is concentrated near the tip with download
near the root. This is because of the reverse flow area shown in Figure 3.22. As
the same blade moves over the tail boom and into the advancing position, the lift distribution
changes so that the lift is now generated more inboard. If the blade is twisted,
the angle of attack at the tip may be negative at high forward speeds resulting in a
download. The situation is now exactly the reverse of the retreating case. Clearly the
blade is being excited by time varying forces and as it is not rigid it will flex.