Wing Spar - design / bonding pultrusions

[Foundationer]

I have a quick question: is the attached basically the best / most foolproof way of doing things?

Red - Pultruded carbon strip spar cap already bonded.
Green - PVC Foam
Black - continuous wrap of carbon overlapping for an inch or so in the middle of the shear web.

And also -bonding pultruded strips. Is the process basically:
1. Cut to length, sand ends to make smooth step down
2. lightly Key with abrasive
3. wipe with acetone
4. Pure epoxy or epoxy and flox? Which is best?
5. Vac down (in an aluminium angle with peel ply both sides?)

 

[BoKu]

I wouldn't bother beveling the foam.

How much overlap you use should depend on how much shear strength you need. You can overlap the full height to get three plies worth of shear.

The weight penalty for using fatter sticks is trivial. The expensive things here are labor hours and the risk of making a bad part.

 

[Vigilant1]

Does epoxy reliably get into the gap between flat pultrusions when using infusion or when applied after the flats are stacked? Or is it necessary/better to manually apply a thin layer of epoxy between the pultrusions, then bag and vacuum?

 

[Victor Bravo]

The beveled foam in your drawing actually weakens the spar in my opinion. The edges of the spar cap that are furthest away from the spar web (right side in your drawing) are somewhat unsupported, and as the spar is loaded I believe this will apply bending ("folding") loads on the thin webs where the beveled foam ends and the web is nearly 2 dimensional. You would be far better off using rigid foam core that is the full width of the spar caps, wrap the spar and webs together, and have essentially a "box beam" instead of a C-channel.

As always, this is the uneducated opinion of a decrepit old model airplane builder without an engineering degree and without ANY formal education in structural beams.

You're in luck, however... one guy who knows this stuff pretty well and builds composite airplanes is already looking at this thread, and at least one other guy who knows this stuff way more than even the first guy (and is also building a composite airplane) will hopefully check in on this discussion soon.

 

[wsimpso1]

I think VB is talking about BoKu and me... BoKu has airplanes in production. Listen to him. I am more nearly academic. If I live long enough I will fly the bird out in my shop...

When you bevel the foam, the shear web can become a hinge as was pointed out above. In addition to that the shear web right at the spar cap is the place where loads in the web are biggest. Adding the "hinge" deformation to the rest of the hard life the web material has right there sounds like really bad juju.

Some folks spend some effort interleaving some plies of shear web through the cap lamination... A man of no less import than Jim Marske trains folks to do this.

The other thing is that you will have hardpoints in the web someplace, and you will substitute something like phenolic plate for the web. Nice beefy cores at those spots make the transitions easier. Web cloth folding up and over bumps is also bad.

As to VB comment about making a rectangular beam, the best reason to do that is when a broad (chordwise) spar is being included to carry big torsion. These sorts of beams end up kind of overbuilt for other tasks, and that is extra weight, but they usually make that trade to avoid something else that they think would cost even more weight or other issues.

The nice channel spar is my preference for several reasons. The big one is that they are easier to make well - straight and well bonded together - and that is important. A poorly built and/or aligned spar can become a disaster many different ways. The other reasons for a channel are almost esoterica: Your thickest web lamination is inside the caps, while your thinner one is outside. In a full rectangular spar, all of the web plies go outside, pushing the caps inboard more than on a channel. Many only put two plies outside. This allows the caps to be further from the neutral axis, and so may allow lower weight. If you are of the persuasion that some web plies should be interleaved in the cap strips, it is easily done with channels too.

How do we carry all that torsion? You have to hang the main landing gear (yeah tricycle), flaps, ailerons and the like on a drag spar anyway. And in my case, I also have to close the fuel bays and have faired slots for the trailing edge devices. Might as well run the drag spar to the fuselage and carry torsion between the two spars...

Billski

 

[TLAR]

Have a look at krsuper2.com
Disregard the choice of foam, just the method that Scott uses.
Look for “the manual”

 

[Geraldc]

Top and bottom carbon caps tapered to wingtip.If you can get a 3/4 by 3/4 inch solid pultruded rod then cut a taper to get 2 lengths.
Full width foam with hard points where needed.Wrap with biax with more layers at root down to one at wingtip.Been done successfully before for a light strong spar.

 

[Lendo]

Geraldc,
Have you used that method, I would be interested to know if it works as I dismissed that approach due to difficulty in maintaining even distribution of Fibre
Fraction.
Due to (Reported) Resin Starvation with Flat Rods under Vacuum, I would include a light scrim between them.
I am currently working on a method of Filament Winding Carbon Caps, but there are issues with that method as well - maybe one day!
George

 

[stanislavz]

I have a quick question: is the attached basically the best / most foolproof way of doing things?

1. Do you have Jim Marske book ?

Photos is better then words some times :

https://www.ihpa.ie/carbon-dragon/index.php/cd-builders/phil-lardner-s-cd

And in LAK - they do not use foam to shear web at all. Just some strips of pultrusion...

 

[Foundationer]

So something more like the attached? 10mmx1mm pultrusions pre bonded for the cap, 10mm thick core running full depth and all wrapped in a single bit of carbon (more wraps of carbon where needed).

I'd not seen the KR Super 2 website before - that's got some interesting stuff!

 

[TLAR]

So something more like the attached? 10mmx1mm pultrusions pre bonded for the cap, 10mm thick core running full depth and all wrapped in a single bit of carbon (more wraps of carbon where needed).

What are you designing?

Scott Watts spent a bunch of time documenting his build and I got an education off his site. Wish he could have finished it, but who knows, he might be flying it now

I'd not seen the KR Super 2 website before - that's got some interesting stuff!

 

[Foundationer]

And this for the spar overlap - full width foam with ply hardpoints with brass bushings all wrapped in a lot of carbon to take the large shear loads between the pins.

Assemble wings on jig, line everything up, drill slightly oversize, insert bushings with a lot of mumpe and pins in place, wait to cure, perfect alignment?

Re. what I'm designing - 300kg unregulated microlight that's supposed to be quite slippery / efficient.

 

[TLAR]

And this for the spar overlap - full width foam with ply hardpoints with brass bushings all wrapped in a lot of carbon to take the large shear loads between the pins.

Assemble wings on jig, line everything up, drill slightly oversize, insert bushings with a lot of mumpe and pins in place, wait to cure, perfect alignment?

Re. what I'm designing - 300kg unregulated microlight that's supposed to be quite slippery / efficient.

Use Phoenolic instead of brass as it will chemically bond
Install bushings during layup, line up and then finish ream

 

[wsimpso1]

So something more like the attached? 10mmx1mm pultrusions pre bonded for the cap, 10mm thick core running full depth and all wrapped in a single bit of carbon (more wraps of carbon where needed).

I'd not seen the KR Super 2 website before - that's got some interesting stuff!

Let's keep in mind that the spar is not some stand-alone bit of kit.

Through most of the wing it is bonded to the wing skins and an occaisional rib. Through the wing, it then includes a glue line between caps and wing skins. Make the spar narrow, as shown and the weight of adhesive is reduced, but narrow flanges mean more rods in the caps. So there is a trade-off here between spar weight and adhesive weight, with a region in the design space where total weight is minimized. Compounding all of this is that internal shear loads in the spar must be carried everywhere, and a little more flange width can help with that job.

Near the root, the spars usually do stand alone and penetrate the fuselage in some way . They have to avoid elastic instabilities through here - fortunately, a cantilever wing airplane's spar tends to be pretty beefy near the fuselage just to carry shear and bending there.

Then there is the illustration of one ply going all the way around the spar. You will have a heck of a time achieving thatwhile making it all lay down without wrinkles. Much more workable is to run the shear web lamina you need inside the channel, onto the caps and a couple plies onto the edges of the caps in one layup, then 2 or 4 plies of shear web lamina around the outside, over the caps and onto the edges of the caps. This gives you two nicely conforming laminations, shear webs on both sides of the wed core, and a couple plies over the edges of the caps to prevent delam or splits in the caps over time.

Billski

 

[Geraldc]

Have you used that method, I would be interested to know if it works as I dismissed that approach due to difficulty in maintaining even distribution of Fibre
Fraction.

This is the method used on Gazaile and other French airplanes.
They use timber strips between the caps and foam to help bonding.

Sujet 1

 

[Foundationer]

Use Phoenolic instead of brass as it will chemically bond
Install bushings during layup, line up and then finish ream

Do you mean phenolic instead of the ply?

 

[BoKu]

We use bronze bushings that we bond into Garolite reinforcing pads. We prepare the bushings by "threading" the OD on a lathe for good mechanical bond, then degreasing them. We inject the bonding paste under pressure to ensure that there are no voids.

 

[wanttobuild]

Foundationer

I am reluctant to advise you about your aircraft. Composites are great provided you know the fundamentals.
I would say to look at the "manual" on the super2 site, items underlined are complete.
the spar section shows a complete reinforcement schedule and step by step instructions. don't use lastafoam.
I am not trying to be rude, just misunderstandings could be a bad thing.
review the super2 spar construction and then lets talk about it

 

[Foundationer]

Thank you everyone - super-helpful as always. Adding about 200g of foam makes my whole spar design really, really simple to build.

I really love the Gazaile 2 but the way the wing joiner hardpoints are made always surprised me - obviously it works, and is really simple but it seems sort of wrong:

http://gazaile2.nmr7.free.fr/images/Ailes/longeron/gd%20format/l30.jpg

 

[Vigilant1]

Thank you everyone - super-helpful as always. Adding about 200g of foam makes my whole spar design really, really simple to build.

I really love the Gazaile 2 but the way the wing joiner hardpoints are made always surprised me - obviously it works, and is really simple but it seems sort of wrong:

http://gazaile2.nmr7.free.fr/images/Ailes/longeron/gd%20format/l30.jpg

I've read frequently that it is hard to get a lasting high-strength epoxy bond with aluminum (oxidizing of the AL over time, differential thermal expansion of AL and CF laminates that causes loosening, etc). But, we do have examples of less severely stressed parts (AL skins to each other and ribs of AL or PVC foam, etc) that have worked well for decades.
Maybe that epoxy pushing through the holes is some part of the Gazaille secret sauce.