238 SYMPOSIUM ON AERONAUTICS. 



the gust are qualitatively as before. The oscillatory motion is not 

 pronounced; the ultimate side-slip velocity is — /; the ultimate dis- 

 placement has the same sign as / because the divergent term in 

 V — iiS-SiA is positive. 



39. Case 5. — Side-gust — oscillatory. When one examines the 

 records made or making at such an observatory as Blue Hill for 

 gustiness in the air, no phenomenon is perhaps more striking than 

 the reasonably periodic side-switching of a reasonably steady wind. 

 A south wind, for example, may whip back and forth between 

 S.S.E. and S.S.W. for hours at a stretch, as Prof. Alexander 

 McAdie has been kind enough to show me on some of his records. 

 In the absence of rotary motion, concerning which I am unable to 

 find satisfactory data, the simplest way to figure this change in direc- 

 tion is as a periodic side-gust. A machine going south in such a 

 wind would experience an alternating side-gust. (The oscillations 

 in the head-on velocity of the wind would be relatively very small 

 except for actual changes in head-on velocity superimposed upon the 

 changes in direction.) It is therefore especially interesting to dis- 

 cuss a periodic side-gust — this being the only periodic gust of which 

 we can reasonably be said to know anything at all definite. 



Let v^ = Je''''K We may assume, from our work above that the 

 rolling motion will be small and that the side-slip velocity v will not 

 be of as much importance in determining the path as the angle \f/ 

 coupled with the large forward velocity. The complex value of r is 



(280.7 — 32.S^pi)p'Ue'^ 



~ 2592/?* — 18000/?- — 854 -1- i(T)46iop — 23780^^) ■ 



If at any one place the period of the complete oscillation is 27r/n 

 with the wind velocity V, the distance traveled by the wind during 

 the time of an oscillation in direction is 2TrV/n, and this is the dis- 

 tance between the nodes of the motion. The time required for this 

 machine {U=^ — ii5-5) to pass over the distance 2TrV/n is 

 27rF/ii5.5«. The periodicity of the gust as it appears to the 

 operator of the machine will therefore correspond to the value p 

 = ii5.5w/F. For instance, if F = 20 and the time of an oscilla- 

 tion at one spot were 10 sees, so that ;; = o.63, the value of p would 

 be about p = 3-6, and the oscillations would appear to the pilot as 



