An optimal ride control system for an executive jet aircraft

Aircraft specifically designed for short take-off and landing (STOL) operations are particularly sensitive to atmospheric turbulence and produce relatively high levels of vertical and lateral accelerations. These acceleration levels cause discomfort, which is unacceptable in modern transport aircraft. Such aircraft ought to have their dynamics improved by the action of a ride quality control system (R.e.S.) which should effectively reduce these accelerations thereby improving comfort. Little attention has been given to date to the problem of designing R.e.S. for executive jets. But with the developing use of such aircraft which are increasingly of the STOL type the demand for an effective R.e.S. has intensified. A few earlier studies used conventional theory to derive the required control laws but so far the use of modern control theory to derive laws based on a multi variable description of the aircraft responses has not been widely tried. Multivariable control theories can be applied to STOL aircraft by making use of the active control technology (A.C.T.) concept. This research has employed both A.e.T. and modern control theory to derive a suitable optimal control system which uses several aerodynamic control surfaces in such a way that the required reduction of the acceleration levels can be achieved. The optimal control law used to provide ride quality control involved the use of elevator, rudder and ailerons, in conjunction with spoilers, and horizontal and vertical canards. The subject aircraft chosen for this work was a specially-modified NASA Jetstar. The uncoupled equations of motion of the aircraft, together with disturbances due to atmospheric turbulence, were simulated on a digital computer. Frequency response methods were also used to provide information for comparison with results from conventional control. The experimental investigations involved consideration of the combination of surface activity, the effects of non-linearities in the surface actuators and the dynamic response to both manoeuvre commands and stochastic disturbances, The best results, expressed in terms of reduction of the levels of the normal and lateral acceleration, were obtained when all available controls were activated simultaneously and reductions of the order of 40% were achieved. The effect of the optimal control law on the aircraft handling qualities was also investigated and compared with idealised models