T-tail flutter: potential-flow modelling,experimental validation and flight tests

Abstract

Flutter of T-tail configurations is caused by the aeroelastic coupling between the vertical fin and the horizontal stabiliser. The latter is mounted on the fin instead of the fuselage, and hence the arrangement presents distinct characteristics compared to other typical empennage setups; specifically, T-tail aeroelasticity is governed by inplane dynamics and steady aerodynamic loading, which are typically not included in flutter clearance methodologies based on the doublet lattice method. As the number of new aircraft featuring this tail configuration increases, there is a need for precise understanding of the phenomenon, appropriate tools for its prediction, and reliable benchmarking data. This paper addresses this triple challenge by providing a detailed explanation of T-tail flutter physics, describing potential-flow modelling alternatives, and presenting detailed numerical and experimental results to compensate for the shortage of reproducible data in the literature. A historical account of the main milestones in T-tail aircraft development is included, followed by a T-tail flutter research review that emphasises the latest contributions from industry as well as academia. The physical problem is dissected next, highlighting the individual and combined effects that drive the phenomenon. Three different methodologies, all based on potential-flow aerodynamics, are considered for T-tail subsonic flutter prediction: (i) direct incorporation of supplementary T-tail effects as additional terms in the flutter equations; (ii) a generalisation of the boundary conditions and air loads calculation on the double lattice; and (iii) a linearisation of the unsteady vortex lattice method with arbitrary kinematics. Comparison with wind-tunnel experimental results evidences that all three approaches are consistent and capture the key characteristics in the T-tail dynamics. The validated numerical models are then exercised in easy-to-duplicate canonical test cases. These parametric studies illustrate the impact of well-known factors in T-tail flutter, namely horizontal tailplane dihedral, flexibility and static deformations. In addition, scenarios are exposed in which the stability behaviour is dictated by typically second-order effects, such as chordwise forces and quadratic modes, revealing drastically different qualitative flutter curves. It is also shown that there is a distinction between angle of attack of the whole tail assembly and incidence of the horizontal tailplane relative to the fin, which might yield very counterintuitive trends depending on the configuration parameters. The paper concludes with flight test results of the Airbus A400M, epitome of modern T-tail aircraft. Tests performed in a wake-vortex encounter campaign complement the virtually nonexistent literature in the topic, demonstrate how T-tail effects can be measured in flight and restate the adequacy of potential-flow models for T-tail flutter prediction