A Sub-Leading Edge is often used when a wing is fully
sheeted or has leading edge sheeting (D-Tube wing). It is glued on
to the front of the ribs then sanded to fair into the ribs before the
sheeting is glued on.
The purpose is to anchor the sheeting to something solid as well as help prevent
from sagging between ribs. It also has the benefit of strengthening
leading edge. Because the leading edge is laminated to the
sub-leading edge and to the forward edge of the sheeting, it creates a
strong, warp-resistant structure with very little weight penalty.
Splitting the Difference
When gluing long parts in place I have found a
method that works well. Obviously if you have added glue then you are
working against the clock and you do not want it to become too dry before the
part is in place.
The method I use is
called splitting the difference. I pin each end of the part and then
center. Then I put pins midway between the center and the ends.
I continue splitting the difference until all the pins are in place. This method
helps to hold the part in areas that are not pinned yet.
Conversely if you start at one end and work your
way towards the other, then the far end is
unsupported and can cause problems such as bowing of the structure, etc.