View Full Version : Lift pins
I have not yet found a description of "lift pins" that I understand but they apparently are pretty important.
Can someone please tell me what lift pins are?
If you try and fly without your Lift Pins, you won’t find ANY LIFT!
;>) JJ
Actually, the 4 lift pins attach the wings to the fuselage............rather important, I’d say!
Tony[_5_]
July 25th 19, 01:09 AM
Yea they are the pins that the fuselage hangs off of.
Scott Williams
July 25th 19, 01:24 AM
On Wednesday, July 24, 2019 at 7:09:32 PM UTC-5, Tony wrote:
> Yea they are the pins that the fuselage hangs off of.
I'll bet they look a lot like SSA Diamond pins:-)
OK- nobody really gave a decent description of the lift pins, so I will give it a try.
Your wings are held together inside the fuselage by large composite spars, either with an interlocking fork and tongue and dual main spar pins (like Schleicher) which carry the load of the glider in flight, or a fore/aft spar arrangement with one main pin, generally located at the center of the spar junction, in the middle of the fuselage. The carry-through load is handled by pins extending through the ends of the spar stub that fit into sockets at the root of each wing. This is a characteristic of Schempp-Hirth gliders..
Both design methods have been utilized by other manufacturers. Interestingly, Jonker Sailplanes use the two main pin configuration on the JS-1, while the single pin and lateral spar pin/socket design is used on the new JS-3.
Now, the lift pins. Since the main spar is designed to carry the load of the aircraft laterally, there must be a way to constrain the wing and keep it in horizontal alignment with the fuselage. This is handled by the pins extending from the fuselage ahead of and behind the main spar. (Although Jonker prefers to have the pins extending from the root rib and fitting into sockets in the fuselage.)
Essentially, the lift pins keep the wing in proper alignment relative to the fuselage while in flight, while the spars and their retaining pin(s) hold the wing together and carry the load of the aircraft.
Surge
July 25th 19, 06:23 AM
On Thursday, 25 July 2019 04:36:26 UTC+2, wrote:
> Essentially, the lift pins keep the wing in proper alignment relative to the fuselage while in flight, while the spars and their retaining pin(s) hold the wing together and carry the load of the aircraft.
I'm not an expert but I thought the lift pins (for and aft pins at wing/fuse junction) carry the full load of the fuselage on gliders. They're not there just for keeping things aligned.
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.
"while the spars and their retaining pin(s) hold the wing together and carry the load of the aircraft."....and transfer this load to the fuselage by AIR!! :) :)
> 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.
Sky Surfer
July 26th 19, 02:25 AM
FWIW:* 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).* These four pins transmit all loads from the wing to the fuselage.* 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.* 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.* So I emailed Jim about the importance of isolating the spar tongues from any fuselage structure.* He answered, "I once wondered about the same thing.* Why is the main spar suspended freely across the fuselage?* The spar is quite flexible - even across the fuselage, especially at the fuselage CL.* So my conclusion was, why throw other unknown forces into a joint when you don't have to.* 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.* Don't fasten the spar to the fuselage on the fuselage centerline."
That all sounds reasonable.* So I'd like to ask the knowledgeable folks on this forum:* 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?* Are there notable exceptions, like spar pins passing through not only the spar tongues but also through a fuselage bulkhead?* How about spar tongues that are pushed into a joining box or tunnel?* That would be the antithesis of isolating the main spar.* These are different but related questions to the original lift pins question.***
Bob Kuykendall
July 26th 19, 02:50 AM
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?* How about spar tongues that are pushed into a joining box or tunnel?* That would be the antithesis of isolating the main spar.* These are different but related questions to the original lift pins question.***
There may be such out there, but they must be pretty rare because I almost never see them in modern sailplanes.
--Bob K.
Tony[_5_]
July 26th 19, 03:59 AM
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.
I think the goal of the wings only touching at the lift pins simplifies the design's engineering. As things expand, contract, and flex there are fewer places where the forces are transferred.
I've always first explained glider assembly to new folks as the assembled wings are a structure all themselves that happen to get assembled passing thru the fuselage. Then mention the 4 pins hanging either the fuselage or wings depending
if flying or on the ground.
John Foster
July 26th 19, 06:39 PM
On Thursday, July 25, 2019 at 7:25:44 PM UTC-6, Sky Surfer wrote:
> FWIW:* 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).* These four pins transmit all loads from the wing to the fuselage.* 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.* 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.* So I emailed Jim about the importance of isolating the spar tongues from any fuselage structure.* He answered, "I once wondered about the same thing.* Why is the main spar suspended freely across the fuselage?* The spar is quite flexible - even across the fuselage, especially at the fuselage CL.* So my conclusion was, why throw other unknown forces into a joint when you don't have to.* 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.* Don't fasten the spar to the fuselage on the fuselage centerline."
>
> That all sounds reasonable.* So I'd like to ask the knowledgeable folks on this forum:* 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?* Are there notable exceptions, like spar pins passing through not only the spar tongues but also through a fuselage bulkhead?* How about spar tongues that are pushed into a joining box or tunnel?* That would be the antithesis of isolating the main spar.* These are different but related questions to the original lift pins question.***
On the Phoebus (1st fiberglass production glider), the pin goes through the spar of both wings at the centerline, but also through the bulkhead behind the pilot's head. But this isn't a structural connection to the fuselage though, I don't think. The main connection is through the lift pins, even on this early model. And the wing is VERY stiff, so not much flex at all.
Martin Gregorie[_6_]
July 26th 19, 08:11 PM
On Fri, 26 Jul 2019 10:39:48 -0700, John Foster wrote:
> On the Phoebus (1st fiberglass production glider), the pin goes through
> the spar of both wings at the centerline, but also through the bulkhead
> behind the pilot's head. But this isn't a structural connection to the
> fuselage though, I don't think. The main connection is through the lift
> pins, even on this early model. And the wing is VERY stiff, so not much
> flex at all.
>
Sailplanes 1945-1965 by Martin Symonds says that the Phoebus was closely
based on the Phonix (the first glass glider) and has a good drawing of
the wing mount (page 253).
This shows sharply tapered wing stubs that fit into a close fitting
structure built into the fuselage. The wings were pulled together by a
single, double-threaded bolt with opposite threads on each end and driven
by a (hex?) flange in the center. The axis of this is at right angles to
the fuselage centre line, so rotating it pulls the wings together and the
stubs into close contact with the matching fuselage structure.
There are two lift pins on each side of fuselage, which appear to lock
into the wing stub support structure. The pins project outwards to fit
into sockets on the root ribs.
--
Martin | martin at
Gregorie | gregorie dot org
John Foster
July 26th 19, 08:47 PM
On Friday, July 26, 2019 at 1:11:43 PM UTC-6, Martin Gregorie wrote:
> On Fri, 26 Jul 2019 10:39:48 -0700, John Foster wrote:
>
> > On the Phoebus (1st fiberglass production glider), the pin goes through
> > the spar of both wings at the centerline, but also through the bulkhead
> > behind the pilot's head. But this isn't a structural connection to the
> > fuselage though, I don't think. The main connection is through the lift
> > pins, even on this early model. And the wing is VERY stiff, so not much
> > flex at all.
> >
> Sailplanes 1945-1965 by Martin Symonds says that the Phoebus was closely
> based on the Phonix (the first glass glider) and has a good drawing of
> the wing mount (page 253).
>
> This shows sharply tapered wing stubs that fit into a close fitting
> structure built into the fuselage. The wings were pulled together by a
> single, double-threaded bolt with opposite threads on each end and driven
> by a (hex?) flange in the center. The axis of this is at right angles to
> the fuselage centre line, so rotating it pulls the wings together and the
> stubs into close contact with the matching fuselage structure.
>
> There are two lift pins on each side of fuselage, which appear to lock
> into the wing stub support structure. The pins project outwards to fit
> into sockets on the root ribs.
>
>
> --
> Martin | martin at
> Gregorie | gregorie dot org
The Phoebus does not have this "feature". The tips of the spars have pins that mate to bracket holes in the wing root rib of the opposite wing. The left spar is forked and the right spar is a single tapered spar that fits between the forks of the L wing spar. Then there is a single pin that goes through all three and holds them together. The wing root ribs also mate to two lift pins on each side of the fuselage.
Dirk_PW[_2_]
July 26th 19, 09:20 PM
What is interesting/fascinating to me is that the fuselage is hanging from the root ribs (via the lift pins) only. That means the entire load of the fuselage, pilot (times 7.5G design loads) are being restrained by the root ribs (one rib per side). I'm just amazed what little material is there to transfer the loads to the rest of the wing structure. I would expect those poor ribs to be ripped out of the wing at high G loads. But alas they don't.. Pretty amazing design actually.
Dirk,
Those are not,”poor little root ribs”, in fact the designers have reinforced them with several layers of cloth running from the spars deep into the root ribs on both sides of the spar, so that they will transfer the fuselage load into the wings.
JJ
On Friday, July 26, 2019 at 5:50:02 PM UTC-4, wrote:
> Dirk,
> Those are not,”poor little root ribs”, in fact the designers have reinforced them with several layers of cloth running from the spars deep into the root ribs on both sides of the spar, so that they will transfer the fuselage load into the wings.
> JJ
Wing skins also play a meaningful part in handling the loads.
UH
Sky Surfer
July 27th 19, 06:02 AM
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.
On Saturday, July 27, 2019 at 1:02:23 AM UTC-4, 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.
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
Frank Whiteley
July 27th 19, 07:28 PM
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.* 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.
>
> 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
Charlie M. (UH & 002 owner/pilot)
July 27th 19, 07:38 PM
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.
Bob Kuykendall
July 29th 19, 06:06 PM
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.
Charlie M. (UH & 002 owner/pilot)
July 29th 19, 11:27 PM
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....
On Monday, July 29, 2019 at 6:27:07 PM UTC-4, Charlie M. (UH & 002 owner/pilot) wrote:
> 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....
More cross section is required to resist buckling.
UH
Bob Kuykendall
July 30th 19, 12:06 AM
On Monday, July 29, 2019 at 3:27:07 PM UTC-7, Charlie M. (UH & 002 owner/pilot) wrote:
> 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...
As Uncle Hank says, the upper spar cap is generally in compression, and the lower cap is in tension. The compression force in the upper spar cap is about the same as the tensile force in the lower cap. But since composites are generally weaker in compression, you might want to give the upper spar cap more cross-sectional area so that the stress (force per unit area) is lower. Or, another way of looking at it is that you are leaving some unnecessary material out of the lower spar cap since you don't need as much there to react the tensile forces.
The result, of course, is a set of wings that can support a greater load factor in normal flight than while inverted. But since most gliders are neither intended nor used for high-g inverted flight, it's a reasonable compromise, especially when you're dealing with relatively low strength-to-weight materials like wood or fiberglass.
With my gliders, I use the same amount of carbon in both the upper and lower spar caps. It makes it easier to fabricate the wing spars, and with the high-strength Graphlite material the weight penalty is negligible.
Thanks, Bob K.
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