QUESTION, KLine-Fogelman Airfoils For Electric or Glider.

[Texas Buzzard]

Have you built and flown a glider or an electric-powered plane with the Kline-Fogelman (KF) airfoil?

I have read quite a lot about KF and small planes/gliders. Some have the thick leading edge on the bottom as the inventers did, but some have the thick part of the wing on the top.

Please - no theory. Have you seen, built and flown a KF airfoil? Is there a DIFFERENCE BETWEEN THE FLAT PLATE AND THE KF WING. WHAT'S THE DIFFERENCE? Ha, ha ; does it work at low speeds?

The pic shows the originators wind tunnel airfoil. The other three show typical 6mm Depron wing X-sections. That 3-D foil used 1/2" thick Pink foam on L.E. I have seen videos and they don't seem to show an advantage. Where do you use KF?

[BMatthews]

I've not tried it myself but way back when there was a magazine article by a guy that tried them. Dismal failure overall was the final result. While it did show some promise in one very narrow range of speed/lift coefficient it was such a narrow range and worked so badly outside of that range that it never went anywhere. Perhaps at supersonic speed the wedge and shock waves off the peak may offer something but last time I checked we don't fly that fast.... :D

Go ahead and try them but I'd suggest you try a sheet balsa plate with a wedge carved depron underwedge to try to simulate the shape more accuratley. A simple little test glider is all you need for these. The design sketch below would work for these trials and the odd looking swept forward fin will provide stability to the flat wing so you won't need dihedral. It's a cheat but it'll work for an airfoil comparison glider. Sizes aren't at all critical but keep the stabilizer area around 1/5 the wing's area and the vertical fin/keel around 1/3 the wing's area. Some way of altering the incidence would be a nice idea. Like glue only the front 1/3 or so of the stabilizer and wedge up the trailing edge. Obviously you'll need to relieve the fin back there as well to allow that. For a simple 12 inch all balsa version if you can't have it flying in about 6 minutes of building and using CA then you deserve to have your Xacto knife confiscated.... :D

[Texas Buzzard]

Thanks for the honest reply to my question.
You don't give the KF airfoil much respect for models, I think rightly so.

I suppose the gliders ( 50 to 60" w.s.) were flying in spite of the KF, or the KF airfoil they used was so thin they essentially had a flat plate.

I have several semi- 3D 32" electric planes. Maybe I could pin a layer of foam to the wing to see if it makes a difference one way or the other. At least I'd have some empirical evidence, huh? Thanks.

[rofujiyama]

http://www.rcgroups.com/forums/showthread.php?t=558321&page=98#post8633572
Hi Buzz,
Here's a link to the foamy guys. they have a whole thread on it , and alot of other threads in the scratch built foamys section,here in RC Groups, It should be a good read. LOL its very long :D Its very popular with the foamy guys.
Roland

[John O'Sullivan]

There is so much practical and well developed data on scientifically proven airfoils, why would anyone waste their time on speculative sections such as these? What is the basis of their viability and where is the proof that they are efficient??
John

[BMatthews]

Kline and Fogelman did up some sort of paper way back. But I think they were not aerodynamicists first and foremost and they avoided some of the pesky things like stall charactaristics or speed range while still performing well or perhaps a few other things that come in so handy when actually flying a real airplane. Obviously it didn't go anywhere in the full sized world but some of us modelers are the eternal optimists.

[soar-ne]

Since you guys are going to throw rocks at the KFm airfoils, I am going to pick em up and chuck them right back at you. I know very little about how a airfoil works and really dont give a crap. I am a practical person and if works it works!!!
I have built many KF style airfoils in the last year or so and have even built the same plane with flat plate, undercamber and clark Y type of foils. The KF is way better than the flat plate and the undercamber in almost any way. The only downside to a U/C is that it might not be quite as slow but has a lot gentler stall. It is about 20-25% faster than either of these airfoils and penetrates a lot better and the glide is much better also. The speed range is almost the same as a clark y, but will fly slower and again has a much gentler stall. I have flown the same plane from goalpost to goalpost wide open with my grandson timeing and the time is just a few seconds difference between the KF and the Clark Y.
The GWS Beaver with the KF step modification to the stock undercamber foil turns a good flying foamy into a great flying foamy on the same power plant.
The same with the Cub wing on the scratchbuild low wing. I made the little foamy with both wings and their is no comparison that the plane flies much better with the KF foil.
It is a easy build with foam and adds structual strength without hardly any weight increase. With small foam planes its a winner in almost every way, that why so many of us "Foamy Guys" have and will continue to use it......Try it you may just scratch your head and say " HUH I guess it really does work"

Bob

[nmasters]

The pic shows the originators wind tunnel airfoil.

Well, it shows one of the models. They also did several RC models with the step cut into a more conventional airfoil section. That flat, sharp leading edge, shape on the first patent was simply to avoid having the Kline Fogleman step associated with a specific airfoil. It’s odd that everybody keeps calling it a “KF airfoil” when in fact Kline & Fogleman did not invent an airfoil section; they invented a feature that can be applied to any airfoil. A step cut into an airfoil dose have positive effects over part of the speed range. Different locations can be useful at different times. For instance some fighter jets have what’s called a “vortex flap”. A vortex flap is basically a drooping nose with an overhanging upper surface skin panel, when you droop the nose the skin panel sticks up and forms a step on the upper surface. The trapped vortex behind the step turns the free stream flow much more effectively than a smooth solid skin thus allowing the wing to operate at a higher AoA. The Kline Fogleman patents only covered a single step at 50%c. A lower surface step acts very much like a split flap. By filling in the area from the flap to leading edge you end up with an airfoil section that looks just like their RC model. This shape has a fairly high L/D at low AoA and an unusually low L/D at high AoA with a delayed stall and stable pitching moment. I’m not at all surprised that power models fly well with that set of characteristics. As Bob said gliders penetrate well because of the improved L/D at 0 AoA but I expect the sink rate to be lousy. See this page for some actual wing tunnel results (http://users.acsol.net/~nmasters/vortex-lift/step.html) .

--Norm

[BMatthews]

Soar NE. What you have done is something else other than a Kline Fogelman airfoil. You've closed in the front of the undercamber on these factory wings. But the correct KF wing is the flat topped wedge shaped lower shown in the first item in the first post. Even the flat foamie options shown in the rest of that diagram are something different from the actual KF airfoil. It's like someone that draws up a flat bottom airfoil and a shoe sole curve over the top and calls it a Clark Y. Nope, it isn't. It's something else just as your capped GWS wings are something else entirely. The curved upper surface makes it so.

I've flown a GWS Beaver and I agree that it's a draggy thing. I don't doubt that your lower cap moves the separation bubble that forms right behind the leading edge further back and it's quite likely the turbulence behind that point makes the airfoil act in a whole different way. Good on ya. But it's YOUR airfoil and not a KF. You'd better name it before someone patents it out from under ya... :D

[adamdb]

I too have built MANY KF airfoiled planes and had great success with them. Please read the other RCG thread shown above. It has all the information you could possibly want. I also don't understand (or care) about the theory behind why they work, I just know that they do.

Towards the end of this thread is some discussion about a glider done with a KF airfoil.

[Texas Buzzard]

There is so much practical and well developed data on scientifically proven airfoils, why would anyone waste their time on speculative sections such as these? What is the basis of their viability and where is the proof that they are efficient??
John
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Practicality? Speculative?

Many of the guys here are building foamies from Depron or Bluecore.
It is so quick and easy to make a flat-plate wing, but if you cup the power they glide like a bird full of lead.

My question was asked for this reason. To double the thickness of the L.E. is easy- even on the completed model. If the added thickness will improve lift at lower speed ( such as on landing approach) the semblance of a glide would be welcome.

[Bob Reynolds]

Back in the 60's when I was one of them there Texan's flying HL gliders, I developed an airfoil that had a steped leading edge that flew rather nicely. I used the standard 5/16 balsa back to 1/3 of the chord with the rear portion as 1/32 tapering to 1/64 on the bottom of the wing. The top part of the thick forward section was a highly polished curve from a rounded LE to the sharp lip. I surmized that durring launch the air would flow over the lip in a smoth fashion giving me both speed and altitude. After transition I felt that the air would buble over the lip giving drag to slow down the plane and the resulting vortex over the lip just might increase the lift (longer air path over the wing). It seemed to work the way that I wanted it to, but the problem was the thin Te that tended to shatter from the bending forces on launch. I did get 5th, (out of over 1,000 contestants) outdoor hand launch glider at the 64 Nats with one of my gliders with this wing. :D

[Texas Buzzard]

Back in the 60's when I was one of them there Texan's flying HL gliders, I developed an airfoil that had a steped leading edge that flew rather nicely. I used the standard 5/16 balsa back to 1/3 of the chord with the rear portion as 1/32 tapering to 1/64 on the bottom of the wing. The top part of the thick forward section was a highly polished curve from a rounded LE to the sharp lip. I surmized that durring launch the air would flow over the lip in a smoth fashion giving me both speed and altitude. After transition I felt that the air would buble over the lip giving drag to slow down the plane and the resulting vortex over the lip just might increase the lift (longer air path over the wing). It seemed to work the way that I wanted it to, but the problem was the thin Te that tended to shatter from the bending forces on launch. I did get 5th, (out of over 1,000 contestants) outdoor hand launch glider at the 64 Nats with one of my gliders with this wing. :D
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Bob, If it works dont fix it. You got a 5th at the Nats with that wing. So it was better than most, right?

Thats called EMPIRICAL EVIDENCE and thats the BEST KIND.

[aman_74]

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Practicality? Speculative?

Many of the guys here are building foamies from Depron or Bluecore.
It is so quick and easy to make a flat-plate wing, but if you cup the power they glide like a bird full of lead.

My question was asked for this reason. To double the thickness of the L.E. is easy- even on the completed model. If the added thickness will improve lift at lower speed ( such as on landing approach) the semblance of a glide would be welcome.

I agree. I'm not sure why John would want to boohoo something just because it hasn't been accepted in for use in larger aircraft. So someone who works for Boeing shouldn't continue to research airfoils because there's already a bunch out there? Makes no sense to me.

Besides, I'm no expert, but what comes to mind is the fact that in RC we have the luxury of being able to fly our aircraft much differently than a manned aircraft. We have really extreme power to weight ratios, we don't have to worry about pilot safety, and if an airfoil was only good under a very narrow set of circumstances it would be a disaster for most commercial uses, but for us, it might be just the ticket.

So, was the KF designed by RC'ers? Someone made mention of that.

[JetPlaneFlyer]

very narrow set of circumstances it would be a disaster for most commercial uses, but for us, it might be just the ticket.

Maybe if your style is flying at the same speed all the time and not pulling any high G turns or manouvres a airfoil that works only over a narrow range will be ideal for you... But for most of us who like to do things such as take off, land, fly slow, fly fast and do all sorts of manouvres and aerobatics then you need an airfoil that can cope with a wide operating range.

[JetPlaneFlyer]

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Bob, If it works dont fix it. You got a 5th at the Nats with that wing. So it was better than most, right?

Thats called EMPIRICAL EVIDENCE and thats the BEST KIND.

Or maybe Bob got lucky and found a few thermals, or maybe his throwing arm was better than most? There are too many variables to draw the conclusion that a fifth place finish for this design in a one off contest that took place 44 years ago is 'evidence' (empirical or otherwise) that this airfoil is superior. What about all the other flying contests that have ever taken place anywhere in the world before or since? You appear to be conveniently ignoring these tens of thousands of events and focusing only on this one event of 44 years ago?
If you want empirical evidence how about the fact that non K-F type airfoils have WON every documented flying contest, full scale or 'model scale', speed or duration, since the dawn of aviation (including the very contest Bob came 5th in). Is this not TOTALLY OVERWHELMING EMPIRICAL EVIDENCE that K-F airfoils are complete rubbish?

I'm sure in reality Bob's glider flew very nicely but if the airfoil had a real advantage over the rest it would probably have won and by now, 44 years later, everyone would be using such airfoils.

The idea of a step on the upper surface, as Bob is talking about here, which may act as a turbulator, has a lot more theoretical merit than the wedge shape, stepped lower surface airfoil that was shown at the start of this thread. In any case just because an airfoil that features a step on the upper surface may work ok on a free flight HLG (i.e. about as good as a standard airfoil) does not in any way support the notion that a wedge shape airfoil with a step on the bottom will perform better than a standard airfoil on an RC model.... In fact 'if' (and that's a big 'if') putting a step on the top enhances the airfoil's lift producing characteristics then simple logic would dictate that putting a step on the bottom (in effect turning the airfoil upside down) would tend to make the airfoil better at 'lifting' downward, not upwards :rolleyes:

[aman_74]

Maybe if your style is flying at the same speed all the time and not pulling any high G turns or manouvres a airfoil that works only over a narrow range will be ideal for you... But for most of us who like to do things such as take off, land, fly slow, fly fast and do all sorts of manouvres and aerobatics then you need an airfoil that can cope with a wide operating range.

I don't know much about this topic at all. I was just thinking that since there are clearly people out there who love it, there has to be a reason it works in RC. People I've heard talk about it seem to land, stunt, etc. without issue.

[Texas Buzzard]

Or maybe Bob got lucky and found a few thermals, or maybe his throwing arm was better than most? There are too many variables to draw the conclusion that a fifth place finish for this design in a one off contest that took place 44 years ago is 'evidence' (empirical or otherwise) that this airfoil is superior. What about all the other flying contests that have ever taken place anywhere in the world before or since? You appear to be conveniently ignoring these tens of thousands of events and focusing only on this one event of 44 years ago?
If you want empirical evidence how about the fact that non K-F type airfoils have WON every documented flying contest, full scale or 'model scale', speed or duration, since the dawn of aviation (including the very contest Bob came 5th in). Is this not TOTALLY OVERWHELMING EMPIRICAL EVIDENCE that K-F airfoils are complete rubbish?

I'm sure in reality Bob's glider flew very nicely but if the airfoil had a real advantage over the rest it would probably have won and by now, 44 years later, everyone would be using such airfoils.

The idea of a step on the upper surface, as Bob is talking about here, which may act as a turbulator, has a lot more theoretical merit than the wedge shape, stepped lower surface airfoil that was shown at the start of this thread. In any case just because an airfoil that features a step on the upper surface may work ok on a free flight HLG (i.e. about as good as a standard airfoil) does not in any way support the notion that a wedge shape airfoil with a step on the bottom will perform better than a standard airfoil on an RC model.... In fact 'if' (and that's a big 'if') putting a step on the top enhances the airfoil's lift producing characteristics then simple logic would dictate that putting a step on the bottom (in effect turning the airfoil upside down) would tend to make the airfoil better at 'lifting' downward, not upwards :rolleyes:
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Jet Plane flier, You usually write a post that's interesting and informative. When I see one of your posts I usually read it. You are a valued poster.

When I answered "Bob" by saying that Empirical Data has to be better than Theoretical Data - or words to that effect - it was directed to a specific application - NOT MEANT TO BE AN ALL-ENCOMPASSING STATEMENT![/U]

Weren't we discussing a FLAT PLATE vs A K-F Airfoil?

Jet Plane Flier you are assuming that I was comparing the K-F Airfoils to every other airfoil in the book, NOT SO!

You and I both know that if we were designing a 2m sailplane we would chose one of a number of "proven" airfoils. We would NOT use a K-F for a well-built sailplane. If I was flying in good air with less than a 6 mph wind I would chose something I have used since 1958 - the NACA 6409 with 2 or 3 turbulators ( 1/8th x 1/8th ) Why? Because it has worked for me - Empirical Data.....right?

[U]So most of what you wrote is acceptable, but you assumed I was comparing the K-F to ALL airfoils....NOT TRUE...... I WAS COMPARING THE K-F TO A FLAT PLATE.