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No one can explain WHY planes fly...

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bmcj

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Very interesting article, if a bit long winded. I wonder if we'll ever come to a right proper conclusion...

https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/?utm_source=digg&utm_medium=email
I’m guessing that your question is a bit tongue-in-cheek. I hate seeing articles like this because the science of lift is very well known, especially if you stay clear of the chaotic (turbulent) nonlinear flows. That is why we can successfully design planes now and so successfully model their performance with computers. These types of articles are written to confound the very ‘lay’men that might read them.

There are several mechanisms of lift, such as Bernoulli, downwash, vorticity, etc. some of them are additive and some of them are just different ways of expressing the same thing (in much the same way that 4 can be expressed as 2+2, 1+3, and 2x2). Engineers know the differences and how to apply them, so articles such as this are intended to confuse or written by someone that is confused.
 
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Aerowerx

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To paraphrase the reason for climbing mountains, planes fly because they can!!

[Edit] And I see they make no mention of the downwash on the trailing edge of the wing!
 

BJC

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That is, unfortunately, a typical Scientific American article. If you want a thorough assessment of the state of the art, I suggest a read of Doug McLean’s Understanding Aerodynamics.

BJC
 

dragon2knight

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That is, unfortunately, a typical Scientific American article. If you want a thorough assessment of the state of the art, I suggest a read of Doug McLean’s Understanding Aerodynamics.

BJC
Actually, that's a major part of the article...and even he doesn't quite explain it fully.
 

Hot Wings

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Actually, that's a major part of the article...and even he doesn't quite explain it fully.
"McLean’s complex explanation of lift starts with the basic assumption of all ordinary aerodynamics: the air around a wing acts as “a continuous material that deforms to follow the contours of the airfoil.”"

IMHO this is where the problem starts - with an assumption that is incorrect. Scientists and engineers (especially engineers) like theories and equations that they can use to predict outcomes. If those equations are good enough for the project at hand we use them - but they may not reflect the true nature of nature.

If you quit thinking about air as "flowing" and think at the molecular level where the air is made up of little particles of O2, N2 and some other bits, it can unify all of the various previous theories.
 

dragon2knight

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"McLean’s complex explanation of lift starts with the basic assumption of all ordinary aerodynamics: the air around a wing acts as “a continuous material that deforms to follow the contours of the airfoil.”"

IMHO this is where the problem starts - with an assumption that is incorrect. Scientists and engineers (especially engineers) like theories and equations that they can use to predict outcomes. If those equations are good enough for the project at hand we use them - but they may not reflect the true nature of nature.

If you quit thinking about air as "flowing" and think at the molecular level where the air is made up of little particles of O2, N2 and some other bits, it can unify all of the various previous theories.
I agree there. It's not like anyone is wrong, they aren't, but too many folks just want a quick and simple explanation and that might not happen simply because there are so many different parts to it.
 

henryk

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If you quit thinking about air as "flowing" and think at the molecular level where the air is made up of little particles of O2, N2 and some other bits, it can unify all of the various previous theories.
=thanks kinetic energy of air (thermic energy)

velocitys of air particles are circa 500 m/s,one qubic meter of
air contains circa 150 kJ energy !


>3:30...
 
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Aerowerx

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If you quit thinking about air as "flowing" and think at the molecular level where the air is made up of little particles of O2, N2 and some other bits, it can unify all of the various previous theories.
Aerodynamics is based on statistics.

The particles behave in a certain way on the average.

But the total system is chaotic. (Has anyone else here tried to read and understand Chaos Theory?) The equations work fine when operating in the linear region, but get too close to a singularity and it is impossible to predict what will happen. That is why computer programs work fine so long as you stay away from the critical angle of attack.

[Edit] The correct term is "node". The term "singularity" is used in Quantum Physics.
 
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bmcj

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But everything is chaotic at a molecular level. Set a block of metal on a table and the contact forces are actually repulsive forces between moving atomic subcomponents. To say that aerodynamics is different than standard physics by calling it chaotic or working at molecular level is not entirely correct.
 

Aerowerx

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But everything is chaotic at a molecular level. Set a block of metal on a table and the contact forces are actually repulsive forces between moving atomic subcomponents. To say that aerodynamics is different than standard physics by calling it chaotic or working at molecular level is not entirely correct.
Not sure I understand what you are getting at, BMCJ.

What do you mean by "standard physics"? Newtonian? Newtonian physics works find so long as everything stays in the linear region of the chaotic system, and the "Goldilocks region"---not too big, not too small.

A block of metal is a solid made of a crystalline structure, not individual independently moving molecules like a gas or fluid. At the scale you are talking about (block of metal on a table) the contact forces are mainly gravitational.

Drifting off topic, by the way.
 
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Norman

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Newtonian physics works find so long as everything stays in the linear region of the chaotic system, and the "Goldilocks region"---not too big, not too small.

A block of metal is a solid made of a crystalline structure, not individual independently moving molecules like a gas or fluid. At the scale you are talking about (block of metal on a table) the contact forces are mainly gravitational.
Science and technology advance in baby steps often with lots of faceplants.

It is Newtonian physics F=MA just like everything else that isn't hot enough to fuse atomic nuclei or cold enough that electrons flow without resistance. A Mass of air is Accelerated more on one side of the wing and exerts a Force. Newton himself didn't understand the problem and applied his math incorrectly when he tried to calculate the force on a sail. About a hundred years later, Daniel Bernoulli, did understand the problem and applied Newton's equations correctly to show the relationship of velocity and pressure so now we can calculate the pressure on a surface if we know the velocity of the fluid (no need to count molecules) and vice-versa However Bernoulli dose not explain why the flow speeds up in one place and slows down in another (circulation), only what happens when something causes the speed or pressure to change. Any object in a fluid flow will have a stagnation point where the pressure of the fluid reaches a maximum (q (dynamic pressure)) and it accelerates away from that point and if the flow field that that object creates is asymmetric a force perpendicular to the flow will result. Another 100+ years and a guy named Ludwig Prandtel figured out that friction causes a boundary layer to form on the surface and that that thin layer is where all the magic happens. Then in the 1950s someone put a little windmill in superfluid helium and proved that without viscosity there is no lift. Here's a video where Alexander Lippisch demonstrates several methods to creat circulation:
 

Aerowerx

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Can Bernouli or Prandtel predict exactly when the stall occurs. That is, when will the flow become turbulent?
 

mcrae0104

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Can Bernouli or Prandtel predict exactly when the stall occurs. That is, when will the flow become turbulent?
Transition to turbulent flow is not the same thing as a stall. Stalled flow is detached (really it's not flowing along the surface at all); attached flow can be either laminar or turbulent.
 

Aerowerx

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Transition to turbulent flow is not the same thing as a stall. Stalled flow is detached (really it's not flowing along the surface at all); attached flow can be either laminar or turbulent.
In either case, the flow has reached a "node" in the system, and switched from laminar to turbulent. That is chaotic behavior.
 

Norman

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Can Bernoulli or Prandtel predict exactly when the stall occurs.
No. Stall is a viscosity effect. Bernoulli's theorem doesn't have anything to say about viscosity, just speed and pressure outside the boundary layer. Not sure about Prandtl but I don't think he quite got to predicting when and where transition and separation would occur. Both of those are really hard problems. The NavierStokes equations can do it but that's super computer territory (or a couple of days to solve for a simple case on a high end desktop). Panel codes use a shortcut to estimate the thickness of the BL and where it will transition to turbulent but separation is still not well modeled.

That is, when will the flow become turbulent?
You seem to be confounding turbulence with stall. Stall is separated flow, a turbulent boundary layer resists separation from the surface better than a laminar BL but it also creates a lot more friction drag. When a boundary layer leaves the surface it separates the potential flow of the atmosphere from a small slug of air between itself and the wing. This isolated slug of air is turbulent but on average almost stationary relative to the wing so creates little friction but it's also low pressure so it creates pressure drag (low pressure on aft facing surfaces sucks in the wrong direction).

Here's a video of Prandtl's original experiment showing circulation. The little vortex that gets shed from the trailing edge is what actually starts the circulation by pulling on the upper surface flow with the low pressure at its core thus accelerating the upper surface flow. After that it's a self sustaining feedback cycle. A new starting vortex will be shed every time the plane pitches up.

 
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