NASA-TP-1043

Abstract

The maximum likelihood parameter estimation technique was used to extract values of stability and control derivatives from flight test data obtained from a light, single-engine, low-wing airplane. The flight tests consisted of 9 runs in which the stabilator was used to excite the longitudinal motions and 28 runs in which the rudder and ailerons were used to excite lateral motions. The various control inputs wee initiated from trimmed level flight with a trimmed airspeed of about 46 m/sec and an initial altitude of about 600 m. <br>The consistency of the derivative estimates as it relates to various inputs was investigated to determine the inputs which provide the most information for identification. Three criteria were used in this investigation: the ensemble variance, the estimated Cram&#233;r-Rao lower bound, and parameter correlations. There were no significant differences in the consistencies obtained by using the various stabilator inputs. On the other hand, for the lateral case, the sequential inputs (rudder followed by ailerons or ailerons followed by rudder) gave noticeable improvement in parameter value consistency over the rudder or aileron inputs individually. Also, some improvement in consistency was noted for both aileron and rudder square waves as opposed to sine waves. <br>The derivatives values obtained by using the maximum likelihood technique were compared with values computer by using an empirical approach. They compared favorably in direct comparison and in comparison of time histories of computed airplane motions, although the extracted parameters provided the better match.