Longitudinal static stability

Since the purpose of the pitch channel of an ASE is to maintain pitch attitude for long periods it should in theory have no effect on longitudinal static stability (LSS) data. If, however, the ASE features trim follow-up and the aircraft is significantly unstable, requiring continuous trim movement to maintain series actuator authority, it may be difficult to quantify CFSS using the conventional test. The validity of CFSS testing must therefore be considered. In normal operations the pilot will wish to select a given airspeed and maintain it with minimum workload. The quality of airspeed maintenance afforded by a typical ASE will depend on the tightness of its attitude hold (this can be assessed during ‘long-term testing’) and the variation of trimmed attitude with airspeed, which can be documented during a conventional trimmed flight control position (TFCP) test. Handling problems may arise during airspeed selection if, for example, movement of the stick to generate a new attitude causes the trim follow-up to activate, which in turn leads to oscillations in airspeed. Thus, precise selection of a given airspeed may be very difficult requiring inordinate pilot workload to match stick position and aircraft attitude with the desired ASI reading. A valid test of airspeed maintenance, provided the ASE does not feature trim follow-up and the cyclic has absolute centring, is the conventional release to trim test. It should be remembered that during this test the pilot is generating a pulse disturbance in attitude rather than airspeed. The ASE should quickly re-acquire the datum attitude, however, the helicopter may regain the original airspeed only very slowly, with many overshoots, or not at all.

7.5.2.3 Cross-coupling

Open loop changes in power should not cause any pitch/roll/yaw coupling if the attitude hold is sufficiently tight. Indeed this test can be used to assess attitude hold performance. The magnitude of the underlying cross-coupling can be gauged by observing series actuator activity. Large changes in power may cause the cyclic to move longitudinally through the action of the trim follow-up system.

7.5.2.4 Manoeuvre stability

Assessing the manoeuvre stability characteristics of an ASE equipped aircraft is difficult and less important than discerning the ease with which the pilot can turn the aircraft. If the role requires aggressive altitude changes (NOE flight) a qualitative assessment of the handling qualities during pull-up and pushover role manoeuvres, such as wire avoidance, should be considered. Formal academic PUPOs cannot be conducted, however, due to the suppression of pitch rates by the ASE. Thus, although level turns and PUPOs may be assessed qualitatively with ASE engaged the resulting cockpit control positions and activity should not be taken as indicative of the underlying characteristics of the helicopter.

Consider performing a level turn with altitude hold engaged and assume that turn co-ordination is active. The pilot will initiate the turn with lateral cyclic by selecting an appropriate bank angle. Since the thrust vector is no longer vertical more collective pitch will be required to maintain level flight and the lever will automatically trim upwards possibly generating a pitching moment. Any tendency to slip or skid in the turn will be eliminated by the yaw channel of the ASE. Movement of the cyclic stick in the longitudinal sense during the turn is dependent on many factors not least of which is the AFCS designer’s perception of the most suitable stick cues for this phase of flight. The tendency of the helicopter to ‘dig-in’ in steep turns should be completely suppressed by the fast-acting series actuators so that no residual movement is seen at the stick, although excessive instability could cause a trim follow-up system to activate.

In balancing the various forces and moments associated with turning flight it is quite possible, however, that the pitch attitude required for a given airspeed in level flight is different to that necessary to maintain the same speed during a turn. The pilot may therefore have to move the cyclic fore or aft to re-acquire the airspeed. The requirement to trim forward in this situation is not necessarily indicative of manoeuvre instability but simply means that a slightly lower pitch attitude is needed to hold speed. In order to provide the pilot with good cues in turning flight some AFCS have been deliberately engineered so that an aft cyclic stick deflection or force is required to increase and maintain the load factor regardless of the actual longitudinal cyclic pitch angle required at the rotor head.