Thank you for interesting article.

Interesting, thought provoking. It simply says Bernoulli principle is not enough to explain the whole story. Everything he said is known in the aerodynamic (or fluid dynamic) community. What is your opinion?

Hard to get the point. For the finite wing, there are trailing sheet vortices generated from air escaping from the bottom to top at the trailing edge due to the pressure difference.

The article is just click-bait; if somebody doesn't understand how the lift is created he/she haven't developed the most basic concepts of fluid dynamics. I'm trying to explain to the layman how the aerodynamic forces are generated around a body; excerpt from a course I'm working on:

"According to the kinetic molecular theory of gases ... static air exerts a pressure of about 101kPa on any object at sea level, created by the collisions between the object’s surfaces and air molecules ... this pressure changes when the air is not static but it moves around the object ... the random movement of the molecules changes, their random speed vectors gaining a component induced by the macroscopic air movement. ... This change of the random movement of the molecules is causing variations of pressure in the air volume, and acting over the object surfaces are creating forces, termed aerodynamic forces."

The article focuses on inability of common layman "lift theories" to account for the decrease of pressure and speed increase on the upper wing surface. I'm trying to correct this using some common sense.

Let's imagine a flat barn door (our "wing"), flying at an angle, through static air. Let's imagine the air molecules are un-moving grains of sand as a first approximation. In such a case the wing interacts only with the "molecules" that is able to hit, in the direct path of the wing. On the top region, the "wing" is not interacting with the "molecules", a void is created between the "wing" and the "molecules". This model is called "Newtonian sine-squared law of lift" and developed by - Newton in 1687. Interesting fact, this model is not at all accurate for normal flight, but it is accurate at hyper-sonic speeds and/or very low temperatures and pressures, such as space vehicles re-entry, as the molecular speeds are much lower than the aircraft speed.

Well, let's replace now the grains of sand with air molecules that are moving, according to the kinetic molecular theory of gases, at typical air at room conditions, the molecules are moving at about 500 m/s. A wing flying e.g. at 100 knots, about 50 m/s, is much slower. The void between the wing and the free stream is quickly filled by air molecules that are "pushed" in by elastic collisions with other air molecules. Since the air molecules are moving in the void in a global downward general direction, this has a macroscopic effect of increasing the airspeed. Since the total kinetic energy needs to be conserved, less molecular speed is available for collisions and that accounts for decreasing the pressure over the top of the wing. According to Bernoulli this is correct, as Bernoulli law is an energy conservation law, as applied to fluids. Since accelerating air downward is creating an upward lift force , the Newton's second law is also correct. So Bernoulli's and Newton's laws are actually the interpretation of the same physical phenomena.

Hopefully this short introduction dispels some misconceptions of how "no one can explain lift"

‘Dusan Stan, thank you. Why did such a reputable journal published that?

My dissertation was on computational fluid dynamic.

This article tries to create a mystery where there isn't one. At the very simplest level you do not even need Bernoulli (the curvature of the wing is a second order effect); as the article points out, a flat sheet will generate lift if there is an angle of attack to the oncoming air. You can explain the higher pressure under wing and the lower pressure on top by gas laws alone. Imagine a closed system with a rectangular tube moving through still air at a velocity V. I now place another flat piece of wood of the same width as the rectangular pipe with its front edge horizontal and on the mid-line of the rectangular hole; meanwhile its trailing edge is horizontal across the back of the rectangular orifice, but now at only one quarter the way up from the bottom of the box (this is our wing). The air entering the front is equally divided and the pressure is the same above and below the wing. By the time the air gets to the back of the rectangular tube, the volume of air that went over the top of the wing occupies a cross-section 3/2 times that it started with, so each volume element of air has the same mass, but must have a lower pressure. Underneath the wing, the cross-section at the rear is only 1/2 that of the front end, so each volume element of air has a pressure twice that it started with. As far as the wing is concerned, if it is not anchored to the sides of the rectangular tube, it will try to rise up to balance the forces acting from below and above. The effect remains even if we move the top and bottom bounds of the box a long way away.

The high pressure under the wing is certainly real and can be measured at the ground, even when the aircraft is flying at 10,000m. 45 years ago I built an unequal path interferometer in my laboratory at Teddington to act as a very accurate barometer, which did just that. I could measure the change in pressure as an aircraft flew over. If I knew the height (by triangulation), I could calculate the weight of the aircraft (assuming it was in level flight)!

Ric Parker, than you so much.

I do believe that, if we go deeper and deeper in whichever knowledge, we always find something that cannot be explained, i.e. we cannot explain the gravity force, but we use it. So at present we do use the Navier Stoke equation to calculate the lift and we can design aircraft always improving their efficiency. Of course we need to create some phisical models that must be described with mathematical equations, and this makes it possible to quantitize the phoenomena and design artifacts that run in our part of reality. The wise engineer knows that these are only models of the reality and not the reality itself, thus leaving always space to improve our knowledge. We must remember Socrates, who said "I have been appointed as the wisest man because I know that I know nothing"!

Claudio Peretti, thank you very much for your educated opinion. Which institute are

you from?

I am from Italy and attended he University of Naples and the Air Force Academy. After this I have been the technical director of some aircraft manufacturing companies and I can assure you that the theory of the arodynamic lift and aerodynamic drag that we use in our calculation to design new aircraft is very reliable and permits a tolerance of up to +/- 2 to 3%, that is good, since the safety factor that we apply in the stress analisys to design the structures is about 20%, depending on the structure we design. In addition, also for the maneouvers of the aircraft, i.e. max speed, stall, g-factor, we do use a large safety factor and the theory at present available is in line with the requirement of good, safe and economic aircraft design. For the theory, I can suggest you 2 books that have been written several years ago by a German aircraft engineer: Sighard Hoerner: Fluid Dynamic Drag and Fluid Dynamic Lift. No other discovery on this matter has been made in the world.. OK, the theory has been programmed in the computers, but this is the best theory I have ever found for the design of new aircraft.

In a simple way, imagine two molecules of air at leading edge of an airfoil. By Kutta theory both molecules should leave at training edge at same time. So it is obvious the upper one should move faster compared to lower one because it should traverse more distance. Hence no doubt that upper surface experience lower pressure than lower surface say by Bernoulli's theory. This will create an upward force called lift force. Of course this discussion is true for ideal fluid. Whereas for real fluid viscosity will change a bit pressure distribution around the airfoil. However upward force will be sustained till the flow get separated say, at high angle of attack. If AOA increases beyond design value separation of flow is inevitable and wing stall will happen.

Mohsen Jahanmiri , not sure how to interpret your answer. It seems you are suggesting the molecules, one travelling on the upper surface, second travelling on the lower surface, should meet at the trailing edge, and that is somehow due to Kutta theory.

There is no physical law that is linking the molecules together to make them reach the trailing edge at the same time. The "Longer Path", or the "Equal Transit Time" "theories" are debunked, see here: https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/wrong1.html

The flow visualisation in this following video shows clearly that on the upper surface air arrives much faster than the bottom surface at the trailing edge. https://youtu.be/6UlsArvbTeo?t=16

The Kutta condition states only that the flow cannot turn around the trailing edge, so the flow from top and bottom should follow the same direction, and builds on the concept of circulation, on which the Kutta–Joukowski theory of lift calculation is based.

Dusan Stan,

Your interpretation about this question: how plane lifts off and fly?

Mohsen Jahanmiri , you probably meant not a molecule, but a patch/blob of air, since Bernoulli's theory is on the macro level.

They claim in Scientific American that though the motion of the air on top of the wing is faster, but there is no strict mathematical proof that it leads to lower pressure on top. That it’s an empirical result.

Also, they claim in this publication that the longer path theory is not supported by the fact that a flat wing with equal paths on top and bottom can lift up an airplane.

Natalia S Duxbury

It is quite clear that unless you apply an angle of attack to flat plate in a free stream you won't get lift. Just in similar way for a symmetrical airfoil.

Regarding your question on Bernoulli's theory. I should emphasize that this theory is true along a streamline. So you should consider a fluid particle trace along a streamline between leading and trailing edge of airfoil.

Thank you.

Unscientific American: Scientific American Promotes Misconceptions in Bungled Explanation of Bernoulli's Principle and Newton's Third Law

A lengthy rant about the article:

https://www.linkedin.com/pulse/scientific-american-promotes-misconceptions-bungled-third-malfitano/?trackingId=VOt1urT2CHzSxg3SfgysVg%3D%3D

Thank you very much.