Sheet Metal Structures

SHEET METAL STRUCTURES INTRODUCTION Materials used in aircraft construction have changed significantly since the Wright brothers built the first practical airplane. The Wright Flyer was constructed from wood and fabric, as were most early aircraft. Later, to increase strength and durability, manufacturers replaced wood substructures with welded steel tubing. However, metallic materials such as aluminum and stainless steel were eventually used not only in the substructure, but also as the outer covering. Today, most aircraft are primarily fabricated from metallic components, although advanced composite materials are being widely used mostly on control surfaces and nonstructural components. In the future, composite materials will constitute a greater percentage of an aircraft's structure, but metallic materials will cer- tainly continue to be used for many years. This chapter serves as an introduction to the design, construction, inspection, and repair of basic airframe structures made from sheet metal. For detailed instructions regarding maintenance practices for specific airframe components, refer to the manufacturer's publications such as the maintenance manual (MM) or structural repair manual (SRM). METALLIC AIRCRAFT CONSTRUCTION During their career, nearly all aviation maintenance technicians are involved to some degree in the fabrication or repair of sheet metal aircraft structures. To fully understand the theories and procedures used in sheet metal work, a technician must recog- nize the construction techniques incorporated into the design of an airframe that allow it to handle different types of structural loads and stresses. In addition, a technician must also be aware of how different physical properties of sheet metal materials increase the durability and strength of an aircraft. With an understanding of these items, a technician Figure 2-1. Modern sheet metal construction techniques can maintain an aircraft to its original condition, or produce airplanes that are strong, streamlined, and eco- properly alter its design to improve aircraft perfor- nomical to assemble or repair. mance or safety. civil aircraft. Heat-treated aluminum alloys have the advantage of being lightweight with the ability to STRESSES AND STRUCTURES carry high structural loads, while being comparably Aircraft structures must be strong, lightweight, inexpensive with regard to other similar strength streamlined, and durable. However, in most cases, metals. These assets make aluminum alloy an excel- all these requirements are unobtainable without lent choice to use for the construction of most mod- compromise. Truss-type structures can be made ern civil aircraft. The remaining 10% of metals used both lightweight and strong; but with a resulting include titanium, stainless steel, and assorted exotic angular form, require a superstructure to make them metals that are predominantly used on military or streamlined. A wooden monocoque design does large transport category aircraft. provide a streamlined form with high strength, but with compromises due to the limited useful life of TYPES OF SHEET METAL STRUCTURES wood, as well as the high cost involved with fabri- There are two basic types of sheet metal structures cating a laminated wood structure. used for aircraft; monocoque and semimonocoque. Both of these are forms of stressed skin structures, Today, the high volume of aircraft production has meaning that the greatest part of the load is carried caused riveted or bonded sheet metal designs to in the external skin. However, these structures dif- become the most common method of construction. fer in the amount of internal components that they This is because sheet metal fabrication has been use. While both designs typically incorporate form- simplified over the years by developing and stan- ers and bulkheads, a semimonocoque airframe also dardizing the tooling and equipment necessary for utilizes stringers and longerons to add rigidity and assembling most structures. Consequently, sheet strength to the structure. metal aircraft are relatively easy to fabricate in a production facility as well as to repair in the field. A thin metal beverage can is an excellent example of [Figure 2-1] monocoque construction. The can has two thin The type of metal most widely used for aircraft structures comes from aluminum alloys, which account for as much as 90% of the metals used for Sheet Metal Structures 2-3 metal ends attached to an even thinner body, but as able to meet FAA requirements. Even in cases an assembly, can support a large load if a force is where the damage is slight, it is permissible for a applied evenly across the ends of the container. technician to use reference materials such as the However, a problem with a monocoque structure is Advisory Circular AC 43.13-lB, Acceptable that just a small dent or crease in a side wall can Methods, Techniques and Practices/Aircraft destroy its ability to support a load. Inspection, Repair, and Alterations. With this publi- cation, acceptable procedures for performing some Semimonocoque structures minimize the problem relatively common sheet metal repairs can be found. of failures caused by dents and creases by support- Although the advisory circular information is not ing the external skin on a framework of formers, FAA-approved data in itself, it can be used to sub- stringers, and longerons. The internal structure stiff- stantiate an approved repair technique. ens the skin so it is far less susceptible to failure caused by deformation. Because of its increased STRESSES strength, a semimonocoque design is used in the In order to determine that a repair will restore full construction of most modern aircraft. strength to a damaged sheet metal structure, you should understand how various stresses act on an Even the skin of an aircraft may be made more rigid airframe. Five types of stress are of major concern. across any large unsupported panels by riveting Two of these are primary, and the other three, for all stiffeners or by laminating honeycomb material to practical purposes, can be expressed in terms of the the panels. Some modern high-speed jet aircraft first two. Tension and compression are the two basic have skins that are milled for increased rigidity. stresses, and the other three, bending, torsion, and With these skins, stiffeners are formed on the inner shear, are essentially different arrangements of ten- surface by either conventional machining with a sion and compression working on a body at the computer-controlled mill or by chemical or electro- same time. chemical milling. TENSION Tension is a primary stress that tries to pull a body STRUCTURAL LOADS apart. When a weight is supported by a cable, the An airplane manufacturer must consider all of the cable is subjected to tension or, as it is often loads to which the structure will be subjected in expressed, to a tensile stress. The weight attempts to order to design each component properly. A safety pull the cable apart. [Figure 2-2] factor is then built in to provide for any unusual or unanticipated loads that may conceivably be encountered. In addition to meeting all of the strength requirements, the structure must be as lightweight as possible and reasonably easy to construct. As a maintenance technician it is your responsibil- ity to be sure that any repair you make to an aircraft restores both the original strength and stiffness to the structure, while maintaining the original shape of the part. These considerations must be met while keeping the weight of the repair to an absolute minimum. In most cases of major structural damage, to guarantee that a repair meets minimum standards for strength, an aeronautical or structural engineer must develop and approve the design of the repair. These engineers have the credentials and experi- ence to analyze the structural loads on the damaged Figure 2-2. A tensile stress exerted on a cable tends to pull area. From their analysis, these engineers can pro- the cable apart. duce drawings and instructions that the technician must follow closely to accomplish a satisfactory repair. However, for some repairs, an aircraft tech- COMPRESSION nician can refer to the manufacturer's Structural Compression, another primary stress, tries to Repair Manual or Maintenance Manual to deter- squeeze a part together. For example, a weight sup- mine if preapproved information is already avail- ported on a post exerts a force that tries to squeeze 2-4 Sheet Metal Structures the ends of the post together, or to collapse it. This is called a compressive stress. [Figure 2-3] Figure 2-3. An example of a compressive stress is realized by analyzing the effect of a column that is supporting a weight. The weight tends to squeeze the ends of the column Figure 2-5. Wing struts are predominantly under tension in together. flight, but under compression when an airplane is on the ground. Outboard of a wing strut, the remaining portion of a wing is subjected to bending stresses. BENDING A bending force tries to pull one side of a body apart TORSION while at the same time squeezing the other side Torsion is a twisting force. When a structural mem- together. When a person stands on a diving board, ber is twisted or placed under torsion, a tensile the top of the board is under a tensile stress while stress acts diagonally across the member and a com- the bottom is under compression. The wing spars of pressive stress acts at right angles to the tension. For a cantilever wing or the section of a wing spar out- example, the crankshaft of an aircraft engine is board of a strut is subjected to bending stresses. In under a torsional load when the engine rotates the flight, the top of a spar is being compressed while propeller. [Figure 2-6] the bottom is under tension, but on the ground, the top is pulled and the bottom is compressed.

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