The Airplane - Jay Spenser [112]
The first U.S. commercial airliner with retractable wheels was in fact a biplane. Equipped with powerful radial engines and billed as a high-speed transport, this was the fabric-covered Curtiss T-32 Condor II. As described earlier, the Condor entered service in 1933 only to be rendered hopelessly obsolete by the Boeing 247 later that year and the Douglas DC-2 the year after that.
The Laird Super-Solution racer in which Jimmy Doolittle won the Bendix Trophy transcontinental speed dash of 1931 had retractable wheels. Flying this green and yellow biplane, Doolittle became the first human being ever to cross the United States in less than twelve hours (seven years earlier, he’d been first across in less than twenty-four hours in a DH-4 biplane). Doolittle’s 1931 triumph—a featured event of that year’s National Air Races—highlighted the value of retractable wheels in the public mind.
Designers turned to retracting the tailwheel or nosewheel, not just the main landing gear, as aerodynamics became more important. In World War II, they also adopted flush riveting for high-performance fighters, a technology also applied to commercial and military transports as their performance increased with the advent of swept wings and turbine propulsion.
Landing gears both drive the design of the airplane and are driven by it. An example of the former is the configuration of a commercial jetliner, which features a low wing in part because it provides a good place to house the landing gear. Gear legs being moment arms, a gear twice as long must be more than twice as robust in order to resist leverage forces. Low wings thus ensure shorter, lighter landing gears.
As for the latter, military cargo jets provide a good example. They must employ a high-wing configuration to keep the fuselage low to the ground for easy loading. This dictates a sharply upswept rear fuselage to avoid striking the runway during takeoff or landing (a particular hazard for low-slung airplanes). It also accommodates rear doors and a boarding ramp for vehicles and cargo.
To avoid having an excessively long landing gear because of the high wing, designers often house the military transport’s main wheels (and sometimes additional fuel as well) in bulged blisters along the sides of the fuselage. This accommodation saves considerable weight and airframe space at the cost of a slight increase in frontal-area drag.
An even more extreme example of how design requirements can dictate landing gear decisions is the Boeing B-47 Stratojet, which flew at the end of 1947. This revolutionary jet bomber’s swept wings were too thin to house either fuel or landing gear. Consequently, Boeing gave the B-47 two sets of retractable main wheels housed in tandem in the fuselage, and small outrigger wheels that extended from the inboard engine nacelles to prevent the airplane from tipping laterally.
Aeronautical engineers who work in landing gear design contend with many challenges. One is materials, since landing gears are subject to high stresses and must be enormously robust. Another is safety and reliability. Yet another is runway loading, which is a measure of how much weight each given area of tire contact imposes on an airport’s runways, taxiways, and flight ramps. As airplanes grow in size, designers must spread their weight over more tires to keep this loading low.
The Airbus A380 provides a good example. This superjumbo, the largest jetliner in commercial service, rides on twenty-two wheels versus eighteen for the Boeing 747 and fourteen for the Boeing 777. It has four main gear posts, two under the fuselage with six wheels each and two more under the wings with four wheels each. The wing gear posts are positioned slightly forward of the fuselage gear posts. As is universally the case with jetliners, the