PROPELLER PERFORMANCE CURVES 17 



character, refer to the condition where the engine is giving both 

 its full torque and its full revolutions in standard density air 

 generally called simply " the design conditions ". 



The Constant Torque Curve at an Altitude. Consider a point 

 on the constant torque curve, i.e. the full torque curve, in standard 

 density air, where the effective horse-power is P T , the velocity V, 

 and the revolutions N, and also a point on the full torque curve 

 at the altitude. 



Let the point on the curve at the altitude be so related to 

 the point on the curve for standard density air that 



V N' 



v~- = ~x 



Then it follows from propeller theory that 



H' P T ' 



H - p; ^ 



T /VV 

 and -^ = <r\jj) . 



p , y,y/ 



also of course -~- = ^T7 (4) 



Also the points we are considering are on the full torque 

 curves, 



Q- <2 ..... (5) 



and in addition it is generally known that the full indicated 

 torque at an altitude is a times that for standard density air, 



.-. Q'-<r<2 (6) 



Also we have I = H + F . . . . (7) 



and I' = H' + F' . . . . (8) 



Now assuming that the fractional loss is independent of. 

 altitude and only varies as the revolutions,* we have 



and -. .-' ( I0 > 



QN QN 

 also of course p- = j- . . . (n) 



* This assumption is justified by the fact that the rate of falling off of engine 

 brake horse-power with altitude can be correctly predicted from it. 



