pooba

hello!
I am thinking of building a discus launched glider wing but don't want to use carbon fibre for the spar. here is my plan... white polystyrene foam cores cut then making a balsa spar covered in fiberglass which is then glued into the foam core at about CG location, then the whole lot covered with fiberglass and then cutting out the ailerons etc. would this work??
also, if i were to do this what weight of fiberglass should i use for covering the wing(wingspan is 1.5meters)? would it be like 1 layer of 1.5 oz/sqft or 2 layers of .75oz or what? (i have never made a fiberglass wing b4...)

thanks.
Blythe.

Ed Smith

If you lay fibreglass cloth straight on to a white foam base you will dent the wing every time you touch it. Foam cores need to be skinned, preferably with balsa at least 1/16 thick and then glassed with say 3/4 oz cloth.

Ed S

davidfee

In addition to Ed's good advice, let me add some extra details. Standard white foam is very weak in compression, which is why Ed warns about dents. He is correct. The skin on a white foam wing must be strong to resist denting. Balsa is the easiest way to do this. Another problem you have with the white foam is the large pore sizes due to the way it's manufactured form expanded pellets. These pores absorb lots of epoxy and often result in a wavy finish to a bagged wing if you don't use something thick like balsa.

The denser foams, blue, pink, grey, green... are extruded and have a much finer cell structure. These can be used with a glass (or other composite) skin without the balsa.

For the skins, this will be determined by the type of foam you use in large part... 2 layers of 0.75 is probably not going to yeild a stiff enough wing for a DLG. I would use a minimum of 1 layer of 1.5 with an outer layer of 0.75. Also consider using the cloth on the bias (+/- 45 deg) to maximize the torsional stiffness. Let the spar carry the bending loads.

For the spars, why do you wish to avoid carbon? Consider that the commercially available DLG's use carbon.... it works well, since the fibers are light and can be oriented spanwise to best carry the bending loads. The fg-covered balsa will make a nice shear web though.

Also, you don't say if you plan to use vacuum or not, but it is nearly essential in order to get a good fiberglass wing.

-David

pooba

thanks people!
i will use pink foam instead of white so i dont have to put balsa on.
david i don't want to use carbon fibre cos its too expensive
also i prob won't use vacuum because that sounds too complicated and expensive so i will just have to do lots of sanding

thanks again.
blythe

Ed Smith

My last word on the subject. The launching of a DLG is pretty severe on the wings, especially the part you hold on to. Pink or blue foam will still dent and dimple very easily with heavy handling. listen to the advice you have been given on this thread regarding wing construction

Ed S

Soar Head

My Feedback: (5)

if you do not use carbon, your wing will be unnecessarily heavy to take the same design loads. The amount of carbon required for a DLG spar is small.

A few plans for DLGs are available at http://www.charlesriverrc.org/

Bob W01

4 words

vacuum bagging made easy

http://www.paonline.com/hayman/video.htm

Bob

Soar Head

My Feedback: (5)

"Bob" - 3 of your total 5 posts have said nothing except "buy this video".