Inspaired by an Air Core a friend of ours was training on, 5 years ago a frind and I folded some coroplast over a yard stick and made a wing that flew. Neither one of us have any background or indepth knowledge of aerdonynamics other than basic theroy of flight. We went through a triangle facet wing (sucked), then a triple facet wing (very successful) and then on to stressing the coroplast in an airfoil shape over the spar. There were days we would go to the field with one airplane and a pile of wings to try on it That was five years and maybe 500 or 600 airplanes (between us) ago. Everything trial and error and TLAR. In an attempt to make a good flat bottomed trainer wing, we clamped the LE and trailing edge flat on the table while the upper wing was stressed over the spar. When the glue was set and the pressure reliesed, the leading edge rises slightly and the lower trailing edge area undercambers naturally. To our suprise, we came up with a wing that blows everything I ever flew in the balsa world away for maneuverability, slow flight... and inverted performance I never though a wing like this could have.

In the last 5 years, and most recently lurking on this forum, one thing I've heard and read is that sharp leading edged cause pitch sensitivity, higher stall speeds, and generally suck. I've seen guys get into coroplast building and go to great lengths to get more rounded leading edges...before even trying our wing. We had doubters at the field amazed at the performance once they see it fly. I never new enough to know it wasn't supposed to work, but what I do know is that I've never flown a sweeter airfoil. Solid as a rock with the CG at the spar. No pitch sensitivity whatsoever, or the pitch "hunt" I've been told it should have. The plane slows down better than my Goldberg Sr. Falcon ever dreamed of doing and It just plane won't stall unless a walking speed bobble counts before mushing back into stable forward flight. I've also been told it would lack any inverted performance...but it's fun to proove naysayers wrong...as it's just as stable and doesn't care weather is inverted or not. Snap rolls are totally lazy, and I have yet to get this plane into any type of tight spin at all.

...ok...the reason for this post:
I don't know what we did right (or wrong), but this wing with a leading edge as sharp as it could be is the best flying airfoil I've ever flown with none of the traits I've read or heard it should have. It would be interesting to hear why from an educated perspective.

I've flown Balsa planes for 20 years, and plastic planes for the last 5 years...and to date, the best flying airplane I've ever had is nothing more than a discarded gas station sign folded over a yardstick, rubberbanded to a couple aluminum rails. Less than 3 hours work, but the look at the field is priceless

Lemme 'splain - or at least try.

Sometimes, when all the factors are right, a sharp leading edge results in a spanwise bound (surface)vortex that sits just behind the leading edge. It's a small cylinder of air that sits there and spins, and deflects most of the oncoming air up and over it AS IF THE LEADING EDGE WERE ROUNDED. THat's why this wing behaves quite well.

I did some work on airfoils with sharp edges and steps in them when I was in school. Once took an Eppler airfoil, doubled the camber, then sliced the "top" of the airfoil off flat. It looked like an airfoil with a flattop hair cut. But it worked, quite well.

That was our first successful type airfoil.

We had tried a triangle facet (flat bottomed with one fold on top at the spar). This airfoil flew fine at higher speeds...but the slower it got, the worse it got...and trying to land the plane looked like the six million dollar man rocking back and forth and crashing!

Then a Cessna engineer flying buddy tried explaining burbles and boundry layers and air seperation...we really didn't have a clue what he was saying...but... we went home and did exactly what you said...gave it a flat top...with a 1" flat section it made for a triple facet (flat bottomed with 3 flat surfaces making up the top wing). This wing flew GREAT, and was the airfoil we stuck with for almost 2 years...and many are still being built and flown. Our Cessna buddy said very much the same thing you did about the sharp leading edge...he said the air flow was close enough to the wing but didn't seperate from it...in effect the air was fooled and "thought" the wing was a true smooth airfoil, as you say the air may "think" the sharp LE is actually round due to the vortex? This is cool...I might actually be able to simi explain it next time it comes up at the field

I'm not so sure you could consider that as sharp. Looking at the side view I see a fairly nice radius thanks to the folding of the corroplast. What I DO see is perhaps a shallower than normal curve on the leading edge...... and gobs of wing area. Perhaps it's flying well thanks to the area? And the strongly cambered section with the "flap" deflection in the rear. Strangely enough what you have for a shape there isn't far off some sailplane airfoils and follows a number of the rules of thumb for free flight airfoils other than thickness.

It would be interesting to run this shape through Xfoil.

PS: considering John's rotor theory I can see how such a little rotor bubble could be filling in the flat'ish upper entry just behind the leading edge.

John, could such a bubble act as a turbulator to encourage high AoA flight without stall separation?

I wouldn't really call it a turbulator. THey just call them bound vortices.

If you see an F-16 or an F-18 at an airshow on a sufficiently humid day, you'll see a chordwise vortex next to the fuselage marked by vapor when he pulls g's . That's a cool way to delay separation, but that's not what I'm talking about here. If you see an F-15 on the same day, when he pulls g's the vapor-vortex will appear on the outer half of the wings, right behind the leading edge and almost parallel. THe F-15 wing was designed specifically to let that vortex build there and delay stall. Sometimes you can see a spanwise vortex somewhere around the slats on the other two types as well.

Um, Check this out:

Oh, hey, a pic just reminded me - how do you think a facet plane like the F-117 flies? On the same principle we are talking about here....

X Foil seems to have a little trouble with sharp LE airfoils, I was only able to get it to run once. The one I did get to run had these weird pressure spikes up to max thickness, then a normal recovery; I think it had more to do with the way I was drawing the airfoil, it didn't come out exactly flat.

Anyways, X Foil also did not show the vortex forming on the LE, but then the vortex will probably be 3 dimensional in nature and X Foil is only a 2-D program, so that's not surprising. With that in mind, X Foil did predict an increasing Cl to about 0.8 at AoA = 3.5, then Cl decreased until 0.6 at AoA = 8, then Cl started going up again until about 1.25 at AoA = 13, and then decreased.

Interesting, but I think you need a true 3-D CFD program to really see what's going on here.

Thanks for some interesting stuff here!

Tattoo,

I have read a few posts here about sharp-edged wings being unstable in pitch and 'hunting'. There could be something going on that I don't know about, but currently, I don't think those are genuine characteristics of sharp-edged wings. A sharp leading edge causes a few things:

1) It 'fixes' stagnation line ( the line at the leading edge where the air divides between the upper an lower surfaces ) so that it doesn't move up and down as the angle of attack changes.
2) At moderate angles of attack, it does result in a small, localized separation bubble just behind the leading edge. This separation bubble is sometimes called a 'rolled vortex', and is what Johng called the 'bound vortex'. In actuality, this is not what the term 'bound vortex' refers to. This separation bubble does have really strong vorticity, but the term 'bound vortex' refers to the 'circulation' which always exists around a wing which is generating lift. This separation bubble can indeed cause the flow to become turbulent, and hence prevent a larger, laminar, separation bubble from forming further back on the upper surface, under some circumstances.
3) It causes massive separation ( stall ) at a lower angle of attack than a rounded leading edge would.

None of these effects results in pitch instability, or hunting, as far as I know, so it is not surprising to me that your wing works just fine. My impression is that these characteristics of sharp leading edges are aerodynamic urban legends, but I would be happy to learn otherwise.

I would guess you get such good results because you have a perfectly reasonble wing shape, and low wing loading. If you needed really high lift out of your wing, which is very rare for our models, then the sharp leading edge might cause some problems.

banktoturn

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The F-117 is what you get when an electrical engineer designs an airplane!
Aerodynamically.. oi!!!
Very very noisy slipstream.
Too many edges.
Somewhat manuverable, but probably not a good idea to spin one.
The sharp leading edge on a symmetrical set of flat plates keeps landing speeds up in the 170 knot region.

Tatoo, not to make fun of your quite nice flying airplane but it sure is ugly! Nothing personal and beauty is in the eye of the beholder and there is beauty in its functionality but ............ How could you possibly keep interest in that many wings on something that .... functional? Just kidding, just kidding...... maybe.

If you did a little rounding on the sharp corners and kept the undercambered aft part it is not all that different from the airfoils a lot of modern gliders are using. Ok maybe stretching it a bit but the undercamber does make a nice airfoil better for good upright work. It cannot be as good inverted since it is optimized for upright. It also depends on your criteria for determining what is good.

The inputs so far have been interesting to read. Pretty good stuff on this site. Almost makes me want to go back to work but not quite!

Years ago I designed a shoulder wing type configuration that had a diamond symmetrical airfoil about 10 percent thick. The planform was similar to the old UC Flight Streak and had coupled flaps-elevator for pitch work.

The airplane was too heavy to fly well butas a result of the weight had an interesting characteristic. When the angle of attack was pulled too high (actually not all that high) at a low speed the air flow would apparently separate. The airplane would pitch nose down since the tail airfoil was still attached and the airplane immediately started flying again.

In flight this looked like a longitudinal rocking motion along a level flight path. I changed the airfoil using the same lower surface and doubling the upper thickness ended up with a semi-diamond shape. Still with the sharp LE. The resulting airplane was nice to fly upright but not too good inverted. Still too heavy. It met its demise due to radio failure much too quickly to learn more.

Mostly it works down to pitch sensistivity is a function of CG location relative to the Neutral Point and elevator power (control throw and size). Light weight increases the airplane response to control inputs. Once you get past the basics and have a nice flying airplane then some small improvements can be made by optimizing the edge radius and wing thickness and those types of things.

I have had a couple of slopers that would exhibit the "stall nibble" when full-up was held. One was the "Rubber Duck", an EPP flying wing. Very benign stall and instant reattach when the alpha dropped a bit.

Ha!!!...don't worry, the plane is so ugly...it's name is actually the Big Ugly Hell on Rails... or Big Ugly HOR for short...as for keeping interest...don't worry, what we did 5 years ago has inspired something really big...and that simple act of folding coroplast over a yardstick inspired every airplane at the following link...all coroplast with basically the same airfoil

http://www.spadtothebone.com/Showcase.html

I'm really enjoying this thread, and thank you for the interesting reading. I've been flying circles around everything at the field for several years...it's about time I try to understand why

Speaking of vorticies and unusual airfoils, here's some interesting stuff a fellow Spadder just showed me ...I've never heard of a steppedfoil before!

http://www.gj.net/~nmasters/vortex-lift/step.html

Klein-Fogelman... I have their book.
Perhaps the "America's Cup" guys might look at it.
Basically it's nothing more than trying to push a sailboat with a fan attached to the sail boat blowing into the sail.
.
Several years ago my nephew got me interested in the thing. I built a wing for a Goldberg Mirage using a 23012 with K-F step on the bottom.
One of the best wings I've built.
I still have it, thinking of covering over the step to make a real wing out of it.
.
Flying exhibited no unusual characteristics such as faster/less fuel needed.
With all the negative results K-F eventually decided it was a "safe" airfoil. Difficult to stall or something like that. As it required more power than a conventional wing to operate, and would therefore be less efficient, I think the aviation community yawned at the idea.

Interesting stuff on the stepped airfoil. It is of course difficult judging something without looking at wind tunnel data and something that indicates the accuracy of the wind tunnel test. It is certainly not something that would give you an immediate "whow that looks like the way it should be done" based on the birds in the backyard.

Certainly there is no advantage in structural weight. For a given load the structure would need to be heavier as the structure in the cutout near the centerline of the airfoil isn't as good at load carrying as the same material at the surface of the airfoil.

Tatoo - and I thought I was being mean, it is a wonder the airplane doesn't bite you at the field. Those prop kick backs are not an accident perhaps!

I experimented with the Klein-Fogelman airfloil on models in the late 80s and found them to be as the originators stated. They were very difficult to stall. I saw no other positive characteristics in comparing them to the Kadet Sr. but, I like to experiment and the experiment was fun.

Tattoos knife edge Le airfoil does fly OK. It will haul a plane thru the air when properly powered, and is cheap and easy to build. It's ugly, but then so are some of us pilots, so no criticism there. I've built many of them, and still build them. (built two wings today.

BUT, the LE is in need of improvement ON SOME airplanes. Sharp LE airfoils are fine on very fast planes, but most of us don't fly very fast planes. Many of us, who still like to experiment, have found the coroplast wings are significantly improved with some rounding of the leading edge. Even a quarter inch radius makes a big difference in the slow speed flight characteristics.

One of the prominenet Spad guys simply puts a 3/8" flat leading edge in his. We have found this to be an improvement over the sharp LE. Why a flat leading edge works is beyond me. One of our guys has built several trainers with the Spad wing only with a well rounded leading edge of about 3/4" radius. It is the best flying Spad wing I have flown, and compares favorably to the Kadet Senior, the plane by which all other trainers are judged.

Until I see a "rolled vortex", or a "bound vortex" on a 45 MPH trainer, I would be skeptical. Yes, I've seen them on supersonic jets, but on model trainers :stupid: :stupid:

I've never seen a perfect airfoil for all airplane configurations. The sharp LE does work Ok on sticks, combat craft, and wild aerobatic, poke holes in the sky ships. Pitch instability in these planes is usually not noticed as we are banging the sticks around most of the time anyway.

But in a slow stable ship used primarily for training and lazy day flying, the sharp LE does want to hunt in pitch unless the ailerons are reflexed up slightly. And reflexing the ailerons does increase the landing speeds and stall speeds. What's the old saying, "nothing is free".

So, until someone finds a better alternative, I will continue to build coroplast wings, but add a little rounding or bluntness to improve the flight characterisitics. No criticism here, just a little difference of opinion.

Cajun

Hey cool! Great to see you here Cajun!
One of the reason I brought this wing, and this ugly airplane to this forum is because it has blown me away. The plane will do a walking speed 45 degree slow pass and never flinch. I can't stall it. It also does the most solid Sloooooooow inverted passes I've ever been able to do with any airplane. There was a Cadet at the field the first time I brought it out, and I blew it away in slow flight. The plane will toilet bowl almost as good as a pizza box, and negitive G stuff is as solid and almost as responsive as positive G stuff, which is unheard of for a Spad airfoil. The only difference from previous Spads is the 4mm stressed over the spar instead of the double facet, and the wing's chord is 14" instead of the normal 12" for .40 sized planes. If you have a Debby handy, slap one of these wings on it and try it. I have no idea what we did right, but I wouldn't change a thing about this wing!

Yea, tattoo, you thought you'd come over here and I wouldn't find you. HA I've got spies all over this web!!

seriously, I didn't know you had developed a new wing. Send me the details and I'll try it out. Of course, I may have to round the LE

What is the difference between it and the 2 mil built the same way.

Cajun

You've gotta hand it to them SPAD guys...they really make an effort in decorating their planes.

Take a look at the nice graphics on TATTOO's plane, must be a lot of work in them...

Cajun,
the instructions for the plane the are here
http://www.spadtothebone.com/SPAD/Buhor/

The 4mm coroplast has a lot more tension in it, and when the pressure is reliesed, the leading edge raises a little more, and it doesn't get as much undercamber as the 2mm wings. You have to do some serious clamping to keep the leading edge down while gluing It also seems like the 2" of extra chord has made a huge difference in performance.

Rudeboy, man...them Coke and Mountian Dew graphics took forever to do!...and I didn't even get any sponsor money!...but I simplified the graphics on my next one

I worked on some stepped airfoil research when I was in college as well. Had a professor that thought it would work especially well on unusually thick airfoils. Tested a step on an airfoil that must have been 35-40% thick. - Like a potato with a flap attached to the back end. The step behind the high point turned the flow considerably more than without the step. Not that it made it a practical airfoil though....

FWIW - There's nothing wrong with the use of the term "bound vortex" in this context. It happens to be widely applied as a virtual element in wing theory considerations, but that's not the only place it may be used. A web search on the phrase turns up it's application to real vortices on anything from airplanes to sailboats to swimmers' hands. The best definition I've found shows how broad the term really is - from an online fluid dynamics course from MIT:

The only requirement for a vortex to be a bound vortex is that it is held in one place wrt the fluid flow. That is what we see in these step airfoils and on some sharp LE's.

I think that the reason sharp LE's are thought to hunt is again, vortices. I would bet that small bound vortices are continually forming and dispersing behind sharp leading edges at low angles of attack - which change the lift in an erratic fashion. I don't have anything to prove it, but that's the explanation I think is most likely for "hunting"


BTW - THat's a cool web site link to the step airfoils from Tatoo. Thanks!

Johng,

I disagree about the term 'bound vortex'. The quote you give is not a definition, and uses the term in exactly the same way that it is used in wing theory, and making the same point. The articles I found, presumably in the same search you did, about swimming and sailing, were clearly written by laymen, who used the term incorrectly. In particular, if one could calculate the circulation around a 'bound vortex', it would allow the direct calculation of the lift. You could not do that with the vortex that exists in the leading edge separation bubble.

I think you may be onto something with the oscillating separation bubbles. It could well be that a separation bubble there would be unstable, and since a bunch of the lift occurs right there, it wouldn't be unreasonable to expect some noticeable change in pitching moments as a result.

banktoturn

Yes, we do disagree. The quote/definition/whatever shows that the term is much more general that for the specific case you use it for. Just because that's the only way you've seen the term used until now doesn't mean that's the only use for it. It is indeed a general term for any stationary vortex.


However, there's no reason a real bound vortex cannot be incorporated into the virtual bound vortex used to calculate circulation. That's more a limitation of the method/derivation than the actual physics.

I'm done.