Meaningless aerodynamics and related physics discussion
 

Sooo... I got into an entirely unreasonably heated argument recently (of course at a bar) on "how planes fly".

I of course have my (as correct as it could be at a bar) opinion, but - how do planes fly?

No, neither drunk or stoned ATM. This is an interesting thing for me regarding teaching reality.

(and it might possibly be driven by the 8yo's "why????" onslaught)

It's witchcraft.

Air velocity and pressure creating lift, maybe?

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Funny, while I've had applied aerodynamics during engineering school, the best explanation that I got was earlier as a kid, and was from an MJohnson who worked as a model maker at Boeing wind tunnel.

Something like, the air, being forced up and over the wing top creates a tunnel of vacuum along the length of the wing. -which also explains why elliptical wings and winglets work to 'pinch-off that tunnel of vacuum.

Hope that helps the 8 year old.

Ask Bernoulli, he knows....


Actually, no one really knows, or so says Scientific American (glad this question came up so I can dispose of this piece of information and store a new bit of pedantic/unnecessary information):

https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/

Bernoulli was a quack.

Anyone who has been involved with airplanes knows that money is what makes them fly. No money, no fly. The more money, the faster, higher and further they fly.

LOL

Ok Mr. Grissom...

https://youtu.be/a7rGA0Zv8R4

Planes get lift by sending air downwards, it's simple Newtonian mechanics. There are two ways of looking at this, which are in a way the same thing.

A. Most wings have a convex top which forces the air to move faster round the outside of the circle. Whenever a fluid moves faster it has lower pressure, so the pressure difference from top to bottom of the wing manifests as lift.

And B. Wings usually have an upwards angle of attack to the direction of travel, I.e. the back of the wing is lower than the front.

Both a and b work by sending air downwards as the plane moves forwards. But a curved wing with a level angle of attack would still generate lift.

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That sums it up.

This got me thinking to my high school days and then I read this - That is what I remember.

I think I agree most with Mr. Frost... Air down = plane up. Works for rockets, too. Bumblebees, I guess we still don't know (JK, I'm aware that we figured that out)

(and thank god I'm not into planes or piloting, but according to some colleagues - money is an absolute requirement to keep those things airborne)

I'll stick with my kid's completely flat winged, rubber band powered, balsa and completely affordable things...

^^^This! It's funny, but the answer to this question is more complicated than it seems (like most things). But, the above is correct.

One interesting thing I read once is that this act of "sending the air downwards" creates a pressure (or force) that can be measured on the ground. When a jet flies over your house, there is a pressure footprint below it as it flies. Logically, there must be. Mythbusters did a good episode on this. IIRC, they ended up putting an RC helicopter in the back of an enclosed 18-wheeler trailer. The question being - if the helicopter lifts off the bed of the trailer, does the truck weigh less than with the helicopter sitting on the bed? The answer was, no - the truck weighed the same. The pressure (force) created by the helicopter rotor sending the air downwards acted on the trailer bed, so the weight didn't change.

That's also my understanding. It's not just what most of us previously learned about the lower pressure over the top due to increased speed. I took a class about 10 years ago, and that was news to me.

Pretty basic stuff:

http://forums.pelicanparts.com/uploa...1666786810.jpg

Where it get really complicated is the multitude of wing designs for various types of aircraft.

I think they were looking for specifically how the lift is generated.

Growing up, this was the only thing that I'd heard.

https://i.stack.imgur.com/xgvJC.png

But then about 2007, I took a course on a whim, and it included the following.
That course taught the following
http://hyperphysics.phy-astr.gsu.edu.../airattack.gif
Essentially that the downwash from the wing and Newtons third law caused the lift on the wing.

I've heard that it's not that simple and is a combination of Bernoulli and Newton that make the whole thing work.

Mebbe ya just needed a few more drinks?

http://forums.pelicanparts.com/uploa...1666787919.jpg

just don't ask about helicopters.

Wings on racecars are upside down.

Like a lot of kids, I got into model airplanes. I built a kit with the classes 0.049 engine on it and learned the basics of U-control. I moved up to a .35 and a balsa wood airplane. It had a wing shape that was the same top or bottom. My Air Force pilot father said it can't fly. But the power to weight ratio was more than enough to fly with ease and I had a lot of fun with it. And yes, even when the engine ran out of gas it would glide down just fine to a safe landing.

Seems that would have to be due to downwash and Newton's 3rd.

So then you two are saying that a flat-bottom (or also convex bottomed) wing with a level angle of attack would not generate lift?

I mean, the leading edge stagnation point of say a NACA 2412 is inline with the trailing edge at zero angle of attack yet it provides positive lift.

How does this wing have a net downward throw? http://forums.pelicanparts.com/suppo...ool_shades.gif

http://forums.pelicanparts.com/uploa...1666793355.JPG

its not a mystery, this is why.

because the air on top is going faster.

net doward throw is a simplified answer, and not a true determiner. useful, but not the be all end all determiner. lift is a force, a force is generated by pressure over an area. you can generate a pressure differential with the shape of the wing, or with the angle of the wing, but both methods create a pressure differential, higher on the bottom, lower on the top (until stall).

get a sail boat to play with

the results of totally screwing up
are you stop or go backwards
vs in an aircraft stall = crash

driving forces are the same

its important to note that static angle of attack is a stability characteristic built into many model and light duty aircraft. ergo, the wing could be flat on both sides (like a balsa glider with stamped flat wings), and with enough power, it would fly. you can make damn near anything fly with enough power.

Great question, and I look forward to the answer. Just looking at and thinking about things (because intuition is clearly the best source of knowledge for physics and aerodynamics), the top edge of the wing comes down to the trailing edge of the wing at a greater angle than the lower edge of the wing. And the air going over the top of the wing is moving faster than the air over the bottom, so that air will follow Newtons 1st law and overpower the slower air under the bottom, still causing a downwash that creates lift?

Like I said before, growing up, I'd only ever heard the Bernoulli speed/pressure theory of lift. Back about 2007 I heard the downwash theory. I've since heard that the truth is a combination of the two, and that science isn't really 100% on it.

Why is it going faster? Because it's getting pushed forward faster by the leading edge?

I mean, the air is still until the plane flies into it. The air is going to be pushed forward and up by the top side of the wing. :cool:

the aero dynamics question that always blows my mind, is the super sonic flows esp around high performance jet intakes. using certain design principles you can arrive at an intake design that significantly multiplies the thrust of the engine at super sonic speeds by basically removing the air in front of the engine, and the whole thing is then sucked forward, rather than pushed by the thrust of the engine. this is counter intuitive because the intake designs primary design goal is to slow the air before the compressor. because the compressor will stall/explode if fed super sonic air. well, at least turbo compressor jets (ie not scram or ram jets)

super sonic flows are facinating and a key reason why i became an engineer. i have wanted to experiment at work with super sonic flows in metal, but the science isnt there yet. someday maybe.

the air is not pushed forward and up until flow separates (ie it stalls). the flow stays attached, and follows the profile of the wing.

ding ding ding...

Yes, multiple forces at play.

IOW, it's not "one thing."

See "Lifting bodies" for example.

Then there are wave pulses that keep bees in the air.

:)

Magic pixie dust keeps bees in the air. Duh! Pixies love honey.

You clearly believe that you have a good mental model of "the flow stays attached" yet I see that you do not. You are simply regurgitating an engineering concept that is used as tool for categorizing general flow types. IOW, all "attached flows" are not equal.

Oh, and the air (even when laminar about the foil) is absolutely pushed forward and upward by the leading edge. (mind your reference frame.)

i didn't say all attached flows are equal.

we were discussing mass flow rate directions (ie "downflow"), not pressure distributions. the mass flow until stall is not upwards and forward, it is rearward.

<iframe width="1280" height="720" src="https://www.youtube.com/embed/VEe7NxB5Vo8" title="Why is the top flow faster over an Airfoil?" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

yup, as i said, its about pressure distribution, not "down flow"

https://peer.asee.org/aerodynamic-performance-of-the-naca-2412-airfoil-at-low-reynolds-number.pdf

You just said the the opposite SMH.

i mean you edited out the part about pressure distributions. its clear you have an ax to grind with me from PARF and didnt read the post and/or dont care what i said.

this is the post which clearly says that pressure distribution is the important thing when computing lift:

"net doward throw is a simplified answer, and not a true determiner. useful, but not the be all end all determiner. lift is a force, a force is generated by pressure over an area. you can generate a pressure differential with the shape of the wing, or with the angle of the wing, but both methods create a pressure differential, higher on the bottom, lower on the top (until stall)."

bolded for emphasis.

either you didnt read it, or you didnt care and decided to lie about my post.