88 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 62 



important, the most general form of the resistance equation which 

 satisfies dimensional requirements is the following : 



R=.Av^f(;^). 



when L is a linear dimension, such as diameter of disk ; v the kinematic 

 viscosity of the air, taken constant ; and / an unknown function of the 



single variable 



This expression may be written 



R = pAV-(f){VD), when v is sensibly constant. 



The coefficient K should then be a function of VD and the curves of 

 figure 37 should coincide. This is not the case, and it may be con- 

 cluded that the effect of viscous drag is not important, or that the 

 disks are not geometrically similar since the thickness remains con- 

 stant. The critical velocity appears by figure 38 to be about the same 

 for each disk. 



For velocities above 27 miles per hour, the value of K remains 

 practically constant for all speeds and all disks. The usual formula 

 can, therefore, be applied if model tests are run at speeds greater than 

 27 miles per hour. Our wind tunnel testing will in future be con- 

 ducted at speeds between 27 and 40 miles per hour. 



The values of K for larger disks appear to be larger than those for 

 the smaller disks. This discrepancy may be eliminated when it is con- 

 sidered that the 6-inch disk obstructs 1.25 per cent of the tunnel and 

 should cause us to underestimate the mean velocity past the disk by 

 about 1.25 per cent, and consequently K as computed will be 2.5 

 per cent too large. The actual discrepancy between values of K for 

 the 2-inch and 6-inch disks is about 3.5 per cent, leaving i per cent 

 to be laid to experimental errors. 



It seems safe to conclude that for speeds above 27 miles the coeffi- 

 cient K remains constant and the same for all disks. 



The same conclusion was reached by Stanton^ and by Riabou- 

 chinski,^ but Eiffel ° states that K is greater for larger surfaces. His 

 tests in the wind tunnel certainly show an increase of K with area, but 



^T. E. Stanton, Proceedings of the Institution of Civil Engineers, Vol. 156, 

 Part II, London, 1904. 

 * Bulletin de I'lnstitut de Koutchino, Moscow, 1912. 

 ' La Resistance de I'Air et I'Aviation, Paris, 1912. 



