The Art of the Helicopter John Watkinson - Karatunov.net

Helicopter performance 327
Fig. 8.1 A density/altitude chart. This allows density altitude to be obtained from pressure altitude. The chart
corrects for ISA standard lapse rate and allows compensation for non-ISA temperature. See text.
subject to considerable variations. Sustained flight is not possible where demand
exceeds available power. In the opposite case, flight in equilibrium is possible with
a power surplus, usually called a power margin, which can be used for climbing or for
manoeuvres.
In general the engine(s) provide power which has a physical limit to the transmission
which has a physical torque limit. For a constant RPM machine such as a helicopter
rotor, power is proportional to torque and engines may be described in terms of torque
available. The engine power/torque available will be a function of density altitude. The
rate at which fuel can be burned is limited primarily by the temperature that the power
turbine blades can withstand. At low altitude, the compressor can produce plenty of
mass flow and the combustion takes place with excess air that limits the power turbine
temperature. As altitude increases the air density falls and the mass flow reduces. At
some point the amount of fuel burned must be reduced to prevent excessive turbine