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Link Posted: 12/19/2013 8:54:05 PM EST
[#1]
As a designer in aerospace for 30 years I can say without reservation that airplanes suck.
Link Posted: 12/19/2013 9:04:32 PM EST
[#2]
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Quoted:
Okay, I'll add a twist;

As a wing moves through the atmosphere, it's obvious that air moves chordwise parallel to the direction of the wing's travel.

What happens at the wing tip?  What does that imply about the flow over the wing?


By the way, the post above that claims the pressure difference between the upper and lower surfaces results in "vortexes" at the trailing edge that create the lift is wrong.  Unfortunately, the connection between vortex theory as a means of modeling the flow, and the actual nature of the flow is poorly taught.


http://www.ar15.com/media/viewFile.html?i=33191

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The cookies on the FBD are particularly entertaining.  

Link Posted: 12/19/2013 9:11:11 PM EST
[#3]
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Low pressure side pulls, (wing, or sail,) up, (or forward.)
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This. No air is required under at all
Link Posted: 12/19/2013 9:14:52 PM EST
[#4]
Air foil

Link Posted: 12/19/2013 9:17:03 PM EST
[#5]
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Quoted:
As a designer in aerospace for 30 years I can say without reservation that airplanes suck.
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Nope, airplane good, FAA bad.
Link Posted: 12/19/2013 9:27:52 PM EST
[#6]
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Quoted:
fast moving air has lower pressure than slow moving air.  Air has to travel faster over the curved top surface of the airfoil (longer distance) so it can arrive at the same time as air moving under the flat lower surface of the airfoil.

Higher pressure on the bottom pushes up on the lower pressure on top, creating lift.
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I can do the same thing with time in my time travel machine. You don't go fwd/backwards in time, you have to move the past/future closer to you.
Link Posted: 12/20/2013 3:44:33 AM EST
[#7]
Link Posted: 12/20/2013 3:47:37 AM EST
[#8]
Bernoulli's Principle, thats how.
Link Posted: 12/20/2013 3:51:42 AM EST
[#9]
The fluid (air, water etc) has to flow farther around one side of an asymmetrical foil than it does around the other side.  

For the fluid to meet up at the trailing edge, it has to go faster.  

The pressure is lower on the fast moving side than on the lower side.

The pressure differential creates force from the slow high pressure side towards the fast low pressure side.  


How'd I do?
Link Posted: 12/20/2013 5:04:48 AM EST
[#10]
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Boom.  

That's it in the simplest terms possible.
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The upper curved surface is longer than the bottom flat one. The air particles moving over the top are moving faster than the ones on the bottom thus creating less pressure on top of the air foil and producing lift.
Boom.  

That's it in the simplest terms possible.



How do airplanes fly upside down then?
Link Posted: 12/20/2013 5:37:11 AM EST
[#11]
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How do airplanes fly upside down then?
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The upper curved surface is longer than the bottom flat one. The air particles moving over the top are moving faster than the ones on the bottom thus creating less pressure on top of the air foil and producing lift.
Boom.  

That's it in the simplest terms possible.



How do airplanes fly upside down then?


Angle of attack, control surface deflection and thrust creates a vectored momentum? Like how a bullet travels but the bullet would have thrust and fins to keep it pointed in the right direction.
Link Posted: 12/20/2013 5:42:03 AM EST
[#12]

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Not hardly, that's a US weapon, and one of its cool attributes is its precision on the target.



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...

I doubt he actually knows.




duck



http://www.ar15.com/media/viewFile.html?i=14897





Alright it's Russian so it will miss anyway!




Not hardly, that's a US weapon, and one of its cool attributes is its precision on the target.







 
Plus, it makes waffles.
Link Posted: 12/20/2013 5:46:22 AM EST
[#13]
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What do you want them to be?  Cookies or pizzas one day, yummie yummie hamburgers on another day, or biscuits made with kernals of corn in the dough and slathered with butter on yet another.

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Okay, I'll add a twist;

As a wing moves through the atmosphere, it's obvious that air moves chordwise parallel to the direction of the wing's travel.

What happens at the wing tip?  What does that imply about the flow over the wing?


By the way, the post above that claims the pressure difference between the upper and lower surfaces results in "vortexes" at the trailing edge that create the lift is wrong.  Unfortunately, the connection between vortex theory as a means of modeling the flow, and the actual nature of the flow is poorly taught.


http://www.ar15.com/media/viewFile.html?i=33191


What the fuck are those things trailing off the empennage?  Pizzas?  Cookies?  I'm hungry.  But if I eat pizza and cookies, I'll need to get on the treadmill.  That won't fly.


What do you want them to be?  Cookies or pizzas one day, yummie yummie hamburgers on another day, or biscuits made with kernals of corn in the dough and slathered with butter on yet another.


Crap!  It's a Rorschach test.  I went into engineering to avoid all this touchy feely stuff.
Link Posted: 12/20/2013 6:05:59 AM EST
[#14]
As can be successfully predicted using potential flow theory, you can approximate one by spinning a cylinder an a transverse fluid flow

Link Posted: 12/20/2013 6:29:28 AM EST
[#15]
I'll give the Common Core answer:

Because white people are racist.
Link Posted: 12/20/2013 6:48:07 AM EST
[#16]
Quoted:
Explain it GD.






I'm curious to see how many of you are wrong...
And no treadmills!
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Bernouli's principal.....look it up
Link Posted: 12/20/2013 6:48:42 AM EST
[#17]
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Quoted:
like this:

<a href="http://s252.photobucket.com/user/Joe49ae/media/wing_zpsbbad0523.jpg.html" target="_blank">http://i252.photobucket.com/albums/hh39/Joe49ae/wing_zpsbbad0523.jpg</a>

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Link Posted: 12/20/2013 6:56:49 AM EST
[#18]
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Unfortunately, that explanation doesn't account for the lift produced by symmetric airfoils (upper and lower surfaces are mirror images), or the ability of a flat plate to produce lift.
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The length of the top side of the airfoil is greater than the length of the bottom side of the airfoil.

The air flowing over the top of the airfoil, if it were to flow over the surface at the same speed as the
air flowing over the bottom side,  would not meet when they both came off the trailing edge.   The airflow
on the bottom gets to the trailing edge first.

The result is that there is an area of partial vacuum (reduced air pressure) on the trailing edge of the upper
airflow.   This creates many effects,  turbulence being one.   The vacuum effect also serves to increase the
airflow velocity over the top of the airfoil.

You end up with a fast moving, low pressure region of air over the airfoil, and comparatively slow moving,
higher pressure air below it.   This creates lift.



Unfortunately, that explanation doesn't account for the lift produced by symmetric airfoils (upper and lower surfaces are mirror images), or the ability of a flat plate to produce lift.



I only fly 'em, I don't know what makes 'em work.



You push the lever thingies forward to fly and pull them back to not fly!


Link Posted: 12/20/2013 8:32:12 AM EST
[#19]

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Air, magnets, unicorns, and flatulence.
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Quoted:

What are the 4 forces that affect an airfoil?






Air, magnets, unicorns, and flatulence.




Finally a correct answer in this thread.    



 
Link Posted: 12/20/2013 11:59:35 AM EST
[#20]
Link Posted: 12/20/2013 12:03:40 PM EST
[#21]
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Not curved, wont work.
Link Posted: 12/20/2013 12:09:36 PM EST
[#22]
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Lots of small hands.
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Small hands push the air into the foils.
 


Lots of small hands.


And those small hands are holding magnets,
Link Posted: 12/20/2013 12:18:39 PM EST
[#23]
Kelly Johnson is spinning in his grave right now!!
Link Posted: 12/20/2013 12:24:47 PM EST
[#24]
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Kelley Johnson is spinning in his grave right now!!
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Because of the magnets?  
Link Posted: 12/20/2013 12:29:08 PM EST
[#25]
Because Ludwig Prandtl fucking said so, that's why.
Link Posted: 12/20/2013 12:57:32 PM EST
[#26]
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Pfft physics.

Answer is the devil
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Airfoils change the m______ of the ___ flowing across the ______e.



Get out of here with your knowledge and logic.

You know that does not work here in GD.


Pfft physics.

Answer is the devil



Magic, they went to Hogwarts..............
Link Posted: 12/20/2013 1:00:48 PM EST
[#27]
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Quoted:
As a designer in aerospace for 30 years I can say without reservation that airplanes suck.
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there is no gravity...............

the Earth Sucks................
Link Posted: 12/20/2013 1:16:05 PM EST
[#28]
Bernoulli's Principle is a huge reason of how they fly.  There's also Newton's third law which plays a big role with angle of incidence and an increasing angle of attack.  A few other reasons as well, but it's all magic and magnets.  But either way, OP is not as smart as he's trying to lead us to believe.  
Link Posted: 12/20/2013 2:01:25 PM EST
[#29]
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Not curved, wont work.
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HA!  You're just jealous because I look so cool!
Link Posted: 12/20/2013 5:28:18 PM EST
[#30]
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Bernoulli's Principle is a huge reason of how they fly.  There's also Newton's third law which plays a big role with angle of incidence and an increasing angle of attack.  A few other reasons as well, but it's all magic and magnets.  But either way, OP is not as smart as he's trying to lead us to believe.  
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I never claimed to be smart I was taught this incorrectly in grade skool, and for many years I struggled with the idea that it didn't take all forces into account, and that air molecules really had no reason to want to reunite at the end of some distance traveled with their other molecule buddies. It mind fucked me for a long time until I got off my ass and researched the real reason and and least gained a paltry understanding of what's really happening.

I'm sure many others were taught incorrectly, and thought this would be a fun way to spread correct info and eat some popcorn.
Link Posted: 12/20/2013 5:34:47 PM EST
[#31]
To all the Bernoulli fans, how does that work with an ultralight or hang glider wing?  They are about  what, 0.1mm thick?





FWIW, back in highschool I took a physics class which had one long lesson on how airfoils work.  As an example they used a kite, which is of course fully stalled all the time.








 
Link Posted: 12/20/2013 9:27:10 PM EST
[#32]
A column of air(earths atmosphere) is thicker at the bottom.  (weighs less at the top, weighs more at the bottom, because gravity pulls air down)  A wing (foil) moving forward forces air down underneath it, increasing the weight of the air on the bottom, compressing the air underneath making it weigh more, and pushing the wing up against the lower pressure above, and repeat add infinitum. Its like a straw, are you sucking the fluid up or is the created vacuum causing the weight of the air to push the fluid up.  Six of one and seven of the other.
Link Posted: 12/20/2013 10:01:59 PM EST
[#33]
The wings are constantly shoving air toward the ground.   The air gets downward momentum from the wings, momentum change takes force.  The opposing force holds up the plane.
Link Posted: 12/21/2013 1:03:24 AM EST
[#34]
Link Posted: 12/21/2013 4:19:15 AM EST
[#35]
Link Posted: 12/21/2013 4:34:27 AM EST
[#36]
Define work.  Not cross product of force and length.





Airfoils are just beams optimized for lowest drag .   Airfoil lift is another thing all together.  Using zero angle of attack drag coefficient principle, lift is just the vertical component of drag.  For symmetrical foils, zero angle of attack generates zero lift.





Now, using basic mechanics, the lift must be the integral of pressure differential and area.   So somehow, the wing develops a pressure differential based on the angle of attack.   It is a momentum effect.

       
 
Link Posted: 12/21/2013 4:57:56 AM EST
[#37]
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Define work.  Not cross product of force and length.


Airfoils are just beams optimized for lowest drag .   Airfoil lift is another thing all together.  Using zero angle of attack drag coefficient principle, lift is just the vertical component of drag.  For symmetrical foils, zero angle of attack generates zero lift.


Now, using basic mechanics, the lift must be the integral of pressure differential and area.   So somehow, the wing develops a pressure differential based on the angle of attack.   It is a momentum effect.
         
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Never heard of it - care to explain?
Link Posted: 12/21/2013 2:20:21 PM EST
[#38]
Link Posted: 12/21/2013 2:33:17 PM EST
[#39]
There's a MAN ON THE WING!










 
Link Posted: 12/21/2013 3:11:47 PM EST
[#40]
If Bernoulli's Principle is the mechanism of action, how does ground effect exist?
Link Posted: 12/21/2013 3:44:32 PM EST
[#41]
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If Bernoulli's Principle is the mechanism of action, how does ground effect exist?
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A little of it is from compressibility but the majority of it (in the average aircraft without special wing tip treatments) is from the inability to fully develop wingtip vortices due to its proximity to the ground.
Link Posted: 12/21/2013 3:56:58 PM EST
[#42]
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Not curved, wont work.
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The way it's sitting, it's making upforce.
Link Posted: 12/21/2013 3:57:26 PM EST
[#43]
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Because of the magnets?  
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Kelley Johnson is spinning in his grave right now!!


Because of the magnets?  

Tip vortices.
Link Posted: 12/21/2013 6:12:31 PM EST
[#44]
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A little of it is from compressibility but the majority of it (in the average aircraft without special wing tip treatments) is from the inability to fully develop wingtip vortices due to its proximity to the ground.
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If Bernoulli's Principle is the mechanism of action, how does ground effect exist?


A little of it is from compressibility but the majority of it (in the average aircraft without special wing tip treatments) is from the inability to fully develop wingtip vortices due to its proximity to the ground.

Bernoulli assumes incompressible air flow.  If that isn't true, it all goes out the window.  (See the P38 Lighting)
Link Posted: 12/21/2013 7:08:43 PM EST
[#45]
Now that I'm in my hotel room for the night, it's my turn to tackle this question.
First off, any of you fools that say Bernoulli is bullshit and that's not where lift comes from, you guys are speshul. But I'll come back to you in a bit.
For all you guys that said that essentially "air goes over top and bottom but top side is longer so air goes faster, faster air means less pressure, higher pressure below... that's lift." Hey, at least I can give you guys props for paying attention in class. But that description doesn't cover all of it. Just as there are two different forms of drag (aerodynamic and parasite) there are two different forms of lift. We will refer to them as Bernoulli's and Newton's. You guys did a decent job of describing Bernoulli's lift but it doesn't account for why a kite flys or why your hand generates lift when you tilt it up and down out your car window. That's Newton's lift. Equal and opposite reaction. Particles bounce off the surface and push it in the opposite direction. Both of these forms work together to make airfoils fly. What percentage of lift is derived from each method depends on the wing design and it's angle of attack.
Now, for all you fucking heretics that claim Bernoulli is wrong, lift is entirely created by air bouncing off the bottom of the wing, the fucking Wright flyer had flat wings as do ultralights... GTFO! If that were the case, why would all aircraft manufacturers go through so much effort and extraordinary expense to design wings with camber? Why does every propellor, rotor and fan blade all have camber? To the low down dirty (COC) that said the Wright flyer had flat wings. Why on Earth would the brothers go through so much effort to design the first wind tunnel and meticulously design, test, and record different airfoil cambers? None of that is necessary for flat wing designs. Do you not realize that the method of steering employed by the brothers was to alter the camber of the wings? The thicker the camber the more lift it generates. That wing goes up and thus the Flyer banked. It was an incredibly difficult though aerodynamically efficient means to accomplish the task but it was the best they had as ailerons had not yet been invented. To prove that Bernoulli's lift is real and not some figment of 6th grade teachers imaginations, all you have to do is look at the average "sectional lift coefficient" graph. Here's some:









These graphs all illustrate typical lift properties of asymmetrically cambered wings with subsonic airflow. You'll notice that at zero angle of attack they all indicate positive coefficients of lift. That means when the chord of the wing is perfectly aligned with the oncoming airstream, and there is no air deflecting off the bottom surface, there is positive lift being generated. That is 100% Bernoulli. There is even positive lift coefficients for slightly negative angles of attack. How's that for Bernoulli breakin' it off in Newton's ass? Only lifting surfaces with zero camber or symmetrical camber will record a zero lift coefficient at zero angle of attack. Of course, airplanes almost never fly at zero angle of attack. So Newton and Bernoulli work together to generate our lift. Bernoulli rules at extremely shallow angles of attack and Newton begins to win out at the very high angles. Where the dividing point between the two is depends upon the airfoil design just as each airfoil has its own fixed critical angle of attack. So the answer is pie.

Link Posted: 12/21/2013 7:15:31 PM EST
[#46]
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Bernoulli assumes incompressible air flow.  If that isn't true, it all goes out the window.  (See the P38 Lighting)
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If Bernoulli's Principle is the mechanism of action, how does ground effect exist?


A little of it is from compressibility but the majority of it (in the average aircraft without special wing tip treatments) is from the inability to fully develop wingtip vortices due to its proximity to the ground.

Bernoulli assumes incompressible air flow.  If that isn't true, it all goes out the window.  (See the P38 Lighting)


It ain't Bernoulli compressing it in this scenario, it's Newton. There is some measurable effect of the airplane riding on a cushion of air trapped between the wing and the ground that slightly decreases the already high angle of attack and artificially increases the coefficient of lift.
And Bernoulli only assumes incompressible airflow at subsonic speeds. Bernoulli still produces lift at transonic and supersonic airspeeds. There is a different Bernoulli equation you have to use for supersonic/compressible scenarios.
This is the Bernoulli equation for compressible regimes:


This is the incompressible equation:



Eta: Do you even lift, bro?

Sorry, I had to...
Link Posted: 12/21/2013 7:34:44 PM EST
[#47]
There sure is a lot of P factor in this thread.
Link Posted: 12/22/2013 4:41:51 AM EST
[#48]
Link Posted: 12/22/2013 5:35:14 AM EST
[#49]
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Now think about all that; does it explain "how airfoils work", or "how airfoils are used"?

I'll answer that, those are charts that describe the behavior of 2D airfoils in isolation, and not as part of a wing.  They are a first step in the application of an airfoil that describe the tail end of "how they work" without understanding of the physical mechanisms.  The hilarious part is that you didn't post a single plot of pressure distribution at any angle of attack over the section to support your argument.

Describing the flow over an airfoil or wing is much more complicated that saying, "derp, it's pressure, duh".  We have to look at the flow in forward of the leading edge, along the chord, and aft of the trailing edge of a 2D airfoil, and spanwise flow along a 3D wing, then modifications to the flow caused by leading edge or trailing edge devices or spoilers and drag brakes installed between the leading edge and trailing edge.

No one will argue that their is a pressure distribution over airfoils, no matter whether they are thick or thin (those have definitions), have a practically zero thickness ratio (a sail, a paper airplane, single surface hang gliders and ultralights), camber or no camber.  But for whatever reason, the momentum mechanism is never discussed in airfoils 101, and that is a mistake.

The lift on a wing is caused by momentum and direction of the airflow spilling off the trailing edge.

Picture two airplanes of equal weight, one with a short fat wing and one with a long narrow wing of equal area (we don't care about spanwise taper and so on at this point), flying at identical speeds.

Now picture circles circumscribing each wing, consider their respective areas, and imagine the amount (the volume and mass) of the air flowing through each circle.  It's obvious that the mass of air flowing through the larger circle is much greater (by the square of the span) than through the small circle around the short wing.  The lift is the same (same airplane weight, same speed), so what is happening at the wings?

Make a sketch, calculate the area in each circle, note how the diameter (span) squared influences the area.

Now, since this is GD science and we're already well into tl;dr territory, we'll jump ahead a little; while operating these airplanes, we noticed that the drag of the longer wing is less than the short wing!  Hmmm, interesting, eh?

Think about this a second; the longer wing produces the same lift as the short wing with less drag, so it appears to be "working" less hard to produce that lift!  Whilst investigating the reason for the lower drag of the long wing, we also noticed that we could break the drag down into two broad sources, one that appeared to be related only to the cross section geometry of the wing (corrected for Reynolds numbers for you pedantics), and another that appeared to be related to the creation of lift!  We'll call that "induced drag", the drag caused by production of lift, and the part caused by the shape we'll call "profile drag" (for now).

So, the long wing produces the same lift with less drag, and we notice that it influences a much greater volume of air passing by, so we look a little closer and we discover that the air flowing off the trailing edge of the long wing departs at an angle to the chord that is less than from the short wing.  "Walla" , we realize the longer wing produces less drag to produce the same lift because it doesn't have to turn the air flow over an angle as great as the short wing.  We also realize at this point that the momentum (direction, speed, and mass flow integrated over the span) of the air coming off the trailing edge of the wing equals the lift required to support the airplane.

In the 2D world, we draw theoretical airfoils and then calculate theoretical chordwise speeds, pressure distributions, and behavior of the boundary layer.  David Bernoulli's principle is merely the jumping in place, it's barely a good start to the problem but does serve as a simple explanation of the basic mechanism.  Mostly.  We also know that there is not a 2D airfoil in the universe that can develop lift!  [Using the lift equation, the area is zero, so the lift has to be zero.)

In the real world, we attempt to measure the behavior of 2D airfoils by testing short 3D wings in wind tunnels, and then correcting all the effects that cause error (3D airfoils, tunnel wall effects, Reynolds Number, Mach Number, compressibility, and a zillion other factors) to learn how well our 2D estimation methods match the real world as best we can measure.  It's very important to understand the magnitude of the errors at this stage.

Also in the real world, we build airplanes with 3D airfoils (wings) with characteristics adjusted to produce desired characteristics for the design flight regime.  We're shocked when we learn how unimportant all those airfoil shapes we fretted about choosing for our light airplane design turn out to be when considering performance, and learn that damn near any airfoily looking cross section shape will work about as well as the next one as long as they are thick enough for adequate structure inside, and that the planform shape dominates the airplane's performance.  (Sailplanes are a little more critical since we need to really squeeze the profile drag part of the equation, too.)

What we've learned is that 2D airfoils are theoretical constructions that exhibit distributions of speed and pressure along their chords but produce zero lift.  

3D airfoils, or wings, produce lift by directing the momentum of the air flow across the surface at an angle to the chord at the trailing edge to produce the force that lifts the airplane.  The chordwise distribution of pressure is interesting, but as long as it is well behaved it's pretty much of secondary interest if the airfoil is remotely suitable for the design flight mission.  In the 3D world, we are at least as interested in the spanwise flow of air and how it behaves at the wing-fuselage intersection and at the tips.

When discussing "airfoils" it is critical to be clear about whether the airfoil is 2D or 3D.


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Now that I'm in my hotel room for the night, it's my turn to tackle this question.
...



Now think about all that; does it explain "how airfoils work", or "how airfoils are used"?

I'll answer that, those are charts that describe the behavior of 2D airfoils in isolation, and not as part of a wing.  They are a first step in the application of an airfoil that describe the tail end of "how they work" without understanding of the physical mechanisms.  The hilarious part is that you didn't post a single plot of pressure distribution at any angle of attack over the section to support your argument.

Describing the flow over an airfoil or wing is much more complicated that saying, "derp, it's pressure, duh".  We have to look at the flow in forward of the leading edge, along the chord, and aft of the trailing edge of a 2D airfoil, and spanwise flow along a 3D wing, then modifications to the flow caused by leading edge or trailing edge devices or spoilers and drag brakes installed between the leading edge and trailing edge.

No one will argue that their is a pressure distribution over airfoils, no matter whether they are thick or thin (those have definitions), have a practically zero thickness ratio (a sail, a paper airplane, single surface hang gliders and ultralights), camber or no camber.  But for whatever reason, the momentum mechanism is never discussed in airfoils 101, and that is a mistake.

The lift on a wing is caused by momentum and direction of the airflow spilling off the trailing edge.

Picture two airplanes of equal weight, one with a short fat wing and one with a long narrow wing of equal area (we don't care about spanwise taper and so on at this point), flying at identical speeds.

Now picture circles circumscribing each wing, consider their respective areas, and imagine the amount (the volume and mass) of the air flowing through each circle.  It's obvious that the mass of air flowing through the larger circle is much greater (by the square of the span) than through the small circle around the short wing.  The lift is the same (same airplane weight, same speed), so what is happening at the wings?

Make a sketch, calculate the area in each circle, note how the diameter (span) squared influences the area.

Now, since this is GD science and we're already well into tl;dr territory, we'll jump ahead a little; while operating these airplanes, we noticed that the drag of the longer wing is less than the short wing!  Hmmm, interesting, eh?

Think about this a second; the longer wing produces the same lift as the short wing with less drag, so it appears to be "working" less hard to produce that lift!  Whilst investigating the reason for the lower drag of the long wing, we also noticed that we could break the drag down into two broad sources, one that appeared to be related only to the cross section geometry of the wing (corrected for Reynolds numbers for you pedantics), and another that appeared to be related to the creation of lift!  We'll call that "induced drag", the drag caused by production of lift, and the part caused by the shape we'll call "profile drag" (for now).

So, the long wing produces the same lift with less drag, and we notice that it influences a much greater volume of air passing by, so we look a little closer and we discover that the air flowing off the trailing edge of the long wing departs at an angle to the chord that is less than from the short wing.  "Walla" , we realize the longer wing produces less drag to produce the same lift because it doesn't have to turn the air flow over an angle as great as the short wing.  We also realize at this point that the momentum (direction, speed, and mass flow integrated over the span) of the air coming off the trailing edge of the wing equals the lift required to support the airplane.

In the 2D world, we draw theoretical airfoils and then calculate theoretical chordwise speeds, pressure distributions, and behavior of the boundary layer.  David Bernoulli's principle is merely the jumping in place, it's barely a good start to the problem but does serve as a simple explanation of the basic mechanism.  Mostly.  We also know that there is not a 2D airfoil in the universe that can develop lift!  [Using the lift equation, the area is zero, so the lift has to be zero.)

In the real world, we attempt to measure the behavior of 2D airfoils by testing short 3D wings in wind tunnels, and then correcting all the effects that cause error (3D airfoils, tunnel wall effects, Reynolds Number, Mach Number, compressibility, and a zillion other factors) to learn how well our 2D estimation methods match the real world as best we can measure.  It's very important to understand the magnitude of the errors at this stage.

Also in the real world, we build airplanes with 3D airfoils (wings) with characteristics adjusted to produce desired characteristics for the design flight regime.  We're shocked when we learn how unimportant all those airfoil shapes we fretted about choosing for our light airplane design turn out to be when considering performance, and learn that damn near any airfoily looking cross section shape will work about as well as the next one as long as they are thick enough for adequate structure inside, and that the planform shape dominates the airplane's performance.  (Sailplanes are a little more critical since we need to really squeeze the profile drag part of the equation, too.)

What we've learned is that 2D airfoils are theoretical constructions that exhibit distributions of speed and pressure along their chords but produce zero lift.  

3D airfoils, or wings, produce lift by directing the momentum of the air flow across the surface at an angle to the chord at the trailing edge to produce the force that lifts the airplane.  The chordwise distribution of pressure is interesting, but as long as it is well behaved it's pretty much of secondary interest if the airfoil is remotely suitable for the design flight mission.  In the 3D world, we are at least as interested in the spanwise flow of air and how it behaves at the wing-fuselage intersection and at the tips.

When discussing "airfoils" it is critical to be clear about whether the airfoil is 2D or 3D.




I don't disagree with any of that. That is an awesome explanation. I also don't see how your explanation refutes my use of the sectional lift coefficient graphs. They were merely being used to illustrate the real effect of Bernoulli's law. What, specifically, about my post do you take issue with? Do you disagree with my statement that airfoils produce lift even at zero angle of attack? Or is it my differentiating between Bernoulli's and Newton's methods of generating lift that you take issue? I do tend to see the two as being distinguishable but when combined the net affect being just what you described; a downward momentum of airflow off the trailing edge of the wing. Tell me this, (this will help to clarify the concept for me); can a cambered wing produce some measurable degree of lift at 0 or even slightly negative angles of attack with no measurable downward momentum off the trailing edge of the wing? What I'm in envisioning is the airs momentum coming directly off the trailing edge of the wing, aligned with the chord, but still producing some small degree of lift concentrated around the first third of the chord. I would have thought this entirely possible in certain circumstances, but perhaps I'm wrong on that point.

Seriously though, tell me everything about my post you think is incorrect as this knowledge stands a good chance of being useful to me in some technical interview somewhere.
Link Posted: 12/22/2013 6:30:13 AM EST
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