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b. The theory of dimensions shows that the coefficients of resistance. 

1’L vary directly as -;-• where 

v=velocity in feet per second. 

L=some convenient linear dimension of the body such as the diameter in the case of a cylinder. 

v= kinematic viscosity coefficient of the fluid. 

a. v, the kinematic viscosity coefficient, is defined as the ratio be› tween the absolute viscosity coefficient and the atmospheric mass density. Hence- e - 

v=~' where v 1s the absolute viscosity coefficient of the air and is a constant. 

vL d. -;’ called the Reynolds number after Professor Reynolds, depends on three variable quantities, p, v, and L : To predict full› scale performance from the model tests, allowance must be made for the fact that the L in the full-sized airship is very different from the L in the model, and consequently the co-efficient of resistance will be different. 

e. To overcome the effect of this difference a wind tunnel has been built at Langley Field in which p may be sufficiently increased to make the product pL for the model equal that of the full-sized small nonrigid airship, thus eliminating scale effect. 

15. Resistance of completely rigged airship.-a. There are• very little data available showing the relative resistance of the various parts combining to produce the total resistance of a completely rigged airship due to the difficulty in obtaining dynamic similarity between the model tested and the full-scale airship. 

b. Total resistance of airships may be subdivided approximately as follows for- 

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