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."

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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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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.

I do visit RAS and will likely post related inquiries there.

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On 6/26/2019 7:21 PM, Sky Surfer wrote:

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.

Interesting question to ponder,,,but I have no idea!
- - - - - -

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.

Difficult to get more succinct than that! "What Jim M. said," is what I had in mind, too.
- - - - - -

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.

I don't remember the HP-14 I had having a centerline anchor tab. In fact, I can't remember the details of its fuselage center structure at ALL...just the configuration of the top/bottom/longitudinally-oriented knuckles and their "overly-long" (unsafetied!) pins. Dick Schreder's reasoning was there were no in-flight fore/aft forces on them. I marked/taped 'em just to keep an eye on Murphy! They never moved...though data/incidents/crashes exist in the
composite sailplane world - whose pins are generally NOT "overly-long" - strongly suggesting (to me, anyway) that Murphy/repeated-spar-movement can do wondrous things...

By the way, I expect you caught a misstatement I made about the Zuni's
spar-end pins. Each goes into a receptacle in the opposite spar's wing root rib, not into the fuselage structure...as in it's possible to completely-structurally-rig the wings without the presence of the fuselage.

Bob W.

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Bob, did you have problems with the Zuni's spar end pins fatiguing or cracking becaus 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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On 6/27/2019 10:50 AM, Sky Surfer wrote:

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.

No problems of which I was aware in 2,000+ hours of flight in the ship. I know the late Stan Hall - who actually DID use his AE degree to do structural
design on airplanes/sailplanes his entire working life - wasn't a fan of spar-end pins for the reason you note, as well as the fact it introduced point mounting loading issues into the spar end...bearing loads induced by spar bending loads where the buried bits of the pin mount in the spar end
structure. Stan was seriously conservative in his structural-design thinking IMO...which is not a criticism!
- - - - - -

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.

Ah. My ship didn't have such a tab.
- - - - - -

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.

After I wrote Dick Schreder about the absence of any safety pins on the main pins of my ship (asking if the builder had merely ignored construction advice) was when he replied with his thinking on the absence fore/aft forces requiring them, though he suggested if I felt differently I could simply drill holes in the ends of each and "do the safety-pin thing." Being both curious and lazy, I opted before each flight for marking with a laundry marker at the interface between each pin and the forward-outer face of top/bottom knuckles (to detect fine movement) and a wrap of vinyl tape about 1/4" from the knuckle (to get a handle on gross movement should my laundry mark disappear); never noticed any movement in ~200 flight hours.

Bob W.

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Thanks for the good and detailed information.  I think I'll migrate over to the homebuiltairplanes.com forums, which are quite active and have interest-specific areas like "Aircraft Design", "Composites", etc.  I don't know if you're there, but I see
at least one of the other soaring luminaries you mentioned (BoKu, a.k.a. Bob K.) posting regularly in those forums.

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