New aircraft designs must be examined for possible aeroelastic instabilities, especially for flutter. The flutter stability must be demonstrated for all operational configurations of the aircraft, especially taking into account various loading conditions. Due to the numerous possible configurations, these investigations are generally carried out in the frequency domain.
Applied methods
The commercial software ZAERO is used as the so-called flutter solver for aeroelastic questions in the frequency domain. This product has, on the one hand, the basic capability to routinely carry out flutter calculations for standard tasks. On the other hand, it allows expansion of the functionality through additional modules that can be written by the user. In this way, the software can also be used to solve research-based tasks.
Engine influences
The first expansions in the aeroelastic modelling of engines were undertaken in the project iGREEN, with the objective of improving the prediction of flutter. This type of advanced modelling includes both the aerodynamics as well as the structure of the engine itself. Apart from the connection of the pylons to the fuselage structure, modelling of the mass effect of the rotating engine components (gyroscopic and centrifugal effects) plays an important role. From the point of the aerodynamic flow around the engine, the time-specific behaviour of the engine jet and its resulting effect on the pendulum are consolidated in a timely staggered force vector. First examinations have shown that these phenomena have only a small effect in comparison to the dynamic primary characteristics of the elastic aircraft used in the tests. Nevertheless, it could be shown that for certain parameter combinations these engine phenomena can highly influence the vibration behaviour in the frequency and natural modes.
To demonstrate the process, a model of the Advanced Technologies Testing Aircraft System (ATTAS) was built and a gyroscopic (centrifugal effect) as well as a thrust vector were included. Due to the skewed symmetry of the incorporated effects, the aircraft structure lost its symmetrical characteristics. Asymmetrical forms of vibration occurred.