Yes Douglas Fir is a strong, hard softwood. Spruce is another. Spruce was used extensively in aircraft manufacture in the first part of the 20th Century. Birch was also used. Ironically, the spruce goose, built by Howard Hughes, the largest flying boat ever made, was made
mostly of birch. Also birch was much used in the
de Havilland Mosquito -- in 1941 one of the fastest planes in the world.
So for the OP, cut down all the spruce and birch and build a plane. Much more fun for the kids than a swing set. However, to use the birch, you'll need a veneer mill as birch is commonly used in plywood form.
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It doesn't really matter what Phil is saying, the music of his voice is the appropriate soundtrack for a bicycle race. HTupolev