AIR PERFORMANCE 55 



the definitions of p and q, and their numerical values, are given 

 on page 19. 



Equations (N ; ) and (F) give for any speed and altitude the 

 revolutions and the ratio of the consumption to full consumption, 

 after V T ' and V R ' have been found from the value of V by the 

 use of the " Throttling Curves ". 



Best Cruising Speed at a Given Altitude. The consump- 

 tion per mile is proportional to the consumption per hour divided 

 by the speed, and, as we see from the last paragraph, the con- 

 sumption per hour is proportional to 



{' + 



therefore the consumption per mile is proportional to 



,/V'N 2 



We can now take a few values of V, find the above quantity 

 for each, and plot to find the minimum and note the speed at 

 which it occurs. 



This is the best cruising speed at that altitude for flight in 

 still air. 



For cruising against a head wind at the altitude of speed v\ 

 we get the consumption per mile made good over the map is 

 proportional to 



which again can be plotted to find the best conditions. 



Best Cruising Altitude. The best cruising altitude can be 

 found by comparing the best consumptions per mile at a range 

 of altitudes. In practice, however, the cruising altitude is limited 

 by navigation problems, so that the calculation, if made, has 

 perhaps only an academic interest. The influence of altitude on 

 cruising with and against the wind can also be investigated at 

 the same time. 



Long Range Cruising. By long range cruising is meant 

 cruising over such great distances that the weight of the fuel used 

 is a very serious item, requiring careful attention in order to get 

 the most out of a gallon. 



