Aircraft wing showing the KFm4 Step In the early s, Richard Kline wanted to make a paper airplane that could handle strong winds, climb high, level off by itself and then enter a long downwards glide. After many experiments he was able to achieve this goal. He presented the paper airplane to Floyd Fogleman who saw it fly and resist stalling. The two men then filed for a patent on the stepped airfoil. Further development resulted in two patents and a family of airfoils known as the KF airfoil and KFm airfoils for Kline—Fogleman modified.
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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? Dick helped create what he and many others believe to be the most effective airfoil ever invented. 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 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. This caught the attention of a group of engineers that worked at NASA. 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 findings by NASA were taken seriously, and the KF airfoil was categorized as a failed design by many professionals. 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.
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. 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 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 airfoil is completed by rounding over the leading edge to decrease drag. 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.
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. 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.
New Technology: Kline-Fogleman Airfoil Design
Mezirr Or for getting started on doing them and finding the data at least. When a television station went off the air at night, people saw airfiol on their tv screens. Fortunately, the step is relatively small in the big scheme of things and the aircraft can aitfoil perform normally. I am perfectly willing to let this one slide if you have the content covered elsewhere, eh? They are less affected by strong winds.
KFM AIRFOIL PDF
I have only tried a couple KF planes, one off some plans and one of my own design. I had a can of spray adhesive in the cupboard and a quick test showed that it gave a very strong airoil quick bond when sprayed lightly over both surfaces without any bubbling of the paper. The KFm-4 airfoil was easy enough to make by measuring the existing wing cord from the center edge and the wingend edge, then halfing both measurments and connecting the dots across the wing to have a line measuring the center of the wing cord. Skip to main content. This slope away from the airstream make it much easier for the airflow to detach itself from the upper surface and when it does Poor lift-to-drag ratio performance kfj wind tunnel testing has meant that to date the KF airfoil has not been used on any full size aircraft. The result is that if the curling and diverging action all depends on the divergence rate of the air. MesarcFoamfighters uses a variety of techniques from score cutting foam airfol for exact folds to glue joints that provide excellent structural strength.
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