This paper reports an investigation of the use of off axis carbon fibers in the all-carbon spar cap of a 37-m. wind turbine rotor blade to induce twist flap coupling. Many studies have been published on the structure of wind turbine rotor blades incorporating off-axis fibers; none has studied optimizing the blade structure simultaneously considering the angle of off-axis material, the fraction of off-axis material, constraints on cross-fiber and in-plane shear strength, constraints on tip deflection, and blade cost. A parametric study has been conducted varying the angle of off axis fibers from 5 degrees to 25 degrees and varying the volume fraction of off-axis fibers in the spar cap from 10% to 90%. In all configurations, the remainder of the spar cap material is 0 degrees carbon fiber: The spar cap thickness has been adjusted in each blade to simultaneously minimize the weight of carbon material, and hence the blade cost, while satisfying constraints on carbon fiber strain and tip deflection. The study also examines the cross-fiber strain and stress and the in-plane shear stress in the 0 degrees and off-axis carbon layers. The conclusion of this study is that the optimal angle,for most cost-effectively achieving twist-flap coupling-considering constraints on fiber strain, cross-fiber strength, in-plane shear strength, and tip deflection-is closer to 7.5 degrees than the 20 degrees that has frequently been reported by prior researchers. As much as 90% of the spar cap carbon fibers can be rotated to 7.5 degrees off axis before in-plane shear strength is exceeded.