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#12
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Lift - Newton/Bernoulli ratio...
On 5/23/2011 3:40 AM, Dave Doe wrote:
Simple answer, there is no "ratio" - Newton 100% + Bernoulli 100%, total = 100% So a wing generates as much lift upside down? For an aircraft flying level, at the same speeds, upright and inverted, the wings produce IDENTICAL lift. To do this, the inverted airfoil needs a higher angle of attack usually. You are really dragging out all the old causes for disagreement: I can see why you would be using a pseudonym Brian W |
#13
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Lift - Newton/Bernoulli ratio...
On May 23, 3:40*am, Dave Doe wrote:
In article 4f56d67d-f259-46e7-8e3f- , , Dudley Henriques says... On May 22, 7:04*pm, Mxsmanic wrote: Dave Doe writes: Does anyone have any figures and references for about what ratio lift is produced by Newton's Laws and Bernoulli's Laws? All lift is produced by Newton's third law of motion. Air is forced downwards by the wings, and this produces an equal and opposite force that attempts to raise the wings, and that is lift. How the air is forced downwards is irrelevant, as long as it happens. In practice, principles discovered by Bernoulli and others play a role in diverting the air flow when this is accomplished by an airfoil. Actually, if I'm reading you right, I would rephrase this just a bit, as it feeds into the problems we as instructors have in "re- explaining" lift to students. STRESSING either Newton or Bernoulli in the lift explanation causes more than a modicum of confusion UNLESS it's done by including BOTH theories in the explanation. You've done that actually. I would just enhance things a bit more :-) Read what Orval says above. He is absolutely correct. BOTH Newton and Bernoulli are COMPLETE explanations for lift, which is interesting in another respect, as neither man had lift in mind with their work. The simple truth of it is that each explanation is totally correct and is interchangeable with the other. Each explains the same thing without relying on the other and BOTH are occurring simultaneously. It's a common misconception that Bernoulli and Newton EACH contribute INDIVIDUALLY to form a TOTAL of the lift produced. This explanation is incorrect and should be discouraged. While it's true that the Bernoulli effect is part of Newtonian mechanics - I want to know what the ratio of (gonna have to rephrase this aren't I) is: * an airfoil where the camber on both sides is equal and opposite (mirroed) vs * an airfoil that is shaped to produce lift via Bernoulli effect. When I dealt with the lift issue with instructors in seminar, my personal approach was to favor the Newtonian explanation as in my opinion student pilots can grasp Newton a lot easier than Bernoulli, but I've ALWAYS made it habit NEVER to leave Bernoulli out in the cold. The correct way to deal with the lift issue is to explain to those asking that BOTH explanations are complete by themselves, and Newton might be the easier of the two to explain. Dudley Henriques I'll rephrase it a second time. *What percentage of extra lift is gained from: a) a plank of wood (can only produce lift via angle of attack) vs b) a plank of wood that is an airfoil - and is getting lift from both angle of attack and the Bernoulli effect. I hope that is clearer. Here are some articles - but they produce no data to show the addidtional lift obtained by the Bernoulli effect. http://en.wikipedia.org/wiki/Airfoil http://en.wikipedia.org/wiki/NACA_airfoil http://en.wikipedia.org/wiki/Lift_(force) http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html And here is a third re-phrase... * A yacht that has a sail made of unbendable stiff material (will not point as high and go as fast as)... * A yacht that has a sail of normal material and has an effective airfoil shape and produces lift perpendicular to the sail (via the Bernoulli effect). Is it not a simple enough question? - I mean, really. *While results will undoubtably vary among plane types and airspeed - I'm just looking for an approximate percentage. Do read that:http://en.wikipedia.org/wiki/NACA_airfoilpage! And I don't want to get stuck on the pedantics of Newtonian physics encompassing the Bernoulli effect - just really looking at, as said (rephrase #4)... - lift produced by an airfoil that has a mirrored camber top and bottom (the zero lift line is the same as the chord line) vs - lift produced by a traditional airfoil -- Duncan. Simple. Newton 100% Bernoulli 100% There is no "extra lift" from Bernoulli OR from Newton. Both are EXACTLY equal and complete explanations for lift just expressed differently. In other words, the total lift being produced on either a barn door or the world's highest performance airfoil can be explained to 100% EITHER by Newton OR by Bernoulli. It's ACTUALLY that simple! If you are trying to explain lift attributing any contribution to the total lift being produced by either Bernoulli or Newton as being less than 100% you are mistaken and using poor information. There is simply not an instant in time when lift is being produced where the explanation for the TOTAL lift being produced can't be shown by EITHER a Bernoulli or a Newtonian explanation as both are equal and total explanations of the SAME THING and are occurring SIMULTANEOUSLY! Wikipedia serves a purpose I guess, but I prefer the Naval Test Pilot School. I'm sure Wiki won't mind. :-)) Dudley Henriques |
#14
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Lift - Newton/Bernoulli ratio...
On May 23, 4:40*am, Dave Doe wrote:
In article , , Private says... "Dave Doe" wrote in message ... In article , , Mxsmanic says... Dave Doe writes: So a wing generates as much lift upside down? * Absolutely..........without question! The EXACT same explanation of lift creation is in play on an inverted wing as on the upright wing. Applies to barn doors as well. Newton= 100% Bernoulli= 100%. Dudley Henriques |
#15
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Lift - Newton/Bernoulli ratio...
Dave Doe writes:
As said in other reply - not looking for a run-down on the physics - looking for the *ratio* of lift obtained by each. I said "all lift" in answer to this. In other words, Newton's third law generates 100% of the lift. |
#16
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Lift - Newton/Bernoulli ratio...
Dave Doe writes:
So a wing generates as much lift upside down? Yes. What I want, is half the difference between a wing up the right way, and the wing up the wrong way. That is, I presume, the additional lifting force from the Bernoulli effect vs a wing with a mirrored camber (obtaining no lift due to the Bernoulli effect). Camber does not produce lift. Angle of attack produces lift. A flat plank will fly as long as it has a postive angle of attack, and it can do that upside-down or right side up. |
#17
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Lift - Newton/Bernoulli ratio...
Dudley Henriques writes:
Actually, if I'm reading you right, I would rephrase this just a bit, as it feeds into the problems we as instructors have in "re- explaining" lift to students. STRESSING either Newton or Bernoulli in the lift explanation causes more than a modicum of confusion UNLESS it's done by including BOTH theories in the explanation. You've done that actually. I would just enhance things a bit more :-) Agreed. The problem is that all these effects interact, and explaining lift is often a matter of preferred viewpoint, as you imply. But it is true that lift always involves the acceleration of an air mass, which is a matter of Newton's third law. How this acceleration is accomplished is irrelevant, provided that it occurs. Bernoulli's effect and many other effects help to explain why air flowing over an airfoil with a positive angle of attack is accelerated at right angles to the direction of flow, but these effects don't produce the lift directly, it's the acceleration that produces the lift. If you build something that accelerates an air mass in the same way without any connection to Bernoulli et al., it will still fly. On the other hand, if you build something that demonstrates Bernoulli's effect but does not accelerate air perpendicular to its flow, no lift results. When I dealt with the lift issue with instructors in seminar, my personal approach was to favor the Newtonian explanation as in my opinion student pilots can grasp Newton a lot easier than Bernoulli, but I've ALWAYS made it habit NEVER to leave Bernoulli out in the cold. Lift is produced by diverting the air flow, thanks to Newton. The diversion in an airfoil is in part produced thanks to Bernoulli. |
#18
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Lift - Newton/Bernoulli ratio...
Dave Doe writes:
While it's true that the Bernoulli effect is part of Newtonian mechanics - I want to know what the ratio of (gonna have to rephrase this aren't I) is: * an airfoil where the camber on both sides is equal and opposite (mirroed) vs * an airfoil that is shaped to produce lift via Bernoulli effect. An airfoil doesn't need a specific shape to produce lift, as long as it's reasonably flat. The lift is ALWAYS produced by diverting the flow of air, no matter what the camber of the airfoil. And in airfoils, Bernoulli's effect ALWAYS has a hand in diverting the air flow, again no matter what the camber of the airfoil. I'll rephrase it a second time. What percentage of extra lift is gained from: a) a plank of wood (can only produce lift via angle of attack) vs b) a plank of wood that is an airfoil - and is getting lift from both angle of attack and the Bernoulli effect. The distinction you are making doesn't exist. A plank of wood is an airfoil when air flows over it and it has a positive angle of attack. Newton and Bernoulli are always involved. There is no lift without positive angle of attack. No special shape is necessary for the plank, but it should be relatively flat and roughly edgewise to the air flow (apart from the positive angle of attack, which is mandatory). Here are some articles - but they produce no data to show the addidtional lift obtained by the Bernoulli effect. Because no additional lift is obtained. It's impossible to dissociate Bernoulli from Newton for airfoils. The lift always comes from Newton, the diversion of airflow that invokes Newton is due to Bernoulli (and other effects, depending on how one looks at things). And here is a third re-phrase... * A yacht that has a sail made of unbendable stiff material (will not point as high and go as fast as)... * A yacht that has a sail of normal material and has an effective airfoil shape and produces lift perpendicular to the sail (via the Bernoulli effect). A flat sail will produce lift just as well as a curved sail. And I don't want to get stuck on the pedantics of Newtonian physics encompassing the Bernoulli effect ... Perhaps that explains why you haven't thus far understood the explanations you've received. |
#19
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Lift - Newton/Bernoulli ratio...
On May 23, 5:20*pm, Mxsmanic wrote:
Dudley Henriques writes: Actually, if I'm reading you right, I would rephrase this just a bit, as it feeds into the problems we as instructors have in "re- explaining" lift to students. STRESSING either Newton or Bernoulli in the lift explanation causes more than a modicum of confusion UNLESS it's done by including BOTH theories in the explanation. You've done that actually. I would just enhance things a bit more :-) Agreed. The problem is that all these effects interact, and explaining lift is often a matter of preferred viewpoint, as you imply. But it is true that lift always involves the acceleration of an air mass, which is a matter of Newton's third law. How this acceleration is accomplished is irrelevant, provided that it occurs. Bernoulli's effect and many other effects help to explain why air flowing over an airfoil with a positive angle of attack is accelerated at right angles to the direction of flow, but these effects don't produce the lift directly, it's the acceleration that produces the lift. If you build something that accelerates an air mass in the same way without any connection to Bernoulli et al., it will still fly. On the other hand, if you build something that demonstrates Bernoulli's effect but does not accelerate air perpendicular to its flow, no lift results. When I dealt with the lift issue with instructors in seminar, my personal approach was to favor the Newtonian explanation as in my opinion student pilots can grasp Newton a lot easier than Bernoulli, but I've ALWAYS made it habit NEVER to leave Bernoulli out in the cold. Lift is produced by diverting the air flow, thanks to Newton. The diversion in an airfoil is in part produced thanks to Bernoulli. Again correct but with a slightly different approach from me. It's fine to quote the need for an accelerated air mass (relative wind actually) as a necessity for lift creation. The statement is absolutely correct, but again we have to be careful when dealing with someone wishing to dissect Bernoulli and Newton. The plane simple truth of it is that YES, we need relative wind to create lift, and YES, we also need a positive angle of attack to create lift. An airfoil no matter how efficient, at rest with no relative wind in play, creates no lift. Same for the plank of wood. Produce a relative wind on either and introduce a positive angle of attack and INSTANTLY you have lift that can be explained completely EITHER by Bernoulli or by Newton. All we do when we stipulate that a relative wind must be present for lift to be created is to stipulate the CONDITION under which Bernoulli and Newton require for either to produce and explain lift. It's a round robin that always ends up with both of these guys staring us right in the puss with neither of them winning OVER the other . Bernoulli 100% Newton 100% Newton the easier of the two to use as an explanation, but NOT at the expense of Bernoulli! :-)) Dudley Henriques Dudley Henriques |
#20
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Lift - Newton/Bernoulli ratio...
On May 22, 10:02*pm, brian whatcott wrote:
On 5/22/2011 5:15 AM, Dave Doe wrote: Does anyone have any figures and references for about what ratio lift is produced by Newton's Laws and Bernoulli's Laws? I appreciate this is not a static figure - but say a yer average C-172, or perhaps a 737. I would hazard a semi-educated guess that lift is *primarily* produced by angle of attack (or deflection if you like) - Newton's Laws - and by a much lesser degree by Bernoulli's Law. *I would guess that Bernoulli's principle might create 20% of the lift a wing generates. *A friend believes it would be much lesser - about 5%. Think of it this way: Newton: force is proportional to the mass and its acceleration. In this context, the meaning is, to produce the aircraft's weight in lift i.e. upwards , an airmass has to move *with sufficent acceleration to provide that up force. Bernoulii: the mass of air flowing through a channel times its speed gives the same product even if the channel then narrows to a waist: the air mass has to flow faster, but its pressure drops.. In this context: air flowing in an airstream over a wing sees it bulging (or waisting) and so that it needs to speed up, and pressure drops over the upper wing. Arguments of this type can be used as evidence that 2/3 of the wing lift is produced at the upper surface, and 1/3 at the lower wing surface. The larger truth: air pressure drops over the upper surface of a wing, and increases over the lower surface of a wing, and the resultant downflow balances the lift on the wing. Brian W Does it matter to anyone posting here that the fluid flow described by Berboulli's equation assumes the fluid is incompressible? Does anyone here really believe there is no change in air density as if flows at speeds of a hundred miles an hour past an airfoil? The equation works well for water flow in pipes and around boat hulls. It does not do such a good job of predicting pressures along an airfoil. Stick with Newtonian Physics and the gas laws. |
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