Selecting Spar Quality Wood

I was looking up references to give a talk on this at our next EAA Chapter meeting and thought I'd share this.  There are more things to look for, any defect should be questioned, but these are the more common things to consider.  The critical area for spars is the upper and lower portions where most of the load is carried.  For the WACO NINE the spars are routed to an I-beam shape so the flanges are critical.

Most of this come from the EAA Video "Basic Aircraft Woodworking" and AC65-15A.  There are plenty of other references to use if your are unsure about a specific defect.

 Sitka Spruce is the preferred wood because it is lighter for the same strength, but Douglas for is an acceptable wood for most plane I care about.  Spruce can have less Annular Growth Rings then Fir and be acceptable, part of why it weighs less.

When looking at the end of the board, the rings need to be at a 45 degree angle or steeper for at least 2/3 of the width of the board.

 Trees grow from the bottom up so the bottom is a larger diameter than the top.  As a result when we saw boards from the tree the growth rings usually run at some slight angle across the board face.  the board is stronger along the growth rings than across them.  Ever split some wood? Because the top and bottom of the spar carry the load, the slope of the growth rings on the outer 1/8 needs to less (15:1) than the interior (10:1) of the spar. 

Not Acceptable for a spar, or much of anything else.

 Once you have good straight strong wood you need to look for small defects. 
Small Hard Sound Knots may be acceptable if they are:
- Not loose
- Not running through the depth of the beam
- No bigger than 3/8"
- Not into the I-beam flange (high stress areas)
- Do not cause the edge to diverge
- Not closer than 20" for 3/8" knots

There is also something called Pin Knot Clusters which might be acceptable.

 Pitch Pockets, little pockets of sap, may be acceptable but there are a lot of limits.

AC65-15A has some tighter limits than the EAA video.  As an example it limits pitch pocket depth and width to 1/8" and length to 1 1/2".

Where the there are multiple criteria, use the the more restrictive limit

 Pitch pockets need to be further apart if they are in the same growth ring.  AC65-15A limits them to 14" in the same ring.

Remember Advisory Circulars and the EAA video are advice.  The person doing the work needs to use their best judgement from all they can learn to make safe decisions.