Dear Mike In the late 1960's I built a biplane flybaby using clear fir spars and structure. I used mahogany door skins for the sheeting on the fuselage sides and for the rib webs. You can see a picture of it on Ron Wanttaja's web site. There was a lumber yard here in the Denver area that would let me hand pick fir stair tread material from their stock. The wood was about a full inch thick and had no knots and had a very straight grain. One edge had a radius for use as the tread on a stairway. I had the planks planed to 3/4 thickness and it was very easy to build with. The information I could get was that clear fir of this type is just a bit heavier than spruce and also is stronger than spruce. You might consider this option for your project. Jim Hulet