Written by Lucas Weakley As seen in the Summmer 2016 issue of Park Pilot.
Most of us are familiar with the basic curved top and flat bottom of a traditional airfoil, and we generally regard it to create the most lift and best flight performance. But what if there were a better design? “When you ask yourself a question, you set your mind in motion looking for answers,” said Dick Kline, one of the credited inventors of the KF (Kline-Fogleman) airfoil design. Dick helped create what he and many others believe to be the most effective airfoil ever invented. The simplicity and performance of the KF airfoil have made it popular in the RC modeling community, but many people don’t know where it came from, or are skeptical about how it works. Let’s dive in and take a closer look at this novel, and possibly revolutionary, wing design. Dick originally designed the KF airfoil to be used in a more effective long-distance paper airplane. In addition to increasing the structure of the paper airplane, the stepped airfoil showed flight characteristics that would be desirable on any aircraft.
Dick Kline is shown here holding his paper airplane called The Condor, which featured the original KF airfoil design. Dick is now 84 years old. Photo provided by Dick Kline.
After the success of the paper airplane design, Floyd Fogleman, a modeler, expressed interest in the airfoil and built a large balsa model for testing. The aircraft flew like the paper airplane and resisted stalling, flew smoother, and glided longer than anything Floyd had ever seen. Dick and Floyd then filed for a patent in 1970. Unfortunately, there were some complications when filing and writing the patent. One of the biggest issues was that the diagramed airfoil design did not accurately resemble the actual wing profile. While these problems were happening in the background, the design gained popularity in the aerospace community from the promising performance of the paper airplane and Dick and Floyd’s claims of its future applications and benefits. This caught the attention of a group of engineers that worked at NASA. The engineers decided to test the design’s performance. The NASA team, however, based its airfoil models directly off of the patent, which was not the correct airfoil shape. The team concluded in a short overall report that the KF airfoil had no advantages over conventional airfoil designs, or even a flat wing. The engineers also stated that the wing increased the overall drag and that their testing never substantiated Dick and Floyd’s claims. The 1974 findings by NASA were taken seriously, and the KF airfoil was categorized as a failed design by many professionals. This is why you can’t find a full-scale airplane with a KF airfoil today. However, many modelers embraced the original findings made by Dick and Floyd and started using the KF airfoil on their models. Designers and pilots such as RCPowers, FPVTrond, RCTestFlight, myself, and many others share our creations online today and swear by the KF airfoil in providing superior performance and flight characteristics over any other wing profile. To understand these benefits, let’s look at how the KF airfoil works. There are several models of KF airfoils, but the most basic consists of a single step on the top surface of a flat wing. The step on a KF airfoil works by trapping a vortex of air. When the air stream flows over the ledge of the step, some of it cascades down over the edge because of surface tension. This begins a cycle where more air is forced down and back. Air is then eventually pushed to the front of the wing by the airstream that passed over the step.
The red spiral indicates the captured vortex of air that the step creates. The vortex in the real world would more wholly fill the cavity below the air stream, contouring with the shape of the flowing air and the step itself. Drawing by Lucas Weakley.
The air moving forward along the bottom of the step is then forced up by the face of the step and back again by the airstream, creating a vortex. This captured vortex allows the air to travel almost frictionless over a portion of the wing, decreasing drag, which in turn increases efficiency. The captured vortex also sucks the airflow down to the wing so that the airstream is less prone to delamination. This makes the airfoil resistant to stalling and keeps air flowing over the control surfaces, even in incredibly high angles of attack. I designed a small airplane for use in an introduction to RC aviation video series called Maker Hangar (makezine.com/author/lucas-weakley). The airplane is called the Maker Trainer 2 and uses the KFm-2 (Kline-Fogelman model 2) airfoil design (a single step on top). When in flight, the Maker Trainer 2 is stable at high speeds for its small size and light weight. When the speed is decreased and flight is maintained by increasing the angle of attack, I am still able to have full roll authority of the airplane, even at angles that visibly exceed 30°. When forced into a stall (which is hard to do), the Maker Trainer 2 tips slightly forward and regains speed.
The KF airfoil on the Maker Trainer 2 allows it to be incredibly stall resistant. With this simple step, the airplane never tip stalls, which allows it to easily do hard banking turns at slow speeds. Photo by Kent Weakley.
Many of my designs that use a KF airfoil have similar flight characteristics, as described above. Flying wings especially benefit from the additional control authority caused by the more adhered airflow over the control surfaces. This makes flying wings easier to fly and more maneuverable in higher angles of attack and low speeds. A KF airfoil can easily be added to dramatically increase the overall performance of almost any airplane. For an RC foam aircraft, this can be done by simply adding another layer of foam to the top of the existing wing. This new layer should be the same thickness as the existing wing or thinner—any thicker and it could start creating drag. The step should begin at the leading edge, terminate between 40% and 50% of the wing chord, and run the entire span of the wing. The airfoil is completed by rounding over the leading edge to decrease drag. That’s it!
This flying wing (originally designed by ProjectFlightDesign) has a deep KFm-2 step that produces too much lift and makes the wing climb with excessive power. This is a good example of how the step doesn’t have to be very deep. Photo by Lucas Weakley.
The design is incredibly simple, making it attractive for easier builds and implementation on any airplane design. More steps can also be added to the top or bottom, which creates different KF airfoil models that vary in performance and advantages. I’ve also been playing around with an adjustable step that can change in flight, depending on how the airplane is flying. All of this is with the aim to find a more effective, stable, and overall better wing design. If this all seems suspicious to you, just give it a try! You might find yourself using the design on all of your future builds. Although general aviation aircraft companies are catching up on the latest and greatest, we might be able to say that the RC community was what finally tipped the full-scale aviation industry to the future of airfoils. -Lucas Weakley [email protected]
