# No one can explain WHY planes fly...

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Doug McLean:

#### henryk

##### Well-Known Member
Earth's gravity is what creates pressure.
-hard to agree...
=Earth gravity generate force of moleculas weight (F=m*g)
and thanks them atmospheric air is close to Earth surface.

The pulse (m*V, Veverage=500 m/s !) of N2,O2 molecules
generate the force,which compensate the weight of them
and air is a gas,not a liquid or ice...

E kinetic=0.5 m*V^2
(1 qm of calm air have circa 150 000 J kinetic energy,
=iff work is 1sec. it can generate 150 kW power)

#### BBerson

##### Well-Known Member
HBA Supporter
Keep going. What is another way to increase the spacing instead of using kinetic energy?
Not sure what you are asking. Do you have an explanation for why streamlines get closer and faster in the Venturi?

#### Norman

##### Well-Known Member
E kinetic=0.5 m*V^2
(1 qm of calm air have circa 150 000 J kinetic energy,
=iff work is 1sec. it can generate 150 kW power)
Exactly, the kenetic energy of molecules of air is much higher than the gravitational attraction between them. If it weren't for the gravitational attraction of earth the atmosphere would just fly away.

#### henryk

##### Well-Known Member

=offtop,but nonstationary aerodynamic too...

#### dog

##### Well-Known Member
We do have only the ONE atmosphere,counting again for accuracy ,ONE,whew.
If you mean that there is a difference in the graviational atraction that varies with distance to the planets resulting in a linear change in density.
And that solar heating results in local changes due to uptake of humidity and uneven mixing.Absolutely horrible trying to keep a slide rule ,err,sliding,with all the variables in reality,best just aproximate it.The bit about the absolute need to aproximate(fudge) reality and then clean up with REAL WORLD testing later needs to be more up front for beginners.Also that NO theory for airodynamics is satisfactory,again it realy realy helps to KNOW that its a counterintuitive ,or perhaps truely unimaginable,and boils down to MEMORISING the important bits,and developing a personal fluency,all the above is an attemp to adress the problem with SHARING that fluency.
Dont forget the butterflys flapping the bloody stuff around either,causing huricanes without permits.

#### poormansairforce

##### Well-Known Member
Not sure what you are asking. Do you have an explanation for why streamlines get closer and faster in the Venturi?
It was a mental exercise. Its hard to discuss streamlines within the confines of a wind tunnel. In the wild the air isn't constricted by walls.

#### PagoBay

##### Well-Known Member
I'm sick of people dissing Bernoulli. If something needs to be debunked, how about incorrect understandings of Bernoulli?
Agree. I just hate it when I see people make science and engineering decisions based on math. Just makes me feel ill. Or is that inadequate, I forget.

#### Jimstix

##### Well-Known Member
Instead of Bernoulli, lets talk Newton, more to the point let’s quote him and say Force = mass x acceleration. Lift is the force that hold airplanes in the sky, so what are the mass and acceleration needed to do that? The best way I’ve found to explain lift from an airplane wing is to get a roll of dimes and imagine the wingspan is equal to the diameter of a dime. Further, imagine that the air (represented by the diameter and thickness of the dime, hence the mass of air) acted upon by the wing.

Imagine the nose of the airplane flying down the centerline of the stack of dimes and as the wing passes through each dime it is deflected downward a small amount with a small acceleration. Here the volume of the dime could be thought of as an air mass (in slugs or kilos) that is pushed down by the wing. To push that air mass down it must be accelerated. Force (lift) = mass (air) x acceleration needed to displace the air.

To fly slow, the dimes (air mass) must be accelerated down faster per unit time than they would be if the airplane were flying fast. This analogy helps explain induced drag at low speed vs. that at high speed.

#### jedi

##### Well-Known Member
Instead of Bernoulli, lets talk Newton, more to the point let’s quote him and say Force = mass x acceleration. Lift is the force that hold airplanes in the sky, so what are the mass and acceleration needed to do that? The best way I’ve found to explain lift from an airplane wing is to get a roll of dimes and imagine the wingspan is equal to the diameter of a dime. Further, imagine that the air (represented by the diameter and thickness of the dime, hence the mass of air) acted upon by the wing.

Imagine the nose of the airplane flying down the centerline of the stack of dimes and as the wing passes through each dime it is deflected downward a small amount with a small acceleration. Here the volume of the dime could be thought of as an air mass (in slugs or kilos) that is pushed down by the wing. To push that air mass down it must be accelerated. Force (lift) = mass (air) x acceleration needed to displace the air.

To fly slow, the dimes (air mass) must be accelerated down faster per unit time than they would be if the airplane were flying fast. This analogy helps explain induced drag at low speed vs. that at high speed.
Finally, a good explanation for why it takes so much money to fly. My plane needs very big dimes and a very long stack. I am trying to imagine flying through a stack of silver dollars thirty feet in diameter and a hundred miles long just to get a hamburger.

Now I know why I frequently just sit at home and have a beer even though I know others are out there flying with their friends.

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#### mcrae0104

##### Armchair Mafia Conspirator
HBA Supporter
Log Member
Is that not the article from the OP? Since we have come full circle, surely this is proof of circulation. The conversation goes fast, "over-the-top", then it goes backwards for a while (relative to where it should be going), and then we're back where we started at the stagnation point.

#### Speedboat100

##### Well-Known Member
Is that not the article from the OP? Since we have come full circle, surely this is proof of circulation. The conversation goes fast, "over-the-top", then it goes backwards for a while (relative to where it should be going), and then we're back where we started at the stagnation point.

Yes it is...funny I never read the OP poster's link. Until I read it in the library today.

#### Speedboat100

##### Well-Known Member
Will you grasp it better by reading a real article printed ?

#### PagoBay

##### Well-Known Member
From the SA article....
-1-
"For these reasons, Newton’s third law is a more universal and comprehensive explanation of lift than Bernoulli’s theorem."
-2-
Cambridge aerodynamicist Babinsky says, “I hate to disagree with my esteemed colleague Mark Drela, but if the creation of a vacuum were the explanation, then it is hard to explain why sometimes the flow does nonetheless separate from the surface. But he is correct in everything else. The problem is that there is no quick and easy explanation.”
-3-
Drela himself concedes that his explanation is unsatisfactory in some ways. “One apparent problem is that there is no explanation that will be universally accepted,” he says. So where does that leave us? In effect, right where we started: with John D. Anderson, who stated, “There is no simple one-liner answer to this.”

Cambridge Univ. Prof.Babinsky is referenced multiple times in this SA article. From Babinsky's paper referenced here earlier in a post ... "A short, but correct, explanation might start by discussing the existence of transverse pressure gradients in curved streamlines and applying this knowledge to the flowfield around an aerofoil in a similar manner to that shown in figure 9. This should explain why pressures on the two sides of an aerofoil are different. There is no need even to introduce Bernoulli’s equation or discuss the rather subtle significance of friction."

Notice the conclusion is NOT that "No one can explain" lift, but rather that the explanations are hard if not impossible to put into words that others will wholly accept. All the while math does the job needed quite nicely. Bernoulli is not even needed there.

#### Speedboat100

##### Well-Known Member

Yes....some notable persons were mentioned there...even Albert Einstein....who also created an airfoil...and an aeroplane.

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#### Chris In Marshfield

##### Well-Known Member
HBA Supporter
Log Member
My opinion: because if it didn’t fly, it would be an antenna.
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Because air plane
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Not ground plane
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#### Pops

##### Well-Known Member
Log Member
I still say its money.
Throw enough money at it and it will fly.

#### BBerson

##### Well-Known Member
HBA Supporter
I think NASA* could do an experiment:
Build an "airless" wind tunnel. In other words a vacuum chamber.
Instead of air, particles of sand or something similar would be slung at the model in the tunnel.
This would eliminate most of the reduced pressure characteristics of air, but the inertia characteristics of particles would remain. I expect the reaction lift would only occur on the bottom.

A device called a "Wheel-a-brator" could hurl the airless sand. (used for industrial sand blasting)

* NASA could "throw" money at the model.

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