138 



CHAPTER XIII. 



GROUND PERFORMANCE. 



Getting off a Deck. Turn to the curve of P T on V and decide 

 on a speed about three-quarters of the estimated minimum flying 

 speed : a reasonable guess will be near enough. Call this speed 

 V miles per hour. Note the value of P T in horse-power corre- 

 sponding to this speed. Call it P T . Then T is defined as 



T = I2,070?i. 



Now decide on the value of X for the run along the deck : 

 this can be got from the model tests and the angle of incidence 

 desired. This angle of incidence may be determined by practical 

 considerations : if not, take the angle which gives minimum 

 wing resistance. 



\ k,m ax , S, Rj, R 2 , W, d, S', L/D are all as defined on page 1 10. 



a = ll'4\k imax T 



K -00149508^ . nnT . OC fR R > '07625X LmflX SW<ft 



Wrf* - aS' L/D(W* - aS'/ 







+ i -- i. 



Now turn to the side view drawing of the machine and set it 

 at the stalling angle (taken from model tests) so that the relative 

 wind speed is now horizontal. 



From the centre of the propeller boss lay in a line aft and 

 horizontal, whose length represents, on any convenient scale, the 

 estimated minimum flying speed which is V. 



Also measure off ^bV aft along the propeller axis. 



Combine these two velocities by the parallelogram of velocities, 

 and note the point A where the resultant meets the line joining 

 the top and bottom leading edges of the machine. Also note 

 the angle of this resultant to the wing chord. 



Now turn to the model tests and note the lift coefficient k } ' 

 corresponding to this angle. 



