38 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 62 



miles per hour. The per cent difference between the velocity at each 

 point and the average velocity of 27.8 miles per hour was then calcu- 

 lated. The results are shown marked on the section of the tunnel in 

 figure 9. It is seen that the variation in velocity over the useful part 

 of the tunnel is from + i.i to — 1.2 per cent. It is believed that this 



48" 

 42" 

 36" 

 30" 

 24" 



lo 18" 

 ui 



I2' 



O" 



TOP o^ "TUNNJ EL 



12" 18" 24" 30- 36" 42" 48' 



J^ 



-2, 



+ Q > 



+ 1. 



A IL 



<- - 



4 



4 I. 



4 0, 



7 -I. 



I 4a 



I -10 4 I. 



t -0 



I -0. 



30 \n 

 3 



4a7 4a 



7 -Q 



o -I 



-o. 



- 1 



a6 



3 +0.4 -I 



4 



8 4Q 



o 0. 



-Q8 10,2 -0 



4 Ifii 



4 0. 



3 4Q 6 I 4 I. 



3 +0. 



7 -/, o ^0 2 404 4^a4 



40 



3 l-»l. | 



jiaz 



O -tQ6 1-0.3 



3 '-0. 



BOTTOKn 



Fig. 9. — Variation of velocity across section of wind tunnel. Points represent 

 per cent above or below mean velocity in dotted square which is 27.8 miles 

 per hour. 



variation is not too great for our purposes, and that the uniformity 

 of flow compares well with that to be found in other wind tunnels. 



With regard to the effect of the variation in velocity across the 

 section, it may be stated that such a small variation in velocity is 

 only of importance in tests on the moment tending to turn a model 

 aeroplane away from or into the wind. An excess of velocity on one 

 wing would give a tendency of the model to show a " lee helm." 

 However, if the model be reversed and the experiment repeated, the 



