Large wind turbine blades are being developed at lengths of 75e100 m, in order to improve energy
capture and reduce the cost of wind energy. Bending loads in the inboard region of the blade make large
blade development challenging. The “biplane blade” designwas proposed to use a biplane inboard region
to improve the design of the inboard region and improve overall performance of large blades. This paper
focuses on the design of the internal “biplane spar” structure for 100-m biplane blades. Several spars
were designed to approximate the Sandia SNL100-00 blade (“monoplane spar”) and the biplane blade
(“biplane spar”). Analytical and computational models are developed to analyze these spars. The
analytical model used the method of minimum total potential energy; the computational model used
beam finite elements with cross-sectional analysis. Simple load cases were applied to each spar and their
deflections, bending moments, axial forces, and stresses were compared. Similar performance trends are
identified with both the analytical and computational models. An approximate buckling analysis shows
that compressive loads in the inboard biplane region do not exceed buckling loads. A parametric analysis
shows biplane spar configurations have 25e35% smaller tip deflections and 75% smaller maximum root
bending moments than monoplane spars of the same length and mass per unit span. Root bending
moments in the biplane spar are largely relieved by axial forces in the biplane region, which are not
significant in the monoplane spar. The benefits for the inboard region could lead to weight reductions in
wind turbine blades. Innovations that create lighter blades can make large blades a reality, suggesting
that the biplane blade may be an attractive design for large (100-m) blades.