All About Aircraft Metals

When the Wright brothers led us into the skies in 1903, they constructed the Wright Flyer from materials that were available and familiar. The requirements were simple?adequate strength combined with the lowest possible weight. The material also needed to be easy to repair, to replace, and to redesign because the Wright Flyer was, after all, a prototype. Their system of using wood structures with metal fittings and fasteners was an excellent choice.

As airplanes became more common and their design became more a matter of experience and science than of trial and error, wood gave way to steel tubing as the primary airframe structural material. Steel tubing offered designers the chance to create airplanes that were strong, stiff, and aerodynamically efficient when covered with fabric. Initially, airframes were constructed of mild (pliant) steel tubing welded into trusses to provide a combination of strength and stiffness with light weight. The ends of the tubes were closely fitted together and then welded to create a unified structure. Mild steel was strong enough until higher-powered engines, higher gross weights, and higher airspeeds became common in the mid 1920s. The mild steel tubing had to be made larger to handle the increased stresses. As the tubing grew larger, the structures grew heavier. Eventually stronger and lighter chrome-moly steel tubing replaced mild steel tubing in structural components. As aircraft progressed, manufacturers produced hybrid fuselages such as in the early Piper Cubs and the Taylorcraft series of aircraft. In these, the cabin was constructed of chrome-moly steel while the empennage and tail sections were constructed of mild steel. The famous Spirit of St. Louis, manufactured by Ryan in 1927, used mild-steel lift struts. As aircraft powerplants evolved and airframe size and speed continued to increase, steel-tube structures became more and more difficult to employ effectively. The outbreak of World War II and the aircraft and manufacturing demands that accompanied it made steel-tube structures virtually obsolete for anything but low-speed aircraft types. Another aircraft construction material was needed. Aluminum was the next logical choice for airframe construction. It provided light weight and high strength due, in part, to the fact that aluminum skin on an aircraft contributes to the strength of the structure while fabric covering does not. Aluminum as a pure metal is very soft and not suitable for use in making structural components. The addition of other elements creates alloy materials of varying structural strength, corrosion resistance, and workability. Aluminum is primarily alloyed with copper to increase strength, magnesium to increase strength and corrosion resistance, and manganese to increase strength and ductility. Aluminum-copper alloys make up the largest family of aluminum alloys used in aircraft construction. The addition of copper provides an alloy of significantly greater strength that can be heat treated, is still very workable, and is fairly resistant to corrosion. When even greater corrosion resistance is required, the aluminum alloy sheet or component can be coated with commercially pure aluminum. The result is called aluminum clad. The cladding process involves coating the aluminum alloy part with a thin layer of commercially pure aluminum, which is actually 99 percent pure. Pure aluminum is virtually impervious to corrosion from any elements that an aircraft is likely to come into contact with. But aluminum-clad parts must be handled very carefully because scratches or other damage to the aluminum coating can allow corrosion to start at the scratch and progress under the coating. Aluminum can be used for very highly stressed parts such as landing gear wheels and reciprocating engine pistons and crankcases. It also is used to fabricate the most common fastener used in modern aircraft?the rivet. Rivets are made of various alloys of aluminum to be compatible with the components they fasten together. A rivet is inserted into a hole drilled through two or more parts, and the end of the rivet is bucked (smashed) over to hold the components together. It is extremely important that rivets be chemically compatible with the materials they join; otherwise, dissimilar-metal corrosion can quickly occur. Other metals used in modern aircraft include stainless steel, titanium, and magnesium. Each of these metals is used when its specific properties are needed. Stainless steel, which is steel containing nickel, is extremely resistant to corrosion and heat. Stainless steel is used in the leading edges of heated wings on many turbine-powered airplanes, in control cables, and in fittings subjected to heat or attack from the elements such as external fittings on seaplanes. Titanium has a very high melting point?more than 2,700 degrees Fahrenheit?which makes it ideal for use in firewalls, turbine-engine shrouds, and wing skins on high-speed aircraft where heat rise is significant. It is 60 percent heavier than aluminum but 50 percent lighter than stainless steel Magnesium is about two-thirds the weight of aluminum and has the highest strength-to-weight ratio of all aircraft structural materials. The drawback is expense and the fact that magnesium burns ferociously if its ignition temperature of 1,200 degrees Fahrenheit is reached. For this reason, magnesium is never used in high- temperature areas. Magnesium has been used successfully in general aviation for skins on control surfaces. As aircraft continue to increase in performance and sophistication we will likely see even more exotic metals used in airframe and component structures. At the same time, we?re witnessing the development of a new generation of synthetic composites such as fiberglass, carbon fiber, and synthetic foam. New general aviation airplanes already are making increased use of composites. Will the trend continue, or will new metals or a composite/metal mix win out? Stay tuned?