This is the exact text from my skypup builders manual.
With the exception of the plywood required for gussets and leading edge skins, the wood for building your skypup may be obtained in a good lumberyard. The airframe is designed to allow some flexibility in choice of wood types for this reason. Before traipsing off to buy a few boards to trust your life to, a discussion of proper selection of wood is in order. Any wood which will go into your airframe must be carefully picked to comply with the following requirements.
1 Must be FREE of knots.
2 Must have grain which runs straight down the length of the board and does not "run out" the side or swirl. The "slope" of the grain (inclination of the grain relative to the long axis of the board) should never be more than one inch across the board for every fifteen inches of board length. Grain slopes of 1:30 or 1:40 are preferable, and can be found with a little persistence.
3 must have atleast 12 grains per inch when cut transverse to the grain. In tighter grained woods, such as fir, 20 or 30 grains per inch is quite common.
4 Must be dry and free of sap, dirt, waterstains, or any other contaminations.
In summary, don't hurry through your selection of wood stock. Take your time and don't let anyone at the lumberyard rush you. When you go to buy the wood, have several alternative board sizes in mind so that you can select the wood more on the basis of the quality of a particular board than whether or not it is the size which you most wanted. If anything, buy boards which are bigger that what you think you'll need, you can always use the excess for other parts later in the project. In cases where you have an option of wood types, make your decision based on the quality of a board...a good piece of pine is better than a poor piece of some stronger wood.
Thought this might be helpful for those trying to source their own lumber.
The selection of wood for an aircraft project also should include species of the tree. There are many different species of spruce, pine, and fir. Some of these species may show up at a lumber yard and be sold as a generic name pine. BUT it may be Red Pine or Scotch Pine. The species in question have much different characteristic . In the north east it is common to find Balsam Fir in lumber and this is not to be confused with Douglas Fir. There is also a condition that exists in wood called "brashness" this condition is hard to actually see in wood until you fracture the wood. It can vary from tree to tree even in the same species. The old TEAM guys had a rig for testing for brashness in their Northern White Pine and it involved a consistent way of breaking a sample of a batch of wood that they had on hand. Please be careful out there.
Strength and Weight are important and grain slope/count etc are complementary.
Too strong wood wont hurt but if its heavy it wont fly.
I was very happy when I made a test by breaking a cantilever stick 1/2 inch square x 24 inch long of a random piece of SPF wood at 13 lbs force average.
A formula I took from the fly baby site showed the wood was good for 12-13 thousand pounds/ square inch or much superior to spruce.
But when I used a crude method to find the density by standing it in a pail of water it came out at 0.5 or much heavier than spruce. This wood piece had superb grain slope and count.
So now I am looking for wood with a 0.3 density and 10 kpsi capability and the grain count/slope etc.
My cousin has a beautful super lazy ace model biplane hanger Queen weighing 50 lbs hanging from the ceiling which needs a further 10 llbs in the nose to balance .The design weight of this plane is about 40 lbs max,It is all a waste due to heavy strong wood with very good grain.
It always reminds me that wood weight is important as well as strength along with grain etc.
For spars, I believe they are strongest when they have the most material the farthest apart; so reducing them in the vertical direction should be better.
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