superstring
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### The Ultimative "Why Do Aircraft Fly?" Question...

Well this question is really a bit long, so take your time to read it...

Why do aircraft fly? This question appears to be so simple, however more than 100 years after the Wright brothers still much controversy is connected to this issue. So for short: What is the physical reason why an wing generates lift?

Now the long part of the topic:

- First of all to make the situation clearer I want to focus on classical airfoils. So I don't want to discuss about blended wing bodies or similiar aircraft (that is because I want to focus on the wings/airfoils alone, without asking how much lift is generated by the fuselage). So if I ask: Why does an aircraft fly, I want to focus on aircraft whose fuselage is not contibuting to the generation of lift.

- What do I mean when I speak of a physical description of lift? If someone comes up with a closed solution for the navier-stokes equations is this then a "physical explanation".
Well, yes from a physicists point of view this would really be an explanation, but it is not satisfactory. It would be the same as if someone said, well my aircraft flies because my 3D-CFD simulation tool has just calculated that it really does. Or it is the same with exp(i x PI) = -1. Many higth school students know this equation, but ask them WHY; you will not get an answer...

- Don't use one of the following explanations for lift:

x Equal transit time theory + Bernoulli --> pressure above the wing is smaller than below --> we have lift.
x Slipping stone theory, or Newtonian theory --> air is being deflected downwards by the lower side of the wing --> Newton's 3rd. law--> lift
x Don't abuse circulation. It is a fine thing to use the Kutta-Joukowski Theorem, but it has one major flaw. If I know the circulation I can calculate lift, but which is the circulation Nature generates about my airfoil?
x Don't abuse Bernoulli + the venturi nozzle thing. The air above the wing does not form a wall, therefore you will never get a venturi nozzle effect with a wing.

- What is the thing with the Coanda effect? Does it play a role or not?
- Do we need viscosity to explain lift? For all experts out there, especially those working with circulation: the regieme up to mach 0.3 is often treated to be inviscous + we have no compressibility. So aircraft engineers come up with the Kutta condition and everything is fine. But why does Nature enforce the Kutta conditon?

- Take care, there are many "the chicken or the egg" dilemmas out there; so consider the causal ordering. For example: Is the pressure difference the result or the cause of the increased speed above the wing, and so on...
Very often different explanations are connected to each other. Pressure difference will lead to moving air, but to move air there must be a a force acting on it, so there must be a pressure difference (of couse I mean to accelerate air from zero velocity)
_____________________________________________________________________________________________
Finally I want to express my personal thoughts:
I think I am right when I say that the mystery connected to lift lies in the upper side of the wing. But what is the mechanism by which nature bends the streamlines above the wing to generate the downwash?
I am a physicist but up to now no explanation has ever answered all of my questions. I find that the mathematical methods like circulation or panel methods are elegant, but it is no explanation rather it is a description.

So everybody who has made it through the whole article --> thanks and I am really looking forward to getting high quality explanations.

Starlionblue
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### RE: The Ultimative "Why Do Aircraft Fly?" Question...

 Quoting Superstring (Thread starter):What is the physical reason why an wing generates lift?

"It's complicated."
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

superstring
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### RE: The Ultimative "Why Do Aircraft Fly?" Question...

Oh yes, indeed...

tdscanuck
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### RE: The Ultimative "Why Do Aircraft Fly?" Question...

 Quoting Superstring (Thread starter):What is the physical reason why an wing generates lift?

The 35,000' view is that the wing imparts downward momentum to the air. Downward momentum change = upward lift.

 Quoting Superstring (Thread starter):Or it is the same with exp(i x PI) = -1. Many higth school students know this equation, but ask them WHY; you will not get an answer...

That's true, but that's just because most high-school students don't know the power-expansion of e^x. It's pretty straightforward to demonstrate once you do know the math...not the same case with lift.

 Quoting Superstring (Thread starter): What is the thing with the Coanda effect? Does it play a role or not?

Yes. Coanda effect is just another manifestation of the same physical principles that govern airflow over an airfoil.

 Quoting Superstring (Thread starter):Do we need viscosity to explain lift?

No.

 Quoting Superstring (Thread starter):But why does Nature enforce the Kutta conditon?

Because air has mass. Even if you ignore viscosity, inertia prevents air molecules from making finitely tight turns, as would be required to go around a sharp trailing edge.

 Quoting Superstring (Thread starter):Is the pressure difference the result or the cause of the increased speed above the wing,

Neither. Nature is "solving" a Navier-Stokes boundary condition problem. The solution is a steady-state flow-field that defines the pressure and velocity at each point...since both pressure and velocity are proscribed, it doesn't make sense to say that one caused they other. The values are what they are because if they were anything else it wouldn't be a steady-state solution.

 Quoting Superstring (Thread starter):But what is the mechanism by which nature bends the streamlines above the wing to generate the downwash?

Pressure transients. When you first start the airfoil moving you get a dynamic pressure distribution. Imagine you have a stationary airfoil and step-change it forward by a tiny amount. For a very brief instant you will get a vacant region along the top skin and an a pressurized region along the lower skin. Naturally, this will very quickly even out by some molecules moving away from the lower skin and some moving towards the upper skin and, if you stop the airfoil right there, that would be it. If you keep moving though, you'll end up with a steady-state flowfield that curves the streamlines generally downwards. Any other flowfield would result in unbalanced pressures somewhere and the flowfield would dynamically adjust towards the steady state flowfield.

Tom.

superstring
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### RE: The Ultimative "Why Do Aircraft Fly?" Question

 Quoting Tdscanuck (Reply 3):Pressure transients. When you first start the airfoil moving...

Well this is of course an analogy, but the best I have seen!

 Quoting Tdscanuck (Reply 3):Or it is the same with exp(i x PI) = -1. Many higth school students know this equation, but ask them WHY; you will not get an answer... That's true, but that's just because most high-school students don't know the power-expansion of e^x. It's pretty straightforward to demonstrate once you do know the math

Yes, but don't tell a real mathematican. He will tell you things like radius of convergence (well which happens to be infinity for exp(x) ) or the convergence of the taylor expansion... However, I am very glad not to be a mathematican...

 Quoting Tdscanuck (Reply 3):Quoting Superstring (Thread starter): Do we need viscosity to explain lift? No.

Well I have already put this question into a thread, but it is never a mistake to ask again. The problem I as a physicist have with "No..." is that if I really forget viscosity how do I consider the effects happening in the boundary layer.
And there is another effect: as I once have asked with my superfluid helium example If We Lived In A World Without Viscosity... (by Superstring Jul 26 2007 in Tech Ops)
I really wonder if an aerofoil would "fly" in superfluid helium. For simplicity lets take an aerofoil which would generate lift at zero geometrical angle of attack. If we put this wing into superfluid helium your analogy you described in

 Quoting Tdscanuck (Reply 3):Pressure transients. When you first...

is not true any more. If there is no viscosity the area being directly behind the wing will have zero velocity, and the area above the projected trailing edge will have a specific speed; and between there will be a sharp velocity "jump".

KELPkid
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### RE: The Ultimative "Why Do Aircraft Fly?" Question...

Well, everyone knows the following about aircraft:

1) No matter how small, they are expensive to operate.
2) They will not fly without some expenditure of fuel, even if they are a glider (and fuel is, as we all know, not getting any cheaper anytime soon).
3) Pilots must pay large sums of money to instructors, flight schools, etc. to retain the privelege of flying unless they are employed as such or are one of the lucky few to be employed as a professional aviator...in which case their employer shells out the money (usually).
4) Mechanics like to eat, so they demand their cut of the money, too...
5) Parts require extra inspection before being certified by the FAA for aircraft usage, so more money goes down that hole...

Which leads me to the inescapable conclusion that aircraft fly because someone threw enough money at the problem, and that, furthermore, money keeps aircraft airborne
Celebrating the birth of KELPkidJR on August 5, 2009 :-)

lehpron
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### RE: The Ultimative "Why Do Aircraft Fly?" Question...

I just tell people that air changes its properties if forced to change direction, like being forced around something. It doesn't matter if it is a wing, a rock, or a building; the properties of the air (density, temperature, pressure, viscosity, etc) will change. Why does this happen? Well, why does it concern anyone? There hasn't been a situation where something different occured. It is possible for air to change phase while going around something, but that requires ultra high speeds.
The meaning of life is curiosity; we were put on this planet to explore opportunities.

vikkyvik
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### RE: The Ultimative "Why Do Aircraft Fly?" Question

 Quoting KELPkid (Reply 5): Which leads me to the inescapable conclusion that aircraft fly because someone threw enough money at the problem, and that, furthermore, money keeps aircraft airborne

And they're airborne for a certain amount of time, generally.

So, following the "Women = Evil" theory:

Aircraft = Money * Time;

Money = SQRT (Evil) ==> Money^2 = Evil;

Time = Money;

Money * Time = Money^2;

Aircraft = Evil.

And for the absolute kicker:

Aircraft = Women.

Which I guess explains why they're generally called "she" and named after women.

Wait, what were we trying to prove again?
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".

ThirtyEcho
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### RE: The Ultimative "Why Do Aircraft Fly?" Question...

I was about to go to bed, so I poured out the remainder of my cup of coffee into the sink. I noticed that the coffee flowed down the side of the cup until I tipped it at such an angle that the flow of coffee separated from the side of the cup.

I am with you that the Coanda Effect explains nothing.

Where did you get your PhD in aerodynamics, Waziristan?

chksix
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### RE: The Ultimative "Why Do Aircraft Fly?" Question...

"No Viscosity" would be like trying to fly in a vacuum. The aerosurfaces need something to bite to be able to produce a lifting force.
The conveyor belt plane will fly

Starglider
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### RE: The Ultimative "Why Do Aircraft Fly?" Question...

 Quoting Superstring (Thread starter):Why do aircraft fly?

Because the runway is only 10,000 ft. long and the destination is a thousand miles away.

Starglider

tdscanuck
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### RE: The Ultimative "Why Do Aircraft Fly?" Question...

 Quoting Superstring (Reply 4):The problem I as a physicist have with "No..." is that if I really forget viscosity how do I consider the effects happening in the boundary layer.

The original question was "Do you need viscocity to explain lift?" and the correct answer is "No." That doesn't mean you'll get the same flowfield if you ignore viscocity...you won't. An invisid fluid won't have a boundary layer. For practical airfoils the Reynolds number is so high that viscocity makes next to no difference, so the answers won't be much different but it will be there.

 Quoting ThirtyEcho (Reply 8): I noticed that the coffee flowed down the side of the cup until I tipped it at such an angle that the flow of coffee separated from the side of the cup. I am with you that the Coanda Effect explains nothing.

The phenomenon with low-speed liquids following a curved surface is largely a surface tension thing...it looks like the Coanda Effect but the physics aren't the same at all.

Tom.

superstring
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### RE: The Ultimative "Why Do Aircraft Fly?" Question...

 Quoting Tdscanuck (Reply 11):The original question was "Do you need viscocity to explain lift?" and the correct answer is "No."

The basic question is whether the Kutta Condition is nothing more than a viscous effect. Isn't the fact that nature "encourages" a flow field to leave smoothly at the trailing edge nothing more than a result from friction effects.

 Quoting Tdscanuck (Reply 3):Because air has mass. Even if you ignore viscosity, inertia prevents air molecules from making finitely tight turns, as would be required to go around a sharp trailing edge.

Well I think that is only the half story. This explains why there won't be tight turns at the trailing edge, but this does not explain why the flowfield stays attached to the airfoil the whole way down to the trailing edge. This can only be explained by the fact that the viscous boundary layer wants to stay attached to the airfoil.

tdscanuck
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### RE: The Ultimative "Why Do Aircraft Fly?" Question...

 Quoting Superstring (Reply 12):The basic question is whether the Kutta Condition is nothing more than a viscous effect. Isn't the fact that nature "encourages" a flow field to leave smoothly at the trailing edge nothing more than a result from friction effects.

Nope. An inviscid CFD will show you that very quickly.

 Quoting Superstring (Reply 12):This explains why there won't be tight turns at the trailing edge, but this does not explain why the flowfield stays attached to the airfoil the whole way down to the trailing edge. This can only be explained by the fact that the viscous boundary layer wants to stay attached to the airfoil.

Not true. You can do all the simulations you like with an inviscid flow (which has no boundary layer, almost by definition) and the flow will still stick to the upper surface.

Tom.

ThirtyEcho
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### RE: The Ultimative "Why Do Aircraft Fly?" Question...

"The phenomenon with low-speed liquids following a curved surface is largely a surface tension thing...it looks like the Coanda Effect but the physics aren't the same at all."

Gee whiz, I guess that all of those wind tunnel smoke tests that show air (a fluid) conforming to the shape of the surfaces of wings were conducted at the same speed at which coffee pours from a cup?

I saw a smoke test of the Bell X-1 wing and, sure enough, the air conformed to the shape of the wing. You could even see the shock wave move over the wing.

I pour out old coffee fast but not supersonically.

Is it your point that the Coanda effect doesn't apply outside of the kitchen sink?

superstring
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### RE: The Ultimative "Why Do Aircraft Fly?" Question

 Quoting Tdscanuck (Reply 13):Quoting Superstring (Reply 12): The basic question is whether the Kutta Condition is nothing more than a viscous effect. Isn't the fact that nature "encourages" a flow field to leave smoothly at the trailing edge nothing more than a result from friction effects. Nope. An inviscid CFD will show you that very quickly.

To be honest a CDF-simulation is no proof. I will give you an example:

Lets assume a designer of rollercoasters who is just interested in the speed/acceleration profile of his new design. Well mechanics tells us that he will be able to calculate those speeds/accelelerations just by analyzing the curve which describes the rollercoaster track in 3D space.
But the point is: Our designer of the coaster will be able to calculate the speed/acceleration profile of the coaster simply by using the curve in space ALONE and without haveing to know the constraint forces EXPLICITELY (for experts Lagrange Formalism II vs. Lagrange Formalism I)
However this does not mean that there are no forces which hold the car on track. Of course it is clear that forces are necessary so that the cars are not "flying away" and the engineer being responsible for the structure of the track will definitely know how big those forces can become.
So what's the point? Well simply the principle of constraints. By constraining the cars onto the 3D curve the designer will be able to find his figures. So he uses a "constraining method" but behind the scenes it are the forces which hold the car on track.
In our inviscous problems the Kutta condition plays the role of those "constraints", but the underlaying principle is visosity. Any textbook on aerodynamics (OK at el

It is the same with the inviscous CFD simulations. If the software uses for example simply the Euler equations, well the results are close to reality as the boundary layer often quite thin (I could read that on a 777 for example we have a . But in the beginning I was asking for an explanation and well not for a simplification appearing to be working very well in some low speed regiemes, and I strongly doubt that an CFD which generates a flowfield without using viscosity explicitely is a proof that the Kutta condition is not a viscous effect (as Lagrange formalism II is no proof that there are no constraint forces)

Well finally the thread has run into a small detail (which is on the other hand quite important). However the thing which appears to me is that such powerful tools and so elegant sets of mathematical principles has been developed by engineers/aerodynamicists that thinking about the reason WHY those tools really work is not necessary any more. Well I as a physicist have another quite opposing perspective; in the physicist's guild many are thinking about the WHY and not about HOW (WHY is the speed sound depending on temperature or WHY is an algorithm stable; engineers are often wondering HOW they build/construct something).
Finally it is nothing more than two different paradigms.

tdscanuck
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### RE: The Ultimative "Why Do Aircraft Fly?" Question...

 Quoting ThirtyEcho (Reply 14):"The phenomenon with low-speed liquids following a curved surface is largely a surface tension thing...it looks like the Coanda Effect but the physics aren't the same at all." Gee whiz, I guess that all of those wind tunnel smoke tests that show air (a fluid) conforming to the shape of the surfaces of wings were conducted at the same speed at which coffee pours from a cup?

I said liquid, not fluid.

 Quoting ThirtyEcho (Reply 14):Is it your point that the Coanda effect doesn't apply outside of the kitchen sink?

No, my point is that the Coanda-looking effect you get with liquids doesn't have the same physics behind it as the Coanad-effect you see on airfoils in fluids which don't have any free surface or surface tension.

 Quoting Superstring (Reply 15):In our inviscous problems the Kutta condition plays the role of those "constraints", but the underlaying principle is visosity.

I've never seen a CFD program which applies the Kutta condition as part of the solution. I'm not even sure how you'd implement that in a CFD framework.

You don't need to use CFD, a wind tunnel will do. The Reynolds number of real airfoils at practical speeds is so high that the viscous forces are functionally zero compared to the inertial forces, yet you'll still see the flow follow the upper surface.

Tom.

Fly2HMO
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### RE: The Ultimative "Why Do Aircraft Fly?" Question...

 Quoting Vikkyvik (Reply 7):Aircraft = Women

So if all aircraft are women, and all women PMS, then all aircraft PMS

Of course, some more than others, as seen by the large puddles of oil under JT8Ds

They really bleed

KELPkid
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### RE: The Ultimative "Why Do Aircraft Fly?" Question...

Congratulations on a mathematical proof of not only my point, but the fact that aircraft are inherently evil (I won't add "and feminine" to that, as my wife would probably make me sleep on the couch...)
Celebrating the birth of KELPkidJR on August 5, 2009 :-)

777236ER
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### RE: The Ultimative "Why Do Aircraft Fly?" Question

You don't need viscosity or Coanda to explain lift (although there's no such thing as an inviscid fluid and you can end up with all sorts of bollocks if you assume no viscous effects).

Why don't you want to explain lift in terms of Newton's third law or circulation or Bernoulli? All are equivalent. The airfoil does deflect air through Coanda and via its geometry (and it would continue to do so without Coanda), so there is a reactionary force. The pressure is lower on the suction side than the pressure side and the flow is faster on the suction side than the pressure side, so there's a resultant force. There is a net circulation from the line integral of the velocity around the airfoil, so there is a force.

Lift can be explained using Bernoulli and the continuity equation, or Newton, or through circulation. They're all just mathematical models describing what happens in real life. All three have flaws: Bernoulli isn't fundamental, although it's derived from bog-standard Newton, the Newtonian theory doesn't go into the detail of the friction and viscosity that causes the deflection and circulation doesn't explain why the circulation occurs in the first place.

If you ignore incorrect assumptions (equal transit time, skipping stone) then all three are valid. Mix and match based on your needs.

Edit: The problem you can end up with is that you can get very metaphysical about the nature of causality. Classical mechanics is reversible, so the question of what 'causes' lift makes little sense. The lift causes the circulation/deflection/pressure gradient, and the circulation/deflection/pressure gradient causes the lift.

[Edited 2007-08-11 02:38:16]
Your bone's got a little machine

ThirtyEcho
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### RE: The Ultimative "Why Do Aircraft Fly?" Question...

I said liquid, not fluid.

Last time I looked, my transmission "fluid" was a "liquid."

Air is a fluid, too.

Wherever you got your degree in aerodynamics, sue them to get your money back.

777236ER
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### RE: The Ultimative "Why Do Aircraft Fly?" Question

 Quoting ThirtyEcho (Reply 20): Last time I looked, my transmission "fluid" was a "liquid." Air is a fluid, too. Wherever you got your degree in aerodynamics, sue them to get your money back.

Tdscanuck was talking about surface tension, something that applies to liquid in ambient gasses, as is the case with coffee flowing out of a cup.

The Coanda effect is caused by different mechanisms if it's a liquid in a gas, or a liquid in liquid/gas in gas. For the liquid in a gas, it can be explained by surface tension. For the liquid in liquid or gas in gas situations, you have to go deeper into the viscosity, friction and momentum.

[Edited 2007-08-12 12:08:13]
Your bone's got a little machine

tdscanuck
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### RE: The Ultimative "Why Do Aircraft Fly?" Question...

 Quoting ThirtyEcho (Reply 20):I said liquid, not fluid. Last time I looked, my transmission "fluid" was a "liquid." Air is a fluid, too.

All liquids are fluids. All fluids are not liquids. Air is not a liquid (at the temperatures we're talking about).

 Quoting ThirtyEcho (Reply 20):Wherever you got your degree in aerodynamics, sue them to get your money back.

Thanks for the tip, but I'll stick with my degree where it came from.

Tom.

AirTranTUS
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### RE: The Ultimative "Why Do Aircraft Fly?" Question...

 Quoting Superstring (Thread starter):x Slipping stone theory, or Newtonian theory --> air is being deflected downwards by the lower side of the wing --> Newton's 3rd. law--> lift

This theory could be easy to test (on a model, of course). Make a wing out of a thin, square box and replace the curved wing. If Newton's theory is correct, the plane should lift off, until...

 Quoting Superstring (Thread starter):x Equal transit time theory + Bernoulli --> pressure above the wing is smaller than below --> we have lift.

It has been proven that an airfoil produces an upward force, but the other day I was thinking, why does an increase in angle-of-attack increase lift? Wouldn't the vector in the direction that lift is acting then point up and back of the wing? If a plane is at 150 knots and level to the horizon, let's assume lift is acting straight up. If the pilot were to pull up though and keep a constant airspeed, lift would be acting less in an upward motion, and have a component aft as well. The upward component would be even less than level flight because the same amount of lift is being produced, but its direction is no longer all vertical. So when an airplane rotates to lift off, I don't think lift increases, but thrust and air deflection (Newton's 3rd law) push the plane away from earth. Thrust also no longer acts horizontally either in a climb. It operates horizontally and vertically, and the vertical component acts with the force of air downward to push the plane higher. (Newton's 3rd again)

Would lift be causing the climb, or would engine thrust? I think thrust is. The airplane is increasing its potential energy, so the mechanical energy of thrust would be used to do it. Also, the plane is no longer in unaccelerated flight (gravity is now greater than lift), so to compensate, more power is needed. I'm sure Newton's 3rd law plays a part here. (I think I answered my own question.)

I have noticed while flying C172's, that if I delay rotation by a few seconds, the airplane gets a bit jumpy on the ground, but will not lift off on its own. If I rotate though and immediately level off, the plane can fly at that speed. I think this proves that Newton's 3rd law is a part of flight (ground effect, which I can see now also happens at cruise), especially the transition from ground to air. When commercial jets are in cruise, they have a positive AOA. The wing would be deflecting air at this moment and the engines pushing in both directions.

Further though, we have all seen pictures of wings flexing on jets, especially heavy ones. They would not be flexing unless there was some force applied to them. Is the force lift ala Bernoulli, or is it their deflection of air towards the ground ala Newton?

My belief is that neither Newton nor Bernoulli can completely explain lift by lift themselves. There are definitely elements of both in lift. I see where both are coming from, but neither is completely correct.
I love ASO!

WingedMigrator
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### RE: The Ultimative "Why Do Aircraft Fly?" Question...

 Quoting Tdscanuck (Reply 3):the wing imparts downward momentum to the air. Downward momentum change = upward lift.

That's the best explanation, based simply on Newton's laws. "All" a wing does is deflect the flow downwards with a minimum of drag.

 Quoting Superstring (Thread starter):First of all to make the situation clearer I want to focus on classical airfoils

Why? Airfoils are not strictly necessary for flight... only for efficient flight.

tdscanuck
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### RE: The Ultimative "Why Do Aircraft Fly?" Question...

 Quoting AirTranTUS (Reply 23):why does an increase in angle-of-attack increase lift? Wouldn't the vector in the direction that lift is acting then point up and back of the wing?

You're mixing up the force vector on the wing with the lift vector. The lift vector is, by definition, the component of force on the wing perpendicular to the air freestream. The other component, by definition, is drag. So changing the AoA doesn't change the coordinate system you measure lift and drag in. Positive lift is always up and positive drag is always aft.

Increasing AoA increases magnitude of the force on the wing and tips it aft. When you resolve the vector into the lift and drag components, you get more lift and more drag just as you see in real life.

 Quoting AirTranTUS (Reply 23):Would lift be causing the climb, or would engine thrust? I think thrust is.

Thrust.

 Quoting AirTranTUS (Reply 23):Also, the plane is no longer in unaccelerated flight

Come again? An aircraft in steady climb has a=0. It's in unaccelerated flight.

 Quoting AirTranTUS (Reply 23):If I rotate though and immediately level off, the plane can fly at that speed. I think this proves that Newton's 3rd law is a part of flight (ground effect, which I can see now also happens at cruise),

Newton's 3rd law is certainly part of flight...it doesn't go away just because you're in an airplane. However, Newton by itself is wholey inadequate to explain flight. Ground effect is caused by an additional boundary condition (the ground) on the flowfield around the airfoil. It can't happen at cruise.

 Quoting AirTranTUS (Reply 23):Further though, we have all seen pictures of wings flexing on jets, especially heavy ones. They would not be flexing unless there was some force applied to them. Is the force lift ala Bernoulli, or is it their deflection of air towards the ground ala Newton?

Yes + some other things. The only thing the wing actually "sees" is pressure. You have higher pressure (on average) across the bottom of the wing than the top. Newton partially "explains" the higher pressure on the bottom and, if you know the velocity field, Bernoilli partially "explains" the pressures. Neither one will tell you what the flowfield looks like.

 Quoting AirTranTUS (Reply 23):My belief is that neither Newton nor Bernoulli can completely explain lift by lift themselves.

Very true.

Tom.

ThirtyEcho
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### RE: The Ultimative "Why Do Aircraft Fly?" Question...

Air is not a liquid (at the temperatures we're talking about).

But air IS a fluid.

Have your instructor demonstrate a high speed stall; that will teach you about the Coanda effect.

superstring
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### RE: The Ultimative "Why Do Aircraft Fly?" Question

Well haven't been here for some days...

 Quoting Tdscanuck (Reply 16):I've never seen a CFD program which applies the Kutta condition as part of the solution. I'm not even sure how you'd implement that in a CFD framework. You don't need to use CFD, a wind tunnel will do. The Reynolds number of real airfoils at practical speeds is so high that the viscous forces are functionally zero compared to the inertial forces, yet you'll still see the flow follow the upper surface.

Yes indeed the simulations dont't use the Kutta condition explicitely. However I want to mention an important thing with inviscous simulations.
First inviscous simulations cannot predict drag (D'Alembert's paradox).
Second point: Boundary conditions. When you put the wing (ore let's stay 2D) the airfoil into a mesh, well what speeds will you assign to the surface? Of course zero (no slip condition), so through the backdoor viscous effects come into play here.

 Quoting Tdscanuck (Reply 25):Quoting AirTranTUS (Reply 23): Would lift be causing the climb, or would engine thrust? I think thrust is.

This is of course valid, but do not forget energy. There is an example to demonstrate the importance of energy
Everyone knows that you can convert horizontal speed (=kinetic energy) directly to get higher up (=potential energy); so if you have enough speed and you want to climb, you simply have to pull the stick. The airplane will climb; in this example no thrust (from the engine) is involved (well of course you will loose speed very quickly...)

 Quoting WingedMigrator (Reply 24):Is the force lift ala Bernoulli, or is it their deflection of air towards the ground ala Newton?

It is important here to give a very illustrating example to prove that there is a pressure difference between the lower side of the wing and the upper side. Wingtip vortices. They are a direct result of air wanting to flow around the wing (coming from the lower side) and get to the region with lower pressure (above the wing).

The thing I mentioned in:

 Quoting Superstring (Thread starter):Slipping stone theory, or Newtonian theory --> air is being deflected downwards by the lower side of the wing --> Newton's 3rd. law--> lift

needs a litte bit of an explanation:
Newton's 3rd law simply states that there must be an equivalent and opposing reaction.
But my statement was aiming at a very wrong theory of lift: That the wing produces lift, because the wing is pushing air downwards and therefore simply by using Newton 3, the wing stays up in the air. This explanation is wrong as it completely "forgets" that the biggest amount comes from the pressure difference.
I think the name slipping stone theroy is a better name instead "Newtonian theory" (somewhere on the web I found this odd name) because simply the whole (classical) mechanics world is a "Newtonian theroy".

 Quoting WingedMigrator (Reply 24): My belief is that neither Newton nor Bernoulli can completely explain lift by lift themselves. There are definitely elements of both in lift. I see where both are coming from, but neither is completely correct.

Well to be very presice every theory you will come up with, which will use mathematical equations will use Newton implicitely (of course as long as you do not consider quantum mechanics or the theory of relativity). The Navier-Stokes equations are derived by assuming mass, energy and MOMENTUM conservation principles.
The next thing is that no supercomputer in the next decade (well who knows maybe in 50 years...) will be able to do the only valid thing: to calculate the lift basing on the interaction of EVERY molecule with the wing's surface. (But if I really wanted an explanation which is THAT exact...). However as long as we have no such tools we will have to use very good approximations.
Maybe in 50 years we will have the following explanation: "Basing on a complete set of Quantum mechanical Fluid equations anyone has demonstrated...."

WingedMigrator
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### RE: The Ultimative "Why Do Aircraft Fly?" Question

 Quoting Superstring (Reply 27):But my statement was aiming at a very wrong theory of lift: That the wing produces lift, because the wing is pushing air downwards and therefore simply by using Newton 3, the wing stays up in the air.

There is absolutely nothing wrong with this 'theory' of lift, unless you wish to call into question Newton's laws. Pushing air downwards is precisely how a wing produces a lift force. Show me a wing that does not impart downward momentum to the air it flies through, and you will have shown me a wing that produce zero lift. Period.

You may be confusing this with Newtonian aerodynamic theory, which attempts to explain the pressure distribution on a body based on assumptions that are flawed (at least in the subsonic regime-- it's OK to first order in the hypersonic regime). That theory is indeed incorrect, but that doesn't mean Newton's laws don't apply to wings.

The man had a mixed record. This is the same Newton who theorized that a prism separated light particles of different 'bignesses and swiftnesses'.

 Quoting Superstring (Reply 27):This explanation is wrong as it completely "forgets" that the biggest amount comes from the pressure difference.

This is not a situation where 'some' lift comes from conservation of momentum and Newton's 3rd law, and 'some more' lift comes from pressure difference. The momentum and pressure explanations are equally valid, independent, and not mutually exclusive.

You meant skipping stone.

superstring
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### RE: The Ultimative "Why Do Aircraft Fly?" Question

 Quoting WingedMigrator (Reply 28):You meant skipping stone.

Thanks thats absolutely right, sKipping stone, not sLipping stone...

 Quoting WingedMigrator (Reply 28):There is absolutely nothing wrong with this 'theory' of lift, unless you wish to call into question Newton's laws. Pushing air downwards is precisely how a wing produces a lift force. Show me a wing that does not impart downward momentum to the air it flies through, and you will have shown me a wing that produce zero lift. Period

+

 Quoting Superstring (Reply 27):The thing I mentioned in: Quoting Superstring (Thread starter): Slipping stone theory, or Newtonian theory --> air is being deflected downwards by the lower side of the wing --> Newton's 3rd. law--> lift needs a litte bit of an explanation:

Well I thought that I have explained what I mean with the "Newtonian theory"; Newton 3rd law is of course playing a role in ANY situation where you have forces. The point I wanted to make is that some (which call the skipping stone thing Newtonian, don't ask me why) think that the wing stays up in the air simply by the fact that the molecules hitting the lower surface are "pushing" the wing up. And that is NOT correct, in the sense that this is the only FACTOR which is contributing to lift production.

 Quoting WingedMigrator (Reply 28): This is not a situation where 'some' lift comes from conservation of momentum and Newton's 3rd law, and 'some more' lift comes from pressure difference. The momentum and pressure explanations are equally valid, independent, and not mutually exclusive.

That is absolutely correct; well it does not change the fact that SOME think that it is the deflection of the airstream by hitting the lower side of the wing ALONE which creates lift. Following image should reveal everything:
http://www.av8n.com/how/htm/airfoils.html#fig-bullets (well another name)

 Quoting WingedMigrator (Reply 28):The man had a mixed record. This is the same Newton who theorized that a prism separated light particles of different 'bignesses and swiftnesses

Well indeed, Newtons laws are the basic thing for our classical mechanics world. However Newton has never put hands onto a CFD simulation, nor did he contribute much to fluid dynamics (his famous sine-square law was such an example; well it holds true for supersonic fluids). Its the same with Einstein, he also "invented" an airfoil which, well was not the best thing...

TheRedBaron
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### RE: The Ultimative "Why Do Aircraft Fly?" Question...

Flux capacitor makes it possible...
The best seat in a Plane is the Jumpseat.

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