On Thursday, July 25, 2019 at 6:25:44 PM UTC-7, Sky Surfer wrote:

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?

That is indeed the case for the vast majority of modern high-performance sailplanes, including the ones I design and make parts for. I have an hour-long sermon about the why of it, but it boils down to performance. The best overall performance is achieved by optimizing the performance of the infrastructure/sailplane/pilot system as a whole, and not just the performance of the sailplane itself.

Isolating the spars from the fuselage incurs a modest weight penalty in exchange for simplified assembly with fewer opportunities for misalignment or misassembly. The payoff is less fatigue accrued during pre-flight phases and more energy available for in-flight strategy and tactics.

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*

There may be such out there, but they must be pretty rare because I almost never see them in modern sailplanes.

--Bob K.