An Integrated Loads Analysis Model Including Unsteady Aerodynamic Effects for Position and Attitude Dependent Gust Fields

Kurzfassung

The modelling approach of an integrated loads analysis model is presented. The model is suitable to calculate gust as well as manoeuvre loads in the time domain. The equations of motion can account for large nonlinear motions as well as small amplitude fexible deformations. The unsteady aerodynamic matrices from the Doublet Lattice Method (DLM) are fitted by an Ration Function Approximation (RFA) for use in the time domain simulation. An alternate form of the RFA is proposed, where the quasi-steady and unsteady aerodynamics are separated in a physical way. The unsteady aerodynamic contributions, namely added mass and wake induced effects, are solely dependent on the time derivative of the normal velocity at the control points. The approach of the physical RFA is verified by analogies with the 2D unsteady thin airfoil theory. In the case of design gusts for load analysis, the wind field can assumed to be independent of the aircraft motion. Hence, the time history of the gust induced loads can be calculated beforehand. However, if the wind field is not independent of the aircraft motion, such as in the case of a wake vortex encounter, this is not possible anymore. The influence of the wind field has to be included in the time domain simulation, since the unsteady gust column of the wind field depends non-linearly on the position and attitude of the aircraft. An example of a wake vortex encounter simulation is presented, using the present physical RFA. The necessity for accounting for large rigid body motions and unsteady aerodynamic effects is demonstrated.