Flying qualities problems of aerospace craft

Flying qualities and control problems concerning long-term dynamics of aerospace craft in hypersonic flight are considered. It is shown that there are specific dynamics characteristics which differ from stability and control properties traditionally associated with aircraft dynamics and related flying qualities requirements. Basic characteristics of long-term dynamics concern the existence of the height mode in addition to the phugoid. Their stability characteristics are described as well as their effect on response properties. Another topic dealt with is concerned with existing flying qualities requirements for prohibition of aperiodic instability and with related flight test methods. It is shown that there are deficiencies in regard to the connection considered to be existent between this requirement and related control gradients when applied in hypersonic flight. As a consequence, the requirement can be overrestrictive in one case and too lenient in another case. A requirement on the correlation between airspeed and pitch attitude for commanded pitch attitude changes does not account for the effect exerted by the height mode. It is shown that this effect is significant so that this requirement is not generally adequate for hypersonic flight. In hypersonic flight, specific problems concerning the simultaneous control of flight path and Mach number exist. It is shown which are characteristics of inherent vehicle dynamics as well as of control augmentation systems. This particularly concerns such augmentation schemes which are considered to generally correct long-term dynamics problems of aircraft. Control schemes using multiloop closures provide an effective means for succesfully coping with long-term control problems.