In 1974 Dan Santich, then at Top Flight, experimented with a set of wings similar to figure 8, they were based on the ideas of Richard Kline and Floyd Fogleman.  In these trials some models had the step on the top surface of the wing and some had it on the bottom.  The  potential flow, shown as blue stream lines in figure 8, follow the outer radius of the vortex as if it were part of the wing's skin.  As the angle of attack changes the vortex can expand and contract in response to pressure changes thereby, according to the inventors, allowing the wing to automatically adapt to flight conditions.

Figure 8Figure 8: Vortex on a Kline-Fogleman wing

Apparently those tests  confirmed Kline and Fogleman's claims but TopFlite had signed a nondisclosure agreement with them and , the results from these test flights were not published.  The concept was tested by at least three other labs in the late '70s and early '80s but none produced results worth pursuing.  One researcher is reported to have said that the Kline-Fogleman airfoil wasn't much better than a flat plate.  Later they would claim that the reason independent lab tests showed such pitiful results was because the wind tunnel models were based on the drawing from the first patent, which showed a shape with flat surfaces and a sharp leading edge because they didn't want a specific airfoil  to appear on the patent.

Figure 9Figure 9: Kline-Fogleman patent

Figure 9 show's the basic shapes from the Kline-Fogleman patents.  The second patent showed a control device that  works  like a flapperon.  The shape in figure 10 is similar to the profile used on the radio controlled models used in the Top Flight tests.

Figure 10Figure 10: Top Flight test section

By the mid 1990s (about the time the patents expired) NASA began showing some interest in stepped airfoils again.  In '98 Fathi Finaish and Stephen Witherspoon published a paper about work they had done involving at least 15 different step configurations.  The basic airfoil section for the wind tunnel models was the symmetrical NACA 0012 and each model had one step at 50% chord.  All models were tested with the cutout on the bottom and on top. Du and Lu are the parameters that were compared in this study.  Figure 11a show's the geometry definition while 11(b, c and d) show the pressure distribution for each cutout geometry.  All stepped models tested had higher drag than the unmodified NACA 0012 (Du=0.00). Most of the modified shapes were significantly worse than the basic airfoil but two showed some positive effect on coefficient of lift and lift to drag ratio.  These two were, the smallest cutout on top (figure 11b+Du=0.19) and the largest cutout on the lower surface (figure 11d+Du=0.50 inverted).   This long step is the same as Kline and Fogleman's consept.
In the case of the small step on top there was a small positive effect on CL that started at zero degrees AoA and increased with alpha. The effect on L/D Starts out positive at alpha = 0, then it is slightly negative at alpha=5 degrees, then L/D is positive again at alpha=10 degrees.  No data was taken for AoA higher than 10 degrees.
In the case of the large lower surface cutout CL at alpha=0 was increased from 0.0 for the unmodified section to  0.15 with the cutout.  Coefficient of lift enhancement was apparent at all values of AoA tested in this study.  At alpha=10 degrees CL was 0.2 higher than for the basic airfoil.  Lift to drag ratio was improved at alpha=0, with the basic and modified wings performing about the same at alpha=5 degrees.  At alpha=10 degrees all the lower surface cutouts caused a significant reduction in L/D.
Figure 11a: geometry definition chord
Xu  Position of upper surface step in %C from the leading edge
Lu  Length of upper surface cutout in %C
Du  Depth of upper surface cutout in % of local airfoil thickness

Figure 11b: Step length = 13% chord

Figure 11c: Step length = 25% chord

Figure 11d: Step length = 50% chord

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