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 8
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 9
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 10
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.
 |
C |
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 |
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