Not so long ago people thought that the air belonged to the birds. If a balloonist landed in a field, he was apt to be attacked by peasants with pitchforks. Today human flight is so commonplace that we would probably rush up to that balloonist waving tickets and wanting a ride.
How much we understand about flight is another story, but we have come a long way from the big mistake of trying to fly like birds do. In fact, with our spacecraft we are outgrowing the need for wings entirely. Nevertheless, there are those of us (I got a D in physics) who can use a little help here.
How, exactly, do things fly? The National Air and Space Museum recently opened a wonderful gallery on this subject. A permanent addition to celebrate the museum's 20th anniversary and the Smithsonian's 150th, it is worthy of "the world's most visited museum," with its eight million visitors a year.
I got a preview with curator Christopher (Kit) Stetser and Don Lopez, a World War II fighter pilot who has bet his life many a time on the deceptively simple physics of flight. Lopez, who joined Air and Space in 1972, retired three years ago but has returned to his post as deputy director. During the war he flew with Claire Chennault's 23rd "Flying Tiger" Fighter Group in China. These were the guys in the P-40s, and the P-51 Mustangs, the planes with the shark teeth painted on their noses. They flew the Hump, as they called the mountain range between India and China, and for my generation they were the last word in heroes — lean, grinning guys with their short leather jackets and their helmet straps hanging loose. Lopez was hit many times by Japanese fire, but he survived to watch a lot of things change in the air above us.
"I came home in '45," he said, "and was a test pilot at Eglin Field. I worked jets. They were so different from prop planes, we could hardly believe it: so smooth, no torque, there seemed to be no reason for them to be moving."
He also flew Sabres in the Korean War, he mentioned casually.
But there we were in the "How Things Fly" gallery. First off, I got a lesson in air pressure. We all live at the bottom of a sea of air, and the stuff pushes against us with astonishing force. There is a brick wall set in the middle of the gallery. It weighs four tons, exactly as much as the air in the room weighs. A hands-on display allowed me to literally push against the force of the atmosphere and measure it.
"We have great photos of animal flight," Stetser noted. "But we don't do much with it here; that flapping business set back aeronautics many years. We're concentrating on fixed-wing flight, with some attention to balloons, helicopters, gliders and spaceflight."
He showed me wind and smoke tunnels, hands-on devices that let even a D student see how the shape of a wing gives it lift and how lift relates to airspeed. If you double the airspeed, you quadruple the lift. The smoke tunnels will run all day, in contrast to the five-minute smoke-tunnel demonstrations many science museums offer.
I had to ask: If the wing surface provides the lift, how can planes fly upside down? "No problem," replied Lopez. "You just keep your angle of attack high, and you get more lift. People fly from one city to another upside down. It's kinda stupid, but you can do it." Acrobatic planes often have symmetrical wings, he added.
Playing with a smoke tunnel, a visitor can see what happens when you come in for a landing. You change the angle of the flaps and the air flows around the surfaces in a new pattern.
This enabled me to lecture my guides a little on one of my favorite subjects, the Wright brothers. They didn't invent flying, of course. Hundreds of people had glided through the air before them. What the Wrights contributed was, in essence, the aileron.
Before, most fliers had thought in two dimensions, as though the plane were sliding around on a vast empty parking lot. The Wrights thought in three dimensions. They learned that, to make a truly controlled turn, you had to bank the plane. And to do this, you warped the wings, bending their trailing edges-one up, one down.
The warping soon developed into ailerons, the movable parts of the wing at the trailing edge. A tail with movable vertical vanes was added to offset the aerial turmoil of a banked turn. "That's interesting," interjected Lopez politely. Clearly, he had learned all of this decades ago.
Some of the gallery's 50 interactive displays involve supersonic flight, including a supersonic wind tunnel run by a giant compressor in the NASM basement. Here, you can see how shock waves, caused by sound waves piling up in front of the plane, look at those speeds.
"One thing we want to do in this gallery is explain why the various aircraft around the museum look the way they do," Stetser told me. "All the different shapes. Why are some planes ultrasleek and others more rounded? Slower planes like the Voyager [the one that flew nonstop around the world] need wings with a large surface area to provide enough lift. But since lift increases rapidly with speed, faster airplanes get by with smaller wings. The wings on planes that fly at nearly the speed of sound are swept back so there's less drag from shock waves. Planes that fly just above the speed of sound can have a very sharp nose — the emphasis is on reducing drag.
But at hypersonic speeds, like Mach 5 and above, the heat caused by air being compressed in front of the plane is extreme. A rounder nose and wings are needed to push shock waves out ahead and thus insulate the plane from the extreme heat. That's why the shuttle's nose looks as stubby as it does. And it's why space capsules reenter the Earth's atmosphere with the large rounded surface facing forward." Heat is always a factor in supersonic flight; on the Concorde, Lopez said, you can put your hand on the inside wall and feel the warmth caused by friction, even though the plane is moving through 65-below-zero skies.
This being a flight exhibit, I kept looking up. And there I saw an 11-foot-long remote-control blimp that cruises the gallery's airspace; a model astronaut demonstrating weightlessness; a barometer that moves from floor to ceiling to show how air pressure changes with altitude; a life-size small-plane cutaway showing how the skin, not just the skeleton, provides strength.
"People don't understand how a rocket engine can work in a vacuum," Lopez said as we examined a cutaway rocket engine, "because they think it has to have air to push against. It's better to think of the engine as creating an explosion that pushes the rocket forward whether there's air around it or not."
I was reminded of the famous New York Times article of 1920 when the paper patronizingly chuckled from the heights of journalism that rocket pioneer Robert Goddard lacked "the knowledge ladled out daily in high schools." Forty-nine years later, with Apollo 11 circling the Moon, the Times ran an equally famous correction: "It is now definitely established that a rocket can function in a vacuum. The Times regrets the error."
This gallery has a large budget for maintenance, Stetser said, because so many of its exhibits are operated — and sometimes abused — by visitors. Volunteer docents and paid explainers are on hand. Supervisors oversee the place from a control tower mock-up overlooking the gallery.
Stetser explained that a team of 15 people at NASM worked on the gallery, which is generously supported by Boeing, NASA, the National Science Foundation, Cessna, the James Smithson Society and others.
We walked into a cutaway of a Boeing 757 that shows, for instance, how the windows are built and the seat structure. "We thought maybe we should have an in-flight movie," quipped Stetser. I climbed into the cockpit of a Cessna 150 and pretended to fly while I learned how to control the ailerons, rudder and elevator.
I told Stetser and Lopez that I had never learned to fly but had once sat in the front seat of a homemade glider watching the ground rush up at me and hoping that the pilot, at my back, was watching it rush up, too. Lopez nodded and reminded me of the old saw: flying is the second greatest thrill in the world — the greatest is landing. "You don't have to take off," he added, "but you've gotta land."