Bernoulli vs Newton (again)

SixPapaCharlie

SixPapaCharlie

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I had given up on bernoulli as a source of lift in planes years ago based simply on symmetrical airfoils and inverted flight.

The Newton version makes way more sense to me but then I got to thinking about accelerated stalls. If Newton was giving us lift there shouldn't be accelerated stalls right? :dunno:

Pulling at a high rate of speed would increase the surface area against the relative wind which should increase lift if Newton is doing the lifting.

Something is definitely going on with that layer of air over the top of the wing but it happens in airfoils of all shapes which I still think makes Bernoulli sketchy with regard to lift.

I am back to not knowing how things fly.


Edit: I know...
horse2.gif
 
SixPapaCharlie said:
I had given up on bernoulli as a source of lift in planes years ago based simply on symmetrical airfoils and inverted flight.

The Newton version makes way more sense to me but then I got to thinking about accelerated stalls. If Newton was giving us lift there shouldn't be accelerated stalls right? :dunno:

Pulling at a high rate of speed would increase the surface area against the relative wind which should increase lift if Newton is doing the lifting.

Something is definitely going on with that layer of air over the top of the wing but it happens in airfoils of all shapes which I still think makes Bernoulli sketchy with regard to lift.

I am back to not knowing how things fly.


Edit: I know...
horse2.gif
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The debate is senseless. Both Newton and Bernoulli apply and even a little bit of Coanda. Lift is a reaction force and a pressure force.
 
CTLSi said:
The debate is senseless. Both Newton and Bernoulli apply and even a little bit of Coanda. Lift is a reaction force and a pressure force.
Click to expand...

Unfamiliar with Coanda. Going to read upon that.
And not really wanting a debate, just satisfy my own internal struggle with lift.
 
Bernoulli and Newton are not mutually exclusive, you know.
 
SixPapaCharlie said:
The Newton version makes way more sense to me but then I got to thinking about accelerated stalls. If Newton was giving us lift there shouldn't be accelerated stalls right? :dunno:

Pulling at a high rate of speed would increase the surface area against the relative wind which should increase lift if Newton is doing the lifting.
Click to expand...

Pulling at a high rate of speed DOES increase lift, right up to the point of stall. Lift is greatest at max AOA.

SixPapaCharlie said:
Something is definitely going on with that layer of air over the top of the wing but it happens in airfoils of all shapes which I still think makes Bernoulli sketchy with regard to lift.
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Of course there is something going on with the top layer over the wing - it becomes turbulent during a stall, and greatly reduces the amount of air that is washing downward for that Newtonian reaction. Stalled wings still create quite a bit of lift, just not enough to sustain normal flight. Despite what some think (and have argued here), stalled wings do not cause the airplane to "fall like a rock". If you don't think stalled wings create lift, analyze the difference in the descent profile if the wings come completely OFF. ;)

And BTW, Bernoulli never said anything about airfoils and wings and lift. A wing is NOT a half venturi. Bernoulli gets misused in this context. As far as lift goes, both Newton and Bernoulli's laws can be used to arrive at the same result. As mentioned above, they are not mutually exclusive. Newton is easier for laypeople to understand - reaction one way, reaction another way. Bernoulli is harder to understand, and can't be dumbed down the way pilots tend to, and have it retain any meaning.
 
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Bernoulli IS Newton. It's just energy conservation along a streamline.

I think you're trying to replace Newton with flat plate deflection. That's wrong; you can't explain any stall that way. Airfoils do not behave like mirrors.
 
Sac Arrow said:
Bernoulli and Newton are not mutually exclusive, you know.
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True. I had been leaning toward Newton eliminating the need for Bernoulli And I am not too worried about those two vs one another.

I am having trouble with the accelerated stall vs Newton at this point

During an acc stall, the plane is going to pitch up relative to oncoming wind and essentially fall through that wind with the break correct?

If so now you have a very rapid moving flat surface moving perpendicular to the force of the wind.
 
SixPapaCharlie said:
I am having trouble with the accelerated stall vs Newton at this point

During an acc stall, the plane is going to pitch up relative to oncoming wind and essentially fall through that wind with the break correct?

If so now you have a very rapid moving flat surface moving perpendicular to the force of the wind.
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Not sure what you are trying to say. A stall is a stall, accelerated or not. Exceed critical AOA, and airflow becomes turbulent on top and the wing's lifting efficiency greatly reduced, but lift is far from eliminated completely.
 
Anyone have a link of a wind / water tunnel test of an accelerated stall? I am having trouble locating one.
 
SixPapaCharlie said:
Anyone have a link of a wind / water tunnel test of an accelerated stall? I am having trouble locating one.
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You seem to be hung up on the "accelerated" stall concept. It looks just like any other stall. All accelerated stall means is that you're stalling at greater than 1G, which simply means the stall will occur at a higher airspeed. Stalling at 1.1G is an accelerated stall no different from stalling at 4G. There's nothing magical about stalling over 1G by any margin.
 
SixPapaCharlie said:
Anyone have a link of a wind / water tunnel test of an accelerated stall? I am having trouble locating one.
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You can't find it because there is nothing special aerodynamically about an accelerated stall. It's ONLY angle of attack. It's identical to a normal stall.

You won't find power-on, cross-control, or trim stalls demonstrated in a wind tunnel either.
 
ok, so post stall, Netwon is still doing his thing it is just not enough so keep the plane flying? It is hard looking at this image and thinking of this airfoil traveling at a very high rate of speed and not climbing even though the airflow has separated on the top.

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SixPapaCharlie said:
ok, so post stall, Netwon is still doing his thing it is just not enough so keep the plane flying? It is hard looking at this image and thinking of this airfoil traveling at a very high rate of speed and not climbing even though the airflow has separated on the top.

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Newton is ALWAYS doing his thing. What do you really mean?
 
SixPapaCharlie said:
It is hard looking at this image and thinking of this airfoil traveling at a very high rate of speed and not climbing even though the airflow has separated on the top.
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Hint - lift goes way down and drag goes way up after exceeding critical AOA.
 
RoscoeT said:
And BTW, Bernoulli never said anything about airfoils and wings and lift.
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I know but CFI's do. There is a lot of "Bernoulli creates lift and that is all you need to know. lets go fly" In my post student pilot life, I am beginning to go back and revisit some of the things we covered but more in depth. I get learning to fly and what you need to know to do it safely but I am also a "need to know" kinda guy.
 
Bernoulli's equation derives from Newton's laws. I don't see why it's a vs. thing. Two equivalent ways of looking at the same thing. Bernoulli is easier to conceptualize when thinking about an ideal fluid, but ultimately, it all reduces to Newton.
 
MAKG1 said:
I think you're trying to replace Newton with flat plate deflection.
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Yes I am. So I need to figure out this as well.

I imagine the bottom of a flat bottom airfoil as identical to a paper airplane wing (which I think is flat plate deflection:dunno:) and I assumed a curved bottom airfoil was just a modified version of this flat plate deflection.
 
SixPapaCharlie said:
ok, so post stall, Netwon is still doing his thing it is just not enough so keep the plane flying? It is hard looking at this image and thinking of this airfoil traveling at a very high rate of speed and not climbing even though the airflow has separated on the top.
Click to expand...

compare the amount of down-flow in the image you posted to the amount of down-flow in a normal, non-stalled, wing...
 
MAKG1 said:
Newton is ALWAYS doing his thing. What do you really mean?
Click to expand...

I know I am wrong but in my mind, in that image the wing should still be climbing or moving to the left like a surfboard on water but I think like you pointed out before I am thinking of it like flat plate deflection which I think is the root of my misunderstanding of lift and stalls.
 
RoscoeT said:
Hint - lift goes way down and drag goes way up after exceeding critical AOA.
Click to expand...

AOA is about moving the Center of Pressure toward the leading edge of the wing. The result is to defeat what's happening below the wing and destroy lift (not all of it, but enough to give gravity all it needs to drop your rear out of the air).
 
SixPapaCharlie said:
Yes I am. So I need to figure out this as well.

I imagine the bottom of a flat bottom airfoil as identical to a paper airplane wing (which I think is flat plate deflection:dunno:) and I assumed a curved bottom airfoil was just a modified version of this flat plate deflection.
Click to expand...

Nope, paper airplanes and hang gliders can stall, too. They have the added complication of being flexible.

Flow across the top of the wing really does reduce pressure.

Read the Denker book, www.av8n.com . Short of a textbook, it's about as good as it gets. And he has a whole chapter on lift generation.
 
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Clark1961 said:
compare the amount of down-flow in the image you posted to the amount of down-flow in a normal, non-stalled, wing...
Click to expand...

Ok cool. I think this lends itself to whomever mentioned "Coanda"
I am watching another one that is a bit more clear and yes, in straight and level and even in shallow climb, there is very little downward flow.
 
It's not Bernoulli vs Newton. The full Bernoulli equations has the Newtonian mechanics in them. EVERYTHING relevant is accounted for.

And it isn't necessary to understand it to be a pilot. It's really complicated stuff that requires college level physics background to understand.

The way I think about it, the aircraft moves forward due to thrust of engine. The wings get lift due to shape and angle to the relative wind. The airplane goes up due to lift. About all you need to know.
 
Question one: What do you mean by "Bernoulli"?

If the answer has anything at all to do with distances or flat vs. curved surfaces, then what you mean is "a fairy tale" that has nothing to do with the historical Daniel Bernoulli, his father Johann or his friend Leonhard Euiler (who was the one who actually published what is known as Bernoulli's equation). The FAA is fond of telling fairy tails.

If your answer is about conservation of energy applied along a streamline of an ideal fluid, then you are referring to the actual Bernoulli's equation which can be applied to calculate the relation between velocity and pressure about a surface (ignoring the boundary layer).

As mentioned by others, there is no vs. Either Newton or Bernoulli can be used to explain lift.

And, the reality is that lift has nothing to do with flat / curved surfaces, differences in distances, or cross sections of venturis. A flat plate generates lift quite nicely (as long as you limit the angle of attach) and has none of these features.

It's all about the flow at the trailing edge.
 
coloradobluesky said:
It's not Bernoulli vs Newton. The full Bernoulli equations has the Newtonian mechanics in them. EVERYTHING relevant is accounted for.

And it isn't necessary to understand it to be a pilot. It's really complicated stuff that requires college level physics background to understand.

The way I think about it, the aircraft moves forward due to thrust of engine. The wings get lift due to shape and angle to the relative wind. The airplane goes up due to lift. About all you need to know.
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Fluid dynamics and Bernoulli is a completely different dynamic and what essentially rules whats above the wing. Pressure differentials.

Newtons laws essentially operate under the wing, and when the wing is at angle to the relative wind. Newton also explains velocity and load factor.

The prop is little more than a wing itself creating lift horizontally as a counter force to drag (induced and parasitic). The same applies to a jet engine except its expansion of hot gases...but Newton 3rd rules the effect of jet engines.
 
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CTLSi said:
The debate is senseless. Both Newton and Bernoulli apply and even a little bit of Coanda. Lift is a reaction force and a pressure force.
Click to expand...


Nope! Lift is a function of Money. Since Alexander Hamilton was the first Treasury Secretary, Alex is the cause of lift.

-Skip
 
CTLSi said:
Fluid dynamics and Bernoulli is a completely different dynamic and what essentially rules whats above the wing. Pressure differentials.

Newtons laws essentially operate under the wing, and when the wing is at angle to the relative wind. Newton also explains velocity and load factor.
Click to expand...

Bernoulli's equation is Newton's 2nd law expressed for an incompressible inviscid fluid. There is no disagreement or 'one or the other' or 'upper or lower surface'. One is a simplification of the other.

Nauga,
And his scratch pad
 
I have two methods of explaining lift:

1) the single method (not SEL/SES, single vs married):
Any quasi-reference to either Bernoulli or Newton is more than enough to get you felatiated at the bare minimum.

2) the scientific method:

Bernoulli Shmernoulli. Any plane with at least 50 hours Hobbs time has exhibited enough lift for me to rent lol.
 
look at it this way.......

Lift is due to the movement of air over the wing. Both Newton and Bernoulli contribute to the efficiency (i.e. amount of) movement required to lift the weight.

Consider why the airplane will speed up when the AOA is reduced in level flight? Less Newton and more Bernoulli. Each contribute in different combinations.
 
Cruiser said:
Less Newton and more Bernoulli. Each contribute in different combinations.
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*sigh* Since I'm betting most of those who think the meanings are different don't have access to aero or fluid dynamics textbooks (like anything by John Anderson) I'll put up a Wikipedia link we can all see:
http://en.wikipedia.org/wiki/Bernoulli's_principle#Derivations_of_Bernoulli_equation
Specifically: "The Bernoulli equation for incompressible fluids can be derived by either integrating Newton's second law of motion or by applying the law of conservation of energy between two sections along a streamline, ignoring viscosity, compressibility, and thermal effects." [emphasis added]

They both mean the same thing and, when applied correctly, yield similar results. It's the correct application/interpretation that trips up most.

Nauga,
and a little curved air
 
But we fly in air, which is NOT an incompressible fluid. This changes many things from the simplifications expressed above. Variable temperature and density make the math more interesting. By then I'm an engineer, but am moving and my aerodynamics and fluid mechanics books are boxed up in a warehouse right now . . .
 
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