Why an Airplane Flies (Bernoulli's Principle vs. Newton's Third Law)

[studiot]

 

MAY develop more lift or more weight in a short time span,

 

That's clever how does that happen as it is flying along, icing?

 

 

There is airflow being turned down off the trailing edge of all airfoils, this is known as Downwash. This downwash is often used to show Newtan's 3rd law. Airflow goes down, airfoil gets lift. Bernoulli is used to describe what happens above the wing. There is a massive low pressure above an airfoil, due to the increased airflow, 'sucking it up.'

 

If you look back at Bignose'e post you will see he mentions circulation.

 

This has physical dimenstions L²T^-1.

 

Multiply this by the mass within the circulation countour and you get the ML²T^-1, which has the dimenetins of moment of momentum.

 

Yes this circulation is going with the stream over the top, down at the rear, against the stream underneath and up in the front.

 

The rule is

 

The thrust at right angles to the flow acts from the side where the circulation and uniform flow oppose towards the side where they reinforce. Hence the lift.

Edited October 18, 2013 by studiot

[Bignose]

There is not a "Law of Lift", we have some good ideas, but no one actually knows how lift works.

I disagree with this, or at least would like it worded differently. The first clause I agree with, 'there is not a law of lift'; In that there is no simple off-the-cuff or even 1 paragraph grade-school level description of how an airplane flies.

 

But I don't agree with 'no one actually knows how lift works" because the models we have of airfoils and the like are incredibly accurate. Yeah, completing those models takes some good knowledge of computational fluid dynamics and the like. Or old school, how to do conformal mappings of non-circle shapes on to a circle and solving the equations there. Sure, a lot of computer and mathematics knowledge will be needed. But considering how accurate those models are, I don't think it is fair to just say that 'no one' knows about lift.

 

It is just that none of these story book explanations are completely right.

[MigL]

If no-one knew how lift works, it would be pretty hard to design aircraft that stay up in the air.

Last time I checked, airplanes weren't falling out of the sky like rain, so we must know something.

[imatfaal]

If no-one knew how lift works, it would be pretty hard to design aircraft that stay up in the air.

Last time I checked, airplanes weren't falling out of the sky like rain, so we must know something.

 

Not sure if the first sentence is at all true - we had plenty of machines that relied on gravity for centuries before Galileo and Newton started to put it on a scientific footing. Things fall - it is in their nature to fall - and we can use this to drive a water wheel etc

[John Cuthber]

If no-one knew how lift works, it would be pretty hard to design aircraft that stay up in the air.

Last time I checked, airplanes weren't falling out of the sky like rain, so we must know something.

Fine, we can ask the Wright brothers.

[ccwebb]

Bignose - you are correct. We do know a great number of things about lift. Yet, lift still baffles even the greatest:

 

On page 4-14 in the Airplane Flying Handbook it has been proven that an aircraft in a spin is in equilibrium! Though the nose is extremely down, and the airplane is following a "corkscrewish" type flight path, the four fundamental forces in aerodynamics are balanced.

 

Even the great Wikipedia does not explain it very clearly:

"There are several ways to explain how an airfoil generates lift. Some are more complicated or more mathematically rigorous than others; some have been shown to be incorrect. For example, there are explanations based directly on Newton's Laws of motion and explanations based on Bernoulli’s principle. Either can be used to explain lift"

 

The point I was trying to make, since lift is not known and often argued, it I try to keep the explanations very simple: You can make lift with large wings or big engines. As you go from one to the other they ratio of engine-wing must change.

 

Even NASA keeps it as simple as possible. "Lift is a force generated by turning a flow. Many different objects can generate a lift force and there many factors which influence the generation of lift."

[Bignose]

On page 4-14 in the Airplane Flying Handbook it has been proven that an aircraft in a spin is in equilibrium! Though the nose is extremely down, and the airplane is following a "corkscrewish" type flight path, the four fundamental forces in aerodynamics are balanced.

So let's look at the exact quote from your link:

 

This is where airplane aerodynamic forces and inertial forces are in balance, and the attitude, angles, and self-sustaining motions about the vertical axis are constant or repetitive. The spin is in equilibrium.

In equilibrium or balanced does not necessarily mean no motion. A parachutist that jumps from a plane and hit terminal velocity is also in equilibrium, but obviously is moving quite fast. And the spin being in equilibrium means that the rate of the spin isn't increasing or decreasing. That is, if the plane is making 1 rev in 5 seconds, it will continue to spin at 1 rev in 5 seconds, not change to 1 rev in 4 or 6 seconds. The angular forces are balanced, but obviously there is still angular motion.

 

The bigger point is that I think you try to use this quote to demonstrate that we don't know what lift it, or how to calculate it. But, we do know what lift is. Even in your example, how they teach you to get out of the spin uses the very fact that the plane will create lift. We obviously know enough about lift to instruct pilots to use it to get out of a spin!

 

And yes, 'flow turning' is that elusive 1-line answer to how lift is formed. It just isn't very satisfactory because the next obvious question is 'well, then, what causes the flow to turn?'. And we're back to no 1-line story-book answers available. It's complex. That doesn't mean it isn't known to a high level of accuracy -- because, really, it is. There is a lot of physics that you can say the same thing about -- quantum mechanics being another good example in my mind. Very complicated, but we do know it can make predictions to very high levels of accuracy.

 

That doesn't mean that there isn't more to learn, and that there can't be a 1-line story-book answer that is discovered some day. Obviously, research into the modeling continues. But, again, I don't think it is fair just to toss your hands up and say things like 'no one actually knows how lift works.' Because I just don't think that is right.

Edited October 24, 2013 by Bignose

[ccwebb]

Point taken. My statement of "... but no one actually knows how lift works." should have been worded "...but no one knows how lift works exactly. We have a lot of very strong ideas, but no cookie-cut simple phrase."

 

 

That doesn't mean that there isn't more to learn,

 

This was the point I was trying to make. We don't have all the answers, and trying to call it "Newton" or "Magnus" or "Bernoulli" or even "magic" isn't the correct either.

 

 

To answer a question from Studiot, post #26 above:

 

That's clever how does that happen as it is flying along, icing?

 

An airplane can become aerodynamically heavier or lighter by simply shifting the Center of Gravity aft or forward. It can become physically heavier with ice, and of course it gets lighter as fuel is burned off.

[MigL]

That's like sayng we didn't know how gravity works when all we had was Newton. We still needed to learn Einstein's viewpoint, and as a matter of fact there is and always will be more to learn ( quantum gravity etc ).

But we put men on the moon with Newton !!!

 

So we don't have exact solutions for the Navier-Stokes fluid flow equations, but we have approximate computational methods which work just fine for the application. Even though sometimes they need refinement in a wind tunnel.

[John Cuthber]

How good a grasp of the Navier Stokes equations did the Wright brothers have?

Do you have details of their wind tunnel?

 

Nobody knew how birds worked- but we could copy them well enough.

Do you consider that the birds know how to build a bird?

It's an interesting philosophical point.

[studiot]

 

How good a grasp of the Navier Stokes equations did the Wright brothers have?

 

To me, the most eloquent enunciation of this principle was

 

"Gentlemen, shall I refuse my dinner because I do not fully understand the process of digestion ?"

Edited October 25, 2013 by studiot

[John Cuthber]

Whatever. The assertion was made that "If no-one knew how lift works, it would be pretty hard to design aircraft that stay up in the air."

Is that assertion credible?

[MigL]

I assume, since you think aircraft design is a crap shoot, you don't like to travel in planes much then John ?

 

Or are you just playng devil's advocate to explore this 'interesting philosophical point'?

Edited October 26, 2013 by MigL

[John Cuthber]

I'm saying that a lot more of it is empirical than theoretical.

The point remains that a model bird, if it was light weight would glide quite well. If you stuck a rocket motor where the sun doesn't shine, it would fly.

You wouldn't need to know "why" to know "how".

 

It still isn't clear to me why people think it's all down to Bernoulli, even though planes fly upside down.

[MigL]

But it has been discussrd at length that its not just Bernoulli's principle which accounts for the lift. Heck even a flat plate at a certain angle of attack will produce lift. as a matter of fact the most efficient supersonic wing profile has zero thickness and a flat top.

 

The point remains that a plane can fly upside down, although Bernoulli would suggest it can't. due to other lift producing effects.

[studiot]

 

 

although Bernoulli would suggest it can't.

 

 

Why on earth (or in the sky) would you suggest this?

 

The theoretical analysis of a phenomenon can often be approached from several viewpoints and calculated in several different ways.

 

However they are always equivalent and compatible if they lead to the same result.

 

 

due to other lift producing effects

 

Again, what line of reasoning leads you to state this?

[John Cuthber]

What he said was

"The point remains that a plane can fly upside down, although Bernoulli would suggest it can't. due to other lift producing effects."

Bernoulli would say that a wing on an upside down plane would force it downward.

But you can fly upside down - all;though Bernoulli 's principle would say you cant.

So there must be other effects.

 

The thing is that Bernoulli's ideas are not compatible with a plane that can fly either way up.

[studiot]

The thing is that Bernoulli's ideas are not compatible with a plane that can fly either way up

 

 

And I asked why not?

 

Or if you prefer, in what way does Bernoulli's theorem fail to hold?

 

Whichever way up the airfoil or other shape passes through the medium, there is a measurable pressure difference between the medium above and the medium below (or to the left and right if the object is rotated 90 degrees) and a measurable velocity difference between the two flows as well.

 

The relationship between measured pressure and measured velocity, insofar as is possible since there is actually a pressure distribution and a velocity distribution, is in accord with Bernoulli's theorem.

 

Bernoulli's theorem does not offer an origin for the difference, only a calculation of how much. That is where explanations take this approach too far and fall down.

 

The difference arises from the local rotation the disturbing object introduces into the flow.

Edited October 27, 2013 by studiot

[John Cuthber]

And I answered.

The wing would push the plane down.

[studiot]

 

The wing would push the plane down.

 

 

Prove it using Bernoulli's theorem alone.

 

I suggest you also reread the excellent post#4 of this thread from pangloss about the subject.

Edited October 27, 2013 by studiot

[MigL]

The rotation and resultant pressure difference suggested by Bernoulli's principle are due to differntial curvature or camber of the airfoil. No one would suggest that a flat plate of zero thickness, parallel to the airflow would induce any rotation or pressure difference.

 

Alternatively place an a Clark Y airfoil ( curved top, flat bottom ) in a wind tunnel with flat attitude and notice that there is decreased pressure on the upper curved surface as compared to the bottom. The lift is then up, towards the direction of the ceiling.

If we now take the whole apparatus and flip it upside down, wind tunnel and enclosed airfoil, are you suggesting the pressure distribution and the resultant lift is still the same and still in the direction of the ceiling studiot?

 

I'm sorry, I have to side with John on this one ( unless of course, you prove me wrong, I always hedge my bets ).

[studiot]

 

The rotation and resultant pressure difference suggested by Bernoulli's principle

 

I'm sorry, where does Bernoulli suggest rotation?

 

He doesn't, period.

 

Jukowski's Thereom 1906 does

 

 

If any irrotational two-dimensional fluid current, having at infinity a velocity Vi, surrounds any closed contour on which the circulation of velocity is G, the force of the aerodynamic pressure acts on this contour in a direction perpendicular to the velocity and has the value

L=piViG.

The direction of this force is found by causing to rotate through a right angle the vector Vi around its origin, in an inverse direction to that of the circulation

 

Bernoulli's equation is a scalar equation.

 

The Lift equation is a vector equation.

 

Jukowski's theorem and the Circulation theorem are both vector equations.

 

You cannot use a scalar equation as a vector one.

Edited October 27, 2013 by studiot

[John Cuthber]

 

Prove it using Bernoulli's theorem alone.

 

I suggest you also reread the excellent post#4 of this thread from pangloss about the subject.

Ho Hum.

The application(s) of Bernoulli's equations that give rise to the simple explanation of lift remain the same if you turn the camera upside down.

[studiot]

 

The application(s) of Bernoulli's equations that give rise to the simple explanation of lift remain the same if you turn the camera upside down.

 

 

Perhaps you don't know what Bernoulli's equation states?

 

Another common misapplication of Bernoulli's theorem is to attempt to apply it to two unconnected places within a flow.

 

Bernoulli's theorem is about energy conservation along a streamline.

 

The streamlines above the object are not the same ones as those below. So you cannot just take values from the streamlines above the object and substitute into the streamlines below.

Edited October 27, 2013 by studiot

[John Cuthber]

From memory it's the eqn you get from conservation of energy in MV, 1/2 MV^2 and MGH which you can convert to density + pressure by division by the volume (for an incompressible fluid).

But I'm rushing to type this before I get a bus and trying to remember it from 30 years ago..