Back in the ‘70s I used silkspan to cover the open bays of my wings and just doped the rest of the bare wood. After the release of Monokote, I never opened another can of dope. Now I’m wanting to cover and paint a few planes with silkspan and dope again. I’m at a bit of a loss as to how to cover a fuse with silkspan. With Monokote, I started at the tail and applied smaller pieces in areas I knew would be difficult to cover with larger pieces. Then once all the small areas were covered, I’d cut out the large pieces and cover over everything. Is this the same approach I should use with silkspan? Maybe there’s a video or video series out there I can watch the process on?
I could just go for it but I’m wanting at least a reasonable finish for a few appearance points at the Nats!

Thanks, Matt

Thanks Ty, I was envisioning having the plane fully assembled with wing and tail fillets already mostly finished before silkspan. So, do I apply fillets over the dope and silkspan?
I’ve always done Monokote like you say but thought with fillets and such it should be all assembled.

   I think you have it backwards. You want to cover the wing from tip-to-tip before installing it in the fuse. I usually cover the fuselage after assembly, to avoid having to patch around the wing cutout, but you could do it before and then just patch the diagonal cuts under the wing. They apply the fillets over the silkspan. You want to have a fair bit of the clear underlying the primer or color before you put the fillets on, so there is not a rough edge where the fillets feather into the covering.

    When I do Monokote on the surfaces and paint on the fuselage, I also put the fillets on after the Monokote, and over the Monokote edge. I would never, ever, attempt to Monokote the fuselage any more, there's just to many small pieces and I can never completely bury the edges. That's really the weak point of Monokote, there is always a visible edge somewhere.

    Brett

   I think you have it backwards. You want to cover the wing from tip-to-tip before installing it in the fuse. 

    Brett

I know that a wing covered with Monokote, needs to have a strip of the monokote removed from the area that will be inside the profile fuselage so that the balsa of the wing centere section is glued to the fuselage.  Does the same apply to a wing covered with silkspan/polyspan?  Does a strip of the silkspan/polyspan need to be removed before gluing the wing into the profile fuselage so that the balsa of the center section balsa glues can be glued directly to the profile fuselage?   

Thanks,

Joe Ed Pederson

I know that a wing covered with Monokote, needs to have a strip of the monokote removed from the area that will be inside the profile fuselage so that the balsa of the wing centere section is glued to the fuselage.  Does the same apply to a wing covered with silkspan/polyspan?  Does a strip of the silkspan/polyspan need to be removed before gluing the wing into the profile fuselage so that the balsa of the center section balsa glues can be glued directly to the profile fuselage? 

 

    No!  Just glue on top of it. That was always the "rap" on this, but if you attempt to remove the silkspan you will create a stress riser where the silkspan is missing, and the wing will tend to fold right along that line. Monokote, you can't glue to, but it also doesn't add any strength, so it's like the entire thing is bare wood. Silkspan or carbon veil is like case-hardening the structure, it adds tremendous strength but if it is not continuous, all the stress will be concentrated right where all the load is applied.

    BTW, for Monokote, you are much better off just covering up to the edge of the bare spot you want to leave, rather than trying to apply it, then remove a strip later. Removing the strip without digging in to the underlying wood is very difficult, and it's tempting to take an exacto knife, lay down a ruler, and just take a cut. That will certainly also cut the wood, now it will have a weak point very near the highest stress area of the wing. If you have to, I lay a razor blade flat against the surface at the line, then pull the covering up against it to cut it. It is a lot harder to get a straight line that way, so it's easier to just not cover where you want it bare than take it off again.

     Brett

On cutting away Monokote, I use a dedicated soldering iron with a tip sharpened like a knife. For straight cuts, I use a thin machinists scale as a guide. No pressure like trying to cut, just let the weight of the iron and the heat cut through the covering. The heat seals down the edge while you are cutting it as well.

   I have seen that, but unless you keep it from touching the wood at all, it still concerns me. Lifting it up to meet the edge (hot or not) seems a lot safer.

   Since the ST60 and then, more so, the tuned pipe era, there has been a very alarming increase in wing failures, so a lot of the stuff that was no problem before is fatal now. The New Jersey guys were the first to run into it.

    Brett

My filet material is called model magic. It sands like balsa. Brett mentioned epoxy. I tried that once with a bunch of talc mixed in. Is that what you recommend, or is there a different epoxy based filet material?
David

   Model Magic (also called Model Tragic, and the same as latex-based spackling) is completely incompatible with butyrate dope, as many people have found out the hard way. It might work with epoxy paint, I don't know

    I use SuperFil for fillets, it's a blue epoxy-based product you get from Aircraft Spruce and Supply. It's kind of like Epoxolite, except for being cheaper and lighter and easier to work and better adhesion and 1000x better in every way.

    The epoxy I was referring to above, however, is the paint - Klass Kote. It's relevant to this issue because, unlike dope, it has minimal shrink and also great adhesion. Dope pulling away from Epoxolite is what causes the "bubbles" or "fillet cancer", because it barely sticks and shrinks a lot.

    Brett

This post was written earlier yesterday, but failed to post. Another try, with this addition: You do not need to perforate silkspan that has been doped on to get a strong wing-fuselage bond! In trying to remove a wing attached as shown below, I had to work very hard to get the pieces of fuselage off the wing shown below being put into a new fuselage the same way. OK, here's the original post, a little of which has been made redundant:

For profile planes, I always cover the fuselage and the entire wing, before I install the wing. I silkspan the wing in four parts: upper and lower surfaces of each side,  with the silkspan overlapping in the root area enough that there are no edges at lines of high stress-concentration, as at the edges  of the fuselage. So the silkspan overlap passes beyond the fuselage on each side. Fillets then come after covering.  My wing cut-outs are usually very accurate, but with any reasonably small clearance, fillets can cover and should strengthen the seams.

So, there is no real challenge to silkspanning the entire fuselage first - other than sometimes having the bad newer silkspan float over the solid wood. There has been much discussion here about silkspan currently available, and it may be better again - I have not used it. Over balsa, I often brush on three or more coats of clear and sand lightly, sometimes until I start to get a sheen. Then, when covering with damp silkspan, I most often brush on thinner, augmenting with more 50/50 clear where needed to first adhere the silkspan. I let set for several hours - preferably a day - before applying further thin coats of clear, until the silkspan surface is filled. The idea - for me - is to dissolve the undercoats as little as possible to prevent weight resulting from floating the covering. Actually, I sometimes seal or even adhere the covering on fuselages or stabs with very thin squeegeed West Systems epoxy resin first, before continuing with clear dope. This ensures that the silkspan (or biased .56-oz fiberglass) stays down against the balsa under succeeding coats.

I usually leave the fuselage-bonding areas of the wing's silkspan a bit rough (less dope) and use epoxy to attach wing and stabilizer after they are doped. I use 30-minute epoxy and micro-balloons for my fillets, all applied after the silk-spanned fuselage and wing are attached. These fillets do not come loose or crack, even in my too-frequent earthly encounters.

One thing to always avoid is trying to dope silkspan over fillets. It will most probably lift as it shrinks. If you use my fillet material, wet sand them until smooth (I use my old Dupont Prep-Sol), but not shiny. You can use 150-300 garnet paper. Then dope a bit inboard of each fillet edge, let dry, and then dope along the center of the fillet. This way, the dope will shrink across only small, un-connected widths at a time and not lift. I haven't had problems with later spray coats (color, clear coats) applied normally, but lightly. Pictures (Edit: Picture #1 is wrong, and #11 was omitted; see next post):

1) Silkspan for one half-span surface with central overlap.

2) Doped wing, ready for insertion. Less dope in center.

3) Trial Fit

4) Masking for fillet, after gluing wing in.

5) Micro-balloons mixed into 30-minute epoxy to form slurry. Stir in as much powder as you think it can take and then add more.

6) Using Popsicle stick to shape fillet (demo pic only; I took this one after the masking had been removed to show what I had done)

7) After fillet slurry applied with Popsicle stick (radius end to suit preference).

8 ) Fillet after masking removed (done before epoxy sets)

9) Sanding fillet with dowel and garnet paper

10) Doping edges of fillet before center-line area

11) Sanded and doped fillet with lighter area of silkspan overlap visible. Feather this by sanding.

12) None of my photos show the completed and painted fillet well. This was shortly after completion.

13) This is today, 3 1/2 years later. With bright sunlight it would be more visible. Dope has shrunk some, leaving the surface less even.

Very nice post on covering. Serge.

I cover my wings before I insert them into the fuselage much like what Serge shows.  I use Polyspan and cover from the wingtip to the opposite side of the center sheeting.  Then I cover the other wing the same way so that I have the center sheeting completely covered with double layers of Polyspan.

Mike