GROUND PERFORMANCE 67 



K 



Equation (/ x ) or (/ 2 ) gives the length of landing run, according 

 to whether a = o or not. 



In practice equation (/ 2 ) would give poor accuracy if a was 

 nearly equal to o : in that case, therefore, equation (/ x ) should be 

 taken instead as an approximation. 



Propeller Thrust at Slow Speeds. For speeds between 

 zero and the getting off speed, a reference to the figures on page 

 41, for instance, will show that the effective horse-power P T is 

 nearly proportional to the speed V. Therefore, the thrust is nearly 

 constant, though it is higher, slightly, at lower speeds. 



Let V be a speed in miles per hour somewhere round about 

 three-quarters of the estimated getting off speed never mind 

 details. 



Let P T be the effective horse-power from the P T curve at this 

 value of V. 



375? 



Then the thrust in pounds = y- 1 , and therefore T, the 



thrust in poundals, is given by 



32-2 x 375P T 

 V 



Total Resistance. If the machine is not designed with the 

 propeller shaft far from horizontal when getting off, we can em- 

 ploy the Third Method of machine performance calculation, 

 remembering, however, that the thrust to which the slip stream 

 is due is the full thrust just calculated. 



Using the same general line of investigation as on page 25, 

 and using the same units as there defined, but simplifying the 

 work by omitting to take account of /, /', and k c) we have 



2 W 



K'V 2 = [(i 



corresponding to equation (2) of page 25 ; 



W == -00237/yS - S' + (i + ) 2 S'](i'467V) 2 . (2) 

 corresponding to equation (3), page 25 ; 



^L = X/ W (3) 



