WATER PERFORMANCE 73 



machine in question : in the latter case, F and V refer to the 

 designer's machine, while /and v refer to the full scale figures 

 he is working from. 



Note. Float tests are usually quoted in knots instead of in 

 miles per hour. Such tests should be converted to miles per 

 hour for facility in working in with other performance calcula- 

 tions. If a speed V knots is given, this is the same as a speed 

 V miles per hour where 



V = 1-151 V. 



Water Performance. Now let V : miles per hour be the 

 getting off speed, i.e. the minimum flying speed for the angle of 

 attack of the wings when running on the water, and let P l be the 

 horse-power corresponding to V x on the machine performance 

 curve. Let V miles per hour be any speed less than V r Then 

 the value of F, the water resistance in pounds, for the speed V 

 is known. 



Now the total machine resistance when flying at speed V l is 



1 pounds. 



Therefore at speed V on the water, the total air resistance is 



Also the water resistance is F pounds, therefore the gross 



375^/Vx 2 



resistance is FT ( ^7- ) + F pounds. 

 \i xVj/ 



Now let P w be the total effective horse-power required to 

 maintain the speed V on the water, then 



Vr 3 75Pi/vy -1 



= vsl'vrW + F J 



(PJ 



With the aid of equation (P w ) we can now plot P w on a base 

 of V from V = o to V = Vj (where the curve runs into the 

 machine performance curve). 



Now let the constant torque propeller curve, i.e. the curve of 

 P T on V, be plotted on the same paper. 



The Getting Off Condition. The criterion that the machine 



