Analytical Correlation of a Flexible Empennage Wind Tunnel Flutter Test at High Transonic Mach Number

Abstract

URRENTLY, industry mainly relies on the conventional Doublet Lattice Method (DLM) to model the unsteady aerodynamics of lifting surfaces and bodies, due to its reduced cost and its relatively good representation of the lags between the surface motion and the aerodynamic forces. As the DLM is based on potential theory, it is not capable of capturing complex three dimensional, compressible or viscous effects. A variety of methods to correct the linear theory exist. It is standard industry practice to correct the absolute value of the aerodynamic forces based on steady aerodynamics, often gained by computational fluid dynamics calculations and rely on the DLM phase lags. This combination is possibly the cheapest unsteady aerodynamic method in terms of computing requirements. This makes the running of thousands of flight cases possible for both flutter and dynamic flight loads. One drawback to this method is the lack of theoretical basis for considering that the unsteady effects are independent of viscosity and compressibility. These effects are traditionally considered as having a major influence on the flutter characteristics of lifting surfaces, even more so when the flutter mechanism involves control surface rotation or deformation, as the shock movement drastically changes the control surface aerodynamic characteristics. Such shock motion is deemed to be highly influenced by the high reduced frequencies of the motion. The complexity of the unsteady aerodynamic phenomena which show up in the transonic speed range make extensive wind tunnel testing highly desirable both to design optimum structures and to reduce the certification risks, as well as to meet the FAA’s mandate for analyses backed up by testing [1]. One way to overcome the aerodynamic uncertainty is through the use of Computational Fluid Dynamics (CFD) codes coupled with a flexible structure finite element analysis (FEA). However the current high computational cost involving a fluid-structure interaction simulation using CFD makes it not feasible to use this method for industrial flutter calculations. For this reason it is often used only in research or cases with conditions including flow separation or buffeting, which would involve many thousands of computations, where the linearized aerodynamic method is not applicable. But in any case, the lack of maturity and validation of fluid-structure interaction codes prevents the trust of the outcome. In this paper a high fidelity computational aeroelasticity method is compared to the acquired test data from a flexible empennage model. Gulfstream conceived and executed a wind tunnel test campaign at NASA Langley Research Center’s specialized aeroelastic testing wind tunnel known as the Transonic Dynamics Tunnel (TDT) [2]. The analysis of this acquired data will not only allow Gulfstream’s Loads and Dynamics department to reduce the uncertainty in unsteady aerodynamic forces at transonic speeds and validate the current state of the art methodology