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The mysteries of aerodynamic lift

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Re: The mysteries of aerodynamic lift

#131

Post by starpilot »

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…

It isn't at all. Bernoulli never asserted that particles are bound in any way, he just discovered a relationship between static and dynamic pressure. I don't know why people apply his name to the equal transit time theory of lift; there is no real association.

Re: The mysteries of aerodynamic lift

#132

Post by mike_ivanov »

glofish wrote: post #130
mike_ivanov wrote: post #109

Earlier quoted context omitted.

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?

conservation of momentum and conservation of energy both take place at the same time, but are independent concepts altogether momentum has nothing to do with energy and vice versa. For example, energy can be stored (transformed into different forms, short or long term) and released later, momentum cannot.

> momentum has nothing to do with energy and vice versa.

o_O In a mechanical system Ek = p^2/(2m), where p is momentum. Please clarify if you meant something else.

> energy can be stored ..., momentum cannot.

Have you ever seen a yo-yo?

Re: The mysteries of aerodynamic lift

#133

Post by starpilot »

rtkwe wrote: post #129
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…

I've always wondered since learning slightly more in college physics. What is supposed to make the air going over the top of the wing suddenly go faster than the air going over the bottom of the wing in the Bernoulli model?

Lower pressure region sucks the flow down to attach to the upper surface. As the pressure is reduced, speed must increase to conserve momentum.

Re: The mysteries of aerodynamic lift

#134

Post by starpilot »

lutorm wrote: post #112
H8crilA wrote: post #93

Earlier quoted context omitted.

Airplane/helicopter mass * g * time Assuming the plane/helicopter didn't accelerate up or down and assuming there's no wind.

Sure, that has to be true if the helicopter can hover. But how do you know that's possible? You can't postulate that something flies as part of an explanation why it can fly...

You can solve for the lift generated by each rotating blade. Then it's roughly the same calculating the lift for a fixed wing, except for the freestream flow speed varying along the span. The lift is different at each spanwise location, so you integrate to get the total lift for one blade, then multiply by the number of blades. Numerous ways to do this, but the simplest accurate way is via lifting-line theory. The resulting system of equations is solved iteratively.

Re: The mysteries of aerodynamic lift

#135

Post by loco5niner »

WalterBright wrote: post #38
simonebrunozzi wrote: post #27

Earlier quoted context omitted.

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

Take a drafting triangle that has 30-60-90 degree corners. Place it between two objects, and squeeze the triangle between them. It'll move to the side faster than the two objects move together. Or you can just think of it like squirting toothpaste. The wind pressure on the sail and the water pressure on the keel form the two "objects" being pushed together and the sailboat "squirts" out the side. Edit: The angle betw…

Kind of like when your car splashes into a puddle and somehow some of the water drops make it onto your windshield.

Re: The mysteries of aerodynamic lift

#136

Post by copperx »

Geee wrote: post #61

I think it's quite easy to understand how planes fly. I figured out this as a kid when I pushed my hand out of a car window and tilted it at different angles. This way you can feel the pressure differential and the how the air pushes the hand upwards or downwards.

So, if a plane wing is like a tilted hand pushing air down, how come a plane can fly upside down with no change to its wings?

Re: The mysteries of aerodynamic lift

#137

Post by munificent »

starpilot wrote: post #131
glofish wrote: post #128

Earlier quoted context omitted.

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…

It isn't at all. Bernoulli never asserted that particles are bound in any way, he just discovered a relationship between static and dynamic pressure. I don't know why people apply his name to the equal transit time theory of lift; there is no real association.

If you don't assume equal transit, then there's no reason to expect the particles above the wing to move faster than those below. And without that, there's no reason Bernoulli's principle would come into play at all.

Re: The mysteries of aerodynamic lift

#138

Post by averros »

The article is pretty much junk because it completely misses the fact that the very existence of aerodynamic lift relies on Coandă effect (i.e. airflow "sticking" to a smooth surface) which in turn is the consequence of viscosity (very close to surface air molecules aren't moving relative to the surface, and far they're moving fast, so viscosity creates a transition layer with different speeds, which creates the force pushing the molecules towards the surface, which results in vertical motion of air downwards, which creates the opposite force (i.e. the lift) on the surface by Newton's 3rd law). The very same effect can be seen as air being pulled down above the wing, resulting in smaller pressure which "sucks" the wing up. Oh, and because air molecules need to fill the space they need to accelerate horizontally - thus resulting in behavior seen as Bernoulli's law.

The two basic explanations of lift are not alternatives; they're simply the different aspects of the same process. It's just that Bernoulli's law is not fundamental, it's a consequence of viscosity and Newton's laws of motion.

A propos: there are fluids which have zero viscosity. Wings do not generate lift in them.

See also: https://en.wikipedia.org/wiki/D%27Alembert%27s_paradox

Re: The mysteries of aerodynamic lift

#139

Post by glofish »

mike_ivanov wrote: post #132
glofish wrote: post #130

Earlier quoted context omitted.

conservation of momentum and conservation of energy both take place at the same time, but are independent concepts altogether momentum has nothing to do with energy and vice versa. For example, energy can be stored (transformed into different forms, short or long term) and released later, momentum cannot.

> momentum has nothing to do with energy and vice versa. o_O In a mechanical system Ek = p^2/(2m), where p is momentum. Please clarify if you meant something else. > energy can be stored ..., momentum cannot. Have you ever seen a yo-yo?

Energy can just as well be E = mgh (potential energy) or rotational energy E = 1/2Iw^2, or elastic energy E=1/2kx^2, chemical energy, nuclear energy etc. with momentum nowhere to be found in those formulas. When we talk about energy conservation we mean the conservation across all the forms of energy that the system can take on - that is what gets conserved.

It just happens that in one particular manifestation of the energy, the kinetic energy, can also be expressed with a squared momentum in the formula - but does not mean that momentum "is" energy by any interpretation.

As I said before momentum and energy are completely different concepts altogether - energy can be stored and transformed. Momentum cannot be stored nor can you transform a linear momentum into another kind of momentum.

If not convinced, consider for a moment (pun intended) that momentum is a vector and it conserves (in each dimension) as a vector! - whereas energy is a scalar and conserves as a scalar.

Re: The mysteries of aerodynamic lift

#140

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…

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 measure the airflow velocity and pressure fields around the wing, you would find both that they conform to Bernoulli, and that the pressure summed over the whole wing would account for the entirety of its lift.

Alternatively, if you were to calculate the rate of change of momentum of the entire airflow affected by the wing, you would find that this also accounts for the entirety of its lift. This works even in those cases where Bernoulli is not applicable.

So we have two different approaches to calculating the lift, and summing them would give the wrong answer.

Both approaches can themselves be explained in more fundamental terms, and in both cases, it comes down to Newtonian mechanics.

Neither approach allows us to calculate what the airflow looks like and how the presence of the wing shapes it, so neither approach offers a complete explanation. For that, we need Navier-Stokes, which is also reducible to Newtonian mechanics.

What's wrong with many attempted explanations of lift, by either principle, is that they don't get the details right: they try to simplify the issues to the point where they are simply wrong.

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