27 J-3 Cub Error in Wing Root Rib
Angle
I have to share the bad stuff along with the good. When I went to trial fit the left wing into the spar boxes on the fuselage and pulled the root rib up tight against the fuselage side, I had way too much dihedral %$#@ The wing's root rib was set at the wrong angle. How could this be?
The picture below shows the dihedral jig that was
modified as per Mike Midkiff to get the reduced 1-1/4 degree scale dihedral
instead of the 4 degrees shown on the plans. The mistake was made when I
used the dihedral jig to set the root rib's inclined angle. The error in my
thinking can be see in the next picture.
Because the fuselage sides are not vertically
straight, using the dihedral jig put in almost twice too much root
rib inclination angle. When the wing was pulled down to the proper 1-1/4
degree dihedral angle, the top of the wing's root rib moved away from the
fuselage and there was big ugly gap on top where the wing joins the
fuselage. In fact, as the 45 degree triangle shows below, the root rib
should have been installed perpendicular to the building board and not
inclined at all!
Therefore, I have spent the day working out the
correction to this problem as shown below. The first step was to cut out the
section of the 3/32" root rib between the rear face of the front spar and
the front face of the rear spar and cut out a new replacement rib section.
The wing was plugged into the fuselage spar boxes and then set at the proper
1-1/4 degree dihedral angle. The new replacement rib section was tacked in
place with CA. The wing was removed and reinserted and the dihedral was
checked with the dihedral jig to be sure it was correct this time. The
replacement rib section was then CA'ed to the two spars and 1/32" balsa
shims were glued to remaining root rib sections in the area in front of the
front spar and behind the rear spar. as shown below.
The picture below shows the corrected fit using
the dihedral jig.
This is a shot looking down at the top of the
wing/fuselage juncture that shows the good close fit I was able to achieve.
Thank God I was able to recover from this stupid mistake. You can bet I'll
know what to do when I build the right wing panel tomorrow. :O)
..................Tandy
