Physics is often a straightforward subject. Usually everything makes sense. So why is there confusion when an aircraft generating lift is explained? Which of the two most common theories of lift, the classic textbook explanation of Airfoil Shape Theory, or the more seemingly censored Attack Angle Theory, is more reliable? If you don’t have any clue what I’m talking about, that would seem to be the place to begin. A basic explanation of each theory is presented below.

• Airfoil Shape Theory states that the upper surface of the wing is curved upward and the lower surface of the wing is flat. It states that to keep up with the air going under the lower surface, which has a shorter path length (distance to the trailing edge, or back of wing, from the leading edge, or front of wing, on the shortest physical path possible), the air going over the upper surface has to move faster. The theory states that the total amount of energy has to remain the same, and as kinetic energy, or the speed the air is moving increases potential energy or pressure, must decrease. This is called The Bernoulli Effect. The higher pressure air under the wing tries to force the lower pressure air of the upper surface, and the whole airfoil with it, upward. In simpler terms, the wing gets “sucked up” (“suction” is incorrect because the true occurrence is that the higher pressure air around the lower pressure air really pushes against it in an attempt to equalize the area), resulting in lift.

• Attack Angle Theory states that the airfoil is pitched slightly upward in level flight. This pitch compared to the direction the air is hitting the wing (or more realistically, the direction the wing is hitting the air) is called attack angle. As air hits the lower surface of the airfoil, it is deflected downward. The air deflected downward deflects more air (with a slight reduction), and so on. The air going over the upper surface tends to adhere to the airfoil. This is called the Coanda Effect. Because of the attack angle, the air has to change direction and move downward. The same reaction as the lower surface, in which the air is affected for a long distance below the wing, occurs with the upper surface, with air being affected for a long distance above. According to Newton’s laws, the air deflected downward will create “an equal and opposite reaction” on the wing, also known as lift.

I strongly favor Attack Angle Theory.

If Airfoil Shape Theory was as almighty as is usually implied, aircraft would not be able to fly upside down. According to airfoil shape theory, if an aircraft were to invert in flight, it would not only fall out of the sky, but force itself into the ground. They can invert in flight and still work almost the same aerodynamically. To be more specific, the only reasons they would operate differently are that the airfoils will stall at a less extreme attack angles (stall is a condition in which the Coanda Effect is broken on the upper surface of the airfoil, causing a severe reduction in lift) and that most engines are not designed to operate upside down (some are. however, especially the engines on aerobatic aircraft).

Wings shaped incorrectly (incorrectly, that is, according to Airfoil Shape Theory) still produce lift. Airfoils can be symmetrically curved, more curved on the bottom, or even flat, and still produce lift, Aerobatic aircraft often have equal camber on both the bottom and the top of their airfoils. Paper planes and balsa-wood gliders have flat airfoils. Do they drop like a rock? No!

Some aircraft reverse some of the thrust produced by their engines to slow down on the landing roll. Turboprop thrust reversers are engaged when the blade angle is reversed (extra oil is also injected to compensate for the extreme friction generated by doing this, but that is somewhat beside the point in this case). No, the prop is not reshaped. If airfoil shape theory is so reliable, using this technique would be an excellent waste of perfectly good time and money. They happen to work just fine.

Some people may still side with Airfoil Shape Theory.

“Why, then,” they may ask, “do so many sources use Airfoil Shape Theory as their explanation if it is so unreliable?”. Airfoil Shape Theory is used to oversimplify lift. For some reason, schools seem to think that young children are not capable of understanding Attack Angle Theory but are capable of understanding Airfoil Shape Theory. First of all, they are more likely to get confused later, when they hear the more correct explanation of lift after they have been taught for years that all theories other than Airfoil Shape Theory are obsolete. Second of all, many or most of the students should be able to understand Attack Angle Theory if they have any interest. In my opinion, it’s easier to understand Attack Angle Theory than Airfoil Shape Theory, especially when Airfoil Shape Theory is explained as it usually is in schools or in textbooks, without a somewhat thorough accompanying explanation of The Bernoulli Effect. College courses on physics and aeronautics teach the theories correctly. Besides, scientific laws are proven, not democratically elected. When people recorded the temperature at which water freezes [under standard conditions at sea level], do you think they had a vote as to whether it was two-hundred twelve degrees Fahrenheit (one- hundred degrees Celsius) or thirty-two degrees Fahrenheit (zero degrees celsius)? No!

They may also retort, “Why do manufacturers still shape airfoils with greater path length on the upper surface if it doesn’t produce lift?” Shaping the airfoils like this doesn’t actually produce lift, but allows the airfoil to move through the air at more extreme attack angles without stalling, Stalling is caused by the air having to change direction too abruptly to follow the Coanda Effect. A stall basically forms an eddie on the upper surface of the wing. When this occurs, the upper surface of the wing is no longer deflecting air downward in the same way and therefore not producing much lift, The upper surface is the primary source of the lift, so when this occurs, the overall lift is severely reduced. By curving the upper surface, you allow the air to undergo a change in direction that occurs over a slightly longer period of time and it is spread over a larger surface of the wing. Because of the ability to endure higher angles of attack, the aircraft can fly slower without stalling. Because of the reduction in stall speed, the aircraft can have a reduction in landing and takeoff distance. Some manufacturers even curve the lower surface of the wing upward to better distribute lift.

These skeptics may argue that aircraft can stay in level flight without an attack angle. This is not true. The shape of many airfoils make them look like they don’t have attack angles. These airfoils do not have a completely flat surface on their underside- The very tip of the leading edge is higher than the lower surface; the surface curves down before flattening out. The upper surface begins at the same point, slopes upward, then curves downward. It doesn’t, obviously, end at the same level as it started at (unless the aircraft owners feel like chopping off the entire trailing section of their wings). It continues in a downward slope meet the lower surface. Therefore, the overall effect is that the air ends up in a lower position than it was in before encountering the wing. The wing is also angled upward in relation to the fuselage. This means that the fuselage can also have a level attitude and the aircraft will still maintain altitude.

Some may still argue that what I wrote about paper planes having flat airfoils is incorrect. Okay, so some paper planes have folds on the upper surface that increase the path length. First of all, in most cases, these folds only increase the path length very, very slightly, and second of all, paper planes can be made with no folds on the upper surface and thick folds on the lower surface. Guess what! These planes glide beautifully. The folds on the upper surface probably do not do very much to improve aerodynamics. It might help initially to maintain laminar flow, but when the air reaches the end of the fold, the fold might act as a less-effective-than-normal spoiler and create a small eddy that creates a small error in laminar flow.

There may be people out there who strongly support The Bernoulli Effect in general and are not too pleased with me supporting a theory of lift that excludes their favorite scientific principal. It is not correct to say that The Bernoulli Effect is completely excluded from Attack Angle Theory. Wind tunnel tests have proven that air moving over the top of the airfoil accelerates and actually reaches the trailing edge before the air below the airfoil (the air below the airfoil also slows down). How does this happen? Another one of Newton’s Laws, you may recall, states that an object in motion tends to stay in motion and an object at rest tends to stay at rest. The air above the airfoil doesn’t want to move downward, but is forced to move downward. Because of this tendency, the air doesn’t want to be deflected downward and, on the upper surface, pulls apart as much as the atmosphere will allow. Hence low pressure. The inverse effect occurs on the lower surface: the air compresses, causing an area of high pressure. As you may recall from earlier in this document, according to The Bernoulli Effect, the air with lower potential energy (pressure) has to have greater kinetic energy (speed) to have a constant energy total and the air with greater potential energy has to have less kinetic energy. In just about any fluid dynamics equation with speed of the fluid and pressure of the fluid as variables, The Bernoulli Effect must be taken into account. Do not interpret Attack Angle Theory the wrong way. This is no exception.

So please, if you ever happen to write a science book that includes an explanation of lift or have any other opportunity to profess this, don’t jump on the bandwagon and use an oversimplified version of Airfoil Shape Theory, Choose Attack Angle Theory and if it is absolutely necessary for you to primarily use Airfoil Shape Theory, give a more in-depth explanation and keep in mind that if you oversimplify, you will quite probably get whatever you are explaining incorrect.

Note: Many explanations in this document described the air moving over the wing. Realistically, the wing moves through the air. It was explained as it was because it is much easier to describe the theories by stating that the air moves over the wing. For all practical purposes, the aerodynamics work the same either way.


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