The Kline-Fogleman Airfoil

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 fibre 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 receiver, 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 Theorem. 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 amount of forward stick, but certainly nothing excessive. Actually, it was very easy to achieve inverted flight and the kid actually did his first successful inverted 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 wind milling 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.