Designing large flat but thin, lightweight and rigid panel

I know this is not about boat-building per se but the expertise is directly applicable and your help is appreciated:

We are building 23' X-wing fighter model to raise money for a local school STEM program and need help with the fabrication - we have little experience in this. We are done with the cabin (see video here:X-Wing Fighter on Twitter https://twitter.com/twitter/statuses/920297413197393921 ) and fiberglassed the body but need to fabricate the wings.

The wings are roughly 8 x 4 feet and ideally 2" thick but can go up to 2.5". They are all square with right angles and same thickness throughout. Can we fabricate a fairly rigid wing this thin that will be relatively light and support its own weight plus the "cannons" (see the picture below) at the ends? They will be attached to the body and will move up and down but will not have to endure any stress.


Here is more info about where we are:
Used polyester resin for the body but getting epoxy for the wings to avoid contraction and ensure better strength.
Used 5/32 cardbox (48 x 96" 200 lb. Corrugated Pads S-3194 - Uline https://www.uline.com/Product/Detail/S-3194) as a core but was not impressed with the strength even after getting one layer of cloth from one side and 3 layers from the other. (adding mat mostly seemed to add weight). So, the cardboard may be fine for the body but may not be strong enough for the wings.
Planned to use coroplast (corrugated plastic Search Results for coroplast at The Home Depot https://www.homedepot.com/s/coroplast?NCNI-5)- as a core for the wings but neither fiberglass nor epoxy seem to adhere to it very well.

Questions: what is the core material we should use and how this should be fabricated? Again, we are trying to keep the cost down (more money for the school) and the wings only need to support themselves and the "cannons" at the end. I was thinking of making 2 inch strips of core and then epoxying them together with fiberglass cloth in between. Then using these as "beams" to go between two flat panels (which would also be laminated from both sides with fiberglass. I have a gut feeling that this may not be the best technique and any guidance will be great.

thank you in advance
Mike

P.S. Here are some of the other core materials we have considered:
1 in. x 4 ft. x 8 ft. R-6.5 Foam Insulation-268417 - The Home Depot https://www.homedepot.com/p/1-in-x-4-ft-x-8-ft-R-6-5-Foam-Insulation-268417/100576728
Owens Corning FOAMULAR 1/2 in. x 4 ft. x 8 ft. R-3 Squared Edge Insulating Sheathing-36L - The Home Depot https://www.homedepot.com/p/Owens-Corning-FOAMULAR-1-2-in-x-4-ft-x-8-ft-R-3-Squared-Edge-Insulating-Sheathing-36L/100320356
Hardboard Tempered Panel (Common: 3/16 in. x 4 ft. x 8 ft.; Actual: 0.155 in. x 47.7 in. x 95.7 in.)-832780 - The Home Depot https://www.homedepot.com/p/Hardboard-Tempered-Panel-Common-3-16-in-x-4-ft-x-8-ft-Actual-0-155-in-x-47-7-in-x-95-7-in-832780/202404545

 

Use styrofoam and laminate with epoxy.

 

Just one piece of 2" styrofoam with fiberglass cloth and epoxy around it? Nothing inside for additional support/rigidity?

thanks

 

Yes. It will be enough for a small part like you are building.

 

In spite of what the "experts" think (they will not lose anything if the wing fails), it is always advisable to perform some calculation that assures a priori that the wing will support the loads to which it must be subjected.

In another thread :

I agree.

 

Amateur .02, beware..

It's expensive however something like a 5 lb Corecell, various types of Airex foam cores, etc.. would be what the application would call for normally. There are other generic brands out there for less $$, I think $100 a 4X8 sheet might be obtainable if you have a local supplier to cut you a deal (and they should for a school, FYI there is a huge markup on this stuff if you are a boat builder versus retail customer). Foam insulation is simply not designed to be structural, although it would be to some extent.

With the real structural stuff you can get away w/ polyester resin, indeed to some detriment of the strength since epoxy or even vinylester is stronger, but likely the cheapest polyester over a true foam core is going to outperform cheap foam with the best 2 part epoxy resin.

Type of fiberglass used is another consideration - this is an issue with polyester resin, you can't just laminate with cloth which has the best strength to weight, you need to use some mat for the mechanical bonding surfaces, which increases weight, however provided the weight is within your design considerations it may not matter. The best thing obviously would be epoxy over structural foam, it just may not be needed.

Plascore would be another option on the core now that I think about it, less expensive than foam core, only issue is more a longevity problem with being able to hold water in a marine application - not a problem you would have.

Jon

 

I hate the use of styrofoam for structural parts.
But it will work just fine for this.

It's cheap enough to just try it.
Use epoxy to glue 1" sheets together, then laminate6 oz cloth around the "wing".
You might want some wood on the inboard and outboard ends to help with attaching the wing to the body and to the "cannon".

Don't use polyester - it will melt the foam.

 

You can use the foam over frame technique and the hot wire cutting method the way model makers use. It is good enough for experimental aircraft.

Since your wing has some weights at the end and will have a tendency to droop, add a built up wooden box spar, (tapered of course) to increase rigidity.

If you use styrofoam, you need to skin it it with fiberglass and epoxy. Skin is about 2 layers thick, less than 20 mils in thickness.

Try your local library for an in depth method of building model airplanes.

 

Personally, I'd prefer extruded polystyrene for it's stiffness over Styrofoam. As for the glass; I don't have a good recommendation through experience, but a guess at 12oz biax. Has to be epoxy or you'll be doing a chemistry experiment instead. Perhaps one of these other fellows can advise a glass that will stiffen the 1" board well (from experience).

I like the Home Depot R-6.5 Super Tuff product you cited a lot for your plan as long as it doesn't have metal or a plastic film on it. Price is unbeatable versus a marine grade core. Sandwich stiffness will be a bit better vs 1/2". I'd do a weight calculation of how much the cannon will weigh and then get back here for the glass recommendation. Definitely a big fan of the 1" extruded. You can also use this product as well. It might be a good idea to make sure you wet the foam with your epoxy before laminating to make sure you don't get a dry joint delam failure, but it'd probably work either way for you. If you build the boards big; you can use a jigsaw blade to cut to size, or you can precut the foam and glass and trim the glass.

Foamular 150 1 in. x 4 ft. x 8 ft. R-5 Scored Square Edge Insulating Sheathing-20WE - The Home Depot https://www.homedepot.com/p/Foamular-150-1-in-x-4-ft-x-8-ft-R-5-Scored-Square-Edge-Insulating-Sheathing-20WE/207179253

 

You might be able to "hot coat" some types of foam insulation to serve as a barrier to prevent the layup (which will have much more resin involved) from eating into the foam.

The method would be mix polyester resin with max hardener, allow it to sit some in the pot until it's nearing gel time, then roll it on to pre coat the panels, and let it cure. If you can get a layer of resin on there without significant damage to the foam you can then laminate without doing any more damage.

I assume the whole consideration here is cost, since otherwise you'd be using a structural foam which is resistant to the resins, so using 2 part epoxy is not going to help cost effectiveness either.

Jon

 

Thank you all very much. This is both extremely helpful and overwhelming at the same time - I guess there is no singular way of doing this. Yes, the "pro" cores may be little expensive but palatable (at $100 per sheet) if they were available locally. Here is the plan and takeaway points:

Will need to do little more testing with mock-ups based on your suggestions above - starting from the 2 insulation foams glued with epoxy and then wrapped in two layers of cloth (with epoxy) Foamular 150 1 in. x 4 ft. x 8 ft. R-5 Scored Square Edge Insulating Sheathing-20WE - The Home Depot https://www.homedepot.com/p/Foamular-150-1-in-x-4-ft-x-8-ft-R-5-Scored-Square-Edge-Insulating-Sheathing-20WE/207179253 and see how it holds up. Will use box spar for further reinforcement since the "canons" at the ends will put uneven weight on the wings (they stick forward from the wing) and will need to make sure the wings do not twist too much.

Again, really appreciate you input and will report back on the progress.

 

Wet the foam first for a much easier and better wetout; then use a 48"+ roller tube to roll the glass on. Once wetted; use laminating roller to remove air. I use 3" pvc, but you can get a suitable tube from the glass vendor. A regular paint roller works to distribute the epoxy, but tends to pull fabric at the end.

 

Just build the stick frame out of wood as illustrated, assemble the "sticks" together and work out the mechanism for demo. Once satisfied, cover with foam, shape, disassemble, cover with fiberglass, then finish. I use the eglass double bias DB7781. It is an industry standard. Sort of a workhorse fabric. About 10 mils per layer.

Styrofoam is cheap but epoxy is expensive. Foam resistant to polyester is expensive but poly resin is cheap. If you can't identify foam, buy a test piece and see if poly will attack it. Work out the materials consumption in advance and compare cost to build. You don't need aerospace grade epoxy nor marine standard poly. Buy the reasonably cheap ones.

 

Do not use polyester or vinylester on styrofoam. It will dissolve it, regardless of how much hardener you add. Since this is not needing to fly, you can simply paint with latex over the foam, let it dry and sand it to whatever finish you need.

 

Why does anyone say "I don't know" and then give a recommendation?
Why would you say "I don't know what layup to use" and then say 12oz biax?

Guessing is not helping. Especially the suggestion about using polyester - when it is well know that will eat the styrofoam?

Again why is there a discussion on 1" foam when the OP said 2" - 2.5"?

A forum is suppose to help, not spread guesses and mis-information.