Hi all,- - - - - -
I started studying Jim Marske's Composite Design Manual to understand composite techniques generally and glider construction in particular. 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. It seems the accepted method involves two overlapping main spar tongues that are pinned together by large-diameter pins in the fuselage. 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. He recommends that the main spar not even touch any fuselage structure. 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. 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)? 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
Thanks in advance. FYI: The bulk of my email to Jim follows below:
I think an "overlap spar with dagger attach pins" arrangement, shown on
page F7 of your manual, would be good. I'm familiar the overlapping spar design only to the extent that I've seen other pilots assemble their
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). These four pins transmit all loads from the wing to the fuselage. 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. So I'm hoping that you
can shed light on the rationale for not touching the main spar to the fuselage structure."
Thanks for the response, Bob W. As a new poster, I did notice the
spammers' infestation of this forum, which I thought was really informative when it was more active. Just wondering if the spammers or a decline in amateur aircraft homebuilding is more responsible for this site's near demise.
At about the time I posted, Jim M. replied to my email. He said, "why
throw other unknown forces into a joint when you don't have to", which mirrors what you said. That reasoning does help my understanding.
Though, I'm picturing the Schreder wing spar "knuckles" joint where one of the connecting pins goes through the knuckles AND an anchor tab welded to
the midpoint of fuselage cross tube. I guess that anchor mostly aligns the knuckles for rigging and maybe makes the spar joint a little less
"plastic". Spar knuckles and tongues are different, but spar movement near the fuselage centerline exist in both geometries.
Bob, did you have problems with the Zuni's spar end pins fatiguing or cracking because of cyclic loads? That's a critique of spar end pins, according to Marske's manual.
The particular HP-14 I have in mind has the bottom of the two long knuckle pins passing through an anchor tab which is just forward of the bottom knuckles. That tab is welded to to the top tube of a steel-aluminum
bulkhead which makes up the front wall of the landing gear box.
The knuckle pins have cross-drilled holes at their ends. After the pins
are pushed rear-to-front through the knuckles and rotated so the small
holes line up vertically, a long 1/16" wire is pushed down through both
holes (just forward of the bottom anchor tab) to retain the knuckle pins.
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