AIR PERFORMANCE 41 



Then in the case of Fig. I since the throttle cannot be more 

 than full open, the speed V 2 cannot be reached in horizontal 

 flight, and Vj is the true top speed of horizontal flight in standard 

 density air. 



Again, in the case of Fig. 2, though the speed V 2 could be 

 reached by leaving the throttle full open, this is forbidden as it 

 would result in over-running the engine, so again the true top 

 speed of horizontal flight in standard density air is V r 



Speed 



VtVzVo 



Speed 



VoV/Vz 



Top Speed at an Altitude. In finding the top speed at an 

 altitude two cases occur corresponding to Fig. I and Fig. 2, but 

 of course P R , P T , P, V , V lf and V 2 are replaced by P R ', P T ', P', 

 V ' and V; and V 2 '. 



In the case of Fig. I, the intersection of the P T ' curve and the 

 P 7 curve gives the top speed at the altitude V/. 



In the case of Fig. 2, however, V/, the intersection of the 

 the P R ' and P' curves is the top speed at the altitude. 



Racing Machines. In the special case of a racing machine, 

 i.e. a machine for which the propeller has been specially designed 

 to give the maximum possible top speed in standard density air, 

 we have the case of top speed = V . 



