GUST ALLEVIATION

The term gust alleviation interpreted in its broadest sense can mean the reduction of any response variable associated with the turbulence. If in these responses we include structural stresses and vehicle accelerations as well as attitude and trajectory variables it may well be that reducing one response increases another. For example, if pitch attitude is controlled to try to keep the lift constant, then a reduction in ДL2 would be associated with increases in AO2 and q2. The term gust alleviation is sometimes used in a more restricted sense, applied to the load factor only.

When one tries to control load factor by a feedback control to the elevator, the inherent time lag associated with pitching motion is usually such as to make this approach not highly effective (ref. 13.15). When a wing flap control is simultaneously used, however, to control the wing lift almost instantaneously in response to aircraft normal acceleration, pitch rate, and pitch attitude, reductions of an order of magnitude in An2 can be achieved (ref. 13.16).

This illustrates the direction in which we must go in striving for ideal gust alleviation (no doubt unachievable in practice). That is, the per­turbations in all forces and moments produced by the gust field should be just cancelled by automatic fast-acting aerodynamic devices, such as flaps and spoilers, circulation control, etc. The ideal result would be a vehicle that would have the same motion and structural stresses in rough air as in smooth—i. e. rectilinear translation and unity load factor, but with its various automatic gust alleviation devices being very active indeed. To be successful
such a system would probably need gust field sensors (perhaps angle of attack and sideslip vanes much like those used in the measurement of turbulence by aircraft) located at strategic points such as wing tips and tail. With suitable input-rate terms incorporated, sufficient lead time for actu­ating the aerodynamic devices might be obtained. There does not appear to be any fundamental technological impediment to achieving very sub­stantial reductions in gust response by this approach. Considerations of weight, cost, and reliability, however, may present serious economic and operational impediments.

[1] = K> v • • • > wo/’

a (9 x 1) vector. The method is therefore similar to the panel method in that the gust vector is defined by the turbulent velocities at a discrete set of points. To compute the aerodynamic transfer functions Skelton assumes