I am about to install the wing pin sockets and find that it is more difficult
than expected to measure wing incidence. The wing template works fine on the
root but attempting to measure incidence at 70% - suggested as producing
better results if the washout is not precisely identical for the two wings -
is much more difficult. I tried using a 4 inch wide section of the inboard
top of P1's jig block. This led to the conclusion that my wing profiles are
not identical in that the buildup of glass over the spar resulted in a bump of
about 30 mils. Since the spars are at different positions relative to the
leading edge, fairing this bump into the wing profile led to slightly
different profiles for the two wings, precluding use of the the jig block for
measuring incidence.
My proposed solution to setting the wing incidence is to use a "bridge"
consisting of a wood straight edge 36" long with two 2" high metal angle
sections attached 30" apart. The goal is to contact two points 30" apart to
measure wing incidence rather than try to make a template since with non-
identical wings it is not possible to make a template which accurately fits
both wings.
The port wing is set to 2.5 degrees using the wing template at the root. The
bridge is then placed chordwise at the 70% point and moved chordwise to attain
a level reading; the two angle sections raise the bridge sufficiently so it
clears the spar area. The exact position of the bridge relative to the LE is
noted and the bridge is then moved to the stbd wing at 70% and its position
relative to the LE set to replicate that on the port wing. The angle of the
stbd wing is then adjusted to achieve a level reading, thus duplicating the
incidence angle of the port wing.
Having made the above measurements, I found that the difference between
setting both wings with the template at the root and using the bridge
technique resulted in a difference of about 3/10 degree (LE down with the
bridge) in setting the stbd wing incidence. This was disheartening in that I
took some pains in building the wings to set washout to the best of my ability
and to otherwise build the wings as accurately as I could.
Should I use the template results or the bridge results? Does anyone have a
better technique for measuring wing incidence? Have others noted any
assymetry between wings? Is it really possible to measure incidence to 1/20
degree?
John A044 Newtown, CT
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