Gust alleviation factor. Parts I, II and III

Abstract

Part I. The report presents the theoretical calculations of gust alleviation factor made for rigid aircraft and with one degree of freedom only (i.e., vertical motion). It is shown that for average gust lengths and for orthodox (tailed) aircraft the influence of thesecond degree of freedom (i.e., pitching) on the value of the gust alleviation factor is negligibly small, providing the pitching moment of inertia and damping are not unduly small. The influence of aspect ratio, the importance of the mass parameter and the gust shape on the values of the gust alleviation factor are shown. The large influence of the gust shape on the value of the alleviation factor makes the gust analysis by simple measurements of maximum aircraft acceleration inadequate, and the full records of the time-history of the aircraft are necessary. An alternative, more direct method of gust measurements is suggested. The inadequacy of the present gust alleviation curve in Air Publication 970 is pointed out and a suggestion for replacement of this curve by the curves calculated in this report is made. Part II. Theoretical calculations of the gust alleviation factor for a range of Mach numbers show an appreciable decrease in its value with increasing Mach number. The reduction in the value of the gust factor at M = 0.7 is about 10 per cent. for sharp-edged gusts and lightly loaded aircraft (μg = 20) and decreases to about 5 per cent. for gust length of 10 chords and heavy aircraft (μg = 100). The analysis of existing flight records indicates that the gust loads at high Mach numbers can be estimated satisfactorily if the gust factor and the lift slope are corrected for compressibility. Part III. The gust loads on swept wing aircraft can be split up into two parts, (a) gust load neglecting pitching motion and (b) correction to gust load due to pitching. Gust loads neglecting pitching can be estimated using the gust alleviation factor of Part I, taking the appropriate aircraft mass parameter and wing aspect ratio and replacing the actual gust length H in wing chords, by an effective gust length Ho~ = H eff + β, where β is the sweep of wing tip expressed in chords. The gust loads computed by this method give satisfactory agreement with gust-tunnel results. For an aeroplane with high wing loading and small aspect ratio the overall effect of wing sweep on gust loads is small.