The Art of the Helicopter John Watkinson - Karatunov.net

332 The Art of the Helicopter
Fig. 8.6 Maximum range speed. In still air this is found by a line from the origin tangent to the power curve.
In the case of wind the origin is offset by the windspeed.
obtained by finding the airspeed at which the rate of power increase is exactly equal to
the rate of airspeed increase. Figure 8.6 shows that this speed can be found graphically
where a line from the origin is tangential to the power curve. The presence of wind
changes the situation. In this case it is necessary to find the airspeed at which the rate
of power increase is exactly the same as the rate of groundspeed increase. Figure 8.6
shows that this can be found graphically by offsetting the origin by an amount given by
the component of wind velocity in the direction of flight. For a headwind, the maximum
range speed will be faster than that needed for still air.
There is a further complication in practice, because a real engine does not consume
fuel proportionally to power produced. This is particularly true of turbine engines that
sustain a loss of efficiency due to driving the compressor. This loss becomes a smaller
proportion of total power as power produced increases and so it is a characteristic
of a turbine that efficiency increases with power. Consequently the best range speed
according to Figure 8.6 is not strictly correct for a turbine helicopter. To be precise, the
machine should be flown at the airspeed where the rate of fuel consumption increase is
exactly the same as the rate of groundspeed increase. Figure 8.7 shows fuel consumption
against power normalized at minimum power speed. At (a) is the curve for a turbine
helicopter. This has the same trend as the power curve that is also shown, but does
not rise as steeply above minimum power speed because the engine is being used more
efficiently. It will be seen that the airspeed for the best range is somewhat higher than
the figure obtained in Figure 8.6. In practice the power curve, or the fuel consumption
curve, remains close to the line from the origin over a range of airspeeds so it is not
necessary to be rigid about the airspeed as the loss of range due to a moderate departure
from the ideal is not serious. In practice the actual airspeed can usefully be faster than
the theoretical maximum range speed by an amount that increases fuel flow by 1%.
This gives a useful reduction in journey time with a negligible range penalty.