TM 1-410. Technical Manual, Airplane Structures 1941: Section 4 - Wings

SECTION IV: WINGS

FIGURE 15-Stressed skin wing construction.

11. General.-Wings of an airplane are surfaces designed to give lifting forces when moved rapidly through the air. The particular design for any given airplane depends on a number of factors; for example, size, weight, and use of airplane, desired landing speed, and desired rate of climb. Frequently the larger compartments of the wings contain or are themselves used as gasoline tanks or for flotation cells.

12. Structure.-a. Wing construction is very similar in many respects to that of fuselages. Variations of design and constructions depend upon manufacturer and specifications outlining assignment and performance requirements. Wing structures of most modern military airplanes are of all-metal construction, usually of the cantilever design, that is, so constructed that no external bracing is required. With few exceptions they are all of the stressed skin type shown in figure 15, where the skin is a part of the basic wing structure and carries part of the wing stresses.

b. One of the numerous methods of general arrangement and f abrication is shown in figure 16. In this case two main spars are used with ribs and bulkheads placed at frequent intervals between the spars to space them and develop wing contour. In this assembly the cross sectional members are stamped in one piece; however, these may be built-up sections or trusses as shown in figure 17.

FIGURE 16.-Two-spar wing construction using stamped ribs.

Other variations of wing construction include monospar and multispar types. During flight applied air loads which are imposed on a wing structure act primarily on the wing covering. From the covering they are transmitted to the ribs and from the ribs to the spars. The spars support all distributed loads as well as concentrated weights such as fuselage, power plants, etc.

FIGURE 17-Wing cross section showing truss construction.

c. Corrugated sheet aluminum alloy is often used as a subcovering for wing structures as shown in figure 15. Corrugations are laid parallel to the spars so as to assist the structure in resisting bending loads. The smooth outer covering is attached with either flush bead or brazier bead rivets placed fairly close together and the joints are carefully fitted so that the stresses on the wing covering are evenly distributed.

d. As in the case of fuselages the metal in general use for wing structures is heat-treated aluminum alloy, Alclad being largely used for the outer covering. Wings of some airplanes, particularly those used for training, are covered with fabric. This covering is made taut and protected against deterioration by several coats of a cellulose base material known as airplane "dope." Cuts, tears, and holes in the covering are easily repaired by patching, but such operations should be done by workmen specially trained for this type of work.

e. Inspection openings and access doors are provided, usually on the lower surface of the wing, and drain holes are placed in the lower surface along the trailing edge. Walkways are provided on areas of the wing where it is intended that personnel will step or walk. The substructure is stiffened or reinforced in the vicinity of the walkways to take such loads. Areas intended as walkways are usually covered with a skidproof surfacing such as ground cork, rubber matting, or carborundum grit, and areas on which walking is prohibited are marked "No Step."

13. Inspection and maintenance of fuselage and wings.a. In inspecting wings and fuselages, it is very important to watch for evidence of corrosion on the inside. This is most likely to occur in pockets and corners where moisture and salt spray may accumulate and therefore drain holes must always be kept open.

b. While an injury to the covering caused by impact with an object is plainly evident and readily identified, a defect such as distortion or failure of the substructure may not be apparent until some evidence develops on the surface, that is, buckled or displaced covering. loose rivets, etc. External indications of internal injury must be watched for carefully and correctly interpreted when found. In this case, a thorough investigation of the substructure in this vicinity should be made and suitable steps taken immediately to correct the trouble. Warped wings are usually manifested by parallel wrinkles running diagonally across the wing and extending over a major area. This condition may develop from unusually violent maneuvers, extremely rough air, or extra hard landings, and while there may be no actual rupture of any part of the structure it might be distorted and weakened. Similar failures may also occur in fuselages.

c. Small cracks in the covering leading away from rivets frequently occur. These are usually caused by vibration of the covering. A small hole may be drilled at extremities of the crack to arrest temporarily its development until a permanent repair can be made by patching. Aluminum alloy sheets in heat-treated condition are springy and hard to bend. No attempt should be made to straighten bent or dented covering of this material as it is likely to crack or break. Repairs of this kind should be done by specially trained sheet metal workers and usually consist of riveting a patch over the damaged area.

d. Aluminum alloy surfaces having protective coating chipped off, scratches, or worn spots which expose surface of the metal should be recoated at once as corrosion may develop rapidly. The same principle applies to Alclad surfaces. Scratches which penetrate the pure aluminum surface layer will permit corrosion to take place in the alloy beneath. Small spots on enameled or lacquered surfaces may be covered using a small brush, but if the condition is poor over a large area it should be refinished properly with a spray gun by a specially trained workman.

e. The special inspections of fabric-covered internally braced wings should be followed as outlined in the Air Corps Technical Orders pertaining to this particular equipment.