20 AEROPLANE PERFORMANCE CALCULATIONS 



but N' and N are equal, since they are each equal to TV, 



.-. V = V . . . (V) 



;*.'-<*, (PRO 



Equations (V) and (P R ') suffice to determine the P R ', V curve 

 for any altitude from the P R , V curve. 



Note that values of V determined in this work will differ 

 from those determined in the previous investigation of P T '. 



It will perhaps make for clarity of thought to remember that 

 at an altitude, if the throttle is left full open, the P T ' curve will 

 be developed. The range of this curve, however, at speeds 

 higher than its intersection with the P R ' curve, involve revolu- 

 tions greater than N: the engine should therefore be throttled 

 at these speeds till the performance falls on to the P R ' curve in 

 this region. 



The P R ' curve between the origin and the intersection speed 

 can never be developed : it is used, however, in later work con- 

 nected with throttling, and that is the reason why we retain it. 



