Posted: 12/12/2010 16:51:16
Can anyone provide guidance on the correct (or preferred) orientation of the grain in spars. I'm familiar with a number of references on grain slope, density and moisture content of spar timber, but I can't find anything on grain orientation.
Imagine you have a solid wood spar that is later spindled out on the front and rear faces. It is rectangular in section with the depth of the spar being the longest dimension. If you look at the end of the spar - what orientation should the grain be? That is, horizontal, vertical or 45 deg? There is no guide provided on the original drawings. All the drawings I have seen that illustrate spar manufacture either show 45 deg hatching or hand sketches of grain which makes it look like the author is using radially cut timber.
Posted: 12/13/2010 10:34:54
If you have'nt found these already, they helped me alot. Clear Vertical Grain, quarter sawn lumber is what you want, these links describe it well. I was lucky enough to find 5 nonairworthy Citabria spars cheap so my hunt was over. If you are looking locally, gooood luck !
Posted: 12/13/2010 16:22:52
Thanks for the response Bob but I'm not sure it really answers the question. It's the end grain direction that I'm interested in. If you view the spar from the end (ie. not from any of the faces), should the grain be vertical? The current consideration is that I have a big piece of lumber that needs to be re-sawn into the right sizes for spars. Orientation of grain affects the amount of spars I'll get from the one piece. I'll go for the strongest spars, even if it waste timber, but I don't know what the correct orientation is. Do you mean by 'Clear Vertical Grain' that the end grain should run vertically?
Posted: 12/13/2010 23:17:07
The drawing above figure #2 in the 'allwoodwork' link shows what it should look like. For lack of any other way to describe it, if you have an 8" X 1" spar laying flat, the grain will be vertical, like this [lllllllllllll] with a minimum 6 rings per inch and max 16 :1 grain slope. That's off the top of me head so don't take my word for it. The EAA wood working book is a good one to have too.
Posted: 12/14/2010 13:44:22
Tony Bingelis had an article in the September 1996 issue of Sport Aviation.
On page 76 of that issue is a drawing indicating the "vertical grain" orientation (the second page of Tony's article.
This is often also shown at 45 degrees in drawings as this is, if my memory serves me correctly, the maximum allowable for spars.
The EAA wood book and Aircraft Maintenance for the Airplane Mechanic by Brimm and Bogess should also have diagrams.
Both can be had for under $20 (and often under $10 each) and good to have on the shelf of any aircraft builder.
Posted: 12/15/2010 11:38:52
here are some more EAA articles on the subject of wood
Posted: 12/19/2010 12:09:00
AC 43.13-1B, Chapter 1 has the information you are looking for. It can be downloaded at the FAA site for free.
Posted: 12/20/2010 12:09:05