I
found a wonderful Windows utility, authored by Martin Hepperle,
in the midst of those zillion internet files
available for download, that is nice to calculate a first guess
for the location of the aerodynamic center of flying objects with
complex shapes.
This
little tool examines a color bitmap image and calculates several
properties of colored areas found in the image. These properties
are:
-
the area of the individually colored region(s);
-
an approximation of the circumference of the individually colored
region(s);
-
the center of the colored region(s), which is also the center
of gravity (CG), if the colored shape would have been cut out
of a flat sheet;
- the
aerodynamic center (AC) of the colored region, which is equivalent
to the 1/4 chord point on a rectangular wing plan form. This
AC is calculated from the geometry of the plan form only and
does not take any aerodynamic effects, like downwash of a wing,
into account. It can be used as a first guess for the neutral
point, which means, that the center of gravity (CG) of a flying
object should be placed in front of this AC. If the CG would
be placed behind AC, the object would be unstable.
- for
a single wing or a canard configuration, the result is pretty
close to the true aerodynamic center; for a typical high aspect
ratio wing of a sailplane, the location will move towards the
main wing by 10% to 20% of the main wing chord. Thus, you should
make the first flight of a typical sailplane model with a first
CG not behind the calculated AC plus a 15% wing chord safety
margin. A more precise calculation of the AC would have to include
the wing plan form and aspect ratio, which is beyond the scope
of this simple tool.
The mechanics:
The
bitmap can be the top view of a plane to define the CG, location
for a longitudinal stability. Another use would be to examine
the side view to check or define the size of the vertical fin
in order to achieve “weathercock” stability around the vertical
axis. This is also useful for the design of model rockets. Since
there is only one CG of an object, the AC of the top view as well
as that of the side view should both be behind the CG.
How-to steps:
- Create
a top or side silhouette image in bmp format of your aircraft
using pure colors based in the combination of the three basic
"amino-acids" of a
video screen "molecule", that is the RGB values;
- Differing
the interest areas by colors would provide different AC and
CG calculated for each of those areas. It can be of use once
you consider that wings and other plane features have a mass
sum smaller than the fuselage's and engine's. Doing this way
you can use your educated guess and assign some different weight
factor to each area and finally infer an average AC and CG;
- Run
the analysis in the AC Calculator
application;
This image shows a post-analysis of a side view for that CBA-123
Vector I modeled, with the AC and CG indicated for each different
colored area;
and
some resulting data from the analysis.
