The downwash angle is expressed in terms of the value 

 obtaining at the position of the idealized lifting line for 

 an elliptic spanwise load distribution. The latter quantity, 

 designated e , is equal to LA/ttA^ . The downwash angle far 

 behind the wing approaches twice the value e . Some reduc- 

 tion, however, occurs from viscous losses. Also the value 

 is reduced in the region above and below the vortex sheet. 

 An estimate of the latter reduction is given by Glauert 

 (Reference 6) for spanwise position as a function of the 

 ratio of height above the vortex sheet to the semispan. 

 The approximate value of the average height of the horizontal 

 stabilizer planes above the zero lift lines of the two wings 

 for the initial configuration is 0.46 — r- , The value of the 

 downwash at that position above the wing was taken to be e . 



On the basis of the equivalence in the induced drags, 



the downwash of the biplane is greater than the downwash for 



CJL' 

 the equivalent isolated monoplane by the amount —r— , and the 



expression for the downwash angle at the tail plane is 



finally e„ (1 + a) (Reference 6, p. 187). 

 o 



Finally, to express equations [la] and [2a] completely 

 in terms of ax. and cjo^, we need only fix p. This was done by 

 imposing the condition that the remote flow be tangent to 

 the mean camber line of the airfoil at the leading edge. 

 Since, for small circular arc camber, this condition is 



71 



