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Building Tips and technical articles. => Paint and finishing => Topic started by: Dennis Toth on August 05, 2020, 01:12:35 PM
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I'm getting ready to cover my Electric S-1 Ringmaster with silkspan and wanted to determine the grain so it would run span wise for the extra strength. I tried searching this forum, there is a lot of how to apply but I didn't see anything about determining the grain direction. In some material it is very easy to determine by looking at it. Silkspan is a bit less obvious. I've heard you can do a tear test in which you take a small piece of material and tear it straight down. If you are tearing in the direction of the grain it will be a smooth tear line, if against the grain it will be ragged. For me this was not cut and dry, both tears were similar.
What I then did was a pull test. For this you take a small piece of the silkspan (noting the direction you cut it from the sheet) and simply grab it with your thumbs and forefingers near the middle of the test piece (I used a piece about 1" sq) and pull apart. If you are pulling in the direction of the grain you will find it takes about 2 -3 times the pressure to pull it apart then if you are pulling across the grain. The results were very obvious. This should work for other materials like doctor paper or other tissue.
Best, DennisT
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Generally speaking, it tears in a straight line in the direction of the grain.
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Interesting. All my silkspan (I have a lifetime's supply of Sig) and decent tissue has a distinct tear direction -- I have some really crappy tissue, though, where it isn't obvious.
I'll remember that test, just in case.
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Generally, silk span and tissue sheets are not square, and one dimension is longer than the other. That is how it comes off the roll. The grain follows this direction, as a place to start. Some times you can see it when held up to the light. Then fall back to the tear test as already mentioned. Model Airplane 101!
Type at you later,
Dan McEntee
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As Dan said it makes sense that the grain runs down the long side of the sheet. However, some have indicated that some of the newer stuff runs down the short length that's why you need to test it. The stuff I have is 20 year old box of SIG. I dyed it yellow and the sheets are almost square so pull testing gave positive grain direction.
For covering wings you do want the grain running span wise for the extra strength. For those of us who have been doing this for a while we knew this but some new comers or those that never used silkspan might not have seen this information in the covering videos so now we have it in the forum.
Best, DennisT
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For covering wings you do want the grain running span wise for the extra strength.
Don't forget the annoying sag between the ribs if you get it wrong. I have never had a problem finding the grain just by looking at it. The squiggly strands are longer with the grain and more aligned. It will also sag more across the grain than with it if you are holding the sheets...or just use MonoKote.
Ken
ps. We have to get over this virus stuff. Even talking about the grain direction in silkspan in the middle of flying season is just unnatural. %^@