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There appears to be some misconceptions regarding the purpose of, and loads
in, fillets in bonded "T" or angle joints. In theory the joint only takes
loads along its edge, (shear,) or is pushed together, (compression,) and
never takes a load in bending or tension. (You can design joints for
significant bending or tension, but that is another story.) Again in theory
the compression load is transferred by direct contact of the skin face
against the rib edges, (assuming a perfect fit,) and the shear load is
transferred through either taping or some other adhesion. If the shear load
is transferred by taping then no fillet is required. (Again, in theory.)

In practice, the rib doesn't fit well enough to bear directly against the
skin, the joint often takes some tension or bending, and one finds it
impossible to push taping into perfect right angle corners. (The taping
must be attached to something to prevent shear buckling, without a fillet
it has to adopt a perfect angle, a difficult proposition at best.) The
fillet is used to solve these difficulties. In compression the fillet
serves to bridge the irregularities of the fit between rib and skin. In
tension and bending the fillet serves as the only tie between rib and skin,
(since the peel strength of the taping is considered negligible.) In shear
the fillet stabilizes the taping at a practically achievable radius.

The tension load is our greatest concern here. (I will presume there are
few instances of joint bending in the Lancair.) As I said, the taping helps
little with tension, since under tension it peels readily. It is therefore
the fillet which does the majority of the work keeping the rib on the skin.
There is nearly a linear relationship between fillet size and joint tension
capability. There is also nearly a linear relationship between fillet
tensile strength and joint tension capability. You therefore should be very
careful when constructing fillets from what you call "micro" as it has very
poor tensile strength. Even worse, it has incredibly poor fatigue
resistance, being in essence a brittle structure with built-in occlusions.

The big question is, how strong should the fillet be? The answer is, I have
no idea! (Very useful.) So why the lecture? Simply to remind everybody to
be wary when making structural modifications. You must make certain that
your modification has all of the life and strength of the original design,
under all environmental conditions. You do this through analysis,
(computers, thought experiments, whatever,) experiment, (meaning someone
else tried it and hasn't crashed in X thousand hours,) or just plain dumb luck.

Good luck,


Guy Buchanan

LML website:   http://www.olsusa.com/Users/Mkaye/maillist.html
LML Builders' Bookstore:   http://www.buildersbooks.com/lancair

Please send your photos and drawings to marvkaye@olsusa.com.