Longitudinal static stability

Since a SAS only responds to rates of change of attitude it will have no effect on static stability data (apparent or collective-fixed). As there will be no pitch rate present when the test data is gathered the stick position will be a true reflection of the longitudinal cyclic pitch demand. A SAS will not, therefore, affect the ease with which a pilot can select airspeed as indicated by the trend from apparent static stability tests. The presence of a SAS will, however, affect the interpretation of collective fixed static stability (CFSS) results. Since in this channel the basic function of the SAS is to oppose pitch rates and hold a given pitch attitude (for a short period) it may also help to maintain airspeed. Thus any assessment of the helicopter’s ability to hold airspeed, as gauged from CFSS, must be modified by an appreciation of the effect of the SAS on pitch attitude maintenance and any subsequent airspeed keeping. If, for example, there is a strong relationship between airspeed and pitch attitude, as measured during the apparent static stability tests, then the SAS may be quite successful in maintaining an airspeed for a short period of time.

7.5.1.3 Collective-to-pitch coupling

Open loop trim changes with power (or collective-to-pitch coupling) are likely to be less ‘vigorous’ with the addition of a SAS. As the collective is raised and a nose-up pitching moment develops the resulting pitch rate will be sensed and opposed by longitudinal cyclic pitch at the rotor head. Since a pitch rate must develop before the SAS can act and it will quickly lose the original attitude datum it is likely that following a power change a residual pitch rate will exist. The SAS will not be able to completely eliminate the coupling since the nose-up pitching moment from collective will remain as long as it is above PFLF and the opposing control input from the series actuator will be directly proportional to pitch rate.

7.5.1.4 Manoeuvre stability

Although maintaining a steady turn is a quasi-static situation (a point of dynamic equilibrium) a SAS can have a significant effect on the pilot’s perception of manoeuvre

stability since any flight at reduced or elevated load factor requires the development of a pitch rate. During a pull-up manoeuvre or steady turn, the resulting pitch rate will be sensed and opposed by the SAS and in order to maintain the desired load factor additional aft cyclic will, therefore, be required. Since additional aft cyclic will be required for any load factor above unity (and vice-versa) the SAS has the effect of increasing the manoeuvre stability as seen in the cockpit, although the amount of longitudinal cyclic pitch required at the rotor head, for a given value of ‘g’, will remain unchanged. If the SAS has sufficient authority it can completely mask any tendency towards manoeuvre instability and can also prevent excessive control activity in regions when the helicopter is manoeuvre neutral/unstable by opposing the tendency to ‘dig­in’. It is usual however for series actuators to saturate at quite modest load factors, typically around 1.5g, due to the low authority inherent in that form of actuation. Consequently although with the SAS engaged there will be a greater stick migration with increasing load factor than with SAS off, the underlying manoeuvre instability of the baseline aircraft is still evident, as shown in Figs 7.10 and 7.11.