How Do Airplane Wings Really Work?


Basic theory

Amazingly enough, this question is still argued in many places, from elementary school 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 it 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. There is an ongoing "religious war" over the way we should understand the functioning of wings, and over the way we should explain them in children's textbooks. The two sides of the controversy are:


  • The physics explanation, PRO-NEWTON or ATTACK ANGLE: wings are forced upwards because they are tilted and they deflect air downwards. 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 (this phenomena is called "Flow Attachment" or the "Coanda effect.") Airplanes fly because of Newton's 3rd law (action/reaction forces,) the law of Conservation of Momentum, and the Coanda effect.
  • The popular explanation, PRO-BERNOULLI 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 therefor 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. THIS EXPLANATION IS SERIOUSLY FLAWED.

This webpage is biased towards the ATTACK ANGLE view. See just about any K-6 science book for the PRO-BERNOULLI AIRFOIL-SHAPE argument.

(There also are two other explanations of lift: the circulation explanation, and curvature of streamlines explanation. These appear in advanced textbooks, and they form the basis of the mathematics used by aircraft designers. The "popular" or "airfoil-shape" explanation commonly appears in children's science books, magazine articles, and in pilot's textbooks. Explanations based upon circulation, on curvature of streamlines, or on Newton's Laws appear there only rarely.)

Explanations which are based on Newton's vs. Bernoulli's principles really aren't as incompatible as many people seem to believe. For the most part they are simply 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 therefor wrong. Which way to crack an egg? It's a war between the Big-endians and Little-endians from GULLIVER'S TRAVELS.

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 divided parcels of air must meet again at the trailing edge. They wrongly believe that wings don't deflect air downwards. These and several other mistakes commonly appear in elementary science texts and in popular articles on aircraft physics. These mistakes change the popular "airfoil-shape" explanation into a system of misconceptions. I explore these below.

Also, those who firmly believe in 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 "Bernoulli" 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.

 


WHAT MAKES AN AIRPLANE FLY: Page 2 - AIRFOIL DIAGRAMS  

Incorrect diagram in grade K-6 textbooks. In the lefthand diagram, the air approaches the wing horizontally and also leaves the wing horizontally. 
This violates Newton's laws, since by F=ma there cannot be a lifting force unless air is accelerated downwards. The wing must deflect the horizontally-moving air downwards, as shown in the righthand diagram.
Actual windtunnel photograph of air flowing around a wing. Pulsed smoke streams illustrate that the parcels of air which are divided by the leading edge DO NOT recombine at the trailing edge. Therefor the "wing shape" explanation of lifting force falls apart.
This image was made by late Aerodynamicist Alexander Lippish, The German Me-163 creator, at Collins Radio windtunnels, USA, in 1953. Alexander Lippish was an expert on smoke in windtunnels and his smoketrails were world-famous.
A flim clip of a windtunnel experiment depicts a single "plane" of smoky air as it approaches a thin airfoil and is sliced into upper and lower portions. Note that this airfoil is NOT "curved above and flat below", i.e., the air path is not longer on the upper surface. Instead the upper and lower surfaces are approximately equal in length. Note that the air flowing above the wing quickly outraces the air flowing below. The air flowing above and below the wing never rejoin again. The real reason for the rapid flow of air above the wing is never explained in "Bernoulli"-based textbooks.
This image was made by Aerodynamist Martin-Ingelman Sundberg at KTH windtunnels in 1992. Sundberg, who first saw syncronized smokepulses when visiting a windtunnel maker in USA 1962, made this smokepulse video to show how ICAO pilot education was wrong in explaining winglift with "airflow longer path over wing".
[GIF: Lift created at zero attack?]

The confusing aspects of "airfoil shape" shown here can totally obscure the true nature of aerodynamic lift.

Many authors point out that asymmetrical airfoils give positive lift even if the angle of attack is zero. They offer this in order to prove that "wing shape", and not "attack angle" should be the explanation of choice. But there is a problem here. To determine if an airfoil is tilted, we cannot rely on construction of the geometrical attack angle. Geometrical attack angle is very sensitive to tiny bumps on the wing's leading edge, since tiny bumps can change where we draw the main 'chord.' Yet tiny bumps on the leading edge can have little effect on deflection of air, while the tilting of the airfoil shown in the fourth section can have an enormous effect upon the deflection of air and upon lifting force. SMALL FEATURES ON THE LEADING EDGE CAN CAUSE US TO TILT THE ENTIRE WING, WHILE WE DENY THAT WE HAVE DONE SO.

To determine the effective attack angle, we cannot trust the simple geometrical rules. To determine whether an asymmetrical wing is REALLY set to zero angle of attack, we instead must inspect the trailing edge of the airfoil to see if it directs the air downwards more than the leading edge pulls the air upwards.
 Simulation of smoke pulses on SAAB aircraft wing Fluid simulation from Saab Aircraft shows phase lag between upper and lower air parcels after an airfoil has passed. Air travels much faster over the top of the airfoil, and then it never rejoins the air which has travelled below. Note that the airfoil has deflected the air downwards.

This image is a Navier-Stokes 2D airflow calculation around a SAAB 340 wing made by Aerodynamist Krister Karling,SAAB Aerospace

    
   
So not only is the common "wingshape / Bernoulli" explanation wrong, but it even covers up some of the most interesting phenomena in airfoil physics: the fact that the time delay between upper and lower airflows is proportional to the attack angle and the lifting force!

 

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