Shaping of Main Spars.
Fig. 27.—Shaping of main spars.
Taking in greater detail the different parts, it is apparent that the spars form the nucleus of the general arrangement. There are two methods of shaping the spar longitudinally, and, as shown by [Fig. 27], the one consists of leaving it parallel for the greater part of its length, while the end forming the tip of the wing is gradually tapered to a comparatively fine edge. This may be said to constitute prevailing practice. The other method which is illustrative of monoplane practice is not used to anything like the same extent, and differs in that it is constantly tapering from root to tip. The advantage of this spar construction is the improved distribution of the material for the stresses involved, and also that a wing built with this spar may possibly possess a greater degree of lateral stability owing to the weight of the complete wing being located nearer the centre of gravity. Against this one must balance the fact that each rib must necessarily be different in contour, entailing a greater number of jigs, an increase in the time taken in building, with a consequent increase in cost. In addition, all strut fittings would differ in size, so that, taking all things into consideration, this construction is hardly justified. It will be noted that at the point of attachment of the interplane strut fittings, or, in the case of the monoplane wing, the anchorage for the wires, the spar is left solid. It is possible to channel the spar right through, from root to tip, and to glue blocks where fittings occur; and although there is a possible saving of labour thereby, it hardly conforms to the standards of modern workshop practice.