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necessary in order to get a rigid structure. It is desirable that the two lower horizontal pins be in line fore and aft so when the third pin or strut is removed the wing will hinge down for ease of assembly or knock-down; otherwise critical loads may be imposed on the fittings due to the dead weight of the wing.
Torsional stiffness.—It is essential that the wing structure have adequate torsional stiffness in order to insure freedom from flutter and other undesirable characteristics. In contrast to metal or plywood covered wings, fabric covered wings usually lack torsional rigidity.
Wing design details.-Wing spars.--Provision should be made to reinforce wing beams against torsional failure, especially at the point of aviachment of lift struts, brace wires and aileron hinge brackets.
Solid wood spars.-Solid wood spars should be made of Grade A spruce. They may be tapered in depth or thickness, but should not be thinner than 1/4-inch at any point.
Built-up wood spars.—Of the built-up types of spars, the box with smooth plywood faces with single upper and lower flanges is the most convenient for attaching ribs. It has half as many flanges to make as the "I" type. In either type, blocking must be provided at all points where fittings are attached. Blocking should be tapered off at the ends to avoid concentration of stress in the flanges. Intermediate verticals are provided in some cases to increase the allowable stress in the webs. These verticals need not be filleted at the ends unless they also carry a concentrated load to be distributed into the spar web. On box spars the attached rib vertical sprovide this stiffening effect on the webs. CAM 18 shows details of spar construction.
Top and bottom spar flanges are usually proportioned according to the relative magnitudes of the critical bending loads in each direction such that the corresponding margins of safety will be approximately equal.
Spar webs are usually made of spruce, mahogany, or birch plywood. These are usually of three-ply construction, but special two-ply and 45-degree constructions are available in spruce and mahogany, providing somewhat higher allowable stresses. The face grains of 3-ply should be laid vertically on the spar. Splices should be vertical, preferably over a stiffener.
Laminated spars may be spliced in either plane, and splices in the various laminations should be spaced well apart. Splices in solid spars, if any, should be in the vertical plane, as shown in CAM 18.
Splices in wood spars.-Splices in structural wood members when necessary should have a 12 to 1 slope or greater. The surfaces should be fitted for perfect uniform contact before gluing. Accordingly, the surfaces preferably should be formed with a planer. The dimensions and type of splice should be similar to those given in CAM 18. Care should be taken in clamping glued splices to use thick "cushion" blocks of the proper slope and size so as to produce clamping action perpendicular to the line of the splice, uniformly distributed, and not such that the pieces tend to slip past each other. A finished splice in wood or plywood should show no change in cross section at the splice.
Wood leading edge.—Ordinarily 3-ply plywood leading edge material is laid with the face-grain spanwise. The best arrangement, however, is to use material having face-plies at 45 degrees, center-ply running spanwise. In any case, the material must be securely fastened at each rib in order to develop its full strength. Splices in the nose covering should be made over a rib. A minimum thickness of Koinch for spruce or mahogany leading edge material is recommended.
On designs where the leading edge covering comes to the spar, the latter can be built full depth of the wing, with a saving of weight over the type which is run under the rib cap strips and then built up flush with spacer strips between ribs. The nose cover serves to gusset the nose ribs to the spar, and an additional gusset with face grains running chordwise should be provided to attach the cantilevered tail ribs. Sufficient glued area must be provided to the face of the spar to carry the vertical shear from the ribs. A smoother covering can be obtained by scalloping the leading edge covering back past the spar between ribs, to keep the fabric from pulling abruptly over the sharp edge of the spar.
Wood ribs.—Rib spacing under a stressed torsion-nose depends on the thickness of the plywood and the curvature. Usual practice is about 6-inch spacing with 12-inch spacing for tail ribs. This can be increased where the limit stress is very low, but sufficient ribs should be provided to hold the surface true and prevent “oil canning.” Roughly, the maximum spacing for nose ribs should not be greater than the nose-rib chord, preferably less.
It is recommended that a leading edge spanwise strip be provided under the cover for support and for drag load in the single-spar wing. The greatest cross sectional dimension of the strip should be fore and aft.
Nose ribs should be stiff enough to allow the nose covering to be clamped down hard for gluing. The lightest practical method of getting the required stiffness is to build the rib up out of approximately square materials and carry the joint gussets the full length of the cap strips so as to increase the bending stiffness of the caps between supports. Nose-rib caps must be heavier in order to avoid splitting when the covering is to be nailed on than when the glue work is only clamped.
End ribs and corner ribs should be braced or specially designed to provide stiffness against fabric tension loads. A double set of ribs
two or three inches apart and connected with a plywood cap top and bottom is effective, or a rib with members four or five times normal width will usually provide the necessary stiffness.
Trailing edges.-Typical laminated trailing edges are shown in fig. 3-1. Bent-up metal trailing edges are also used, but are more difficult to attach satisfactorily to the ribs. In any case, trailing edges must be strong enough to withstand considerable rough handling when setting up the glider.
Wing tip bows.-Wing tip bows are frequently made of wood, laminated, or steel tubing attached by welded clips bolted to the spars. The lower surface of tips should be covered with metal or plywood for protection. External brace struts. • Struts may be wood, or metal tubing, either round with fairing
or streamlined in shape. Material is either steel or aluminum
alloy. . The wooden struts are usually tapered and are made up solid
and of square or rectangular section with plywood fairing. The end fittings must be carefully designed to transmit the
tension loads into the wood portion. • Steel struts usually have the end fittings welded on so that
the load is carried through the weld in shear. However, in the case of aluminum alloy struts the fittings are riveted or bolted onto the end of the strut. A typical end fitting for a streamline steel strut is shown in fig. 3-1. For a single strut, it is desirable to provide universal end fittings similar
to fig. 3-II, b. • Care should be taken in the design of the attachment of struts
and wires to avoid eccentric loads tending to roll the spar or bend it in the weak direction. This can be avoided on twospar wings by providing a deep drag strut near the strut point capable of stabilizing the spars. On single-spar wings it is desirable to keep the strut axis under the spar axis.
. • In connection with single-apar braced wings employing air
foils which have a large center of pressure travel, it is desirable to provide for some freedom at the strut attachment to allow for flexing of the wing without straining or introducing dangerous secondary loads into the strut. A serious secondary load could be imposed on the strut when flying at a low angle of attack into a “down” gust unless the wing is exceptionally rigid or freedom to flex is provided by means similar to a universal joint. Ball and socket joints are difficult to design
and are not recommended. Jury struts. When clamps are used for the attachment of jury to lift struts, the design should be such as to prevent misalignment or local crushing of the lift strut.
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