Scarfing and laminating wing spars

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davidb

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Jun 3, 2008
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1,702
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Vacaville, CA
AC 43.13 Section 4 1-39 on page 1-15, talks about the scarf joint. In that section, it states that the No grain deviation steeper than 1 in 15 should be present in an outer eighth of the depth of the spar. The scarf joint can be 10:1 according to the inspector that checked me. I may have gotten off easy as my airplane only weighs 560lbs MGW.
Hmm, ok. That's where I'm looking and the diagrams depict 15:1. The doublers are 10:1. If the inspector is happy, I'm happy.

Can you post some pics of your scarfing jig? I'm trying to figure out a good way to make one. Thanks.
 

flyboy67109

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Apr 24, 2008
Messages
56
Location
Mesa, AZ
Hmm, ok. That's where I'm looking and the diagrams depict 15:1. The doublers are 10:1. If the inspector is happy, I'm happy.

Can you post some pics of your scarfing jig? I'm trying to figure out a good way to make one. Thanks.
That's referring to the grain deviation. I took it to my local EAA meeting and asked and they agreed that 10:1 was what was wanted and I wasn't just lucky.

The way I made the scarf joint was I made the jig first from these plans: http://home.comcast.net/~kathymatsu****a/amateurluthier/htmlpages/scarfjigplan.html

I had to change it a bit to make sure the ratio was 10:1 but I don't remember what it was. I think these plans are a 8:1 ratio so check before you cut the wood to make the jig. I also changed it so the blade didn't cut into the jig as these plans show but right next to it. I thought that was stupid as there was a guide that kept it straight underneath and a risk of the jig kicking if I didn't have the blade adjusted just right.

Here's my picts:
After the cut -


Clamped and glued:


Sanded and done:


This was actually one I did for the rear stinger. I did a test one earlier to see if it would break. I broke outside the joint. I tested this with a 50" long piece of 3/4" by 3/4" cut in half and the glued back together with a scarf joint. I then used a scale to see that it would take 150 lbs. I then got on top of it and started jumping up and down on it (showing off ) and that's when it broke. I hear that if you do the joint right, it is actually stronger then the wood itself. I believe it.
 
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flyboy67109

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Apr 24, 2008
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56
Location
Mesa, AZ
The censor tried to clean up what it thought was a dirty word but was actually a Japanese last name. :) Use the bad word for doo doo to replace the stars and it will work.
 
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