c l = 2 π α {\displaystyle \ c_{l}=2\pi \alpha } where c l {\displaystyle c_{l}\!} is the section lift coefficient, α {\displaystyle \alpha \!} is the angle of attack in radian, measured relative to the chord line. d c m , c / 4 d α = m 0 {\displaystyle \ {dc_{m,c/4} \over d\alpha }=m_{0}} where c m , c / 4 {\displaystyle \ c_{m,c/4}} is the moment taken at quarter-chord point and m 0 {\displaystyle \ m_{0}} is a constant.