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Flexible Aircraft

Over the past years a close and successful cooperation with Airbus-D has been established within the framework of technology projects Flexible Aircraft II and III, and MODYAS. The goal of this on-going cooperation is to adapt and extend the optimisation-based control design process to loads alleviation and comfort augmentation problems, which comprises analysis of loads control potential, definition of feasible control law structures, definition of significant criteria and derivation of appropriate models for fast and efficient computations.

The active loads control functions to be designed should be a control law extension to the standard electronic flight control system which is mainly designed regarding handling qualities. As control surfaces the conventional aerodynamic surfaces are used.

From the overall requirement "robust load reduction at certain structural segments without interfering loads at other positions and without interfering handling qualities too much" the following computational criteria have been developed and successfully applied in mode and loads control function design:

• amount of acceleration at all structure segments due to gust or manoeuvres,
• amount of loads (forces and moments) at all structure segments due to gust or manoeuvres,
• eigenvalue damping and stability of approximated linear aeroelastic models, derived automatically from aeroelastic models with unsteady aerodynamics,
• preserve handling properties (like step responses due to stick input) by comparing augmented and un-augmented aircraft.

Design of load control functions

The Multi-Objective Parameter Synthesis- (MOPS)- approach is applied for synthesis of loads control functions, providing a convenient way to handle the amount of particular demands by means of the vector valued criteria (more than 300 criteria). Descriptive criteria vectors can be formulated, combining accelerations, forces and moments for the individual structural components, whereas each position can be weighted separately.

A typical problem in synthesis of load control functions is to reduce loads at some structural points but not to increase loads at other positions up to a defined level. The min-max optimisation strategy of MOPS provides a convenient way to handle such problems. This strategy allows either to achieve the demanded load values everywhere on the structure or to detect design conflicts easily.

Model development

In the framework of loads analysis and active loads control design, models are needed that accurately describe aircraft dynamics and that allow numerical criteria of interest (including component loads, handling and stability) to be computed from various types of model analysis.

To this end, a process for integrating flight mechanics and aeroelastic models and model data has been developed and applied. This process results in nonlinear integrated flight and aeroelastic dynamics models for analysis in the time and frequency domain. The process removes overlaps between both types of model data, and transforms unsteady aerodynamic loads from the frequency into the time domain. Structural data are obtained from lumped-mass condensations and modal analysis of a finite element model of the airframe.

Flexible aircraft dynamics have become and integral part of our Modelica Flight Dynamics library .