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Old June 27th 19, 12:33 AM posted to rec.aviation.homebuilt
BobW
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Posts: 504
Default Sailplane wing joint

On 6/26/2019 1:13 PM, Sky Surfer wrote:
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
maintenance.

Thanks in advance. FYI: 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. 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).
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."

- - - - - -

Coupla notes, Sky Surfer...

1) rec.aviation.homebuilt is moribund...maybe not quite as "indubitably dead"
as Monty Python's parrot, but darned close. Has been for years. The only
reason I caught your post is because my Thunderbird is set up to notify me
when any posts appear in RAH.

2) rec.aviation.soaring IS still active, and at least one "does it for a
living" sailplane designer lurks and occasionally posts there. (Bob Kuykendall
- designer of the carbon fiber, 15-meter, HP-24.) Additionally, several people
with beaucoup years' experience rebuilding/repairing plastic sailplanes
regularly check/chime in. (UH - Hank Nixon; JJ - John Sinclair)

3) My 2-cents (as a NON-practicing, degreed aerospace engineer) regarding Jim
Marske's guidelines is - from a structural perspective - it's difficult to
design a simpler, "continuously-aerodynamic-load-distributing" wing than
imagining single-spar plastic sailplane wings ONLY bearing flight loads across
the span. Arguably, Jim Marske's suggested approach seeks to approach this
load-bearing condition...no struts; minimal introduced point loads along the
spar span (2-dagger pins/side); etc.

FWIW/FYI, the 15 meter sailplane in which I have most of my time pretty much
does as Marske suggests (the late George Applebay's 15 meter,
1st-generation-fiberglass, Zuni): fore/aft lift/anti-torque/"dagger" pins in
each wingroot rib connecting with receptacles in each side of the fuselage
structure, while the tongue-and-tongue spars both have spar-end pins
connecting into similar receptacles in the opposite wing's root rib. The wings
are held together with a single through-pin located on the apar's
neutral-load-axis at mid-span (rather than two pins each located near the root
rib along the neutral axis; some with this design dispense with the
spar-end-pins).

HTH, and try rec.aviation.soaring.

Bob W.

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