Aces High Bulletin Board
General Forums => Aces High General Discussion => Topic started by: SunTracker on December 27, 2003, 11:09:17 PM
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I was reading about laminar flow wings a few days ago. The website said that a laminar flow wing is symmetrical (top half being the same as bottom half). So this would rule out Bernoulli's Principle of Lift.
So either a laminar wing isnt totally symmetrical, or there is some other factor here.
How does it produce lift?
Found a very neat article on the P-51's wings http://yarchive.net/mil/laminar_flow.html
but it still didn't answer my question.
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Angle of attack, incidence, and thrust generally help.
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Mr. B's principal still applies here !
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If you can and are interested get a copy of " Aerodynamics for Naval Aviators " it does a good job of explaining lift.
NAVWEPS 00-80T-80
Patty Wagstaffs plane also has a symetrical wing and flys very well, how the airstream is circulated over the wing is an impotant factor, the chord of the wing where the leadin edge break point is etc. But for a symetrical wing probably the biggist factor is angle of attack.
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Ok, so the wing is just tilted upwards to deflect air downwards?
How does a symmetrical wing get air to flow faster over the top and slower over the bottom? That is where I am confused.
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http://www.mste.uiuc.edu/davea/aviation/bernoulliPrinciple.html
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Thanks. I found the answer guys, its deflection. A symmetrical wing is cambered to reduce drag. Or in the P-51s case, to produce laminar airflow (which didn't really work).
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All wings produce lift by the same method-- by pushing air molecules down and thereby pushing themselves up. Even a flat piece of cardboard generates lift, if you hold it at an angle.
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Also some wings produce additional lift by have high air pressure on the bottom of the wing, low air pressure on the top.
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These are different faces of the same thing. The high pressure on the bottom exists because air is being pushed down, and the air "pushes back". The fundamental principle here is action-reaction, Newton's third law. That is the principle of the wing and the rocket alike.
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Actually, it's a decreased pressure area above the wing that forms as a result of forward motion through air. The aircraft doesn't create lift, rather it is sucked into the air. Comforting isn't it?
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Through the huge positive lift generator powered by numerous 115 volt / 400 cycle vortex generators, of course.
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It is true that there is a pressure difference between upper and lower surfaces. But this pressure difference is caused by the downward deflection of the air, not the other way around.
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You guys are all wrong. Its skyhooks. They do it all (well, except Paris Hilton, but I hear they are working on that)
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Originally posted by SunTracker
Thanks. I found the answer guys, its deflection. A symmetrical wing is cambered to reduce drag. Or in the P-51s case, to produce laminar airflow (which didn't really work).
And my ma 'n pa tol me dat it wuz lil flyin' munkees under the plain... :(
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Lift is a result solely of the pressure under the wing being higher than that above the wing and the wing is pushed up by the higher pressure. The old adage that nature abhors a vacuum is false.
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That didn't make too much sense AKIron. Why does the higher pressure push it up if nature doesn't abhor a vacuum? ;)
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The big thing on the nose turns really fast and just yanks that dood thru the air.
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Originally posted by mold
That didn't make too much sense AKIron. Why does the higher pressure push it up if nature doesn't abhor a vacuum? ;)
I know you're joking and I'm sure you know this but it's really all about curved space, the thing we call gravity, the thing that "pulls" air to the earth and creates air pressure. If nature abhored a vacuum we wouldn't have so much more of it than anything else and the universe would be filled with evenly distributed matter. And be a much more boring place. :D
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Originally posted by AKIron
Lift is a result solely of the pressure under the wing being higher than that above the wing and the wing is pushed up by the higher pressure. The old adage that nature abhors a vacuum is false.
All this time I thought lift was the result of the pressure above the wing being lower than the pressure below the wing.
Duh, just goes to show how much I know.
;)
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Originally posted by AKIron
And be a much more boring place.
Oh I dunno, might be kinda surreal. Life would be so much easier without the bother of differentiated matter. :D
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Wow this is all very confusing. Let's get back to basic principles. Airflow over the 'top' (top meaning the low pressure side. The wing can be facing any direction up, down left or right. It's all three dimensional) of a wing is faster with a corresponding drop in pressure. This contributes most of the lift. Suction effect. Angle of attack is important too. The greater the angle of attack the more lift generated until the airflow breaks away at the critical angle and the wing stalls. A symmetrical airfoil depends on angle of attack for generating lift. That is the Bernoulli principle, crudely put. However there is a big controversy raging. There are those who state that is the interaction of the airflow as it meets the trailing edge of the wing which imparts thrust to the wing which translates into lift. Battle lines are drawn. It's funny here we are a hundred years later and we are still arguing just how wings work.
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Originally posted by cpxxx
Airflow over the 'top' (top meaning the low pressure side. The wing can be facing any direction up, down left or right. It's all three dimensional) of a wing is faster with a corresponding drop in pressure. This contributes most of the lift. Suction effect.
Actually, NASA recently did a study where they measured air speeds above and below a wing using dust particles...they found that the dust going over the top trailed the dust going along the bottom by the difference in surface lengths, i.e. the air on top was not going appreciably faster. Anyone remember that study? Oops.......damn, whose gonna clean up all these jets? :D
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Exactly Mold, One hundred years later and we still don't know how them airplanes stay up in the air for sure:( :confused:
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Good thing air isn't made out of dust particles!
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I found this (http://www.av8n.com/how/) page to be helpful.
Chapter 3 talks about airfoils.
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Originally posted by cpxxx
Exactly Mold, One hundred years later and we still don't know how them airplanes stay up in the air for sure:( :confused:
Sure we do. ;) It's really simple at the most basic level. Take a piece of cardboard and hold it at an angle like a wing, and move it through the air. That makes a downward breeze. How does it do that? By deflecting or pushing the air down. The air hits the board and is forced along the angled side, which means it is going down. A wing works the same way.
Ask yourself--why is the pressure lower on the top side? The only way to create a pressure difference (without heating or cooling or helium balloons) is to take some of the air above and shove it below. When you take air from one place and put it in another, you get a pressure difference. The laws of fluid dynamics, including Bernoulli's Law, are derived from Newton's equations of motion.
Suntracker-- the dust particles follow the air particles, and there's no reason to think that they don't travel at the same speed. ;)
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Did a quick Google
(http://www.ebtx.com/mech/tubes.gif)
The action of bulk air and molecular air. (http://www.ebtx.com/mech/mech03.htm)
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One thing to keep in mind about Bernoulli's Law. It is an application of the law of energy conservation. As such, it can be misapplied. In particular, it can be misapplied if energy is added or removed from the system in question.
Try this experiment. Take a piece of paper and hold it horizontally in front of your mouth, hanging down. Blow across the top of it--you will see it flutter up. Now blow across the bottom of it--and it will still flutter up. Why doesn't it flutter down? Now hold it vertically downward and blow across the top of it--it doesn't move at all. Why not? Where is the Bernoulli effect?
The answer to this question is that the increased kinetic energy from the higher air velocity does not need to be compensated by a drop in potential energy (lowered pressure), because you are adding energy to the system by blowing on it. So the pressure stays the same, and the speed increases. Energy is not conserved, so Bernoulli doesn't work.
Now consider a venturi, such as can be found in a carburetor or a spray bottle for household cleaners. These devices do not work without a venturi. A venturi is a double-ended funnel that squeezes the air and then releases it. Clearly the Bernoulli effect is what makes these devices work. How? The moving air itself is not enough, because again the increased velocity is caused by energy added to the system. But a venturi, now that is something different. The venturi funnel is able to change the velocity of the air without changing the energy of the air. Since the force of the funnel walls is applied perpendicular to the airflow, no energy is added that way. And since the air in question is incompressible, it must travel faster down the narrow venturi throat. This increased kinetic energy must come from somewhere, and in fact it comes from the potential pressure energy. So pressure drops. Here is a real example of the Bernoulli effect at work.
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OK apparently everyone talking about lowered air pressure and Bernoulli's principle didn't stop to read the article described in this link
Article of why things fly and stuff (http://www.aa.washington.edu/faculty/eberhardt/lift.htm)
This was the 5th post in the thread by Dingbat.
Read it...
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this is the SAME kinda fediddlein headache I used to get in trig. class
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FINE!
Very short version would go like this...
The low air pressure caused by the top surface of the wing and high pressure caused by the bottom surface isn't quite what is going on. Lift is not coming from the HIGH pressure under the wing. Lift is coming from the mass of air above the wing racing down to fill the low pressure caused by the top surface of the wing. Newton's law... for every action there is an equal and opposite reaction. This air above the wing surface is racing down and oddly enough pushes the wing UP. In a way the aircraft is not lifted up so much as it is sucked upward! I'm over simplifying a hell of a lot but if you read that article that Dingbat posted and follow it all becomes clear.
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This pic from the article looks to be showing the pressure from under the wing forcing it up to me.
(http://www.inettek.com/stuff/lift.gif)
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Laminar flow means smooth (no turbulence) airflow, for any given aerodynamic shape.
Laminar flow wing profile (P-51) doesn't neccesarily mean a symmetrical wing profile, does it?
Did the P-51 have a symmetrical wing?
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Since we're on the subject of wing profiles:
Have any of you had the opportunity of seeing (and touching) the leading edge of a F-104 Starfighter wing?
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I emailed my father about this and without referencing this is what he wrote.Hope it helps.
A laminar-flow wing section(airfoil) is shaped so that the air flows over it in uniform layers(i.e in laminars) without any outward cross-flow that creates turbulence and,therefore,drag.
Any wing section has some laminar flow that is maintained until just aft of the maximum thickness position when it slows down and starts to separate,creating cross-flow,instead of following the wing contours.Compared with a conventional wing airfoil,the thickness point of a laminar-flow airfoil is moved further aft-to,say,70% chord instead of 30% chord-so that laminar flow may be maintained to perhaps80% chord.
Laminar flow cannot be maintained to 100% chord without some means of sucking the boundry-layer flow down onto the wing surface,such as using a porous surface through which the air is drawn to be vented overboard.Alternatively,air may be drawn in through a slit near the leading edge and expelled through another slit near the trailing adge to re-energize the slowing airflow.This boundry-layer control technique has been done experimentally(notably by Handley Page back in the 50's),but is impractical for operational aircraft since the small holes or slits tend to become plugged by insects,ice,rain,grease,dirt,etc.Also,an insect or bird that sticks to the leading edge produces a wide V-shape wake behind it that destroys laminar flow(and therefore lift) over that part of the wing.
Natural laminar flow,obtained by careful shaping,is very effective and may be applied to the fuselage from the nose back to the wing.The best example of this is the Dornier 328 turboprop which has fairings added to the fuselage cross-section.I thought that this might have been achieved so,when I attended the 1992 Berlin Air Show,I asked the Dornier cheif designer if that was the case,and he confirmed it was so.(He was very pleased that someone had realised what had been done.)
A laminar-flow section--or any other high-speed wing profile--may be symmetrical or near symmetrical to minimise cruising drag.Lift is obtained by increasing the angle of attack so that the air moving over the top of the wing has further to travel than air passing underneath it and so it speeds up to cause lift.
For lower-speed aircraft,it is usual to set the wing at an angle of incidence to generate extra lift for takeoff when the aircraft is heaviest.If this fixed angle is too large,then the aircraft will fly nose-down in cruise which effectively increases fuselage cross-sectional area and thus creates profile drag.(The classic case was the Armstrong Whitworth Whitley bomber.)
Nowadays,when any desired engine power is available for take-off,use of full power combined with wing flaps permits the wing to be set at a small angle of incidence not far from that angle of attack which is needed for cruising flight where the aircraft spends most of its time.Likewise,wing area is a compromise between the bare needs for cruise and the size needed for take-off and landing.Here,speed range is the key.For a light airplane that stalls at 60 knots and has a top speed of 140 knots,the range is small.But in a Mach 2 fighter,the speed range is very large so engine afterburning is used in combination with leading-and trailing edge flaps for take-off,while some special means of creating lift(e.g blown flaps) is needed for landing.Even then,it may be necessary to use a drag parachute and an arrestor hook(e.g Lockheed F-104 Starfighter) connected to cables that drag paddles in water channels on either side of the runway.
Another way to obtain laminar flow is to construct the wing so that--like a dolphin or manta ray swimming--it changes its shape according to speed.This is a dificult thing to do and so far is still experimental.In great confidence,I was told about this "adaptive skin" technology in 1970 when I was helping the corporate vise-president of engineering,Al Cleveland,to write his Write Brothers Lecture that year for the American Institute of Aeronautics and Astronautics.This year,a test aircraft was flown for the first time--more than 30 years later--and it's still nowhere near being applied practically.
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I thought it was fairy dust.