5.3.3 Manoeuvring at reduced load factor
For rotors where rotation of the thrust vector is the only source of a control moment (such as teetering rotors) the ability to control the helicopter will disappear if the load factor should ever reach zero. Hingeless rotors or those with some effective hinge offset will maintain more control power at low rotor thrust, but it might be noticeable that more cyclic control is required to return to level flight from a 0.5g push-over than from a 1.5g pull-up. Not only is control power reduced at low rotor thrust, but it can set up a potentially dangerous condition leading to ‘mast bumping’ on teetering rotor helicopters or ‘droop stop pounding’ on those helicopters with articulated rotors of low hinge offset. In either case, at a low load factor the tip path plane will respond normally to cyclic inputs, but the low rotor thrust will have little effect on fuselage attitude. The ultimate effect of this critical situation is that the rotor may flap outside design limits, leading to blade/fuselage contact or rotor separation due to mast shearing. A further degradation of handling qualities at low rotor thrust results from the reduction in the rotor’s contribution to rate damping (in both pitch and roll) since the damping effect of thrust tilt is reduced.
5.4 DOCUMENTING DYNAMIC STABILITY CHARACTERISTICS
5.4.1 The longitudinal long-term mode
All dynamic stability testing, but especially long-term response testing, demands that the initial trim flight condition be set up as accurately as possible. If this is not achieved then any control offset will bias the response and may cause, for example, a lightly damped oscillatory response to diverge aperiodically. If a very accurate engineering assessment of long-term response is required, then the tests are conducted in conditions of zero turbulence. In addition to quantitative tests, qualitative testing is conducted during role manoeuvres and comprises observing the pilot compensation necessary to suppress any undesirable dynamic effects such as a lightly damped longterm mode. Often it is these qualitative tests that are the most important part of the test programme as they show how the aircraft’s stability characteristics will affect the operational pilot when conducting the role.
To avoid damaging the aircraft during the large excursions from straight and level flight that can occur when documenting an aggressively unstable long-term mode it is often necessary for the aircraft to be instrumented with stress parameters monitored by telemetry. Practice recoveries from unusual attitudes followed by an incremental approach with some form of in-flight prediction of the severity of the next response, similar to that described in the section on lever delay testing, should prevent any exceedence of limits.