No one can explain WHY planes fly...

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Dan Thomas

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Lift is one of those things most of us just use without getting all bent out of shape in trying to explain or understand it. Like time. Or magnetism. Or gravity. Or light. (Is it a particle? Or a wave? It clearly acts like both, yet it can't be.)

In flight training there are so many bogus explanations out there that students get confused. Some instructors will hammer away on Newton, saying that downwash creates a reaction that lifts the wing. Others say that Bernoulli's pressure/velocity stuff explains it all and Newton doesn't enter into it. Yet both are at work there.

One of the stupid theories that should have been booted out a long, long time ago is the equal-transit-time idea that says that air molecules above the wing must meet up with the bottom molecules they were acquainted with before the wing came along. One of my favorite videos that proves that idea so very wrong:

 

jedi

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No one can explain WHY planes fly...

Airplanes do not fly. Whenever I drive by the airport all the planes are just sitting there.

Google 737 max grounding photos as an example.

Sorry to many images to copy here but there sure is a lot of money sitting on the ground and Boeing still does not seam to have enough time, money or expertise to get those planes into the air.
 
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Aerowerx

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You seem to be confounding turbulence with stall.
Whatever.

In either case, the presence of turbulence indicates the chaotic system has changed state even if the wing is still producing lift. That was my main point.
 

Aerowerx

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No one can explain WHY planes fly...

Airplanes do not fly. Whenever I drive by the airport all the planes are just sitting there.

Google 737 max grounding photos as an example.

Sorry to many images to copy here but there sure is a lot of money sitting on the ground and Boeing still does not seam to have enough time, money or expertise to get those planes into the air.
You mean like this one, sitting on the ground??? Must be computer animation, right? :D
 

Erik Snyman

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Oct 9, 2019
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KASPERwing=
AoA>30 deg, Cl>5 (Vmin >5 m/s)
Correct!!! Angle of attack. You are all WRONG!! ONLY relative angle of attack matters!!
Read Wolfgang Langewiesche`s `Stick and Rudder- An Explanation of the Art of Flying` and get wise. Only angle of attack matters. A Mirage F1 or F16 has very little top curvature on the wing to help Bernoulli`s explanation. They fly **** good last time I checked.

Ok, I`m running for cover..:D:D:D

Erik in Oz.
 

Erik Snyman

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To paraphrase the reason for climbing mountains, planes fly because they can!!

[Edit] And I see they make no mention of the downwash on the trailing edge of the wing!
I always thought they are running away from the fire you light in their backsides! I would also run away...
Erik in Oz

....and yes, here in Oz we fly upside down. If we were to fly right side up, we would fall off....

Sorry, my painkillers are finished..
 

dog

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Dec 29, 2019
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why?why?why? do planes fly?
generaly its because someone decides to go flying.
why?
again generaly because they want to.
why?
generaly they will tell you if asked.
seven year olds love that game.
 

Dan Thomas

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No one can explain WHY planes fly...

Airplanes do not fly. Whenever I drive by the airport all the planes are just sitting there.

Google 737 max grounding photos as an example.

Sorry to many images to copy here but there sure is a lot of money sitting on the ground and Boeing still does not seam to have enough time, money or expertise to get those planes into the air.
There's an old saying: "There's never enough time to do the job right the first time, but always enough time to do it again the right way."
 

Monty

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M DV/DT
DKE/DT
DPE/DT
DU/DT=alpha Del^2U
Navier-Stokes
Viscosity
Heat
Entropy

Everything else is an obsolete math model that relies on crap assumptions. Navier-Stokes is the only approach that starts from first principles. Problem is there is no known closed form solution to the equations. A computer and descretized model is needed to obtain a solution, and then you need an appropriate turbulence model for the regime you are in if you want to look at transition and turbulence. You also need the control volume/solution space to be big enough that all energy is dissipated as heat. In other words it's not a continuum and momentum is NOT conserved. It's dissipated!!!! WHY IS THIS SO HARD???? They can't find the momentum which doesn't exist.....because it becomes heat! The temperature of the air goes up, a very tiny amount. So does entropy. Everything else is just a work around....and a bad one. But that is the aerodynamics we teach undergrads and a lot of grad students....because deriving Navier-Stokes and applying the equations is beyond most of them. So we teach them garbage, and they are never told it's all bunk. There is an entire bunk book, bunk teaching, and bunk spewing industry built around this.

I almost had a seizure when I saw Mclean with that picture of vorticity and Biot Savart.....fortunately he was correct in tearing it apart. I've calmed down now. I thought he was going to be defending it. The biggest problem with aerodynamics is the over-mathification of the field, without using sound physics and thermodynamic principles or even understanding such things exist. If I hear the word continuum one more time I'm going to scream! Of course the stupid integrals don't exist, but even if they did they would still be crap. I kept shouting Reynolds number! viscosity!!! HEAT!!!!!! he never got there. Aerodynamicist like to throw those terms out....And that whole Pradtl pancake thing....good grief. No viscous loss, no dissipation of momentum as heat. That pressure field only goes so far before being dissipated.

I also think these guys spend so much time looking at airflow in a wind tunnel they forget the airplane is moving through the fluid, not the other way around. You have to burn fuel to make that happen, and it all winds up as heat in the end. Even gliders....the sun heats the ground, the glider takes some of the energy from the thermal and then spreads it around as it descends. We are all riding the potential energy wave and spreading entropy hither and yon.

I think this is where Mclean was going....at least I hope so. But Aero guys are strange-you never can tell.

Fuel costs money.

Pops is right....money makes airplanes fly.

QED

I can't believe I got drawn into this.....I'm weak...must be the fever.
 

Monty

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Look up the thread "Lift and Conservation of energy" if you really must know more about this topic. I just re-read it and stand by most of what I said. The stuff I would quibble over would require more space and energy than any of us have.
 

dog

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Dec 29, 2019
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Thanks Mr Monty
I knew that all wind is dissapated as heat, and see things around wind energy where claims are made that turbines add heat, which seems to me impossible.Hadnt thought of lift bieng ultimately converted the same way as friction into heat.
And the stats at the bottom of the page attest to
many hundreds of people quietly reading and learning,some few will be grad students going
,aha!
Some other few will be going into acute unearned
tenure shock.
 

Monty

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Joined
Jul 15, 2010
Messages
1,294
Location
Fayetteville, AR / USA
Thanks Mr Monty
I knew that all wind is dissapated as heat, and see things around wind energy where claims are made that turbines add heat, which seems to me impossible.Hadnt thought of lift bieng ultimately converted the same way as friction into heat.
And the stats at the bottom of the page attest to
many hundreds of people quietly reading and learning,some few will be grad students going
,aha!
Some other few will be going into acute unearned
tenure shock.
I'm glad someone got something out of it. I read a couple of posts at the end of the "Lift and Conservation of Energy" thread just now that happened after I forced myself to never look at it again. There was comment about Ilan Kroo saying the wing uses a change in inertia of the lift medium to counteract the gravitational force, and is therefore an anti-gravity device...I agree, but it requires energy input and has losses just like everything else. No magic in the airfoil. Rockets work the same way. In fact one of the scariest thoughts I've ever had is that the only way to interact with spacetime is through inertial means......that would really suck. It would imply we are stuck here in this solar system for all practical purposes.

Anyway, Dr. Kroo's comment is about the best ending to that discussion I could imagine.
 
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