a088008

I think that most of us know how a flat bottom airfoil works (high pressure under the wing and low pressure over the wing), but how is lift achieved with a symmetric airfoil? Does the incidence of the wing change with speed to create a variable semi-symmetric airfoil?

-Q.

FHHuber

The BASIC version of the manner in which lift is developed never covers the symetrical airfoil because the math gets somewhat extreme.

I'm trying to avoid going into calculus here...

You have part of the idea with the "variable semi-symetrical airfoil" though. With the AOA incrasing, a larger amount of the LE is above the "line" separating the upper surface airflow from the lower surface airflow. This does give an asymetric airflow pattern.

There's also a downward deflection of air passing under the wing. The conservation of energy law will show that the air being forced to move downward means something else had an upward force given to it... that was the wing.

The best way to look at it from the modelers perspective: It works. The models fly just fine.

If you want/need more than this... you probably need to get some aeronautical engineerig textbooks. (maybe a degree in aeronautical engineering..) I used to read thes texts for fun.

BMatthews

Simple angle of attack. It just happens to have a very, very low camber value...

If you look at polars for symetric and cambered types you'll notice that the airfoil has a range of lift and that as the camber curve rises the lift range is shifted towards the positive values and the max lift coefficient before the high drag knee that we know as a stall moves up as well.

In the pic below you can see the 8020 fully symetrical has a range of lift up to about Cl= 0.4 to 0.5 before it gets kind of draggy. And, of course, being symetrical it does the same thing when upside down. The very low camber 7003 FAI glider type has a slightly better range up to 0.8 and a low end that is really fast in a vertical dive when the Cl is 0 but would be draggy if you actually flew upside down (that part is missing here but it's in my Soartech 8 book). The relatively high camber 7037 gets up to about Cl 0.9 before the drag gets very high. But look at the bottom end. See that recurve back? That means that in very fast flight it's actually making MORE drag than when it's cruising quickly but not speeding.

So you CAN fly and even thermal soar with a symetrical airfoil but it's going to have some limitations.

There's another item but since it has another pic to go with it I'll break this into two. But in the meantime here's the polars to go with the above...

BMatthews

Parte Deux....

But you also put in an interesting point...

------------

"...how a flat bottom airfoil works (high pressure under the wing and low pressure over the wing),..."
---------------

I've written this part about 4 times now. You're really bending my mind and I'm learning that I have some bad misconceptions.

OK. I'm going to take a stab at this now and hope that I'm not TOO wrong. Looking forward to hearing from Ollie and Ben and the others here.


Drop on by this site for a very interesting app that lets you play with an airfoil in real time.

http://www.grc.nasa.gov/WWW/K-12/aer...let/vj402.html

Start with the symetrical one and add 6 degrees of angle (of attack) using the slider. Use the slider on the right side of the airfoil to zoom back for a wider view. Note that the streamlines have a little bit of deflection downwards at the trailing edge and there's a pressure difference in the chart on the right. Somewhere about a 1/4 psi at 6 degrees if you average it out. Now add .4 on the camber slider. Note the volume between the pressure curve grows. That's lift but it's not a large amount. But look at the way the streamlines have changed. There's now a very healty curve up to the airfoil and a strong downsweep off the trailing edge. That's lift too. You've redirected the air coming up to the foil and squirted it downwards. The acceleration of the air downward in the wing wash is a large part of the lift off a wing.

And here's the kicker.... (FINALLY.... )

If I remember one write up accuratley this downwash is the major contributing factor to lift. The pressure differential being a player but a minor one.

So THIS is why symetrical airfoils can generate lift but not as well as cambered airfoils. They both have the ability to redirect the air flow into downwash but the cambered foil has a greater angle of redirection.

PHEW..... Gotta put on my Nomex underwear now and let the experts punch holes in this one.

Having to explain this helped me a lot too but I just hope I'm on the right track.

PS. check over the polars below but they are part of one of the other explanations that I tossed out. I forgot I'd put the graph on this and here it is... Oh well.

Ollie

A wind tunnel with a smoke generator reveals the physical reality of what takes place.

Start by considering a flat plate airfoil. It has no camber and little or no thickness. At zero degrees angle of attack, the flow splits at the leading edge and the air goes over and under the airfoil. Increase the angle of attack a bit and the stagnation point moves aft a bit on the lower surface. the increase in angle of attack also results in an upwash ahead of the leading edge. The upwash gives the arrival angle just ahead of the stagnation point a greater angle than the angle of attack. The stagnation point where the flow splits is no longer at the extreme leading edge. The flow that goes over the top has to first reverse itself almost 180 degrees twice to negotiate the leading edge. This peculiar and non intuitive behavior of the flow is part of the mechanism that allows a flat plate to generate lift. The longer path of the flow over the top also arrives at the trailing edge before the flow over the bottom of the airfoil. The higher velocity of the flow over the top generates a lower pressure. The down wash behind the flat plate results in a vertical acceleration of the air mass which produces the lift force ( force equals mass times acceleration). Circulation is what produces the up wash and down wash which is also part of the lift generating mechanism.

A symmetrical airfoil is just a flat plate with a streamlined form wrapped around it which adds thickness. The movement of the stagnation point aft along the lower surface near the leading edge occurs with angle of attack increase. The main difference compared to a flat plate is that the reversal of direction of the flow around the leading edge is less extreme and the flow is not stressed as much. That's why a symmetrical airfoil with a large leading edge radius can generally achieve higher angles of attack without stalling. The lower stress in the flow results in delayed flow seperation.

All this funny stuff can be seen in a windtunnel equipped with a smoke generator.

banktoturn

a088008,

A symmetric airfoil generates lift in the same way as a non-symmetric airfoil. A flat bottom airfoil is just one kind of non-symmetric airfoil, and there is nothing special about the flat bottom, except that it makes the wing easier to build. If you look at the way air flows over a symmetric airfoil when it has a positive angle of attack, which it must have in order to generate lift, you could see that the airfoil actually 'looks' non-symmetric to the air. I say this because when a symmetric airfoil is at a positive angle of attack, the 'stagnation point' ( fancy word for the point at the front where the airflow splits between the top and bottom surfaces ) moves down toward the bottom surface. As a result, the 'path' taken by the air that goes over the top is different from the path taken by the air that goes under the bottom. Because the trailing edge is sharp ( and it should be sharp ), the point where the air leaves the airfoil does not change with the angle of attack, until the wing stalls. To paraphrase your post, as the angle of attack varies, you do sort of get a variable non-symmetric airfoil. There is no such thing as a semi-symmetic airfoil. A thing is symmetric or it is not.

You will read a bunch of explanations for the creation of lift by a wing. The bottom line is that the pressure on the top is lower than the pressure on the bottom. That's all there is to it. There are some aspects of the flow phenomena that help explain why the pressure on the top is lower, and looking into them can help you gain some intuition, but I see a lot of mistaken notions presented when the discussion goes past the simple difference in pressure between the top and bottom.

banktoturn

Ollie

Banktoturn,

I would appreciate it if you would be more specific about the source of the mistaken notions you are referring to and exactly what they are.

banktoturn

Ollie,

My remark was a general one, and reflects a large number of explanations I have read, including a bunch of posts here. I wasn't specifically responding to or disagreeing with other posts in this thread. I guess I don't have a list of mistaken notions and their sources to give you. I will say that many of the explanations that I've had issues with have left the impression that there are multiple 'sources' of lift, besides the pressure difference between the upper and lower surfaces. If you tell someone that the pressure difference is where lift comes from, and then rattle off some other 'sources', it could lead someone to some mental double-dipping in tallying up lift for an airfoil.

I didn't mean any offense; I hope none was taken.

banktoturn

a088008

I never expected such a wealth of info on symmetric airfoil lift.
Thanks for all your input.

The whole discussion on the stagnation point has raised a question in my mind. I currently have a symmetric airfoil that will be used on ribs in a open structure wing. The problem is that there will be sections of the wing that have a more sharply defined leading edge than that of the rib section (i.e. the center section between 2 ribs). This will be due to the covering pulling tight and down. What should I expect from a symmetric airfoil that has a fairly sharp leading edge (compared to the rib section)? Will it stall more easily, fly faster, have less drag, more penetration, ... ?

I have attached a drawing of the wing rib for reference:

-Q.

banktoturn

a088008,

Generally, an airfoil with a sharper leading edge will stall at a lower angle of attack than one with a blunter leading edge. I think that's the most significant difference.

banktoturn

Ollie

The leading edge contour is the most important part of the airfoil. it should be maintained as accurately as possible. Covering sag will caues the airfoil in question to stall at a lower angle of attack than it would if the leading edge of the airfoil contour were maintained all along the span. at low angles of attack the airfoil should behave more or less normally. The main effects will be that the plane can't be landed as slowly nor will it be able to turn as tightly as one where the airfoil contour has been maintained. Also, the wing with covering sag will enter maneuvers involving stall, like snap rolls and spins, much easier.

JimTrainor

I have an old flying book, I think from the 30's, called "Stick and Rudder"... I can dig up the authors name if requested. The pilot who wrote the book didn't have much patience for highly technical explanations of lift. The book explains it simply: "The plane goes up because the air goes down.". No laws of physics broken by that explanation.

So, the net result of a symmetrical airfoil at a positive angle of attach is to move some air down...

a088008

Thanks for the explanations.

As a result, I'm considering adding more intermediate ribs to the wing on the leading edge only. I have some already, but they might not be enough. This should help solve the sag problem and maintain the leading edge profile more accurately. I do, however, notice that those indoor aerobatic electric planes use very few ribs and essentially have a sharp leading edge due to sag in-between ribs. How do these types of planes perform? Does anybody have experience flying these type of planes?

Here is a picture of what I'm proposing to add more of (I currently have one in-between each full-length rib):

-Q.

a088008

I am fairly comfortable with the explanations given thus far, but I'm always interested in getting as many viewpoints on a subject so that I can extract what applies to me and form a complete picture of what is going. I like to get a "feel" for the dynamics involved and any additional information would help this. I also like the aspect of getting a non-technical viewpoint that might give some insight the person has gleamed from actual experience, especially a pilot (model or not).

Please forward me the author, if you it's not too much trouble. I would appreciate reading some of the old-school know-how on a topic so abstract as aerodynamics.

-Q.

Ben Lanterman

Pretty good explanations given here so far, online sources I would suggest looking at include

http://www.monmouth.com/~jsd/how/htm/how.html#contents

for a balanced view of lift, I consider it very good.

Look at

http://www.aa.washington.edu/faculty/eberhardt/lift.htm

for a view fo lift as diverting air downward. The writer of the second one says it was designed to present to non engineers and I personally don't like it much.

Rudeboy

I'm defenitely not an aerodynamics pro, but my idea about these fun-fly type planes is this: keep the wingloading low enough and the plane small enough and it will do whatever you want it to do...regardless of aerodynamics and wing sections.

I once read this in an article on aerodynamics: "Imagine how a fly thinks about air..." I find this a very nice statement. A fly must be experiencing air as we do water for instance. This is of course the Reynolds number at work...

BMatthews

I FOUND THE SITE I WAS LOOKING FOR ! ! ! !

This is the site I found a few months back that broke me of a lot of my misconceptions.....

http://www.allstar.fiu.edu/aero/airflylvl3.htm

A very good read.

The only little nitpick I have here is that the "Clark Y they show with the bottom surface being level and no downwash is just not true. The camber in these sections produces some downwash at 0 degrees AoA as shown in the windtunnel "simulator" from my post above.

But in that simulator if you have a cambered airfoil and keep making the AoA more negative until the lift value just under the "window" is 0 then you'll see that the streamlines have gone to level coming off the trailing edge. This being also the zero lift AoA for that airfoil.

BMatthews

That's one option or you could mold up some 1/16 sheet around a form carved from some clear 2x4 that closely matches your leading edge shape. Either way the ides is to prevent any bad covering ridges where it transitions from a spar or leading edge to the open bays. Unless you have WAY too much power, then you don't care...

On funfly types that are doing super tight loops to the point that you see them start to porpoise in little bobbles as they go round this is where a slightly better leading edge shape would help them do THAT diameter. It'll still porpoise with a "good" wing but it will happen at a tighter radius.

A good dollop of household ammonia in hot water and a 5 minute soak will make 1/16 sheet as pliable as a rubber band. Wrap it with an elestic bandage and let dry and the wood is right back to it's old stiffness but with a curve.

Rudeboy was very close IMO to the answer on the very thin wings. Us free flighters have known for some time that "thin is IN" when it comes to very low speed airfoils. Also the rule about sharp leading edges falls on it's nose at those speeds too. Many a free flight airfoil is meant to have either a sharp angle or at least a very tight leading edge radius. At the speeds we fly these models the sharp leading edge helps generate a turbulent boundry layer on the upper surface to delay stalls. And since microfilm models fly slower than anything else they can get away with just a single surface.

I actually think the IFO's and other carbon rod outline models could benifit from a slightly thicker airfoil what with their flying speed being more like our lighter outdoor free flights but this may be where the toughness, lightness and simplicity is worth the 5 or 10 percent difference. But that's just a guess as no one that flies these is serious enough to do any proper testing.... They are having too much fun just doing what a bumble bee does.... Flying even though they shouldn't be able to....

Whirley Bird

My Feedback: (1)

.Someone quoted Simple angle of attack. It just happens to have a very, very low camber value... .
I'm not an engineer but I did use symmetric air foil blades from a small helicopter and cut then root sections down and used them on my full sized Gryo.
Big mistake.
Gyros are not made to fly with ful symmetrical foils.
I lost a lot of sweat bringing it down.
The Gyro doesn't have advancing blade stall and retreating blade lift like a powered helicopter.
Live and learn

William Robison

My Feedback: (3)

"Stick and Rudder" was written by Wolfgang Langschweisse. 50 years ago it was close to being required reading for a private pilot's license.

And for the micro fliers, when you combine low speed and small airplanes you get to a Reynolds number where a flat plate works as well as a cambered airfoil. True.

Whila Wolfgang knew a lot.
. Go small, his ideas go to pot.

Bill.

a088008

Again, you guys have given me a wealth of info on the subject.
Thank you very much for all the input.

Just to fill you in on the purpose of my airfoil, I'm using it in a park-flyer of my own design. It has some unique design principles that should make for an interesting model to look at and to fly.

It uses the latter mentioned (see my attachments) symmetric airfoil in a wing that is 47" in wingspan and has an aspect ratio of 1:6.3 i.e. cord:wingspan. The plane is built light and uses a 380 direct drive electric motor. The model is not overflowing with power, hence my concern over stall characteristics. It is however quite light and should be fine from a power-to-weight ratio standpoint.

The size of the wing does not really warrant using 1/16" balsa sheet for the leading edge, as was suggested. I always use that method on any of my larger designs. It works great and can even be used with a leading edge block of balsa with balsa sheeting joining the block on the top and bottom. I have used this method successfully time and again. All that is needed is to shape the balsa block to smoothly transition to the balsa sheet and you have a very accurate leading edge contour. See my avatar for my previous model's wing construction.

I'll post some pics when I'm done with the model, which should not be long now. And then, more importantly, some after flight analysis for your enjoyment. Hopefully it won't be a crash report

-Q.

JimTrainor

Stick and Rudder

It is a how-to-fly book. Not really an aerodynamics book. His(Langschweisse) discussion of aerodynamics is on a sort of "need to know" basis. As in, forget about all the theory... here is what you need to know to fly a plane.

It is popular enough to remain in print. Here is an Amazon link:

http://www.amazon.com/exec/obidos/tg...books&n=507846

Whirley Bird

My Feedback: (1)

].I'll take your word on that.
But I know many that will disagree.
Thanks for the post

banktoturn

[QUOTE]Originally posted by William Robison

And for the micro fliers, when you combine low speed and small airplanes you get to a Reynolds number where a flat plate works as well as a cambered airfoil. True.

Bill,

As a general statement, this is false. For very low Reynold's numbers, a cambered plate becomes less inferior to a 'traditional' airfoil shape, but the best performing airfoils for most low Reynold's number planes are not flat or curved plates.

banktoturn

William Robison

My Feedback: (3)

banktoturn:

As you wish, sir. I've built many a micro sized rubber job to play with in the yard on calm days, and a flat 1/16 balsa wing frame with Jap tissue stretched over one side flies every bit as well as a cambered airfoil, and it's a lot easier to make. I've tried both.

Scientifically, granted, my statement is false. Empirically, however, it is true.

Build your airfoil cambered or flat,
. Small flat flies well, and that is that.

Bill.

PS: Check the camber of a bee's wing. Special case? Nope. Low Reynolds number. wr