A theoretical investigation of the response of a highspeed aeroplane to the application of ailerons and rudder

Abstract

The response of a fast moving aeroplane to a lateral gust, and to applied rolling and yawing moments, is examined by means of the differential analyser, taking a range of values of the principal lateral stability parameters, and including sufficient ranges of the other stability and inertia parameters to make the conclusions of general validity for high-speed flight. The motion following a sharp-edged side-gust is shown to be of a markedly oscillatory character, with an unpleasantly short period, particularly in small aeroplanes. The shortness of the period is probably the worst feature. A general survey is made of the dependence of the motion upon the various parameters, the differential analyser results being supplemented by the use of approximate formulae, which were developed with a view to this application. Particular attention is paid to the amplitudes of the motion in roll, yaw, and sideslip, and it is seen that it may be difficult to make the motion less unpleasant. The period may be lengthened by reducing nv, but the improvement that is possible in this way is limited. Damping can be improved by reducing dihedral, or by increasing body side area: the addition of a forward fin, ahead of the centre of gravity, would therefore be doubly helpful, lengthening the period and improving the damping. In studying response to applied moments attention is chiefly concentrated upon response to ailerons, and the theoretical results are compared with a theoretical standard motion produced by a constant roiling moment together with a yawing moment varied so as to suppress sideslip. Response at high speeds is shown to be insensitive to changes in lv and nv within their normal ranges, and good response to pure rolling moment is assured for all lateral stability characteristics other than those associated with the combination of small fin with large dihedral: this combination is worst at high values of the lateral relative density. The effect of adverse yawing moment from the ailerons is detrimental, and becomes worse as the dihedral is increased or fin area decreased.