Airplane design and construction is far more complicated than I originally
gave it credit for. Differences in performance could arise from a vast
number of sources and discerning which variables are important is difficult.
I saw an article which said that Vari-Eze's varied much more in performance,
one to the next, than Long-Eze's, apparently because the Vari-Eze's were more
sensitive to build variations.
I expect that the Europa wing is relatively tolerant to minor variations in
contour since it has been designed to be tolerant to bugs and rain. In this
area, I haven't seen mention of contouring the foam before glassing. I noted
that the foam wing sections did not fit together perfectly in that adjacent
sections were slightly different in size, leaving a lip of a few thousandths
at each joint. In addition, there was a noticeable curve in each foam block;
a straightedge across the top of a block showed a low spot in the center
while a straightedge across the bottom of a block showed a high spot in the
center. This curvature is apparently due to catenary sag of the wire while
hot wiring the foam rather than stresses in the foam. I spoke with Roger
Sheridan about this and he said to use a 3 foot straightedge to judge
discontinuities and to profile the foam to reduce the maximum error to 1/64
inch. He said that each hour spent profiling the foam would save 10 hours
profiling the filler. My profiling tool was 2.5" wide 1/2" thick and 3 feet
long with a 9 inch piece of #100 sandpaper in the middle. The wood was
slightly bowed lengthwise; the sandpaper was placed in the concavity so that
it didn't touch unless slight pressure was applied. The wood was recessed
slightly at the ends of the sandpaper to prevent them digging in. This tool
worked amazingly well and allowed profiling the front or back half of a wing
in less than two hours, most of which time was spent with an aluminum
straightedge judging progress. The difference as judged by eye, as well as by
straightedge and by fishline was quite noticeable. I haven't filled the wings
yet so can't say how they will look ultimately.
Now, I wonder if I affected the efficiency of my airfoils and if so was it
for good or bad?
Did anyone else profile their foam or is it just me? Why isn't it mentioned
in the manual?
I wonder how much of the performance difference being discussed is due to the
build process vs aerodynamic cleanup. I saw Kim Prout's machine at Copper
State '96 and it had quite a number of minor details which aren't part of the
standard Europa and I'm sure more have been added in the last year. Kim's
father is an aeronautical engineer and I think Kim has used that to advantage
in his drag reduction program along with a lot of enthusiasm and hard work.
Graham's machine also performs well and he also has gone a bit beyond the
norm on drag reduction.
Speed measurement itself is subject to so many variables that the only real
way to compare aircraft is side by side and then only after comparing
tachometers. Perhaps Alan and Graham can get together sometime for an in the
air comparison plus an on the ground comparison of cleanup efforts. Easy for
me to waste their time and fuel, I suppose, but it seems that there is much
to be learned here.
Regards, John A044 Newtown, CT
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