** Kline-Fogleman (KFm) Airfoils - Building/Flying Discussion ** - Page 16 - RC Groups
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Nov 05, 2009, 08:50 AM
High Altitude Flyer
viking60's Avatar

I think of the KF steps / notches which we use in building these sctatch-built foamie aircraft as 'wing structure elements' or as 'airfoil elements' - as one of many wing design elements which we can optionally implement when designing / building a wing structure. I personally do not think of these step / notch discontinuities as 'airfoils' per se, because there is so much else which is also going on aerodynamicly *in front of, and behind* these stepped discontinuities. The shaping of the rest of the wing's airfoil exerts major dynamic influences on the way that air moves over a wing. Vortex-trapping discontinuities, when implemented properly, can be useful elements in achieving the performance we might want to end up with from a given aircraft design.

That's my understanding of why we now commonly refer to all of the wide variety of step-discontinuity-modified-airfoil wings & aircraft as "KFm" - "Kline Fogleman modified", giving recognition to them. We say THANKS!

A stepped / notch discontinuity appears to act as a type of vortex-trapping element. (It can be desirable to trap vortexes for a variety of purposes.) Foamie scratch-builders are free to build these in a wide variety of ways and in various relative dimmensions / proportions, located in many positions on an aircraft structure (including using stepped discontinuities on the fuselage and elsewhere, as may prove useful- not just on the wing surfaces.)

When building wing structures with layers of foam, it's very easy to implement KF steps / notches. That's why those of us who participate on the 'Foamies- Scratchbuilt' discussion forum have experimented more to date with implementing stepped discontinuities than anyone else. But we can sometimes fall into the trap of letting the materials dictate to us, as to how we use them.

I continue to build prototype wings and aircraft to investigate and confirm my own concepts, just because I've always had this life-long fascination with things which fly, and with designing examples that do it very well. It's a fascination and an obscesion which I suspect many of us share to some degree.

Last edited by viking60; Nov 08, 2009 at 09:26 AM.
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Nov 05, 2009, 09:09 AM
Registered User
My wing is a success! My spar is tall in the middle to 0 at the tips. Fanfold is thick so actual root thickness is 1 inch and tip is inch. Step is tall at the tips and tall in at the root. Wing chord 13 root 6 tip no sweep 31.75 span.

Cg 27.7% 2.75 from LE CG 40% = 4 from root le

My CG is around 53% or 5.25 from the LE. I glued the wing on too far forward. Anyway it does fly fine like this. Elevator is twitchy but only down but this is on high rates. May need more expo. The flat plate would not fly with a cg this far back. It was all over the place. This wing flies very stable. It does tend to pitch up and requires a tad of down elevator to fly level.

Overall I am impressed with this KFM2 wing. Roll rate is great rolls are nice and axial. Flies inverted well. Glide is good too. CG could be moved forward.
Nov 05, 2009, 12:14 PM
Registered User
davereap's Avatar
grant ...youve tried a deeper step than I would have used..my max so far is about 1/2" that was using 1/4" balsa spar and a 6mm layer of depron...

well done good info
Nov 05, 2009, 12:25 PM
High Altitude Flyer
viking60's Avatar
Originally Posted by Cybernaught
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.

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.


You may want to figure out how to do some thinning of your thicker foam material for use in certain areas, in order to build an aircraft with a more efficient glide. (I even use a sharp fillet knife for some of my foam carving jobs, then do more sanding.)

I first started building foam wings with KFm type steps / notches for the Slim Beagle / Dancer series wings by using the RG15 and RG14 airfoils as their basis back in early 2007 or before. I designed and built the wings in that series, experimenting with wings from just over 7% thick to over 9% thick. The one KFm3 wing that I built that was over 10% thick (The "Hot Dog" wing for the Slim Beagle) had noticably poorer gliding abilities.

In late 2008 / early 2009, I designed the airfoil and wing for the Me163-e Komet starting with the MH32 airfoil. I refer to it as a MH32/KFm2. It actually uses a modest sized secondary step panel just behind the higher sections of the primary upper wing panel's aft edge for roughly the inboard third of each wing. The web pages for all of these designs provide more photos, build details, airfoil structure diagrams, and flight evaluations:




Last edited by viking60; Nov 05, 2009 at 12:41 PM.
Nov 05, 2009, 01:38 PM
Registered User
Dickeroo's Avatar

Some analysis of the KFm step concept.

San Le was kind enough to run an computer analysis of the KFm airfoil concept. Here is his email to me along with links to the study.


I'm finished with my analysis. You can see the video for
pressure at:

pressure for Kline-Fogleman airfoil for compressible flow (0 min 40 sec)

and the velocity magnitude at:

Velocity Magnitude for Kline-Fogleman airfoil for compressible flow (0 min 40 sec)

I have also set up a page of still images at:


I hope this will inspire someone to do a better analysis.
Here are some impressions I have which you can see on the
still images page.

The Good:

1) There is no region of low pressure behind the step. This
is very surprising and good. As you can see behind the
tail of the airfoil, there is a very big low pressure region
but the step seems to have the same pressure as its surroundings.
So the step does not add to the drag. We may have hoped for
a positive pressure there, but that may be optimistic.

2) For the Mach Number, which is very similar to the velocity
magnitude, you can see the blue region which seems to mirror
the shape of lower part of the airfoil. This seems
to be the stagnant air we expected. So this may
act as a region of still air which acts as a virtual upper
airfoil that has less friction than an actual solid upper
region. This may have the effect of reducing drag.

On the other hand, if you look at the analysis I did for a traditional

mach number for airfoil in compressible flow (0 min 14 sec)

you would see this same stagnant air at the back-top, so
I'm not sure how significant this is.
The Bad:

1) I don't see lower pressure above the airfoil. The top and
bottom look the same. This could be due to crudeness of my
meshes, or that my software isn't solving an appropriate version
of the equations for airfoils.

2) I tried to do a streamline plot hoping that there would be circulation
at the back of the step. But this compressible flow doesn't capture
it well, like my incompressible code.

Again, I am not very confident about these results. At best, they
are a very preliminary starting point for someone who can do a real
calculation. Someone should also do an analysis with a similarly
shaped airfoil without the step for comparison.

San Le
Nov 05, 2009, 04:44 PM
Onward through the fog.
Cybernaught's Avatar
Originally Posted by davereap
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...

...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
Spot on Dave, We think very much alike.

Nice going Grant and thanks for your input. Keep at it. The KF approach is very worth while. Please post pictures and video if you can.

Lee, I always enjoy your inputs and comments and your tutorial video on construction of the Roswell has done so much to improve and simplify my building techniques with the soldering iron recesses for servos and other gear and the razor knife slits for wiring. My planes are looking much better now and the build time is actually lower, if you can believe that!

Bruce, I'll check out your links when I get back from Tagbilaran. Things that work for you at over 10,000 feet should be even better for me at sea-level as long as I stay out of the water. May even try to curve the wing tips like you have done in lieu of flat plate tips.

Any of you using the Chewing Gum cam please post pictures and/or more video. I'm trying to order two of them from a seller in HK. I used to be into the spy cameras years ago. I had a few of the Minox cams in my collection before I moved to the Philippines. Has anyone tried this Turnigy camera for AP?

6:30am and no wind so I'm off to the field for a bit with the new Pagan. Wish me luck. I want to see how she does with a bit of crank and bank.

Last edited by Cybernaught; Nov 05, 2009 at 05:32 PM.
Nov 05, 2009, 07:34 PM
jackerbes's Avatar

A search on YouTube for "gum pack camera" will bring up bunches of hits in numerous uses and environments including a lot of RC. Here is one of mine, you can see the others I've posted from the "More From JackErbes" link:

091030 2 (1 min 55 sec)

I don't think the Turnigy camera is any better than the gum pack was at $20 or less. It looks like a bigger package and maybe a better protected lens but I'm not sure.

I've not flown in almost a week because of chores and the weather. We got our first visible snow today. It was an 18" to 24" snow for the most part. This early in the season we measure the distance between flakes in the air as nothing measurable can be found on the ground.

You remember snow, right? Do you know what a snowblower is? Here is a photo of mine:


You'll have to come up and visit sometime, bring your snorkel and fins too:


Nov 05, 2009, 08:13 PM
just Some Useless Geek
You are a rather sick man, Jack.
Nov 05, 2009, 09:39 PM
Onward through the fog.
Cybernaught's Avatar
Originally Posted by A Useless Geek
You are a rather sick man, Jack.
Yeah! But he's likable.

I remember snow. It's white. Right? But then sometimes it's yellow and you don't mess with it if it's that color! It's coming back to me... Slowly! I think I like the way we snorkel here better and I can see why the early Indians called us "White" men!

Flew the Buzzard in wind yesterday and it performed fine. Here's an example.
BuzWind 11 4 09am (3 min 30 sec)

Then this morning I got in a few flights with the Pagan to test the performance and agility. It's OK in my book. A good sport plane but not 3D .
Pagan agility test (4 min 25 sec)

Overall a good flier. I'd like to try it with a KFm4 wing just to see the difference.

Last edited by Cybernaught; Nov 05, 2009 at 10:15 PM.
Nov 05, 2009, 09:44 PM
Registered User
Dickeroo's Avatar

The maiden flight of the Jupiter Project.

Paul of RCFoamFighters has posted a video of the maiden flight of his cargo plane, which will be testing the lifting capacity of the KFm2, KFm3 and Clark-Y airfoils. Needless to say, the maiden flight went extremely well. He clocked the Jupiter at 74 mph on his initial flight. The outcome of these test should be very interesting...

Project Jupiter Part 5 (KFm3 Airfoil RC Plane) (10 min 9 sec)
Old Nov 06, 2009, 12:21 AM
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Nov 07, 2009, 07:38 PM
High Altitude Flyer
viking60's Avatar

Voyager ready for test flights


The new prototype 'Voyager' is ready for it's test flights.

This aircraft implements a KFm3 variant (double step on top) lifting airfoil. The steps on this wing build are limited to 6mm in height or less. Part of the objectives of this experimental prototype are to evaluate the lifting characteristics of this airfoil build, as well as to evaluate this wing's glide efficiency (lift versus drag) in power-off glide mode. This is the lowest thickness KFm3 build I've done to date, as far as the % of chord, so it will be interesting to see how it actually performs.

Wingspan is 36", built with minimal wing tip plates. Average chord of the tapered wing is 10", for 2.5 square feet of wing. Flying weight, as shown in the photos, when carrying a 3S 1000 mAH battery pack is 16.5 ounces right now, for a wing loading of 6.6 Oz./ Ft^2.

The Voyager also implements KF step structures on the sides of the upper fuselage- square steps at the rear edges of the forward fuselage doublers. Servos are buried flush with the surface of the three-layer FFF forward fuselage for minimal drag. 1mm solid CF rods are used in several places, inset flush with the surface of the foam, to provide stiffness and structural strength; they are inexpensive and very light weight, and very effective when installed in opposing pairs.

For initial test flights, the flight battery is temporarily mounted with velcro on the right side of the upper fuselage, until final ideal flight CG for this particular KFm wing build / airfoil is defined. Then the fuselage will be modified to carry the battery within the fuselage on the lateral centerline for better lateral balance and best handling across a wide speed range.

Since the later mounting of the set of full floats (with their added area in front of the CG) does require additional vertical stabilizer area for good stabile handling, the vertical stabilizer and rudder are quite large on this prototype for now. The nice thing about designing with FFF is that it can be easily modified & morphed!

There are 29 photos and more information in the build log on the Voyager's web page, for those who might be interested:


UPDATE: The Voyager is flying very well! I'll post details to the Voyager's web page soon. (The first videos did not turn out, so I'll see about shooting more soon.)

Last edited by viking60; Nov 09, 2009 at 10:02 AM.
Nov 08, 2009, 02:35 AM
Account Closed

SkyFly on KFm3

Thought you might get a kick out of this video. I'm using the pod and boom from the SkyFly and the rest is custom. I decided to use this bird to test out the different KF airfoils.

This one is flying with ailerons. I had built one wing without ailerons and did not like the way it flew. It felt like I was flying a sailplane.

I bet it would carry that little gum pack camera around without any problems.

Anyway, I could only get video of one flight. Lots more to come...


Drawing showing wing build.
Last edited by sharpewing; Sep 07, 2012 at 05:23 PM.
Nov 08, 2009, 11:12 AM
jackerbes's Avatar
That little bugger really moves out! You gave your camera person quit a workout there.

That is pretty far removed from the starting point but when I did the same kinds of things it really helped with the learning curve on both building and flying.

My Sky Fly's pod finally got to the point where it was beyond practical repair and I moved on to other things. And at that point the pod was about the only thing that was original.

Thanks for sharing that.

Nov 08, 2009, 02:19 PM
Registered User
Steve, re the Turnigy mini cam, have you checked out the reviews on that page?
I ran this

MiniDV RD52 Turnigy embarqué sur une aile volante (3 min 21 sec)

and the quality looked ok though I don't know much about mini cams. I presume that it would lose resolution if blown to full screen, perhaps someone with experience could advise.


Just realised I could view full screen, a little loss of resolution but ok.
Last edited by JimW; Nov 08, 2009 at 02:30 PM. Reason: New info

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