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Sep 07, 2010, 03:45 PM
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Originally Posted by A Useless Geek
are you saying that the trailing edge wedge might be a useful feature to test out on R/C planes?
I was just pointing out that it doesn't infringe on the Kline and Fogleman patent as you implied. You are quite right that the trailing edge wedge is a bandaid.

BTW I mentioned vortex flaps. These have been used on deltas since the 1950s. They're interesting because a wing with an open VF looks a lot like a Kline Fogleman.
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Sep 09, 2010, 09:43 AM
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Excellent illustration on the vortex flap! I was wondering what chord length and airspeed would need to be achieved to see these kinds of effects? I understand this drawing is probably just a general representation to show the airflow effects.


I've attached a picture of my next test wing. This bizarre looking thing has been sitting in my shop for months. I havn't had a chance to build or get out over the last couple of months to do any testing. The picture was taken a few days back and it's now ready to go. This is the third completely different build to test flight control characteristics for this airfoil design. I've been waiting to see what this one could do and then compare and post the details. BTW, this is not something I would fly on a regular basis.

Wind tunnel testing would be nice, I can only test the airfoils this way.


When I found the KF airfoils, I also found viking's website. Lots of good, informative stuff there. Anyone getting into the KF building needs to visit his site.

I will post what I have felt when flying the Horten wing after I can hopefully get this next wing up. We've had a lot of rain here over the last couple of days and and more is expected. Hopefully I can get out soon.

Last edited by sharpewing; Sep 07, 2012 at 04:23 PM.
Sep 09, 2010, 11:21 AM
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Thanks for the heads up. I put it on my list of do it soon agenda.

Sep 09, 2010, 12:44 PM
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Bill, you had me going for a sec. I saw the wing in the foreground and the Radian fuse in the background, and wondered why you would use that for a glider wing.

That diamond saw tooth pattern looks beautiful. I am really interested to hear how it flies. Do you plan to put tufts on it and photograph the air flow, or just do flight testing?

Best of luck.

Sep 11, 2010, 09:02 PM
Ok. Recently I've had my attention turned to something that is new to me. These KF Airfoils. Untill Vikings post nobody had mentioned the point that has bothered me most about much of the testing, and seat of the pants results we have.

Most of the designs are out of folded foam with little attention to the shape of the foil at the LE prior to the step. The wood spar in many of the pictures somewhat unpredictibly creates a shape and also changes the shape of the lower surface of the wing as well.

While I understand this data and construction is very usefull and used by many people. I personally would prefer the airfoil as made "properly". If I can get my situations ironed out. I would like to get my hotwire foam cutter into these KF airfoils and even use the diagrams provided of the various versions by the creators of the foil as templates.

Through most of my reading I kept getting the feeling that some portion(be it a great or a small) of the extra drag "caused" by the KF foil over the flat or other foils was caused by the lack of attention the LE shape of the wing.

That is kind of where i'm at with this. I'm more interested in how these perform when made with solid "proper" construction vs flat foam and spar construction.
Sep 12, 2010, 09:38 AM
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The KFm3-SAV Swept Flying Wing did great yesterday. The SAV (stepped airfoil variation) designation is a term Iím using on my builds. It allows me to use any variation and then put the under one category. I post the video over on Jackís KF build thread last night. The video quality is pretty bad, due to the fact I had to use my old camera. I try to get some better video.

Iím also working on a comparison chart, comparing a plank wing, the Horton wing, and this wing design using the modified airfoil.
There have been discussions on airfoil sections, here and there on the various KF threads. Most of the results are coming from repetitive builds and the comments are based on this. Some do like the simple instructions on building a KF airfoil using the early illustrations. They can build quickly and get outside and fly. I have never tried a fold over airfoil, so I canít comment on the flight characteristics of these builds. My theory is that the light weight builds are benefiting more from an increase in airfoil thickness and thrust than they are the steps. I should mention that all my builds so far have had a smooth symmetrical leading edge tapper with a round dowel taped to the leading edge. This allows for a smooth transition to the remainder of the airfoil.

The original patent deals with a wedge shaped section with a step on the bottom surface. Dick Klineís illustration is far more aerodynamic than the earlier drawings. Pick a foil and start cutting. A smooth transitioning leading edge will always lead to a better flying build.

There is a lot of talk when it comes to drag and the KF airfoils. Iím not so sure itís drag as it is the design of their builds. I find it hard to think that a light weight floater could generate enough airflow over the steps to create drag. Reynolds numbers are too low. This also goes back the airfoil section design.
Sep 12, 2010, 09:50 AM
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Sorry if I'm butting in here, but I have gotten access to a small wind tunnel (AeroStream 21 by Applied Educational Systems). It appears pretty basic, but will do AoA from -10 to +20 degrees and wind speed from 0-45 mph-- basically the range of the average KF sport flyer. So it would be possible to make wing sections and attach tufts of string to them to do some rudimentary airflow testing. I'm not sure how I could measure lift, but there has got to be a way to jury-rig something. And since I have access to a 1080p video camera, I should be able to record everything very clearly. Comparative airfoil testing (KF vs. Clark Y, UC, etc.) also comes to mind. It might even be possible to measure KF wingtip airflow losses with and without tip plates/winglets.

Hopefully I'll have some updates soon.

And is there anybody here who knows about the smoke systems used in wind tunnels? That would be extremely useful in evaluating the post-step vortex.

Sep 12, 2010, 09:57 AM
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Spend some time in the threads, build a few wings, and I think your "concerns" or "criticisms" (I'm not sure which they are) will abate. Look at the process here in the KF winged scratch build foamie community:

1 - Use readily available materials.

2 - Devise a way to assemble those into an airfoil shape that is only approximately or roughly defined and for which that is little or no hard scientific testing data.

3 - Go fly it and see if it meets your personal standards and expectations.

It all turns into a process with a lot of "TLAR" (That Looks About Right) judgment calls and produces a lot of stuff that is as good as you can expect for the level of expertise and materials.

If you compare that to taking thoroughly tested and well defined airfoils and cutting them out as foam cores, in the latter process you are likely to find much less deviation from the specs and wings that are more likely to meet long established and well defined and documented performance specs.

There is nothing here that is as well defined or proven as a specific Clark, Clark Y, Selig, or NACA airfoil.

But getting the KF airfoils to the point where they are well defined is what this thread is about. Each of us has, to some extent, been operating in a bubble and many of the bits and pieces of KF construction (shapes, techniques, materials, etc.) are not described, or even easily found or compared.

And the standards of performance for KF wings have been very subjective. They usually emerge from the opinions of a small number of builders and from builds that can vary a lot

This is a friendly discussion and I'm not arguing with what you say nor am I angry. But when you say:

"..Most of the designs are out of folded foam with little attention to the shape of the foil at the LE prior to the step. The wood spar in many of the pictures somewhat unpredictibly creates a shape and also changes the shape of the lower surface of the wing as well..."

I would argue that that is not the case at all. For starters it implies that folded foam is an inherently deficient material and I can't agree with that unless there are some standards specified and it is clear that they are not met.

And my feelings are a little hurt too as it seems to slur my standards of workmanship. I do pay attention to the leading edge and I have been working the materials to produce the shapes I want. To address one of those specifics:

I don't care for sharp leading edges as they are more fragile and easily deformed. But there are those among us (viking60 for example) who have lavished considerable attention on leading edges, made them thinner and stronger at the same time, and taken them to a much higher plane than you describe.

The blunt leading edges have proven to perform admirably on these generally slower flying wings and be damage resistant too. There has even been some speculation that too sharp a leading edge even detracts from the performance of a KF wing but that is unproven.

So I would encourage you to move on with your plan to cut KF foam cores and make them to the standards you want to use or evaluate. And eventually you will have some cored wings with "better" leading edges and "more predictable" shapes. And if the testing and comparing plans here have evolved to that point, maybe they can be tested, evaluated, and compared to the stuff that is going on now or is yet to come.

If you want to be perceived as a contributor to the efforts here it would be helpful if you did not keep mentioning that you perceive the ongoing efforts to be "improper" relative to what you deem to be "proper."

There is no clearly established "proper" yet as far as I know, just a number of different way to do things.

Sep 12, 2010, 10:11 AM
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Originally Posted by jackerbes
I don't care for sharp leading edges as they are more fragile and easily deformed...Jack
Can you please clarify your comments on "fragile and deform". Are you talking about pointed airfoils or steamlined symeterical airfoil leading edge?

Sep 12, 2010, 10:28 AM
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I really don't want to turn this into an argument as to what is or is not sharp, I was just discussing or maybe even defending they way we typically form our leading edges against criticism.

Here are what I consider to be "not sharp" and adequate leading edges for KF wings. One is from a KFm4 Zagnutz and has a 1/4" square spar, the next two are from the polyhedral wing on the KFm3 wing on the Blu Sail II glider.

The KFm4 wing used the factory folded edge between two sheets of FFF as the leading edge.

As far as I am concerned these leading edges are certainly adequate for those builds. Neither build was focused on high speed flying or on creating a minimum drag airframe, just planes that flew well and were fun to fly.

Sep 12, 2010, 10:40 AM
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Further to the above, here is what is probably my least sharp leading edge, again on a KFm4 wing. It is three layers of FFF (3/4" thick nominally) sanded to rounded curve, no attempt at all was made to sharpen it.

And again it performed very well. I was not pursuing speed of flight and no sharp leading edged versions were made to pursue comparisons of the effects of a sharper leading edge on either general flying qualities or speed.

A classic example of TLAR engineering.

Sep 12, 2010, 10:52 AM
I honestly Didn't have the purpose of implying you or anyone elses build methods were not sufficient. However When you are trying to gather any sort of useful information on a subject that is already either poorly..or controvertedly understood. I know a lot of time, effort, and care were put into the comparisons. I'm not saying that the builds are flawed in any way. I'm certainly not trying to put you down but you have to understand that with an already poorly understood subject any variable could have a major effect if the purpose is to gather usefull information on performace differences.

Time permitted I'd like to be able to take somewhat standard airfoils and cut steps into them and see what sort of difference that alone creates.It seems obvious to me that airfoils with different shape and thickness LE are going to perform slower/faster or be more/less draggy and have different stall characteristics. I'm curious to know how two identical airfoils, one with just step/steps cut into it would perform compared to each other. look into the prospect of adding the KF step to airfoils that could benefit from what we see as KF's strong points. Maybe we could get a step into a standard semetrical airfoil or a clark-Y and see if we add usefull properties to it and make up for deficiencies with out damaging the already proven characteristics that make them popular as standard airfoils.

I'm more looking down the avenue of trying to cut these out of foam obviously with a hotwire cutter as stated before.
Sep 12, 2010, 11:31 AM
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An easier approach might be to take a standard airfoil, like a flat bottomed Clark Y and simply attach a layer of foam to the top of it that will follow the curve and create a step where it ends. That would let a before and after comparison.

The Kline-Fogelman wings simply are what they are. They were not built to compete with the well established airfoils and readily available/low cost nature of their materials has played a important part in their rise.

You could build a Clark Y with FFF but it wold take a lot more work and time. It has been done in fact.

There is an attraction in planes that fly slow, are inherently stable, have gentle stall sharacteristics, and are easy to fly. I can't think of anything you could do to a Clark Y to make it better in those areas than a KF.

Easy to fly is subjective but I can't think of a Clark Y winged basic trainer that was a better basic trainer than the KFm2 winged Blu Baby. Add to that a comparison of the time to build and the complexity of the build and the KF wings as we know them today become a "proper" build in the opinion of many.

Factor in costs and the KF winged scratchbuilt foamie starts to come out the clear winner.

Consider the history of it and why it is here. The pursuit of flying inexpensively lead to the evolution of a number of flat plate winged airplanes in recent years. These are the planes that fly more on angle of attack and surface area than on any generated lift. Many of us who had flown before, or seen other planes that had shaped airfoils fly, were not particularly attracted to the flat plane wings.

Then it was found that the KF wing could turn flat plate wing into something entirely different with the addition of a second flat plate that created a step. We tried it, even some that scoffed at it first, and it worked. It added something to the quality of hobby, and that is where we still are except that we are trying to better understand it all.

As I said, this is all just about friendly discussion and, as always, we can agree to disagree on any or all facets of it. My feelings were not hurt, I was not angry or looking for a flight. Not was I trying to offend or chastise you.

Sep 12, 2010, 11:39 AM
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Thread OP
Leading edge shape is an important part to an airfoil, as is trailing edge and overall profile. Change any part, even slightly, and you can get very different flying characteristics. Viking has for some time now spent considerable effort in sculpting sharp training edges using a covering iron. Those sharp edges reduce drag. How much, and how important that drag is over all to a specific model can only be determined by making identical airfoils (except for TE shape) and testing them.

Attempting to make repeatable airfoils using a hot wire is a commendable task. The question is which airfoil. Dick Kline will tell you that he and Mr. Fogleman did not patent an airfoil. They patented a step. The airfoils in the "KFm Family of Airfoils" image above are just artist representations. They do not represent any specific airfoil. What this means is that when you go to cut out KFm airfoils, you first have to pick an existing airfoil shape to imitate (maybe the MH32 which looks like the airfoils that Viking and I have been making, one of the NACA symmetrical airfoil shapes, or maybe one of Mark Drela's excellent designs). There are thousands to choose from. The chosen airfoil should match the purpose of the overall wing (aerobatics, high lift, etc). The airfoil you compare to, should have similar characteristics. The venerable Clark Y has well known characteristics, but is seldom used now days. By the way, a "flat bottomed" Clark Y is not a Clark Y at all, but it is what many trainer type model airplane wings have used for years.

I think Viking and others will tell you, that very repeatable, predictable shapes can be made by folding sheet foam. Sharp, large, and small radius curves and be achieved, depending on the method used. It comes down to what one is trying to do, and how much effort one wishes to employ.

Gigaah, I personally think it would be great, if you could work with others on this forum to build agreed upon airfoil shapes that can be tested in the air and in yellow13's wind tunnel.

The shape of the airfoil is only part of the equation. We are dealing with 3 dimensional airflow, and that means the flow at the tips is very important. Tony65x55 used tip plates to reduce KF tip drag. Many of us use tapered tips, while others use no tip treatment at all. When testing wings, not just airfoils, the tests should include likely tip variations.

It all gets so very complicated.

Sep 12, 2010, 12:07 PM
Well I'm happy that we are all just trying to figure this out and not butting heads.

I would gladly ship off hotwire cut airfoils to anyone that has a wind tunnel.

You are right in that this is a very complicated subject. The reason I've started posting in the theroy/science thread ot he KFm foils is I'm looking to take the ease of build out of the equasion. I understand how much use it is to be able to build a useful airfoil in the mannor specified and it is a great boon.

I'm looking at attempting to determine what sort of effect the step alone has. Doing my best to isolate the qualities imparted ONLY by the step. Not by the thickness or LE shape of the airfoil. Personally, I feel this may be the only way to actually have even semi-solid data on the KFm step design, by taking every other aspect we are able out of the equasion.

I *think* once we get an apples to apples comparison like that we can at that point make an attempt to put the KFm step into differnt foils to try and tailor a refined airfoil that includes the step.

Personally I'd love to find that a symmetrical airfoil with the step has improved stall AOA and more docile handling at low speed and not effecting negativly the other aspects we like about the standard version of the airfoil. Or a flat bottom foil has more lift or is even more docile at stall so we could make it thinner. Some sort of quantifiable bennefit aside from ease of build.

That is kind of what I'm looking for out of the testing. A wind tunnel test would be quite usefull to see how the step effect airflow over the wing vs the same foil minus the step.

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