Steped Airfoils

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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 somemodels had the step on the top surface of the wing and some had it on the bottom. Thepotential flow, shown as blue stream lines in figure 8, follow the outer radius of the vortexas if it were part of the wing's skin. As the angle of attack changes the vortex can expandand contract in response to pressure changes thereby, according to the inventors, allowingthe wing to automatically adapt to flight conditions.

Figure 8

Apparently those tests confirmed Kline and Fogleman's claims but TopFlite had signed anondisclosure agreement with them and , the results from these test flights were notpublished. The concept was tested by at least three other labs in the late '70s and early '80sbut none produced results worth pursuing. One researcher is reported to have said that theKline-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 tunnelmodels were based on the drawing from the first patent, which showed a shape with flatsurfaces and a sharp leading edge because they didn't want a specific airfoil to appear onthe patent.

Figure 9

ped airfoils http://www.acsol.net/~nmasters/vortex-lift/st

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

Figure 10

By the mid 1990s (about the time the patentsexpired) NASA began showing some interest instepped airfoils again. In '98 Fathi Finaish and Stephen Witherspoon published a paperabout work they had done involving at least 15 different step configurations. The basicairfoil section for the wind tunnel models was the symmetrical NACA 0012 and each modelhad one step at 50% chord. All models were tested with the cutout on the bottom and on

top. Duand Luare the parameters that were compared in this study. Figure 11a show's thegeometry definition while 11(b, c and d) show the pressure distribution for each cutoutgeometry. 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 buttwo 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 atzero 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 theunmodified section to 0.15 with the cutout. Coefficient of lift enhancement was apparent atall values of AoA tested in this study. At alpha=10 degrees CL was 0.2 higher than for thebasic airfoil. Lift to drag ratio was improved at alpha=0, with the basic and modified wingsperforming about the same at alpha=5 degrees. At alpha=10 degrees all the lower surfacecutouts caused a significant reduction in L/D.


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C chord

Xu

Position ofupper surfacestep in %C fromthe leading edge

LuLength of uppersurface cutout in

%C

Du

Depth of uppersurface cutout in% of local airfoilthickness


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Steped Airfoils

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