Abstract: This paper examines the static and dynamic behavior ofinitially twisted composite spars for tilt-rotorapplications, as well as documenting the fabrication offive pairs of initially twisted advanced compositesingle-cell spars. Spars having five different materialstacking sequences were fabricated for a givengeometric and pretwist definition so as to determinethe maximum and minimum spar untwist for a given unitload (stiffness approach) and a given maximum load(strength approach). Analytical static stiffness andstrength results are presented using a detailedshell-type finite element model, along with atwo-dimensional elasticity approach to show how themaximum elastic beam untwist varies as a function ofthe composite spar geometric parameters and materialply orientation. Experimentally measured free-free andcantilevered natural frequencies and damping levels ofthe spars are presented along with a correlation to ahighly refined shell-type finite element model. Thesevibration results reveal that both ply angleorientation and initial twist significantly alter thespar natural frequencies, where the effects are largestin the higher bending and torsion modes. Damping levelchanges were clearly observed with ply orientationchanges (i.e. increases in extension-torsion couplingproduced bending mode damping increases and torsionmode damping decreases). !9
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