Lift pins

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....

On Thursday, July 25, 2019 at 7:25:44 PM UTC-6, Sky Surfer wrote:
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 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.

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

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.