Performance envelopes for preliminary sizing of aircraft structures
Two fundamental stages within aerospace structural design are loads development and sizing. The first deals with the development of all the load cases that an aircraft may encounter in service, whereas the latter is responsible for the design of the structural components required to carry the predicted loads. A significant milestone within the airframe development is the preliminary sizing process. It aims to identify the critical loads driving the design and optimise the aircraft structure for minimum weight. Due to the resource limitations imposed by a large number of load cases and structural design constraints, there exists a gap between the loads development and sizing activities. This results in lengthy and costly design iterations, which hampers the aircraft development process. A novel method to bridge this gap is utilised in this thesis. It is based on the concepts of characteristic loads and performance envelopes. The characteristic loads are a small set of linearly independent loads acting on the aircraft, identified using Singular Value Decomposition (SVD), which can represent all the load cases of interest. Performance envelopes are created within the characteristic load space and define the load bearing capacity of a structure. These two concepts enable the engineer to relate the different aircraft loads with the structural failure constraints. This research aims to further the two concepts, specifically focusing to address the challenges arising from the preliminary sizing process. The first contribution towards improving the state of the art is in the construction of the performance envelope. A robust and efficient strategy is developed to reduce the computational costs of building the envelope. A second novelty is in the development of a framework to update the envelope as the structural design evolves in the sizing process. These two contributions improve the feasibility of industrialisation of the method The third and fourth contribution of this research is in developing and demonstrating applications possible with a performance envelope. One such application aiding the loads development is to drive the loads optimisation using structural indicators obtained from the performance envelopes. Another application supporting the sizing activity is in computing the sensitivities of the critical structural constraints to loads. These sensitivities are essential within the preliminary sizing problem and are expensive to compute using traditional finite-difference methods. The investigation illustrates that it is indeed possible to reduce the cost of computation through an envelope-based approach by ~42%. The above concepts have been explored and validated using examples based on real world load cases and standard industry analysis tool sets. The next step is to implement the above developed methods in a full aircraft preliminary sizing along with a benchmarking with other alternative methods. However, this has not been addressed in the current work. The results from this research indicate that a performance envelope can indeed be a cost-effective approach to bridge the gap between the loads development and sizing processes. The outcome of the thesis demonstrates that the overall efficiency of the aircraft preliminary sizing stage can be improved by exploiting these new concepts.
Preliminary sizing, Performance envelopes, characteristic loads, aircraft design, aircraft loads, constraint sensitivity, reserve factor
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