On Thursday, July 25, 2019 at 7:25:44 PM UTC-6, Sky Surfer wrote:
FWIW:Â* Jim Marske's Composite Design Manual, page F7, says of the lift/dagger pins, "There are four such pins located near the leading edge and rear spar (or trailing edge).Â* These four pins transmit all loads from the wing to the fuselage.Â* The main spar does not touch the fuselage structure anywhere."

Having seen (mostly from afar) plug-in wing spar tongues that are pinned together in the fuselage, that last sentence about the joined main spar not touching anything surprised me.Â* I wish I could look at many fiberglass gliders to see if the industry "standard" (if there's such a thing) is to float the joined spar freely accross the fuselage, but I don't have that ability.Â* So I emailed Jim about the importance of isolating the spar tongues from any fuselage structure.Â* He answered, "I once wondered about the same thing.Â* Why is the main spar suspended freely across the fuselage?Â* The spar is quite flexible - even across the fuselage, especially at the fuselage CL.Â* So my conclusion was, why throw other unknown forces into a joint when you don't have to.Â* However, motion near the fuselage side rails is quite small as it is close to the dagger pins so you may get away with it.Â* Don't fasten the spar to the fuselage on the fuselage centerline."

That all sounds reasonable.Â* So I'd like to ask the knowledgeable folks on this forum:Â* Is it true that the main spar tongues are pinned only to themselves and maybe to the opposite wing root, but they don't touch anything else in the fuselage?Â* Are there notable exceptions, like spar pins passing through not only the spar tongues but also through a fuselage bulkhead?Â* How about spar tongues that are pushed into a joining box or tunnel?Â* That would be the antithesis of isolating the main spar.Â* These are different but related questions to the original lift pins question.Â*Â*Â*

On the Phoebus (1st fiberglass production glider), the pin goes through the spar of both wings at the centerline, but also through the bulkhead behind the pilot's head. But this isn't a structural connection to the fuselage though, I don't think. The main connection is through the lift pins, even on this early model. And the wing is VERY stiff, so not much flex at all.