** Kline-Fogleman (KFm) Airfoils - Advanced Theory/Science ** - Page 6 - RC Groups
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Sep 12, 2010, 04:31 PM
just Some Useless Geek
You know, the science aspect of this may be just a little out of our reach. I'm tempted to once again suggest that we limit ourselves to the experimental results that can be replicated using conventional materials, adhesives, and building techniques. Talking about hot wire cutting of complex curvatures with a step is all well and good for those equipped with multiple axis CNC controllers on our hot wire cutting jigs, but for guys like me who are using an X-acto #1 handle and a supply of #11 blades this discussion attains rare air very quickly.

The research of complex airfoil curves with KF steps inserted can continue apace from the research of much more conventional wing builds. Nothing is preventing this study from taking place. However, the results can't be tested and confirmed by any but a tiny fraction of the participants in this discussion. Therefore, any progress made in the research of such shapes must be done by those capable of replicating the original test shape. Oh, well.

In the mean time, we have hundreds of RCGroups members who have built numerous KF variations and report all kinds of benefits. In fact, the only negative feedback I've seen on KFs is from those who have obviously misapplied the KF to an application on which it is inappropriate. The best example is those who use a KFm3 and put ailerons way too close to the second step, resulting in a wing that performs horribly. By contrast, KFm3 wings without ailerons result in uniform lift benefits over many other wing forms.

This is the kind of research we need to focus on, methinks. The results of this research can yield immediate benefits to those of us actually building aircraft to fly out at the park. It would be nice to know that a Clark Y with a 40% step inserted at 37% of the wing underside produces a 6% drop in total drag, but how the heck do you expect me to build such a thing into my 24" STC or 28" Blu Baby? On the other hand, a 50% KFm2 step with a rounded front edge of 1/2 wing thickness radius -- now, that even I can build. Therefore, I want to know if it should be a 1/2 thickness radius, or a taper to a point, or 45%, or if 6% step height is close enough for rock and roll, etc., etc., yada yada yada.

N'est ce' pas?
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Sep 12, 2010, 05:04 PM
Addicted to building...
Freddie B's Avatar
A Useless Geek,

I agree with so much of what you are saying. Science can be stuck in the trench like what NASA does, or used in simple, somewhat subjective ways, here, for RC modelers, which is what we want it for!

I'm guilty for making a KFm3 wing, steps too close to an aileron, but so much info is spread out so far and in-between, I missed that one. Sad thing it was built by using both a published plan (that has thousands of followers), and the basic diagrams published by the inventors Kline and Fogleman. Stated steps at 50%, 75%. Great learning lesson though, and if others have known about this I do hope thousands more do not have to find out the hard way by 're-inventing' the wheel.

That said, I hope some concise, all in one place, actual hard data, including "Building do's and don't's" would also be compiled.

We are here for RC, we build like RC'ers, using RC gear and common RC building materials. So I'm with you on 'science for RC aircraft'. Bound to be some further great discoveries that ALL Builders can duplicate, fast and easy. Heck, fast and easy is one of the attributes everyone brags about with the KFm family of airfoils.

Sep 12, 2010, 05:24 PM
jackerbes's Avatar
The Kline-Fogelman airfoil, and the discussions about it, really got started on rcgroups.com with tony65x55's ** Kline-Fogleman Airfoiled Flying Wing ** thread back in August 2006. While that thread was about a plane it also stimulated a log of speculation and discussion as to what it was and why and how it worked. There is still some good and interesting reading in that thread.

On page six of that thread, in post #84 tony65x55 posted his thoughts on how and why the KF airfoil works the way it does along with a good figure that illustrates and explains his theories.

I still think that is one of the best explanations of what is going in here. It has not been proven to be dead right but Tony is a pretty good amateur aeronautics engineer, he speaks in laymans terms so I can understand what he is saying, and the Lift and Stall figure help me with the concepts.

That explanation is good enough that I am going to add it to the opening posts in the Building & Flying thread so that it will help enlighten the inquisitive. Maybe some day we will be able to better explain it.

Last edited by jackerbes; Sep 12, 2010 at 08:07 PM.
Sep 12, 2010, 06:37 PM
Account Closed
Thanks for the clarification. When you stated “fragile and deform”, I pictured something completely different than the pictures you provided. I do like the wing labeled Blu Sail II KFm3 polyhedral tip root.

A Useless Geek,
I see the topic over the hinge location for the KFm3 airfoil has found its way over to this thread. All of my KFm3 builds have been hinged behind the step. I’ve yet to have any failures or erratic flight behaviors due to this method. The only modifications I’ve made to any KFm3 using this method is small adjustments in the control throws, by either decreasing or increasing the throws depending on the design. Where is this coming from?

None of the current airfoil sections to date look like Dick Kline’s patented drawing. I would however like to see the testing done on the first four airfoils. These seem to be the building blocks for all other airfoils.
Sep 12, 2010, 07:45 PM
just Some Useless Geek
Okay, Fred, I'll bite: let's start a KF Do's and Don'ts. #1 -- DON'T put deep ailerons on a KF. Or maybe, DO use shallow ailerons on a KF. The point is to avoid having the aileron hinge too close to the last step so that the vortex can form properly? Or something like that. Heh. What do I know?

Bill, you aren't the only guy who has suffered grief at the hands of the KF with ailerons. The users of the mod 3 do seem to be the ones having the most problems, though. So far we don't have science to explain what's going on with that. Perhaps this is a topic for some of that very useful, perhaps even empirical, research. Certainly any light shone on why the ailerons on KFm3 wings are problematic would improve the lives of a lot of glider builders out there.

Roger is certainly correct in saying that this is a potentially large can of worms when trying to ascertain the interactions between variables in the KF. For instance, is it possible to make ailerons behave differently on a KFm3 if the leading edge is shaped a certain way? The trailing edge? Do winglets/tip plates have some effect? Oy!

I worry that we will bite off more than we can chew if we attempt to answer too many of these questions at once. Thusly, we gotta attack one problem at a time and see if we can isolate the attributes of the wing that directly affect that problem without affecting each other too much. Edison and his crew tried something like 3000 combinations of materials and vacuum and such to get a repeatable, reliable electric incandescent light bulb. Let's hope we don't have a similar experience trying to isolate the characteristics of the KF.
Sep 12, 2010, 10:00 PM
I wasn't really talking about a Hotwire CNC. I make my wings with two templates and a bow by hand so its not particularly difficult or expensive.

The angle I was looking at was pure comparison between an airfoil and the same airfoil with the step to try and determine the attributes the step impart to flight. I guess in this arena I'm a bit apart from most of the pack in that I really don't have much interest in making a wing out of a few pieces of flat foam. Albiet easier and effective to build I have a difficult time seeing how we can draw direct or concrete theory/science information from such experiments.

What we can find from those build methods is a well behaved wing shape for those that do like to use those build techniques...which is obviously most of you guys.

I'll just wander merrily down my path and if I come up with any information I'll let you guys know
Sep 12, 2010, 10:12 PM
Viking 60 did just post some testing method that could prove useful. Using a depron cover over the steps to see the difference between the wing with and with out the steps. The CG looks like it needs adjusted after the change as well.
I think this might yield some very useful information but still using the quick and simple and repeatable building techniques you guys are after. So I might have to eat my words on some of the information gathering capabilities using the flat foam building techniques. Could also have me building wings with flat foam when trying to gather information at times.
Sep 13, 2010, 09:14 AM
High Altitude Flyer
viking60's Avatar

One Method for testing a wing with & without KF stepped discontinuities


I've been posting information in the other "KF Airfoils- Building & Flying" thread; you can check page 88 for the latest which Gigaah referred to:


(Some discussion on various wing tip outer end treatments & their affect on both lift & drag of a wing is also in another recent post there. )

I'll re-post some of that info here:

More modifications & test flying

Winds became gusty & irregular early in the day as a weather front was passing through the area, so I prepared a couple of modifications for test flying while watching a football game.

The first idea I had concerned the wing's center area between the primary step and the wing's trailing edge, between the ailerons. I have been thinking that smoothing the airflow over this section of the wing, just where the high speed air from the prop passes over the wing, might smooth out the airflow over the tail group surfaces a bit, and thereby smooth out the handling a bit. Maybe the KF type stepped discontinuities are not needed in this area... and maybe it was time to do a test of this concept.

So I added the three foam strips as supports, then ironed on a section of the ~1.5 mil thick document laminating film over this area. (This added very minimal weight.)

Since this didn't take long, the winds were still up, and the football game was still on, I prepared for another test:


Many have speculated as to which might fly 'better'- a wing with the KF steps, or one without but otherwise as close to each other as possible. I figured to approach testing this question by simply fashioning filler panels to match this KFm3 variant wing, and simply taping them in place after flying the KF'ed wing without them.

The white filler panels shown in the photos below are the result of this brainstorm, made from 2mm Depron sheet. With a 4mm depron spacer strip under the filler panel's leading edge, there's less than 1mm of drop / discontinuity at the back edge of the primary KF step at 50% of chord. The trailing edges of these filler panels extends to within 3/8" of the aileron hinge line, but it is only 2mm high..... yes, it's still a stepped discontinuity, but a fairly modest sized one. I figured that this experiment would at least give me some idea of the affect on this particular wing when the stepped area was covered over, and the KF stepped discontinuities were no longer trapping the circulating vortexes behind them. (How would we ever learn these things if someone dosen't try something like this???)


The football game was finally over, & the winds had settled down... it was time to fly! A light haze was in the air, but with enough sun shining through to do some nice warming of air, & generate some thermals. Just right!

I first started flying the wing with it's newly 'smoothed' center section, but without the filler panels. SWEET!!! I caught a few different thermals, and had to duck out of a couple of them to keep from being carried too high. After flying only a modest part of one battery pack for over a half hour (brief climbs to altitude in 6 to 10 seconds, then shut off motor, hunt thermals, cruise, grin!) I landed. I then quickly taped the Depron filler panels in place, mounted a fresh battery pack, and launched again.

With the steps covered, the wing wanted two clicks of down trim; the wing's center of lift (& ideal balance point) had also shifted noticeably forward, leaving the aircraft flying like it was a bit too tail-heavy. This left it very sensitive to slight control inputs, and more easily destabilized by irregularities in the air. But the wing also now seemed to not glide as efficiently as it had been while the KF steps were exposed, despite being trimmed for best glide...

More test flying with the filler panels in place, and with the balance shifted farther forward would be needed to draw any further conclusions... but I'll say for now that, within the parameters of the test flying I did, the wing seemed to actually handle better with the KF steps exposed & working, than it did when the filler panels were in place. This was far from a 'perfect' test, especially in view of the shift of the wing's center of lift when adding the filler panel, but I thought that some of you might be interested with the test methodology I used and the observations I was able to make while flying. It may be rather subjective, but I now have a better idea of how I'll proceed with further testing in the future.

For the next wing of this series which I build, I'll likely change the upswept wingtips to curve a bit more gradually, and to extend farther above the wing's upper surface - (more like thosse on the earlier DANCER series wings.) This should result in more 'virtual dihedral' affect on the wing, and result in a wing which is more self-stabilizing, and which does not have to be flown so attentively.

More Later!

Last edited by viking60; Sep 13, 2010 at 09:20 AM.
Sep 13, 2010, 09:25 AM
OpenTX University Staff
maguro's Avatar
Gigaah, you can do the same thing that Viking is doing. If you make two sets of templates: one with the step and one without. You can cut the wing without the step, swap templates, then cut out the step. You can then test the wing with the step, and then tape on the step and test again.

The vortices created by the KF step do change the location of the aerodynamic center and the pitching moment of the wing. This necessitates a relocation of the CG.

Once we select an airfoil shape, we need to determine what thickness the airfoil should be and what the step height should be. What is the optimum combination of thickness and step height for a particular airfoil for a particular KFm type? Unless we conduct multiple tests we have no way of knowing that adding a step of X height on airfoil Y with thickness of Z on a KFmQ wing with the step at W percent of chord, will have the same results if we change any of Q,W, X, Y or Z.

Ailerons! Did someone mention ailerons? I've been watching Fred's posts about his Blue Baby adverse yaw problems. Others complain of aileron ineffectiveness. I've seen the adverse yaw problem on my KFm2 SE5a biplane. The problems are almost exactly the same as Fred describes. If I turn with enough speed, more shallow angle of attack or apply rudder (as I should), the plane turns beautifully. The SE5a has tapered tips not tip plates. Is this the problem? I have no idea. I can say that aileron shape and placement is a non-trivial aerodynamics subject. Big surprise there. We should have an entire test program just to understand aileron, step, tip interactions.

Let me just say that testing folded foam wings, and testing cut foam wings at the same time can produce meaningful results. We just need to conduct well thought out tests, and collect as much data as we can. With enough data we can make educated guesses (that's the best we are going to get) as to what's happening in certain situations, what best shapes of wing, aileron, airfoil work best with which KFm variant, etc. I urge everyone interested in testing to do so. Don't spend time arguing which is the best method. Just make a plan, document it, and start making wings.

I am planning to use my old Slow Stick as a wing platform for now. I started drawing a wing plan form (including ailerons). The wing is 42" long, 10" wide, rectangular, and with no dihedral. The wing will have about 15% less wing area than the Slow Stick. It will not be a floater.

I plan to test a KFM4, and a KFm2 that later becomes a KFM3 to start. The KFm4 will have a 50% top step and a 40% bottom step (that can be changed to a 50% bottom step). This should give me more than enough to be getting on with. I plan to add yarn tufts and use a key chain video camera (under $10 on Ebay) to video the tufts. I sure hope I can get some useful data. I expect at first I will learn a lot of things I should or should not have done in testing. Fred, if you want to provided a cut wing to compare against, I would be delighted. I can provide the airfoil shape.

Sep 13, 2010, 09:30 AM
OpenTX University Staff
maguro's Avatar
Viking, have you considered putting tufts on your wings, so you try to visualize what is happening at the tip? Might be helpful as you go from tip shape to tip shape. Just a thought.

Your wings are beautiful by the way. I've been watching you threads for a couple years now. I got more than a few ideas from you. Your help in testing will be invaluable I am sure.

Sep 13, 2010, 09:54 AM
“There’s no place like Foam”
gpw's Avatar
You guys ever thought of trying this Mythbusters style ??? Make some sample KF wing panels , tufts and all ... and a "control" wing (flat plate with tufts) ... stick those out the window of a car moving at model speeds (two hands)... Have someone in the back seat video the results by varying the Angle of attack... Quick , simple !!
Sep 13, 2010, 10:01 AM
High Altitude Flyer
viking60's Avatar

Sounds like the essence of simplicity- I like it!

My '63 Studebaker Commander station wagon with the sliding rear roof would be really handy for this... but it's been 'in storage' off the road for quite a while now, & it's not quite ready for the job. Ah, well...

I'm wondering how far out away from the sides or roof of a moving vehicle a test wing section would need to be, in order to be out of the disturbed / compressed air affected by the vehicle's body?

Sep 13, 2010, 10:16 AM
“There’s no place like Foam”
gpw's Avatar
That's why I mentioned both hands , so you could lean way out and hold it in the "clean" air...
You know there is another way ... about 15 years ago (or more?) we built a wind tunnel out of a Huge appliance box, some scrap cardboard and a big box fan... worked ... we could suspend a plane with some string in the back of it and do simple testing and maneuvers ... Now a bit more engineering with some lights , a video camera, mounted wing panels with tufts , and a wind speed indicator (like the kind the glider guys use) ... We could get some usable visual data ... just another thought /remberence

Car thing would be EASY !!!!
Sep 13, 2010, 10:21 AM
“There’s no place like Foam”
gpw's Avatar
PS. Had an egg crate style "flow straightener" made of cardboard in front of the fan ..
Sep 13, 2010, 10:23 AM
High Altitude Flyer
viking60's Avatar
Originally Posted by maguro
Viking, have you considered putting tufts on your wings, so you try to visualize what is happening at the tip? Might be helpful as you go from tip shape to tip shape. Just a thought.

Your wings are beautiful by the way. I've been watching you threads for a couple years now. I got more than a few ideas from you. Your help in testing will be invaluable I am sure.


I've got that in mind- thanks for the timely reminder! I have an 808 'Car Keys' camera that has the focus set for closeup work.

I ran into a bit of a delay in my ongoing test flying late yesterday ... (the rest of the story...) I had installed the Corona receiver with a shortened base loaded antenna setup, buried in the foam of the side of the fuselage. While playing with chasing thermals, I ended up flying close to 1000 yards out, and began to experience radio glitches at that distance.

With the setup of the filler panels taped in place in the wing resulting in the wing being hyper-sensitive - (flying as if it were tail heavy due to the shifting forward of the center of lift), I had my hands more than full, and the image of that airplane is mighty small at that distance..... Short story, I didn't manage to get it back into better radio reception range, and so the glitches continued, and the plane went down.

I got in the vehicle and drove over quite a loooong ways before I got out to walk the area where it went down to recover the aircraft.... So I have some repairs to the wing structure to do, and need to reinstall a full length antenna on the receiver before I am back out flying thermals again.

But Ahhhh, it wouldn't be as much fun flying within a small patch of sky when I have the incredible wide open skies to fly out here in South Park... a guy just has to range out and crowd the limits of visibility some days... and yesterday was another of those days!


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