A sliding mode approach for control and estimation in active aircraft

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Monitoring the effectiveness of any drag reduction scheme is vitally important to instil a level of confidence in its effectiveness. The active or passive drag reduction schemes will represent a 'hidden' technology on the aircraft in the sense that their effects and proper functioning will not be readily visible to the naked eye. Monitoring the efficiency of any flow control or drag reduction scheme over long periods of operation allows the pilot to estimate more accurately the required fuel reserve and assists the ground crews by indicating issues requiring inspection/maintenance which may have arisen during a flight. The objective is to utilize sliding mode observers to monitor changes in the performance of the aircraft which can be correlated to changes in the overall drag performance. Changes in drag will be formulated as unknown (and unmeasurable) changes in the aircraft system and will be estimated utilizing the equivalent output error injection signal necessary to maintain sliding. Direct development of nonlinear observers based on the nonlinear longitudinal equations of motion eliminate the errors associated with the approximations arising from linearizing - which appear to be significant when considering the level of accuracy required to estimate 1% changes in the overall drag. Sliding mode observer schemes are ideally suited to cope with such scenarios since the design process is not limited to linear system representations. All the different strategies that will be developed can be viewed as monitoring systems working independently of each other. The 'central nervous system' can then adopt a higher level monitoring objective, confirming genuine drag reduction, detecting sensor faults, and raising alarms when confronted with issues of inaccurate measurements or faults.