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The mysteries of aerodynamic lift

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Re: The mysteries of aerodynamic lift

#141

Post by starpilot »

munificent wrote: post #137
starpilot wrote: post #131

Earlier quoted context omitted.

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.

Yes there is. Suction on upper surface to maintain attached flow -> lower pressure -> increased speed to conserve momentum. Bernoulli's relationship holds true along a streamline, equal transit time doesn't.

Re: The mysteries of aerodynamic lift

#142

Post by [deleted] »

knolan wrote: post #126
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…

Viscosity is responsible for drag via the boundary layer and flow separation. You can absolutely account for lift with an ideal inviscid fluid, tools like Xfoil with inviscid solvers accurately match experimental data but ignore stall. It’s a pressure effect, see my other post in this thread. The viscous solvers can handle drag, separation and stall.

[deleted]

Re: The mysteries of aerodynamic lift

#143

Post by numpad0 »

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…

> Thus if you instead lay that teardrop so the bottom is flat

The thing is, both sides of a wing are defined by some nonlinear polynomials, and aft edges are sharp. Machinists don’t/can’t use such surfaces for any reference purposes so centerline can’t be there.

Re: The mysteries of aerodynamic lift

#144

Post by GuB-42 »

copperx wrote: post #136
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?

tl;dr just point the nose up.

The thing is that what really matters is the orientation of the trailing edge. And in airplanes the trailing edge points downwards. It is not always obvious because of the way wing profiles are designed but it is almost always the case. We could have it level, but that would mean the plane will need to always fly nose up in order to create lift, it would be uncomfortable, and most likely not ideal from an aerodynamic perspective.

When you are flying upside down the natural orientation of the wing goes the opposite way, so the leading edge points up, and indeed you can't fly level with the nose pointing straight ahead. You need to point the nose up in order to compensate the natural wing orientation, and then point up even more to create actual lift.

Of course, there are some more advanced considerations. Performance will generally be reduced when flying upside down, because the wings are not designed to do so. However some wings have a symmetrical profile, and in theory, they could be flown equally well in any orientation.

Re: The mysteries of aerodynamic lift

#145

Post by mike_ivanov »

glofish wrote: post #139
mike_ivanov wrote: post #132

Earlier quoted context omitted.

> 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 = 1/2 I w^2, or elastic energy E=1/2 k x^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…

> nuclear energy

I beg your pardon?

> does not mean that momentum "is" energy by any interpretation

I never said that. You literally said "momentum has nothing to do with energy", and I gave you one example where they are directly related.

> can you transform a linear momentum into another kind of momentum

Yes, you can. This is exactly why I mentioned yo-yo.

> momentum is a vector and ... energy is a scalar and conserves as a scalar.

That's a good point, it's 100% correct and I'm not arguing with that. But I insist that saying that they are unrelated is still wrong.

Let's go back for a second to where we started. My claim was (and still is) that the only source of aerodynamic lift is the kinetic energy of the air molecules acting on the airfoil. There is just nothing else, after all. This is not conceptually different from how [solar] sail works. Now, in this specific case, the energy of particles acting on the air/solar foil is directly related to their momenta.

So, what are we actually arguing about?

Re: The mysteries of aerodynamic lift

#146

Post by antod »

munificent wrote: post #137
starpilot wrote: post #131

Earlier quoted context omitted.

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.

There are other ways to expect differences in velocity. A bit like the venturi effect in a pipe constriction, a constant flow rate will have to involve faster velocity when the area of flow reduces.

If you trace a line on the wing cross section between the stagnation point and the trailing edge you will see the upper surface restricts/squeezes the flow more than the lower surface. So with a constant flow rate, the upper velocity will be higher even without equal transit.

Re: The mysteries of aerodynamic lift

#147

Post by glofish »

mike_ivanov wrote: post #145
glofish wrote: post #139

Earlier quoted context omitted.

Energy can just as well be E = mgh (potential energy) or rotational energy E = 1/2 I w^2, or elastic energy E=1/2 k x^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…

> nuclear energy I beg your pardon? > does not mean that momentum "is" energy by any interpretation I never said that. You literally said "momentum has nothing to do with energy", and I gave you one example where they are directly related. > can you transform a linear momentum into another kind of momentum Yes, you can. This is exactly why I mentioned yo-yo. > momentum is a vector and ... energy is a scalar and conse…

> I beg your pardon?

Not sure what could be unclear there at all. Nuclear energy can be turned into any other energy and vice versa. As long as something has mass it has energy - whether or not we can readily transform that is beside the point.

Your recurring yo-yo example only demonstrates that you don't understand the physical phenomena in the first place. The linear momentum is conserved when the yo-yo pulls on your hand and through that your body, you are either pushing or pulling on Earth via gravitational force, the Earth wobbles opposite of the yo-yo (albeit infinitesimally, thankfully). That's the conservation of the momentum.

The rotational energy of the yo-yo has nothing to do with the linear momentum, that rotation comes from the chemical energy of your muscles that have first lifted, pulled or tossed the yo-yo. With that, you have transformed chemical energy stored in your muscles into the rotational energy of the yo-yo. Momentum has nothing to do with energy. Just because both exist and both get conserved. It is quite profound actually that they are not related at all.

As for this discussion we were talking about you conflating momentum with energy, a common misconception actually, your reticence of even remotely entertaining the idea that you did indeed misuse these concepts diverted into a lengthy discussion that slowly drifted away from the actual points to flawed analogies and yo-yos - also not surprising and a common predicament

Why am I still replying? Because it demonstrates why it is so hard to discuss flying (the very point of the original post) the majority of participants conflate and misuse scientific concepts - then go onto lengthy roundabouts to avoid owning up to these mistakes.

Re: The mysteries of aerodynamic lift

#148

Post by glofish »

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.

Perhaps because it is always drawn that way, that the same streamline gets separated then goes over the wing, then goes back together. Then in big letters it says Bernoulli principle.

Now that I read about it more, it looks like Bernoulli's principle is just conservation of energy - and as you say, there are no other requirements.

Re: The mysteries of aerodynamic lift

#149

Post by taneq »

luckyscs wrote: post #52
WalterBright wrote: post #38

Earlier quoted context omitted.

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…

It's like our brains aren't meant to handle resolving that not only is it pushed through, it's sucked into a thin and ever moving void. It's pushed and pulled at the same time, in otherwords, part of the continuum.

Suction isn't a real thing, though. It's just our description of when lower pressure on one side of a thing allows the thing to be moved by the higher pressure on the other side.

The pressure that the air above the wing applies to the top of the wing is lower than the pressure that the air below the wing applies to the bottom of the wing. The net force is upwards.

Re: The mysteries of aerodynamic lift

#150

Post by redis_mlc »

thrustmaster wrote: post #88
zarmin wrote: post #3

Earlier quoted context omitted.

Wright you are.

Really? I got a few down-votes... I also know the angle of attack of the wing is important, no angle of attack, no lift?

In aviation, we consider deflection and Bernoulli's principle as both important.

In physics, it's mostly about pressure differentials.

When you try to get more specific than that in a forum comment, it means you probably don't really know what you're talking about.

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