I am considering using foamboard or something like it for ribs.  Has anybody already done this and have a suggestion as to thickness.  Weight - is this heavier than 5lb balsa?

Ken

Steve,
Thanks for posting that, I have thought of using the foam board and stripping off the paper, your method of building the box then hot wire cutting the core looks like a winner. Maybe Bob Hunt can comment on how to set this up for a stunt wing with a spar and bellcrank. Could solve our balsa crises?

Best,   DennisT

I am exploring using Bob's lost foam jig and replacing just the ribs with blue foam.  I have found references to blue foam sheets as thin as 1/8" and some 1/16 and 2mm foam board but I am having a devil of a time finding where to buy it for less than balsa.  I have a feeling that blue foam with a paper backing put on with thin finishing resin *might* give you equal strength to balsa but probably not the flex strength.  I am also exploring thin pvc foam.  It is readily available and cheap compared to balsa but I have no idea about the weight.  Being an old fart in a brave new world really sucks! 

Bill Smith and Mick Castell from the UK made a tutorial on using blue foam for the bulk of the wing construction. Clever self jigging system with foam ribs, carbon tow reinforced spar and balsa sheeting. 
https://web.archive.org/web/20160705084830/http://www.clapa.org:80/BLUE%20FOAM%20CONSTRUCTION%20FOR%20CONTROL%20LINE.pdf

Bill Smith and Mick Castell from the UK made a tutorial on using blue foam for the bulk of the wing construction. Clever self jigging system with foam ribs, carbon tow reinforced spar and balsa sheeting. 
https://web.archive.org/web/20160705084830/http://www.clapa.org:80/BLUE%20FOAM%20CONSTRUCTION%20FOR%20CONTROL%20LINE.pdf

   A fellow club member, Bob Arata, built a Tucker Special years ago using a similar method and common white expanded bead foam for the ribs and for anywhere a balsa block was called for. He cap stripped the ribs with balsa and applied gift wrap to the foam turtle deck, and bottom block foam with white glue. The model was them finished in clear dope tinted with a dash of white in it and colored tissue trim on top of that. It looked really nice but after a few years the tissue trim started to fade from sitting in the sun. The bottom of the model still looked like new! he flew that in classic and in advanced class for years and put untold number of flights on it, but despite his best efforts to seal everything, fuel soaking got to it eventually and it fell apart after a hard knock to the ground. Just n way to repair it, but he sure did get a lot of use out of it!!

  Type at you later,
    Dan McEntee

I get foam board from Dollar Tree, 3/16" for $1.25 a sheet. I use it to build E1000 Rc combat planes. It's the lightest foam board I've found.

Found this on line for 1/8". 
https://mirageoutlet.com/1-8-FOAMCORE-WHITE.html

My flying buddy in the late 70's till mid 80s, Jim Cochran built most his stunt ships using ribs cut from foam meat trays given to him at his grocery store.  (he was just cheap!).  The wings were close to the same weight as balsa.  Jim flew a lot and down the road the cap strips would begin to peal up from the foam.  He used thinned white glue to attach the caps.   I can say from my own experience now the white glues don't soak in and stick to balsa very well.  Needless to say in time his beautiful wings became a mess of lumps and dips....but he usually had a new one coming along to replace them.

Dave

Dear Friends,
Working with extruded polystyrene foams (blue and pink) for more than 40 years, I can tell you some useful facts, experiences and know-hows:
-both of them (plus the white Depron, yellow Barradur and green BASF) made in different densities (specific weight)  and the pressure strength correlates with density. As I overwiewed the datasheets, the lightest make 21 grams/liter and the heaviest make round 50 grams/liter. ( g/l right correspondes to oz/cu.ft).
-it is interesting, that the doubled density (e.g. 22 to 44 g/l) makes not doubled, but almost 4 times higher
 strength! It means, when the structure is a "built up" type, it deserves to use harder, heavier thin sheets, but when the structure is "sculptured" type*, without spars and ribs, but thicker walls, it deserves to use lighter density foam.
-I cannot tell you which brand is "better", or "worse", since the strength/density parameter is almost identical  by every brands, taking the same density. Choose by price, or color, or accessibility.
-there is no problem, if you cannot find some thinner (3-4-5-6 mm, 1/8", 3/16" 1/4" ) sheets on market. Buy one 4" thick and slice it yourself! The slicing rig is more than primitive, take a straight, smooth and slippery board, and span a solid .015 or .018 steel control line between two wooden blocks, above the board, at the proper height. Just a small laboratory supply unit is needed to heat ( 5 Amps, 30 Volts ). Choose middle temperature, (when not too cold, pulling hairs onto the surface, and not too hot, melting grooves when the moving slowes down. Glossy, silky surface is best: everybody can hit it within 3-4 attempts.)

*if you remember, see topic "The bluefoam model, or how to..."
Istvan

Ken,
I think you might be able to first cut slots for a spar, like an I beamer, then with the spar or a short spar like jig you could stack the rib blanks and cut the set cored out around the spar slot. Then put the spar in a wing jig and build the wing I beam method. For the cap strips I like the foam safe CA maybe medium vs thin or the epoxy.

Please show us your progress.

Best,   DennisT

I chickened out.  I made up several test ribs out of 1/8" and 3/16" foamboard and compared to 1/16" balsa they just aren't anywhere near as strong.  Are they strong enough - probably, but as long as I can get decent 6lb wood I am more comfortable with balsa ribs especially since my next wing is a Geo-XL that has some pretty long ribs in the open bay.

Ken

PS - this decision is not final.  The 3/16 when cap stripped with a center post added is pretty stiff and basically the same weight as the balsa.  I need to get the plans for the Geo-XL on paper so that I can layout the line requirements and see if I can get a second vertical post.  Problem is that the foam has very poor vertical crush strength.   Maybe using 5/16" cap strips?  The arch in the strip adds quite a bit of crush strength.  My real concern is the inboard wing and the leadouts.  I typically have a 3" range of movement for my leadouts 1 1/2 each way from 3/4" behind the intended CG.  That makes for removal of most of the spar on several ribs, maybe half of them.  I need a "stiff upper lip" rib to not crush with that much hollowing out.  One idea I have is to leave the ribs solid until I have the controls in and put a small hole to get the leadouts to the tip then "saw" out the foam using the leadout wire from full forward to full aft.  Then I could hollow out just what I need to remove and no more and put in two posts at the outer edges.  Does anybody think this might work?  More to come...