Lift pins

That is certainly the case on my Nimbus 2 where the main spar is joined and secured with a main pin but can still essentially float around in the fuselage if it were not for the lift pins on the wings.
Hence the name "lift pins" - they carry the fuselage load.

Yes, I guess that's probably more correct, as the lift pins are mounted to the internal structure of the glider. (Generally, a steel tube cage that incorporates the landing gear and all other structural components.) The spar carry-through hole is primarily surrounded by composite structure only.

My bad.

FWIW:Â* Jim Marske's Composite Design Manual, page F7, says of the lift/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."

Having seen (mostly from afar) plug-in wing spar tongues that are pinned together in the fuselage, that last sentence about the joined main spar not touching anything surprised me.Â* I wish I could look at many fiberglass gliders to see if the industry "standard" (if there's such a thing) is to float the joined spar freely accross the fuselage, but I don't have that ability.Â* So I emailed Jim about the importance of isolating the spar tongues from any fuselage structure.Â* He answered, "I once wondered about the same thing.Â* Why is the main spar suspended freely across the fuselage?Â* The spar is quite flexible - even across the fuselage, especially at the fuselage CL.Â* So my conclusion was, why throw other unknown forces into a joint when you don't have to.Â* However, motion near the fuselage side rails is quite small as it is close to the dagger pins so you may get away with it.Â* Don't fasten the spar to the fuselage on the fuselage centerline."

That all sounds reasonable.Â* So I'd like to ask the knowledgeable folks on this forum:Â* 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?Â* 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*

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.

Salto? I think it's spars pin into a fuselage fitting.

What about Diana 2? I know the stubs stick out the fuselage and the wings have sockets. Don't know how it pins.

Interesting discussion.Â* Another question for Bob K. and other experts:Â* If you saw crosswise (cross section) through a typical sailplane's main spar tongue/stub what would it look like?Â* Mostly foam surrounded by many layers of fiberglass?Â* About how much glass versus foam?Â* Just trying to envision a vital structural component that most of us take for granted and never see the inside of unless one is a designer or fabricator.

On Saturday, July 27, 2019 at 1:02:23 AM UTC-4, Sky Surfer wrote:
Interesting discussion.Â* Another question for Bob K. and other experts:Â* If you saw crosswise (cross section) through a typical sailplane's main spar tongue/stub what would it look like?Â* Mostly foam surrounded by many layers of fiberglass?Â* About how much glass versus foam?Â* Just trying to envision a vital structural component that most of us take for granted and never see the inside of unless one is a designer or fabricator.

Two very strong beams(caps) at the top and bottom to handle tension and compression. Fillers(spacers?) between that handle the shear loads between the caps and also provide for strong pins that handle bending loads on many ships like Schleicher. Others(the "Glasflugel method) use pins on the ends of the spars that plug into sockets on the root rib of the opposite wing to handle bending.
In all cases there are many plies of cloth to tie the 2 webs together and handle shear loads. The "spacers" are commonly plywood or solid fiberglass for portions of high(pin)loads, and foam for the balance. Shear layers are most commonly glass.
UH

On Saturday, July 27, 2019 at 6:13:06 AM UTC-6, wrote:
On Saturday, July 27, 2019 at 1:02:23 AM UTC-4, Sky Surfer wrote:
Interesting discussion.Â* Another question for Bob K. and other experts:Â* If you saw crosswise (cross section) through a typical sailplane's main spar tongue/stub what would it look like?Â* Mostly foam surrounded by many layers of fiberglass?Â* About how much glass versus foam?Â* Just trying to envision a vital structural component that most of us take for granted and never see the inside of unless one is a designer or fabricator.

Two very strong beams(caps) at the top and bottom to handle tension and compression. Fillers(spacers?) between that handle the shear loads between the caps and also provide for strong pins that handle bending loads on many ships like Schleicher. Others(the "Glasflugel method) use pins on the ends of the spars that plug into sockets on the root rib of the opposite wing to handle bending.
In all cases there are many plies of cloth to tie the 2 webs together and handle shear loads. The "spacers" are commonly plywood or solid fiberglass for portions of high(pin)loads, and foam for the balance. Shear layers are most commonly glass.
UH

28 layers of glass/resin bind the spar caps and shear web of my Kestrel 19.

Frank Whiteley

I will state, I am not an expert on internals of a FRP spar.....
My knowledge.....mostly wood laminations (not including balsa at the root for spar) with glass or CF for ultimate strength.
Further out, yes, maybe balsa for reasonable strength and lighter weight. Engineering basic is...."the more you make 3 dimensional, the better...", thus Origamy works with thin paper.
There is always diminishing returns...eventually you get past one engineering segment and start to lose on others....


I will let real engineers (ME or aeronautical) to dispute my comments.

On Friday, July 26, 2019 at 10:02:23 PM UTC-7, Sky Surfer wrote:

Interesting discussion. Another question for Bob K. and other experts: If you saw crosswise (cross section) through a typical sailplane's main spar tongue/stub what would it look like? Mostly foam surrounded by many layers of fiberglass? About how much glass versus foam? Just trying to envision a vital structural component that most of us take for granted and never see the inside of unless one is a designer or fabricator.

Spanwise fibers for tension and compression in the spar caps at the top and bottom to react accumulated tensile and compression forces due to bending. Bias fibers in between to react shear. Localized reinforcements to react discrete loads at main pins and root rib. Some manufacturers use some wood, but not me.

The shear web between the upper and lower spar caps is usually a composite sandwich similar to the wing skin; for most of the span it's 6mm to 10mm PVC foam with one to three plies of bias cut (+/-45) fiberglass or carbon on each side.

The unidirectional fibers in the caps have in the past mostly been fiberglass or carbon fiber tapes or raw tows. These days carbon fiber spars are usually made of prepreg carbon and autoclaved. In my shop we use strips of a pultruded unidirectional carbon fiber product called Graphlite.

Composites usually have lower compression strengths than tensile strengths, so sometimes the spar caps are asymmetrical in depth, with a deeper section for the upper spar cap than for the lower.

At the spar stub, the spar is wrapped with several plies of fiberglass or carbon fiber to react shear due to the moment applied by the force couple between the attachments between the two wing spars.

The wing spar tapers in depth (top to bottom), to fit inside the tapering wing. The spar caps are also usually tapered in one or both dimensions to tailor their strength and stiffness to bending loads that diminish rapidly as you go from root to tip. In my gliders the spar caps are about 0.37" deep from root to tip, and taper in width (front to back) from 1.76" at the root rib to about nothing at the end of the 18m tip extension. Out there the loads are so low that the wing skin itself has enough strength to handle bending loads.

If you want to see this stuff in action, come by our workshop in Arnold CA. We are just now dressing out a pair of spars to go into the next wingset.

Thanks, Bob K.

OK....stupid question....most FRP works better in tension (from what I know) than compression.
Sooooo.....why is top layer of spar thicker than bottom (I will assume.....yes, I know what assume can mean....) since I will assume bottom of spar has more tension than top (which I will assume is in compression).

Not picking, just asking based on your post....