On scaling and system identification of flexible aircraft dynamics.

Abstract

The use of subscale models has been common practice in the industry and has helped engineers gain more confidence in their design processes. However, each subscale model is developed for a specifc test, and consequently, different types of models are needed for observing aerodynamic, structural and aeroelastic characteristics of a full-scale aircraft. Yet, traditional aircraft design methods face serious challenges when a novel aircraft de- sign emerges and a proof-of-concept is needed for investigating this multi-disciplinary problem. An example of such a problem is the development of aircraft configurations with high aspect ratio wings for which the disciplines of aeroelastic and flight mechanics are strongly interconnected. Moreover, if the prediction of dynamic behaviour is of interest, a method that utilises system identification for analysing experimental data is of importance. Therefore, this thesis aims to develop a methodology to investigate the complex flight dynamic behaviour of flexible aircraft by combining techniques for developing subscale models and methods with the field of system identification. This aim is achieved through three objectives: 1) assessment of system identification methods for subscale flexible aircraft, 2) theoretical development of subscale modelling in terms of scaling laws and aeroelastic simulation framework and, 3) wind tunnel testing of the subscale model. Aspects of System Identification have been explored through use-cases where experimental data for a rigid aircraft both in full-scale and subscale configuration is used. The results highlight the fact that in testing a subscale model, dynamics are more prone to exhibit non-linear behaviour when compared to the full-scale model. It followed by the application of system identification for a flexible aircraft based on a simulation framework. This study emphasised the need for non-linear identification methods, such as an output error method, to characterise a flexible aircraft system. The work continues with the exploration of scaling laws applied to a simple aerofoil that is free to pitch and plunge. These results build the foundation for the development of a subscale high aspect ratio wing for wind tunnel experiments. The work highlights the trade-o s and compromises faced during the development of a dynamically subscaled model and the practice of system identification. The main contribution lies in the development of a low-cost methodology in building a subscale model that allows the use of dynamically scaled models at the early design stages. This practice provides the designer with a means to de-risk novel aircraft concepts as early as possible and in doing so, reduce overall development costs.