DURAND— MECHANICAL ASPECTS. 173 



Regarding question (2) we may state the problem thus. For a 

 series of aeroplanes of increasing area, how closely will the increase 

 in weight vary with the % power of the rate of area increase? 



Broadly speaking the relation seems to hold within a significant 

 degree of approximation. The weight of skin covering itself will 

 increase as the surface. All structures subjected to cross breaking 

 and in general all elements which tend to constitute the structure as 

 a whole into a truss or girder will, except as the character of the de- 

 sign may change increase in their own linear dimension nearly with 

 the overall increase in linear dimension, and hence in weight 

 nearly as the cube of the linear dimension or with the % power of 

 the surface. Time does not permit any detailed analysis of this im- 

 portant problem, but broadly speaking we may expect that in a 

 series of aeroplanes of the increasing area the weight will increase 

 somewhat more rapidly than the area but somewhat less rapidly than 

 the % power of the area. 



The practical question is this. To what degree of approximation 

 in a series of aeroplane structures will the structural weight vary 

 with the % power of the area of wing? We know that for a given 

 speed, wing area and gross weight vary nearly in direct linear ratio. 

 Hence if the structural weight increases more rapidly than the area 

 but somewhat more slowly than with the % power, it is obvious that 

 for any given speed there will be some area which will insure the 

 maximum net lifting capacity and beyond this area the next lifting 

 capacity will decrease. 



Actual experience seems to indicate an increase in weight re- 

 lated to wing area according to an index lying between i and % and 

 varying somewhat irregularly according to the changing type of 

 construction with increasing size. Hence we may conclude that for 

 a given speed continued increase in size of wing alone will not insure 

 indefinite increase in the net carrying capacity, but that instead 

 there will be some area for which the net carrying capacity may be 

 expected to reach a maximum, after which further increase in size 

 at the same speed will involve a loss in carrying capacity. 



It follows again that in order to increase carrying capacity the 

 following steps are indicated. 



