Tissue/Dope over epoxy and glass?

[JBF]

Im just finishing the fuselage of my ‘Introduction F5J’ and after sanding/shaping the balsa planking on the bottom of the fuse is awfully thin and delicate (particularly soft balsa).  I probably should have left it relatively square but I can’t help wanting these things to look more streamlined than a box with wings attached.

My intention is to finish the fuse in tissue and dope but I don’t feel confident that will add enough strength to the more delicate parts of the fuselage belly.  I’d like to cover the belly on the front 1/3 of the fuselage with a few layers of lightweight glass cloth, carbon toes and laminating resin.  I feel that would significantly strengthen the belly.

The only flaw in this plan is that I’m concerned the dope might not bond to the epoxy.

Before embarking on some time consuming testing I thought it worth asking whether anyone has experience with this?  My other option is to glass the entire fuselage but I’d rather avoid that if possible.

 

Edited February 26 by JBF

[PeteMitchell]

Epoxy does not bond well to cellulose doped surfaces unless the surface is well sanded.

Best to epoxy glass cloth to balsa first. A few carbon tows at each corner of the fus underside will increase strength and rigidity far more than doped tissue can.

If weight is a worry, then best to remove excess epoxy by 'blotting' the fus with kitchen paper before it starts to cure.

What weight glass cloth are you going to use ?

 

 

 

[JBF]

I have both 18g/m2 and 48g/m2  so I was considering a layer of 48g, carbon in the areas you described and the an 18g layer.  I’d be looking to sand to a Matt finish and then tissue/dope the entire fuselage before painting with automotive paint.

I guess my main concern is if the dope didn’t bond at all with the epoxy/glass and the tissue ended up delaminating/peeling in those areas.

Could glass the whole thing and then paint but I’m relatively new to composite layup and would be concerned about ruining the fuselage or making it unnecessarily heavy with excess resin.

[PeteMitchell]

I missread your question and assumed you meant to dope and tissue cover your fus first and then glass the nose

Dope and tissue on top of glass/epoxy is better but could 'flake' off as you prepare the tissue surface to paint if you are not carefull.

18g glass cloth will give pretty good 'ding' proofing. 48g will be a bit overkill, add more weight and wont really save damage if you should have a sudden arrival.

[JBF]

Thanks, I may just consider two layers of 18g with the carbon reinforcements.  Sounds like I’ll have to test on a scrap piece of balsa.

[oipigface]

Personally, I wouldn’t consider putting tissue on it at all, using glass cloth and epoxy will add all the strength you need, and can give an excellent finish if you haveenough patiene and you grease your elbow sufficiently. The carbon tows won’t add much either, although you might consider putting some light kevlar cloth under the glass, where you feel you really need it to tough. Kevlar is impossible to finish, so you would need two or three layers of lightweight glass over it.

[JBF]

My concern with that is the possible weight that I’ll add covering the entire fuselage in glass cloth and resin.  Imagine it will take a fair bit of resin to fill the weave, that or a lot of primer?

[mikef]

Do some test pieces in the possible finishes and weigh them at every stage - strength and stiffness test them too.  I used glass over a balsa dbox on an F1A glider to stiffen it about 30 years ago and I think I still have the results of my tests somewhere - couldn’t find them quickly.  I used two different glass cloths and epoxy or dope to attach them - I tried tissue alone too.  I ended up doping the glass to the wood and then doping the covering tissue over it.  That gave the best stiffness per weight.

[Dick Whitehead]

I can't help with dope/tissue over glass/epoxy, but I do have some notes about weights of glass/epoxy covering.

I built a 55" span scale Spitfire following a flight in a real one (70th birthday present!) and covered the wings in 28gm glass and epoxy. My notes tell me that I mixed 56gm of epoxy to cover both sides of the wing, and the total weight gain was 62gm. There is a lot of area in a wing, and my notes tell me the equivalent values for the fuselage was 28gm epoxy mixed and 25gm total weight gain (epoxy mix left over!).

I did quite a bit of glass/epoxy on gliders in the past but unfortunately can't find any data or photos from those days.

Dick

[JBF]

So I tested a piece of balsa sheet covered in lightweight glass cloth and epoxy.  I then gave it a couple of thinned coats of dope before painting on a piece of tissue with some of the same thinned dope.  Now it's dry I don't think there is any chance of it peeling off, seems secure.

I could just go with the glass but the tissue/dope gives a nice smooth finish to take paint.  I suspect I'll have to use a fair amount of extra primer to fill the surface pattern of the glass cloth.  That might still be lighter than the tissue and dope, but I really hate painting/sanding/painting/sanding - this approach seems like it might be easier and perhaps quicker too what with drying times etc.

[Shane A]

Just to throw a spanner in the mix, I covered my Introduction with Solarfilm, and three years on and lots of flying, including some hit the spot competition flying it’s still going strong