Aeroplane Construction and Operation

by John B. Rathbun

Fig. 11. Built-Up Wooden Wing Spars, Commonly Used with European Aeroplanes.

The front end of a Hansa-Brandenburg wing is shown by Fig. 12, the box spar and its installation being drawn to scale and with dimensions in millimeters. The top and bottom are sloped in agreement with the rib flange curve, and the rib web is strengthened by stiffeners at either side of the spar. The hardwood dowel strips are at top and bottom as in Fig. B, and when placed in this position the glued joint is not subjected to the horizontal shear forces. The walls are thicker at top and bottom than at the sides, in order to resist the greater vertical forces. For the same reason is deeper than it is wide. As will be remembered, the drag is very much less than the lift, and again, the drag stress is greatly reduced by the internal drag wire bracing.

Leading Edge Construction. In the early Bleriot monoplanes the leading edge was of sheet aluminum, bent into "U" form over the nose of the rib. In modern biplanes, this edge is generally of "U" form hollow spruce, about 3/16 inch thick. Another favorite material is flattened steel tubing, about 1/2" x 1/4", and of very light gauge, the long side being horizontal. The tube has the advantage of being much thinner and much stiffer than the other forms, and the thin edge makes it very suitable for certain types of aerofoils. The wing tip bows are generally of hollowed ash and are fastened to the spar ends, leading edges, and trailing edges with maple dowels, the joint being of a long scarfed form. When the scarfed joint is doweled together, it is wrapped with one or two layers of glued linen tape. In some types of machines the top surface of leading edge is covered with thin two ply wood from the extreme front edge to the front spar. This maintains the aerofoil curve exactly at the most critical point of lifting, and also stiffens the wing against the drag forces.

Fig. 12b. An Old Type of Curtiss Biplane Strut Socket, at Left. At the Right Is a More Modern Type in Which the Bolts Do Not Pierce the Spar.

Trailing Edges. These are either of thin beveled ash or Steel tube. On army machines, the rear part of the trailing edge fabric is pierced with holes about 3/16 inch diameter, the holes being provided with rust proof eyelets. This relieves an excess of pressure due to rips or tears; one opening being located between each rib and next to the body.

Fig. 12c. A Standard H-3 Interplane Strut Socket Is Shown at the Left, the Bolts in This Case Passing on Either Side of the Spar. Note the Stay Wire Attachment Clips and Pinned Strut Connection. A German Strut Socket at Right. Courtesy "Aerial Age."

Protection of Wing Wood Work. In protecting the wood framework of the wings from the effects of moisture, at least three coats of good spar varnish should be carefully applied, with an extra coat over the glued surfaces and taping. Shellac is not suitable for this purpose. It cracks with the deflection of the wings and finally admits water. The steel parts of the wing should be given two coats of fine lead paint, and then two coats of spar varnish over the paint. Wires are treated with some flexible compound, as the vibration of the thin wires, or cables, soon cracks off any ordinary varnish. The use of shellac cannot be too strongly condemned; it is not only an indifferent protection, but it causes the fabric to rot when in contact with the doped surface.

Monoplane Wing Spars. A few representative monoplane wing spars are shown by Fig. 13, the R. E. P., Bleriot XI, and the Nieuport. Except at points where the stay wires are connected, the Bleriot spar is channeled out into "I" beam form as indicated in the figure. It will be noted that the top and bottom faces of the spar are slanted to agree with the curvature of the ribs. A. steel connection plate is bolted to the sides of the spar by through bolts, and with a lug left top and bottom for the top and bottom guy wires. The R. E. P. is also an "I" type, the section. "A" being taken through the channeled portions, while "B" is taken through one of the connection points where the beam is a solid rectangle. The channeling should always stop at connection points; first, so that the plates have a good bearing surface, and second, to allow for the material removed by the bolt holes.

Fig. 13. Typical Monoplane Wing Spar Construction.