The Art of the Helicopter John Watkinson - Karatunov.net

Introduction to helicopter dynamics 97
The never exceed speed for the machine, Vne, will be set at a value which avoids
advancing blade compressibility stall.
3.24 The speed limit
The pure helicopter is forever trapped in a forward flight region where it can retain
control and contain blade forces. Figure 3.29 shows the constraints. The airspeed is
added to the tip speed on the advancing blade, and this must be kept below about 0.92
of the speed of sound to avoid excessive noise and blade forces. On the other hand the
airspeed is subtracted from the tip speed on the retreating blade and the advance ratio
has to be kept below about 0.5 to avoid a large reverse flow area and retreating blade
stall. As a result the pure helicopter with ideal tip speed is unable to exceed about 200
knots at the extreme right of the envelope of Figure 3.29.
If outright speed is not a priority, the possible range of tip speeds is greater. The
upper limit is set by noise and the lower limit by the requirement to store enough
kinetic energy to handle an engine failure. If a rotor has to provide thrust as well as
lift it will be tilted well forward at high speed and the hull will be creating a lot of drag
because of its nose down attitude. Higher speeds can be reached if an auxiliary form of
forward thrust is available because the rotor thrust can then be vertical and minimized
and the hull attitude will be better.
Westland modified the turbine exhausts to produce thrust on the speed recordbreaking
Lynx, and the Lockheed Cheyenne had a second tail rotor facing rearwards
Fig. 3.29 Pure helicopter speed is forever limited by the coincidence of advancing blade compressibility
and retreating blade stall at the right of the diagram. Only compound helicopters can break this limit using
auxiliary thrust and/or wings to offload the rotor. At lower speeds tip speed is constrained by noise and stored
energy limits.