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Aug 18, 2006, 11:03 PM
I'm not flying backwards!
Tony65x55's Avatar
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** Kline-Fogleman Airfoiled Flying Wing **

After reading the comments regarding the Kline-Fogleman airfoil, examining the patents and thinking of the possibilities of this super simple airfoil, my experimental nature got the better of me and I had to try it. The patent papers clearly show a pronounced step approximately 40-50% of the chord, similar to the step on a set of floats.

I elected to try it on a Zagnutz type airframe, both for it's simplicity and figuring , if there was not much but wing, any benefits and deficiencies of the airfoil would be clearly apparent. I made the wingspan 48"

The first thing it showed was the simplicity of the airframe as I built the whole airframe in 2 hours (and I'm a slow builder).

Secondly, it was pretty strong for it's weight. I used a 1/4" square spruce spar for 36" of it's span, leaving the tip 6" on each side unsparred. It was light and strong. Carbon fiber would be lighter but of course, much more expensive and I'm cheap. All adhesive was hot melt and occasional epoxy.

The power plant was an old, half worn out GWS 300 "c" drive with a 9070 prop I had lying around. I didn't care about power, I only needed enough to move the wing through the air.

Radio equipment was my kid's GWS transmitter and reciever, an old elevon mixer, two cheap servos and two generic 1300 mah lipos, all cannibalized from the kid's Blu Baby. Of course, if the plane flew, he could have it (lol)

The Kline-Fogelman airfoil should not fly. The pronounced step on the underside creates a low pressure pocket UNDER the wing, totally contrary to Bernoulli's Theorum. Kline and Fogleman claim the low pressure zone under the wing actually propels the wing forward, but truthfully, I had serious doubts. After all, 100 years of aerodynamic theory can't be wrong.

A few test glides at the flying site showed some interesting characteristics. I had built the plane with a standard elevon reflex but the plane nosed up dramatically. I had way too much up elevon on the wing. I reduced the elevon reflex and was able to get a mushy glide out of it.

Cocking my arm back, I perused the moment of truth, tossed the wing and applied power and guess what... IT FLEW! Not pretty, but flying. It limped around the field in a poor mushy condition, always looking like it was on the edge of a stall BUT, it never broke through the stall, just mushed.

I landed and reduced the elevon reflex again, reducing it essentially to zero reflex! For those unfamiliar with the flying characteristics of flying wings, they almost always have to have SOME reflex to fly stably.

I tossed the plane, hit the power and she leaped into the air and began a steady climb. She was very sensitive to the sticks and a little elevon was all that was needed to produce rolls and loops. All in all, very controllable. A little bit of trimming and she was flying like a champ and I climbed to about 200' to begin serious testing.

She required a little up elevon in the turns but would track through the turns easily and smoothly. Stalls were non-existent. As stated before, I had way too much servo throw on it but I was able to fly around with full back stick and about 3/4 throttle, the nose just gently porpoising up and down as she stalled and unstalled, very similar to a well trimmed canard stall. Even violent whip stalls only pushed the nose up at about 80 degrees and then she gently lowered it and kept flying. Loops were easily achieved by feeding in about 1/2 stick and inverted flight required a small amont of forward stick, but certainly nothing excessive. Actually, it was very easy to achieve inverted flight and the kid actually did his first successful inverteds tonight.

Power off, there were two ways down. Simply letting the plane glide down revealed a fairly flat glide, by no means a sailplane but pretty good with the drag of a windmilling 9070 prop. The other way was to slowly feed in back stick until the wing achieved about a 60 degree angle of descent, virtually parachuting down, but still with full elevon effectiveness and no stall. At about 30' you go back to regular flight trim and she was easy to grease in. Even the kid had no problem.

Kline and Fogleman maintain their airfoil has radically reduced stall potential and has excellent low speed handling. They are right, it is exceptional.. even with zero reflex on the elevons. The also claim the airfoil to have excellent lift. Once again, right.

They forgot to add: it is super simple to build and seems to have a wide latitude for CG location. Although it is very early in the testing phase, this airfoil looks like it has tremendous application for many of our planes, such as park flyers and aerial photography planes. I will continue the testing and report back in this forum. Until then, try it yourself, it's really neat.

And don't worry about Bernoulli.


UPDATE NOVEMBER 2007: See the list of developed KFm Airfoils added below. This will be updated as necessary.

UPDATE FEBRUARY 2008: Please read the excellent study done by Rich Thompson (Kaos2) on the flying characteristics and qualities of the various KFm airfoils. Thank you Rich! Great Work.

- Tony
Last edited by Tony65x55; Jun 26, 2008 at 07:21 AM.
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Aug 18, 2006, 11:16 PM
Registered User
aceldama's Avatar
Wow, very informative.

I've been thinking of building a flying wing based AP platform and have been looking at the various designs that have been implemented.

I really like this airfoil for the simplicity... and I have a stack of fan fold in the next room...

How large a wing do you think it would take to lift around 12oz of camera junk into the air with this design?
Aug 19, 2006, 12:11 AM
Registered User
LLoo's Avatar
This wing reminds me of the building method of the BlueFunder:

Aug 19, 2006, 12:58 AM
Very cool!

I wonder how it would do with a thicker, more dramatic step?

Aug 19, 2006, 01:14 AM
Use the Force!
LBMiller5's Avatar
I have studied aerodynamics for many years, and I have designed several dozen RC aircraft. The Kline-Fogelman airfoil does work for RC aircraft and for small ultra-light type aircraft. The reason that the airfoil never caught on in general aviation is that it has a very high induced drag at higher airspeeds. At the low airspeeds and low Reynolds numbers that we fly our models at, induced drag is not much of a problem, and ultralights, that have a regulated fixed maximum speed of 63 MPH don't go fast enough to see much induced drag.

A lot of wind tunnel testing was done on this wing, and on a plane the size of a Cessna 172 or Piper Cherokee, the induced drag at cruising speeds around 120-150 MPH was much higher than a conventional airfoil. Since most general aviation aircraft spend the majority of there time flying at 65-75% cruise power, that is where the designers try to optimize the performance. The small gains in low speed stability do not make up for the large drag losses at cruise speed, so the end result was that the airfoil was not suitable for general aviation use.

The good news is that it works great on foamy planes, so give it a shot!

Aug 19, 2006, 01:16 AM
Registered User
Interesting! I had done a KF stepped airfoil wing for a combat plane a few years ago. Most people at the field were amazed that it flew at all, but it did fly and not too badly!

I am curious though, most of the ones I had seen have the top and bottom surface of the wing parallel to each other. Your configuration is more of a V shape. Is that what you had seen in your research?

Incidentally the SPAD guys had also done a stepped airfoil on the Gnat model. You should be able to find info and discussions about it at

Aug 19, 2006, 02:29 AM
Here is a link on the airfoil.

I originally ran across it in the original Great Paper Airplane book that Scientific American put out long ago.

Aug 19, 2006, 06:26 AM
I'm not flying backwards!
Tony65x55's Avatar
Thread OP
Aceldama, I am not a huge fan of wings for AP as I find they need constant flying but that said, people like CenTexFlyer (the designer of the ZagNutz) have made a living out of AP and use wings all the time. Do a search for his stuff and you'll find tons of info.

Lloo, it is similar to the Blu Funder but the Funder's spar butts against the step and this one helps create a larger step i,e,. 1/4" spar and 1/4" foam= 1/2" step.

Larry, this quest for knowledge is very much in its infancy. Give it a shot. You can always change it later.

Lucien, thanks for weighing in. I had hoped you would. I think this airfoil has serious potential for RC models with its simplicity and performance. Lots of work to do on it yet. Perhaps the drag penalty of the K-F is outweighed by the requirement for zero degrees of elevon in the case of wings. I would also like to experiment with a conventional configuration K-F wing with flaps to change the camber at the trailing edge.

Adamdb, if you look at the patent info posted by Larry in this thread you will see the wedge shape although the K-F patent shows a slight fillet on the step. The paper airplane model however, has no such filet and flies great so for the sake of simplicity, I eliminated the filet. After all, if the small gain of performance over a conventional airfoil requires complexity, we're no further ahead. It has to be simple to be worth bothering with.

Aug 19, 2006, 08:01 AM
Registered User
[QUOTE=Tony65x55 people like CenTexFlyer (the designer of the ZagNutz) have made a living out of AP and use wings all the time. [/QUOTE]

Well... working on it anyway!

We hotwired some KF foils out of EPS and tried them about a year ago. They did fly OK, but didn't haul the freight like a convetional foil so we abandoned it. Sure is fun to proble the limits tho', huh?

Aug 19, 2006, 09:12 AM
Dismembered Member...
arx_n_sparx's Avatar
There's another thread going on about this 'foil right now here in this forum:
Aug 19, 2006, 09:38 AM
I'm not flying backwards!
Tony65x55's Avatar
Thread OP
Thanks arx, that is where I first heard of the airfoil. A liitle research, a little fanfold... you know.

Gene, just returned from it's #2 flight and it was also interesting. Besides confirming last nights results, I hung a 4 1/2 oz weight on the CG and she flew just fine, lacking in power but nonetheless, no problems. While three mistakes high, I slowly fed in full back stick to see if the extra weight would assist in a stall.

No way!! She slowed way down, sat back on her haunches and gently porpoised the nose. Didn't even lose any altitude. AMAZING

I know 4 1/2 oz doesn't seem like much but remember, the powerplant is an old, worn out GWS 300 with a C gear and a 9070 slow fly prop (flies way better on 1080 but it's hard on the kid's cheapo batteries and Dad has to replace them when they burn). We also tried this exact weight on the kid's old Blu Baby (42" undercambered 'foil, 10" chord) and this same motor could not lift the weight. I know there are several differences involved but clearly, the airfoil is taking away little, if any, performance.

Still at 0 degrees elevon reflex and she flies perfectly inverted with a tiny amount of forward stick.

Back to probing the limits. More testing...

Aug 19, 2006, 12:32 PM
Registered User
Nice work, Tony,
Keep at it & be sure to post your results as you go.
Bob G
Aug 19, 2006, 01:17 PM
RIP Azarr - "Old age is not for sissies"
Azarr's Avatar
Interesting, that's how Mike Glass from Horsefly Hobbies has been building his wings for years. They definitely fly different from your typical foamy, particularly in the glide.

Aug 19, 2006, 02:16 PM
I'm not flying backwards!
Tony65x55's Avatar
Thread OP
Here's a short clip from this morning's flight. Nothing much, just flying around.
Aug 19, 2006, 02:39 PM
I'm not flying backwards!
Tony65x55's Avatar
Thread OP
Originally Posted by Azarr
Interesting, that's how Mike Glass from Horsefly Hobbies has been building his wings for years. They definitely fly different from your typical foamy, particularly in the glide.
Sort of. The manuals on the Horsefly Hobbies website show a folded leading edge construction and while it is very similar in its construction methods, it is not the same as the K-F in cross section. See pic below.


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