The air foil used on the BD-4 is a modified 64-415. The following are the coordinates
of the air foil. You can also download an CAD drawing of the air foil as DXF
and DWG file. (These drawings are currently
plotted to the OLD cordinates.)
See: sk21-rib.dxf for an updated drawing. Sk21-rib.dxf contains
two layers. One layer is the rib shown on
SK-21, and the other layer is a drawing of
the cordinates below.
| Original | Upper Surface | x | 0.000 | 0.1435 | 0.2525 | 0.4781 | 1.059 | 2.243 | 3.438 | 4.638 | 7.047 | 9.463 | 11.88 | 14.31 | 16.73 | 19.15 | 21.58 | 24.00 | 26.42 | 28.83 | 31.25 | 33.65 | 36.06 | 38.45 | 40.84 | 43.23 | 45.62 | 48.00 |
| y | 0.000 | 0.6197 | 0.7579 | 0.9782 | 1.384 | 1.978 | 2.436 | 2.815 | 3.419 | 3.872 | 4.210 | 4.445 | 4.58 | 4.615 | 4.519 | 4.328 | 4.059 | 3.726 | 3.338 | 2.906 | 2.440 | 1.950 | 1.450 | 0.9514 | 0.4685 | 0.000 | ||
| Lower Surface | x | 0.000 | 0.3365 | 0.4675 | 0.7219 | 1.341 | 2.557 | 3.762 | 4.962 | 7.353 | 9.737 | 12.12 | 14.49 | 16.87 | 19.25 | 21.62 | 24.00 | 26.38 | 28.77 | 31.15 | 33.55 | 35.94 | 38.35 | 40.76 | 43.17 | 45.58 | 48.00 | |
| y | 0.000 | -0.5237 | -0.6235 | -0.7728 | -1.027 | -1.371 | -1.622 | -1.822 | -2.126 | -2.343 | -2.492 | -2.579 | -2.602 | -2.558 | -2.416 | -2.210 | -1.956 | -1.669 | -1.360 | -1.040 | -0.7219 | -0.4214 | -0.1574 | 0.04128 | 0.1382 | 0.000 | ||
| Modified | Lower Surface | x | 0.000 | 0.3365 | 0.4675 | 0.7219 | 1.341 | 2.557 | 3.762 | 4.962 | 7.353 | 9.737 | 12.12 | 14.49 | 16.87 | 19.25 | 21.62 | 23.40 | 25.86 | 28.32 | 30.78 | 33.24 | 35.70 | 38.16 | 40.62 | 43.08 | 45.54 | 48.00 |
| y | 0.000 | -0.5237 | -0.6235 | -0.7728 | -1.027 | -1.371 | -1.622 | -1.822 | -2.126 | -2.343 | -2.492 | -2.579 | -2.602 | -2.558 | -2.416 | -2.267 | -2.04 | -1.814 | -1.587 | -1.36 | -1.133 | -0.9068 | -0.6810 | -0.4534 | -0.2267 | 0.000 |
The modified coordinates are red.
Source: Cordinates posted to bd4@northwestaero.com
by Mark Napier on Feb. 13, 2002
The upper surface was quite different from
the original points posted here. The lower
surface was the same.
The table now lists 4 significant figures
for all points.
The following are three messages by Mark Napier to the mailing list on Febr., 7, 2002:
...I just called and talked with Jim Bede for a couple of minutes. I've spoken with him before and one thing he likes to do is talk about airplane design. The airfoil used would be better called a 64-415a. The paper that talks about these modified sections was published in 1948 and was (NACA ?) TR 903. The modification is simply to draw a strait line from the trailing edge of the airfoil to a tangent point on the lower airfoil. That's it. This removes the cusp on the trailing edge and makes the aileron forces much more linear. The publication claims that there is very little difference in lift or drag from the original section. Found the following on the NASA tech report server: Loftin, Laurence K , Jr , Theoretical and experimental data for a number of NACA 6A-series airfoil sections , NACA Report 903 , 1948 , pp. 21 . Abstract: The NACA 6a-series airfoil sections were designed to eliminate the trailing-edge cusp which is characteristic of the NACA 6a-series sections. Theoretical data are presented for NACA 6a-series basic thickness forms having the position of minimum pressure of 30, 40, and 50 percent chord and with thickness ratios varying from 6 percent to 15 percent. Also presented are data for a mean line designed to maintain straight sides on the cambered sections. |
| ...Seems like there is about 1/4" between top of the spar and the skin which is what
Bob said. I just finished doing some linear interpolation with the help of MathCad. My result is not exactly correct because the exact point of tangency is either a little after or a little before 24" after the leading edge. I would have to run a cubic spline to approximate the actual point. But for numbers as close as I think it is possible to build a real rib this is what I get. Also, considering the computer technology readily available to Jim Bede for a homebuilt project in 1968 I can't imagine he did much different. Anyway, here it is. ... |
| ...I did run a cubic spline on the data and
find an exact point of tangency. I'll admit
this is a little ridiculous since Jim Bede's
modification is only an approximation of
an "A" series airfoil anyway. If
anyone wanted a truly "exact" solution
it would be by crunching through the equations
in TR 903 but I doubt the result would differ
any in flying characteristics. ... |
[Ed: The result of this computation is the "Modified lower surface ordinate" column in above table.]