Static droop measurement

Measurement of static droop is achieved by setting the nominal rotor speed on the ground or in flight and noting the variation of Nr with power setting at a variety of airspeeds. For each test point the aircraft is flown accurately and the condition allowed to stabilize before data is recorded. For the zero airspeed point the NR value for a series of power settings is recorded as the aircraft is raised into an OGE hover and then higher powers are achieved during vertical climbs. If the aircraft has an avoid area then the OGE hover and vertical climbs are performed above the danger zone. An alternative to the zero airspeed point is to use the tethered hovering technique (Section 3.5.6.3) which will permit the full range of power settings to be tested without climbing. Forward flight tests are flown using a saw-tooth profile around the datum altitude starting at the power for level flight for the chosen airspeed. Static droop testing is often combined with aircraft performance and static stability testing. Test results are usually presented in the form of a plot of power or torque against NR for each speed condition, as shown in Fig. 7.7.

For aircraft without droop cancellation the rotor speed will depend only upon the power setting. The amount of droop will not be affected by the collective lever position and therefore a single test airspeed will be all that is required. For systems that

incorporate droop cancelling, airspeed becomes a factor as it will affect the collective position for any given power setting due to the different inflow through the rotor. In the same way the collective position will also be affected by the rates of climb and descent achieved for any given power setting which means that the aircraft weight will affect the results. To fully document the static droop of such a system requires, in theory, a large matrix of test points at various density altitudes, weights and airspeeds; in practice however, a zero airspeed, minimum power, and VH point at low and high density altitudes normally suffices.

To the operational pilot static droop is never a desirable system characteristic. A large amount of static droop at high power settings will cause the tail rotor to operate at a reduced RPM and may lead to a loss of tail rotor effectiveness. Some systems require the pilot to compensate for droop by providing a manual ‘beep’ control. However, this has the serious disadvantage of requiring the pilot or co-pilot to direct his or her attention inside the cockpit for relatively long periods often at critical moments such as when transitioning to the hover. Even if the pilot is not required to compensate and the aircraft handling is not affected, large variations in NR with power demand are a distraction for the pilot and should be considered to be a deficiency. A series of role manoeuvres that requires large changes in power setting such as lifting external loads or rapid transitions to the hover are flown to determine the effect of any droop.