An Integrated Three-Dimensional Aeromechanical Analysis for the Prediction of Stresses on Lift Offset Coaxial Rotors

This paper presents the first application of an Integrated Three-Dimensional aeromechanical analysis-defined as the coupling of three-dimensional finite element-based structural dynamics with a three-dimensional Reynolds-Averaged Navier-Stokes-based fluid dynamics-to predict the stresses on a lift-offset coaxial rotor. The coupling was carried out with the University of Maryland structural dynamic solver X3D and the U.S. Army CREATE^TM-AV Helios suite of fluid dynamic solvers. A modern four-bladed hingeless coaxial rotor model-inspired by the gross dimensions of the Sikorsky S-97 Raider but generic and open source otherwise-is developed as a demonstration test case. The new structural analysis is both enabled and driven by advanced high-performance computing, parallel and scalable solvers, high-order three-dimensional brick finite elements unified with multibody dynamics, integrated aeromechanics, and a special 3D-to-1D fluid-structure interface that refines the power of the delta-coupling procedure while retaining the advantages of existing CFD mesh motion schemes. The analysis predicts the three-dimensional stresses on the rotor blades and hub, together with the deformations, airloads, and wake, in an integrated manner. Three flight conditions are studied: a low-speed flight at µ = 0.1, a high-speed flight at µ = 0.35, and a very high-speed flight at µ = 0.5. Interesting three-dimensional unsteady stress patterns are revealed all across the blade but particularly inboard of 50%R-patterns that change from flight to flight, with and without lift offset, and have remained invisible until now-since they could neither be predicted nor measured in flight. The key conclusion is that such analysis is now indeed possible, and the stress patterns they reveal provide deeper insights into the dynamics of advanced rotors, and these might provide a path toward mitigating them in the future.