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Location: Episode 97: Airplane on a Conveyor Belt
Discussion: Bernoulli Priciple
Keyword tags:
aerodynamics
air speed
airplane
bernoulli
lift
Watch
 cknox |
Bernoulli Priciple
Jun 20 2008, 3:04 PM EDT The Bernoulli Principle describes the relation of a fluid speed to its pressure. Air, being a fluid, experiences this same correlation and because such, the Bernoulli Principle has often been misrepresented as what causes the lifting affect on an airplane wing. In reality, the Bernoulli Principle has negligible affect on an airplane's wing, generating lift or otherwise. I'll qualify this as being below mach speeds. The common explanations are: 1. Air has farther to travel across the top than across the bottom of the wing, so it goes faster, which generates lower pressure, relative to the bottom. The difference in pressures produces the force known as lift. 2. A cutaway of an airfoil is compared to the cutaway of a venturi. Because the Bernoulli Principle explains the lower pressure at the greatest restriction, say in a carburetor, that similar shapes mean the airfoil works the same way. Both explanations are flawed. Ex.1 - Air does not travel faster over the top of a wing. This misconception was probably someone thinking that two air molecules need to come back together at the tail end, but they don't. This is what generates the vortex behind the wing. Ex.2 - A pure stretch of the imagination. Similar in shape, but not environment. The venturi encircles the air flow, the airfoil does not. This changes how the air reacts to these different surfaces. Air above the wing actually has to compress to get out the way of the wing. The faster the wing speed, the greater the compression. 3 out of 9 found this valuable. Do you? |
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cknox |
Bernoulli Priciple (cont.)
Jun 20 2008, 3:34 PM EDT The real cause of lift is the acceleration of air downward. Yes, good ol Newtonian physics at play, F=ma. Acceleration does not mean a change in speed. Well, yes, but this is the air being sped up in a downward direction, which means it comes from somewhere, which is the air being sped up in the direction of the wing travel. This is what causes the induced drag. This does not mean the air is traveling faster across the top of the wing... The acceleration is the change in direction. The energy to do this comes from the kinetic energy of the airplane relative to the air. This energy is replenished by the thrust of the airplane engine. This 'relative' is could be the airplane traveling forward to stale air, a strong headwind with the plane stationary, or any combination in between. To demonstrate this, take a block of material, clay, wood, metal, plastic, ice, what have you. Then shape the top into an airfoil. A little mockup of a wind tunnel will show this generates some lift, even if not the most effiecient. Next, take the same material and carve the shape in relief (concave). Obviously, if this is the same shape, just inverted, it will have the same length across the top. With the same mock up wind tunnel, this shape should also generate lift if the top length had any affect. But you will quickly discover that it doesn't as this shape generates no more lift than a brick. What's the difference? The first shape, air is directed downward. Simple math will show how much air is required to produce the mass and the acceleration needed to counter the affect of weight of the plane. Now that you know where the real lift comes from, I'd like to see MB demonstrate this so everyone can disspell Bernoulli as the source of lift. 2 out of 4 found this valuable. Do you? |
 weaponsboy |
RE: Bernoulli Priciple (cont.)
Jun 20 2008, 4:10 PM EDT "The real cause of lift is the acceleration of air downward."If that's true, explain how a plane with a perfectly level wing gets into the air. Both Bernoulli's Principle and air deflection cause lift. 1 out of 2 found this valuable. Do you? |
 NODOUDT |
RE: Bernoulli Priciple (cont.)
Jun 21 2008, 12:58 PM EDT Hi, Here is a link to the FAA’s Pilot’s Handbook of Aeronautical Knowledge. http://www.faa.gov/library/manuals/aviation/pilot_handbook/ To further your discussion you may wish to use this book and reference chapters 2 & 3. Chapter 2—Principles of Flight Newton’s Laws of Motion and Force............2-2 Bernoulli’s Principle of Pressure...................2-3 Chapter 3—Aerodynamics of Flight Forces Acting on the Airplane.......................3-1 I’d suggest that at least the first three chapters should be read and generally understood so the various details can be discussed and placed in their proper context. The document can also serve as an excellent reference source for starting off most threads related to the topic of subsonic flight in general. Happy Mythbusting! 3 out of 3 found this valuable. Do you? |
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cknox |
RE: Bernoulli Priciple (cont.)
Jun 23 2008, 6:12 PM EDT My point, exactly. The FAA publishes these concepts and accepts the Bernoulli Principle on their tests as to what causes lift. The NASA website does also. I agree with the explanation for the Magnus Effect, when applied to a rotating cylinder or baseball. The viscosity of the air 'drags' on the lower surface slowing it down. The air across the upper surface is also dragged, pulled, accelerated up, over, then downward. In this case, there is a combination of downwash and Bernoulli's Principle affecting the airstream. Note, however, that the airstreams do not rejoin and resume at identical velocity's as described in the link. This would require a shape that would cause no eddies, which most shapes produce. Additionally, the next paragraph takes the Magnus Effect a step further, which I cautioned about, and correlates circulation to lift as applied to an airfoil or airplane wing. This is what probably lead into applying Bernoulli's Principle to airfoils, described later in the same link. This infers that the air accelerates in speed because of the longer surface vs. accelerating as in a change in direction. The link goes on further, citing the typical venturi example and garden hose. Again, the air is not constricted or restricted to a venturi tube or hose, which enables the downwash. The saving grace of the link is that on p. 2-6 it states that both Bernoulli's Principle and Newton's laws are in operation whenever lift is being generated by an airfoil. But this is such a small sentence among six pages of wrongful application of Bernoulli's Principle. Do you find this valuable? |
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cknox |
RE: Bernoulli Priciple (cont.)
Jun 23 2008, 6:13 PM EDT Rather than Magnus or Bernoulli effects, the Coanda Effect does a much better job explaining the downwash. Hold a spoon by the handle and bring the convex side against a stream of water from a faucet. The water will follow the curve of the spoon and no longer drop straight down (downwash, sidewash in this orientation). The affect is that the spoon is drawn into the stream (lift). It's only drawn in until it reaches equilibrium. If Bernoulli were at play, turning the spoon around so the water hit the concave side would have an equally long surface to follow, accelerating and generating lower pressure, and would also draw the spoon into the stream. But this doesn't happen. What happens is that the water follows the curvature of the spoon and creates a sidewash (downwash if oriented for an airfoil in an airstream) pushing the spoon away from the stream. I'm not disputing Bernoulli. I'm debunking the misapplication of his ideas to the theory of flight. I've been interested in fight and flying for over 30 years and have my pilots license over 15 of those. I had, through independent study, come to discover that Bernoulli's Principle was insufficient in describing the complete amount of lift required to sustain flight of the modern aircrafts. I had bounced my Coanda Effect paired with Newton's second and third laws to describe a very close approximation (90+ percent) of lift and was urged to formalize and publish this. Because of my lack of ambition toward this prospect, someone beat me to the punch. About 3-4 years after discussing my concepts, "Understanding Flight" by David Anderson and Scott Eberhardt was published and does this explanation better justice than I have in these past few sentences. (I don't think they know any of the people I talked to and their book covers many more interesting aspects that I hadn't thought about, so they can have full credit) Do you find this valuable? |
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cknox |
RE: Bernoulli Priciple (cont.)
Jun 23 2008, 6:24 PM EDT The typical debunk for "flight according to Bernoulli advocates" is inverted flight. The upper surface, just as long but now on the bottom, would 'lift' the aircraft into the ground. However, the angle of attack is changed to create the negative lift in inverted flight (double negative = positive). The application of BP doesn't work for inverted flight... But there's still F=ma... Do you find this valuable? |
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EnderGT EnderGT |
RE: Bernoulli Priciple (cont.)
Jun 24 2008, 10:14 AM EDT "The typical debunk for "flight according to Bernoulli advocates" is inverted flight. The upper surface, just as long but now on the bottom, would 'lift' the aircraft into the ground. However, the angle of attack is changed to create the negative lift in inverted flight (double negative = positive). The application of BP doesn't work for inverted flight... But there's still F=ma..."All this talk about Bernoulli is great, but it has little to no impact on the myth. No one, and I do mean no one, disagrees that air flow over/under the wings is necessary for lift and thus flight. The disagreement that keeps popping up is whether or not a conveyor/treadmill can prevent a plane from moving. Someone needs to do a better writeup than I've been able to do to really explain this once and for all. 0 out of 1 found this valuable. Do you? |
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cknox |
RE: Bernoulli Priciple (cont.)
Jul 21 2008, 12:25 PM EDT This isn't about the conveyor/treadmill being discussed on another thread. Simple math demonstrates that the thrust an airplane generates easily overcomes any bearing friction or rolling resistance imposed by a treadmill. However, you will always find exceptions, like an ultralight on a treadmill moving at 7000 mph. But keeping things realistic, there is no need to demonstrate this and it can be put to rest. What I was proposing involves educating the aviation, and general public, that Bernoulli is being given more credit than due, as his principles are being misapplied as an explanation of aerodynamic lift for an airfoil/wing. The 'myth', in this case, being that the Bernoulli Priciple causes lift. 1 out of 1 found this valuable. Do you? |
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weaponsboy |
RE: Bernoulli Priciple (cont.)
Jul 21 2008, 1:03 PM EDT I just tried the thing you said about the spoon, wasn't drawn in. I might not have held it right (at its center of gravity) though. 0 out of 1 found this valuable. Do you? |
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weaponsboy |
RE: Bernoulli Priciple (cont.)
Jul 21 2008, 1:13 PM EDT I think that if flight was not caused by Bernoulli's Principle, there would be more people than just you saying that it's something else. I've read books on aviation, seen shows on it, and did a unit (we called it a module) in an engineering class last year on it; they all say it's Bernoulli's Principle. I'm not saying you're wrong, I'm just saying that there should be more people saying this if it were true. 0 out of 1 found this valuable. Do you? |
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cknox |
RE: Bernoulli Priciple (cont.)
Jul 21 2008, 2:38 PM EDT Holding the spoon by the end works fine. Note how it hangs. Turn on the faucet so there's a smooth stream (no airator). Note where the water lands. Bring the back of the spoon into contact with the stream, slowly. As it comes in contact, the spoon will move further into the stream and the stream will move toward the spoon (after the point of contact, of course). My point, exactly. There's nearly a hundred years where this principle has been used, accepted, and regurgitated in one form or another, without question. I wish I could find the PDF on the web which is a excerp from the book mentioned above. It did a fabulous job of explaning this. So much so, I bought the book. Do you find this valuable? |
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cknox |
RE: Bernoulli Priciple (cont.)
Jul 21 2008, 2:58 PM EDT This is a link which contains a slide show which are excerps from the book. http://vmsstreamer1.fnal.gov/Lectures/Colloquium/Anderson/index.htm Do you find this valuable? |
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EnderGT EnderGT |
RE: Bernoulli Priciple (cont.)
Jul 21 2008, 11:09 PM EDT "Simple math demonstrates that the thrust an airplane generates easily overcomes any bearing friction or rolling resistance imposed by a treadmill. "You'd like to think so, but the sheer volume of idiots that keep popping up out of the woodwork to dispute it amazes me (and leaves me with very little hope for the future of humanity). I'm not sure exactly how much of a wing's lift is due to Bernoulli vs deflection, but I agree with you that Bernoulli is not the entire answer. Do you find this valuable? |
