Background:
I had always understood that as a general principle, a strut-braced light aircraft wing using a single strut (Cessna 100) relies on a structural "D-tube" leading edge for torsional rigidity... and that on the old style fabric covered wings using two struts (Cub, T-craft, Champ, etc.) the torsional rigidity is essentially provided by the rear strut... and a structural D-tube is not required. This is why, I understood, that the leading edge "sheeting" on two-strut airplanes is usually non-structural aluminum tacked on with small nails. I understood this is also why you can routinely set and change the twist (washout) using an adjustable fork on the rear strut.
Situation:
I'm refurbishing a Legal Eagle UL/LSA, which uses stick-built ribs, C-channel wood spars, and a glued plywood leading edge D-tube. The plywood is very thin (0.8mm!), the rib spacing is large (15 inches), and the plywood is sagging / drooping / "starved horse" between the ribs. I want to go in and put some blue foam false ribs in place to push the plywood back out, and hold the airfoil shape. Peeling the plywood up to insert the false ribs is not likely to work; the thin wood is epoxied onto the ribs and spars with T-88, and will assuredly be destroyed while attempting to peel it back.
Proposed Approach:
I figure I can cut 2 inch wide chordwise slots in the plywood lower leading edge, to insert the foam ribs. If the plywood D-tube on this two-strut wing is not providing part of the torsional structure, then covering up these slots is cosmetic. But if the plywood leading edge was intended and designed to contribute significantly towards the wing's torsional rigidity, then this plywood repair is structural, and I have to figure out a way to splice across these slots (whether using scarf cuts or overlap patch strips).
Question:
What are the odds that a two-strut ultralight wing is actually making significant structural use of 0.8 mm plywood and 15 inch rib spacing.... and that a non-structural plywood repair will reduce the flight safety of the wing?
I had always understood that as a general principle, a strut-braced light aircraft wing using a single strut (Cessna 100) relies on a structural "D-tube" leading edge for torsional rigidity... and that on the old style fabric covered wings using two struts (Cub, T-craft, Champ, etc.) the torsional rigidity is essentially provided by the rear strut... and a structural D-tube is not required. This is why, I understood, that the leading edge "sheeting" on two-strut airplanes is usually non-structural aluminum tacked on with small nails. I understood this is also why you can routinely set and change the twist (washout) using an adjustable fork on the rear strut.
Situation:
I'm refurbishing a Legal Eagle UL/LSA, which uses stick-built ribs, C-channel wood spars, and a glued plywood leading edge D-tube. The plywood is very thin (0.8mm!), the rib spacing is large (15 inches), and the plywood is sagging / drooping / "starved horse" between the ribs. I want to go in and put some blue foam false ribs in place to push the plywood back out, and hold the airfoil shape. Peeling the plywood up to insert the false ribs is not likely to work; the thin wood is epoxied onto the ribs and spars with T-88, and will assuredly be destroyed while attempting to peel it back.
Proposed Approach:
I figure I can cut 2 inch wide chordwise slots in the plywood lower leading edge, to insert the foam ribs. If the plywood D-tube on this two-strut wing is not providing part of the torsional structure, then covering up these slots is cosmetic. But if the plywood leading edge was intended and designed to contribute significantly towards the wing's torsional rigidity, then this plywood repair is structural, and I have to figure out a way to splice across these slots (whether using scarf cuts or overlap patch strips).
Question:
What are the odds that a two-strut ultralight wing is actually making significant structural use of 0.8 mm plywood and 15 inch rib spacing.... and that a non-structural plywood repair will reduce the flight safety of the wing?
