No one can explain WHY planes fly...

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BBerson

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Yes, Feynman was talking about atoms. Molecules are made of atoms. So Feynman was talking about everything.
O2 and N2 are diatomic, and interact as Feynman said. The electric force is what holds air, water, your chair and body more or less loosely together.

excerpt: "The ultimate basis of an interaction between the atoms is electrical. Since this force is so enormous, all the plusses and all minuses will normally come together in as intimate a combination as they can. All things, even ourselves, are made of fine-grained, enormously strongly interacting plus and minus parts, all neatly balanced out. "
 
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lr27

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Exactly. Now that we see we are dealing with a compressible and expandable gas we can finally leave Bernoulli behind.
Well, if we're going to drop all simplifying assumptions, even when they still give reasonably accurate results, we had better take relativity into account as well. For instance, at high altitudes, the red shift is less. What will that do to the flow?
 

lr27

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Ŵhen it comes to gases, there is no pull. The molecules are just bouncing off each other. Chamces are the air molecules have already reacted with anything they'd stick to, unless there's some newly introduced, reactive substance. (Ozone, fuel, etc.) O2 doesn't stick to N2 or Argon.
If air molecules aren't being shoved together by pressure, I don't know what shove means. While flying at 10,000 feet, in an unpressurized, moderately warm aircraft cabin, fill a balloon with air and measure the diameter. After landing, with the cabin at the same temperature, measure the diameter. It will be smaller, because the air molecules have been pushed together.
 

BBerson

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I guess you are talking about chemical bonds not related to aerodynamics. Air is very thin, but still has viscosity, not as sticky as honey, but not zero friction.
Air flowing interacts with a surface with attractions and shearing but not bonds. That attraction is also seen as the Coanda effect.

https://en.m.wikipedia.org/wiki/Viscosity
 
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Aerowerx

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Ŵhen it comes to gases, there is no pull. The molecules are just bouncing off each other. Chamces are the air molecules have already reacted with anything they'd stick to, unless there's some newly introduced, reactive substance. (Ozone, fuel, etc.) O2 doesn't stick to N2 or Argon.
If air molecules aren't being shoved together by pressure, I don't know what shove means. While flying at 10,000 feet, in an unpressurized, moderately warm aircraft cabin, fill a balloon with air and measure the diameter. After landing, with the cabin at the same temperature, measure the diameter. It will be smaller, because the air molecules have been pushed together.
It took me a while to reason this out.

Air molecules have little or no attraction to each other, under normal conditions. If they did, then the air would spontaneously liquefy. (I am excluding water vapor here). Any attraction they do have is dwarfed by the kinetic energy of motion.

The pressure comes from the "attraction of gravity" (put in quotes because I don't want to get in to relativistic effects. Newtonian physics is good enough here.). The high-up molecules pile on top of the lower ones, and the resulting pressure is the result. There is also an increase in temperature as the molecules bounce off each other more.

It is only under extreme conditions where the kinetic energy is lower than the attractive force, and the air would liquefy. Oxygen requires about -183c degrees, Nitrogen -196c, for example. Note that this also depends on pressure. At normal pressures, Carbon Dioxide will spontaneously solidify at about -57c (this might happen in the polar regions).
 

Dan Thomas

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AIr molecules repel each other only because they are in a gaseous state, well above their boiling points, just like steam. Steam generates pressure as the water molecules bang against each other much harder at elevated temperatures. Get any gas cold enough and it liquifies and comes together in a puddle. Colder yet and it solidifies.

Gravity, as others have said, keep the molecules forced into closer contact and pressure is generated. The density of air is a balance between the repulsion between gaseous molecules and gravity's pressure. As we climb, we get above some of the atmosphere and when there is less of it above us, the pressure falls and the density decreases. It gets colder, too, but not cold enough to condense the gases, and the molecules get a long way apart at the edge of space.

Compression raises temperature because the heat content of a parcel of gas is concentrated.
 

poormansairforce

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A couple of good posts here.
Once we understand the above we know we can use the mass and kinetic energy available to us and send that mass around a convex curve or spin up a vortex which will expand it against the local pressure. You can also deflect it off an inclined surface, concave or otherwise, increasing pressure which will quickly lead to induced drag because you have left the drag bucket since you are putting more energy into wingtip vortex. This is why barn doors can fly, just not as well or as efficient and takes lots of control inputs. This is why landings and takeoffs are so dangerous.Go back and see the video I posted. That KF 1 airfoil wants to be 'right side up'!
 

Aesquire

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See? Gravity!

The above folk are correct. Air molecules don't have special attraction, electric, strong, weak, whatever force. They bounce around. Any nuclear effects are frankly unmeasurable outside a special lab. If then. Gravity dominates on the scale we live in.

That's unless we're talking about the forces holding O2 & Ozone, N2, etc. together. That can be affected if you are fast enough. ( that your plane gets really hot ) And of course, the slow oxidation of exposed aluminum, steel, human flesh, etc. exposed to the air. None of which matters to the limited question of How Things Fly.

Honestly, I don't get the hate for the limited and incomplete explanations that are common... But feel free to rant about how too many people don't understand that the Science Is Never Settled. And There's More To IT Than That.

They teach basic physics in grade school with "The Greeks thought atoms were the indivisible elements of matter" and move on to the cute picture of the planetary atom model, and then the next limited model, and maybe get into the electron cloud of quantum probability and quantum shell states... It's been a while since I was in Grade school. They probably stop at ( POLITICAL RANT DELETED ;) ) and how tiny demons cause disease, now.

Not one of those explanations of atomic theory that I learned in school are complete, since they still haven't finished figuring out how many bits make up a Quark ( the previous indivisible ) that make up the Protons ( the previous indivisible ) that are at the nucleus of the Atom ( the previous...oh, never mind )
 

BBerson

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Air molecules stick to wings and get dragged along, called skin friction.
"Skin friction drag arises from the friction of the fluid against the "skin" of the object that is moving through it. Skin friction arises from the interaction between the fluid and the skin of the body, and is directly related to the wetted surface, the area of the surface of the body that is in contact with the fluid. Air in contact with a body will stick to the body's surface and that layer will tend to stick to the next layer of air and that in turn to further layers, hence the body is dragging some amount of air with it. The force required to drag an "attached" layer of air with the body is called skin friction drag. Skin friction drag imparts some momentum to a mass of air as it passes through it and that air applies a retarding force on the body. As with other components of parasitic drag, skin friction follows the drag equation and rises with the square of the velocity."
 

dog

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And with really light wing loadings. Not a valid argument.
Of course its valid.
Any explanation of flight is going to satisfy all forms of flight.A unified description of all aerodynamic phenominon is the minimum requirement.
Lift does not exist as a seperate function,except mathimaticaly,which is a fudge used to build airplanes.
Watch spiders,"balloning" for some strange reason the silk goes up,direct evidence of the molecular forces responsible ,why on earth doesnt the silk work as a drag line and hold them down?
There are other phenominon assosiated with vortices that are giving hints.Namely the "vortex tube" a device that can mechanicaly sperate a vortex and yield a 40degree celcius temp difference between the inner and outer part of the vortex.
This is Scrodeners cat that you are trying to skin here,tricky beast.
 

henryk

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.Namely the "vortex tube" a device that can mechanicaly sperate a vortex and yield a 40degree celcius temp difference between the inner and outer part of the vortex.
=Ranke-Hilsch tube...?

-in tornado (vortex too !) this different (auter/inner parts) can be much moore,

thus it is possible to condense water from air,
and propan gas condens too !!!
 

BBerson

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Watch spiders,"balloning" for some strange reason the silk goes up,direct evidence of the molecular forces responsible ,why on earth doesnt the silk work as a drag line and hold them down?
I like that. Direct evidence that air is "sticky". Especially for insects at low RN.
It isn't that the electric force in the neutral atoms has "attraction" at a distance like an opposite charge has distant attraction. But they do stick when close enough by the electron interactions. Gravity is millions times weaker at atomic distance.
 

Aesquire

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Interesting, but at hang glider and 747 scale electrical forces don't dominate, much less measurably contribute. ( correct me if I'm wrong )

"Gravity is millions times weaker at atomic distance."

Really? Or is it that other forces only work at short range or fade very quickly with range?

Because we don't understand gravity worth a dang, yet. But it's defining character is long range.
 

BBerson

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Air atoms only attract at a very short distance. Only on the inside lamina of the boundary layer. So yes, on 747 scale the boundary layer is proportionally insignificant.

Feynman: "If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generations of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis (or the atomic fact, or whatever you wish to call it) that all things are made of atoms—little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. In that one sentence, you will see, there is an enormous amount of information about the world, if just a little imagination and thinking are applied."

See where he said "attracting each other when they are a little distance apart?
So air is attracted to the wing surface. And above that surface air layer the next air lamina is attracted to the surface air layer but not the wing itself which is too far, so these layers slide past. The surface layer stays attached. This way the attraction effects goes up to about a semi-wing span above the wing, I think.

Feynman Basic Physics chapter two: https://www.feynmanlectures.caltech.edu/I_02.html
 
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