Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: SgtPappy on October 10, 2008, 11:01:22 PM
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Here I am with yet another question. Kind of makes me look like I do no research of myself but hey. I can only find so much.
Various sites and books state a bunch of different theories of lift. How does an airfoil create lift? Some theorists (like my gr12 physics teacher) claim that the air at the top of the wing must catch up to the air traveling at the bottom of the wing once those air molecules reach the trailing edge of the wing. Since the top part of the wing is curved, the air molecules must travel faster and the pressure difference creates lift. Honestly, there's nothing to support the assumption that air molecules somehow stay together after being split by an airfoil. NASA, too disproves of this theory. It would essentially mean that no matter what the AoA is, the airflow over the top is always faster than the flow at the bottom. Clearly impossible since planes can fly upside down and negative elevator creates negative lift.
(http://upload.wikimedia.org/wikipedia/en/2/2d/Equal_transit-time_NASA_wrong1.gif)
Theory:http://www.lerc.nasa.gov/WWW/K-12/airplane/wrong1.html (http://www.lerc.nasa.gov/WWW/K-12/airplane/wrong1.html)
Another theory (the most promising one) is that the chord line of an airfoil is angled upwards (leading edge facing up, trailing edge of the wing pointed down). As air travels over the top of the leading edge, the obstruction of the upward-pointed leading edge causes air molecules to be directed upwards. As these air molecules are pushed up, they are pushed down by the air molecules above them in result of Newton's 3rd law (not fully a Venturi principle since the air is pushing on itself and the wing is causing this). This compression speeds up the air over the top of the airfoil. The air at the bottom of the wing is less obstructed and is travels a near straight line. The upward motion of the air above the wing, however, pulls on the air at the bottom of the wing (since air molecules, like water molecules tick to each other) 'loosening' the air and actually spreading it out over the bottom of the wing, slowing it down, causing a high pressure effect. The resultant pressure difference causes the lift. NASA states that the compression of the air cannot be accredited for the velocity field, but the theory seems completely logical. The NASA version of the incorrect Venturi theory, does, however only take airflow on top of the wing into account, when we know that the bottom must be designed to give a slower airflow.
(http://www.ivorbittle.co.uk/Section%206%20The%20keel%20and%20bulb%20with%20compressed%20graphics%2015.10.2006_files/image026.gif)
Theory:http://www.lerc.nasa.gov/WWW/K-12/airplane/wrong3.html (http://www.lerc.nasa.gov/WWW/K-12/airplane/wrong3.html)
So how is lift produced?
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The clearest (correct) explanation I have seen is from late Richard Shevell, a former aeronautics professor at Stanford:
"A simple feeling for lift can be obtained by realizing that the wing is a device for pushing air downward. The wing gives the air a downward momentum. The air 'pushes' back on the wing, producing a lift. The resultant downward velocity of the air is the downwash.
There is nothing wrong with this thinking except that one cannot get any useful information from it. Questions such as "how much lift for a given angle of attack?", how much drag to produce the lift?", " how much change in the local air velocity on the wing ?", cannot be answered except through the use of potential theory combined with the theory of circulation.
Potential theory is beyond the scope of this discussion. It allows the calculation of flow around bodies of various shapes, including airfoils. Forgetting viscosity of the air for a moment, i.e. considering a 'perfect' fluid, the theory is excellent for symmetrical shapes but failed for airfoils at some angle of attack because it showed zero lift and zero drag. For over 150 years this fact was used to show that the theory was useless because it was known that there is lift and drag in experiments. Then in 1902 a German scientist named Kutta realized that the theory showed the flow on the bottom surface of the wing curled around the trailing edge and moving forward for perhaps 5% to 10% of the chord before turning aft in the direction of the freestream. But observation showed that the fluid flowed smoothly to the trailing edge of the airfoil (except of course when the airfoil is stalled at too high an angle of attack, a viscous phenomenon).
Kutta assumed that some correction must be applied to the theory so that the flow continued to the trailing edge.This is the well known "Kutta condition".
The correction was to assume a "circulation", a vortex flow, superimposed on the airfoil. When a circulation is applied of an amount necessary to make the flow go smoothly to the trailing edge on both top and bottom surfaces, (meet the Kutta condition) the theory showed a lift very close to experiment and calculated the velocities along the surfaces very well. A wing can be thought of as a vortex extending from one wing tip to the other. At the wing tips the vortices trail aft as many of you have actually seen on a high humidity day. Having established the local velocities which are generally higher on the top surface, the Bernoulli equation can then be used to determine the local pressures and the lift.
The lift produced by the wing 'bound' vortex (so called because the center of the vortex is bound to the wing) is similar to the forces produced on spinning golf, baseball, or tennis balls to make their trajectories curve.
For further information on lift and circulation refer to "Fundamentals of Flight", R.S.Shevell, Prentice-Hall 1989 , "Aerodynamics of the Airplane",C.B. Millikan,McGraw-Hill, circa 1943, or similar textbooks.
Some people including some authors, think that circulation is a mathematical theory not a physical thing. However, it is truly a physical process and the theory explains not only lift but drag due to lift also known as induced drag, ground effect, why birds fly in an approximate 'V' formation, etc. If you want to see circulation develop on your dinner table, place a soup spoon in soup with fat particles or chopped parsley. Set the spoon at a low angle of attack and move it through the soup, and watch the vortices develop. World's cheapest windtunnel!"
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Kutta's theory is probably the best to explain lift. there is a good explanation in this online book:
John Denker's See how it flies (http://www.av8n.com/how/). The author is a pilot and a physicist, it's not an easy reading (at least for me :) ) but I think you'll find Chapter 3 and 4 very interesting.
Another good site for flight theory and piloting, is the Australian Ultralight Federation's, where there are a lot of info about flying:
AUF's Flight Theory guide (http://www.auf.asn.au/groundschool/contents.html). The "Airfoil and wings" section is what you need.
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best thing to do.......head out to your local airport, and find a CFI. they can explain it to ya very clearly.
try here too......http://www.allstar.fiu.edu/aero/airflylvl3.htm
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Here's the best website I've ever seen on the topic:
http://www.av8n.com/how/htm/airfoils.html#sec-airfoils-summary (http://www.av8n.com/how/htm/airfoils.html#sec-airfoils-summary)
I've linked to the summary, but be sure to scroll up!
EDIT: just realised Gianlupo had posted a link to the overall website.
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Don't hook up on the airfoil alone, for lift requires area, thrust and angle of attack.
Think of it as holding a metal sheet in a storm, - if you apply angle of attack you may be airborne....
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Don't hook up on the airfoil alone, for lift requires area, thrust and angle of attack.
Think of it as holding a metal sheet in a storm, - if you apply angle of attack you may be airborne....
easier yet........stick your hand out the car window, palm down and flat. now tilt the leading edge up slightly, then down slightly. :aok
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easier yet........stick your hand out the car window, palm down and flat. now tilt the leading edge up slightly, then down slightly. :aok
That's how I did it the first time I was brooding over lift! :)
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A cool demonstration of an airframe featuring complete flat wings are current RC indoor aerobatic planes. The reason for this is that the flat wing starts to stall (ie airflow separates) at very low AoA - just few degrees - so there is no sudden separation or stall at higher AoA as with normal wing profiles (typically somewhere around 15-20 deg). In practice this means that RC indoor planes can be steadily flown through high AoA maneuvers because the separation has allready happened unnoticed at lower AoA and this also demonstrates well how the AoA is the key factor for the lift generation despite there is no profile and wing is actually stalled during maneuvering.
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A cool demonstration of an airframe featuring complete flat wings are current RC indoor aerobatic planes. The reason for this is that the flat wing starts to stall (ie airflow separates) at very low AoA - just few degrees - so there is no sudden separation or stall at higher AoA as with normal wing profiles (typically somewhere around 15-20 deg). In practice this means that RC indoor planes can be steadily flown through high AoA maneuvers because the separation has allready happened unnoticed at lower AoA and this also demonstrates well how the AoA is the key factor for the lift generation despite there is no profile and wing is actually stalled during maneuvering.
Yep, and the fact they can be flown like they are flown is the power to weight-ratio which is "out of this world" compared to the WWII fighters, for example. Good example is the "slowing to hover - move" when a flat plate rc-plane translates from normal controlled flight into a hover. "It just can do it" because of the rediculous thrust to weight ratio.
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A cool demonstration of an airframe featuring complete flat wings are current RC indoor aerobatic planes. The reason for this is that the flat wing starts to stall (ie airflow separates) at very low AoA - just few degrees - so there is no sudden separation or stall at higher AoA as with normal wing profiles (typically somewhere around 15-20 deg). In practice this means that RC indoor planes can be steadily flown through high AoA maneuvers because the separation has allready happened unnoticed at lower AoA and this also demonstrates well how the AoA is the key factor for the lift generation despite there is no profile and wing is actually stalled during maneuvering.
well...that and even the smallest ones are severly overpowered. :D
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Funding.
That's what makes your birds go up.
No bucks, no Buck Rogers.
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Funding.
That's what makes your birds go up.
No bucks, no Buck Rogers.
lol. Love it!
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No bucks, no Buck Rogers.
:lol
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Yep, and the fact they can be flown like they are flown is the power to weight-ratio which is "out of this world" compared to the WWII fighters, for example. Good example is the "slowing to hover - move" when a flat plate rc-plane translates from normal controlled flight into a hover. "It just can do it" because of the rediculous thrust to weight ratio.
Some WWII planes could do sort of stalled maneuvers, like the one in your signature; at full power it could hang on propeller, mushing forward 130-140km/h at around 60deg angle (see Kokko's report). Probably some other planes like the P-38 could do similar things. Anyway, the control is probably very limited at such condition.
Harrier style maneuvers does not require ridiculous thrust to weight ratio, just good control at slow speed. I remember when the first IFOs came around, these had thrust to weight ratio some what below 1 with the power setups available that time. However, these could easily do 60-70 deg harriers with good control despite being unable to hover due to limited power (or too heavy batteries, that was before the high current lipos).
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Wow that's an amazing site. I'll be sure to show my physics teacher. :aok
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Most problems visualizing lift are because of associating air with 'nothing' but once you look at air as a 'fluid medium' (similar to water but compressible also) it all begins to make more sense.
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Most problems visualizing lift are because of associating air with 'nothing' but once you look at air as a 'fluid medium' (similar to water but compressible also) it all begins to make more sense.
Agree.
That's why you see scientists and engineers using smoke when when checking drag on an object inside a wind tunnel.
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The Venturi effect, Bernoulli’s principle and their effect on an airfoil are not just theories, but observable facts:
http://www.youtube.com/watch?v=13eoSasj4hw
http://www.youtube.com/watch?v=6UlsArvbTeo
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Defies my logic how a deflected airflow can move faster than undeflected but as is seen in second link it apparently does. So does that mean that air actually "moves faster" through the low pressure area on top of wing?
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This one is interesting too: http://www.youtube.com/watch?v=QaamEo6WyI4&feature=related
And this: http://www.youtube.com/watch?v=0q3OlZWpOww&feature=related
-C+
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Some WWII planes could do sort of stalled maneuvers, like the one in your signature; at full power it could hang on propeller, mushing forward 130-140km/h at around 60deg angle (see Kokko's report). Probably some other planes like the P-38 could do similar things. Anyway, the control is probably very limited at such condition.
Yes, I'm aware of the 109's ability to briefly "hang on it's prop".
Harrier style maneuvers does not require ridiculous thrust to weight ratio, just good control at slow speed.
Yep, but like I said, "slowing down to hover" does require thrust to weight over one when it's done without altitude loss and the hover is sustainable, as after all it's called a hover. And thrust to weight over one is "out of this world" when talking about WWII fighters. :)
As you probably know Turbo Raven had and Turbine Toucan has a very interesting performance. :) Toucan's t/w is around 1.65 at display weights. :D Even piston engined (MP-14) Python Pitts has t/w just over one. Interesting planes indeed. :)
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About turbo prop biplanes...
Didn't bother to look up the exact t/w on this one but the article sure is a fun read! :rofl
(http://www.airbum.com/pireps/GreatLakesSpread.jpg)
http://www.airbum.com/pireps/PirepGreatLksTrbn.html (http://www.airbum.com/pireps/PirepGreatLksTrbn.html)
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Agree.
That's why you see scientists and engineers using smoke when when checking drag on an object inside a wind tunnel.
ACTUALLY,
those of you with r/c models can do that very same thing.
i did a demo for my cadets, showing them what happens when a wing stalls. i used a diagnostic smoke machine that i use at the shop for finding vacuum leaks. had 2 cadets hold my scale T34 in front of a low speed fan, and i put the tip of the hose infront of the leading edge by a couple inches. they could see the smoke going over and under, comming back together at the trailing edge. then i had them slowly pivot the plane tipping the nose up. at around 17 degrees of so, the smoke got very turbulent going across the top of the wing.
it's a very good visual aid.
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Defies my logic how a deflected airflow can move faster than undeflected but as is seen in second link it apparently does. So does that mean that air actually "moves faster" through the low pressure area on top of wing?
It's more intricate than this but yes essentially the air moves faster through the low pressure area on top of the wing. "Newton" and "Bernoulli" are two different ways of describing how this occurs. Both are correct.
The bottom line is that an airfoil changes the direction of airflow around it with the result being lift.
Using Newton (air diversion and reaction) to describe this when you change something's direction you're exerting force which means the wing is exerting force on the air, thus by Newton's F=ma relationship the air must be accelerated. Since the air is being diverted much more by the upper surface of the wing due to positive aoa the air must experience more acceleration there as well. The more you divert the air, the more the air is accelerated.
The Bernoulli school of thought looks at this issue through the lens of conservation of mass and energy. The airfoil changes the direction of the streamtubes of air. Where you have air streamlines being compressed according to conservation of mass and energy you get lower pressure and higher velocties. Again thanks to positive aoa the upper surface of the wing changes the air streamline flow more than below and thus higher velocities and lower static pressure.
The key point is that lift is a result of a wing's ability to change the direction of airflow around it which both the Newton and Bernoulli school's of explanation rely on.
Tango, XO
412th FS Braunco Mustangs
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More about downwash and upwash, or why birds fly in a V -formation:
http://www.aerospaceweb.org/question/nature/q0237.shtml (http://www.aerospaceweb.org/question/nature/q0237.shtml)
(http://www.aerospaceweb.org/question/nature/formation/formation.jpg)
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Why I said it defies my logic was that since the airflow above wing is deflected and thus needs to work more (accelerate) to get to the trailing edge it would still be late compared to that "air mass" that started from the leading edge at the same time running the unobstructed lower side thus resulting in a lower pressure above (and aft) of the wing and thus creating "lift". Apparently to video I referred to this is not so, but the pressure dimension and airflow dimension are separate as is also evident in the links I posted which give some explanation to effects of wing profile in drag and lift generation.
I'm fully aware that aerodynamics is not a piece of cake to comprehend but I thought that I'd have such a basic phenomenon as lift generation in my grasp. Well I don't...
-C+
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Wow that's an amazing site. I'll be sure to show my physics teacher. :aok
Which one?
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My personal favorite method with which to consider it, is through the use of symetrical airfoils. Since both the top and bottom of the airfoil are the same shape and length, you can get away from some of the velocity type definitions. And, for the same reason, you see that zero degrees angle of attack, the airfoil theoretically produces no lift. But, create one degree of angle of attack, and you have lift!
So, while it doesn't explain many of the other phenomena associated with the way an airfoil/wing interacts with the relative wind, you can answer Sgt Pappy's original question with a simple "angle of attack". Its generalistic and brushes over a few other facets of airfoil theory, but in a single sentence, it sums it up nicely, IMHO...
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My personal favorite method with which to consider it, is through the use of symetrical airfoils. Since both the top and bottom of the airfoil are the same shape and length, you can get away from some of the velocity type definitions. And, for the same reason, you see that zero degrees angle of attack, the airfoil theoretically produces no lift. But, create one degree of angle of attack, and you have lift!
So, while it doesn't explain many of the other phenomena associated with the way an airfoil/wing interacts with the relative wind, you can answer Sgt Pappy's original question with a simple "angle of attack". Its generalistic and brushes over a few other facets of airfoil theory, but in a single sentence, it sums it up nicely, IMHO...
No, not really. The more the angle of attack, the more of the rounded leading edge curves to the top of the airfoil, and the flatter the bottom of the airfoil. Simply by changing the angle of attack you also change the effective shape of the airfoil.
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No, not really.
Yes really. Sgt Pappy's initial question was "what creates lift?" My answer was "angle of attack". And you cannot change the "effective" shape of the airfoil by increasing angle of attack. Airfoil theory is probably best served in another thread.
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Your answer is far too simplistic to be meaningful in any way. It would be more correct to say that lift is created by deflecting air downwards.
And yes, changing the angle of attack on a laminar flow airfoil does indeed change the effective shape of said airfoil.
(http://content.screencast.com/users/Lumpy/folders/Jing/media/40afb3d8-838c-405b-90b3-5c1fc1d1d484/2008-10-14_1245.png)
You can clearly see how the effective upper surface of the airfoil is considerably larger/longer than the lower surface measured by airflow.
Over-simplifying something as complex as aerodynamics really doesn't help at all.
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Pressure.
Bernoulli.
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...by deflecting air downwards...Over-simplifying something as complex as aerodynamics really doesn't help at all.
:confused:
I just don't understand that combination of statements.
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Stoney.. about the AoA theory .. That's what I found out by reading all your posts and answering questions that those answers spawned. Thanks again! :salute
I told my physics teacher about how his theory was wrong. I elaborated and he then asked 'then how is it possible that symmetrical supersonics airfoils have lift?' At first I was puzzled. I looked at my diagram on the board and found.. well. The Spitfire and P-38 books I have show cross sections of the wing. Their wings are close to symmetrical around the chord line. The only thing giving them lift is the fact that the chord line is tilted up.
Rooting a symmetrical airfoil to the aircraft while giving that airfoil a positive angle of attack will produce lift. The faster you are, the less AoA needs to be applied to have the same amount of lift at lower speeds. And there we have it.
Thus my final theory is that air is deflected upwards when in contact with the airfoil (plane is at zero AoA, but wings are ALMOST always at a positive given AoA). The deflected air is pushed downwards by the surrounding fluid/air streams and is accelerated down and along the top of the airfoil, since the plane is moving forward (downwash). The air near at the bottom of the wing is pulled upwards by the viscosity of the air itself. As the air is deflected upwards, it pulls more air with it. Once the stagnation point is apparent, the air has 'split' and the air traveling under the wing is spread out since it was 'pulled' on. Thus, high pressure at the bottom.
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SgtPappy & Charge,
The air above does not "catch up" with the flow from below, period. It has little to do with the "longer path" either. That is one of the urban myths that are very hard to root out since it has been quoted so many times (wrongly). It is almost as bad physics as the "CO2-caused global warming" that itself became truth, just by being quoted so many times (wrongly).
What you have to realize is that there is a distribution of velocities above the wing, and more importantly - toward the trailing edge. What is said above about air being pushed downward is half true. Air is being pushed downward, but continuity requires that somewhere air will be pushed upward as well - air does not accumulate below the plane. If you really are interested in this approach, the more accurate thing to say is that the air acquires net downward momentum.
I will try to take a different approach and try to explain the basic idea of lift with just a "drop" of physics:
It is the trailing edge that is responsible for the creation of lift. The flow around a corner creates a vortex, or a whirlpool if you like. Move a flat surface, such as a teaspoon in your tea and you will see it easily - a vortex behind each edge of the spoon. Leaving out the detailed physics, the direction of circulation is normally (if you are not too wild) "into" the corner. Translating this to the airfoil - the trailing edge, "lowered" into the airflow, creates this corner and the direction of circulation is "upwards" around the corner. This is the trailing-edge vortex. The front of the wing is thicker and more curved in an attempt to prevent a vortex from begin formed there, the way it does around both edges of the teaspoon.
Now consider a physical principle: "In a flow around obstacles, the total vorticity of an inviscid fluid is conserved".
Vorticity is a measure of rotation in the flow. This is related to conservation of angular momentum. A fluid is a strange thing. You disturb it in one place and it affects the flow everywhere (when not supersonic), because it pushes against itself. A vortex created in one place can affect circular motion in another place.
Inviscid means that it has no viscosity and a vortex will not decay. "But the air is not inviscid", you may claim. That is true, but the thing that is called the Reynolds number, if it is high (in our case it is), means that the time it takes to a vortex to decay due to viscosity, is much longer than the time it takes to travel the size of the "system" (i.e. the wing). So the total rotation of the air (of the wing-size scale flow) does not change as it flows over the wing.
What does this mean? It means that there has to be another rotational flow that counters the rotation created by the trailing edge vortex. This is the global flow around the wing. The trailing edge vortex creates a flow with its upper part directed against the direction of the wind. If the flow is from left to right (as in most pictures), the direction of rotation will be counter-clockwise. The global flow around the wing will have a component of clock-wise rotation to counter it. This global "rotation" is superimposed on the bulk flow and so the effect is faster air above the wing (rotation speed added to the flow) and slower air below (rotation speed subtracted from the bulk flow). From this point Bernoulli's principle kicks in and you get upward force.
If you increase the angle between the wing and the flow (before it is affected by the wing) too much, the curvature of the leading edge is not enough to prevent a vortex being formed there as well. The direction of this vortex will be opposite to the one at the trailing edge ("inward" direction around a corner). This subtracts from the large scale rotation around the wing and reduces the lift. Your wing is becoming a teaspoon. If you push it even further, the edge vortices will breaks into several smaller scale vortices of opposing directions (cancel each other out in terms of total vorticity) and a large scale rotation will not be produced. Keep going, and the rotation will break into vortices of all scales - turbulence.
This is the simple explanation. What it hides under the rag is the how/why the vortex is formed in the first place, or worse - how it breaks. This is an annoying problem that is not solved neither analytically, nor numerically. It is not a problem of not understanding the physics - it is a problem of mathematical "instability" or chaos.
I hope this helped.
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Three more remarks:
1. Die Hard's picture of airflow is completely un-physical. It seems to have some Re number being stated on it, so I guess it is from some numerical code? It is still nonsense.
2. Tell your physics teacher that supersonic lift is a different thing. In supersonic flow what happens at the trailing edge cannot affect the flow before it hits the shock front. This is a huge difference.
3. "Angle of attack creates lift" is a poor answer, as absolute AoA is poorly defined and not every surface will create lift if you place it against the flow, regardless of the angle. Personally, I'd define AoA=0 when the wing creates no lift, by I know engineers like to define it differently.
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My answer of "Angle of Attack" is consistent with Prandtl's Theory of Thin Airfoils. This is an accepted method with which to model lift, even though it does it through using the concept of infinite aspect ratio, which we all know doesn't exist in real life. I'm sure Mr. Prandtl knew that too, and yet, his theory is accepted.
Given that many assumptions are made in order to model aerodynamic forces, I chose to include this statement in my original post: "So, while it doesn't explain many of the other phenomena associated with the way an airfoil/wing interacts with the relative wind...Its generalistic and brushes over a few other facets of airfoil theory, but..."
Sorry that the "Bubba" answer doesn't satisfy everyone.
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1. Die Hard's picture of airflow is completely un-physical. It seems to have some Re number being stated on it, so I guess it is from some numerical code? It is still nonsense.
Your comment is unnecessarily confrontational, and also untrue. The picture is a visual representation of a temporarily reattached airflow due to the formation of vortexes at extreme angles of attack. I intended however only to use it to show how a change in AoA also change the effective shape of the airfoil due to the changes in airflow around it.
Here are some NACA wind tunnel tests on the phenomenon my previous picture illustrated:
(http://content.screencast.com/users/Lumpy/folders/Jing/media/21831933-2f6d-4446-97a6-1d71b7305c57/2008-10-15_1524.png)
These photos are from experiments using plasma to reattach airflow at high AoA's:
(http://content.screencast.com/users/Lumpy/folders/Jing/media/eda6eaba-e864-45bf-888e-5662b2ae66d2/2008-10-15_1521.png)
You can clearly see how the effective upper surface of the airfoil is larger than the effective lower surface despite the upper and lower camber being almost identical.
This is a simulation of both pressure and airflow changes while increasing AoA. It may help people to intuitively understand how lift is created.
(http://www2.icfd.co.jp/examples/naca0012_2d/image/2dn0012.gif)
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Your comment is unnecessarily confrontational, and also untrue. The picture is a visual representation of a temporarily reattached airflow due to the formation of vortexes at extreme angles of attack. I intended however only to use it to show how a change in AoA also change the effective shape of the airfoil due to the changes in airflow around it.
No confrontation was intended. My gripe against that picture was that it it shows the airflow COMPLETELY undisturbed behind the wing. This cannot be a state in which lift is produced (assuming constant air velocity). The plots you posted now are much more realistic.
My answer of "Angle of Attack" is consistent with Prandtl's Theory of Thin Airfoils.
Perhaps, but it doesn't explain why my keyboard makes a lousy wing.
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Perhaps, but it doesn't explain why my keyboard makes a lousy wing.
Touche'...I suppose I should have made a caveat that my answer applied to inherently aerodynamic objects only. Spheres, cubes, keyboards, et al excepted...:)
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It is the trailing edge that is responsible for the creation of lift. The flow around a corner creates a vortex, or a whirlpool if you like.
Where is the trailing edge of rotating tennis-, base- or football. ? No corners, and still lift.
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Where is the trailing edge of rotating tennis-, base- or football. ? No corners, and still lift.
More inertia then anything else.
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Where is the trailing edge of rotating tennis-, base- or football. ? No corners, and still lift.
The tennis ball, unlike a static wing, generates vorticity by its rotation - no rotation, no lift. Instead of distributing vorticity in a "smart" way like the wing does, it adds vorticity to the system at the expense of its own angular momentum. The end result is similar - the global flow around the spinning ball resembles a superposition of a bulk flow and a rotational flow (in the ball's spin direction). In both cases the objects leave a wake of vortices in the flow behind them due to this sheer in the flow - the flow from above does not rush to "catch up" with the flow from below.
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... double post
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Where is the trailing edge of rotating tennis-, base- or football. ? No corners, and still lift.
what you may find is that tennis balls , footballs (soccer) do in fact have a "trailing" edge in-flight . They do not travel as pure spheres but are flattened out . then after the the first moments of flight the spin of the ball creates the lift that causes it to change direction or flight path
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Not to mention the effects of laminar flow...
-C+
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Wow Bozon. I think I'm starting to get it. I read something on NASA recently ... about 'turning' the airflow and more 'turning' meant more lift. They actually gave the teaspoon example as well.
Don't worry about the physics; if you ever want to post the physics, that's fine since I really like the subject and I'm trying to get into it. Keep it simple though haha. At any rate, I'm having a little trouble visualizing what you said. Is the curved edge of the spoon supposed to be the leading edge and the back side the trailing edge? Sorry I'm a tad slow.
Perhaps you could explain to me in terms of Die Hard's animation of the airfoil.
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Don't take the teaspoon example too seriously. Any flat surface that you put inside the liquid will do. What this comes to demonstrate is that the flow, when it meets a "corner", will tend to form a circular vortex flow behind it, and the direction of rotation will be "into" the corner.
Dip the "teaspoon" (or you other flat surface) only a little, so the widest area is in the water. When you move the teaspoon in the liquid, so that the direction of the flow is against the wide surface (concaved or convexed, doesn't matter), you will notice the circulation patterns on each side of the spoon, around the "corner". This is NOT like a lift-generating wing. What the wing tries to do is create this circulation only on one edge (trailing edge) but prevent a local vortex from forming at the leading edge. This breaks the symmetry and forces the counter rotation (that counters the vorticity in the trailing edge) to be formed elsewhere - around the entire wing.
It is a fragile (and unstable) pattern and this is why the wing is limited to a very small tilt (angle of attack) range of ~15 degrees. Beyond that, the large circulation will break into smaller circulations - typically forming the counter rotation only above the wing (behind it, from the flow point of view), instead of around it.
If you want to use Die Hard's image. Look at the yellow background one that says "accelerated" flow. Ignore the "accelerated" part and look only at the bottom left figure. You can see the trailing edge vortex, but no leading edge vortex. The "global" circulation is the air flow velocity difference between above and below (which is not drawn unfortunately). Any sheer in velocity is a kind of circulation. The mathematical definition is that the path-integral of the velocity vector field, around the wing, is non-zero.
When you understand this type of vortex flow, you will realize that there has to be more, smaller, vortices in the flow (places where the velocity sheer is strong). This is where the real difficult physics is hiding and what makes this problem so difficult analytically. A professor of mine, an expert in hydrodynamics, used to say that he still thinks it is a miracle when he sees a plane actually lifts off the ground.
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Hah it is difficult but I suppose this is why I'm so interested in physics class. Thanks for the explanation Bozon. I'm understanding it much better now.
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"Don't take the teaspoon example too seriously. Any flat surface that you put inside the liquid will do. What this comes to demonstrate is that the flow, when it meets a "corner", will tend to form a circular vortex flow behind it, and the direction of rotation will be "into" the corner."
Or expand the low pressure theory from there. Not that you could actually do it or notice any lifting forces but if you would move the spoon sideways through the fluid in straight motion, turned the way you normally use it with leading edge slightly up and according to low pressure theory there would form a low pressure in the "cup" which would create "lift" causing the spoon to slightly go upwards. But thats not how it happens but the spoon would want to go down according to vortex generation theory.
Or think about frisbee. Try throwing it upside down and see what happens. Frisbee is interesting because the shape effect is very evident there but how would frisbee fly if it did not rotate? Is rotation mandatory to its lift generation or is form effect enough. Or does rotation simply stabilize it in flight. If it does, why? One would think that due to laminar effect the frisbee would drift to opposite direction from its rotation unless thrown slightly tilted to compensate but I don't think it does. Why does not laminar effect work here if if works for a golf ball?
This is interesting approach to aerodynamics too: http://www-scf.usc.edu/~tchklovs/Proposal.htm
Sorry if I'm degrading the conversation but I'm a simple man and I try to reach complex things through understanding simple examples first. ;)
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(http://lh4.ggpht.com/qiqi.wang/ROZMCm-4ABE/AAAAAAAAACw/4-KLPHDA0uM/VortexContail.jpg)
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if it works for a golf ball?
Golf ball has dimples.
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Or think about frisbee. Try throwing it upside down and see what happens. Frisbee is interesting because the shape effect is very evident there but how would frisbee fly if it did not rotate? Is rotation mandatory to its lift generation or is form effect enough. Or does rotation simply stabilize it in flight. If it does, why? One would think that due to laminar effect the frisbee would drift to opposite direction from its rotation unless thrown slightly tilted to compensate but I don't think it does. Why does not laminar effect work here if if works for a golf ball?
A frisbee can fly upside down. Not so well though. When you flip the frisbee over, you change the "corners" that the airflow meets. The flow going over the leading edge now meets a sharper edge instead of the curved edge (now curved downwards). This encourages a vortex to be created over the leading edge - not good for lift generation. On the other side, the trailing edge is now curved upwards and produces a weaker vortex behind the frisbee.
The rotation of the frisbee serves the purpose of stabilizing it in flight, not generating lift. A sudden tip of the disc while in flight will lead to a precession that (if not to large) will decay quickly (it dissipates the energy to create small fluctuation/vortices in the airflow). The rotation does create some sideways "lift". Notice that the flight path of the frisbee (if not tilted) tends to curve to the side that rotating "into" the airflow.
As Stoney mentioned, the dimples in the golf ball serve to increase the lift generated by the rotation (better stirring of the air due to ball's spin). The stripe on the baseball serve a similar purpose and allow effective curve-ball pitches. The knuckle-ball pitch tries NOT to rotate the ball as it leaves the hand. This creates a very unstable airflow due to the interaction of the air with the stripes on the ball, that shifts the ball in random directions as it travels.
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And, primarily, the dimples keep the boundary layer flow attached, reducing drag and increasing the distance the ball will fly. I'm not so sure that "lift" has as much to do with a golf ball's trajectory/distance.
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"I'm not so sure that "lift" has as much to do with a golf ball's trajectory/distance."
The point with the golf ball and laminar attachment to those "dimples" is that while the laminar airflow is attached to ball's surface it will have effect on surrounding airflow at certain speed range IF the ball rotates around its horizontal axis which will alter its trajectory which can be described as lift, especially if it pulls it upwards.
"How a Golf Ball Produces Lift
Lift is another aerodynamic force which affects the flight of a golf ball. This idea might sound a little odd, but given the proper spin a golf ball can produce lift. Originally, golfers thought that all spin was detrimental. However, in 1877, British scientist P.G. Tait learned that a ball, driven with a spin about a horizontal axis with the top of the ball coming toward the golfer produces a lifting force. This type of spin is know as a backspin.
The backspin increases the speed on the upper surface of the ball while decreasing the speed on the lower surface. From the Bernoulli principle, when the velocity increases the pressure decreases. Therefore, the pressure on the upper surface is less than the pressure on the lower surface of the ball. This pressure differential results in a finite lift being applied to the ball."
"So, why dimples? Why not use another method to achieve the same affect? The critical Reynolds number, Recr, holds the answer to this question. As you recall, Recr is the Reynolds number at which the flow transitions from a laminar to a turbulent state. For a smooth sphere, Recr is much larger than the average Reynolds number experienced by a golf ball. For a sand roughened golf ball, the reduction in drag at Recr is greater than that of the dimpled golf ball. However, as the Reyn olds number continues to increase, the drag increases. The dimpled ball, on the other hand, has a lower Recr, and the drag is fairly constant for Reynolds numbers greater than Recr.
Therefore, the dimples cause Recr to decrease which implies that the flow becomes turbulent at a lower velocity than on a smooth sphere. This in turn causes the flow to remain attached longer on a dimpled golf ball which implies a reduction in drag. As the speed of the dimpled golf ball is increased, the drag doesn't change much."
What I'm trying to bring into this discussion is the consideration of how air itself can work on an object (other than a wing) to produce lift. Ball, pole (spear) etc.
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