Flight idle power contribution

When helicopter pilots complain about the powerplants and governing system of their aircraft it is usually because power is not supplied quickly enough to match demand. There are occasions, however, when more power is supplied than the pilot wishes. This may happen when the pilot has lowered the collective lever fully or to a low position to descend or to reduce speed rapidly. Any power supplied by the engine(s) to the rotor when the collective lever has been lowered fully or to the collective position used in power-off autorotation is known as the flight idle glide power contribution. This situation occurs because the minimum fuel flow of the engine(s) at flight idle is set too high to allow the engine(s) to back off fully. This high minimum fuel flow may be set by the manufacturer deliberately either to prevent the poor acceleration characteristics that would occur at lower engine speeds or to avoid problems with governor stability once Nr is higher than Nf. It is important to realize that once a needle split (Nr indication higher than Nf indication) has taken place then the rotor is turning faster than the engine(s) power turbine (in percentage terms) and there can be no power contribution.

A FIG power contribution can present some serious problems. For example, if the pilot wishes to reduce speed rapidly and lowers the collective fully the undemanded power from the engine(s) will combine with the autorotational force to cause Nr to rise. The pilot will be forced to raise the collective to prevent a rotor overspeed and this will significantly reduce the rate at which airspeed can be bled off. During a quickstop manoeuvre this will result in a much increased stopping distance. A FIG power contribution will also have operational implications for rapid descents and for forced landing practice, as it will reduce the rate of descent compared to the real power-off case.

Documenting a FIG power contribution is achieved by comparing the rates of descent for a variety of collective lever positions with the engine(s) at flight and ground idle. The test is usually performed at the speed for minimum rate of descent and is conducted by timing the descent at each collective lever position through a band centred on a datum altitude. The results can be presented in the form of a plot of torque, ROD and Nr against collective lever position, as shown in Fig. 7.9. Of course the power-on line will be the static droop line. The important point on the plot is the point at which the collective position equates to the nominal Nr for a true autorotation. The manufacturer normally only gives a range of permitted Nr, therefore the test team has to decide on the nominal rotor speed to use in autorotation. This will usually be a compromise between a speed which provides sufficient rotor kinetic energy for an engine(s)-off landing and that which maintains a margin below the maximum Nr limit to account for rises in rotor speed during manoeuvring. Comparing the ROD at the collective lever position that gives the nominal Nr in autorotation with the ROD at the same lever position in a FIG allows the power contribution to be quantified both in terms of torque and decrease in ROD.