The useful performance of any helicopter depends on the amount by which the power available exceeds that required. Helicopter performance is therefore measured in terms of the power required to maintain steady flight for various atmospheric conditions, over a range of weights and external configurations. Engine fuel flow data is also gathered to determine the helicopter’s range performance. It has been shown that the factors affecting the performance of a family of geometrically similar helicopters with similar rotor blade profiles are:

• engine power;

• helicopter weight;

• rotor speed;

• forward speed;

• rate of climb;

• ambient atmospheric conditions.

Although dimensional analysis is used to show how these quantities can be related the basic non-dimensional groupings are rarely used. Since usually the performance of a single model of helicopter is considered at any given time, the linear dimensions of rotor radius, chord and disk area are omitted. For a similar reason ambient pressure, temperature and density are expressed as ratios of the standard sea-level conditions (8, 0, a). Likewise, rotor speed ()) is expressed as a percentage of some reference or standard value ()). This, of course, means that the groups have become dimensional although they still contain the required information. These modified groups are often termed ‘normalized’, ‘referred’ or ‘reduced’.

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