Advanced linear methods for T–tail aeroelasticity

Abstract

Flutter is one of the primary aeroelastic phenomena that must be considered in aircraft design. Flutter is a self-sustaining structural vibration in which energy is extracted from the air flow and transferred to the structure. The amplitude of the vibration grows exponentially until structural failure occurs. Flutter stability requirements often influence the design of an aircraft, making accurate flutter prediction capabilities an essential part of the design process. Advances in computational fluid dynamics and computational power make it possible to solve the fluid flow and structural dynamics simultaneously, providing highly accurate solutions especially in the transonic flow regime. This procedure is, however, too time-consuming to be used in the design optimisation process. As a result panel codes, e.g., the doublet lattice method, and modal-based structural analysis methods are still being used extensively and continually improved. One application that is lagging in terms of accuracy and simplicity (from the user’s perspective) is the flutter analysis of T-tails. The flutter analysis of a T-tail usually involves the calculation of additional aerodynamic loads, apart from the loads calculated by the standard unsteady aerodynamic codes for conventional empennages. The popular implementations of the doublet lattice method do not calculate loads due to the in-plane motion (i.e., lateral or longitudinal motion) of the horizontal stabiliser or the in-plane loads on the stabiliser. In addition, these loads are dependent on the steady-state load distribution on the stabiliser, which is ignored in the doublet lattice method. The objective of the study was to extend the doublet lattice method to calculate the additional aerodynamic loads that are crucial for T-tail flutter analysis along with the customary unsteady air loads for conventional configurations. This was achieved by employing the Kutta-Joukowski theorem in the calculation of unsteady air loads on lifting surface panels. Calculating the additional unsteady air loads for T-tails within the doublet lattice method significantly reduces the human effort required for T-tail flutter analysis as well as the opportunities for introducing errors into the analysis. During the course of the study it became apparent that it was necessary to consider the quadratic mode shape components in addition to the linear mode shape components. Otherwise the unsteady loads due to the rotation (“tilting”) of the steady-state load on the stabiliser, one of the additional aerodynamic loads that are crucial for T-tail flutter analysis, would give rise to spurious generalised forces. In order to reduce the additional burden of determining the quadratic mode shape components, methods for calculating quadratic mode shape components using linear finite element analysis or estimating them from the linear mode shape components were developed. Wind tunnel tests were performed to validate the proposed computational method. A T-tail flutter model which incorporated a mechanism for changing the incidence angle of the horizontal stabiliser, and consequently the steady-state load distribution on the horizontal stabiliser, was used. The flutter speed of this model as a function of the horizontal stabiliser incidence was determined experimentally and compared to predictions. Satisfactory correlation was found between predicted and experimentally determined flutter speeds.