** Kline-Fogleman (KFm) Airfoils - Building/Flying Discussion ** - Page 15 - RC Groups
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Nov 03, 2009, 01:42 AM
just Some Useless Geek
I tried to rescue a misbehaving flat Midwest Dart with a KFm2 patch on it; flew like kaka. Oh, well.
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Nov 03, 2009, 06:00 AM
Onward through the fog.
Cybernaught's Avatar
Too windy to fly. I have 7 planes ready to go with 2.4G receivers, which are all working well, and I can't fly because of the weather.

RE: current discussion...
My thickest chord percentage is on my 36" Assassin. It's a KFm4 with a 7" tip chord 1.5" thick. (3 pieces of 1/2" foam) 21.4% and it flies just fine.
Just had a thought. With a tapered wing like the Assassin and a constant chord thickness the tip's larger relative % of thickness may generate more lift, proportionally, than the center. This may provide a "washout" effect that helps to prevent tip stalling.

Thinking of new project See pix. Needs to be "fine-tuned" just a bit...
...Or how about a Lazy Bee (side view) Camera plane?? With the camera on CG for flying with or without??? HMMMMMMM!
When I can't fly I have to build. Does that make me compulsive/obsessive???
Now where did I put my Andy Clancy PDF files? Hey! I'm serious here...

Last edited by Cybernaught; Nov 03, 2009 at 06:09 AM.
Nov 03, 2009, 07:15 AM
Registered User
So the 7 to 9% thickness is not applied? Root chord 13 to 6 tip chord on 31.75 span.

Root would be 13*.07 = .91 or .09= 1.17
Tip would be 6*.07= .42 or .09= .54

Step taper from 6.5 to 3 at the tips. The foam is 1/4 thick so deduct it from the rest and the spar would be roughly 1/2 inch tall at the root to 0 at the tips.
Nov 03, 2009, 09:13 AM
Registered User
davereap's Avatar
After my bad experience with even thicker tips (30%), I felt the tips were loosing lift and adding drag, and then when I thinned the wing down the difference was noticable, with the thinner wing gliding much better and the overall flying speed increasing..
My best flyers are wings having between 8% and 12% at their thickest point

For your wing I would use 1/4 balsa strip under the step and taper from about the half way mark going down to nothing at about 3" from the tip , so then the ends can be just the two layers laminated..
Having a low step at the root doesnt apparently cause any problems, mine have been down to as low as 4%
Nov 03, 2009, 01:16 PM
jackerbes's Avatar
"..So the 7 to 9% thickness is not applied?..."

It is a suggestion or starting point and it is always a percentage of the root chord. The percentage is not applied to both the root and tip.

So it is suggesting that, in your case, the wing thickness be in the range of 7-9% of 13", or the .91 to 1.17 you came up with. But read on, the thickness of the KFm4 is better at 6-7% than it is at 7-9%.

Since the thickness is measured at the thickest part of the wing, that would be from the top of the top step to the bottom of the bottom step on the KFm4.

The KFm4 was originally recommended to be 9-12%. But it was quickly learned that it worked much better if it was thinner so when the unofficial "specs" figure for the variations was updated a few months ago it was changed to 6-7%. You can see all of those (both the old and the new) here:


That figure is in post #2 of the ** Kline-Fogleman (KFm) Airfoils - Discussion ** thread:


Since we start with materials of various thicknesses we just try to find a combination of layers of materials that will keep us in the range that works. As an example, my KFm4 Zagnutz is three layers of FFF (Dow Performance Board III), it has a thickness of .710" and a chord of 11" making the wing thickness about 6.5%.

Sometimes spars and/or spacers of various heights are used in combination with various foams to get to a desired combination of step height and wing thickness. It has been proven that almost anything is worth trying and that the learning curve on KFm wings is not over yet.

Nov 03, 2009, 03:07 PM
Registered User
Sounds like a toss up.
Jack you say the thickness percentage is a percentage of the root chord only so that would mean a constant height spar with no taper in height from root to tip.

Dave you said your best flyers had the root(thickest part of the wing 8 to 12% thick tapering in spar height down to the tips.

May try the tapered spar.
Nov 03, 2009, 07:28 PM
jackerbes's Avatar
I'm not sure and will let Dave speak for himself on it.

He has posted a lot of photos of his builds here and over on the Zagnutz thread and it seemed to me that the majority of them had constant wing thickness and KFm step heights.

And he was talking about tapers, I'm sure if he was talking about wings wings with a constant thickness but a taper in the step height (steps that tapered down towards the tip). Or if he was talking about both the wing thickness and step height both tapering down towards the tip.

I'm not much on aerodynamics but it has been suggested that the combination of vortex at the step, and the constant step height are major factors in the stability, slow flying, and gentle stall of the the airfoil. Or something to that effect. I'm probably not expressing it well. I'm more of a mechanic than an engineer.

For a first build I would not experiment with it. About 99.9 percent of the KFm wings have been made with a constant wing thickness and step height both. The only real variable for most builds has been the percentage for locating the step and the height of the step.

So if you start playing around with tapering wing thickness and step heights it will be exploring new ground for the most point.

We certainly don't discourage experimentation with the airfoil and love to hear about that and how it goes. I think the best way to start on that is by having something that is flyable and then working in the changes to see the difference.

There are no KFm rules! And I'm not say that there are any.

Nov 03, 2009, 09:24 PM
Lee's Avatar
There is a lot going on with these airfoils. I have been trying to get one of the graduate students to take the testing of the KF airfoils on as a masters research project but so far have been unsuccessful.

Dick and I have been bouncing ideas off of each other for months outside the threads trying to come up with some conclusions. First of all let me say that I believe that there is something to the KF airfoils. I'm designing some planes around them and you may see them appear on our home page before I die, but maybe not since I am lazy and there is fishing to be done.

We often get so focussed on the step of the airfoil that we don't show appropriate respect for the leading and trailing edges of the wing. I am sure that the leading and trailing edges of the wing will have as much effect on the airfoil as the step will so as we try different configurations at least try to make them look like airfoils.

I don't like the way flat foam planes fly unless they are extraordinarily light wingloading and aren't over or under powered. In my own trials I have seen a relationship with the drag created by an airfoil and the stability. This may be part of the reason that the KF airfoils are improving flying characteristics over the flat foam. The adding of the KF airfoil is not only adding a step but it is creating drag that may be stabilizing the inherently unstable flat wing.

Just because it has a step doesn't mean it will fly better. As I look at the 9 or so KF airfoils that have been identified recently I don't believe that they will all fly great. There is still a science to the madness and we need to take a scientific approach not just a looks good approach.
Nov 04, 2009, 05:58 AM
Registered User
davereap's Avatar
Personally I believe the KFm steps work over a wide range of wing thicknesses using steps going from 3mm up to 12mm. But an over deep step in relation to the width of chord is not good whereas a proportionally over shallow step will still work..

Like any wing the overall thickness determines the effective speed the model can acheive

That leads you into constructing your wing and what KFm you build in.. and the %ages for the thickness you will end up at, when you laminate the layers
Its mostly common sense , using depron usually the shallowest step will be 3mm the thickness of a single layer.... (I dont think anyone has tried much less, but I could be wrong.). Then we can add spars or use 6mm depron or a combination of both to let us get a variety of step depths.
When I build, I like to keep it simple, so where possible I use a constant depth step/spar along the whole wing... I look at the tips which is where the %ages will be the highest to determine the layers I need..
My preference these days is for KFm4 sections and the use of balsa spars..So usually I aim at getting a reasonable tip thickness and the rest of the build follows that... If the wing has a taper the lower %age thickness at the root will not matter, as long as a step is there the KF works ok

Ive only come unstuck in a couple of builds..the latest had too thicker tips..30% of the chord and kFm4 12mm steps top and bottom... which caused high drag and loss of lift...
Thinning the wing by removing the lower step gave an 18% tip thickness and a good flying wing...
A previous disaster using a KFm3 had the second top step right on the elevon hinge line... this flew or glided ok untill you moved the surfaces and then it didnt..it just went weird..

Ther have been lots of numbers quoted and these should be used as a general guide, nothing is exactly determined, smallish %ages up or down wont kill the plane.. Its better to think what do you want out of the wing..then work from there..
example speed... keep it thin...
Nov 04, 2009, 07:29 AM
jackerbes's Avatar
The KFm airfoils could really use some thoughtful study in a wind tunnel. It would answer some questions for a lot of us. Stuff that is just fun to know...

"..As I look at the 9 or so KF airfoils that have been identified recently I don't believe that they will all fly great..."

If you look at each of them in one build or another they have all flown great, that is really why they are there. If you're thinking in terms of using them all for one flying venue or another or one type of aircraft of another then you probably would be right.

"..A previous disaster using a KFm3 had the second top step right on the elevon hinge line... this flew or glided ok untill you moved the surfaces and then it didnt..it just went weird..."

That same thing was experienced with the Zagnutz in one of the early builds and had me telling people that getting the steps too close to the ailerons was a bad thing to do. And then someone showed me their Blu Baby (it was a 52" or 60" AP variant I think) that was a perfect flier with the ailerons hinged at the step. So that is just another one of the questions that would be lovely to sort out a little with a wing tunnel and some wing sections.

In today's world of engineering simulations, is there not a wind tunnel simulation that would let us draw airfoils, run them through a range of speeds, incidence angles, etc., etc., and come up with something helpful?

Nov 04, 2009, 07:48 AM
Lee's Avatar
The trouble with wind tunnel testing is what are they going to test???

When you say I have a Clark Y anyone can reproduce one. When you say test a KF airfoil what are they supposed to put in the wind tunnel? I'm not sure a KF airfoil can be described in a single sentence. To the hundredth of an inch draw one for testing. Can't do it can you?

Study the airfoils in the data bases below. They test different in a wind tunnel. One or two percent in thickness or a slight variation in shape makes the difference of a popular airfoil or an airfoil that is forgotten. The same thing is true with the KF foils. I could force any airfoil in the data bases to fly. That's not the point I'm trying to make. Airfoils are precision designed for maximum performance. That precision is missing in the KFs making it impossible to test them.

I have also linked a simulator to play with with the airfoils.

You would need choose a specific step configuration and have 25 different airfoils similar steps varying by by thousandths of an inch to compare against each other if we want results that mean anything. A random sampling gives random results. The engineers I have talked to are looking for the precision and exact plotting of the airfoils as I have tried to get wind tunnel data.

There is so much variation in how they are being built that there is going to be a drastic variation in how they perform.

Airfoil Data Base


P.S. It is OK to disagree with me. My kids do it all the time.


Last edited by Lee; Nov 04, 2009 at 08:16 AM.
Nov 04, 2009, 09:23 AM
High Altitude Flyer
viking60's Avatar

My first RC aircraft was a glider which I received as a Christmas present from my wife back in 1981. I have always had a fondness for wind ships- aircraft which perform well in winds without any power system. I have been designing and scratch-building them since the early 1980s. These many years of glider design projects contribute to my perspectives and my approach to studying airfoils, and to utilizing turbulator spars, trip strips, discontinuities, and step structures to control airflow over wing surfaces.

Building aircraft with layered foam and implementing the KF steps effectively has been an interesting process. Over the last four years I have focused on watching power-off glide performance and efficiency on various KFm wing build designs as a primary indicator of how well a given wing build is performing.

(I'm not talking about the symmetrical airfoil aerobatic / powered wing designs here- that's a topic which should be approached separately. While I've built & flown KFm4 type wings for other performance tasks, I'd like to offer some thoughts right now on effectively implementing the KFm2 and KFm3 variants of stepped airfoil lifting wing design. The other types of KF step implementation certainly have their place, too.)


Many quick-built foamies using stepped structures seem to focus only on the middle of the wing, where their steps are located, while giving little attention to the leading and trailing edges. I've been writing about the importance of the rest of the wing for quite a while, and Lee has just commented on this important concept again.

Since it is known that the shaping of the forward 25% of an airfoil potentially creates 50% of the wing's lift, it also follows that a poorly shaped leading edge area will result in a wing which does not generate as much lift as it could if shaped more extensively. This is one of the 'traps' of the quick-built foamie approach of stacking layers of foam....

I have done a lot of careful shaping work on the leading edge contours; the Dancer and Komet wings are good examples of what can be accomplished as far as wind penetration and glide efficiency. At higher speeds, shaping of trailing edges can also contribulte to cleaner control response and reduced drag. Of the two, the leading edge area contouring may be far more significant (from the perspective of foamie scratch-builders) than the trailing edge treatments.

The Soarbird 4 wing modification experiment demonstrated that thicker 3 layer wing tip structures on short-chord wing tips suffer from a significant drag penalty, and that reducing the thickness of those structures would reduce drag quite significantly. Recent discussions and experiments confirm this observation. (Too thick = excessive drag = poor air movement = degraded performance.)

My Next Experiment & Prototype

I'm presently working on my 14th KF variant stepped airfoil wing design. This one further explores the possibilities of how the dual step / KFm3 type of wing can be built to produce higher lift while minimizing the drag component.

I have a specific performance envelope in mind for this aircraft design; it's being built to be mounted on (removable) floats for water and snow flying. From this task-specific perspective, I wanted to create a lifting airfoil wing with some aerobatic capabilities. I also wanted to design the wing structure so that the floats could be mounted directly to the wing- no external frames and struts to be used at all.

Below are a couple of preview photos to show how I am building this new aircraft from Bluecor PP, using the fold between two panels of the FFF at the wing's leading edge. I have built in some Phillips Entry (raised leading edge position) while building the basic structure shown; I will do further heat-shaping and contouring of the leading edge.

I prefer to keep the steps shallow on my KFm3 wings; with shallow steps, the trapped vortexes may in fact *Reduce Drag*..... Taller steps may create additional drag under certain flight conditions; this may be especially noticable on narrow chord wing tips. And while adding drag can increase the stability of some aircraft [see Lee's Assassin, with it's drag wing tip cuts, for a superb example of this!] the bottom line is that increased drag is increased drag, and reduced glide performance efficiency.

Maybe there are better ways of gaining stability that by adding drag... ????

I'll offer more food for thought as time allows... have to go for now.

I have just uploaded the project page for the "VOYAGER" to my web site with the first 18 photos, and will continue to update the build process and eventual flight evaluation there.


The FUN Continues!!!

Last edited by viking60; Nov 08, 2009 at 08:35 AM.
Nov 05, 2009, 05:43 AM
Onward through the fog.
Cybernaught's Avatar
I gotta jump in here folks.

For each of the basic notched airfoil KF types there are no base-line standard parameters. The notch position (40% to 50% usually) and the percentage of chord MAC to maximum thickness vary a great deal and seem to produce acceptable results. I'd love to see things standardized and some serious testing and analysis done but only to satisfy my intellectual curiosity not for any practical purpose. As a lazy, casual, designer/builder/flier, specializing in "inferior-materials" construction techniques, I let the size of my building material determine the chord thickness percentage and my whim, the style and shape of the plane. I'm motivated by KISS and TLAR much more than aerodynamics.

Having said that I do hope you and Dick can come up with a way to definitively test the KF variants against standard airfoil shapes and that you can make your findings public here because I know they work and I would like to know why.
BTW: The points you made about sweep being used to control "Plank roll and pitch" problems is what helped to motivate the building of the Pagan.

I have been on your page quite a few times in the last year or so since getting back into RC. I particularly like your Snoboat and Soarbird designs. I plan on a canard at some point in the future and since I live on the ocean I've tried a few ROW planes with limited success but I'm obsessed with flying wings at the moment so all that is on the back burner for now.
I have a question for you. You seem to have incorporated a lot of KF notched wings in your many designes. Which of the KF foils have you used on your gliders and do you have a "success-formula" for getting the best glide ratio? My best gliding plane currently is the Pagan version of my Vesper and the surprising thing is it's a low aspect wing KFm2. Check out the landing glides in this video.
Pagan2n3 11 2 09am (2 min 4 sec)

The reason I ask is that we have some Philippine Eagles in my area and they have come over to see whats up on occasion but lost interest in my noisy Buzzard and Assassin wings. I'd like to throw together a good gliding slow AP plane with a side view camera and your Soarbird is along the lines of the shape I was considering. Good glide = less motor = less noise so maybe the eagles will stick around and let me get some video of them.

Last edited by Cybernaught; Nov 05, 2009 at 05:52 AM.
Nov 05, 2009, 07:08 AM
Registered User
I have had one of those birds chase and claw up one of my foam planes. Even with the motor going. After that, whenever they come over busybody-like, we all chase them with our plane and watch them scoot out of the way. Fun.

Nov 05, 2009, 07:46 AM
Registered User
davereap's Avatar
Originally Posted by grant31781
Dave you said your best flyers had the root(thickest part of the wing 8 to 12% thick tapering in spar height down to the tips.

NO the thickest %age is at the tips.... note When I make a wing, the thickness of the wing is usually constant...so when the chord tapers at the tips the %age gets higher even though the wing is no thicker...
But sometimes I will taper the spar under the step out towards the tips to reduce the thickness of the wing..thus reducing the %age at the tips aiming at the target numbers..

overall if you aim at 8-12% you wont go too far wrong...but there is a lot of leeway + and - that will still work

Shaping .... I always carve the LE into a nice profile... symetrical for KFm4 ..or clark Y shaped for KFm2... and leave the rear edges square..This is ok for general sports planes.. the trailing edge shaping is a worth while future experiment, especialy for higher performance glider wings and the like..
I also want to try using thin slightly undercambered stepped, and fully shaped wings on a glider..thats another area that wants looking at

Lee you are correct, our builds are variable and all our results subjective... which makes it hard to get numbers that are precise, however we are lucky in that the results we get mostly make for good flying wings, when compared to flat plate wings and even profiled airfoils, and the construction of a KF wing is far simpler than that of a built up wing and is easier than hot wire cutting for a lot of us..
I am a sucker for easy builds, and a sucker for cheap flying.. and it doesnt get any better than this... all thanks to Dick Kline and flat foam sheets
Last edited by davereap; Nov 05, 2009 at 08:18 AM.

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