Bernoulli, yes or no?

Does Bernoulli’s law apply to an airfoil?

  • Yes

    Votes: 42 73.7%

  • No

    Votes: 5 8.8%

  • Maybe

    Votes: 2 3.5%

  • I don’t speak Italian but I like pizza

    Votes: 8 14.0%

  • Heretic!

    Votes: 0 0.0%
  • Total voters
    57
Checkout_my_Six said:
there are a few other variables to it ....than that.
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Oh jeez! I've already taken Physics, I really don't want to see that stuff again.
 
Does it apply to airfoils? Yes.

Does it apply to lift? No, there is no force vector in the bernoulli effect. It is the bernoulli EFFECT, not the bernoulli CAUSE.
 
mscard88 said:
Sicilian, thick as hell! My grammom made it that way and she came from Italy. Of course she called it tomato pie and it only had sauce and a couple spices on it. Pizza is so American. But, I like that too. Damn I'm hungry...

How's that for thread drift? He did have pizza though as an answer...
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Been to Sicily multiple times, never saw a "Sicilian" pizza. It's all thin crust.
 
Cool video of pulsed smoke streamlines in a wind tunnel:


You can really see how the pepperoni improves airflow as it approaches stall.
 
Cooter said:
I watched a B52 takeoff yesterday and I would almost swear he was nose low on climbout. I don’t know what his AoA was but it wasn’t much.

Blowing across the top of a straw brings the fluid up. That seems kind of Bernoulliish.
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He probably was. The lighter the jet, the worse that deck angle gets. At very light weights the risk of wheelbarrowing the jet is real enough to be part of a caution on the Tech Order (our version of your NATOPS).
There was plenty of AOA however and that is because.....
GRG55 said:
Wouldn't that have something to do with the wing angle of incidence?
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This. This guy gets it. The angle of incidence in the BUFF is positive 8 degrees. That's a lot. High altitude flight is also much more robust in the buff than bombers like the Bone.

Cooter said:
Yes, I don’t know what it is but it looks really flat.
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See above.
Tarheelpilot said:
And flaps
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Indeed. The wing area is almost effectively doubled in the case of the BUFF flap deployment. Though they can be partially extended, the TO calls for all extended or all retracted for critical phases of flight (TO and landings). The deployment takes 60 seconds, 120 if one motor has to drive both sides due to failure of the other motor. 120 seconds is an eternity in category D airplane, and they don't want you to turn while you retract flaps either, given consideration to the idea that losing half your wing area before reaching L/D min airspeed is probably a doozy. So you have to no kidding stop flap track to change heading, then resume retraction when not turning. Rinse and repeat. A royal PITA, which is why you don't fly that POS where civilized airplanes fly (aka civilization). And I digress, my nuke-qual PTSD is getting triggered.
 
hindsight2020 said:
He probably was. The lighter the jet, the worse that deck angle gets. At very light weights the risk of wheelbarrowing the jet is real enough to be part of a caution on the Tech Order (our version of your NATOPS)...
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Is this because the effective center of lift with those long swept-back wings is well back on the longitudinal axis?
 
nauga said:
Does the airfoil cause the fluid to move or does the airfoil pass right through without disturbing the fluid?

Nauga,
undisturbed
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The airfoil certainly doesn’t cause air to flow from leading edge to trailing edge. The airfoil pushes air around and more importantly downwards.

Please don’t offer greater insult by continuing to propose simplistic items such as you have.
 
paflyer said:
Been to Sicily multiple times, never saw a "Sicilian" pizza. It's all thin crust.
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Yeah I dunno, thought they called it Sicilian. My grandparents were actually from an area east of Naples, small village.
 
GRG55 said:
Is this because the effective center of lift with those long swept-back wings is well back on the longitudinal axis?
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It serves a double purpose. The Buff is bicycle-geared (everything about that airplane is wonky), and the rear trucks are aft of the bomb bay. The inclusion of a high angle of incidence on the wing lessens the need for a rather large tail force in order to attain the required angle of attack at the design takeoff weights. Otherwise the takeoff speeds would be in the ludicrous regime or the stab would have to be redesigned. There is simply not enough moment arm between the tiny elevator (a mere 10% of the chord of the horizontal stab) and the aft gear trucks. Interesting side note about that system, the trim is the entire horizontal stab, and it is large enough in range of motion to render the elevators ineffective. Out-of-trim condition is a real killer in that airplane; runaway trim is a boldface item on it (what airline doods call memory recall actions or immediate action items).

At any rate, second effect was the desired design mission of the aircraft: high altitude strategic bombing. In 1955 that meant carpet bombing in the almost 40s. As such, deck angle of the bomb bay could be normalized by increasing the angle of incidence of the wing at the mission-design bomb run true airspeeds. This is no longer relevant in the age of -1760 bay weapons and SDBs and those dumb "rules of engagement" that no longer allow us the freedom to rain hate by sheer tonnage, but that's not here nor there. So there ya go.

Lots more interesting nuances about that airplane's systems. It's old, but if you didn't act like a complete buffoon with it, it would limp ya home on 5 or six engines :D

ETA: to your original question, the aircraft is indeed of longer wingspan than its overall airframe length, which is the reason for both the outrigger gears and the lack of wing low method on landing, which necessitates the crosswind crab system (that's for another episode of story time).
 
Thank you. Interesting airplane alright. And even more impressive design exercise when one considers much of it was probably done with slipsticks and recorded on vellum.

A couple of years ago had a chance to chat with a Buff commander who had brought it to the airshow from Barksdale. She said it was interesting she was flying some of the same tail numbers as her career USAF father.
 
Clark1961 said:
It matters a great deal.
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In what way? Relative movement is all that is required, same as with a wire in a magnetic field generating current. Move the wire, move the field: whatever. Same effect occurs.
 
We were taught that it is applicable. That the airfoil was acting in the same way as the classic tube that is narrow in the middle, with one tube "wall" being the layers of air above the airfoil.
 
nauga said:
Bernoulli's equation is derived from Newton's second law (conservation of momentum) for an inviscid, incompressible fluid, a reasonable assumption for a typical GA airfoil. They are not different, they are different ways of stating the same thing given the same assumptions.
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Actually, the Bernoulli equation is derived from Euler's equations. I no longer care much about mathematical derivations, but google knows the answer.

The way Bernoulli plays into wings is via pressure. The air moved up by the leading edge of the airfoil creates a low pressure area behind the wing. That low pressure area is abnormal in nature, so air rushes in (and downward) to fill the low pressure. Bernoulli is describing the differential and intensity of the low pressure. Newton describes the equal and opposite upward force to compensate for that air moving down - we call that force lift.

The low pressure effect happening on top of the wing accounts for approximatly 30% of lift. Obviously that varies based on many factors. But it is not because of air moving faster or directly because of lower pressure, it is because of air moving downward. 100% of lift comes as an opposite force to air moving down - newton's 3rd law.
 
Dan Thomas said:
In what way? Relative movement is all that is required, same as with a wire in a magnetic field generating current. Move the wire, move the field: whatever. Same effect occurs.
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Does a magnetic field behave the same as air? Does it have viscosity and mass? Is an induced current the same as lift?
 
LongRoadBob said:
We were taught that it is applicable. That the airfoil was acting in the same way as the classic tube that is narrow in the middle, with one tube "wall" being the layers of air above the airfoil.
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Yes, the FAA used that bit of nonsense to"explain" lift. Now, how can you fly upside down? Why do so many wings have a curve on the bottom? Wouldn't that suck you back down to the runway?
 
Clark1961 said:
Does a magnetic field behave the same as air? Does it have viscosity and mass? Is an induced current the same as lift?
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A magnetic field is not air so it does not behave the same. But the example was for relative motion, which you seem to have missed. It does not matter which one is standing still, it matters what the relative motion is. That is true for air/airfoils as well as magnetic fields/currents.
 
bflynn said:
I no longer care much about mathematical derivations, but google knows the answer.
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I care about the math - I have to. If anyone here would prefer to use Google to cut to the chase, I include this from NASA Glenn (https://www.grc.nasa.gov/www/k-12/airplane/bernnew.html)
"So both "Bernoulli" and "Newton" are correct. Integrating the effects of either the pressure or the velocity determines the aerodynamic force on an object. We can use equations developed by each of them to determine the magnitude and direction of the aerodynamic force."

Don't trust my quote. Read the site and check it yourself. Pay particular attention to the "What is the argument?" paragraph.

Nauga,
going with the flow
 
nauga said:
Read the site and check it yourself.
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You know, when I said that I really don't care about derivations anymore it isn't because I can't - I have a BS in Math. I have no interest.

I believe the difference between "both" and "Bernoulli for pressure / Newton for reaction" is largely semantic. Bernoulli is based on Euler, which is an idealistic implementation of Newton for fluids.
 
Clark1961 said:
Does a magnetic field behave the same as air? Does it have viscosity and mass? Is an induced current the same as lift?
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That doesn't answer the question as to why you think it matters whether the air or the airfoil is moving.
 
LongRoadBob said:
We were taught that it is applicable. That the airfoil was acting in the same way as the classic tube that is narrow in the middle, with one tube "wall" being the layers of air above the airfoil.
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I think what some are saying is that the layers of air above the airfoil can't be a "wall" since that air can be displaced.

It makes logical sense to me that the answer is "both", and that the percentage assigned to each depends on the angle of the airflow to the wing or other object.

People have tried to explain sails to me as being just like an airplane wing. I can buy that when sailing close to the wind. However, sailboats can also sail with the wind behind them. I think this is akin to put your hand out the window of a car. It gets pushed by a Newtonian force, and does not have anything to do with Bernoulli.

I suspect this is a very difficult question to answer. If it was easy, there would be papers published, and we would not be debating the issue.
 
Capt. Geoffrey Thorpe said:
Yes, the FAA used that bit of nonsense to"explain" lift. Now, how can you fly upside down? Why do so many wings have a curve on the bottom? Wouldn't that suck you back down to the runway?
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1) I can't fly upside down. So far just learning right side up. :)

2) but maybe you have. Some have. I don't see the problem, the airfoil is still the bottom side of Bernoullis pitot tube, the "laminated" air, being less affected as you look further away from the airfoil layer makes up the top side, and low pressure above the wing?

Edit to add....I see my error. Wasn't thinking if the camber, but aren't acro planes made to fly inverted designed with more equal camber? I think there are other forces countering the Bernoulli effect to keep a "normal" aircraft flying inverted, but I don't know much about inverted flying. Just that for normal flight it made sense to me, while also there is the downward push of air also at play?
 
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LongRoadBob said:
1) I can't fly upside down. So far just learning right side up. :)

2) but maybe you have. Some have. I don't see the problem, the airfoil is still the bottom side of Bernoullis pitot tube, the "laminated" air, being less affected as you look further away from the airfoil layer makes up the top side, and low pressure above the wing?
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The number 1 myth about airfoils is that they are curved on the top and flat on the bottom. A brief visit to the airport should make it obvious that this is not the case. If someone starts out with an explanation of "lift" that claims that airfoils are flat on the bottom and curved on the top, you can stop listening, because what follows will be bull hockey.

The number 2 myth is that what happens on top of the airfoil is somehow different than what happens on the bottom (in terms of fundamental aerodynamic principals). Say, Newton vs. Bernoulli. If this were true, all the people doing CFD (Computational Fluid Dynamics) would be coming up with the wrong answers and Boeing would have gone out of business years ago.

Newton, Euler, Bernoulli, Navier-Stokes, lifting line, circulation, etc.; all that stuff applies above, below, in front of and behind the wing.

If you like Bernoulli's equation, you apply it both above and below the wing to get the pressure distributions on both sides (lower than static over much of the top, higher than static over most of the bottom).
If you like circulation, you apply it to both the top and bottom (faster than free stream velocity on top, slower on the bottom, upwash in front, downwash behind).
If you like Newton's equations, you have to apply them to the air all around the airfoil to get a total result ( upwash and downwash)
If you like to the integrate the Navier-Stokes equations, you are either working on your aero PHD, are getting paid very well to do CFD, and/or are a couple bricks shy of a full load.

Everskyward said:
People have tried to explain sails to me as being just like an airplane wing. I can buy that when sailing close to the wind. However, sailboats can also sail with the wind behind them. I think this is akin to put your hand out the window of a car. It gets pushed by a Newtonian force, and does not have anything to do with Bernoulli.

I suspect this is a very difficult question to answer. If it was easy, there would be papers published, and we would not be debating the issue.
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Bernoulli and Euiler did their work in the mid to late 1700's - well before the idea of airplane wings and lift were developed. Bernoulli's equation simply applies to flow along a streamline in an invicid fluid and really is not directly related to wings, venturis, etc.
Now, in your example, when you are operating on the back side of the lift curve ("stalled"), the vortex behavior gets very messy and detailed calculations are exceptionally difficult at best. But to say that "This is Newton and that is Bernoulli" really doesn't work given the actual nature of the equations.

Now, the question about lift does get complicated to answer in a simple intuitive way (without resorting to vector addition and/or calculus) which is probably why there is so much mythology out there. Also, there seems to be a tendency to discount some of the anatomically correct resources out there because they pretty much tell you that the stuff that your primary instructor (and the FAA literature) taught you is nonsense. But, if you are willing to look, you will find. Even the Wikipeda article is pretty good. https://en.wikipedia.org/wiki/Lift_(force). Some NASA resources have been linked earlier in this thread but were apparently dismissed because NASA is run by scientists who actually understand this stuff. Even my smart ass versions have been linked and ignored as well (not that I blame anyone for that one ;-) )
 
Everskyward said:
People have tried to explain sails to me as being just like an airplane wing. I can buy that when sailing close to the wind. However, sailboats can also sail with the wind behind them. I think this is akin to put your hand out the window of a car. It gets pushed by a Newtonian force, and does not have anything to do with Bernoulli.
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The hand out the car window thing made it really clear to me when I started flying that the books were leaving something big out. But you take the tests and answer the way the tester wants you to or you don’t pass.
 
Everskyward said:
I think this is akin to put your hand out the window of a car. It gets pushed by a Newtonian force, and does not have anything to do with Bernoulli.

I suspect this is a very difficult question to answer. If it was easy, there would be papers published, and we would not be debating the issue.
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Bernoulli's equation is equally applicable on the lower surface as on the upper. The problem seems to be the misapplication of the equation and the principle to apply it only to the upper surface and "suction." The airflow deflection results in lift. The pressure distribution (high on the bottom, low on top) causes airflow deflection. Newton and/or Bernoulli are different means to the same end. You might not find a 'scholarly paper' on the difference (or you might, I don't know) because the 'difference' is largely one of misunderstanding.

Nauga
and misapplied science
 
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