The Art of the Helicopter John Watkinson - Karatunov.net

Helicopter performance 333
Fig. 8.7 Fuel flow for a turbine helicopter (a) and gasoline powered helicopter (b) superimposed on power
required. Note these are not the same as the efficiencies of the two types of engine change differently with
load. The turbine is inefficient at low power and most efficient at full power. The piston engine is most efficient
at less than full power.
Gasoline-engine helicopters behave differently. At high power, the mixture may have
to be rich so that some additional cooling is provided by fuel evaporation and this
reduces efficiency. In cruise, the mixture can be leaned out to make the engine more
efficient and improve range. Thus a gasoline engine may be most efficient at some
fraction of full power rather than at full power as is the case for the turbine. As a result
the fuel consumption curve rises more steeply than the power curve away from minimum
power speed as shown in Figure 8.7(b). Thus the spread of speeds for maximum range
with a piston engine will be smaller than for a turbine. The gasoline engine does not
run on excess air and so available power will fall off rapidly with altitude. However,
if a turbocharger is fitted, engine power can be maintained at altitudes sufficient to
reduce parasite drag.
Where the utmost range is to be achieved, flying at an appropriate altitude may be
useful. There are two related factors to consider. The first is that all real helicopters
need an available load factor significantly above unity so they can manoeuvre safely.
The greatest efficiency is obtained by running the rotor blades at a CL of around
0.5, suggesting that the available ISA MSL load factor of a typical helicopter will be
around 2. By definition cruise takes place at unity load factor where the lift to drag
ratio changes little with CL. The second is that helicopters need engines whose power
is appropriate for the load factor and in cruise these will be running inefficiently at part
power. For the best range anything that reduces the load factor and improves engine
efficiency should be considered. Reducing the air density may do both. In the case
of a rotor running at constant RRPM, reduced air density will mean that a greater
angle of attack is needed. However, if this changes CL from 0.5 to 0.6, the increases in
induced drag and profile drag will be minor. However, there is a significant reduction
in parasite drag.
A turbine engine has no actual throttle and so operates with excess air, like a Diesel
engine. Unlike a Diesel engine, the compressor suffers profile drag. An efficient way of
throttling a turbine engine back to cruise power is to reduce the density of the inlet air.