Spar depth and weight

Abstract

In all classes of structural design it is the usual practice to employ deep rather than shallow beams. This arises from the fact that, within limits defined partly by the web construction, the deeper the beam the less the quantity of material used. Also for beams of a given length and strength a deeper beam is stiffer. In the design of aircraft spars, where weight saving is of primary importance, and where too low a flexural stiffness might be a disadvantage, the greatest spar depth and the shortest span consistent with good aerodynamic properties are used. In discussions of wing design, it is customary to consider aspect ratio and thickness/chord ratio as primary design parameters, these quantities being intimately connected with the drag of the wing. The influence on wing weight of changes of either of these quantities is associated mainly with their effect on the semi-span/spar-depth ratio. For structural discussions, therefore, it is convenient to consider the ratio of wing semi-span to root thickness as the basic design parameter (root thickness is chosen here as the most representative depth and is most readily defined).