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Lets Stir Some Worms, Airfoil Myth

Old 09-16-2014, 04:19 AM
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CHELLIE
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Talking Lets Stir Some Worms, Airfoil Myth

http://warp.povusers.org/grrr/airfoilmyth.html

The airfoil myth

This is one of the most persistent and widespread aerodynamic physics myths in modern history. Not only do they teach this wrongly in schools around the world (all the way from grade schools to even some universities), but it's a myth strongly believed even by professional engineers dealing with the subject (for example, it's a rather common myth among Formula 1 car design engineers). This myth is even more persistent and widespread than the solar windmill myth and the weight-of-air-proven-with-balloon myth.
I once saw a Formula 1 engineer recite this myth on a F1 broadcast. Recently this myth was indirectly repeated in the show Mythbusters (episode 100). Curiously they were not testing this myth, they simply mentioned, and I quote (emphasis mine): "Kari is concerned that MacGyver's wings look too flat, not the classic airfoil shape that makes powered flight possible." It's clearly implied that flight is impossible with flat wings which do not have the airfoil shape.
This myth is so prevalent and persistent that even Albert Einstein, one of the most brilliant physicists of all time, believed it and got it all wrong.
The so-called "airfoil shape" is this (with the classical but wrong explanation):
The myth is: The lift is produced solely because the air travels a longer path on the upper side of the wing than the lower side. Allegedly the Bernoulli effect this causes is strong enough to lift the airplane. Usually this is the only explanation given for the lifting force.
Moreover, some people (like in the Mythbusters episode) go further ahead and claim that flight is not possible without wings with the airfoil shape.
Of course both claims are laughably easy to disprove: If this was so, then it would be impossible to fly a plane upside-down. Flying a plane upside-down would mean that the lift is reversed and would then point towards the ground, which would make the plane fall like a rock.
Naturally anyone who has seen an airshow knows that airplanes can indeed fly upside-down.
Also, most scale model airplane aficionados know from experience that a plane with flat wings can indeed fly. The cross-shape of the wing is not really all that critical for flight to be possible (either normally or upside-down).
Also, the Bernoulli effect requires the air to move at different speeds, not different distances. The air going over the wing does not move significantly faster than the air going under the wing (which means that the air going over the wing lags behind a bit). Thus there's no significant Bernoulli effect caused by this.
So what is it that causes the lift if not the shape?
The classic airfoil shape actually does affect the lifting force, but it doesn't do so in the way as erroneously described, nor is it all that critical for flight to be possible. Also, the Bernoulli effect is part of the physics involved, but again not in the way as erroneously described, nor does it require the airfoil shape.
There are two main phenomena which in conjunction cause the lift:
  1. The first one is quite simple to state: The wing changes the direction of incoming air, deflecting it downwards (from both the lower side and the upper side). This is called downwash. In other words, air arrives at the front edge of the wing horizontally, and leaves the back edge of the wing slightly deviated downwards. By Newton's law, since there was a force which caused the air to be deflected downwards, an equivalent force in the opposite direction causes the wing to go upwards. The so-called angle of attack affects greatly how much this force comes into play.
  2. The second one is slightly more complicated. The front edge of the wing splits the air flow into two streams: One goes underside the wing, the other goes over the wing. If the wing has an angle of attack or if it has the classic airfoil shape (which is where it comes into play), the air stream going over the wing will traverse well above the wing's upper surface, ie. making an arc. Here's where Bernoulli steps in: The air which is closer to the wing's upper surface is travelling slower than the stream of air making the arc over the wing. Because of the Bernoulli effect this causes a low-pressure pocket on the upper surface of the wing.
    At the same time the pressure of air on the lower side of the wing is much higher (because the air is more compressed, more "packed" there because it's colliding with the wing's lower surface). The low-pressure pocket on the upper side of the wing causes a significant lift.
    As said, the low-pressure pocket forms because of the Bernoulli effect, which is why Bernoulli plays a role in wing lift, but not in the way it's classically explained.
Possible differences in airflow speed between the upper and lower streams might play a very minor role, but it's probably insignificant (and could not possibly all by itself lift the plane).
Fixing the previous image, this would be more correct:
A flat wing works too, but needs a steeper angle of attack for the low-pressure pocket to form on the upper side of the wing (and to cause a stronger deviation of the air downwards to compensate for the weaker lifting force caused by the weaker low-pressure pocket). The disadvantage of this is that a steeper angle of attack causes more drag. The airfoil-shaped wing is more economical because it causes the low-pressure pocket without the need for a steep angle of attack, thus reducing drag.
This is what happens with a flat wing:
A flat wing requires a steeper angle of attack to achieve the same amount of lift, and the consequence of this is increased drag.
That is the reason why the airfoil shape is optimal for flight, not because it would be mandatory for flight to happen.
In practice planes tend to have more a tear-shaped cross-section in their wings, making it almost symmetrical. The tear-shape becomes close to the airfoil shape when positioned appropriately. This is the reason why planes can fly upside-down: Both lifting phenomena still work.
Obviously a plane with flat wings can fly equally well normal-side-up and upside-down. A tear-shaped wing makes it more economical, though.
Note that helicopter rotor blades are basically flat wings in a steep angle. The mechanism by which they form lift is exactly the same. Rotor blades are usually not airfoil-shaped or tear-shaped because drag is less of a concern. On the other hand, the angle of attack of the blade is very critical because the vertical lift of the helicopter is controlled by changing the angle of the blades. In this case flat blades might even work better for this purpose than airfoil-shaped ones would.
If the myth was correct, however, then flat rotor blades would not produce the lift necessary for helicopters to fly.
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Old 09-16-2014, 07:22 AM
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Yep, it's a persistent myth. The only bit of the above article I'd take issue with is the idea that anyone with aerodynamic qualifications actually believes it. The 'real' way wings work is perfectly well understood by anyone who has studied the subject and is covered in many good online resources. As your article points out, the idea that the curved top/flat bottom shape is necessary for wings to make lift is patently obviously wrong because as we all know planes can fly upside down and they fly fine with flat plate wings.

Wattflyer's own Wiki thread (since I revised it in 2009) explainst the correct theory and debunks the mythical one: http://www.wattflyer.com/forums/vbgl...y&id=2&catid=4

The Wiki discussion thread covers the edit: http://www.wattflyer.com/forums/showthread.php?t=45589
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Old 09-16-2014, 04:10 PM
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Outside of the airfoil aspect......Flight (motion) through the atmosphere/space can be achieved (sustained) by any aerodynamic reaction related to "enough "force" that supports weight against gravity and enough propulsive thrust against drag"............

As exhibited by the many types of models builders here at Wattflyer have tossed in the air....the NASA Space Shuttle was a prime example of defying the common "airfoil" principles........many great minds in that industry (at the time of it's conception) thought it would be a "brick" once it entered atmospheric conditions and be unmanageable....!
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Old 09-16-2014, 04:52 PM
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Default The real forces of flight

These are the real controlling factors of flight (at least for most of us)

The only comment I will make about the OP, is about helicopters.

Quote

Note that helicopter rotor blades are basically flat wings in a steep angle. The mechanism by which they form lift is exactly the same. Rotor blades are usually not airfoil-shaped or tear-shaped because drag is less of a concern. On the other hand, the angle of attack of the blade is very critical because the vertical lift of the helicopter is controlled by changing the angle of the blades. In this case flat blades might even work better for this purpose than airfoil-shaped ones would.
If the myth was correct, however, then flat rotor blades would not produce the lift necessary for helicopters to fly.


Unquote

Every helicopter blade I have ever seen is an airfoil shape. Military or civilian it has made no difference.
Normal flying heli's (non aerobatic types) have always had a flat bottom "normal" airfoil. More aerobatic ones had a semi symmetrical airfoil.
Full insane aerobatic model helicopters, have a full symmetrical airfoil.

Try and see if you could get Curtis Youngblood to fly with 1/4" x 1 1/2" plank blades.
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Old 09-16-2014, 05:51 PM
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Yep, that's true, all helis i've seen have some form of traditional airfoil blade, and drag is very much a concern in heli blades as it is in fixed wing.
This shouldn't detract from the 'correctness' of the overall message of the original post though. Possibly the Wattflyer Watt-Wiki article explains it better?
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Old 09-16-2014, 06:20 PM
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I've only physically flown in or been around two types of helicopters, CHP Bell 206 L4 and a CHP AS 350.......As part of two different service patrol and surveillance exercises. Both had "airfoil" shaped props......

As for model heli craft, with only one exception, every model I've seen in the last 15years, flown or repaired/built used "airfoil" shaped props......the one exception was a very old gasser Hirobo Falcon 505 1980's vintage, that used flat wooden blades......it actually flew pretty well but had many frame and head gear failures and was a beast to set-up and repair!

I'm not quite sure the Wiki link provided explains the issues "better" but it is a decent resource to consider.
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Old 09-16-2014, 07:40 PM
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Ive seen quite a few helicopters from the 1980's with flat blades as well.

Wasn't all that long ago, I was getting told a propeller wouldnt work backwards because airfoil.
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Old 09-17-2014, 01:13 AM
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Then there is the "flat plate" effect = generate enough horsepower and you can fly a barn door.
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Old 09-17-2014, 01:15 AM
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kyleservicetech
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Originally Posted by carpetbagger View Post
Then there is the "flat plate" effect = generate enough horsepower and you can fly a barn door.
As far as that goes, the wing loading of a modern day jet fighter is about double that of a cast iron man hole cover, launched like a frisbee.

As you've indicated, enough power, and anything can fly.
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Old 09-17-2014, 01:19 AM
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kyleservicetech
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Originally Posted by JetPlaneFlyer View Post
Yep, it's a persistent myth.
Years ago, I knew a fellow that was certified in jet engine repair. He told me that 75% of the thrust on those jet engines came from SUCTION from the front of the engine. Had to ask him how those clam shell thrust reversers used on engines worked.

As indicated, there are more than a few myths about the way our models, and full scale airplanes fly.
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Old 09-17-2014, 02:27 AM
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Originally Posted by kyleservicetech View Post
Years ago, I knew a fellow that was certified in jet engine repair. He told me that 75% of the thrust on those jet engines came from SUCTION from the front of the engine. Had to ask him how those clam shell thrust reversers used on engines worked.

As indicated, there are more than a few myths about the way our models, and full scale airplanes fly.
Lol! I'll bet someone told him that to make sure he stays clear of the intakes during ground runs!
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Old 09-17-2014, 04:51 PM
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Another good one is the myth that a stalled wing loses all lift. It doesn't, but the lift-to-drag ratio gets unmanageable for most airplanes. Except for 3D planes that have enough engine power to keep them going...
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Old 09-22-2014, 06:16 AM
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Lift is redirected air. If no air is moving over the wing, then there cant be any lift. I wonder why all the 3d planes have large props and control surfaces...
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Old 09-22-2014, 09:00 AM
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A stalled wing still re-directs air and the air still flows over the wing, the only difference is that the flow is no longer closely attached to the top wing surface. When this attachment breaks down (a stall) lift initially reduces and drag increases massively, but with enough thrust you can overcome that increase in drag. By the time the wing gets to about 45 degrees AoA it's usually making about the same lift, or maybe more, as it did just before stalling, but at the expense of enormous drag.

Here's a lift drag-graph plotted for a 180 degrees rotation of an airfoil, Some things to note on the graph:
  • Stall occurs at about 12 degrees AoA and a coefficient of lift (Cl) of about 1.1
  • Lift initially drops to about Cl=0.7 at 17 deg AoA
  • Lift then increases again and by 45 deg AoA it's actually higher than pre-stall (Cl=1.2)
  • Drag is about 30 times higher at 45 deg AoA than it was pre-stall.


So the biggest effect of stall is actually not the decrease in lift but the massive increase in drag. So when you see 3D models flying at about 45 deg AoA the wing is making a lot of lift, of course as the prop is now pointing up then the prop thrust also contributes (roughly equally) to the lift.
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Old 09-22-2014, 12:15 PM
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Flat wings fly just fine. I built a Fokker DR1 with 3 flat wings that did and still does fly great. A very knowledgeable member of my flying club despite the evidence said that it would not fly which was confusing as it was up in the air at the time. Hope the Fokker doesn't here it!
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Old 09-22-2014, 05:21 PM
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Originally Posted by Kvar View Post
A very knowledgeable member of my flying club despite the evidence said that it would not fly which was confusing as it was up in the air at the time.
So he wasn't really very knowledgeable at all then, at least not on the subject of aerodynamics anyway!

It is incredible that people can still be convinced that flat wings cant fly despite the overwhelming evidence of their own eyes that they actually do, and perfectly well. It says something about how the human brain works, confirmation bias and all that good stuff.
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Old 09-22-2014, 05:44 PM
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Originally Posted by JetPlaneFlyer View Post
So he wasn't really very knowledgeable at all then, at least not on the subject of aerodynamics anyway!

It is incredible that people can still be convinced that flat wings cant fly despite the overwhelming evidence of their own eyes that they actually do, and perfectly well. It says something about how the human brain works, confirmation bias and all that good stuff.

Flat airfoils can't fly? Just look at the simple folded up paper airplane we made as kids.
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Old 09-22-2014, 07:51 PM
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Default Flat airfoils

You can imagine the pioneer builder aviators arguing over NACA, Eppler etc airfoil shapes before constructing their aircraft! A close look at the Bleriot wing does show a curve which probably came about by tensioning the covering with dope for shrinking and the myriad of flying wires. What is also amazing guys flew aircraft long before there was any established flight physics! Often when I prototype a model before I commit to chopping balsa a foam board proof of concept flies first!
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Old 09-22-2014, 08:12 PM
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The Wrights don't get the credit they deserve sometimes. Far from just stretching fabric over a frame and seeing what shape they ended up with, they actually did painstaking wind tunnel research to determine the optimal airfoil for their flyer. Although they did kinda screw up a little because they neglected to factor in Reynolds number effects (which was why they ended up with such a thin airfoil) but still it was pioneering work and a big part of their success: http://wright.nasa.gov/airplane/tunnel.html

Most aircraft builders up to the end of WWI basically followed the Wright's lead on airfoil shape, including their omission of Reynolds number effect. It was only toward the end of WWII that it was realised that without correction factors small scale wind tunnels provided misleading results and thicker airfoils actually worked better at full size aircraft scale than the thin highly cambered Wright brothers type airfoil.

And as for the physics .. Nikolai Zhukovsky got there with the theory before the Wrights put it into practice. The true way wings work has been understood for well over 100 years but for some reason the bogus (but simple to understand) explanations that came much later gained popularity and still wont die.

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Old 09-22-2014, 09:20 PM
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Now we should talk about how turbulators and Kline-Fogelman airfoils can move the airflow separation of the boundary layer farther aft delaying the normal increased drag of separation that usually contributes to a stall.

Two awesome technologies that really work.
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Old 09-22-2014, 09:29 PM
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Kvar
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Default What is an airfoil?

Funny thing guys I know I am guilty of sometimes "designing" my own airfoil using the LAR technique and have had good success, Makes me a little confused sometimes! But hey, all good fun!
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Old 09-22-2014, 10:49 PM
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Originally Posted by ggunners View Post
Now we should talk about how turbulators and Kline-Fogelman airfoils.....
Hmmm.. my pet subject Turbulators certainly have merit, but not so the KF airfoils. The only real advantage of the KF is that they are easy and quick to build out of foam. Aerodynamically they have nothing to commend them.

That's why you have never seen a KF airfoil in any full size plane or in a genuine high performance model... too much drag.
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Old 09-23-2014, 12:10 PM
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I would disagree re: KF airfoils, but only regarding low Reynolds numbers for RC flying. The different KF airfoils do fly "better" than flat plates on RC planes some with better glide paths, others with more efficient flights. I like to think of KF designs as improved flat plates for foamies.

As for KF being easier to build, hmmm.... it's pretty easy to bend foam over a spar for a simple airfoil. Many like KF construction because the spars are part of the construction and make the wing more rigid. I see a lot of the pusher profile jets with KF.
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Old 09-23-2014, 07:48 PM
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Accepted, if we are comparing to flat plates.

The KF can perform better than a flat plate because they at least have a rounded nose. My previous comment was made with a comparison between the KF and a good standard streamline airfoil in mind.

I've done a bit of testing on this and the difference in flight duration on a glider between a good 'normal' airfoil and a KF is huge. Testing was done with the same glider fitted with different wings of the same area, planform and weight, one KF and one a Drela glider airfoil. All testing done back to back and repeated numerous times. Typically KF flight time was around 60% of the Drela.

http://www.rcgroups.com/forums/showthread.php?t=1542598

Last edited by JetPlaneFlyer; 09-23-2014 at 08:34 PM.
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