AIR PERFORMANCE 



117 



and at an altitude * 



I96W/ 



Gliding Angle. 



R is the total body resistance as found by the method of 

 Chapter VIII. , page 77. 



k^ m ax an d L/D as found by the method of Chapter IX., page 



8 5 . 



S is the total wing area in square feet. 



is the gliding angle in still air, i.e. the inclination of the 

 gliding path to the horizontal. 



Now write down the numerical values of R, k^ max , and S. 

 Then work out and write down the numerical value of 



R 

 a = 



Then construct a table in the following form (see next page). 



Next fill up the X column as shown and the L/D column as 

 explained in the definition of L/D given above. Then work out 

 the columns in succession. 



The last column gives the tangent of the gliding angle in 

 still air, and the result is independent of altitude, that is to say j 



* As a matter of fact this formula is not quite useless, as in hot countries the 

 average air density is below standard : the same applies to England in the summer, 

 and applies strongly to countries like Mexico and South Africa which lie at a great 

 height above sea-level : on the other hand, the accepted figures give the normal air 

 density in England as rather above standard (see curve on page 104). The matter 

 becomes of importance when designing machines for foreign countries or quoting 

 performance of existing types of machines to foreign buyers. The following table, 

 kindly communicated to the author by G. E. Petty, will be found very useful in this 

 connection : it gives what may be called the effective height in feet of the local sea- 

 level at the worst time of year for a number of places. Every feature of the per- 

 formance of a machine is, of course, affected by these considerations: 



