Lift: Probably my last question

[superyuby]

Maybe there are some questions like this... But I would like to listen to your answers... Thank you!

As many people say, there are many ways that describe incorrectly the generation of lift. Two models survive today which are supposed to explain this aerodynamic force, or at least, a part of it: the Newton and the Bernoulli model. Both models are widely spread today among universities.

You can get deeper in the topic understanding the Kutta-Joukowski Theorem, and the role of the circulation in this. But I think this is a less intuitive way to understand it and (as Bernoulli), it requires some assumptions (potential flow...), so I think the best way to explain this effect is the Newton explanation, as Anderson thinks too, just because we only use the third law as an axiom.

Every one of these explanations require the flow to curve, due to the body shape or due to an angle of attack (as in flat plate). My question deals with paper planes. How does the lift generate? Where does it come from?

The wings of a paper plane are approximately flat plates, so they require a positive angle of attack to produce lift.

Some explanations I have heard:

- You must throw the paper plane with an angle of attack so as to get the lift produced.
I don't agree with this. I (and I'm sure you too) have seen paper planes flying with a zero angle of attack, with both the aerodynamic velocity and wings parallel between them. The classic paper airplane can make good flights parallel to the ground, with its wings with a zero angle of attack.

- When the paper plane is in the air and it starts to fall, the induced velocity creates an angle of attack (that is one of my professor's explanations).
This is an interesting question, but I don't agree neither. When we see a paper plane flying, it flies in an straight trajectory, it doesn't need to fall to create the lift, so there is no induced velocity (and no induced angle of attack).

- The angle of attack is not zero despite of throwing it with zero angle of attack. This is due to the relative position of the mass center and the pressure center. The aerodynamic forces create a positive pitch coefficient and then the wings get an angle of attack.
I don't agree with this question due to the same facts explained before. I have seen paper planes with zero angle of attack (and without any pitch moment) flying.

You can search for some answers (http://aviation.stackexchange.com/q...airplanes-create-lift-if-their-wings-are-flat, http://www.scientificamerican.com/article/bring-science-home-paper-planes-drag/), and even some scientifical papers:

http://msc.tsinghua.edu.cn:8090/down/1.pdf
http://www.enu.kz/repository/2009/AIAA-2009-3958.pdf

In these both papers, there are some results in the Cl vs AoA graph that show that with a zero angle of attack, there is a small amount of lift created (Cl>0).

So, I think I have explained my question the best I can. Can a flat plate with a zero angle of attack (and by extension, symmetric airfoils) create lift in some manner? We have always heard (and studied, some of us) for ages, that for a symmetric airfoil with a zero angle of attack is impossible to create lift.

Of course, there is an explanation of how the lift works in a paper plane based on turbulence (it has to do with vortex created in the leading edge and the lower pressure in the extrados), but I am looking for a more wide explanation (like Newton or Bernoulli). If there is no change in the direction of the flow, why does a paper plane fly? In which way I'm wrong?

Thank you everyone for reading such a long post!
And of course... Sorry for my English! I would like to explain this topic as well as using my mother language.

Pd.: I would like you to show me the best explanation of generation of lift you have ever heard. It may help me.

 

[berkeman]

Have you read the PF Insights article on lift? https://www.physicsforums.com/insights/airplane-wing-work-primer-lift/

 

[russ_watters]

I think it is important to note:
1. You can't measure the aoa of a paper airplane accurately.
2. Since paper is light, it takes a very small amount of lift.
3. A paper airplane is not an airplane, it is a glider. So it cannot maintain level flight. So its attitude may be pointed down, but that doesn't necessarily indicate a negative aoa.

And all of that is accepting your premise that paper airplanes have flat plate airfoils, which may depend on the airplane, but I would generally disagree.

 

[CWatters]

+1 to what Russ said.

And no it's not possible for a perfect flat plate to produce lift at zero AOA. But very few paper planes are perfect.

Aside: some cambered sections can even generate positive lift at small negative AOA but not a flat plat section.

 

[boneh3ad]

The explanation can really be any of your three proposed explanations that you seem to not accept for some reason. You could thrown a paper plane with an angle of attack and it would generate lift. Of course it would also generate drag, and without propulsion, it will likely lift up, stall, and then come crashing down. Also, depending on the design of the paper plane (there are many possible means of folding paper planes) you can certainly have some whose natural positions are pitch-up.

Really, though, angle of attack is, for the purposes of lift, defined as the angle the lifting surface makes with the free stream. In this case, paper airplanes are gliders and tend to lose altitude while the fly. If the wings are at zero angle of attack relative to the ground, then they still have a positive angle of attack relative to the air since they are moving forward and down (meaning the air is moving backward over them and upward against them in the wing's frame of reference). That is how paper airplanes (and gliders) work.

To your more general question: no, a symmetric airfoil at zero angle of attack cannot produce lift, at least without an external influence of some sort. More generally, a symmetric flow field cannot generate lift on a surface. There are means of doing it using various flow control techniques such as surface blowing/suction or plasma actuators or other methods, but then that breaks the more general symmetry rule I just proposed, so it still works.

 

[ImStein]

I've reviewed a number of interesting PF threads on lift, including the Insight/FAQ. Most are closed to further comment though some is warranted. I admit a strong bias to the Bernoulli explanation but will accept downdraft (i.e. Newton's 3rd) as equivalent whenever it's apparent. Mostly, I have fun thinking about and experimenting with lift.

And no it's not possible for a perfect flat plate to produce lift at zero AOA.

I see this comment a lot and must disagree. Lift from a wing with zero camber and zero angle of attack is commonly observed with a Bernoulli grip. In fact that lift arises from a continuous positive pressure source above the wing!

What's more, the most efficient form of lift derives from zero camber, zero angle of attack and zero net flow! This is achieved by ignoring Bernoulli's restriction to steady flow (DC) and instead relying on his equation's squared velocity term to employ periodic flow (AC) as I demonstrate below. Both methods also take advantage of zero flow beneath the wing.

 

[CWatters]

I see this comment a lot and must disagree. Lift from a wing with zero camber and zero angle of attack is commonly observed with a Bernoulli grip. In fact that lift arises from a continuous positive pressure source above the wing!

but a Bernoulli grip is a very different set up to what's being discussed in this thread.

 

[russ_watters]

This thread is old and the new posts are not on topic. Closed.