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Mar 31, 2018, 11:22 AM
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Bernoulli's Principle Always?


I have been trying to determine if Bernoulli's Principle still has an effect on a symmetrical airfoil or a flat plate for that matter when there is significant angle of attack to create lift strictly from Newton's Law? Or is Bernoulli's Principle only involved with an airfoil with a curved top and flat bottom?

https://en.wikipedia.org/wiki/Lift_(force)
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Mar 31, 2018, 11:30 AM
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pmackenzie's Avatar
As far as velocity differences resulting in pressure differences, and those pressure differences causing a force perpendicular to the flow defined as lift, then yes the conservation of energy described by Bernoulli always applies ( in ideal or near ideal fluid like what we call "air" at subsonic speeds)

Nasa web site has a great little app that shows flow around flat plates and symmetrical sections, the resulting velocity and pressure distributions and resulting lift.

https://www.grc.nasa.gov/www/k-12/airplane/lift2.html
Mar 31, 2018, 06:59 PM
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richard hanson's Avatar
Good ol NASA
The spinning cylinder will create lift
As long as it is moving
Mar 31, 2018, 07:53 PM
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BMatthews's Avatar
Keep in mind that any airfoil needs to have some angle of attack. And a flat plate is a symmetrical airfoil that happens to be very thin. And with any symmetrical airfoil that has an angle of attack the path around the airfoil is longer on the top than on the bottom. But more than that it's the fact that the top has a rather different shape because of the way the air splits at the stagnation point of the air hitting the airfoil.

There's a number of great wind tunnel smoke streamlines testing on airfoils on You Tube. Here are just two of them.

how wings work? Smoke streamlines around an airfoil (1 min 54 sec)

Aerodynamics: Airfoil Camber, Flaps, Slots-Slats & Drag: "Smoke Lifts" circa 1938 NACA Langley (10 min 0 sec)
Apr 01, 2018, 06:03 AM
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I guess the top moves faster regardless of airfoil shape due to the fact its airflow goes down unimpeded and the airflow underneath gets compressed and must have résistance due to friction.
Apr 01, 2018, 06:45 AM
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pmackenzie's Avatar
It is more complicated than that. Flow is faster on top than the bottom to satisfy the equations that define fluid flow ( Navier-Stokes) combined with the fact that flow can't be into or out of the surface defined by the airfoil (Boundary conditions).
The end result is circulation of the airflow imposed on the steady stream, which results in higher velocity on top and lower on the bottom.

Unfortunately no amount of "hand waving" type of analysis allows you to get an understanding of this.
So we get things like the "longer path" theory that is often falsely tied to Bernoulli.
Best you can do is to know that in order to satisfy the above maths, a bound vortex results.
Apr 01, 2018, 11:18 AM
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So regardless of what cause's the faster moving air on top, Bernoulli's does come into play?
Apr 02, 2018, 12:06 AM
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BMatthews's Avatar
There IS a lower pressure over the top. And as some will be quick to say a change in direction of the air as it comes off the wing. And depending on which camp you prefer some will say it's a Bernoulli cause and others a Newtonian reaction. And when you get down to it both are happening at the same time.

And there IS more to it. Even if you only look at the pressure distribution around the airfoil there's a rise in pressure along the bottom and a drop in pressure over the top. I'm not sure Bernoulli allowed for the slight but positive rise in pressure below the wing.

And there's been more than a few others wade into the issue with things like Prandtl's lifting line theory and another (name escapes me at the moment) that came up with the theory of circulation around the airfoil to evaluate the lift of a wing. And going back to Newton apparently if you look at the change in momentum of the air coming off the trailing edge you'll find enough of a change to add up to the lift needed. So that's led to some folks saying that it's just a Newtonian matter.

In the end though it's a bit of this and a bit of that. My own non mathematical interpretation is that the wing's shape and angle of attack produces an acceleration in the air mass flowing around the wing. And this change is tied in with a Bernoulli like pressure change on top and bottom. It's that pressure difference acting on the skin of the wing's surface that keeps the plane aloft.
Apr 02, 2018, 09:12 AM
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Quote:
Originally Posted by BMatthews
There IS a lower pressure over the top. And as some will be quick to say a change in direction of the air as it comes off the wing. And depending on which camp you prefer some will say it's a Bernoulli cause and others a Newtonian reaction. And when you get down to it both are happening at the same time.

And there IS more to it. Even if you only look at the pressure distribution around the airfoil there's a rise in pressure along the bottom and a drop in pressure over the top. I'm not sure Bernoulli allowed for the slight but positive rise in pressure below the wing.

And there's been more than a few others wade into the issue with things like Prandtl's lifting line theory and another (name escapes me at the moment) that came up with the theory of circulation around the airfoil to evaluate the lift of a wing. And going back to Newton apparently if you look at the change in momentum of the air coming off the trailing edge you'll find enough of a change to add up to the lift needed. So that's led to some folks saying that it's just a Newtonian matter.

In the end though it's a bit of this and a bit of that. My own non mathematical interpretation is that the wing's shape and angle of attack produces an acceleration in the air mass flowing around the wing. And this change is tied in with a Bernoulli like pressure change on top and bottom. It's that pressure difference acting on the skin of the wing's surface that keeps the plane aloft.
IMHO, Bernouilli follows from Newton: if air flows over the top of a wingfoil, it has to follow the surface (AKA change direction). The airfoil surface cannot exert any force on that air, so it has to be something different that makes the air move downwards. Only thing that makes air move is a pressure difference, and the pressure in the wider surroundings above the wing does not rise, so the pressure near the airfoil surface has to drop, in order for the air to follow the curve of the foil.
Whether the airfoil is "tugging" the air downwards, or the pressure drop is causing the air to flow downwards, the result is the same, and whether you use Newtons formula of mass flow times vertical acceleration or Bernouillis formula for the pressure drop due to change in velocity, the outcome should be the same.

IMHO, Bernouilli "only explained how the Newtonian reaction comes to happen".... Or in other words: Bernoulli and the Newtonian reaction are the same thing.
Apr 02, 2018, 10:20 AM
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Montag DP's Avatar
Bernoulli's equation is simply a restatement of Newton's 2nd Law along a streamline (or anywhere if the freestream is uniform) in an incompressible, irrotational flow. Therefore, if you think that somehow "Newton" and "Bernoulli" are mutually exclusive, it is because you have a false understanding of at least one of them.
Apr 03, 2018, 04:48 AM
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It's actually Aristotle's principle: nature abhors a vacuum...
Apr 03, 2018, 05:09 AM
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I am glad to hear that Bernoulli's is a major factor in lift. In Middle School I did a Science Project on How A Plane Flies and was very proud of learning about Bernoulli's Principle. Through the years I often tell others about it as it relates to planes, chimneys, hurricanes and other interesting effects.
Apr 03, 2018, 05:38 AM
An itch?. Scratch build.
eflightray's Avatar
Don't forget the greater contribution ---

http://bernoullisicecream.com/


Ray.
Apr 03, 2018, 05:58 AM
Launch the drones ...
Quote:
Originally Posted by Lithium Polymer
I have been trying to determine if Bernoulli's Principle still has an effect on a symmetrical airfoil or a flat plate for that matter when there is significant angle of attack to create lift strictly from Newton's Law? Or is Bernoulli's Principle only involved with an airfoil with a curved top and flat bottom?

https://en.wikipedia.org/wiki/Lift_(force)
No one has demonstrated heavier than air lift, without accelerating enough air downwards, to overcome gravity.

IOW, lift is a Newtonian reaction, by an object, to the action of having accelerated air downwards.

Most simply demonstrated experimentally by hovering a quad copter over a scale.

Or via analysis ... and in that regard, NASA has several web pages on the subject. Here‘s three of them ...
https://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html
https://www.grc.nasa.gov/WWW/k-12/airplane/wrong1.html
https://wright.nasa.gov/airplane/newton3.html


And a snippet from the first link ...

Quote:
HOW IS LIFT GENERATED?

There are many explanations for the generation of lift found in encyclopedias, in basic physics textbooks, and on Web sites. Unfortunately, many of the explanations are misleading and incorrect. Theories on the generation of lift have become a source of great controversy and a topic for heated arguments. To help you understand lift and its origins, a series of pages will describe the various theories and how some of the popular theories fail.

Lift occurs when a moving flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction. Because air is a gas and the molecules are free to move about, any solid surface can deflect a flow. For an aircraft wing, both the upper and lower surfaces contribute to the flow turning. Neglecting the upper surface's part in turning the flow leads to an incorrect theory of lift.
And a pic from the 3rd link ...
Last edited by Tim Green; Apr 03, 2018 at 06:16 AM.
Apr 03, 2018, 08:24 AM
An itch?. Scratch build.
eflightray's Avatar
Does a lightweight model plane rising in a thermal have 'lift' ?, and if it does, how is it, "accelerating enough air downwards, to overcome gravity". ?

Actually, many things, not just model planes can rise in a thermal.




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