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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• • 

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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. 

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H ence Vol=0.6562X1,485X200=195,000 cubie feet. 

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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. 

