Physics - The enigma of aerodynamic lift in Scientific American of February 2020

This document contains comments about the article The enigma of aerodynamic lift by Ed Regis In Scientific American of February 2020.
No one can completely explain why planes stay in the air.



Two competing Theories

page 38

The flawed Classics

On a commonsense, every basis, two theories have been advanced to explain what keeps an airplane aloft.
The wording "On a commonsense, every basis" should be removed.
One is Bernouille's theorem, which associates lift with the area of higher speed and lower pressure atop the wing.
To understand this sentence you need a clear definition of what means lift and pressure.
The other is Newton's principle of action and reaction which explains lift as an upward push on the wing from the moving air below.
To understand this sentence you need a clear definition of what means action, reaction, lift and push.
Still neither theory by itself provides a complete explanation of lift, nor both of them together, because each leaves something out.
For example ?
A complete explanation must account for all the forces and factors acting on the wing, with no issue, major or minor, left dangling.
Maybe we want to much

Bernoulli's Theorem

As applied to an airplane wing - technically called an airfoil - Bernouilli's theorem attempts to explain lift as a consequence of the wing's curved upper surface.
The word 'attempt' implies doubt and failure?

Newton's Third Law

Air has mass.
Air has mass because it consists of particles, molecules, which each have mass.
Therefore, Newton's third law would say that the wing's downward push results in an equal and opposite push back upward.
Newton's third law does not, generally speaking, explain the cause of any force.
Also generally speaking more forces can be involved, which should all be discussed.

page 40

Nevertheless, there are several reasons that Bernouilli's theorem does not by itself constitute a complete explanation of lift.
This is a honest comment.

page 41

Historical Understanding

Toward a Complete Theory of Lift

Contemporary Scientific Approaches to aircraft design are the province of Computational Fluid Dynamics (CFD) simulations and the so-called Navier-Stokes equations which take full account of the actual viscosity of real air.

page 42

Still, they (the solutions) do not by themselves give a physical, qualitive explanation of lift.
The equations as such, do not give an physical explanation why an aircraft can fly.. This is a very important physical consideration.
McLean's complex explanation of lift start with the basic assumption of all aerodynamics: the air around the wing acts as a "a continuous material that deforms to follow the contours of the airfoil"
Such a sentence explains nothing.
The wing pushes the air down, resulting in a downward turn of the airflow.
Same comment.
The air above the wing is sped up in accordance with the Bernouilli's principle.
Nice, but this means nothing.
And what causes this mutual, reciprocal, dynamic interaction? McLean's answer: Newton's second law of motion.
This law can never be the physical explanation of how physical processes operate and interact.
Physical laws are mathematical descriptions, something like an abstract model.

page 43

Newton's second law states that the acceleration of a body, or parcel of fluid, is proportional to the force exerted on it.
This Law allows you first to calculate the mass of the all the planets around the sun, assuming that the sum of all the forces involved is zero.
Secondly to calculate the positions of the planets in the future.
Aren't we getting something for nothing?
McLean says no: If the wing were at rest, no part of this cluster of mutual reinforcing activity would exist.
If the speed of the aircraft is zero, it will not fly.
But the fact that the wing is moving through the air, with each parcel affecting all of the others, brings these co-dependent elements into existence and sustains throughout the flight.
Very difficult to comprehend and understand.

Turning on the Reciprocity of Lift

Whether Mc'Lean's section 7.3.3 and his follow-up article are successful in providing a complete and correct account of lift is open to interpretation and debate.
It should be easy to understand.
There aare reasons that it is difficult to produce a clear, simple and satisfactory account of aerodynamic lift.
What you should try to explain is: what is the reason that an airplane can move upwards.
For one thing, fluid flows are more complex and harder to understand than the motions of solid objects, especially fluid flows that separate at the wings leading edge and are subject to different physical forces along top and bottom.
Fluid flows specific are very difficult to decribe mathematical. At the same time IMO fluid flows are not the cause that an airplane can fly.
Drela himself concedes that his explanation is unsatisfactory in some ways.
If that is the opinion of Mark Drela, then it is no explanation.
"One apparent problem is that there is no explanation that will be universally accepted", he says.
Apperently Drela does not accept the explanation of Babinsky.
So where does this leave us? In effect, right where we started: with John D. Anderson, who stated: "There is no simple one-liner answer to this"
That is a too simple answer.

Reflection 1 - Overall

The overall article is very good, specific because it tries to answer a seemingly easy problem, but fails.
Part of the problem is that there are two options, but neither is accepted. The problem with that is, even if one answer is accepted, is that answer correct. The whole issue boils down to a more phylosophical discussion: what is involved to explain something?

IMO in order to explain something you should explain each concept used, as simple as possible. You can start from a set of mutual accepted concepts, but for the rest it is more like a bottom up approach.
Existing laws should not be used, because each law,in some way, is a description of a physical process and the whole question is, insofar that law applies.
In that sense Bernouilli's law should not be used. See also:

Reflection 2 - What is required for an airplane to fly.

To understand why an something can fly i.e. move upwards in the air, you have to explain all the concepts used.

  1. The most basic thing required is an engine with a propeller. The propeller is required to move your aircraft along the runway. Without enough speed in horizontal direction your aircraft is never able to fly.
    It is interesting to see how a swan starts to fly. First he more or less starts to run.
  2. The next step is: in order to move an object into the air you need an explosion, which gives a force in vertical dicrection to move the object in the air.
      x    ^    x
      x    |    x
      x    |    x
      x    |    x    
      x    |    x
      x    |    x
      x    |    x
      x    v    x  
      Picture 1
    • The picture at the left shows a round cylinder.
      The top side is closed and the bottom side is open.
      At the inside there is an explosion.
      General speaking all the forces to the cylindrical shaft will cancel each other.
      The resulting force will be in the vertical direction, which will lift the object.
    • A better example is to consider a balloon filled with air. That means there is more air inside the balloon as outside. The density inside is higher as outside.
      When there is an opening in the balloon, air will escape through that opening (downwards) and the balloon, in total, will move in opposite direction (upwards).
  3. The third, and most important part is the shape of the wings.
    The following picures shows 4 possibilities:
      Picture 2a
      ^  |  |
      |  |  v
      |  |  ^
      V  |  |
      Picture 2b
      Picture 2c
      Picture 2d
    1. Picture 2A shows a wing in horizontal direction. The wing moves towards the left. This is the same in all pictures. The direction of the wing does not influence the surrounding air, nor the direction of the wing is influenced.
      The letter E (Equilibrium) indicates that there are no pressure diffences as a result of the movement of the wing.
    2. Picture 2b shows a wing in vertical direction. The wing moves towards the left. The result is that the amount of air (preasure) increases in front of the wing and the amount of air decreases behind the wing. You get as a matter of speaking a void immediate behind the wing. The letter H indicates High pressure. The letter L indicates Low pressure.
      As a result of this pressure difference there should be an extra air flow from left to right. (In fact this is a compansation for the air build up in front of the vertical wing.
    3. Picture 2c shows a wing partly vertical and partly horizontal.
      The result of this of this shape is that something like a balloon (pile) of air builds up in front of the wing and a void at the back of the wing. Those two areas are shown in red and green. It is important to understand that the whole wing moves towards the left.
      That means the only thing that the aircraft can do to compensate for the air balloon/pile in front is to move upward and the only thing that the aircraft can do to compensate for the void/emptyness at the back is also to move upward.
    4. Picture 2d also shows a wing partly vertical and partly horizontal, but axis-symmetric compared picture 2C
      The result of this of this shape is an airballoon in front of the wing and and void at the back of the wing. Those two areas are shown in red and green. It is important to understand that the whole wing moves towards the right.
      That means the only thing that the aircraft can do to compensate for the air buil up in front is to move downwards and the only thing that the aircraft can do to compensate for the void at the back is also to move downwards.
    The following picures shows 3 possibilities:
     ^ H->\->L   
     |  H->\->L ^
      Picture 3a
     |  H->/->L
     V H->/->L |  
      H->/->L  V
      Picture 3b
     ^ H->\->L   
     |  H->\->L
      Picture 3c
    1. Picture 3a is a more improved version of picture 2c and shows a more practical realisation of a wing.
      It identifies that there are two places which demonstrate an upward force towards the wing.
    2. Picture 3b is a more improved version of picture 2d.
      It identifies that there are two places which demonstrate a downward force towards the wing.
    3. Picture 3c shows what happens (in the situation of Picture 3a) when the wing at the back is pulled down. In that case the upward force at the back of the wing is removed and the aircraft will descend.
    This more or less explains why a moving aircraft can fly.

Reflection 3 - Explanation

The easiest way to explain why an airplane can fly (by itself) and a glider not is because an airplane has a propellor and a glider not. As a result an airplane can become airborn by itself and a glider needs an extra airplane for take off, to do the same.
An airplane continuous, collects air under its wings (in front of the wing) at the expense of the air above its wings (at the back of the wing). To compensate for the increased density the airplane will rise. To compensity for the decreased density in the back the airplane will also rise. Both movements are caused by the shape of the wings.

This explanation is as much as possible not based on any law.

If you want to give a comment you can use the following form Comment form
Created: 24 February 2020

Back to calling page Comments About Scientific American
Back to my home page Contents of This Document