28 



InU /U 

 that the ordinate — approaches the limit of zero at the origin more gradually than by 



a linear relationship. Until additional test data are available, it is suggested that a value of 

 g' = 7 be used for drag calculations. If the value of h = 1.42 (from the data for the Mark 13 

 torpedo) is also used in Equation [95], there results finally 



1/ \3.38 



fl„ = n 



(t) 



[96] 



CONCLUDING REMARKS 



• 



The method described in this report gives a relatively simple procedure for computing 

 the viscous drag of a body of revolution to an engineering degree of accuracy. Any marked 

 deviation between the calculated and measured values of drag should be expected only in the 

 case of a shape differing radically from the usual streamlined figure. 



To test the method of this report, drag coefficients of the model of the airship AKRON 

 were computed at various Reynolds numbers for comparison with measured values. '*°''*^ 

 Measured values of the pressure distribution and measured locations of the transition point 

 were used in the calculations. The computed values were found to lie between the measured 

 values as shown in the accompanying table. 



TABLE 2 



Comparison of Measured and Computed Values of Drag Coefficient of 

 1/40-Scale Model of Airship AKRON 



Reynolds 

 Number 



Drag Coefficient C^ 



Experimental 



Computed 



Wooden 



Model 



Metal 

 Model 



12.3x10^ 

 15.0 X 10^ 

 17.3x10^ 



0.0198 

 0.0193 

 0.0190 



0.0228 

 0.0223 

 0.0219 



0.0222 

 0.0216 

 0.0211 



In conclusion, it is noted that in the case of low Reynolds numbers the viscous drag of 

 bodies of revolution depends to a large measure on the position of transition, and that in the 

 case of high Reynolds numbers, it depends on the detailed development of the turbulent bound- 

 ary layer in pressure gradients. The current theories on turbulent flow are semi-empirical in 

 nature. Accordingly, Before all the processes determining viscous drag are fully elucidated, 

 there is need for accurate measurements of such factors as velocity profiles and shearing- 

 stress profiles in turbulent boundary layers especially at high Reynolds numbers. 



