CHAPTER II. 



WING CHARACTERISTICS. 



General. The properties of the wing with which we are chiefly 

 concerned in performance calculations are k v the absolute lift 

 coefficient, L/D, the ratio of the lift coefficient to the drag co- 

 efficient, and k c , the centre of pressure coefficient* 



The original practice among designers was to note the values 

 of these quantities at a series of angles of incidence. This 

 practice, however, is unsatisfactory, as it does not lend itself to 

 the application to a model test of the corrections for aspect ratio, 

 gap/chord, stagger, wing tip shape, and dimensions, which have 

 to be applied to the test results on a model before they can be 

 used in calculating the performance of an actual aeroplane. 



The procedure here employed is to define a quantity X as 

 kj^imaxi the ratio of the lift coefficient to the maximum value 

 of the lift coefficient (i.e. to the lift coefficient at the stalling 

 angle). A series of values of X is then taken, -I, -2 . . . *9, I'O, 

 and k c , and the corrected value of L/D is noted for each, while 

 the corrected value of fc^ max is also noted (the last, in conjunction 

 with X, gives k l , of course). 



This procedure enjoys the advantage of lending itself satis- 

 factorily to the application of the necessary correcting process : 

 the reason is that the available tests on wing tip shape, for 

 instance, were done on a certain wing section, which wing 

 section has a certain range of lifting angles of attack and a 

 certain range of L : when, therefore, an attempt is made to 

 estimate on an angle of attack basis, with the help of these tests, 

 the wing tip corrections for a wing section which perhaps has a 

 longer range of lifting angles and a higher k^ max , no result can 

 be obtained : correspondingly, failure will result from an attempt 

 to estimate on a L basis. When, however, the value of X is 

 taken as the basis of estimating, no such difficulties arise, since 

 the wing and all the models made use of in the process have 

 values of X from o to I. 



* The angle of incidence comes into performance calculations only in connection 

 ith certain corrections for the action of the propeller slip stream on the wings. 



wi 



These will be dealt with later. 



