Insights How Does an Airplane Wing Work: a Primer on Lift - Comments

[rcgldr]

It isn't really a cause and effect thing. It's two sides of the same coin.

The shape of the airfoil, angle of attack, and free stream velocity could be considered as the causes. As you stated, everything else, pressure gradient normal to and in the direction of flow, change in velocity of the affecte air, ... , are coexistent.

conservation of energy

Using the glider model, from the air frame of reference (no wind), the decrease in gravitational potential energy is offset by the increase in the air's energy (mostly mechanical, some thermal). From the glider's frame of refernce, it's not clear to me.

 

[boneh3ad]

The increase in gravitational potential energy is negligible in a flow like that. It is air, so it has very low density and very low change in height. It's perfectly reasonable to just ignore that effect.

Otherwise, I'm honestly not sure how I can be any more clear. The flow fields in the two frames are exactly the same only offset by the free stream velocity. Forces such as lift are the direct result of the change of momentum, not energy, and the momentum change is identical in each case.

 

[rcgldr]

The increase in gravitational potential energy is negligible in a flow like that. It is air

I meant the decrease in gravitational potential energy of the glider, which is offset by the increase in the energy of the air (since the glider is in a steady descent and not accelerating). It's a 1500 lb glider descending at 1 mph, 4 hp, 550 ft lbs of energy every second. Sorry I didn't make it clear I was referring to the potential energy of the glider.

However, the same issue exists for something simpler, like a parachutist with parachute deployed in a vertical and steady descent. From the ground / air (no wind) frame of reference, the decrease in gravitational potential energy is offset by an increase in energy of the air. From the parachutist's frame of reference ... ? So I'm not sure this was going anywhere.

... but I see your point, the energy issue shouldn't matter, it's the momentum change that corresponds to lift (and drag).

and thanks for creating the primer.

 

[markuzi]

Go here:

https://en.wikipedia.org/wiki/User:Rolo_Tamasi

Lift on a wing is the result of differential radial forces over the wing surface.

 

[Chestermiller]

Part of the issue regarding energy is that, although the forces and the velocities are the same in the two frames of reference (aside from a constant offset equal to the free stream velocity), work is not frame invariant. Fortunately, the net sums of the energy contributions is frame invariant (i.e., the energy balance is satisfied in all frames of reference). The differences in the work are exactly offset by the differences in the kinetic energy.

 

[skrat]

What level of material interests you?

For example, the go-to text for introducing undergraduate engineers to the basic principles of flight is Introduction to Flight by John Anderson. It's pretty expensive but the basics haven't changed so you can certainly look into an older edition or check it out at a library if you have access.

I have finished my first Bologna in Physics and am currently working on a masters degree. My idea is to someday study the very details of fluid dynamics and use my knowledge in flying industry. So, perhaps not the very basic literature, however PHD articles are probably (at the moment) too advanced for me. Something in between would be perfect.
I will for sure check that Introduction to Flight you suggested! Thank you! If you remember any more, don't hesitate!

cheers

 

[boneh3ad]

Go here:

https://en.wikipedia.org/wiki/User:Rolo_Tamasi

Lift on a wing is the result of differential radial forces over the wing surface.

That person's article actually just proposes a different way of calculating the pressures given the velocity field. It is essentially functionally identical to using Bernoulli.

 

[John_RB]

If you get two sheets of A4 (letter) sized paper and hold them parallel with a gap between them and blow into the gap the sheets are sucked together, not blown apart. That's my practical demo or where lift can come from just by the flow of air

 

[boneh3ad]

If you get two sheets of A4 (letter) sized paper and hold them parallel with a gap between them and blow into the gap the sheets are sucked together, not blown apart. That's my practical demo or where lift can come from just by the flow of air

The problem with that approach is that you can't directly use something line Bernoulli's equation to compare the two streams of air (between the papers and outside of them) because they originate from reservoirs with different total pressures.

It turns out that human lungs are only typically capable of producing on the order of 2 psi of pressure above atmosphere. It just works out, then, that if you use Bernoulli's equation on the air in your lungs separately from that outside the papers, that the stream of air between the papers still may end up with a lower static pressure than ambient. If you had an air source with higher pressure blowing between the papers, that wouldn't necessarily be true and very well could push the pages apart.

 

[markuzi]

If you get two sheets of A4 (letter) sized paper and hold them parallel with a gap between them and blow into the gap the sheets are sucked together, not blown apart. That's my practical demo or where lift can come from just by the flow of air

If the venturi effect is applied to the lift on a wing, then an aircraft would never get off the runway because the venturi airflow under the wing would cause the aircraft to be sucked downward to the ground. This is obviously not the case.

The fact that an aircraft takes off from the runway and flares on a cushion of pressure upon landing is evidence the venturi effect does not cause lift.

Anyway ... has anyone actually seen this mythical venturi around a wing.

 

[DoobleD]

Awesome article. I encountered the NASA pages a while ago and was completely amazed when I discovered that lift is often explained using wrong arguments. I then got completely lost in a ton of different explanations of lift. This article cleared some things.

 

[boneh3ad]

Awesome article. I encountered the NASA pages a while ago and was completely amazed when I discovered that lift is often explained using wrong arguments. I then got completely lost in a ton of different explanations of lift. This article cleared some things.

Excellent to hear that. It's certainly not perfect, though, and I am open to editing it and/or writing a follow-up if more misconceptions arise.

 

[DoobleD]

Excellent to hear that. It's certainly not perfect, though, and I am open to editing it and/or writing a follow-up if more misconceptions arise.

For me, there are two parts I didn't completely understood :

- the one circumstance where the venturi effect could actually work (I didn't see the difference with the other venturi case described above this part)
- the explanation of why the flow is deflected downward

However it's important to say that I am not always comfortable with English language. Neither I did a lot of fluid mechanics. Also I understand that to really understand the downward flow wouldn't be that easy. That is to say the article might be actually crystal clear for most people, I couldn't tell.

 

[lomidrevo]

Many thanks for this article, it helped me a lot to understand the topic!
However, I think there is missing ρ (density) in the last term of your first equation: gz. (When I compare the units, I miss kg/m³ in this part of equation)

 

[boneh3ad]

Many thanks for this article, it helped me a lot to understand the topic!
However, I think there is missing ρ (density) in the last term of your first equation: gz. (When I compare the units, I miss kg/m³ in this part of equation)

Good catch. I can't believe that's been hanging out like that for a year and a half with no one noticing.

 

[Robert M]

Vorticity is so much more productive than primitive variables in explaining and analyzing low Mach number flow, and it fits naturally with the actual physics of a standard airfoil.

https://en.wikipedia.org/wiki/Kutta–Joukowski_theorem

For potential flow--no vorticity, or, equivalently, the curl of velocity is everywhere zero--there would be no lift and no drag, so the introduction of vorticity is more than just a pedagogic nicety or a pedantic detail.

https://physics.stackexchange.com/questions/46131/does-a-wing-in-a-potential-flow-have-lift

I suspect that the reason that elementary discussions of lift lean so heavily on primitive variables, which are themselves problematical for this problem, is that actually using vorticity to do calculations introduces a fair bit of mathematical baggage.

 

[Joseph M. Zias]

Suggest looking at the web book: See How It Flies, by Dr. John Denker, a physicist and pilot.

https://www.av8n.com/how/

 

[Robert M]

A different thread on lift offers a talk from someone who doesn't underestimate the problem. In the end, if you stay with the talk that far, the difficulties of even the best discussions of lift don't become apparent until you examine whether or not integrals over reasonably obvious control volumes converge.

 

[boneh3ad]

Look, guys. This is a primer on lift, not a treatise on lift. Are there more sophisticated ways to describe it? Of course. Do you think the average person who is still bickering back and forth about Bernoulli versus Newton or who believes in the equal transit time fallacy is going to appreciate the concept of vorticity, the Kutta-Joukowski Theorem, D'Alembert's Paradox, or conformal mapping? Somehow I doubt it.

The point here is to illustrate that both of those concepts are equally related to lift and neither is any more correct than the other. I was also trying to do so in a way that the average person visiting this site who isn't likely to be familiar with vorticity transport would understand it reasonably well.

 

[Robert M]

Look, guys. This is a primer on lift, not a treatise on lift. Are there more sophisticated ways to describe it? Of course. Do you think the average person who is still bickering back and forth about Bernoulli versus Newton or who believes in the equal transit time fallacy is going to appreciate the concept of vorticity, the Kutta-Joukowski Theorem, D'Alembert's Paradox, or conformal mapping? Somehow I doubt it.

The point here is to illustrate that both of those concepts are equally related to lift and neither is any more correct than the other. I was also trying to do so in a way that the average person visiting this site who isn't likely to be familiar with vorticity transport would understand it reasonably well.

One possible purpose of a discussion like this is to clear the air. Mathematical fluid mechanics is a very mature discipline, but, in order to understand what has been concluded and how it has been concluded, you need some mathematics beyond what many will be unwilling to commit the time to learning. Even if many will not be interested in following the details, it would be a good idea if everyone with a professional stake in the problem understood that the details exist.

 

[boneh3ad]

Sure, but one of the most important things to learn as a researcher and educator is to know your audience. No matter how beautiful or complete the theory, it is useless if your target audience does not understand it.

 

[Robert M]

Sure, but one of the most important things to learn as a researcher and educator is to k ow your audience. No matter how beautiful or complete the theory, it is useless if your target audience does not understand it.

Your audience included me. I didn't write an article. I made a comment. The idea that truth (or whatever you choose to call it) should depend on an audience's tolerance for detail or for being told that the necessary detail may be beyond any likely audience, is--in my opinion--dangerous.

The mathematical theory of lift is solid, but you need some advanced tools that many learn in elementary electricity and magnetism. If vorticity and circulation are beyond an audience, then so are the Biot-Savart Law and electric currents as a source of the magnetic field.

 

[boneh3ad]

I didn't say they were beyond any audience. I said they were beyond a large chunk of the target audience. The purpose was a brief introduction to lift and to sort of quell a very common argument that erupts on this site about lift. The purpose was not to lay out a complete mathematical description of aerodynamic forces. The present article serves it's intended purpose. Feel free to write a more complete Insight on lift if you desire, but that wasn't the point here. A line had to be drawn somewhere for a short article.