T-tail flutter simulations with regard to quadratic mode shape components

Dominik Schäfer

doi:10.1007/s13272-021-00524-8

Abstract

It is known that the dynamic aeroelastic stability of T-tails is dependent on the steady aerodynamic forces at aircraft trim condition. Accounting for this dependency in the flutter solution process involves correction methods for doublet lattice method (DLM) unsteady aerodynamics, enhanced DLM algorithms, unsteady vortex lattice methods (UVLM), or the use of CFD. However, the aerodynamic improvements along with a commonly applied modal approach with linear displacements results in spurious stiffness terms, which distort the flutter velocity prediction. Hence, a higher order structural approach with quadratic mode shape components is required for accurate flutter velocity prediction of T-tails. For the study of the effects of quadratic mode shape components on T-tail flutter, a generic tail configuration without sweep and taper is used. Euler based CFD simulations are applied involving a linearized frequency domain (LFD) approach to determine the generalized aerodynamic forces. These forces are obtained based on steady CFD computations at varying horizontal tail plane (HTP) incidence angles. The quadratic mode shape components of the fundamental structural modes for the vertical tail plane (VTP), i.e., out-of-plane bending and torsion, are received from nonlinear as well as linear finite element analyses. Modal coupling resulting solely from the extended modal representation of the structure and its influence on T-tail flutter is studied. The g-method is applied to solve for the flutter velocities and corresponding flutter mode shapes. The impact of the quadratic mode shape components is visualized in terms of flutter velocities in dependency of the HTP incidence angle and the static aerodynamic HTP loading.

Highlights

Environmental as well as economical aspects motivate aircraft jet engine manufacturers to continuously improve their jet engine’s efficiency
T-tails are in a certain way unusual compared to conventional tails in that the stability strongly depends on the static aerodynamic loading and unsteady aerodynamic forces induced by inplane motion, which can usually be neglected in flutter simulations
A typical T-tail flutter mechanism can be described as a coupling between elastic vertical tail plane (VTP) and horizontal tail plane (HTP) deformations, usually involving out-of-plane bending and torsion of the VTP

Summary

Introduction

Environmental as well as economical aspects motivate aircraft jet engine manufacturers to continuously improve their jet engine’s efficiency. T-tails are in a certain way unusual compared to conventional tails in that the stability strongly depends on the static aerodynamic loading and unsteady aerodynamic forces induced by inplane motion, which can usually be neglected in flutter simulations. A typical T-tail flutter mechanism can be described as a coupling between elastic VTP and HTP deformations, usually involving out-of-plane bending and torsion of the VTP. As the HTP performs roll motion when the VTP undergoes out-of-plane bending, the steady lift force vector is tilted in lateral direction, inducing a lateral force component. For VTP torsion, the HTP yaws, which results in asymmetrical aerodynamic span loading and a corresponding rolling moment. Van Zyl pointed out, that the consideration of the additional aerodynamic terms alone

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Conclusion