I am designing a model airplane and plan to use a spruce spar. I used the schrenk approximation to calculate the the moment and shear stress along the semi-span. I found the moment at the wing root to be 2681 lb-in, which seems about right considering review of other design reports. For simplicity I modeled the wing spar as a simple cantilever beam and considered this moment of 2681 lb-in. Using a circular cross sectional area (.75 inch diameter spruce wood dowel) I found the stress at the surfaces of the dowel to be 65,661 psi. This far exceeds the compressive or tensile yield strength of any wood spar materials. At this point I am kind of stuck and am not sure how to accurately size the cross section of the wing spar. Any help would be appreciated.