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.
Hollow Spar Construction.
The advantages of the hollow type of spar summarized are (1) greater strength for a given weight; (2) it can be produced from wood of small section, and is therefore a better manufacturing proposition. On the other hand, the strength of a hollow spar is greatly and almost entirely dependent on the glue used. Now, however well the joint may be made, the glue is susceptible to a damp atmosphere, and if so affected is of greatly reduced strength, while possible depreciation in the glue due to age renders the life of the spar a problematic quantity. Where the various fittings occur it is also necessary to place blocks before the spar is glued up, which is rather an unmechanical job. The practice of forming vertical sides of a hollow spar from three-ply is not to be commended, by reason of the doubtful character of the glue used in its manufacture. However, in spite of these disabilities, there is a future for hollow spar construction in the manufacture of the big commercial machines of the future, for with these the question of maximum strength for minimum weight, to permit the carrying of the greatest possible useful load, will be a primary consideration. This, of course, assuming that the era of the all-steel machine has not arrived.
Strut Sections.
In the construction of the interplane and undercarriage struts, one does not find a very decided preference for any one particular method, although the interplane strut spindled from the solid to a streamline section is common to many types of modern aircraft. The strut shown in section by [Fig. 13] is in use for both interplane and undercarriage struts. This consists of ordinary round section steel tubing, to which is attached a tail piece or fairing of wood, this being bound to