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Bernoulli's principle - some myths

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Hi fellow pilots!I've been researching some information from NASA about how aircraft actually fly, and it turns out that lift generated from pressure differentials between the underside of the wing and the upper wing only accounts for about 2 percent of the lift!! The rest of the lift is actually generated from the reaction caused (Newton's third law of motion dictates that every action is directly opposed by a reaction of equal magnitude) by "pushing" the air downwards.Many experiments have been performed on wings and it has been found that the overall shape of the cross-section of an efficient wing works extremely well at displacing air downwards, hence causing a lift reaction in the wing.NASA have known about this over 50 years, which makes me wonder why most aerodynamics texts purport to state a "fact" when claiming that the pressure differential caused by the Bernoulli effect is what constitutes most of the lift.What do you think?James

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James, The angle of incidence and subsequent angle of attack of a wing seems to be good testimony to the theory that air forced downwards produces more lift than differential pressure. But, don't you think that as air is forced downward, under the wing, that there is a pressure differential created? There definately is and the pressure is higher where the air is being forced to go. As air is forced downward the area where the air was drops in pressure and the area where the air goes rises in pressure. What NASA is doing is allocating percentages of lift to the effects of pressure differential and air flow. You can't have one without the other and it's kind of like saying that a propeller doesn't actually push its way through the air but sucks its way along. Six of one, half dozen of the other. NASA does a lot of experimenting with high-lift wings, boundary layer control, etc. In their world of unknowns and unproven theories the difference in press diff and airflow sometimes makes big differences. To all pilots of all conventionally accepted flying craft the differences are mute and undetectable.Glenn

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Hi Glenn,It turns out that most aerodynamicists have their own theories, and some aspects of aerodynamics are still a bit of a mistery. I personally believe that the pressure differential caused by air moving faster over the top of the wing is actually minor, and does account for only 2 percent of the overall lift. The downdraft caused by the cross-sectional shape of the wing, as you suggest, causes a greater pressure differential, and I agree.The phenomenon of coalescense is in fact the main factor in generating lift. This is what causes a fluid such as air or water to "cling" to a curved surface. When you blow over the top of a sheet of paper it is pulled up, which is caused by this very effect. Another experiment is to run a tap over a spoon. The spoon will want to move towards the stream while the stream follows the contour of the spoon. This is caused by the reaction of water being forced to flow away from its natural state.Henri Coanda discovered this effect. His "reaction" jet caught fire with him inside it at the Paris Airshow in 1910 (that's right, 1910!).Another myth is that air travels over the top of a surface because it needs to meet up where it left of and join the same molecules it pulled away from. The reason air travels faster is because it has longer to travel.James

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When air pressure pushes against a wing, the air will be pushed away from the wing. The vertical forces on both the wing and the air will be in direct proportion to the air pressure.Since the pressure against the top of a wing is lower than the pressure against the lower surface, the air against the top of the wing will be pushed away at a lower velocity than the air under the wing. Net force against air is downward, while net force on wing is upward.Some aircraft have leading edge flaps, which are sometimes referred to as slats. These surfaces extend the wings forward edge downward and sometimes forward. Combined with conventional trailing edge flaps they must create a significant pocket of air that is accelerated forward by the wing and therefore has a lower relative velocity across the lower surface of the wing. Not all leading edge flaps have openings between the flap and the wing. The US Navy

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>>air forced downwards produces more lift than differential pressure.Glenn,This statement makes little sense - jus analyze it on a purely logical level.Lift is force. This force is a difference between the force that acts on the wing from underneath and from above. The forces in this cases are pressures - you could put pressure gauges at every square inch of the wing and measure them. You can argue what physical laws generate those forces and what factors contribute more/less but the final force is but one. So saying that air forced downward produces more lift than differential pressure is equivalent to "air forced downwards produces more lift than lift".Michael J.http://www.reality-xp.com/community/nr/rsc/rxp-higher.jpg

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Sailboats use this same means for forward motion into the wind. The angle of attack makes all the diffrence, if you trim a sail to loosely the sail begans to luff(ie stall). 70% of a sailboats drive is from lift. The sail is just a wing turned sidways. this is how a sailboat can sail into the wind, along time ago, in the days of square riggers the could only really run downwind.Hope this helps.

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Michael, Read my post again and you should realize that I never said that air forced downward produces more lift than differential pressure does. That statement is simply not there. What I did say was that the AOI and AOA SEEM to be testimony to the statement you quoted. In my next sentence I stated that it was not true. You took a few words from a complete sentence and called that a statement. The complete sentence is what you should read and understand. I stand by everything I said in my post and nothing I said can be proven to be otherwise. Read the post again. Glenn

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>A climb converts velocity into the kinetic energy of higher altitude too.The climb is the kinetic energy, the increased altitude is an increase in potential energy.Paulhttp://www.strontiumdog.plus.com/sbird.jpgOfficially licenced by British Airways plc for use of name and logo[p]AMD XP2800+ Barton, Gigabyte GA-7NNXP nForce2, 1Gig Crucial PC3200 DDR 400MHz, Gainward 128 MB GF4-4200, SB Audigy, 3 x WD Caviar SE[/p]

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Just pick up the book Stick and Rudder, Bournoulli's principle and Newton's law take part it producing lift. As the air ON TOP of the wing is excellerated AND deflected downward from ABOVE the wing, the wing is forced UP. The majority of lift is from this downwash from above the wing.The downwash from above the wing is the downwash NASA is talking about, not downwash from below the wing.Without Bournoulli's principle the air above the wing wouldn't accelerate and be deflected down, without Newton's law the wing wouldn't be pushed up by the downwash from above the wing, it takes both to make lift.

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