TM  1-320  8  AIR CORPS  (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.  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?  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 :  Percent  Square plate--------------------- -------- ----- ------------ 104.5  Cylinder, horizontal--- ---------------------------------- 65. 5  Sphere--------------------------------------------------- 25.4  Cylinder, capped ends------------- ---- - - ------------------ 21.0  Airship model --------- - - •- -•- ---------------------- 3. 0  22  (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.  FIGURE  15.-Air  stream fiowing by  airship~  K=.OOl (average value) 