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Old 06-18-2012, 06:15 AM   #1  
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Default How to explain Bernoulli's principle

Ah the age old question of who is more responsible for lift. As a CFI, I feel more confident explaining Newton's action/reaction concept, upwash/downwash, and the wing's positive angle of attack to the relative wind. It makes sense to me, and I see that it makes sense to my students. BUT there is a reason for that cambered upper part of the wing. To say that the wind traveling on the top surface of the wing has to travel a longer distance than the bottom surface, and therefore it needs to "speed up" to "catch up" to the wind flowing the bottom part of the wing, just feels weird. Then I move on to draw a venturi tube and explain how air needs to speed up when flowing through the venturi. I have already explained Bernoulli's princple, which states that when a velocity of a fluid increases, its pressure decreases. Then I draw a line across that venturi and say, "what does this look like"? A wing indeed! The student is lost at this point and probably thinks I have no idea what I am talking about. At this point, I feel like a jerk for wasting his/her time. I can however, use Bernoulli to explain wing tip vortices and wake turbulence, but to explain lift via Bernoulli is a mighty feat. Am I alone here? As a side note, I have just begun to read Aerodynamics for Naval Aviators, hopefully it will help me better understand how to explain Bernoulli.
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Old 06-18-2012, 06:26 AM   #2  
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It's all about air pressure. Take two flat pieces of paper, hold them 2 inches apart vertically, blow between them, and they draw together. Explain the faster air above the wing creates a lower pressure over the area of the wing, with a higher pressure under the wing. Then differential pressure (lb/sq ft) times area (sq ft) equals upward force perpendicular to the wing.
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Old 06-18-2012, 07:33 AM   #3  
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Quote:
Originally Posted by PearlPilot View Post
To say that the wind traveling on the top surface of the wing has to travel a longer distance than the bottom surface, and therefore it needs to "speed up" to "catch up" to the wind flowing the bottom part of the wing, just feels weird.
That's because it's not quite correct, a particle doesn't need to "catch up" to any other particle. But if you picture a wing in a tank of water a foot below the surface, and then move it forward, the water above the wing is "constricted" at the top of the curvature/camber, and therefore must travel faster at that point, to have the same amount of water traveling at the LE and TE. That means low pressure. And yes, submarines have wings too.

Engineers hate it when you talk about newtonian lift and upwash/downwash. I think upwash and downwash patterns can make controls further back more or less effective, but it's not a "lifting mechanism" IMO, at the least it would mean the TE of our wings would have to be the strongest part, as that's where all the particles are "thrown" from?
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Old 06-18-2012, 08:03 AM   #4  
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If the student speaks math, here's a decent discussion that compares Newton and Bernoulli

Bernoulli's Principle - Advanced Discussion
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Old 06-18-2012, 08:08 AM   #5  
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Ask him/her if they have ever had a ride in a soft topped jeep. At a stop, the top is straight or little sag. As acceleration occurs what happens to the top? if they have witnessed this they can now visualize what is happening. Hope this helps.
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Old 06-18-2012, 10:24 AM   #6  
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This is an article I wrote on this. You are free to print it out and use it for teaching, if you so desire. I am not sure if "Cubdriver" will work as a publishing name, but it's not about the credit!
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~
Lift
by Cubdriver

What causes lift? Most will quickly say it is due to Bernoulli’s Law. But can we just add up the pressure differences from top to bottom on our wing arrive at the correct lift? Well sure but what really caused those pressure differences? Bernoulli’s Law alone, maybe because the air parcels need to meet back up at the back of the wing at the same time? Action and reaction per Newton, perhaps like little bullets impacting the bottom of the wing? There’s more to the story than this and by the way, both of these theories are in error, as is the one about the wing being a huge one-sided venturi. Yes, the air above the wing goes faster and Bernoulli is part of the reason for this, but Bernoulli’s law comes no where near to the whole story because it can be shown that Bernoulli’s Law alone cannot cause the pressure differences we know are actually there through testing. So what else is it? Bernoulli plus Newton’s Law, maybe? Newtonian physics might say air is being thrown down as it passes the wing and this means you have action and reaction. But why is it being "thrown" anywhere? Pressure maybe? That’s all certainly true, but again you have a gaping hole in your theory. Newton and Bernoulli alone cannot explain lift. Here's why.

“Circulation” is what you are missing. Without it you will never be able to account for all the lift. So what exactly is circulation? Think of it as an additional flow added to the linear (straight) air flow we already know is there. It’s an imaginary flow on the theoretical level, a way to account for what we know is happening around the wing and can test, what we can measure and if there is a smoke trace, also see. And maybe that’s why most people tend to dismiss it. They want real, tangible, simple facts. But the effects of circulation are real. So it does exist. Circulation is an abstract concept that says if you have lift, then it is proportional to the amount of circulation around the wing. What do you mean, circulation around a wing? It is both a mathematical idea and a physical idea. Mathematically we must add a clockwise flow to the linear flow, one plus the other, to get the summed effect of both parts. The circulation “kicks” the top air along a little faster and it retards the air on the bottom side of the wing a little bit too, since it only turns on one direction. Namely, clockwise if you are looking at the left side of a wing on an airplane going to the left. It also gives drag through the action of the vortex spirals that must leave a wing due to its presence. So we know circulation exists, and we know it works.

Air must leave the trailing edge of a wing because it cannot come around the bottom at the rear. Simply can’t do it. This is why the top air is faster and the bottom air is slower. Little packets or parcels of air must decide whether they are going over top or under bottom but they do not have to both arrive at the trailing edge at the same time. The top air gets there first correct, and although they do not meet parcel for parcel, when the bottom air gets there it is going slower and the combination of these two speeds produces a vortex or spinning twirl. It’s just like water going down a tub drain. Slower mixes with faster and spinning is introduced at the drain. This spin is circulation around a moving central axis.

But what about the physical aspects of circulation? Can we see them? Yes you can, and not just in the vortex trails. This is where circulation meets Bernoulli meets Newton. All 3 aspects happen at the same time, they are one and the same physical process. It is a complex thing to grasp. Let’s talk about the Newton part a little more though. When an air parcel decides whether to go under or over a wing, the decision is made based on how much pressure is felt by the air parcel. It wants to go to the low pressure zone, high to low. But when the wing sets up a low pressure zone more or less in front of the wing because of angle of attack, circulation is established because it suddenly has to lunge forward around the leading edge of the wing to get to the top. To do this, it absorbs energy in the form of centrifugal force. It sucks forward against the wing and the wing pulls backward. The air turns toward the front of the wing to go around the leading edge while seeking low pressure. Bear in mind we are talking about 3 simultaneous effects that make up one complete, continuous behavior, and no particular one can be identified as being the cause and no particular one can be named as the effect of the other two. They all happen at one time, they are one and the same thing, and there is no “first this, then that” chain of causality.

We are not quite done explaining the physical action of circulation. Now the air is on top and the additive part of the clockwise flow when viewed on an airplane that is traveling to our left occurs. You might say ok, fine the air circulates around the front of the wing then why does it not just circulate around the back side and add back to the loss of speed felt on the bottom side? Why do we need these vortices shedding from the back? This is because the friction of the air is such that a sharp trailing edge will not allow the kind of energy levels air needs to make the turn back to the bottom around the edge. A vortex is born and the wing sheds the vortex to the rear. The vortex then drags the wing from behind.

  • angle of attack makes the air have to make the turn around the front of the wing
  • circulation is set up, which adds to the speeds on top and subtracts from the bottom speeds
  • this leads to the pressure differences top to bottom
  • this connects with the acceleration of the mass or momentum changes in the flow, inasmuch as flow leaving the wing at the back goes downward creating a force balance on the wing through the momentum of the air as it changes direction

Finally, you can’t say it is the momentum only, the pressure only, or the circulation only. It is possible to calculate lift based on any of these things, but it depends on where you measure these non-separated aspects. For example, at the surface of the wing you feel only pressure differences, right? There is no flow “through” a wing, only around it. But if you do the math a few feet away from the wing, you will find momentum and circulation are movers and shakers. So if you want to measure using only pressure you can do so, but you will not be able to find the right number using only Bernoulli.
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Old 06-18-2012, 10:42 AM   #7  
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Wait wait, is the air what's moving or the plane?

I honestly think that was a little overly complex, Cubdriver.
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Old 06-18-2012, 01:22 PM   #8  
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F=Ma

The wing accelerates air in a downward direction. Accelerate more air (larger wing or faster) in a more extreme direction (greater angles of attack) equals more lift. The wing is curved to prevent burbling which would prevent the air from smoothly deflecting downward.

That's the short answer. There are many variables but none as important as the mass of deflected air.
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Old 06-18-2012, 02:14 PM   #9  
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No answer is adequate unless as a minimum includes the concepts related to Bernoulli's Law, Newton's Laws, and circulation. These are the fundamentals beyond which the subject cannot be reduced. Most people are ok with some Bernoulli and some Newton, heck we get that in grade school. But circulation requires calculus to truly get because you have to integrate the flow field around a circle to arrive at the sum. That is what sorts the men from the boys, although in truth the explanation is not that hard once you see a 2D thin airfoil derivation. Second year college physics. Like I said above in the article, you can go so far as to set up a test chamber with a wing in it, but you will come up short of predicting how much lift and drag it makes until you introduce the complementary concept of circulation. Bernoulli and Newton's Laws are not the whole deal. If they were, we might have had airplanes in the 1700s. Saying an elephant has a trunk, and that's all, does not negate the fact it also has legs and a body. It is a partial explanation, and a partial one only. You simply must include all three fundamental concepts to get the whole solution.
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Old 06-18-2012, 03:22 PM   #10  
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Stick and rudder says to ignore bernoulli, focus on newton.
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