The pressure from society and politics on the aviation industry to develop, build and operate energy-efficient and low-noise aircraft is growing. For future aircraft conceps, T-tails are increasingly envisaged, which require special attention with regard to dynamic aeroelastic stability in the context of aircraft certification. A numerical investigation of the flutter oscillations serves the detailed understanding of relevant physical effects for the safe prediction of critical flight ranges.
For the reduction of emissions towards a green and socially acceptable air transport, jet engines with high bypass ratio, turboprop engines or distributed (hybrid) electric propulsion concepts are increasingly used for concept designs of future aircraft generations. The usual low wing configuration of cruise aircraft, such as the A320, no longer provides sufficient space under the wing to safely operate turbines with such large diameters. High wing configurations or engines mounted on the tail of the fuselage are therefore being considered as an alternative. For reasons of better controllability and flight mechanical stability, as well as aspects of engine integration at the tail of the fuselage, these configurations usually require a tail with the horizontal stabilizer mounted on top of the vertical stabilizer. Because of the "T" formed in this way, such a design is also called a T-tail.
The interaction of aerodynamic forces in flight with the mass and stiffness of the airplane structure can, in unfavorable cases, lead to oscillations of the structure, which can grow with time and shortly result in severe damage to the structure and even loss of the airplane. Such dynamic aeroelastic instability is called flutter and occurs at even the smallest disturbances above a critical airspeed, the flutter speed. Within the scope of aircraft certification, it must be demonstrated by flight tests and analyses that this flutter speed is at least 15% greater than the maximum flyable airspeed for all possible operating conditions and that the occurrence of flutter can thus be reliably excluded. However, the flutter speed may well depend on how strongly the structure is deformed, e.g. by a gust. A well-known phenomenon in this context are Limit Cycle Oscillations (LCOs), in which the oscillatory motions after an excitation initially increase in amplitude, but ultimately reach only a limited amplitude due to structural and/or aerodynamic nonlinearities [1, 2]. In principle, such phenomena can even occur below the flutter velocity. For accurate flutter analyses it is therefore necessary to have a comprehensive knowledge of the possible flutter mechanisms of the aircraft as well as of existing nonlinearities and thus of the behavior of these mechanisms even at larger deformations.
To get a first insight into the influence of a nonlinear and thus more accurate description of the structural deformation on amplitude-dependent air forces it is useful to first consider a generic horizontal stabilizer in isolation. In a CFD procedure, the horizontal stabilizer is excited harmonically at different frequencies and deformation amplitudes in a motion typical for T-tails [3]. The deformation is described geometrically linear as well as nonlinear. The normalized aerodynamic forces plotted against the normalized deformation initially show a clearly nonlinear characteristic in the case of linear deformation, which is evident from the deviation of the shape of the hysteresis from a pure ellipse. However, when a geometrically nonlinear deformation is used, the aerodynamic nonlinearity is substantially reduced. Consequently, only the nonlinear description of the structural deformation avoids an artificial nonlinearity in aerodynamics. This emphasizes the necessity of such a deformation description both for the numerical investigation of physical effects in T-tail flutter, and for the accurate prediction of critical flight regions. This is a contribution to the design of the new generation of aircraft for the Green Deal at the Institute of Aeroelasticity.
References and recommendations for further reading
Author: Dominik Schäfer, DLR-Institute of Aeroelasticity, Department Aeroelastic Simulation