General test conditions
Tests are normally made at three or four values of referred weight, which are selected to give a good coverage of results from the maximum to the minimum permitted AUM. Normally only a central CG is used, unless the CG range of the helicopter is large as changes in CG may affect the results significantly. Datum testing is conducted with the helicopter in a clean configuration before it is evaluated in its normal role fit. Accurate testing can only be conducted in clear air, away from clouds, fronts and turbulence. The best results are often obtained late in the evening, or at dawn, when the air is generally more stable [3.6].
A limited performance trial, targeting only three values of referred weight for example, may be conducted economically by using the same mass and testing across three altitude bands. To get the best coverage of results, one height should be as low as practicable and one should be close to the ceiling of the aircraft, with one intermediate altitude. The rotor RPMs to be used will depend upon the test methods selected and the aircraft’s engine and rotor governing system. If the rotor RPM cannot be varied in flight, tests are done at the normal governed RPM using the W/am2 weight group. Relative rotor speed (m) may also be ignored in the case of a constantspeeding rotor governing system, such as a FADEC equipped helicopter, as opposed to the system described above that does not allow the pilot to compensate for the effects of static droop in flight. Use of the W/a group permits a more compact presentation of results but does not portray the effects of changes in rotor Mach number. It should therefore be borne in mind that significant errors could arise if the results are used to predict performance in temperatures markedly different from those in which the tests were conducted.
Data is normally gathered throughout the speed range from the lowest speed that gives a sensible indication up to VH, in 10 kt increments, with 5 kt increments around VMP and close to VH. Engine air bleeds and other secondary controls (cabin heating, engine anti-icing, oil cooler shutters, blade de-icing and other heavy electrical loading) should be pre-set as specified by the test schedule or adjusted to maintain optimum engine conditions. Datum results are initially obtained with all secondary controls off, and fuel consumption penalties then determined with these systems switched on.