Multipoint Model for Flexible Aircraft Loads Monitoring in Real Time

Kurzfassung

At present state-of-the-art operational Structural Health Monitoring (SHM) systems have no real-time capability for modern transport aircraft. Moreover, these systems are based on global flight parameter thresholds or subjective pilot judgment regarding for example the severity of in-flight turbulence in order to address critical events for the structural integrity. Furthermore, this offline SHM is conducted using computational models that are expensive to be developed and maintained while also requiring rigorous validation. Considering these issues, two problems can clearly arise from using offline SHM systems: firstly, the rate of flight operations can be adversely affected and secondly, the airlines' financial success can be drastically compromised (e.g. unforeseen extra maintenance costs). Therefore, current SHM practices could be further improved to ensure cost-effectiveness and safe continued airworthiness for modern transport aircraft operations. Flight loads are usually monitored by direct loads measurements or through analytical aircraft design models fed by flight data. The first approach allows real-time monitoring, but requires infeasible reliability of numerous sensors during the long aircraft lifecycle in the harsh operational environment. The second approach has the drawback of the necessary timedemanding analysis, which also requires high computational efforts that also hinder real-time applications. Principally, most previous work has focused on developing operational SHM systems based on direct loads measurements (e.g. using classical strain gauges or modern optical fibers) or complex analytical processing of flight data in non-real time. The research presented here investigated the feasibility of developing an effective loads modeling approach that enables operational loads monitoring using standard aircraft sensors, which it is envisaged will contribute to future real-time SHM systems. This concept is demonstrated by modeling of the loads developed for seven load stations distributed across the Discuc-2c sailplane. That is, local loads are modeled at different positions of the aircraft structure; which is referred within this thesis as a multipoint loads modeling approach. The current model development pursued the best mathematical model, which contains unknown parameters that need to be determined indirectly from the measured flight test data (i.e. by means of system identification). The model needs to satisfy conflicting requirements: on the one's side the model should accurately represent the behavior of the real systems (which can be verified using the flight test data) and on the other's side it should be kept simple enough for further use (computation cost / real-time capability) and for interpretation (e.g. flight mechanics/dynamics). To enable this research a complete flight test instrumentation including many strain sensors was installed in the DLR Discus-2c high-performance sailplane. The strain sensors were calibrated by means of extensive ground tests to allow the accurate calculation of inflight local loads. A comprehensive flight test campaign was performed. The gathered flight test data permitted to identify the numerous parameters of the multipoint loads model for flexible aircraft. A novelty of this research is the extension of the time-domain system identification of a six degrees-of-freedom aircraft motion (rigid body model) by the inclusion of local loads measurements as additional observation equations. Moreover, the local dynamic effects of the structural flexibility on the local aircraft loads are also considered and therefore complement the equations of motion by another six degrees-of-freedom (i.e. multipoint loads model for flexible aircraft). To summarize the main contribution of this research is to present a new modeling approach for the development of flexible aircraft loads models in order to contribute to future real-time SHM systems, which could permit to reduce operational costs and to improve safety of the modern transport aircraft operations.