AIR PERFORMANCE 129 



Best Cruising Speed at any Given Altitude. In a calm. 

 Take the values of a and V from the table of page 128 and work 



a 



out a column of y-,. Plot this column against V and note the 



value of V which gives a minimum. 



This is the best cruising speed in a calm at the altitude in 

 question, i.e. the speed at which the machine will fly most miles 

 to the gallon. 



Against a Head Wind of v Miles per Hour. Somewhat as 



before, plot yr~^ / and note the value of V at which it is a 



minimum. 



This is the best cruising speed at the altitude against the 

 wind v ' . 



Doubtless it will be desired to repeat this operation for a 

 range of wind speeds (some of them negative, i.e. following 

 winds), and for a range of altitudes. 



a 

 Best Cruising Altitude. Find y-, as above, plot it against 



CL 



V and note the minimum value of y-,- 



Repeat the work for a series of altitudes and plot the minimum 

 values of ^7 against altitude. Then note the altitude at which 



this curve has a minimum. 



This is the best cruising altitude in a calm. 



The best cruising altitude against a head wind v is got in the 



a r a 



same way, but using y, _ -, instead of y?. 



Cruising Range, In a Calm. First determine the altitude. 

 This is probably governed by considerations bearing on naviga- 

 tion, the avoidance of inconvenience to passengers at high 

 altitudes, or some such practical considerations. If not, take the 

 best cruising altitude found above. This determines cr from the 

 curve of page 104. Then find the best cruising speed at this 

 altitude as above, and call it V '. 



Now refer to the plottings of P T ', P R ', and P' on V for this 

 altitude, and, with the throttling curves, find the values of V T ' 

 and V R ' corresponding to V = V ' (this may involve interpolat- 

 ing by eye between two of the throttling curves, but that is easy 

 enough to do). 



