A simplified spar sizing method—along with a basic wing mass estimation scheme—is outlined in this document. The wing geometry, along with other basic aircraft parameters should be defined: the wingspan, airfoil thickness along the wingspan, wing area, and the estimated maximum takeoff weight (MTOW) are taken as inputs.
The spar is split into the spar caps (flanges) and shear web. We assume that the spar caps take the bending moment, and the shear between to be taken by the shear web. The upper and lower spar caps are defined as identical thin laminates of unidirectional carbon fiber. They have a constant (chordwise) width, with the height varying across the wingspan.
By fixing the stress developed in the spar caps to be less than the ultimate compressive stress of the carbon material, the thickness of the spar caps may be determined throughout the wing.
Updates specific to DBF are made in red.
The load is uniformly distributed along the span of the wing. Thus, the loading equation is:
$$ \begin{equation}\omega= \frac{N(m_0)g}{b} \end{equation} $$
$N=$ Ultimate Load Factor
$m_0=$ MTOW
$g=$ Acceleration due to gravity
$b=$ Wingspan