Present at the Future (35 page)

It’s got to be something to do with the air, right? There must be some lifting force to make the airplane rise off the ground. There is, but it’s not very obvious. And because you really can’t see the air at work, aeronautical experts have had a tough time explaining to the public why an airplane flies. Right from the time that air travel came to the masses—around World War II—teachers were looking for
a quick and simple, easy to remember, handy-dandy, user-friendly, not-too-complicated way to explain why airplanes fly to a public that was about to trust life and limb to some heavier-than-air contraption. So somebody—I don’t believe anyone knows who—came up with the explanation that still echoes through classrooms today.

A MYTH IS BORN

The explanation goes like this: Look at the typical textbook diagram of the cross section of a wing below. (Diagrams like this one have appeared in countless explanations of why airplanes fly.)

What do you notice about the wing? The top side is curved while the bottom is straight. This shape is very important to this classical explanation. Since the shortest distance between two points is a straight line, the distance from the front of the wing to the back is shorter on the bottom side of the wing than the top. If you were to follow the paths of the air flowing over the wing, you would notice that air particles hitting the front of the wing at the same time will split, some on top of the wing and some on the bottom. Since the top of the wing is curved and therefore its path is longer, in order for the air on top to reach the back of the wing at the same time as air particles on the bottom, they have to move much faster.

This means, in effect, that the air on top is being stretched thinner to make the same amount go farther. In physics, Bernoulli’s principle says that air stretched thinly on top exerts less pressure than air
packed thickly under the wing. So you’d have greater air pressure on the bottom of the wing, giving the plane “lift.” You might look at it in a different way and say that the thin air on top is “sucking up” the wing in the same way in which you suck up soda in a straw.

As I said, that’s the classic textbook explanation that has been taught for decades. And it’s quite wrong. Why? Let’s take it point by point.

“One common myth,” write David Anderson and Scott Eberhardt in Understanding Flight, “is the principle of ‘equal transit times.’” That’s the myth that says the particles of air—top and bottom—must reach the back of the wings at the same time. “But in reality, equal transit times hold only for a wing without lift.” And we all know no plane is going anywhere without lift. So much for that myth.

Now for that idea that the shape of the wing is important. Dave Anderson points out that high-performance acrobatic planes and helicopters have wings whose top and bottom sides are identical. No rounded top and flat bottom. In fact, says Anderson, your wing could be the shape of barn door or a garbage lid. It’s not the shape of the wing that gives it lift but rather how smooth it is and in what direction it’s pointed. That’s because a wing develops lift when air is smoothly flowing over the top of it and is forced downward toward the ground by the tilt of the wing. It’s the ability of the wing to make the air flow over the top and down to the ground—the angle of attack—that
results in an upward force on the wing. It’s Newton’s third law of motion: Every action has an equal and opposite reaction. Air goes down, wing goes up. And Newton’s second law of motion tells us that the greater the amount of air being diverted downward, the greater the lifting force. Sounds simple!

And you can stand under a helicopter and feel the wind blowing or watch those videos of helicopters landing in the desert or rescuing folks from boats. There the wind is quite visible.

As for Bernoulli’s principle, it does play some role in shaping the way the air flows over the wing. But Anderson says that the “suction” of Bernoulli is so weak that a tiny single-engine plane such as the Cessna 172 would need to fly 400 miles per hour just to get off the ground if it relied on Bernoulli’s principle for lift. My flight instructor would have a fit if I tried to fly a Cessna Skyhawk that fast; its normal takeoff speed is around 55 miles per hour, fully loaded. Another myth busted.

THE COANDĂ EFFECT

There is a fascinating detail about how the air is diverted downward on the wings that elegantly explains why airplanes fly. For a plane to fly, the wings have to attack the air at an angle that forces the air downward. One would assume from looking at how the wing is angled that the air should be deflected downward the same way that tennis balls bounce off a racket: bouncing off the bottom of the wing. But that is not the case. The air is diverted downward by something called the Coandă effect, something we experience every day but don’t even know it exists.

Henri-Marie Coandă was a distinguished aeronautical pioneer who began experimenting and designing airplanes in 1905. Born in Bucharest, Coandă is probably best known for his Coandă-1910, an “air reactive” airplane that was powered by what we would today call a jet engine.

Coandă noticed that in testing his flame-spewing jet that the
smoky gases that came out of the back of the engine tended to stick to the sides of the plane. This was a puzzlement. Why did the gases hug the fuselage? He investigated and noticed that any stream of gas, be it burning like that in the exhaust, or just your ordinary flow of air, tends to follow the curvature of a smooth surface as long as the surface doesn’t have to make a sharp turn.

You’ve seen this happen many times. When you pour milk out of a glass into your bowl of cereal or cup of coffee, if you tilt the glass of milk but don’t pour it very quickly, it dribbles down the side of the glass or cup and makes a mess on the table. Or notice what happens with your wet hands as you hold them while you’re looking for a towel. What happens to the water? The water flows down your arms. But it doesn’t drip to the floor! It follows your elbow around to your upper arm and then drips. It follows the smooth curve of your elbow. That’s the Coandă effect in everyday life. Nice little detail, huh?

The same thing is happening on the wing of an airplane. The air is hugging the curvature of the top of the wing. And because the smooth wing tilts back and down, the air follows that path too. It is forced down to the ground. In fact, when pilots take off, they have to rotate the nose of the plane upward to increase this angle of attack, directing even more air downward and increasing lift.

The plane also achieves lift because the leading edge of the plane is higher than the back (trailing) edge. No doubt when you were young and eager to experiment, you noticed this effect when riding in a fast-moving car. When you hold your hand horizontally out the window with your fingertips pointed slightly up, your hand takes off like a plane.

Notice: it is the air on the top of the wing that is doing the useful work. Not the air on the bottom. The air flowing beneath the wing is not bouncing off the wing bottom, like a ricocheting marble, and providing lift that way. In fact, the airflow below the wing is so unimportant to lift that the bottom of the wing is the place where airplane
weapons designers stick all the bulky military hardware—rockets, fuel tanks, and so forth. If that air was needed for flight, you couldn’t put that airflow-disturbing hardware there.

If you look out the window of your airplane during takeoff and landing, you’ll notice that the wing changes shape. It appears to
grow. Flaps extend backward and beneath the wings. During takeoff and landing, they give the plane added lift because they extend the size of the wing. On really big planes, you’ll notice that the flaps actually separate from the wings and extend slightly below them. The air is funneled over the top of the flaps to create extra lift. Remember, the air on the bottom of the wing is not really doing any work, so it has not lost any energy and can be used by the flaps for lift.

So why not keep the flaps deployed all the time? Too much wind resistance, called drag. Whenever lift is created, so is drag. The flaps are not needed at high speeds; they would slow the plane down and could actually be ripped off the wings at the 600-mile-per-hour speed that a jet airliner flies. (My flying instructor, Richard Orentzle, had to check on the condition of the flaps on my Skyhawk 172 when I went into a dive during stall practice with flaps fully extended. That was a no-no. It could have buckled the flaps, but there was enough margin of error built into that sturdy little plane to absorb the abuse.)

Myths die hard. And this one is no exception. Teachers continue to use Bernoulli’s principle in classrooms; I got into quite an argument with my daughter’s physics teacher over this. But progress is advancing slowly as enlightened textbooks and encyclopedias begin to modernize their thinking. And the Internet, that great online debating arena, is host to quite a number of discussions about the truth about why airplanes fly.

CHAPTER THIRTY-TWO

THE GREAT CHAMPAGNE BUBBLE MYSTERY

Tiny bubbles in the wine,

Make me happy, make me feel fine.

—BY LEON POBER, AS SUNG BY DON HO

You’ve had a wonderful wedding, bar mitzvah, graduation, or New Year’s Eve party, and now you’ve got a few open and partially filled bottles of champagne left over. Who would want to waste that bubbly? Not me! But is there a way to keep the brut from going bad overnight, to keep the champagne bubbly so that you might enjoy that mimosa the next morning?

This is a question that has been debated for centuries, since the French started making champagne. And the French thought they had the answer: the old spoon trick. Hang a silver spoon, handle down, in the neck of the bottle, pop the bottle into the fridge, and the booze should hold its bubbles for a few more days.

A team of Stanford University researchers put the idea to the
test—all in a thirst for knowledge and digging into their own pockets for research funds. They found that the spoon theory fell flat. Would any other technique work? They decided to find out.

The idea for their test came in 1991, when a reporter from Germany called Stanford University chemistry professor Richard Zare to find out whether and how the spoon theory works. Zare, a “bubbleologist,” had just published an article in Physics Today about the physics of the bubbles rising in a glass of beer. The question intrigued the physicist but left him doubtful about the magical powers of a spoon.

“I thought it might be a bubblemeise, said Zare. That’s a takeoff on bubbemeise, Yiddish for “grandmother’s tale.”

Then in 1994, renown food expert and author Harold McGee made a guest appearance on my radio program, Science Friday, where I asked him the same question. McGee, no stranger to science sleuthing, was a perfect choice for this question. He had published a paper in the journal Nature on why the froth of a soufflé stabilizes when you beat the eggs in a copper bowl. In McGee’s words, the two friends realized an obligation to human knowledge: Here was an experiment that cried out to be done.

Rising to the challenge, they convened an informal team of eight amateur taste-testers, including Zare’s wife, Susan; McGee’s wife, Sharon Long, a professor of biology; two more biology professors; a law professor; and a physician. A true cross section of scientific brainpower.

In the true spirit of scientific investigation, they tested and rated 10 bottles of champagne, carefully controlled for temperature and with a single glass of champagne removed to make sure all were the same at the start. The bottles received five different treatments:

• One bottle was opened just before the test.
  
• One bottle was opened 26 hours earlier and left uncorked.
  
• One was opened for 26 hours with a silver spoon in the neck.
  
• One was opened for 26 hours with a stainless-steel spoon in the neck.
  
• One was opened and recorked overnight.
  

The results were highly unexpected.

“What we found was a surprise—at least to us,” Zare said.

The spoons, silver or stainless, were not especially successful in maintaining the sparkle of the wine. But spoons and all other treatments worked better than recorking the bottles. Hard to believe, but at least in this test, recorking the bottles seemed to be the best way to make champagne lose effervescence and taste.

The hands-down winner? Simply leaving the bottle open—uncorked—in the refrigerator. In fact, the two bottles left open in the refrigerator for 26 hours averaged a higher score than any other treatment—including just-opened champagne!

Why such a surprising result? No one knows for sure, but there may have been a complicating factor or two. Perhaps the testing method itself influenced the results—that is, the state of the observers by the time a glass of champagne had been sipped (in some cases, more than sipped) from each of 10 bottles. As research scientists, several members of the team noticed what Zare called “fatigue of the instrumentation.” In this case, the instruments (the human beings) might have—how should we put this?—been influenced by the alcoholic content after so many glasses of wine.

“Our palates were not as fine as at the beginning. Eventually we didn’t feel quite right about letters and numbers,” McGee recalled. “You hear of the observer influencing the observed, but not often the observed influencing the observer,” McGee said. “I think we have a reverse Heisenberg principle here.”

One team member had a philosophical disagreement with a test that used bubbles as the mark of quality. “I am unable to disaggregate the gestalt of the wine,” he declared, setting down his scorecard.

Zare and McGee concede that their results are very preliminary
and that their data set is small. As Zare puts it, “We are struggling to achieve statistical significance.”

As reported by Stanford University, “After their study was completed, McGee learned of a French study that seems to confirm their results, conducted under the auspices of the Centre Interprofessionnel des Vins de Champagne. French science journalist Hervé This-Benckhard told McGee by e-mail: “I think we can affirm now that a spoon, made of silver or stainless steel or of aluminum, has no effect on what the French term éventage, or the loss of gas.”