GROUND PERFORMANCE 69 



off the deck. Generally, however, the angle of attack for the 

 run will be determined by practical considerations, but as near 

 as may be to the angle of least resistance of the wing. 



In this case, find the value of X corresponding to this angle 

 on model tests (this is quite near enough) and then find K' from 

 the above equation. 



For landing, we may take X = ro and T = o, . . a = o and 

 we have 



K , _ R! + R 2 'OOSifcjjuuS 

 ~^tf~ ~T/D~ 



where L/D is the value corresponding to X = I "O. 



In the case where the machine is designed with the propeller 

 axis at an unusual angle, little error will follow from using the 

 same value of K' as is found above. It would be very laborious 

 to carry the approximation any further, but if desired a method 

 corresponding to that used in the following investigation may 

 be employed. 



K' is in engineer's units and we have to find K, which is in 

 dynamical units. This is done by the equation 



K = I4'95K'. 



Minimum Flying Speed when Getting Off. We are deal- 

 ing only with the case where X = I -o, i.e. the machine is flying 

 at its stalling angle. We will work in the engineer's units used 

 generally in other chapters than this. 



Consider a side view drawing of the machine, so set that the 

 wings are at the stalling angle of incidence : to do this we take 

 the stalling angle of incidence of the model tests as being suffi- 

 ciently accurate. 



Then the wind speed, V miles per hour (of which an esti- 

 mated stalling value must be taken at this stage) is horizontal. 



Now we know the thrust in pounds T', and we can write 

 down the equation 



io 5 T 



= * 



where B = (i + bj- - I, as on page 68. 



.*. b =-- JB + i -- i . . (2) 



Now consider the direction taken by the slip stream on 

 leaving the propeller : it has a component V in the direction of 

 the relative wind (horizontal) and a component bV parallel to 

 the propeller shaft. 



