38 AEROPLANE PERFORMANCE CALCULATIONS 



It will be noticed that cr has dropped out : from this we 

 deduce that the gliding angle in still air for a given value of \ 

 is independent of altitude. 



Equations (tan 0) and (a) are in a convenient form for 

 tabular treatment for values of X from -I to ro (see Chapter 

 XII., page 117). 



Such a table can be drawn up and then, by plotting, the 

 minimum value of tan 6 can be found : this is the tangent of 

 the "best gliding angle in still air" often referred to as the 

 tangent of the " gliding angle," simply. 



Gliding in a Wind. If there is a steady horizontal head 

 wind* of v miles per hour at the altitude in question, it will 

 be clear that a glide will probably reach further if the above 

 "best gliding angle in still air" is departed from in the direction 

 of faster speed relative to the air. For this reason the case of 

 gliding in a wind has to be treated separately from the case of 

 gliding in still air. 



We shall consider the problem at a definite altitude, and 

 shall employ the same notation as in the previous section, re- 

 membering that and V are now relative to the air. Let </> be 

 the angle of glide relative to the ground. 



As before, we have 



. . . (L) 



tma 

 and -- + -0051 



and in addition, by resolving vertically 



W=Lcos 0+T sin<9 . . (W) 



The last equation can be written 



* Winds are seldom steady in magnitude or direction, are often not horizontal, 

 and generally vary considerably both in magnitude and direction with altitude. 

 Consequently the investigation which follows, like other later investigations into 

 which a wind enters, has an appearance of getting down to hard facts which is 

 largely spurious. 



The results of such investigations, therefore, though a better guide to the pilot 

 navigator than they would be if wind was neglected, are only a guide after all. 



For information on the variability of wind with altitude, see " Manual of 

 Meteorology," Part IV., by Sir Napier Shaw, published by The Cambridge University 

 Press. 



