Flutter Characteristics of the Interfering Wing and Engine Nacelle Determined from their Mean Aerodynamic Powers.

Abstract

The problem of unsteady aerodynamic interference between n interfering bodies or surfaces is solved for the limiting case of small amplitudes. The mathematical part of the solution rests on a higher order panel method approximating the vorticity transport equation in incompressible flow. The concept of the numerical procedure is briefly outlined and applied to the computation of unsteady aerodynamic forces and moments of an engine nacelle suspended from a wing. Unsteady forces and moments resulting from prescribed kinematics of the individual interfering bodies define a mean power during one cycle of motion for any of the degrees of freedom involved in the problem. From this point of view, flutter is determined to be the state of equilibrium of all powers contributing to the balance of total mean power for each body. Thus, the set of all flutter solutions (phase shift and amplitude ratio of each individual degree of freedom with respect to a motion of reference) is given completely by the analysis of unsteady aerodynamics. The mechanical parameters of a particular body (mass distribution, parameters of its suspension such as pitch axis, stiffness, etc.) merely select distinct solutions from this set. The final goal of the flutter computation, e.g. a plot of reduced flutter velocity versus the ratio of selected natural frequencies, isobtained from a straightforward linear analysis within this set of solutions. Thereby, forces and moments for each individual reduced frequency provide one complete flutter point in such a plot. The method is applied to the wing/engine interference by including selected high-accuracy solutions of critical flutter regions to systematic investigations computed with lower precision.