Even though I know something about how airplane flight works (and you will too, soon), it never ceases to amaze me.
I remember landing after a transatlantic flight in a Boeing 747 and being directed by the crew to deplane directly onto the ground and into a waiting bus, instead of through one of those people tubes. I looked up in utter dis-belief at the four-hundred-ton monster that had just wafted me across the Atlantic Ocean at an altitude of more than five miles above Earth’s surface.
My awe was magnified by the fact that back when I was “taught” what makes airplanes fly, I was misled. In spite of the fact that most flight training manuals attribute an airplane’s lift to something called Bernoulli’s Principle, that is not the main reason airplanes stay up. It just happens to be a quick, easy explanation, but like all simple answers it is misleading, bordering on downright wrong.
First, let’s put the Swiss mathematician Daniel Bernoulli (1700–1782) on the witness stand and see what he has to say for himself.
In 1738 Bernoulli discovered that as the speed of a moving fluid (gas or liquid) increases, its pressure on adjacent surfaces decreases. For example, air that is blowing by as a horizontal wind doesn’t have the time or energy, so to speak, to press very hard upon the ground.
How does this affect airplanes?
The top surface of a conventional airplane wing is humped upward, while the bottom surface is relatively flat.
As the plane flies, air sweeps over these two surfaces. On its way to the back (trailing) edge of the wing, the air on the top surface has farther to go because of its curved path. The Bernoulli-Makes-Planes-Fly advocates claim that the top and bottom air must reach the wing’s back edge at the same time, that’s called the equal transit time assumption, and that inasmuch as the top air has farther to travel it must move faster. According to Mr. Bernoulli, then, the faster top air exerts less pressure on the wing than the slower bottom air does, so the wing is pushed upward by a net force called lift.
That’s all very well except for one thing: The top air and the bottom air don’t have to reach the trailing edge of the wing at the same time; the equal transit time assumption is just plain wrong, in spite of all the arm-waving that physics teachers and flight instructors do to try to justify it. You and I can both forget our embarrassment at never having understood that point in school. There is simply no good reason that the top air has to arrive at the trailing edge at the same time as the bottom air.
The Bernoulli effect does contribute some lift to an airplane wing, but acting by itself it would require a wing that is either shaped like a humpback whale or traveling at an extremely high speed.
Thank you, Mr. Bernoulli. You may step down now.
We now call Sir Isaac Newton to the stand.
Newton’s three laws of motion are the ironclad foundation of our understanding of how things move. Newtonian mechanics (as distinguished from quantum mechanics and relativity) can explain the motions of all objects, as long as they are not too small (smaller than an atom) and are not traveling too fast (near the speed of light). Newton figured out his laws for the motions of solid objects, but they can be applied as well to the interactions between airplane wings and air. Let’s see how.
Newton’s Third Law of Motion (again) says that for every action there must be an equal and opposite reaction. So if the plane’s wing is being pushed or lifted up, then by gosh something else is being pushed down. It is. The air. The wing must be whooshing a stream of air downward with a force equal to the lift it is getting. We’ll call it downwash.
How?
When a fluid such as water or air flows along a curved surface, it tends to cling to the surface more tightly than you might expect. This phenomenon is known as the Coanda Effect. (See the explanation, but instead of water flowing over a curved glass surface, think of air flowing over a curved airplane wing.) Because of this clinging, the air flowing over the surfaces of the wing is constrained to hug the shapes of the wing; the top-of-the-wing air clings to the top surface and the bottom-of-the-wing air clings to the bottom surface. The streams not only take different paths, but as a consequence of the wing’s shape they wind up flowing in different directions at the back of the wing. It’s not as if the wing were simply cutting through the air like a flat knife blade, with the airstream parting to let it through and then closing back to its original direction after the wing passes.
As the top-of-the-wing air meets the leading edge of the wing it flows first upward over the surface and then downward again as it leaves the trailing edge. But the shape of the wing leads it farther downward than where it began; it leaves the trailing edge of the wing in a net downward direction. In other words, the top-of-the-wing air is actually being thrust downward by the wing’s shape. And according to Newton’s Third Law, the wing is therefore thrust upward with an equal amount of force. Voilà! Lift!
Do you think this can be only a small amount of force, coming as it does from a push by “thin air”? Hah! Think again. Even a small plane like a Cessna 172 flying at 110 knots (204 kilometers per hour) is pumping three to five tons of air downward every second. Just think of the hundreds of thousands of tons of air that an 800,000-pound (360,000-kilogram) Boeing 747 is pumping downward every second to get off the ground and stay there.
We can give Isaac Newton still more credit for lifting airplanes, because the lift doesn’t all come from downwash (with a slight assist by Mr. Bernoulli). Some of it comes from yet another application of Newton’s Third Law. Airplane wings are not parallel to the ground; they are made to be tilted slightly upward in front, usually about 4 degrees when the plane is in level flight. That makes more pressure on the bottom surface than on the top, thereby pushing the wing upward and contributing to the lift. The pilot can tilt the plane even farther upward in front (Flyspeak: He can increase his angle of attack) to get even more lift from this effect. Sir Isaac’s Third comes in because as the plane moves, the wing is pushing the air down in front of it, so the air responds by pushing the wings up.
We see, then, that two different wing actions create lift: the wings’ shape, the “airfoil”, and their upward tilt, or angle of attack. Both must be used to maximum effect in order to grunt a heavy plane off the ground during takeoff. That’s why you see planes taking off from the airport at such steep angles of climb; the pilots must increase their angle of attack to gain extra lift while the plane is so loaded down with fuel, not to mention that fat lady in the seat next to you.
And you thought the pilot was simply pointing the plane’s nose in the direction he wants it to go in, as if it were a horse.
Bonus: Have you ever wondered why ski jumpers bend over so far forward when they’re in the air that their noses practically touch the tips of their skis? Two reasons. First, if they stood straight up they’d encounter more air resistance, which would slow them down. But second, their arched backs simulate an airfoil. Their upper surfaces are curved like an airplane wing, and they actually gain some lift that keeps them in the air longer.
