DURAND— MECHANICAL ASPECTS. 171 



In its essence, however, the aeroplane is a wing or a combination 

 of wings fitted with one or more engines and propellers. 



The chief structural problems are therefore concerned with 



(i) The design and construction of the wing. 



(2) The design and construction of the members necessary and 

 sufficient to join the wings together into a coherent structure suited 

 to the purposes in view. 



The wing again presents two problems. The surface and the 

 frameivork necessary to give form and strength to the whole. 



The amount of surface to be provided is dependent, according 

 to well known laws, on the weight to be supported and on the speed 

 at which support is to be realized. In the outlook ahead the insistent 

 demand wall be for the largest practicable size. W'e may there- 

 fore put the question bluntly, w^hat is the largest attainable size, 

 what elements tend to limit size and how may we hope to remove, 

 in some measure, the effect of these limitations. 



If we consider a series or family of aeroplane structures, homol- 

 ogous in all dimensions and differing only in size, we shall evi- 

 dently find a ratio of surface to weight decreasing with increasing 

 dimensions. The weights will increase as the cube of the linear 

 dimension, the surfaces as the square, and hence the ratio of surface 

 to weight will vary as the inverse ratio of increasing dimension. It 

 follows that for such a series of structures the weight of the struc- 

 ture itself will tend to absorb an increasing part of the total weight 

 which the surface should sustain at any given speed, and with cor- 

 responding reduction in the surplus lifting capacity available for 

 power plant, crew, armament, express freight, etc. 



Let X denote any linear dimension of the plane. 

 A the area. 

 W^ the weight of the plane and auxiliary structures. 



Then for a family of structures such as are here considered we 

 shall have 



A =5.r^ 



W^ = Cr^ 



where B and C are two coefficients connecting respectively area 

 with the square of .r and weight with the cube. 



