BODY RESISTANCE 79 



area, of course, not the product of their length and maximum 

 width. 



Now take the geometrical shape of the cross section (or take 

 the shapes one at a time if more than one type of section is used 

 on the machine) and plot it on tracing paper to a scale which 

 will make it i" x 4", reducing the section in different ratios 

 horizontally and vertically if it is not of 4 : I fineness ratio : before 

 doing so, however, if the section has a pointed tail this should 

 be rounded as may be judged most suitable, see C, page 80. 

 Next, lay the tracing over shapes A and B, page 80, and then, 

 from a consideration of the shape of the traced section relative 

 to the shapes A and B, particularly at the tail end, and taking 

 account of the actual fineness ratio of the struts on the machine, 

 a suitable value of x can be read off from the curves (or at some 

 interval between the curves, if the strut is of intermediate type) 

 at the top of page 80. It is to be noted that in considering the 

 aspect ratio of inclined struts such as chassis struts and the gap 

 struts of staggered biplanes, the downwind dimension is to be 

 taken and not the dimension at right angles to the length of the 

 strut : also when obtaining the frontal area, the length is taken 

 in front view not the true length. 



Then if a is the aggregate area of front view in terms of a 

 unit i" x i' - o", the aggregate value of r for the batch of struts 

 is given by 



r = ax. 



Correction for Shielding. The shielding effect between front 

 and rear gap struts is inappreciable, and no allowance should be 

 made for it. 



Correction for Interference. If any pairs of struts occur side 

 by side in the machine at a distance apart which is not great 

 compared to the width of either strut, then the resistances of 

 such struts are to be increased by multiplying them by the value 

 of the correction read off the curve at the top of page 8 1. If 

 the two struts are of different widths, the mean of the two widths 

 should be used in reading the curve. 



Wires. First note the overall length / of each wire in 

 inches ; this may conveniently be done by scaling off a front- 

 view general arrangement drawing of the machine in fact, the 

 front view length and not the true length is to be taken : also 

 note the diameter d' of the wire in inches in the case of piano 



