Designing large flat but thin, lightweight and rigid panel

How badly it gets eaten is dependent on how much resin is on there and how long it takes to cure, if you damage the top 1/32 of the surface of the foam it likely would be of no consequence.

I've certainly never tried to fiberglass over it (not much sense in this generally I don't care what you are doing ), but I use the lowes sort of foam for all sorts of things when working on a boat (patterns, closing off areas to hold heat in, catching drips if there is a scrap sitting there when I want one, protecting areas from scratches) and on a number of occasions I've had bits and pieces of wetout PE fiberglass laminated to it unintentionally during construction. I've also seen it get dissolved pretty good if you dump resin onto it, but I do not think I've ever seen even the 1/2 inch stuff actually dissolve straight through under any circumstance.

Even the structural foams for boat building come with inherent possibility of getting a little dissolved by styrene if you do something unusual, at least one manufacturers write up I read said that.

Jon

 

Well, the OP suggested 1" himself firther down.

And we don't know the weight of the cannons; so the glass I cited, which is similar to the glass rx cited is a best guess. However, if you want to get upset with someone recommending polyester and styrofoam; I'm with you.

 

wI'll the model be seasoned above public?

Will public be allowed to touch or climb on it?

Will it fit through door?

What is its expected lifespan?

What is the anticipated weight of "cannon"?

 

Styrofoam boats do OK

 

I have customers that do this stuff all the time.

You can get low density urethane foam at a box store, it's yellowish and frequently has foil on it, it can be had in 2". I think Atlas makes it, it's compatible with polyester.

A 2" thick foam core will be stiff enough for a wing even with the stuff mounted on the end of it, one layer of 1708 would be strong enough, or a layer of CSM with cloth. It won't sag.

More glass will make it more durable if it needs to take some abuse.

There are polyester primers that can be used over EPS foam if needed, then you can go with a polyester laminate over it.

There are polyester high build, easy sand primers that can be used over the laminate to achieve the final shape and detail desired. You can paint right over it.

 

What are the dimensions of the wing? Chord at root, chord at tip, span. How heavy are the "cannons"?

 

Following the spirit of trueness posted by Upchurchmr, I took some time to analyze the design of the X wing fighter model. Rather than guessing at layups and foam use, I worked on preliminary design. this also to assure myself that I am not just guessing on my posts.

Following the imaginary dimensions of the x wingfighter, it is 12.2 m. (40') long and 8.4 m (27.5') wide.

The wing spans 10' long from root to tip (minus cannons) and as the OP said the wings are about 8 ft long, I assumed he is building to 80% scale. From a model drawing, the wing is a trapezoid, 7' wide at the root (fuselage), 4.2' at the tip with a half span of 8 feet.

Since I am interested only in the wing loading, I simplified my initial calculations to 16' total span. Found a supersonic thin airfoil with about 6% thickness of the chord (not that it will work in "hyperdrive" but just to give some semblance of "engineering"). This gives me 5" thickness at the root and 3" at the tip. This just preliminary so I adjusted it to 4% thickness which would give me a 3.4" and 2.0" thickness. Very thin wing but creates havoc on stresses.

Using a 3 lb/ft3 styrofoam core and 1 layer of 8.8 oz/yd2 of glass, the weight of the finished wing is about 67 lbs total with about 15% of that in primers and paint.

The skin laminate is about 0.010" per layer. Using 33,000 psi ultimate strength of an Eglass/epoxy laminate, and a 3.4" height at the core, the required thickness is 0.009" at the upper skin laminate in tension. The bottom (in compression) requires 0.015" of laminate. This requires two layers of laminate. The 2 layers is required only at about 30% of wing span as going toward the tip, the stress is reduced. Of course, when the "cannons" are attached, a concentrated load cantelever calculation must be superimposed on the design.

The 3lb styrofoam has a shear strength of 22 psi. With a wing load factor of 2 plus a FoS of 2, I get a Vshear of 67 psi. This way above core shear of 22 psi the styrofoam core can handle. To solve this, a box spar is required to handle the load and shear stress and let the skin handle the torsional stress.

 

Great - real information.
Much better and more informative than my "guess" that Styrofoam will work at 2.5".

I still don't really like Styrofoam.

We once made a styrofoam/ glass model of a the wing trailing edge for a full sized model at work.
The shape was a long tapered triangle in cross section.
There was a requirement to survive some wind loads, so we put sand bags on the finished part.
It survived, but the tip of the trailing edge curled up, contrary to the expectation.

Our standard analysis for a core structure was based on equal modulus of stiffness - aluminum honeycomb and aluminum skins.
Apparently the significantly lower stiffness of Styrofoam verses the glass changed the actual deformation under loading.

Pursuing this later we build a flat panel of Styrofoam and glass sandwich.
Under bending load it was observed that the deflection was "almost" the same as two separate skins without the foam.
The styrofoam deflected so much that the skins took "almost" the entire bending load.

Subsequent tests with marine grade foam core showed that the higher core stiffness was much closer to the deformation expected of a core structure (historical examples of aluminum and aluminum honeycomb), but the skin still provided some individual bending support. Normally you would expect the skin to be so thin that it only provided tension in the top skin and compression in the lower skin (loaded with the weight on top).

I'd suggest anyone using Styrofoam over design the skins, or better yet, do a test - then adjust the design.

Sorry for the long post.

Regarding the proposed design, it might be easier/ cheaper to just put some wood webs in the middle of the foam to take the shear stress and still use the foam to get skin support for the large area.

 

Upchurchmr- I chose the 3 lb styrofoam over the 2 lb because the 2 lb is sensitive to heat. Aggressive sanding will create heat and deform it. Even when glassed over, exposure to sun will shrink it. 2lb is great for flying models though as it is light. 4lb is better all around for static model.

That curling up on the trailing edge tip is due to inability of the foam to stay rigid. I keep it blunt and square, something like 1/8" to 3/16" and double the glass. Balsa trailing edge solves the problem. Styrofoam likes it thick.

I will post a built up wood web/box frame if needed.

 

Actually the curling was due to the shape and the softness of the foam. The triangle cros-section meant the skins were joined at the aft point. The top skin bent a great deal due to the sandbags being directly in contact. The lower skin didn't bend as much due to compression of the foam, and the aft tip position was a combination of the two skins bending.

Agreed, the square section should have no problem. I just used that to say why I don't like styrofoam. I shouldn't have got on a tangent.

 

Is this what is happening? Attached. That is the property of a low modulus plate when subject to bending forces. The outer edge curls.

The OP seems to have vanished but I wanted to know if a spar is needed.

I made an error on the shear stress. It is still in lbs not psi. To get shear, Vload must be divided by area to get psi o the height x shear stress of foam to get shear. The 22 psi of foam will suffice but I went ahead and tried to find out if hanging a "cannon" on the tip will require a spar.

With a 6 lb cannon on the tip, I get a constant Vshear load along the height of the wing. The calc shows me that a 0.30" width of 22 psi foam will suffice. Since the foam is wider than that, the 22 psi foam can take the concentrated cantelever load. The caps of about 4" wide (top and bottom) is needed along the span at the spar position. I chose 2 layers of unidirectional fiber as this gives me the greatest strength of 66,000 psi. That takes care of the cantelever load.

Because the wing tapers at the leading and trailing edge, reducing the EI (modulus), curling will occur under load, the leading edge must be reinforced with an additional layer and the triangular trailing edge replaced by a higher modulus solid wood. With a more solid leading and trailing edge and a spar cap, the wing can now handle torsional load and the skin laminate can be a biax 2 layers thick. This will handle lifting where the stresses are reversed.

Final lamination will be;

2 layers UD (0 degree) fiber 4" wide for spar caps (top and bottom) located about 40% from leading edge.

I layer extra 6" wide BD (0/90) fabric on the leading edge.

1 layer extra (0/90) on the trailing edge to cover the triangular wood.

2 layers Biax (+45/-45) top and bottom skin.

 

No, the anticlastic bending of honeycomb would be completely different.
The piece I was working on was just a test shape for RCS reduction. There were no flying loads, just the need to survive on a test stand in the wind.
The test also did not replicate wing bending loads.

 

Dear all, thanks again for all the responses. Once again, it is overwhelming and extremely helpful. I am trying to digest (and "translate" to a layman terms) all the recommendations above while, at the same time, exploring supplies available to us.

The scale is 0.61 and the dimensions are below in the picture. While the wing total is >9', initial portion along the engines will be supported with welded steel frame and affixed/hinged to the body; the "hanging" portion will be about 8'.

Resin:

• To avoid any shrinkage or other issues, already ordered epoxy (so, no polyester - which should make things easier).

Core:

• The easiest foam to obtain seems to be Foamular sheathing from home depot (Owens Corning 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/Owens-Corning-FOAMULAR-150-1-in-x-4-ft-x-8-ft-R-5-Scored-Square-Edge-Insulating-Sheathing-20WE/207179253). HD has them in 2in thickness
  
• ondarvr mentioned yellowing polyurethane foam but could not find it. But, the Foamular should be OK since I am using epoxy?

Cloth:

• We purchased 5.8oz fabric (#3733 E-GLASS FIBERGLASS CLOTH 60 from Aircraft Spruce http://www.aircraftspruce.com/catalog/cmpages/3733.php).
  
• More than one person here recommended getting the heavier 7781 cloth at 8.95oz (#7781 FIBERGLASS CLOTH from Aircraft Spruce http://www.aircraftspruce.com/catalog/cmpages/7781.php) and may get this instead? (we can use the 3733 elsewhere and it will not be wasted)

Box:

• It seems that the wing will need a box. Wood was one option but we could buy 2" wide 1/8" thick aluminum and weld them together. Will this be a better alternative to a wooden box? Welding will be no problem
  
• Also, do we need to create just a box along the perimeter or something more complicated?

"Cannons"

• Since all of this is so overwhelming, we have been trying to "cross one bridge at a time" and have not thought much about the canons yet (which obviously is a terrible strategy, considering how much it may impact the wing design). The plan is to make it as light as possible and PVC would be too heavy. One idea was to cut PVC pipe in half and use the inside as a mold for two half circles and then glue them together. Finding long ultralight tube would make things 100-fold easier.
  
• The "cannons" are 10' long from the leading edge of the wing (could make them 8' if needed and probably nobody would notice). Thicker portion of the cannon tube (the gray part) is 4.75" and thinner portion towards the front is 2.3".

 

I don't really like EPS foam for longterm use due to the things mentioned above, but you can overcome low density foam with a few modifications.

Use two pieces of foam for the longer spans with the seams running lengthwise, cap each piece of foam with glass, use a rebate (shave it down a bit to allow for the thickness of the glass) so after glassing it's level with the surrounding area. Each piece of foam has the edge caped like this ][ , the foam fits into each side of the channel, when these are butted it makes for an I beam, when the wing is glassed over it ties everything together. This should make the wings strong enough without much effort.

Go to the craft store, you can get round lengths of foam that you can coat with epoxy after you shape them.

 

You can simply build a frame or skeleton in wood and some metal brackets and then glue styrofoam over it.