The Kline–Fogleman airfoil or KF Airfoil is an airfoil design with single or multiple steps induced along the length of the wing.
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The KF airfoil (in American English) or aerofoil (in British English) was designed by Richard Kline & Floyd Fogleman.
Also known as the KF airfoil and KFm airfoils (Kline–Fogleman modified). The two patents, US Patent # 3,706,430 and US Patent # 4,046,338, refer to the introduction of a step either on the bottom (KFm1) or on the top of an airfoil (KFm2), or both on top and bottom (KFm4). It can also be used with two steps on the top (KFm3), or two steps on the top and one on the bottom (KFm7). The steps all work extremely well on radio-controlled aircraft. The purpose of the step, it is claimed, is to allow some of the displaced air to fall into a pocket behind the step and become part of the airfoil shape as a trapped vortex or vortex attachment. This purportedly prevents separation and maintains airflow over the surface of the airfoil.
In the early 1960s, Richard Kline wanted to make a paper airplane that would be able to handle the strong winds outdoors and be able to climb high, then level off by itself and go into a nice long glide. After many experiments he was able to achieve this goal. One day he showed the paper airplane to Floyd Fogleman who watched it fly and resist stalling. The two men decided to file for a patent on a stepped airfoil.
They went to see Dr. John Nicolaides, the first head of NASA, who was teaching at Notre Dame. Dr. Nicolaides was intrigued with the idea. The early wind tunnel testing showed that the step on top had higher lift over drag ratios than when it was placed on the bottom. It was decided to show the step on the bottom in the patent because when an aircraft goes from subsonic to supersonic the L/D characteristics reverse themselves. Thus, supersonically the higher L/D would be available.
Time published an article, The Paper-Plane Caper, on the paper airplane and its Kline–Fogleman airfoil in the Science section of its issue of April 2, 1973.
Then, CBS 60 Minutes did a 15 minute segment on the KF airfoil in 1973, and repeated the show again in 1976.
In 1986, Kline wrote a book entitled "The Ultimate Paper Airplane".[1] To publicize the book, he went down to Kill Devil Hills, NC to the site where the Wright Brothers first flew where the first manned flight went 122 feet. A crew from Good Morning America came along to film the event. The longest flight by Kline with his paper airplane traveled 401 feet, four inches.
In 1979, P.K. Pierpont, then manager of the airfoil-research program at NASA’s Langley Research Center in Virginia, revealed information on three studies, one of them partially funded by NASA. All had come up with the same results: The Kline-Fogleman wing was found to have a poor lift-to-drag (L/D) ratio --- a standard measure of wing efficiency. These results indicated that the airfoil had no practical application, Pierpont said, so no further tests were made.
And according to Bud Bobbitt, chief of NASA’s transonic-aerodynamics division, test showed that the Kline-Fogleman wing was inefficient. The L/D numbers weren’t encouraging, so studying the wing’s resistance to stalling became a low priority.
Max Davis, of the Air Force Flight Dynamics Lab at Wright-Patterson Air Force Base in Dayton, OH, told a similar story. A few tests were performed after all the publicity in 1973, he said, but preliminary studies indicated that the wing was not suitable for a full-size aircraft because it has too much drag and not enough lift.
Recent testing of trapped vortex airfoils indicate that it is impossible to maintain a stable trapped vortex without active control of the vortex cavity by suction and/or blowing[2] Such scientific test results oppose the previously stated (unverified) speculation on the KF airfoil.
Poor L/D performance in wind tunnel testing has meant that to date the KF airfoil has not been used on any full size aircraft.
The KF airfoil and derivative 'stepped' airfoils have gained an enthusiastic following in the world of foam constructed radio controlled model aircraft. The simple KF airfoil shape lends itself well to construction in foam resulting in an easy to build wing that often has improved performance and handling characteristics compared to the 'flat plate' wing often used in foam radio controlled models. The Airfoils illustrated previously in this article are examples of those used in radio control foam models.