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.
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.