Gimbaled Tiltrotor Conversion Flight Loads Prediction Using Three-Dimensional Structural Analysis

This paper presents a method for modeling gimbaled rotor aeromechanics in Experimental 3Dstructural dynamics solver, which is a next-generation three-dimensional finite-element-based rotor structural dynamics solver. The rotor is modeled using a single blade with a free flap hinge at the hub, the motion of which is suppressed at integer multiples of the blade passage frequency by introducing harmonics of the joint rotation angle as additional trim variables, efficiently simulating gimbal kinematics. Rotor frequencies are examined for a three-bladed gimbaled rotor model and are found to combine the modes of both free flap hinge and fixed flap hinge (cantilevered) one-bladed models, similar to a teetering rotor, justifying the gimbal modeling methodology. Gimbal flapping is examined for a proprotor in edgewise flight: successful suppression of steady and 3/rev flapping indicates the gimbal model accurately represents the rotor kinematics. Air loads and blade loads are examined with and without with the gimbal model, revealing an increase in 3/rev sectional normal aerodynamic force on the blade but a decrease in the 3/rev flap and lag bending moment when the gimbal is added. Three-dimensional axial stress distribution at the blade root is examined, revealing only minor differences with and without the gimbal model, primarily in the flex beam.