Methods for measuring the steady-state performance of gas turbine-engined helicopters commonly use non-dimensional parameters [3.1]. These parameters consist of groups of relevant dimensional quantities arranged by means of dimensional analysis. Performance flight test then involves determining the relationship between pairs of nondimensional parameters whilst the others are held constant. We shall see that this experimental method of testing reduces any limitations in the applicability of forward speed, rate of climb, power and fuel flow test data. This is because data converted into a non-dimensional form can be used to produce information relevant to atmospheric conditions and aircraft masses different from those actually tested. Consequently, with few exceptions, a relatively small number of tests at carefully chosen test sites can produce information relevant to much of the helicopter’s flight envelope. The experimental method does however have some disadvantages in terms of planning and in the choice of non-dimensional grouping:
• Although the test method can yield large quantities of relevant data from a few test points [3.2] it requires detailed and careful pre-flight planning if the full utility of the method is to be achieved;
• The method can appear vague with alternative groupings possible;
• It is possible to require flight conditions, in terms of the non-dimensional groupings, that are outside the limitations of the aircraft;
• Performance limiting factors that depend on actual conditions may not be fully replicated although matching non-dimensional values have been targeted successfully.