Spar Sections.
Fig. 7.—Solid spar.
The I section form of wing spar, shown by [Fig. 7], is in general use, being spindled from the solid. It is comparatively easy to produce, which in a measure explains its popularity, and it also disposes the material in probably the best manner for the stresses involved. The laminated spar, [Fig. 8], is an improvement on the solid channelled spar; it is stronger, will withstand distortion to a greater degree without injury, and the strength is also more uniform than with the solid spar. An additional point in its favour is that it is much easier to procure three pieces of small section timber free from defects than one large piece, which, in view of the increasing scarcity of perfect timber, is an important consideration. In order to minimize the risk of the glue between
Fig. 8.—Laminated spar.
the laminations failing, the usual practice is to copper rivet or bolt the flange portion, while both spars are left solid at the point of attachment of the interplane strut fittings and wire anchorages. The spar shown by [Fig. 9] is of the hollow box variety, chiefly used for machines of large wing surface, where weight reduction is an important factor. The two halves of channel section are spindled from the solid and glued together. The joint is strengthened by the provision of small
Fig. 9.—Hollow box-spar.
fillets or tongues of hard wood, and in some instances the complete spar is bound with glued fabric. Comparing the hollow spar with the solid, and neglecting the cost factor, the writer contends that the advantage is indisputably with the former. The tendency of the I-section spar to buckle laterally is of much lesser moment in a hollow spar of the type shown by [Fig. 9], while for a given weight it shows an increase in strength, and for equal strength it is much lighter. A different version of the hollow spar system is that indicated by [Fig. 10], consisting of two channelled sections, tongued together at the joint, the sides being stiffened with three-ply. The disposition of the joint in a vertical plane is a distinct improvement on the hollow spar previously considered, mainly in that better resistance to a shearing stress is afforded.
Fig. 10.—Hollow spar with
stiffened sides.
Fig. 11.—Hollow spar with
multi-ply sides.
The principle underlying the construction of the spar shown by [Fig. 11], is that in its manufacture the lengths of wood necessary are of small section. The sides of this spar are built up with a centre of spruce about ⅛ in. thick, to each side of which is glued thin three-ply, these being glued, screwed, and bradded to the flanges. The wing spar shown in section by [Fig. 12] is unique in that it really constitutes two spars placed closed together, the connection being formed by the top and bottom flanges of three-ply. This spar was used in a machine with planes of small chord, but of very deep section, and in which no interplane wiring occurred, the wings functioning as cantilevers. Its chief advantage is great rigidity for a low weight, but such a spar necessitates a deep wing section, and is not in general use.
Fig. 12.—Twin box spar.