FIGURE 1
Technique | Safety | Training | Flights
AirFOILS THE REAL REASON OUR WINGS WORK In 1944, a pilot and writer named Wolfgang Langewiesche published the first edition of his flying manual–Stick and Rudder. It was highly controversial because his description of the way wings produce lift was at odds with the popular theory of the time. Langewiesche encouraged his readers to forget nearly everything they learned in flight school about airfoils and focus on a simplified, useful theory. Even though the complete story of airfoil lift is much more complicated, his explanation gradually became accepted as the correct one, and as the most effective way to teach the idea to pilots. Strangely, though, the old theory that Langeweische debunked in 1944 persists to this day in high school textbooks and even in some flight manuals. The popular writer David Macaulay even presented it in his 1988 book, The Way Things Work. Why is this happening? Some have suggested that it’s entirely possible to learn the incorrect theory as a child, grow up to be an accomplished pilot in spite of your misunderstanding, and then write your own book from a position of authority. Another possibility is that one correct, but very complicated, mathematical explanation of lift is based on the same equation as the incorrect explanation, and so the two theories are given equal credence by some amateur scientists. The common explanation is based on two ideas: the Bernoulli equation, and the principle of equal transit times. Figure 1 shows the essential points. We have an airfoil (wing cross-section), flat on the bottom, curved on top, fat in the front and pointy in the rear. The lines represent an airflow coming from the left, which, for our purposes, is equivalent to a wing moving to the left through a stationary air mass. The lines show that the air mass splits at the leading edge (front) of the wing and then comes back together at the trailing edge, where each air molecule is reunited with its estranged neighbor. Since the molecules going over the top have to take a longer path from 26
and the oncoming air, but you can also see it as the degree to which the back of the wing is tilted down. Figure 2 shows a more accurate picture of the airflow: it encounters the leading edge at the angle of attack by Tom Webster (roughly the angle between the dashed line and the air stream lines on the left, exaggerthe front to the rear than the ones going ated for clarity), splits apart at some point, under the bottom, they need to go faster to and, most importantly, is forced downward reach the trailing edge. According to Daniel by the tilt of the wing. The air going under Bernoulli, a Swiss mathematician from the the bottom is deflected downward, too, and 18th Century, increasing the speed in a local adds something to the effort, but the heavy airflow lowers its pressure. This low pressure lifting is done by the flow over the top. creates suction which pulls the wing up. Or, What makes the air stick to the top so if you like, the relatively higher pressure on it can be shot downward is a curious thing the bottom pushes it up. called the Coanda effect. This effect causes Bernoulli’s equation is correct, but it’s a moving fluid to adhere to a nearby surface, being used the wrong way here. The lift as long as the surface is smooth enough and force that it predicts is not strong enough to doesn’t curve away from the flow too sharpexplain flight: a Cessna 152, relying entirely ly. You can see the Coanda effect in person on the lift predicted by the Bernoulli equa- by holding a spoon under the kitchen faucet tion, would need to fly more than 300 miles like I did in the picture. The water stream an hour to stay aloft instead of 55. And the is pulled sideways by the back of the spoon, principle of equal transit times has been the spoon is pushed in the opposite direcproven wrong, far beyond any doubt, in tion by the water stream, and Isaac Newton wind tunnel experiments. Air goes over the is happy. Rotate the picture until it reminds top of a wing so much faster that the neigh- you of an airplane wing, and you’ve got one boring molecules split apart at the leading of the most important concepts in aerodyedge never see each other again. Here are a few more problems with the popular explanation: * Airplanes and gliders can fly upside down and still generate lift. * A wing with a very concave underside, like a single surface hang glider, should produce very little lift according to this theory, but in the real world it produces a lot of lift. * Some airplanes use a symmetrical airfoil, or even one that is longer on the bottom than the top. * Paper airplanes (and vintage hang gliders) have no airfoil, yet they seem to fly just fine. As Langeweische says, you’d do well to forget about Bernoulli-the 18th Century mathematician you should be concerned with is Isaac Newton. Newton’s famous Third Law states that for every action there is an equal and opposite reaction, and explains how rockets work, why a pistol recoils, and what would happen if you used a fire extinguisher while standing on a skateboard. Applying it to flying, we find that a wing goes up because it forces air down. It’s all about angle of attack. That’s basically the vertical angle between the wing July 2008: Hang Gliding & Paragliding – www.USHPA .aero