Engine Placement Effect on Nonlinear Trim and Stability of Flying Wing Aircraft

The effects of engine placement and sweep on the flutter characteristics of an aft-swept flying wing resembling the Horten IV are investigated using the Nonlinear Aeroelastic Trim and Stability of HALE Aircraft. The analysis was validated against the published results for divergence and flutter of swept wings and found to be in excellent agreement. Moreover, the modal frequencies and damping obtained for the Goland wing were found in excellent agreement with published results based on a new continuum-based unsteady aerodynamic formulation. This aircraft exhibits a nonoscillatory yawing instability, expected in aircraft with neither a vertical tail nor yaw control. More important, however, is the presence of a low-frequency "body-freedom flutter" mode. The aircraft center of gravity was held fixed during the study, which allowed the aircraft controls to trim similarly for each engine location and minimized flutter speed variations along the inboard span. The maximum flutter speed occurred for engine placement just outboard of 60% span with engine center of gravity forward of the elastic axis. The body-freedom flutter mode was largely unaffected by the engine placement except for cases in which the engine is placed at the wing tip and near the elastic axis. In the absence of engines, aerodynamics, and gravity, a region of minimum kinetic energy density for the first symmetric free-free bending mode is also near the 60% span. A possible relationship between the favorable flutter characteristics obtained by placing the engines at that point, and the region of minimum kinetic energy is briefly explored.