experiment with fin papering

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bjphoenix

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I've been papering my fins for LPR, using Titebond II and normal 8.5x11 printer paper. I do this for strength as much as anything else. I decided to do some experiments to see if other papers might be better.

I found some scap balsa in my stash and picked out the weakest feeling 1/8" thick piece I could find. I cut some specimens, perpendicular to the grain, about 5/32" wide. I cut strips of the different papers, glued on and clamped the specimens between some flat boards to dry. It is fairly difficult to saw narrow strips of balsa perpendicular to the grain and after cutting them they feel very fragile. They felt a lot better after papering but still weren't real stiff, not as stiff as I expected compared to my results with actual fins but these things are after all only 0.15" wide.

For testing I made a little basket to hold the weights and used old nuts as weight. I measured and marked a 2" distance on each specimen so that was the moment arm.

Here are the papers I had readily available for testing, along with test results for each one.
1. normal copier paper, cut so it is stressed parallel to its grain, max. load 3.7 oz.
2. normal copier paper, cut so it is stressed perpendicular to its grain, max. load 3.4 oz.
3. very thin paper from an old sketch pad I had, looks like vellum, max. load 4 oz.
4. paper from another sketch pad, it looks like copier paper but i thought it might be higher quality, max. load 3.3 oz.

This is a pretty crude test so a few tenths of an ounce variation might be testing error. Only one of the samples failed by tearing the paper on the tension side. Initially this was what I expect for all of the tests then I remembered that typically wood fibers in direct tension will fail in compression before failing in tension. That meant it was hard to determine the actual failure point for the other 3 specimens. I did a final test with a piece of plain balsa, 1/8" wide, and it held all the weight I could put into the cut without breaking.

My conclusion is that this will strengthen a fin tremendously against bending perpendicular to grain, but the thickness of one sheet of paper is not enough to reach the limit of what you could get. Paper thickness may be more important than type of paper. I think thicker paper and/or a piece of thin cardstock might be much stronger if you have an application where you want more strength such as complicated fin shapes or fins that sweep back a lot.
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I'm guessing that if you're getting to a point where you need to use cardstock or other paper materials with sufficient thickness, you might as well use a different material altogether for your fins, like plywood or basswood.

The above statement assumes you only paper balsa or other weak types of wood materials. Do people paper plywood or basswood fins?
 
I'm guessing that if you're getting to a point where you need to use cardstock or other paper materials with sufficient thickness, you might as well use a different material altogether for your fins, like plywood or basswood.
Not necessarily... I would think cardstock-laminated balsa might be stronger than un-papered basswood in some situations (that's another good experiment), and balsa retains the benefit of being easy to work with.

But yeah, at some point balsa is no longer the appropriate material.
 
I'm guessing that if you're getting to a point where you need to use cardstock or other paper materials with sufficient thickness, you might as well use a different material altogether for your fins, like plywood or basswood.
That's a good point.
It is easy enough for me to paper the fins that come with the kit, I wouldn't have to buy more material or cut out new fins. I wanted to strengthen my fins but I think for most of the rockets I build and where I launch this is overkill. I've been staying away from swept back fins until recently. One of my friends has a Baby Bertha that has been launched a lot. At a recent launch we had a lot of wind and it landed on a street, breaking one of the swept back fins. I think a design like that needs stronger fins so papering would help. You probably wouldn't want to cut new basswood fins for a kit like that. Now I have some new builds with really swept back fins so I was getting more concerned about strength.
 
For my Double Shuttle I used 20-pound stationery paper soaked in white glue with paper on both sides of the 1/8" thick balsa. The final product seemed light and strong to me. I found it easy and convenient to cut with a scroll saw. The down side was that it was very tedious to make a large wing. I needed to put wax paper down on a cutting board and weigh the glued material one side at a time with heavy books on top.
 
Here is an example where I think papering is useful. I'm building a kit bash and thought about restyling the fins just a little bit. I glued some small pieces of balsa on the outboard edge and then papered both sides. The piece I added is not stressed on landing so I think it will be OK.
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I'm guessing that if you're getting to a point where you need to use cardstock or other paper materials with sufficient thickness, you might as well use a different material altogether for your fins, like plywood or basswood.

The above statement assumes you only paper balsa or other weak types of wood materials. Do people paper plywood or basswood fins?

Actually, I have papered plywood for a mid-power rocket. The plywood fins that were supplied were of very low quality and were warped in 2 directions. They flexed a lot. I had a hard time believing that it was actually 3-ply plywood. So I sanded smooth and papered them with glossy label paper. And pressed them flat. Then they were fine and I mounted them on the body tube.
 
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