Sailplane wing joint

Hi all,Â*

I started studying Jim Marske's Composite Design Manual to understand composite techniques generally and glider construction in particular.Â* My current topic of interest is the best design for joining two plug-in wings for a small sailplane, say in the 13 meter class.Â* It seems the accepted method involves two overlappingÂ* main spar tongues that are pinned together by large-diameter pins in the fuselage.Â*Â*Â* Typically, each wing root also has two short pins, near the leading and trailing edges, that slide into receptacles in the fuselage sides for alignment and for imparting lifting forces to the fusefage.

Jim's Manual refers to these pins as "dagger pins" and says that they transmit ALL the wings' loads.Â* He recommends that the main spar not even touch any fuselage structure.Â* This last idea is counterintuitive and begs for clarification for my limited understanding of the topic.

I recently emailed Jim and hope to get a response, if he's willing to do so..Â* In the meantime, I hope the very knowledgeable people on this forum can weigh in with your thoughts about the reason for structurally isolating the joined-spar from the fuselage. Is this how it's actually done (asking those who really know gliders)?Â* I would also appreciate other details about how you would join two lightweight sailplane wings, considering structural integrity (of course), weight, ease of rigging, and maintenance.

Thanks in advance.Â* FYI:Â* The bulk of my email to Jim follows below:

"Hi Jim,

I think an "overlap spar with dagger attach pins" arrangement, shown on page F7 of your manual, would be good.Â* I'm familiar the overlapping spar design only to the extent that I've seen other pilots assemble their "glass" sailplanes.

The note at the bottom of page F7 says of the 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."

Can you please explain the importance of isolating the main spar from the fuselage?

What is the downside, for example, of passing the two spar tongues under reinforced fuselage longerons so that the wings can lift there (where the spar touches the longerons/fuselage) in addition to the four pins?

This is probably my top question from the manual.Â* So I'm hoping that you can shed light on the rationale for not touching the main spar to the fuselage structure."