AIR PERFORMANCE 125 



In consequence, the accurate calculation of climbs at all alti- 

 tudes is very laborious. 



Fortunately, however, the curve giving the rate of climb 

 plotted against altitude is generally quite a fairly straight one, 

 so that it is sufficient to calculate the rate of climb at three 

 altitudes only, namely, one a little below the estimated ceiling ; 

 one for standard density air, and one intermediate altitude. 



Times to Altitudes. C is the rate of climb in feet per 

 minute at an altitude. Plot C' against altitude. Then, if the 



plotting is not approximately a straight line, plot against 



\*s 



altitude in feet. The time to any required altitude is then the 

 area under the curve from the altitude of ground level to the 

 altitude in question. 



But if the plotting of C on a base of altitude z> approximately 

 a straight line, draw in that straight line which is the closest 

 approximation to the curve, and let it give C' = c at ground 

 level and let it give C' = o at a : feet above the ground. 



Then the time t in minutes up to any height a feet above 

 ground (where a is between o and a x ) is given by the formula 



t = - log 



w 



- a 



Ceiling. If these are not already available in curve form, 

 plot the machine performance curve (P on a base of speed) and 

 the constant torque propeller performance curve (P T on a base of 

 speed) both for standard density air. 



Then lay over this the celluloid throttling curves * so that 

 the axis of speed of the throttling curves lies on the axis of speed 

 of the P and P T curves, while the axis of horse-power of the 

 throttling curves lies on the axis of horse-power of the P and P T 

 curves. 



Now consider say four or five of the throttling curves which 

 cut the P curve towards the lower end of its speed range. Let 

 the intersections be at speeds V x , V 2 , etc. 



Now let these same individual throttling curves cut the P T 

 curve in points where the speeds are V T1 , V T2 , etc., respectively. 



Now draw up a table in the following form, allowing for as 

 many horizontal rows of figures as there are points to consider : 



* For the method of making these see page 126. 



