Debunking lift theories

[Nathan Parker]

Originally Posted by G-EMMA

I hold a cigarette still there will be a portion of perfectly laminar flow as the smoke leaves the cigarette. An inch of so above the flow becomes turbulent, why does that happen? Got an equation for it, will CFD tell my why it happens?

The existence of turbulence is not a prerequisite for the existence of lift.

Still, your whole criticism is off-target. You're claiming that the difficulty in calculating lift in any particular situation is the result of a conceptual problem, when it's really a matter of practicality.

CFD doesn't give an explanation of lift

Not to you, perhaps, but if it can calculate it, then it establishes that the factors that contribute to lift are well-known.

Apply simple mathematical equations to the wing of a model aeroplane. Work out mathematically how much lift it will produce (use Bernoulli if you really must).

What right have you to ask for "simple" equations? Reality isn't always simple, particularly when you're trying to predict the mass behavior of large numbers of independent actors.

You seem to think something can't be easily explained to you means that it isn't understood.

[Henry Hallam]

I think some of you guys should really take an aerodynamics course or read a textbook (Anderson is good) before saying that nobody understands how a wing works. If it could all be explained in a forum post then we wouldn't have extensive college courses on it, but that doesn't make it any less understood or understandable.

[bookworm]

What I'm saying is that it isn't the whole picture. If we had a really good explanation for lift, a real mathematical model that worked all the time, we could use it to design aerofoils? I could say, right I want it to do this, and then design the shape. As it is that doesn't work does it? We have some good bits of theory that let us work out some things such as predicting quantative lift at low Mach numbers for what I would call for want of a better term 'classic aerofoils'... but it isn't all of it.

Can you give any example of a physical model that works all the time without "some fudge factors thrown in"? I can't. The nature of physics is that we start with a simple model that crudely approximates reality and we add complexities to it so that the approximations become better and better.

[bjornhall]

Originally Posted by bookworm

Can you give any example of a physical model that works all the time without "some fudge factors thrown in"? I can't. The nature of physics is that we start with a simple model that crudely approximates reality and we add complexities to it so that the approximations become better and better.

Of course there are such models! In beam and ray optics, just to give one example, models are usually good enough that you can design from a model and get a predictable and useful result on the first attempt. Electronics usually as well. And those were just two...

Thing is though, when you have a model that is developed and understood at such a level, you tend to overlook it in discussions about "the unsolved problems of physics and engineering"... It's too simple.

If aerodynamics was understood at such a level (and I doubt it can ever reach that level, due to the nonlinearities of the Navier-Stokes equation), we could design a useable wing from first principles without using wind tunnels or even CFD, build it, fit it on a plane and it would work as designed on the first attempt.

But we are not on that level with aerodynamics. I suspect that is what G-EMMA means; not that we have no clue how our planes really work...

[Andy_RR]

yeah, but CFD is simply a numerical solution to the Navier-Stokes equation. Just because you can't find an analytical solution, doesn't make it a bad (or poorly understood) method.

A

[IO540]

In electronics, one can model resistors and capacitors 'exactly' but semiconductors are all approximations full of fudge factors. Digital logic can be modelled exactly for function of course but not exactly in timing.

Incidentally, one sentence in that letter caught my eye

In two-dimensional flow (no tip effects) there is no net downwards air movement.

How can this be true, while the weight of the aircraft is being supported? There has to be some downward acceleration of air mass.

[bookworm]

Of course there are such models! In beam and ray optics, just to give one example, models are usually good enough that you can design from a model and get a predictable and useful result on the first attempt.

As we do in aerodynamics. Beam and ray optics is fine until you get to distances of the order of the wavelength. Can you use it to explain diffraction aand interference? And try applying "beam and ray optics" to near-field situations! Incompressible flow is fine until you get velocities of the order of the speed of sound. But it's good enough to design aerofoils with.

Physics is about peeling the layers of the onion to get closer to reality. Aerodynamics is no different.

[Contacttower]

That's what the US Creationists say, as if to dismiss the theories they don't like!

Maybe, but what US Creationists say is not really relevant to this, science draws the distinction between laws and theories because some ideas are more proven and established than others. Ideas relating to lift tend to fall into the latter, and I think one is being naive if you say hand on heart we know exactly how a wing works and can predict with total accuracy what it will do throughout the speed and angle of attack range.

If you think there is some inconsistency in the science of flight due to the "Downwind turn" debate, you're very mistaken.

The poster who constantly advocated the "downwind turn" claimed to be an RAF helicopter pilot (and I have no reason to believe he was not) and was basing his ideas on an observation.....now I don't believe in them...and I doubt many do but I just thought it demonstrates that this is far from a closed debate.

[Henry Hallam]

"In two-dimensional flow (no tip effects) there is no net downwards air movement.

How can this be true, while the weight of the aircraft is being supported? There has to be some downward acceleration of air mass."

This is true for two-dimensional, *inviscid* flow. There is no circulation (flow turning) and therefore no lift. There is also no drag (d'Alambert's paradox). Of course all actual flows are viscous, but the inviscid equations are easier to solve, so it's often convenient to approximate them as inviscid over almost the entire volume except for point or sheet vortices that we add in to provide the circulation.

p.s. the Wikipedia article on D'Alembert's Paradox is full of crap.

[Islander2]

This is true for two-dimensional, *inviscid* flow.

John Gibson's comment is not referring to inviscid flow, however, he is describing what one observes in a wind/smoke tunnel when simulating two-dimensional flow by taking the wings to the tunnel walls or beyond (no tip effects). There is no net downwash (i.e the downwash is balanced by the upwash), yet the measured lift force is greater than for the equivalent three-dimensional flow.

As I said in an earlier post, this is a highly inconvenient fact for those that wish to built a lift theory around Newton's third law. Of the various inventive arguments put forward by the proponents of modern-day lift theories to explain away this inconvenience, the prize has to go to Anderson and Eberhardt who state that there is a net downwash in two-dimensional flow, it's just that it's produced by the wing diverting "an infinite amount of air at zero velocity!"

[Genghis the Engineer]

Oh no, not again! The dreaded Newton .v. Bernoulli debate.

If anybody wants a really clear explanation of the various forms of aerodynamic theory read Glauert's book "aerofoil and airscrew theory", published in 1926, still in print, and far more complete than anything I've seen in PPL or ATPL theory books.

G

[IO540]

I do not believe that it is necessary to have a wingtip (i.e. that it is necessary to have a wing of a finite length) in order to get lift.

To me, it seems intuitively obvious that one cannot get lift unless one is accelerating some air downwards.

Is that wrong?

[Henry Hallam]

"To me, it seems intuitively obvious that one cannot get lift unless one is accelerating some air downwards.

Is that wrong?"

That's right, at least as far as I'm concerned and explicitly what Prof. Darmofal taught my class.

[bjornhall]

As we do in aerodynamics. Beam and ray optics is fine until you get to distances of the order of the wavelength. Can you use it to explain diffraction aand interference? And try applying "beam and ray optics" to near-field situations! Incompressible flow is fine until you get velocities of the order of the speed of sound. But it's good enough to design aerofoils with.

The crucial difference, in my view, that made me pick that example, is that beam and ray optics is self-contained. It is not complete, there are limits to its applicability, but within its range of applicability you can use that and only that to obtain a useful result.

Simple aerodynamics, such as inviscid flow approximations, as far as I am aware, is not thus self-contained. For instance, ignoring viscosity you have to pull things like the Kutta criterion out of your hat to explain what the circulation will be; details about the boundary layers such as thickness and where they go turbulent influence the results (especially at high angle of attack) but you can't readily calculate those details from first principles, etc.

I think that is the main point about these "pilots' explanations" of lift, such as the equal-time argument etc: There is no simple and correct way of explaining it. It's either simple, or correct, or most commonly neither. Whereas better understood and more easily understood (and thus far less fascinating! ) topics can be explained to a sufficient level within some parameter range.

I also take the view that just because one can calculate something does not in-itself mean that one understands it. The engineer-me is happy with just an equation and a numerical solution, but the physicist-me wants more...

[BackPacker]

So... If one of my non-pilot friends asks me how this whole flying thing works, what do I tell them?

So far I've tried a combination of "equal distance over the top and bottom" (hence the shape of the wing) and "impact lift" (hence angle of attack), and that the truth is somewhere in between, but not completely understood from a theoretical point of view. And then added that a lot of empirical data has shown that aircraft, indeed, can fly. And, so far, they've bought it. But what if a really clever chap comes along?

(I did not spend the better part of an hour retyping Gibsons article just to hear the phrase "we really don't know" over and over again... )

[BackPacker]

So inbetween the pax briefing about doors and seatbelts, if the question comes up, would this be a more or less correct reply?

"Originally there were two theories, commonly referred to as Bernoulli and Newton, although both scientists never put forth those theories themselves. Very briefly put, the Bernoulli camp uses the equal transit time theory (hence the shape of the wing) to explain a pressure differential between the top and bottom of the wing, while the Newton camp uses the impact theory (hence angle of attack). Both theories hold some merit of truth but on their own are not enough to explain lift, and elements of both theories have been proven to be wrong in any case. Euler took the work of Bernoulli and Newton and put forth a comprehensive set of equations that explain lift in a lot of cases, and Navier-Stokes expanded on that to include viscosity effects, which is needed to explain boundary layer behaviour, among other things. Euler and Navier-Stokes are now the essential formulas used in CFD, which is used to design modern wings. All this is essentially for laminar flow: turbulence is still far from understood from a theoretical point of view."

Let's hope their eyes start glazing over by the time I speak that second sentence so that I can start extending that stick below the fuselage to hold the plane up.

[soay]

Originally Posted by backpacker

Let's hope their eyes start glazing over by the time I speak that second sentence so that I can start extending that stick below the fuselage to hold the plane up.

Mine uses a sky hook. Vorsprung durch Technik!

[barit1]

BackPacker - you need to edit that tome so it's a single sentence with a minimum of punctuation

[BackPacker]

Nervous pax: "How does this whole flying thing work then?"
Me: "Well, you see, onions have layers, just like Ogers."

Anyway, "The wing, due to its shape and forward motion, displacing the air downwards, thus generating an aerodynamic counterforce more or less upwards (let's just mention lift at this stage and ignore drag) which keeps the plane in the air" works for me. And then add "all the slightly more complex explanations have all been proven wrong somehow and the theory that explains almost all of it correctly (except turbulence), Navier-Stokes, is so complex that it takes a mathematical genius to understand and apply it. And, by the way, the sickbags are located in this here pocket."

[bjornhall]

Should add to your nervous pax tho' that Navier-Stokes explains turbulence just fine!

The problem is not finding an equation that contains turbulence in its solutions; Navier-Stokes already does that. The problem is solving the equation...