40 AEROPLANE PERFORMANCE CALCULATIONS 



that v'^/o- remains constant, then tan </> is constant for a given 

 value of X 



That is to say, that if v *J<r is kept constant, tan </> is a function 

 of X only, and therefore the minimum value of tan </> and the 

 value of X at which it occurs are both fixed. 



Hence in practice we can find the minimum value of tan < 

 and the associated value of X for a range of values of v in standard 

 density air, then assign any required value to <j and immediately 

 obtain these quantities for a range of values of v by making 

 v'^/a = v for each of the values of v originally chosen. 



From this it follows that the work of finding the best gliding 

 angle for a range of altitudes and wind speeds is far less laborious 

 than would appear at first sight. 



II. FULL POWER FLIGHT. 



General. By full power is meant that the throttle is full 

 open, thus allowing the engine to develop its full torque for the 

 altitude in question unless that would cause the engine to exceed its 

 normal revolutions, in which case the engine is supposed throttled 

 down to its correct full revolutions. This last case is not con- 

 sidered to belong to Throttled Flight but to Full Power Flight. 



Another way of looking at it is to define Full Power Flight 

 as the condition when the engine is developing either its full 

 torque for the altitude in question or its full revolutions, but is 

 not exceeding either. 



From the point of view of safety of the engine from damage 

 there would be no objection to the engine developing more than 

 its full torque for the altitude, provided that that did not involve 

 the engine developing more than its full torque for standard 

 density air. Actually, however, as the throttle cannot be more 

 than full open, this case cannot arise. 



Top Speed. Let us consider a typical machine performance 

 curve with a typical pair of propeller performance curves plotted 

 on the same paper, limiting ourselves for the moment to the case 

 of standard density air. 



Let V be the speed at which the P T and P R curves (found in 

 Chapter III., page 16) intersect, let V x be the speed at which the 

 P curve intersects whichever is the lower of the P T and P R curves, 

 and let V 2 be the speed at which it intersects the higher. 



Then we are confronted with two types of case, illustrated in 

 Fig. I and Fig. 2. 



