Sunday, March 29, 2015

Mid Power: Building the Quest Big Dog (Part 5) - Experiments in Papering Fins

Click here for Part 1

I'm going to digress from this series for a moment, because this is the point where I needed to make some decisions about how to proceed. In order to make that decision, I had to try a few things.

On most of my rockets, I make the fins smooth for painting by filling them with Elmer's Carpenter's Wood Filler, commonly abbreviated CWF.

This works, and it's pretty common among rocketeers looking to hide the wood grain that will show through the paint job. The common way to do this in the old days was to use sanding sealer, which would fill in the pores in the wood, and after several coats and several times sanding, you'd have a smooth, plastic-like finish.

Sanding sealer seems to have fallen out of fashion these days. I don't really know why - I've heard that it smells bad, but so many materials we work with in rocketry smell bad and irritate the eyes (I'm looking at you, cyanoacrylate!). Perhaps it's not as good as CWF, I don't know. I've never tried using it, though I'm considering doing so, just to see what it's like.

CWF works, but it takes time, produces a lot of dust (the sanding produces such a fine dust that it gets all over everything in the spare bathroom where I work), and often requires multiple coats; and, since it's water-based, CWF can warp fins if you're not careful.

A lot of rocketeers "paper" their fins. That is, they cover the fins with paper skins, carefully glued on and flattened out. It takes less time, and can make fins much stronger, as the paper reinforces the balsa fins. With paper skins, you're much more likely to have a fin break off  long before you risk snapping one in half.

People have their different methods of papering fins, but here's a basic method by Tim Van Milligan, of Apogee Components:

The quicker build time, less mess and added strength are very appealing to me. And papering fins is supposed to be easy. But I've had a bit of a time with it, so I decided to try it out on the practice fins I airfoiled in the last post.

Now, I've tried papering fins before, with mixed results. Some have turned out well; some have not. I've tried different methods, all of which seem pretty straightforward, using different adhesives, from wood glue to white glue to spray adhesive, and using two skins (one for each face of the fin) and one skin (which wraps around the leading edge of the fin).

I figured if I could get this papering thing down, I'd do that for the kit fins; if I couldn't, I'd stick with the CWF. I'm not willing to risk making a $45 dollar rocket look bad because I decided to try something new.

Well, I get a good result on about 1 out of 4 fins:

The six big fins are practice airfoils for the Big Dog. The bottom right three fins, from left to right, are from
my scratch builds: Sounder I, the Ceres B booster, and the Janus II two-stage rocket
with a camera payload. The upper left is just a scrap I tried papering.
The basic method is to apply some kind of glue to either the surface of the fin itself, or to the paper, wipe off all but a thin layer, then glue it to the fin. Do both sides, allow to dry, and then seal all the edges (except for the root edge) with thin CA (superglue). Once the CA is dried, you sand off any extra paper from the edges.

Some people use spray adhesive, and some people like to wrap one skin around over the leading edge of the fin. I liked this idea, because it means that you have once piece and don't risk showing a seam under the paint job, or having exposed wood grain on the edge. But the trouble with that is that the skin tends to pull away from the wood before its dry, leaving a bubble underneath.

I've tried spray adhesive, and my best fin seems to be with that method.

I was also able to get one skin wrapped around the leading edge. Unfortunately, other fins I attempted to do this way had a gap at the leading edge between the paper and the edge of the fin. And on at least one rocket I've built using the spray adhesive method has a bubble I only noticed when I started painting.

Hard to see, but if you press on the fin on the left, you can feel it give; there's a bubble
under the paper skin where it pulled away from the fin either during papering or afterwards.
OK, so the Trident A fins were done using a weaker spray adhesive than I used on this Big Dog fin. Perhaps spray adhesive would work just fine. But there's also the problem of the leading edge. Even on one fin that didn't have a gap between the edge of the fin and the paper, there was some wrinkling where I folded it over the edge.

Well, that's not going to look good! So I tried curling the paper first, or folding it. Folding it would probably have worked pretty well, if I had gotten the leading edge of the fin in there perfectly. But working with sticky materials is hard - you keep getting your fingers stuck, then the material moves, then you don't get the part placed the way you want to. I ended up with such a bad gap on that one, I ended up cutting it, sealing it down with CA, and treating it as if I had used two skins:

This one might turn out OK - I guess we'll see when I sand off the excess.
Working with white glue and wood glue turn out alright, provided the fin isn't too large.

But the Big Dog fins are quite large, and I kept dropping them while holding them by the edges. This means I get blobs of glue where I don't want them, such as on the paper itself.

And with glue, you have to work fast, so wrapping the fin around the leading edge doesn't seem to be an option for me, at least until I get better at this. Look at this horrible sight!

Ripples and tears, from the paper becoming saturated with the glue. I wasn't able to work fast enough with this fin, and this happened. When you paper fins with glue, it's a good idea to work quickly and then press the fins under a book to prevent them warping. Between sheets of waxed paper, of course - you don't want fins glued to your Shakespeare!
I had one fin turn out reasonably well with glue, except that the leading edge is prying away, despite being sealed down with some CA.

Even with a successfully papered fin, if you're fussy and want to hide all wood grain, you still need to deal with the tip edge, which shows the actual capillaries running through the wood.

This can probably be done with a minimum of CWF or some other sealer. And of course, you won't know how things really turned out until you put on primer and paint - and by then, what's done is done.

All of this is to say that with the Big Dog, I decided in the end to go with CWF. It may not be perfect, but I know I can get good results with it. I'll keep working on papering fins on my own scratch built rockets, until I get comfortable enough with it that I am willing to paper the fins of a kit I've paid money for. Papering fins is probably one of those things that simply take doing until you get less clumsy with it. Once I feel good enough about my ability to paper fins, I'll show you how I end up doing it here on the blog.

As for the three fins that came out reasonably well...

...well, they're not perfect, but they're plenty good enough to be put on a scratch build. It's an E-motor quad cluster which uses the Quest Big Dog fin planform, so until I can come up with a better name for it, I'll call it the Quad Dog.

It'll be a simple, big rocket that should have some awesome flights. And cheap to build!

I've now finished filling the fins, so in the next post, I'll show that part of the build, and attach the fins to the rocket. I'll also decide what size rail buttons I'm going to use. Stay tuned.

Click here for Part 6.

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  1. 2nd on the glue stick. That's what I use when not using self-adhesive paper. Also, I CA the entire fin, not just the edges.

    1. I forgot to about the glue stick! I did get one, but forgot to try it out on these fins.

      I may try the Avery labels. It seems like it's probably the easiest option. I was just stubbornly trying the glue method, because it seems to work well for others.