content_image/592041.jpg content_image/592033.jpg
(10) Struts of strecmWirw form.-It is fou~d in practice that the best fineness ratio for struts is 4 to 1. Inclining the strut to the vertical does not have the effect of reducing the resistance for streamline forms, but for blunter shapes (shorter than the true streamline) inclination reduces the resistance considerably. A group of strut sections are shown in figure 14 and the value of K for each shape is shown. It can be seen that the effect of yawing is to increase greatly the resistanoo by placing the strut sidewise or at a different angle to the air stream. 

(11) AirshVp cars.-All cars are built to take advantage of stream› line form. This is especially true of the inclosed models for which an average value of K is 0.001. However, there is a wide variance in the shape of airship cars and a corresponding variance in the value of K. 

K=.OOl (average value) 

<img src='content_image/592041.jpg'>

For each different shape a new value of K must be determined by wind tunnel test. 

c. The following problem illustrates use of the resistance formula: 

( 1) Problem.-(a) What is resistance of a fiat. plate 1 foot square placed at right angles to direction of motion when moving at a velocity of 30 miles per hour in air of standard density? 

(b) What is resistance at 60 miles per hour~ 

(2) Sol!ution. 

(a) Rv= KAV 2 =0.00328X 1 X900=2.95 pounds. 

(b) Rv = KA V $^{2 }$= 0.00328 X 1 X 3600= 11.81 pounds. 

This problem illustrates rapidity with which resistance increases with increasing velocity. 

d. Based on resistance of a fiat disk, the following shapes have the relative resistance shown below : 

<img src='content_image/592033.jpg'>

