33 



The equation for the wire drag as a fraction of the computed frlctional re- 

 sistance of the model is: 



D s 2 C D d 3 'R 



— — = — [221 



R' f 15 C' f xLS l J 



where R is the model Reynolds number and L is the model length. As a conse- 

 quence of Equation [22] the smallest possible wire for adequate stimulation 

 should be used, since the drag of the wire varies directly as the cube of its 

 diameter. 



For the range of wire Reynolds number, 1 .5 x 10 2 = — ^10 3 the value 

 of C D is found to be about 1.2, (see, e.g., Reference 15)- Substituting the 

 values, d = O.OO267 ft, I = 4 ft, S = 128.8 ft 2 , x = 1 .25 ft and L = 25.5 ft 

 into Equation [22] and inserting the results into Equation [17] gives 



^..i?t.oVBO,lO-|r- [23] 



Curves of AC t /C. and AC f /C' from Equation [23] are plotted in Figure 22. It 

 may be observed that the curve of measured resistance corrected for wire drag 

 agrees quite well at the higher Reynolds numbers with the calculated curve of 

 ACt. /C' . This ratio is obtained by the evaluation of Equation [13] using the 

 data from the boundary-layer surveys made with and without the trip wire. 



CORRECTION FOR SAND-STRIP DRAG 



Similar calculations have been made with the data obtained from the 

 resistance tests with and without a 7/8-inch wide sand strip. The sand-strip 

 drag is assumed to be that of a trip wire, having a diameter equal to 0.00233 

 ft, the mean height of the sand strip. Thus for the sand strip, the corrected 

 curve for the increase in resistance was calculated from 



AC. AC. R 



7^ = pv-5- - 0.013 x 10' 10 4- [24] 



u f °f °f 



4C f AC t 

 The curve of Equation [24] is given with that for (f~~ = c 1 ln Fig- 

 ure 22 where it may be observed that the corrected curve from drag measure- 

 ments agrees fairly well with the curve computed from the boundary-layer sur- 

 vey data with and without the sand strip. 



