Why we can fly

2007 » April 2007
With enough power, you can make a barn door fly.
Sunday, April 1, 2007 - 5:00am

Ask a pilot what keeps an airplane in the sky and he'll most likely talk about the forces of lift, thrust, gravity and drag; power-to-weight ratios; and possibly "airfoils"-the word used to describe the wings' shape. Or maybe he'll answer simply, "Your credit card." Either way, it's not much comfort when you're eight miles above terra firma with no visible means of support.

The truth is, basic aerodynamics is a largely black art even to us pilots. The education we receive teaches us all the math and physics we need, which is really not much. What's far more important is learning to feel the effects of the controls during critical phases of flight. And that just takes practice. While it might be helpful for your teenager to study a car's steering and suspension systems, what he really needs to learn is how it feels when a turn gets too steep and a skid is imminent.

Of course, career pilots are much more savvy when it comes to advanced aerodynamics-if only because they must fully understand the aeronautical equivalent of those automotive systems as part of their training. In fact, though, most of what they need to know, they learned from their first textbooks. And unlike most people, they now find "how an airplane flies" to be second nature. And that makes it harder for them to explain it.

Conversely, almost everyone intuitively understands a ship's ability to float on water. Maybe that's because we all grew up playing with toy boats in bathtubs. The fact that a vessel will float as long as it holds enough air is learned and then becomes ingrained. We accept the science without completely understanding it because we've seen it over and over. As a result, we're less likely to fear an ocean cruise than our first airplane ride.

Still, anyone who has seen a balsa-wood toy airplane in action grasps most of the vital principles of flight. All it takes are a few more pieces of the puzzle to gain an appreciation for how a full-size airplane stays aloft.

Consider a falling leaf. It's easy to see that it won't plummet, even in calm air, but rather will sway from side to side as it floats to a soft landing. You don't need a scientific explanation for that.

The balsa glider works on the same principles. It, too, gains buoyancy-or lift-from the air it passes through. The difference is that the glider is balanced with wings and a weighted nose so that its route is less random and more stable than the leaf's-at least until my sons get their hands on it.

I remember years ago attaching a bottle rocket to one of those balsa gliders. I lit the fuse and launched the glider, with the rocket charge lighting off just as the plane slowed to the ideal speed. Instead of the usual swoop-and-fall, swoop-and-fall flight path, it climbed smoothly in a straight line for a few seconds-until the firecracker part of the bottle rocket exploded and the experiment came to a smoking end.

So if you can view a full-size jet as a better designed, grownup version of the balsa glider, with (let us hope) non-exploding engines of much greater power, you might feel more comfortable about soaring through thin air.

And notice that the wings of a toy glider are flat. Haven't we all read that a wing needs to be curved in order to fly? Actually, that's not true.

Like the leaf, the glider's flat wing gets buoyancy from gravity-induced momentum pushing against the air. Remember Newton's law: "For every action, there is an equal and opposite reaction." Push down on the air and it will push back as it flows around you, cushioning your descent. It's the same principle that pushes your hand skyward when you stick it out the car window and tilt it up. Add a source of thrust and surfaces to control all three axes of flight, and you've got a flying machine.

If that sounds unsophisticated, consider that the 1940s book Stick and Rudder by Wolfgang Langewiesche makes that exact argument for explaining to pilots how a wing flies. It's not that he disagreed with the curved-wing theory. He just found it less relevant to the person at the controls.

So what do we get from the curved-on-the-top design that we all know has something to do with our ability to fly? We get a refined, more efficient version of this upwardly mobile force called lift. There's a saying in aviation: "With enough power, you can make a barn door fly." It's true. Some specialty aerobatic airplanes have symmetrical wings-the same shape on top and bottom-the better to fly upside down. A regular airplane can fly upside down, too: it just does it a lot less efficiently, working hard to push enough air down to support the wing. When rightside up, the curved wing provides lift more efficiently and under better control.

Daniel Bernoulli, an 18th century Swiss physicist, sparked the debate when he wrote, quite correctly: "The pressure of a fluid, liquid or gas decreases at points where the speed of the fluid increases." Make a wing teardrop-curved on top and flat on the bottom, and air speeds up over the top, lowering the pressure. Air pressure on the bottom is unchanged, and voila: the wing wants to rise.

In fact, Bernoulli doesn't contradict Langewiesche's view of how the wing works. They're both correct, and the theories intertwine. When the pilot pulls the airplane's nose up, he's increasing lift by pushing more of the air down as in Langewiesche's scenario; and by increasing the distance of the flow over the top, as in Bernoulli's.

Tilt the wing up too far, and under both theories it loses all its buoyancy. Langewiesche's believers might say it's like trying to climb a hill that's too steep. Bernoulli fans would point to the breakdown of smooth airflow over the top of the wing. Again, they're both right. Either way, when the wing gives up, it's called a "stall" and the airplane will drop until speed builds up and enough airflow is reestablished.

One element that neither Langewiesche nor Bernoulli can escape is that creating lift also creates drag; and in level flight, drag must be overcome by thrust from the engines. It takes energy to push the wing through the air. The greater the volume of air that is disturbed, the greater the energy required. That's where today's computers can make a huge difference in efficiency. In the early days, aeronautical engineers designed wings with big curves and large reserves of lift-huge margins against the stall. Today's wings are much more refined, and computer design programs can milk more lift out of less drag.

But that's all part of the fine print when it comes to understanding what keeps the airplane safely up among the birds. If you look at the crude wings that supported some of man's earliest flying machines perfectly well, it's clear that flying is one of the more mundane scientific "miracles" we've achieved.
Just ask one of those birds.

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““We didn’t take no fancy rock-star airplane [to get here]. [Pause.] All right, all right, we did take a fancy rock-star airplane. But we thought about driving.” —Bruce Springsteen, talking to the audience during an April 23, 1988 concert at Los Angeles Sports Arena ”

-Bruce Springsteen