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The mysteries of aerodynamic lift

scientificamerican.com

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Re: The mysteries of aerodynamic lift

#101

Post by namirez »

This is a confusing take on lift. To explain lift intuitively we need two ingredients: the Laplace equation and the Kutta condition.

Most people have an intuitive understanding of the Laplace equation. For example lightning usually hits the peak of mountains. The reason is that in solutions of Laplace equations, field gradient is proportional to curvature. In fluid dynamics, this field is called the stream function. The top of airfoil is more curved than the bottom so the stream function gradient is higher on top which results in higher wind speeds over airfoil.

But the second ingredient is the Kutta condition which represents viscosity. If there were no viscosity, there would be no lift. The Kutta condition is applied to the tail (trailing edge) of airfoil. Without Kutta condition, the speed at the trailing edge would be infinity (because of Laplace equations. Speed around sharp corners is inevitably infinite). Viscosity prevents infinite velocities so we apply another condition at the trailing edge to make the air velocity smooth.

It's kind of complicated and I agree that there is no simple explanation to lift, but if you think about it for a little while, it's not that hard to grasp.

Re: The mysteries of aerodynamic lift

#102

Post by namirez »

lmilcin wrote: post #77

I don't think there is anything mysterious about lift. We can model it very well, we have precise equations that predict the exact results (though we can't solve them) and we know where these equations come from. The fact that I do not understand the equations doesn't mean there is anything mysterious behind it. There might be some artistry with regards to actually designing the aerodynamic shapes. We have no way of…

"We have no way of finding the best possible shape yet"

In general, you're right but for certain class of problems we absolutely have solutions. Just look up inverse methods whereby the input is a pressure distribution and the output is a shape. This is used often for designing airfoils, ducts, and other simple geometries.

Re: The mysteries of aerodynamic lift

#103

Post by nimish »

mannykannot wrote: post #90
nimish wrote: post #75

Earlier quoted context omitted.

This doesn't explain all of the induced pressure differences. You need Bernoulli (Conservation of Energy, more or less) as well. They interact in a complicated way. You really can't "just" explain lift simply.

It is more like that there is a hierarchy of explanation. There is nothing going on that cannot be explained by Newton's laws, conservation of energy does not need to be introduced as an extra constraint, and Bernoulli's law is itself explained by Newton's laws. The issue is that once you recognize that lift is the reaction to accelerating the airflow downwards, you still don't know how the air moves around the wing,…

> The issue is that once you recognize that lift is the reaction to accelerating the airflow downwards

You need bernoulli to explain why the flow field is changed beyond just the area in contact with the flow. This induces the measured pressure differential, explaining part of lift along with the reaction effects of deflected flow for momentum conservation (NS, Newton's 2nd law). It's simply not _enough_ to say that it's purely angle-of-attack or geometry, and it's definitely not enough to say it's just pressure difference caused by Bernoulli, it's _both at once_.

>One significant issue is that if you do this without taking into account friction at the surface of the wing, and the boundary layer that results, you will find that there is no lift at all! Your solution will show the air that passes under the wing turning around the trailing edge, and flowing forward for some distance over the upper surface. In practice, the presence of a boundary layer causes the flow to separate at the trailing edge (if not before).

Not really true for 2d inviscid flows, a very useful approximation -- you need to enforce the K-J condition IIRC (which includes viscosity in a sense)

In short: fluid dynamics is complex, and you need more than just newton's laws to explain it thoroughly (newton's laws don't explain conservation of mass either...)

Re: The mysteries of aerodynamic lift

#104

Post by mannykannot »

o_nate wrote: post #89
frank2 wrote: post #82

Earlier quoted context omitted.

A flat wing at 45 degrees produces massive turbulence above and behind the wing, which in turn greatly increases the drag-to-lift ratio compared to a standard airplane wing. Also I suspect that ailerons and other control surfaces become much less useful in turbulent flow.

Sure, but now explain drag and turbulence using Newton's laws.

Via the Navier-Stokes equations, which are the application of Newton's laws to viscous fluids.

Re: The mysteries of aerodynamic lift

#105

Post by usrusr »

Retric wrote: post #35
munificent wrote: post #24

This is a somewhat confusingly written article about a famously confusing topic. It directly parallels arguments about how sailboats are able to sail. Sails are also airfoils so similar mechanics come into play. Interestingly, because a sail has effectively no thickness, both sides of the sail always have the same length, which immediately calls the Bernoulli argument into question. Sailboats are also interesting bec…

The billiard ball model works fine when you include the impact of other billboard balls on each other resulting in vortexes etc. It’s simply computationally expensive to do so. Anyway, absolutely flat wings generate lift as long as the angle of attack is non zero. But, by changing the wings shape they get more efficient. The reasons for that are complex differential equations that don’t really have simple plain Engli…

A nice first approximation extension of the billiard ball model is to look at a stalled wing in comparison to an un-stalled: the down side will continue to "deflect the balls" just fine. But while the un-stalled upper side of a wing will also draw air downwards in an orderly way, the stalled upper side pulls a tail of vortices forwards. Not strictly a billiard ball model anymore, but even the crudest model of turbulent flow is enough to get the general idea.

The purpose of an airfoil, in this approximation, is giving the pulled air a longer, gentler acceleration compared to the short peak of acceleration a flat wing would require at the same angle of attack. You won't be able to calculate optimal airfoils with this model, or even a winglet, but it's good enough for many of the usual little riddles like symmetric airfoils, inverted flying etc.

Re: The mysteries of aerodynamic lift

#106

Post by mannykannot »

munificent wrote: post #58
simonebrunozzi wrote: post #27

Earlier quoted context omitted.

> can also sail faster than the wind at times. Really? Can you elaborate?

Imagine a sailboat pointed 90° relative to the wind so that the wind is coming right at its side. The sail is curved so that it takes that wind and redirects it towards the rear of the boat, giving it forward thrust. The boat starts moving forward. But, because the wind is perpendicular to the boat, even when its moving the wind is still coming in at the same velocity, so it's still producing thrust. If you can get t…

There is one complication here: as the boat accelerates, the relative wind moves forward.

Consider a boat on a beam reach, where the wind over the water is at 90 degrees to its track. If the boat is travelling at wind speed, the apparent wind over the deck is at 45 degrees to the bow.

The useful angle of attack for airfoils goes up to about 15 degrees, so let us assume that the sails are set to this. Therefore, the chord of the sail is at 30 degrees to the boat's track, its lift is at 60 degrees to its track, and so half the total lift is in the direction of motion [1]. So long as this exceeds the total drag of the boat, from the water and the air, then it will continue to accelerate.

[1] To simplify (and to go faster!) assume a multihull sailboat, hydrofoil, or a dinghy with its crew hiked out so that is not heeling appreciably, as when a boat is heeled, a component of its sails' lift is directed downwards.

Re: The mysteries of aerodynamic lift

#107

Post by msandford »

nimish wrote: post #44
opwieurposiu wrote: post #40

A wing is a device that pumps air downward, which in turn pushes the wing upward, by newton's third law. For a large plane, the wing will be pumping many tons of air per second. Start with a cube of still air, with zero mean velocity. Fly a plane through it, and that cube will have a mean downward velocity. http://www.aviation-history.com/theory/lift.htm

"Pump" is doing a lot of work in that explanation. It's a very subtle thing to explain the pressure differential of a non moving airfoil, and marry it up with such real-world conditions like flying aircraft upside down, trailing edge vortices, and the like. You need both Newton and Bernoulli here, and Euler when viscosity is irrelevant (most airfoils except on the boundary layer) -- the Kutta-Joukowski theorem demand…

> It's a very subtle thing to explain the pressure differential of a non moving airfoil

I'm not sure that's a real thing that happens in real life or theoretical physics.

If the airfoil isn't moving, is anything happening? Or are you talking about a non-moving airfoil with air moving around it? It's impossible to tell the difference between a non-moving airfoil with air moving around it and a moving airfoil with still air around it, given that they're equivalent provided you define the reference frame correctly.

Re: The mysteries of aerodynamic lift

#108

Post by mannykannot »

nimish wrote: post #103
mannykannot wrote: post #90

Earlier quoted context omitted.

It is more like that there is a hierarchy of explanation. There is nothing going on that cannot be explained by Newton's laws, conservation of energy does not need to be introduced as an extra constraint, and Bernoulli's law is itself explained by Newton's laws. The issue is that once you recognize that lift is the reaction to accelerating the airflow downwards, you still don't know how the air moves around the wing,…

> The issue is that once you recognize that lift is the reaction to accelerating the airflow downwards You need bernoulli to explain why the flow field is changed beyond just the area in contact with the flow. This induces the measured pressure differential, explaining part of lift along with the reaction effects of deflected flow for momentum conservation (NS, Newton's 2nd law). It's simply not _enough_ to say that…

No, you do not need Bernoulli, it is merely convenient (but only if compressibility is not an issue, which it is, of course, for cruising airliners as well as supersonic aircraft.) Bernoulli does not give you the velocity field. If you are looking for just one thing that is sufficient, it is Newton's laws applied to viscous fluids - i.e. Navier-Stokes.

I may have made one mistake in that the the separation at a sharp trailing edge may be inevitable simply because of the high acceleration that would be needed to go around it, together with the impossibility of negative absolute pressures, but in more general cases, such as the flow around a sphere, the effect of the boundary layer in triggering separation is important - as it is when we consider a stall, for that matter. The Kutta condition is merely a way of putting trailing-edge separation into Kutta–Joukowski. It is not a law of nature, it is a rule of thumb that makes K-J a realistic and useful model.

Conservation of mass? well, you need that, but who doubts it? You mentioned conservation of energy, and I simply pointed out that you do not need it as an additional constraint, whether or not you are using Bernoulli.

Re: The mysteries of aerodynamic lift

#109

Post by mike_ivanov »

phdp wrote: post #98
mike_ivanov wrote: post #95

Earlier quoted context omitted.

> the fundamental reason airfoils in boats and places work the way they do requires viscosity There is no viscosity in solar wind, yet the solar sail is expected to work. EDIT: s/solar wind/solar radiation/

A solar sail doesn't work via aerodynamic lift. It works on conservation of momentum. Also, the biggest contributor to force on a solar sail is not solar wind, but radiation.

Right, radiation -- thank you. Nevertheless, you think that conservation of momentum is not the ultimate source of aerodynamic lift? It's not an electromagnetic phenomenon, obviously, neither gravitational -- so it has to be mechanical. Where that energy is otherwise coming from? Or are you claiming that aerodynamic lift is a fundamental force?

Re: The mysteries of aerodynamic lift

#110

Post by mike_ivanov »

H8crilA wrote: post #39

I'm surprised people don't start with the basics on this confusing topic. The third law of Newton's mechanics tells us that for the plane to get an up force to counteract the gravity, the air must receive and equal amount of down force. Therefore what planes must be doing is deflect air masses down. A plane must be applying a downward force to air masses, with total force value of "mass * g", i.e. supply "mass * g *…

I also don't get it. People talk about aerodynamic lift as of a magical force, somehow different from regular physical matter interactions. Just exclude what it obviously is not (strong force, gravitation, etc), and what is left has to be it.
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