Ben Diss

My Feedback: (23)

I'm building a wing from plans that were drawn for a balsa built-up wing. I'm going to cut a foam core and sheet it. I need to know how to build the foam core wing to match the strength of the built-up wing.

On the plans, each wing half is comprised of two panels totaling 45". The plans call for 1/2x1/4 spruce spars, top and bottom, 1/16 shear web, 1/4 sq spruce LE spar and two 1/4 sq spruce TE spars. LE is shaped from 1/2" balsa.

I can sheet the wing with 1/64 ply, 1/16 or 3/32 balsa, or even obechi. Some options for spars include just adding everything just like the original at the expense of weight, or building a balsa/carbon spar and skipping the LE and TE spars.

I'm not a math wiz so I can't figure out the stress loads myself. Without doing a bunch of math, can someone suggest a strong and light structure?

Any help is appreciated.

-Ben

AlexBrasil

Ben,

Which kind of plane is it?
I´ve built an own design .40 Fun-Fly, it´s wing was built with low density foam and sheeted it with 1/32 mogno, a dark, light wood which is used, here in Brasil, to sheet some mobiles. This Fun-Fly performs snaps, tight loops, and some other stressing manuvers and the wing seem not to have failures.

I hope it helps you,

Alexandre

Ben Diss

My Feedback: (23)

This is for a 1/4 scale L-19 Birddog. There is no wing tube as this plane uses a functional strut.

-Ben

tonyc

My Feedback: (4)

What I did for an 84” Hots, one piece wing.
¼ by 3/8 spruce spars top and bottom set into a grove with Elmer Probond or Gorilla glue. Two strips of Dry Wall tape from the local Hardware store. Sheeted with 1/16 balsa using Elmer Probond glue. Cut out the ailerons used 1/8” balsa for end caps. Hinged the ailerons using Robert hinge points, glued into the foam with the Elmer Probond, no blocks.

Works for me

tony

ilikeplanes

I like tonyc's advise. The drywall tape is a good idea. It's strong, cheap, has its't own adhesive so laying it out is easy, has the right width, and is very open so the skin adhesive will still bond to the foam. Depending on the thickness of the wing, you may opt out of the spar completely. I have used epoxy and aliphatic resin. I prefer epoxy spread with a squeegee on the skins only. I think 1/16 balsa is a good skin material for general purpose use. It has good strength to weight and is easy to cover. Are you going to paint? If so, you can go all glass skins. It's a much more difficult process I believe.

Building foam wings is very rewarding. It's so easy and you usually get very straight and true wings. The airplane will be quieter too.

Ben Diss

My Feedback: (23)

The root chord is 16" and 2" high. With 3/32 balsa do I even need a spar?

tonyc

My Feedback: (4)

The functional strut needs a hard point to attach to. You don't need the spar but how much of a hard point will you put in? On a wing the size you are making I would go with a 2 inch square by 1/4 thick piece of hard balsa set inside the sheating, a short spar. I would then glue a 1/2 inch square by 1 inch tall hard wood block to this balsa "short spar". This gives you a larger foot print of holding power. Your plane is not fast but it has a ton of lift with the flat bottom wing. The hard point needs to be up to the task.

good luck

tony

Flypaper 2

Ben: Check out my thing on foam wings in Question & Answers. Just scale up the sizes. The foam ribs make it more scale like. I built a 1/4 scale Cap 10 among other planes using this method. Light as a wood wing only much stronger.

Ben Diss

My Feedback: (23)

Tony- That brings up a good point. I was thinking of going with full length spruce spars, top and bottom. For the strut attachment, I was thinking of adding a 1/8 ply short rib on one side of it. I was looking at some of the designs for wings with wing tubes and they often have a ply rib at the end of the tube.

Now that I think about this more, I think your idea is better. The rib would spread the load fore and aft, while the stress is from root to tip.

Flypaper- Thanks. I'll check it out.

-Ben