Fig. 8. Typical Biplane Wing. Gap for Aileron Shown at Right End of Wing. Left End Rests Against Fuselage, the Observation Port Being Cut Out at the Upper Left Hand Corner. Drag Wire Bracing Clearly Shown. Courtesy "Aerial Age."
Fig. 8 is a photograph of a biplane wing with the framing members completed and ready for the application of the fabric. At the right is the opening left for the aileron, and at the left is the observation port, the latter coming next to the body. As this is a lower wing, the sockets for the attachment of the interplane struts can be seen on the upper and near edges of the main spar. Between the spars are very thin wood strips running with the length of the wing. These are the "Battens" used for stiffening the ribs between the points of support at the spars. As the distance between the spars is comparatively great, in respect to the thickness of the rib flanges, some sidewise support of this kind is necessary. The drag-bracing cables cross three rib spaces, or are fastened to every fourth rib. Between the front spar (at the bottom), and the entering edge, are the small strips that serve as sub-ribs. Double cross bracing is used at the inner end of the wing (left), while additional knee braces are placed at the aileron opening, and at the outer tips. This is necessary to withstand the stresses due to assembling and handling, rather than for the flight stresses.
Fig. 9 is a Standard Wing ready for covering. Before the fabric is applied, a narrow cloth strip is wrapped over the trailing edge, as shown, and is stitched to the frame. This forms a means of stitching the main covering at the rear edge, where the ends of the upper and lower surfaces meet.
Wing Fabric or Covering. At the present time unbleached Irish linen is used almost exclusively for covering the wing structure, although in the early days of flying rubberized fabrics were used to a great extent.
Fig. 9. Standard H-3 Wing Ready for Covering. Opening for Aileron Flap Shown at Upper Left Hand Edge (Trailing Edge).
After the linen is stretched on the wing frame, it is given several coats of a special preparation commonly known as "Dope" to proof the fabric against moisture. In addition to waterproofing, the dope adds considerably to the strength of the fabric and shrinks it tightly on the ribs—much more evenly than could be done by hand. When completely "Doped," the linen should be proof against the effects of salt water, moisture, or extreme dryness, and the fabric must be "Drum tight" at every point on the surface of the wing.
The linen should have a tensile strength of at least 75 pounds per inch of width in any direction, and weigh from 3.75 to 4.4 ounces per square yard. It must be wet spun, free from filling matter and uncalendered. As a usual thing, the width should not be less than 36 inches, although the width can be altered to meet conditions of rib spacing, etc. The U.S.A. seaplane specifications (1916) require a minimum strength along the warp of 75 pounds per inch width, and 85 pounds per inch of width along the weft. The following table gives the properties of well known wing fabrics:
Wing Dope. Wing dopes are in nearly every case based on cellulose—either cellulose acetate or nitrate being the most common base. This has proven far superior to the resin, copal, gum or oil bases contained in ordinary varnish, since the cellulose of the dope seems to amalgamate with the cellulose of the flax fiber and bond the whole into an integral structure. The fact that the dope must be elastic bars the use of shellac or other hard resin solutions. The solvents used for the cellulose dopes vary with the makers, some using amyl-acetate, tetrachlorethane, etc., while others use special secret compounds that are best adapted for their bases. Many of the solvents give off poisonous gases in drying, and this must be guarded against by good ventilation. The vapor of tetrachlorethane is particularly dangerous, and has resulted in many deaths.