The Kline-Fogleman airfoil, also known as the KF airfoil, wing was designed by Richard Kline and Floyd Fogleman, hence the name. The Kline-Fogleman airfoil is an airfoil that is designed with a single or with multiple steps induced along the length of the wing. The steps that are induced are primarily located on the top or underside of the wing with the intent to assist with greater lift and stability during the flight process.
The Kline-Fogleman airfoils and the Kline-Fogleman modified airfoils refer to the introduction of a step either on the bottom, top or both of an airfoil. It can also be used in conjunction with two steps on the top or bottom of the airfoil. The steps all work extremely well on smaller airfoils such as radio controlled aircraft.
The purpose of the step on the Kline-Fogleman airfoil 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. This prevents separation of the boundary layer and maintains a smooth and constant airflow over the surface of the airfoil.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 trials and error experiments, he was able to achieve this goal. One day he showed the paper airplane to Floyd Fogleman who watched it fly and resisted stalling. The two men later decided to file for a patent on a stepped airfoil.
Richard Kline and Floyd Fogleman went to see Dr. John Nicolaides who was teaching at Notre Dame and was the first head of NASA. Dr. John Nicolaides was extremely intrigued with the idea. And so were the people at Time Magazine. On April 2, 1973, an article was written and published about the Kline-Fogleman airfoil.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 lift over drag characteristics reverse themselves. Thus, supersonically the higher lift over drag characteristic would be available.
The idea behind the airfoil is a new and improved airfoil for aircraft adapting to a greater variety of airspeeds. Characteristics of the Kline-Fogleman airfoil include having a continuous upper surface and a lower surface having a stepped discontinuity. The surfaces being angle related and space related to define a chord section that increases in thickness from the leading edge of the airfoil to the discontinuity. Conventional airfoils are generally designed to either generate a lot of lift in order to carry more weight, which means they must be thicker to produce more lift, or they need to be thinner in order to fly faster thus sacrificing lifting heavier loads. The characteristics of Kline-Fogleman airfoil can do both of these tasks extremely well.
Read more about how innovative designs have changed the way we think about aviation in Aerospace Manufacturing: How the Aviation Industry Shaped American Manufacturing by Carter Mathews.