34 AEROPLANE PERFORMANCE CALCULATIONS 



v '-4 v =i v 



and 



Hence we have the rule that to obtain the machine perform- 

 ance curve at an altitude, we find the value of or from the curve 

 on page 104, and then divide the values of the speed and horse- 

 power in standard density air (i.e. V and P) by Ja to get the 

 values of the speed and horse-power at the altitude (i.e. V and P'). 



The same rule would have resulted if we had followed the 

 proof of either of the other three methods, but there is no need 

 to go into that fully, as the fact is obvious. 



