Beat around in the underbrush of aerodynamics and you'll encounter an interesting question:
"How do aircraft wings REALLY work?"
Amazingly enough, this question is still argued in many places, from club classrooms all the way up to major pilot schools, and even in the engineering departments of major aircraft companies. This is unexpected, since we would assume that aircraft physics was completely explored early this century. Obviously the answers must be spelled out in detail in numerous old dusty aerodynamics texts. However, this is not quite the case. Those old texts contain the details of the math, but it's the *interpretation* of the math that causes the controversy. The two sides of the controversy are:
(1) The physics explanation, NEWTONIAN or ATTACK ANGLE: wings are forced upwards because they are tilted and they deflect air. A wing's trailing edge must be sharp, and it must be aimed diagonally downward if it's to create lift. Both the upper and lower surfaces of the wing act to deflect the air. The upper surface deflects air downwards because the airflow "sticks" to the wing surface and follows the tilted wing. After the wing has passed by, air remains flowing downwards.
(2) The popular explanation, PATH-LENGTH or AIRFOIL-SHAPE: wings do not deflect air, instead they are sucked upwards because the "airfoil" shape has a longer surface on top. Airfoils are curved on top and flat below, and therefore the air follows a longer path above than below. Air that is divided at the leading edge of a wing must rejoin at the trailing edge. Since the upper surface of the wing is longer, it causes the air to flow faster over the upper surface, which (by Bernoulli's principle) creates lower pressure above. Because lift is caused by the shape of the wing, wings can create lift at zero attack angle. They can create lift simply from path length difference which leads to pressure difference, and no air needs to be deflected. After a wing has passed by, the air does not remain moving downwards. (THIS EXPLANATION IS SERIOUSLY FLAWED.)
Note: Newton and Bernoulli do not contradict each other. Explanations which are based on Newton's and on Bernoulli's principles are completely compatible. Air-deflection and Newton's Laws explain 100% of the lifting force. Air velocity and Bernoulli's equation also explains 100% of the lift. For the most part they're just two different ways of simplifying a single complicated subject. Much of the controversy arises because one side or the other insists that only *THEIR* view is correct. They insist that only a *SINGLE* explanation is possible, and the opposing view is therefore wrong. In other words, which is the single best way to crack an egg? This is a war between the Big-endians and Little-endians from "Gulliver's Travels." They simply refuse to acknowledge that there are several valid yet independent approaches to solving the problem. They insist that their version must be the single right answer.
However, there are also several serious mistakes usually associated with the "popular" explanation described above. People who believe the "popular" explanation wrongly insist that any parcels of air divided by the wing's leading edge must meet again at the trailing edge. This is incorrect; experiments easily show that the air above a wing far outraces the air below, and parcels never meet again. They also believe that wings don't deflect the oncoming air downwards. Also incorrect. These and several other mistakes commonly appear in science texts, as well as in popular articles on aircraft physics. These mistakes change the popular "airfoil-shape" explanation into a system of misconceptions.
Also, those who firmly adhere to the popular explanation have been successful in convincing many authors that there can only be a single best method for explaining aerodynamic lift, and that the "Airfoil-shape" method is far better than the "Attack-angle" method. I strongly disagree with this, and believe that the correct versions of both explanations should be in constant use. Since the Newton method gives a better intuitive grasp of the issues, that method is more appropriate for elementary explanations aimed at the public and for introductory material for science students and pilots. On the other hand, the "Airfoil Shape" explanation is less intuitive, yet it dovetails very well with lifting force calculations, so it is very useful in mathematical modeling, for physics students, for aircraft design, fluid flow simulation software, etc.
Parts of the Airfoil Misconception
1. Wings create lift because they are curved on top and flat on the bottom.
Incorrect because only some wings look like that, while other wings are symmetrical (they're the same on top and bottom,) while still others are flat on top ...and curved on the bottom! And don't forget the hang-gliders and the Wright Brothers' flyer, both of which used thin fabric wings with equal curvature top and bottom. The lifting force does not vanish if an airplane flys upside-down. Explanations for flight involve other things, and not airfoil asymmetry.
2. Part of the lifting force is due to Bernoulli effect, and part is due to Newton.
Incorrect because ALL wings, regardless of shape or degree of tilt, must create 100% of their lift because of Newton. To say otherwise would mean that a wing could violate Newton's Laws! Yet at the same time, ALL wings create 100% of their lift because of the Bernoulli Equation. This is true because 100% of the lifting force comes from pressure differences on the wings' surfaces.
In fact, we can explain the lifting force by "Newton," by ignoring the pressure differences and instead measuring the dense deflected air and calculating the change in momentum.
And of course we can explain 100% of the lifting force by "Bernoulli",by looking at air speeds and then calculating the air pressure on every part of the wing surface.
3. In order to generate lift, the upper surface of an airfoil must be more strongly curved than the lower surface?
Incorrect, since lift can be generated by symmetrical airfoil such as those used on acrobatic aircraft. Lift can also be generated by thin fabric airfoils, by sheets of paper (paper airplanes), by tilted pieces of flat plywood, or by "supercritical" airfoils which are more curved on the BOTTOM than the top.
4.The upper surface of a wing will deflect air, but the lower surface is horizontal, so it has little effect.
Incorrect, but for an interesting reason.
If a thin flat wing deflects air downwards, diagrams show that the air above the wing and the air below the wing are equally deflected. Both the upper and lower surfaces create the lifting force.
If we then make this wing thicker and streamlined, the total amount of deflected air and the lifting force remain the same... but the air below the wing APPEARS less deflected, and the air above the wing appears more deflected. Also, the pressure below the wing APPEARS to provide less lift. This happens because a thick wing must push air out of its way, and as the flowing air splits up and down to make a space for the oncoming wing, air below the wing takes a straighter path. It takes a straighter path because the thickness of the wing bends air upwards at the same time as the tilt of the wing bends air downwards. This has no effect on the lifting force, since the air above the wing takes a more curved path, so THE PRESSURE DIFFERENCE REMAINS THE SAME AS IT WAS FOR A THIN WING. The thick wing is making us confused. The thick wing SEEMS to get more lift from the curved streamlines above the wing than from the straight streamlines below, but this is an illusion. The thick wing distorts the stream- lines. Examine the streamlines surrounding a thin wing to see the truth. The lift comes from the DIFFERENCE between the two flows, and changing the thickness of the wing will alter the appearance of the air flows without changing the difference or changing the lifting force.
5.The low pressure above an airfoil produces suction. The lifting force is an upwards suction force.
Incorrect. Air molecules produce pressure upon a surface by colliding with that surface. They do not attract that surface. In other words, SUCTION DOES NOT EXIST. When you suck air through a straw, you are lowering the pressure within the straw. There is no suction. Instead, the outside atmosphere PUSHES the air into the straw. So, while it is true that the pressure above the wing is low, it is not true that the lifting force is caused by suction. Instead, the lifting force is caused by the pressure-difference. If the pressure above the wing should fall, then the ambient pressure below the wing will force the airplane to move upwards.
6.A properly shaped airfoil gives increased lift because the air on the upper surface moves faster than the air on the lower?
MISLEADING! Not entirely wrong. This is only half the story. A properly shaped airfoil gives increased lift because the airflow does not easily "detach" from the upper surface, so the upper airflow can generate lift even at large angles of attack and at low aircraft speeds. A sheet of plywood makes a poor wing because the airflow will "detach" from the upper surface of the wood when the sheet is tilted more than a tiny bit. This is called "stall", and it causes the upper surface of the wing to stop contributing a lifting force. A properly designed wing must spread the net deflection of air widely across its upper leading surface rather than concentrating all the deflection at its leading edge. Hence, the upper surfaces of most wings are designed with the curvature which avoids immediate flow-detachment and stall. The shape of wings does not create lift, instead it only voids stall.
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