Considerations of Gravity Effects on VAWT Rotor Configurations Which Minimize Flatwise Moments and Stresses

Abstract

Anticipating that composite materials will permit greater freedom when future VAWT rotor blades are configured, this paper further examines the benefits of adopting true Troposkien shapes for blades which do not use horizontal struts. Two parameters are identified which control blade geometry under combined rotational and gravitational loads. Representative zero-bending-moment shapes are calculated, both for constant mass distribution and for several new variable-mass families. The latter include one case whose mathematical solution proves remarkably simple. Steady design stresses are found to be severely affected by deviations from the ideal shape. The concept of “aerodynamic Troposkien,” which preserves zero bending moments in the presence of 1-P cyclic normal forces, is extended to account for gravity as well as inertial effects on blade shape. This concept is limited to unstalled flow and it is found to require unrealistic blade chord distributions near the tower when gravity effects are included. It is noted that both steady gravitational loads and nonsteady aerodynamic normal force loads will become increasingly important in achieving high-cycle fatigue life with larger-scale rotor blades. It is indicated that this achievement will depend primarily on configuring the rotor blades to minimize the cyclic flatwise bending moments and stresses due to the nonsteady normal forces.