A century of progress in aircraft materials - Part 2 Metals

Nov. 6, 2003
Aircraft designers have gone from wood and fabric to metal and composites in the last 100 years.

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This second installment of a three-part look at the first 100 years of powered flight focuses on the materials used by aircraft pioneers and manufacturers. As always in engineering, the drive has been to lighter, stronger materials that are easily machined, assembled, and repaired.

Currently, metal is king, having inherited the crown from wood and fabric, but composites seem to be the "next big thing" waiting in the wings. Technologists and futurists assure us composites will dominate the industry by the end of the next decade, but no one can point to solid evidence that composites are any better than metal, or wood and fabric, for that matter. They might be stronger or lighter, but they're more expensive to make, more difficult to inspect, and trickier to repair.

A 1929 Ford Trimotor, a "Tin Goose," touches down at the Ford Motor Co.'s Dearborn Proving Ground this year, the first time in 56 years the test track has been used as an airport. The plane belongs to the Experimental Aircraft Assoc., and was in town celebrating Ford's centennial. One unusual feature of the plane is that control cables for the flight surface on the tail run outside the fuselage to allow for easy and frequent inspection and maintenance.

The Tin Goose takes to the air

In 1925, Ford Motor Co. bought an aircraft company to supply planes to its airline. And based on Henry Ford's belief in using three engines for safety, a monoplane to avoid icing, and metal to simplify manufacturing, the company built the 4-AT. It was quickly dubbed the Tin Goose for its corrugated metal fuselage. (AT stood for Air Transport.) The plane was among the first all-metal planes and the first metal airliner.

It has a metal fuselage with a rectangular cross section, several longitudinal spars, and a corrugated-aluminum skin to support structural loads. The high-mounted wing was constructed using several spars and also covered with corrugated metal. But corrugated skin was problematic. It gave the thin-metal fuselage compressive strength, but was difficult to shape and attach. And corrugated skin had little strength perpendicular to the corrugations, so it couldn't strengthen the airframe. The wavy skin also added drag, even though corrugations ran lengthwise, same as the direction of flight.

Through 1932, Ford built 200 Trimotors, and even though the company was the acknowledged master of mass production, it could not make money on the plane. The technology to form and assemble an all-metal plane just wasn't ready. Ford did, however, popularize the idea of all-metal planes, stressing their safety to a public not that enthused about flying. Just three years later, the DC-3 was introduced, and that all-metal, multiengine monoplane became the first money-making airliner.

Boeing's P-26 Peashooter

America's first metal fighter plane

Boeing's P-26 "Peashooter" mixed old designs and new materials to come up with a fighter fast enough to keep up with the Army Air Corp's latest bomber, the 190-mph B-9, also built by Boeing. The Peashooter was an all-metal low-wing monoplane with wire-braced wings. Its 600-hp Pratt & Whitney Wasp engine let the plane fly 235 mph.

The fuselage was a semimonocoque design with aluminum bulkheads, longerons, skin stiffeners, and skin. A cantilever wing was considered too weak for a fighter, even though the Peashooter's horizontal tail was fully cantilevered. The wing's two main spars were built of sheet and angle duralumin. The wing also had duralumin ribs and closely spaced spanwise stringers riveted to them to support the metal skin. The wing's wire bracing might seem like a carryover from biplane days, but it let Boeing design a lighter wing. The fixed landing gear might seem like another step backward, and it did add considerable drag. But it also reduced weight and structural complexity and provided structurally well-placed anchor points for support wires.

The P-26 was the last fighter to have an open cockpit and a fixed landing gear. Coincidentally, the bomber it was designed to keep up with, the B-9, was also a transitional design. It was the first twin-engine bomber with a cantilevered low wing, retractable landing gear, and an all-metal skin. It was also the last to have open cockpits.

The F-86 Sabre jet built by North American Aviation was operational in1948. Design for it started in 1944, but was delayed as engineers tried to incorporate technology from German aircraft designers. It is one of the first planes to be built with significant amounts of titanium. The relatively exotic metal was used on the airframe and engine.
Lockheed's SR-71 Blackbird, which first flew in 1964, is considered by some to be the first all-titanium plane. It has a titanium skin and monocoque airframe.

Metal of choice

In 1964, Dick Tracy cartoons talked about titanium, the metal that "makes space travel possible." That's not exactly correct, but the metal does play a large role in military planes and spacecraft where performance is all that matters.

The metal was quickly developed into new alloys and special machining and joining methods were devised thanks to Cold War pressures and the Space Race. It has a high strength-to-weight ratio, corrosion and heat resistance, and a propensity to become stronger as it is heated. Today, 10% of the weight of a commercial airliner is titanium. Military planes use a higher percentage. The 1980s-era B-1B Lancer, for example is 20% titanium, which amounts to 40,000 lb.

Technicians at the Naval Research Laboratory prepare to test an aircraft canopy.

Curing cracks

Acrylic cockpit canopies of World War II fighter aircraft had a nasty habit of cracking after being hit by a sharp object or tiny pieces of shrapnel. The small crack would grow, and eventually the canopy would explode in what the Navy termed "blow-out failures." To J. Kies, a scientist working at the Naval Research Laboratory in the early 1950s, it looked like an opportunity to apply the fracture mechanics being developed there. Kies worked with acrylic manufacturers, shattering hundreds of canopies made of hot-stretched acrylic and carefully reassembling them to retrace the crack propagation paths.

Kies discovered that the critical stress for a given crack depended only on a material property that could be computed by measuring the applied stress and resulting crack size. In recognition of Kies, aerospace engineers testing stretch-toughened glazing materials use results measured in Ks. The NRL, working with the Air Force, increased the toughness of stretched acrylics, as well as reduced its weight and extended its service life as a canopy. The material is now used in military and civilian planes.

The NRL also helped solve problems of catastrophic failures in commercial jet aircraft in1953 and fracture problems in the Polaris and Minuteman missile programs 1957. The lab was established by Thomas Edison in 1923 to explore technologies important to national security.

Petroleum based lubricants (top) could not withstand the high operating temperatures in gas-turbine engines used toward the end of World War II. Engineers at the Naval Research Laboratory developed hydrocarbon ester fluids as lubricants that don't break down under high temperatures.

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