Double the size of an Airbus A380? No problem, aerodynamicists say.
- By Michael Milstein
- Air & Space magazine, July 2006
How big can an airplane get?
Airbus Industrie’s A380, undergoing certification tests now, has racked up some impressive statistics: a length of 239 feet, a tail as high as an eight-story building, and a 262-foot wingspan. It can carry 853 passengers on two levels, if you cram them all into economy seats. It has 17 restrooms, is heavier than 16 semi trucks (1.23 million pounds fully loaded), and can accommodate 81,893 gallons of fuel and 6,492 mini-bottles—or thereabouts.
But the A380 is not as big as an airplane can get.
An airplane can be as big as you want, say researchers who have tested the question against the laws of aerodynamics. The Russian Antonov An-225, a six-engine jet produced in 1988 to carry the Russian space shuttle, is bigger than the A380, with a span of 290 feet and a takeoff weight of 1.32 million pounds. Howard Hughes’ Spruce Goose is even bigger. And Airbus is already hinting at a stretch A380. When it comes to hauling people through the air, size is an advantage. But at some point, does size become a handicap?
At about the time Airbus committed to the A380, Ilan Kroo, a Stanford University professor of aeronautics and astronautics and a leading aircraft designer, tried to answer that question.
Kroo worked with several other aerodynamicists on a NASA-sponsored study to evaluate the effects of size on aircraft performance and cost. He and his colleagues first believed that the growth of an aircraft would bump up against the square-cube law, a principle first outlined by Galileo that suggests that everything has a maximum size. In mathematical terms, the law states that when an object increases in size, its weight multiplies faster than the strength of the structure that supports it. In the case of an airplane, the engineers feared that the weight of a hypothetical craft would grow faster than the lifting ability of its wings until at some point you couldn’t build wings long enough and sturdy enough to get the whole thing into the air.
But the square-cube law turned out not to limit the size of airplanes until the craft grow much bigger than the A380. “The basic physics that makes flying insects common and flying elephants impossible is not the main factor limiting the size of future aircraft,” Kroo says. He measured the lift-to-drag ratio—the aerodynamic efficiency—for aircraft ranging in size from a 92,000-pound weakling with a span of 75 feet to a whopper that would weigh about 2.5 million pounds on takeoff, about twice as much as the A380. Its wings would stretch 392 feet, half again as long as the Airbus’ and almost twice those of a 747. You’d need a roadmap to find your seat. It had a better lift-to-drag ratio than the smaller designs.
That a larger wing is more efficient may seem counterintuitive, since a longer wing on a heavier aircraft will need added structure to handle the increased loads from lifting that weighty fuselage. But Kroo found that the weight added to strengthen the wing was only a modest fraction of the airplane’s overall weight—modest enough not to impose a significant penalty. Given a fixed span, in this case a very long one, the aerodynamicist would strengthen the wing by lengthening its chord (the distance from its leading to its trailing edge) and making it thicker—creating a deeper, structural box. Increasing size also confers some advantage in Reynolds number, a parameter that reflects how the size and speed of an object affect the resistance it meets from the fluid (in this case, air) it moves through. The airplane’s larger wings experience less drag per square foot of area.
“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.
“If you’re going to do a big plane, why don’t you do something grand?” McMasters says. “The airlines like big planes because they move lots of people. Why not build something people will like too?”
McMasters’ seagoing Super Clipper is a fanciful extrapolation from his study of large aircraft configurations. He has also studied the prospect of a giant flying wing, a design that NASA has continued to study. At this point, Boeing’s interest is purely in military applications, and the company will fly a scale model of the Blended Wing Body aircraft at NASA’s Dryden Flight Research Center in California this fall. If a full-scale commercial version were to be developed, it would face the same problem the A-380 faces: airports.
London’s Heathrow is the third busiest in the world (after Atlanta and Chicago) and, as the airport handling more international travelers than any other, probably the most cramped for space. When Airbus designers approached airports around the world starting in 1996, Heathrow was enthusiastic because the A380’s capacity could squeeze more people into each of its coveted landing slots. The airport now expects 65 superjumbo flights a day by 2015.
But big airplanes create big demands on the ground: Heathrow is spending about $800 million to rebuild itself, widening runways, adding taxiways, lengthening baggage conveyors, and renovating a terminal to create four superjumbo-size gates almost as wide as Big Ben is tall. The airport is also building a new terminal, to be completed by 2011, that will include 14 gates that can accommodate the A380; two of the 14 opened this year.
In Singapore, where Singapore Airlines has ordered 10 A380s, with options for 15 more, costing a total of $8.6 billion, and where the first commercial A380 flight will likely take off early next year, Changi Airport is expanding 19 gates with extra seating, restrooms, and three jetways to handle A380s. Crews at Changi, which promises to be a hub for the megaliners on crowded Asian routes, have widened runways, expanded runway-taxiway intersections, and added shields to block the intense blast from the A380’s engines.
Airbus designed the A380 so airports wouldn’t have to strengthen existing runways. Though it weighs far more than a 747, its tires put no more pressure on runways because its weight is spread out over more wheels: 22. The immense weight of the new airliners does require some airports to reinforce bridges they will use. Its span and engine placement require them to widen runways from 150 feet, the standard for a 747, to 200 feet. Some airports, such San Francisco International, will work around that requirement by closing adjoining taxiways when an A380 arrives.
Airbus has strained to coax every bit of per-passenger performance possible from its superjumbo, and that’s the reason the airplane has two decks. Airplanes that pack people onto two levels—like the A380—or even three levels—like Kroo’s superduperjumbo—make more efficient use of their space. Kroo figured that by stacking 1,500 passengers in an airliner’s triple-deck fuselage, for example, he would reduce the ratio of nonlifting fuselage area to lifting wing area, which would reduce the power needed to carry each person. (Pity the poor pilot who’d have to stand there and say goodbye as all those people file off.)
But the payoff multiplies not only because per-person thrust is optimized but because the basic costs of flying any airliner—the salary of our friendly pilot, for instance—are spread across the ticket price of more passengers, so the cost of moving each person is smaller.
“You get some help from the aerodynamics [in a larger airplane], but you get even more help by putting another 50 to 80 people on it,” says William Crossley, a professor at Purdue University’s School of Aeronautics and Astronautics in Indiana. “That’s why the airlines really like it. They’re in the business of moving people, and the more people they can move, the better.”
Moving more people through the air, however, sometimes seems easier than moving them on and off an airliner. The A380, says Airbus, is focused on efficient loading and unloading. The exits were located so that once the airplane pulls up to the terminal, passengers can gather their belongings and get off quickly, and the next load can get on and fasten their seatbelts—all in 90 minutes, keeping time on the ground to a minimum.The ultimate test of exit efficiency, however, would come in an emergency, and the FAA and the European Aviation and Safety Agency required Airbus to load the A380 to maximum capacity and then get every last one of its 853 passengers off within 90 seconds. The test had to mimic real emergency conditions: At least 35 percent of the “passengers” were over 50 years old, at least 40 percent were women, carry-on baggage and pillows were strewn about the cabin, and only half the airplane’s doors were workable. And the evacuation had to be performed in total darkness.
Last March in Hamburg, Germany, 853 volunteers and 20 crew members left an A380 in 78 seconds. One man broke his leg and there were several other minor injuries, but the airplane passed the test.
Risk analysts wonder if a test can truly simulate all real world conditions. Emergency exit slides are rated to inflate within 10 seconds even in a 25-knot wind, but a critical question for the A380 is whether passengers will balk at sliding almost 30 feet from the uppermost deck. Passengers must leap onto the slide faster than one per second, so more than a blink of hesitation will clog the flow.
Exit slides, made by North Carolina-based Goodrich, are designed so the slope doesn’t look as steep as it really is, Champion says. But exit slides were a problem for at least one passenger very familiar with airliners. Juan Trippe wanted the Boeing 747 to be a double decker for the economic advantage, but it ended up with only a partial upper level in its hump after Trippe was invited to try out an exit slide—and chose the stairs instead.
One of the more contentious issues in the A380’s test program was the danger of the vortices the airliner will trail. Air swirling around the wings not only cuts into performance, it also trails the wings in the form of invisible whirlwinds. Every airplane creates them, but the heavier the airplane and the shorter its wingspan, the more powerful they can be.
To an airplane moseying behind, a vortex may seem like no more than a little speed bump, but the whorl could also act like an unseen hand, suddenly flipping an airplane upside down. Aviation authorities manage the problem by imposing buffer zones between aircraft, especially on takeoff and landing, when vortices are most dangerous. A light aircraft, like a Cessna 172, must stay at least six miles behind a 747.
Through wind tunnel tests and computer modeling, Airbus adjusted the A380’s wing design to keep vortices in the same range as those of a 747, but the International Civil Aviation Organization said early simulations and preliminary flight test data found much more powerful vortices, and ordered aircraft to stay at least 10 miles behind the Airbus. Congested airports like Heathrow would have to space flights out so widely they’d lose the advantage of packing more people onto the bigger airplane. An anxious Airbus has now committed to measuring vortices as the A380 continues its early flights at various locations around the world. It’s doubtful the 10-mile buffer will stick; a similar buffer imposed on the 747 when it first entered service turned out to be stricter than necessary.
Kroo’s imaginary 1,500-seater would no doubt trail monster vortices. Long wings dampen the whirlwinds, but no little Cessna would want to get within many miles. Perhaps airports and regulating authorities would find a way to channel traffic to lessen the impact of a superduperjumbo.
Richard Marchi of Airports Council International says that airports are adjusting to airplanes with larger, more efficient wingspans. “If aerodynamics are going to drive you to a bigger wing, then the airports are going to accommodate it eventually,” he says.
Boeing’s humped 747 entered service in 1970 as the biggest airliner in history amid fears that it too would be more trouble than it was worth. But in an era of booming air travel, it proved so popular with airlines that it turned out to be one of the most successful airplanes in history—1,117 are still in service today.
No doubt there is some size at which an airplane would be too big to get off the ground, Ilan Kroo says. But he hasn’t found it yet.