Introduction to GRASP—General Rotorcraft Aeromechanical Stability Program—A Modern Approach to Rotorcraft Modeling
Abstract:The General Rotorcraft Aeromechanical Stability Program (GRASP) is described in terms of its capabilities and development philosophy. The program is capable of treating the nonlinear static and linearized dynamic behavior of structures represented by arbitrary collections of rigid‐body and beam elements that may be connected in an arbitrary fashion and are permitted to have large relative motions. The main limitation is that periodic coefficient effects are not treated, restricting the solutions to rotorcraft in axial flight and ground contact conditions. Rather than following in the footsteps of other rotorcraft programs, GRASP is more of a hybrid between finite element programs and spacecraft‐oriented multibody programs. GRASP differs from standard finite‐element programs by allowing multiple levels of substructures in which the substructures can move and/or rotate relative to others with no small‐angle approximations. This capability facilitates the modeling of rotorcraft structures, including the rotating/nonrotating interface and details of the blade/root kinematics for various rotor types. GRASP differs from standard multibody programs by considering aeroelastic effects, including inflow dynamics (simple unsteady aerodynamics) and nonlinear aerodynamic coefficients. The main structural element is the aeroelastic beam element which may possess arbitrarily more than the 12 degrees of freedom common in beam elements. Although it is assumed in the analysis that the strain components in the aeroelastic beam element remain small compared to unity, no kinematical limitations are imposed on the magnitudes of the displacements and rotations. Numerical results from GRASP are presented and compared with results from an existing, special‐purpose coupled rator/body aeromechanical stability program and with experimental data far large deflections of an end‐loaded cantilevered beam. The agreement is excellent in both cases.
Document Type: Research Article
Publication date: April 1, 1987
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