Wing Spars. In American aeroplanes these members are usually of the solid "I" form for medium size exhibition and training machines, but for small fast aeroplanes, where every ounce must be saved, they are generally of the built up type, that is, made up of two or more members. In Europe, built up construction is more common than in this country, and is far preferable for any machine that justifies the additional time and expense. The wing spars are the heaviest and most important members in the wing and no trouble should be spared to have them as light as the strength and expense will permit. They are subjected to a rather severe and complex series of stresses; bending due to the load carried between supports, compression due to the pull of the stay wires, bending due to the twist of non-central wire fittings, stresses due to drag and those caused by sudden deviations in the flight path and by the torque of the motors. These should be accurately worked out by means of stress diagrams if the best weight efficiency is to be obtained.
Fig. 9. Types of Wing Spars. (A) Is the "I" Beam Type. (B) Box Spar. (C) Is Composite Wood and Steel, Wrapped with Tape.
A number of different wing spar sections are shown by Figs. 9, 10, 11. Spar (A) in Fig. 9 is the solid one piece "I" type (generally spruce), channeled out along the sides to remove the inefficient material at the center. The load in this case is assumed to be in a vertical direction. In resisting bending stresses, it should be noted that the central portion of the material is not nearly as effective as that at the top and bottom, and that the same weight of material located top and bottom will produce many times the results obtained with material located along the center line. At points of connection, or where bolts pass through the spar, the channeling is discontinued to compensate for the material cut away by the bolt and fittings.
Spar (B) is of the hollow type, made in two halves and glued together with hardwood dowel strips. The doweling strips may be at the top and bottom as shown, or on the horizontal center line as shown by Spar (J). The material of the box portion is generally of spruce. This is a very efficient section as the material lies near the outer edge in every direction, and offers a high resistance to bending, both horizontally and vertically. Unfortunately a great deal depends upon the glued joints, and these require careful protection against moisture. There is absolutely no means of nailing or keying against a slipping tendency or horizontal shear. The best arrangement to insure against slipping of the two halves is to tape around the outside as shown by spar (E). This is strong linen tape and is glued carefully to the spar, and the whole construction is proofed against moisture by several coats of spar varnish and shellac. In addition to the strengthening effect of the tape, it also prevents the wood from splintering in accidents.
Fig. 10. Four Types of Wing Spars, the Spar D Being a Simple Steel Tube as Used in the Caudron and Breguet Machines.
Spar (C) consists of a central ash "I" section, with steel strips in the grooves. Two spruce side strips are placed at either side as stiffeners against lateral flexure, and the entire construction is taped and glued. This is very effective against downward stresses, and for its strength is very compact. Since spruce is much stiffer than either the thin steel strip, or the ash, it is placed on the outside. Spar (D) in Fig. 10 has been described before.
Fig. (F) consists of two spruce channels placed back to back, with a vertical steel strip between them. Again the spruce is used as a side stiffener, and in this case probably also takes a considerable portion of the compression load. Spar (G) is a special form of box spar used when the spar is at the entering edge of the wing, the curved nose being curved to the shape of the aerofoil nose. In Fig. 11 (H), a center ash "I" is stiffened by two spruce side plates, the ash member taking the bending moment, and the spruce the compression. Spar (I) has a compound central "I," the upper and lower flanges being of ash and the center web of three ply veneer. The two outer plates are of spruce. This should be a very efficient section, but one that would be difficult and costly to build. Fig. (J) is the same as (B), except that the parting lies in a horizontal plane. Spar (K) has ash top and bottom members, and spruce or veneer side plate. The resistance of this shape to side thrust or twist would be very slight. The sides are both screwed and glued to the top and bottom members.
