Double the size of an Airbus A380? No problem, aerodynamicists say.
- By Michael Milstein
- Air & Space magazine, July 2006
(Page 2 of 5)
“It’s hard to beat a bigger wingspan,” says William Mason, an aerodynamics professor at Virginia Polytechnic Institute and State University. “If you wanted to make your design more efficient, that’s probably the first thing you would do.”
Airbus didn’t have that luxury, and one number came to dominate the airplane’s design—and perhaps its future: 80 meters, or 262.5 feet, the limit placed on its wingspan by airport authorities. In 1999, when Airbus and, at the time, Boeing, were contemplating superjumbo airliners, the International Civil Aviation Organization worked with manufacturers, airports, and its member agencies—including the U.S. Federal Aviation Administration—to establish standards for aircraft with wingspans greater than 65 meters, the limit established when the 747 was introduced. The participants determined the dimensions of arrival and departure gates, widths of runways and taxiways, spacing between parallel runways, and strength of bridges that the larger airplanes would need to traverse. One result of their work is the “80-meter box” at terminal gates. It’s spacious for most airplanes, but for the A380 it’s a struggle. Smaller Airbus airliners, scaled up to the size of the A380, would have quickly punctured the sides of the 80-meter box. The big brother, therefore, ended up with stubbier wings than its siblings. The A380’s wingspan, 79.75 meters, squeaks about as close to the boundary as Airbus could get. It leaves less than six inches on either side. Pulling into a gate will be like squeezing into a garage with an eighth of an inch on either side of your car.
Charles Champion, the number-two officer at Airbus and head of the A380 program, acknowledges the company paid an aerodynamic penalty for the 80-meter box. The A380, he says, is a giant flying compromise between the aerodynamic and financial forces that say that say bigger is better and the practical realities of assembling the behemoth and bringing it back down to earth.
With the A380 limited in wingspan, its designers looked to power: They’d need four engines blasting a total of 280,000 pounds of thrust—more power than any other commercial airliner—to lift it into the air. The Engine Alliance, a joint venture of General Electric and Pratt & Whitney, makes one of the two engines available for the A380. (Rolls-Royce makes the other.) Engines that power modern airliners use large fans at the front to suck in huge amounts of air. The bigger the fan, the greater the volume of air forced rearward and therefore the more thrust an engine can produce. The Engine Alliance fan has a diameter of 9.7 feet and is rated at 76,500 pounds of thrust (and produced 94,000 pounds during testing). That’s big, but it’s not the biggest jet engine. That title goes to the one on the twin-engine Boeing 777, with a fan measuring 10.7 feet and blasting up to a record 122,965 pounds of thrust.
If Kroo’s superduperjumbo had to take off on today’s international airport runways, it would need somewhere between 500,000 and 750,000 pounds of thrust to get off the ground in the runway length allotted. With a reasonably efficient wing, an aircraft needs thrust equal to between 20 and 30 percent of its takeoff weight to lift off from a runway of about 10,000 feet, the typical length of runways at today’s international airports. (If the engines could provide thrust equal to 100 percent of the takeoff weight, says Kroo, “you could go straight up.”) To provide enought thrust to the superduperjumbo—more than 500,000 pounds—you’d have to equip it with eight of the massive engines that power the A380—compared to the four on the Airbus. (Hanging eight engines has been done before—on Boeing’s B-52, for example.)
The cost of a bigger engine is more weight, requiring the airplane to carry more fuel to fly with the extra weight, and still more fuel to carry the weight of the extra fuel, says Bruce Hughes, Engine Alliance president. To compensate, designers scoured the A380 engine for ways to shed weight—thinning its walls, shaving its airfoils, and using hollow titanium blades sculpted like sinuous modern art. After dozens of trade-offs, engine and airframe designers came up with the power-to-weight ratio needed.
“There’s no reason you couldn’t just keep building bigger and bigger planes,” says Dennis Bushnell, chief scientist at NASA’s Langley Research Center in Virginia, who has studied designs for very large passenger aircraft. “The problem you run into is whether the airport infrastructure can handle them, and whether you have the margin of safety that you need to have.”
John McMasters, an aerodynamicist at Boeing who worked with Kroo on his 1996 analysis of large aircraft, contemplated the problems of fitting the beasts into existing airport infrastructure and found a way out: He designed a seaplane. That way, he reasoned, he wouldn’t have to worry about finding a runway big enough to land on. He calls his concept the Super Clipper, a modern successor to the luxurious flying boats that Pan Am designed in the years before World War II, when runways were scarce. The vast, 239-foot wings of his design would be supported with floats made from the fuselages of 747s, which would themselves carry some of the Super Clipper’s 1,200 passengers. Though it might fly slower than today’s aircraft, it would make up for its leisurely speed by offering its passengers enough room for jogging tracks, ballrooms, and wine bars—featuring Las Vegas-style lounge acts on selected routes.