AIR PERFORMANCE 121 



II. FULL POWER FLIGHT. 



Top Speed. Let the curves of machine performance and 

 propeller performance be plotted over one another. Then the 

 top speed is given by the intersection of the machine performance 

 curve with whichever is the lower of the two propeller perform- 

 ance curves. 



For this purpose the First Method of calculating a machine 

 performance curve gives only approximate results, and the Third 

 Method should be used instead. 



Top Speed at an Altitude. The procedure is the same as 

 above, but the curves for the altitude in question are to be used 

 instead of those for standard density air. 



Racing Machines. In the case of a machine solely designed 

 for best top speed in standard density air, the intersections of 

 the P T and P R curves with the P curve are identical in standard 

 density air, and at altitudes no ambiguity will be found to arise : 

 the ordinary method given above will be found to apply. 



Rate of Climb. There are three methods available, but, 

 as has been explained in Chapter V., page 35, only the First 

 Approximation should ordinarily be used. The Second Ap- 

 proximation is advisable for machines whose rate of climb is 

 exceedingly large, but to obtain accuracy logarithms should be 

 used with this method rather than a slide rule. The Third 

 Approximation is really hardly worth while : it is, however, 

 included here since, if concluded by the use of the method of the 

 Second Approximation, it is the most accurate at present avail- 

 able. 



First Approximation. Referring to the above-mentioned 

 curves for P, P T , and P R plotted on V, and calling P p the value 

 either of P T or of P R , whichever is the lower at each value of V 

 considered, and calling the climb in feet per minute C 



where W is (as usual) the weight of the machine in pounds. 



There is a value of C corresponding to each value of V, and, 

 therefore, a plotting is necessary to determine the maximum value 

 of C, and the value of V at which it occurs. 



The procedure at an altitude is the same, but the curves for 

 P', P T ', and. P R ' on V are used, of course, instead of the curves 

 for P, P T , and P R oa V. 



