Variation of static droop with airspeed
Having seen how the collective lever position varies with airspeed in level flight and with airspeed at constant torque it is now possible to surmise the effect of collective anticipation on the static droop recorded during climbs and descents at a range of fixed airspeeds. First the variation of collective position with airspeed, for constant torque, can be expanded for a range of torque values, see Fig. 6.12.
Now consider the basic function of collective anticipation; as the collective lever is raised the NF datum is increased in order to compensate for static droop. Suppose that for the example helicopter the standard rotor speed is 330 RPM and that with the lever fully lowered (0%) 0° of collective pitch is applied. Now assume that with the lever fully raised (100%) a collective pitch of 30° is set at the rotor head and that the basic static droop law causes a reduction of 20 RPM between 0% torque and 100% torque. Likewise assume that the engine control system designer intended to eliminate static droop by arranging that the NF datum be set to a rotor speed of 330 RPM with the collective lever fully lowered. As the lever is raised the NF datum is increased linearly to a maximum value equivalent to a rotor speed of 350 RPM. With these simple assumptions in mind it is now possible to determine the actual effects of the collective anticipator in flight. In the hover, 30% torque requires a collective pitch of 7.5° (a lever position of 27.2%). This will signal a NF datum equivalent to 335.7 RPM. The basic static droop law will result in a reduction of 6 RPM at 30% torque and so the rotor speed with collective anticipation will be 329.7 RPM. Similar calculations for 50% and 70% torque can be made; these are summarized in Table 6.1.
If the process is repeated for 140 KTAS different results arise due to the slightly
Table 6.1 Variation of rotor speed with torque – hover.
different collective lever positions required for the same three torque values, see Table 6.2. If, therefore, a static droop assessment is conducted on this aircraft in the hover and at high forward speed the results will not be exactly the same due to the influence of the collective anticipator.