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The mysteries of aerodynamic lift

scientificamerican.com

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Re: The mysteries of aerodynamic lift

#161

Post by benhurmarcel »

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

While that's correct, that doesn't answer at all the question of how the wing pushes the air downwards.

It's like answering "how does a lightbulb work" by saying you put current in and it produces photons.

Re: The mysteries of aerodynamic lift

#162

Post by savrajsingh »

The way I think about lift is that at a certain point, the air can’t get out of the way fast enough, so lift is created. If you’ve ever skydived, you know the air at high speeds feels thick —- slight deflections and imbalances produce drastic movements.

Re: The mysteries of aerodynamic lift

#163

Post by mikorym »

> There is little, if any, serious disagreement as to what the appropriate equations or their solutions are. The objective of technical mathematical theory is to make accurate predictions and to project results that are useful to aeronautical engineers engaged in the complex business of designing aircraft.

> But by themselves, equations are not explanations, and neither are their solutions. There is a second, nontechnical level of analysis that is intended to provide us with a physical, commonsense explanation of lift. The objective of the nontechnical approach is to give us an intuitive understanding of the actual forces and factors that are at work in holding an airplane aloft. This approach exists not on the level of numbers and equations but rather on the level of concepts and principles that are familiar and intelligible to nonspecialists.

It's funny to read this after many years of teaching myself to think "mathematically". The situation described above is in some ways the true mathematical ideal: to be able to describe our surroundings so precisely that intuition and perception blurs away, giving way instead to something stronger than what we can describe in human words. When you describe something to two different people, the objective is to adapt each explanation to their personal framework, much in the way an artist or musician would adapt to their audience.

However, the objective of mathematics is to take away exactly that: the variability in perhaps equally valid social or human explanations.

I do think that intuition and perception is a very important of mathematics, but I think it forms the first steps of the scientific method behind mathematics. Identify perceptions and intuitions and after that try to get to precision and rigour.

Mathematics aims not to be intelligible, but it is in fact a fortunate state of affairs that mathematics is intelligible to humans at all! It reminds me of what Eugene Wigner wrote about mathematics, titled The Unreasonable Effectiveness of Mathematics in the Natural Sciences. [1]

One thing that should be more clearly stated in the article is that it esentially claims that the mathematical descriptions themselves seem somewhat incomplete, the explanation of which I don't know, as the article tries to avoid mathematics in the first place.

[1] http://www.dartmouth.edu/~matc/MathDrama/reading/Wigner.html

Re: The mysteries of aerodynamic lift

#164

Post by mannykannot »

nimish wrote: post #154
mannykannot wrote: post #140

Earlier quoted context omitted.

In my previous reply, I overlooked this sentence, which gets to the heart of the misunderstanding: > 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_. Given a situation where Bernoulli is applicable (steady-state flow and inignificant compressibility effects), if you were to measur…

Not quite. You can't just derive the equation of state that's equivalent to Bernoulli's principle from Newton's laws. Only for incompressible flow is it a consequence of Newton's second law (NS) See https://en.wikipedia.org/wiki/Bernoulli%27s_principle#Deriva... -- the broader (applies to compressible flow too) derivation is via conservation of energy and mass

That is a fair point, but to be clear, it does nothing to rehabilitate the notion that Newton and Bernoulli provide independent components of lift that have to be added (or, for that matter, that one is right and therefore the other is wrong, which is another common misunderstanding that has shown up elsewhere.)

Re: The mysteries of aerodynamic lift

#165

Post by p_l »

glofish wrote: post #128
Rury wrote: post #54

Earlier quoted context omitted.

>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 Bernoulli's principle is often misunderstood. The principle itself doesn't say anything as to why different speed effects are observed around airfoils. It only states that within a steady state of fluid flow, increases in speed are associated wit…

my understanding is that Bernoulli's principle also involves equal transit times. Meaning that the same two positions need to rejoin later. When the sail has no thickness the outer flow cannot go faster and still be a valid Bernoulli effect, because the outer and inner paths have the same distance. That being said, there might still be another similar principle at work, though technically speaking it should not be ca…

The "Equal transit time" is one of the "lie to children" explanations of lift. In fact, classic wings depend on inequal speed caused by a vortex behind the wing - which is also an oversimplified model. Such "bootstrap" vortex causes an opposite vortex to form around the wing, giving difference in speed of air on both sides. Once you have the airflow in the right way, Bernoulli's principle gives you lift.

That's the very simplified version of a simplified explanation used in aviation teaching materials :)

Re: The mysteries of aerodynamic lift

#166

Post by p_l »

Sharlin wrote: post #80
o_nate wrote: post #72

Earlier quoted context omitted.

The obvious follow-up question is : why does a wing deflect the air down?

Why it deflects air below the wing downward is trivial: it is an inclined plane (the angle of attack is nonzero) Why it also deflects air above the wing downward – pulling the air down so to speak – is more complicated.

An inclined plane provides enough force... at ridiculous speeds - generally you need to start with either very, very light vehicle, or supersonic speeds.

The angle of attack is important in kickstarting and keeping running various phenomena that ultimately result in pressure differential between both sides of the lifting body (be it wing or other shape).

Re: The mysteries of aerodynamic lift

#167

Post by jvanderbot »

lutorm wrote: post #85
jvanderbot wrote: post #49

Earlier quoted context omitted.

Yes, and in rotating-wing configurations (e.g., helicopters), the lift is basically calculated using the momentum of the column of air being forced downward + the momentum of the chassis. By accelerating air downward in a column below the rotating wingspan, the column of air gains a net negative momentum (downward), necessitating a net positive (upward) momentum to the chassis to keep the system's momentum conserved.…

Sure, that's just momentum conservation. But how do you know how much downward momentum the column of air has?

This may help. It's a guide to the equation describing lift as a function of blade angle of attack, air density, and true airspeed.

https://sciencing.com/calculate-lift-rotor-blades-7680704.ht...

Re: The mysteries of aerodynamic lift

#168

Post by p_l »

The article is... bad. Very bad. Doubleplusungood bad, in fact.

I recommend checking out https://www.grc.nasa.gov/www/k-12/airplane/lift1.html which is much more complete explanation, and written for school age children - without the usual "lie to children" part that is common till you hit fluid dynamics at university level.

Re: The mysteries of aerodynamic lift

#169

Post by [deleted] »

dhxjncdggxghbh wrote: post #151
Retric wrote: post #35

Earlier quoted context omitted.

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…

Symmetric airfoils (which do exist) are considered to have a centerline in the obvious (symmetric) place, but this doesn't have anything to do with angle of attack. For various reasons, aerobatic planes sometimes have symmetric airfoils with an angle of incidence of zero—that is, the aircraft centerline and airfoil centerline are parallel. https://aviation.stackexchange.com/questions/7560/what-are-t... Angle of attac…

[deleted]

Re: The mysteries of aerodynamic lift

#170

Post by Retric »

dhxjncdggxghbh wrote: post #151
Retric wrote: post #35

Earlier quoted context omitted.

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…

Symmetric airfoils (which do exist) are considered to have a centerline in the obvious (symmetric) place, but this doesn't have anything to do with angle of attack. For various reasons, aerobatic planes sometimes have symmetric airfoils with an angle of incidence of zero—that is, the aircraft centerline and airfoil centerline are parallel. https://aviation.stackexchange.com/questions/7560/what-are-t... Angle of attac…

I am saying the choice of centerline for an airfoil is arbitrary in terms of aerodynamic forces. The angle of incidence is then based on that arbitrary centerline. The aircraft’s angle of attack is independent of the above and therefore confusing.

To be clear I am not saying how the wing is mounted arbitrary, just what angle is written down in the documentation. Chose a different centerline and that angle changes even as the aircraft stays the same.

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