V 



THEORY OF AN AEROPLANE ENCOUNTERING 

 GUSTS, II. 



By EDWIN BIDWELL WILSON. 



1. This discussion is an immediate continuation of my previous 

 treatment of the subject published in the First Annual Report of 

 the National Advisory Committee for Aeronautics, Washington, 

 191 5> PP- 52-75 (Senate Document, 268, 64th Congress, ist Session, 

 reference to which will be by pages). The notations of that work 

 will be continued without change except as hereafter noted. 



Periodic Longitudinal Gusts. 



2. That there is a certain degree of periodicity in gusts is obvious 

 from casual observation, from the records of scientific observatories 

 hke Blue Hill, and from the familiar fact that all such phenomena 

 in nature reveal a general tendency toward periodicity. Needless to 

 say the periodicity is not mathematically exact in its regularity nor 

 indefinite in continuance. 



The object, however, of an investigation of the effect of periodic 

 gusts on an aeroplane can for practical purposes be no other than 

 to reveal any exceptional effects that periodic, as compared with 

 single, gusts may have upon the flight of the machine ; and these 

 exceptional effects will probably be indicated with sufficient practical 

 completeness by an analysis built on the assumption of strict 

 periodicity, long continued in operation — the phenomenon most to be 

 feared being resonance. 



3. The longitudinal gusts are in, 1°, head-on velocity Mj ; 2°, 

 vertical velocity Wj ; 3°, rotary velocity q^. Very little is known as 

 to the nature of rotary gusts (p. 65) and hence 3° may be left aside. 

 It is not easy to see how vertical gusts can have any pronounced 

 periodicity; the disturbance of the aeroplane's motion by vertical 



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