On Wednesday, August 12, 2015 at 1:52:27 AM UTC-7, krasw wrote:
Is there a reason this damage cannot be repaired by putting simple multilayer patch (right size, of course) on the inside surface? With proper overlap it gives you original strength without time-consuming surface finish work. Why not take advantage that fuselage shell is not sandwich structure and inside of the shell is easily accessible?

The conventional reasoning for not going with a simple patch like that is that it results in a steep stiffness gradient at the site of the damage. The more widely accepted scarf repair, when properly done, restores not only the strength, but also the stiffness distribution of the original. That helps ensure that the repaired area will not be prone to delamination or cracking as it ages.

At issue is that gliders tend to be very limber structures--they bend a lot.. As they bend, some parts of them get stretched out, and other parts get pressed together. And where there are abrupt changes in stiffness, for example, where there is a change in sectional thickness, the stretching or shrinking happens unevenly. That uneven stress distribution can cause cracking in the most brittle layer, the gelcoat. Sometimes the cracking can go deeper, into the structure below, but that's pretty rare in a well-designed glider. Typically you'll see this effect in gelcoat cracks that form at the corners of airbrake boxes, along ribs and bulkheads and other internal reinforcements.

Thanks, Bob K.