/sci/ - Science & Math

>>12207410
>Airplanes: how do they work? If you think that the air goes a longer distance on the top of the wing and the resultant lower pressure exerts a buoyancy force on the airframe, then how come airplanes can fly upside down? Also magnets. How do they work?

Disparity/pressure of the medium already present.

>>12207598
Airplanes work due to conservation of momentum. If the air isn't moving down, and then it gets forced down by the angle of attack of the wing, the airplane has to go up or else the downward momentum of the air deflected by the wing would be an anomalous momentum. In terms of Newton's laws, if the wing is exerting a force on the air to make it go down, then the air is exerting an equal and opposite force on the wing. This is the lift force and the plane will fly is the lift force is greater than or equal to the gravity force.

>>12207669
This. Cool pic also.

They can only fly upside down of they can change their angle of attack, which is mostly only true for planes of flight artists.

>>12207410
Theres no such thing as "flying". You're simply being attracted to mass via gravitational forces in a fluid medium, better known as "falling"

>>12207410
>If you think that the air goes a longer distance on the top of the wing and the resultant lower pressure exerts a buoyancy force on the airframe, then how come airplanes can fly upside down?
Because that's not how aerodynamic lift works
>>12207669
>it gets forced down by the angle of attack of the wing
That's an oversimplification, to the point it's wrong.

Aerodynamic lift is generated by a difference in speed of the air traveling on the upper and lower side of an airfoil, which translates in a difference in pressure which results in a net force. Such difference can be seen as a vortex surrounding the airfoil. Said vortex is generated by the geometry of the airfoil, in particular the pointy end at the rear: due to air viscosity when the airfoil starts moving it generates a vortex that detaches from the airfoil itself and due to the conservation of momentum an equal and opposite vortex is then generated around the airfoil itself. You can see what I just said in this visualization:
https://www.youtube.com/watch?v=VcggiVSf5F8

>>12208162
Also when inverten the aileron in the back is fighting the downward force from the wings.

>>12208322
>Aerodynamic lift is generated by a difference in speed of the air traveling on the upper and lower side of an airfoil, which translates in a difference in pressure which results in a net force
That's just plain wrong. I know it's what they always taught but we know now that it just isn't true.

>>12207410
You want to know how magnets work because you want to dodge any work in universe possible?

>>12208908
>That's just plain wrong.
>I know it's what they always taught but we know now that it just isn't true.
>no further explanation provided
>source: trust me bro
Lift is the result of a net imbalance in the pressure distribution around the airfoil. In incompressible inviscid flows this is always associated to a velocity field around the airfoil that can be summarized without loss of accuracy in "on the upper side air is faster than the air on the lower side".
Source: I'm a fucking aerospace engineer, trust me bro.

>>12209254
>I'm a fucking aerospace engineer
Aerospace engineering PhD student here. I repeat, this has been taught in the past but was shown to be incorrect.
You'll see for yourself when answering this question: why can planes fly upside down?
I'm not saying the effect doesn't exist. It's just not the effect that makes planes fly.

>>12209467
What? I have gotten my PPL 10 years ago and we were taught Venturi theory. I know a guy who even tattooed lift formula on his peck lol

>>12209467
>Aerospace engineering PhD student here
>but was shown to be incorrect.
Show the correct theory then, we both have the knowledge to understand each other, don't we?
Personally, I'm talking about the classic thin airfoil theory, in particular the vortex sheet, the strength of which is explicitly expressed in terms of jump of tangential velocity between the two sides of the airfoil (equation 4.8 at page 336 of the Anderson 6th edition), and once integrated along the chord it gives the total circulation which then translates in lift through Kutta-Joukowsky. Now, I'm perfectly aware this is just a handy mathematical model, but said difference in tangential velocity do exists and Bernoulli still applies.
>why can planes fly upside down?
Because they have an AoA negative enough that the lift points in the useful direction. It doesn't change what I said, the theory is not limited by AoA, within stall limits of course.

>>12209598
https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html
https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html
https://www.grc.nasa.gov/www/k-12/airplane/wrong3.html

>>12209613
None of those three cases are representative of what I've said.

https://www.grc.nasa.gov/www/k-12/airplane/right2.html
>For a body immersed in a moving fluid, the fluid remains in contact with the surface of the body. If the body is shaped, moved, or inclined in such a way as to produce a net deflection or turning of the flow, the local velocity is changed in magnitude, direction, or both. Changing the velocity creates a net force on the body. It is very important to note that the turning of the fluid occurs because the molecules of the fluid stay in contact with the solid body since the molecules are free to move. Any part of the solid body can deflect a flow. Parts facing the oncoming flow are said to be windward, and parts facing away from the flow are said to be leeward. Both windward and leeward parts deflect a flow. Ignoring the leeward deflection leads to a popular incorrect theory of lift.
https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html

This says that my explanation was an oversimplification and only works in certain cases to give the right answer about lift.
>BUT..... this theory is not totally inaccurate. In certain flight regimes, where the velocity is very high and the density is very low, few molecules can strike the upper airfoil surface and the Newtonian theory gives very accurate predictions.

https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/dynamicsofflight.html

So what is the answer to why planes can fly upside down then? Do they have adjustable flaps that reverse the velocity difference?

Ahhh... I think maybe the discrepancy regards flight at a constant altitude versus increasing altitude. Certainly if the plane is not going up, then net momentum vertical momentum of the air has to be zero. So it seems these NASA slides are not referring to the case in which the plane is going up. Is that right?

If the plane is going up, what is going to down to converse momentum?

>>12209770
The point about looking at lift as the deflection of the flow, in my opinion, is that you're looking at the problem from too far away: sure the air gets pushed down and so the wing must be pushed up, but it makes much more sense to look at what happens close to the wing and see how the air changes speed and what pressure distributions are consequence of that, because in the end it's the pressure distribution that IS the lift.

As for the
>certain flight regimes
Yes, it refers to hypersonic regimes (M>5). Re-entry capsules see Mach numbers as high as 30~32 in the upper atmosphere and there you must consider the interaction between body and air in the classical newtonian way. It's really an edge case.

>>12209787
>So what is the answer to why planes can fly upside down then? Do they have adjustable flaps that reverse the velocity difference?
No. Simply when the plane is upside down it must be angled in a way that points the lift in the right direction. Pic related, blue is air speed, green is lift.

No one knows. But they're experimental. They keep the industry going.

>>12209809
Disregard the conservation of momentum of the air, it's misleading. Just look close to the airfoil: on one side the air is faster than the other side (for the reason why see >>12208322), and that creates an imbalance in pressure, with the pressure being lower where the air is faster, which is applied to the wing area creating the lift force. If the lift is greater than the weight, then the plane goes up, if it's lower the plane goes down, if it's equal the plane doesn't change altitude.

>>12207410
>RH Thread
>Tooker
>Planes

Tooker, why are you so interested in airplanes all of a sudden? Please don't tell me you are planning a 9/11.

>>12209920
Not just planes but also flying upside down.
Hmm.

>>12209920
I started playing chess online instead of Nuclear Sim City and I think one of my detractors was saying that because I am a novice at chess it means that he is smarter than me, and he is someone who had a misconception about flight. I got irked by a comment about how if you want to judge whether or not someone is smarter than you, you should play strategy games with them. Obviously playing strategy games measures how much you have played and studied the game far more than anything else, and I brought up this thing about lift as a fuck you because me not knowing about chess is pretty similar to him not knowing about airplanes.

>>12209886
>Disregard the conservation of momentum of the air, it's misleading.
>>12209850
>The point about looking at lift as the deflection of the flow, in my opinion, is that you're looking at the problem from too far away
These both sound like terrible advice. You can find a lot of explanations for mechanical precession online. I am 100% certain all of them are wrong and there definitely exists a misinformation campaign online with respect the nature of mechanical precession. I wonder if lift is also like that. Certainly, "Disregard conservation of momentum," is not good advice. Since fluid mechanics is a lot more complicated than rigid body mechanics, I can't say for sure about the explanations for lift being bullshit like I can with precession but something seems fishy to me here.


>>12209850
Here this pic related does not explain anything. Why would he even put such an obvious explain-nothing diagram

Here's another question: Why does a kite fly? What generates lift on the kite? It can't be the shape of the airfoil because it's just a 2D surface. Clearly this:
>https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html
is the reason why kites generate lift. Why would an airplane generate some other kind of lift?

>>12207410

You just have to look at it with the lens of rocket science, and not fluid dynamics.

Instead of interpreting lift as a force, look at it as the airplane gaining vertical momentum over time.

The the airfoil gains vertical momentum whenever air particles collide with the airfoil with a non-180 degree angle. When the air moving past the foil gets faster and faster, the average air particle collision angle gets shallower and shallower.

The top of the airfoil has an aggressive downward slope, preventing a lot of shallow angle collisions from the top. However, the shape of the bottom of the airfoil helps enable shallow angle collisions from the bottom. The faster air current provides more air particle collisions, and the difference in the collision rate is what provides vertical momentum in one direction.

>https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html
>However, for most normal flight conditions, like those on an airliner (35,000 feet, 500 mph), this theory does not give the right answer.

I went and did this calculation and I think I got the right answer. I looked up the specs on the 737 and then computed what angle of attack is needed to generate lift equal to gravity force. I found that the angle of attack is 12.7 degrees which seems just right.

So... either I have an error in my calculation or that is obvious misinfo on NASA's website preying on people's ignorance of the principles of elementary mechanics.

Seems like a real good estimate, IMO.

>>12210321
>>12210334
Looks like air is 100 times less dense up there.

YEAH... that turns into a big problem I think.

>>12210361

>>12209598
Just for the record, I'm not the guy who replied to you.

>>12210044
>I can't say for sure about the explanations for lift being bullshit like I can with precession but something seems fishy to me here.
Oh, you're a retarded schizo. My bad, I thought you were in good faith.
>>12210321
>says fluid mechanics is a lot more complicated than rigid body mechanics
>uses newtonian mechanics
>So... either I have an error in my calculation
>or that is obvious misinfo on NASA's website preying on people's ignorance of the principles of elementary mechanics.
Yeah, you're clearly smarter than NASA, you got them, good job.

>>12211171
>you're clearly smarter than NASA,
Yes, I know and that's obvious. However, in this case I was using the density of air at sea level in my first estimate.

>>12211171
>you're clearly smarter than NASA,
NASA consists of beaurocratized NPC brainlets these days. It's absolutely possible.