The accompanying text is headlined "Physics of Flight," and it says:
Birds use their wings to push themselves through the air. But if birds relied on flapping alone, they would tire quickly. Flying is made easier because wings provide lift, even without being flapped. The key is the shape of the wing.
You may have noticed that an airplane's wings are not flat. The upper surface is rounded. Look at Figure 18.3 to see the shape of an airplane wing. Notice how it is similar to the shape of a bird's wing.
When a wing with this shape moves through air, the air has a longer way to go around the curved upper surface than it does across the flat bottom surface. The air above the wing must move faster to cover this longer distance in the same amount of time. This difference in air speed above and below the wing creates a difference in air pressure. The pressure under the wing is higher. So there is more force pushing up, under the wing, than there is force pushing down, on top of the wing. The result is lift. The larger the wing, the greater the lift. Birds with large wings can soar and glide for a very long time. Once they are airborne, they can cover great distances without flapping.
Neither the illustration nor the text has any basis in science. Neither has any connection with physical reality. Both present fantasies that were conceived long ago by hacks who knew nothing about the physics of flight and who guessed that the induction of lift by an airfoil was a reflection of Bernoulli's principle -- i.e., the principle which states that the pressure exerted by a moving fluid decreases as the fluid's speed increases. These fantasies (with or without explicit references to Daniel Bernoulli) have been printed in schoolbooks for decades, although they have been denounced repeatedly by scientists, engineers, and competent teachers.
Read again the text that appears in Scott Foresman - Addison Wesley Science Insights: Exploring Living Things, and notice the claim that an airfoil's ability to generate lift depends wholly and specifically upon the airfoil's shape -- "When a wing with this shape moves through air, the air has a longer way to go around the curved upper surface than it does across the flat bottom surface. The air above the wing must move faster to cover this longer distance in the same amount of time. This difference in air speed above and below the wing creates a difference in air pressure. The pressure under the wing is higher. So there is more force pushing up, under the wing, . . ."
Now read why the claim is absurd:
First, an airfoil need not have more curvature on its top than on its bottom. Airplanes can and do fly with perfectly symmetrical airfoils; that is, with airfoils that have the same curvature top and bottom. Second, even if a humped-up (cambered) shape is used, the claim that the air must traverse the curved top surface in the same time as it does the flat bottom surface . . . is fictional. We can quote no law of physics that tells us this. Third -- and this is the most serious -- the common textbook explanation, and the diagrams that accompany it, describe a force on the wing with no net disturbance to the airstream. This constitutes a violation of Newton's third law.
That neat refutation of "the common textbook explanation" comes from an article that Norman F. Smith, an aeronautical engineer, contributed to the November 1972 issue of The Physics Teacher. The article was called "Bernoulli and Newton in Fluid Mechanics." Smith examined Bernoulli's principle, showed it was useless for analyzing an encounter between air and an airfoil, and then gave the real explanation of how an airfoil works:
Newton has given us the needed principle in his third law: if the air is to produce an upward force on the wing, the wing must produce a downward force on the air. Because under these circumstances air cannot sustain a force, it is deflected, or accelerated, downward.
There was nothing new about this information, and Smith demonstrated that lift was correctly explained in contemporary reference books. Here is a passage which he quoted from the contemporary edition of The Encyclopedia of Physics:
The overwhelmingly important law of low speed aerodynamics is that due to Newton. . . . Thus a helicopter gets a lifting force by giving air a downward momentum. The wing of a flying airplane is always at an angle such that it deflects air downward. Birds fly by pushing air downward. . . .
Nearly 30 years later, fake "science" textbooks continue to dispense pseudo-Bernoullian fantasies and continue to display bogus illustrations which deny Newton's third law and which teach that wings create lift without driving air downward.
By the way: In the last paragraph of the "Physics of Flight" passage in Scott Foresman - Addison Wesley Science Insights: Exploring Living Things, the stuff about birds that "soar and glide" is irrelevant rubbish: Birds soar by riding on updrafts.
Educators who want to learn about the physics of flight should be sure to read Smith's article in The Physics Teacher. Another useful source is this Web page sponsored by NASA: www.lerc.nasa.gov/WWW/K-12/airplane/wrong1.html
William J. Bennetta is a professional editor, a fellow of the California Academy of Sciences, the president of The Textbook League, and the editor of The Textbook Letter. He writes often about the propagation of quackery, false "science" and false "history" in schoolbooks.
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