Atmospheric Density and Power Required

HELICOPTER AERODYNAMICS WORKBOOK

CHAPTER 1

2. Induced power. This is the power associated with producing rotor thrust and must be

sufficient to overcome the induced drag which increases proportionally to thrust. In order to

maintain rotor thrust as air density decreases, angle of attack (AOA) must be increased by

increasing pitch on the rotor blades. The resulting increase in rotor drag requires an increase in

induced power to maintain a constant Nr. Increased density altitude affects induced power

significantly.

3. Parasite power. This is the power required to overcome the friction drag of all the

aircraft components, rotor blades being the exception. Parasite drag is constant for a given IAS.

As density altitude (DA) increases, TAS increases, and parasite drag will decrease slightly.

The combination of these ups and downs result in greater power required at a higher density

altitude.

Power required, the amount of power necessary to maintain a constant rotor speed, is

adversely affected by increased DA and decreased rotor efficiency. The pitch angle of the blades

must be increased to increase the AOA during high DA conditions in order to generate the same

amount of lift generated during low DA conditions. Increased pitch angle results from an

increased collective setting, which demands more power from the engine.

DA also affects power available, or engine performance. Turbine engine performance will

be adversely affected by an increase in DA. As DA increases, the compressor must increase

rotational speed (Ng) to maintain the same mass flow of air to the combustion chamber; and the

bottom line is, when maximum Ng is reached on a high DA day, there is a lower mass flow of air

for combustion, and therefore (because of fuel metering) a lower fuel flow as well. Thus, with

increased DA, power available from a gas turbine engine is reduced.

Since DA affects helicopter rotor and engine performance, it is a necessary consideration for

safe preflight planning. It can be determined in two ways: deriving a value from NATOPS

charts (figure 1-1) or a "rule of thumb" which can be used in the aircraft when no chart is

available (see figure 1-2).

THE ATMOSPHERE 1-3