TM 1-320  12-14  AIR CORPS  12. Index of form e:fficiency.- In general, in design it is desired  to get the greatest volume from the. least surface area as this  reduc~  weight and diffusion.  Fortunately, good streamlined shapes usually  have high prismatic coefficients, but of course some shapes are r.nore  efficient in this regard than others.  In studying relative efficiency  of shapes, both the resistance coefficients and the prismatic coefficients  must be considered.  The ratio of the latter to the former is called  the index of form efficiency,  H  t•  •  13. Dlustrative resistance problem.-a.  Proolem .. -Given  an  airship whose hull has a length of 200 feet and a major diameter  of  43.5 feet ’vith the hull offsets those of the  C  type airship envelope.  ( 1)  w•hat is total volume of  envelope~  (2) "What is hull resistance at 60 miles per hour in standard atmos› ph ere~  b. Solution.  ,(1)  Vol=Q.vAL  From Table I,  Qv  is 0.6562.  H ence Vol=0.6562X1,485X200=195,000 cubie feet.  14. Scale effect.-a. One great reason why so much difficulty is  encountered in determining prior to construction the resistance o£ the  completed hull lies in the fact that the resistance of the model cannot  be multiplied by the ratio of the linear dimensions of the model and  the completed hull to determine the resistance of the latter.  The  discrepancy between the calculated resistance and the actual resistance  of the full-sized airship is attributed to scale effect.  Often errors  in calculation due to :faulty data or bad theories are so explained away  by those responsible :for the mistakes.  There are several reasons how› ever, why, even with proper data and theory discrepancies will•exist  between calculated and actual resistance.  28  7rd 2  3.1416  A=4=  4  (43.5) 2 = 1,485 square feet.  (2) R=0Dp(vol)2/3vl.86  22  60  MP  H=60X15=88 feet per second.  GD  from T able 1=0.0136 at 60  M PH  R=0.0136 X0.00237X (195,000)$^{213}$X881• 86  R=455  pounds.  H ,= & 