The Art of the Helicopter John Watkinson - Karatunov.net

328 The Art of the Helicopter
temperature. From then on available power must fall. However, the fall in power is not
as steep as might be expected because intake air temperature falls with altitude and
this offsets the loss of density due to reduced pressure to some extent. As a result the
engine power/torque available curve may show the characteristic knee of Figure 8.2.
At low density altitudes the torque must be kept within the constant transmission limit
whereas above a certain density altitude the available torque falls.
The rotors in turn require power that is a complex function of density altitude and
airspeed. The power will be delivered to the main and tail rotors and will be absorbed
by various forms of drag. Power required to hover is clearly important and the worst
case will be at zero airspeed. Figure 8.3 shows a typical chart relating hover power
Fig. 8.2 Power available may be constant at low altitude owing to a transmission limit until turbine temperature
limits dominate at higher density altitude.
Fig. 8.3 Simple hover power chart.