COFCLUO - Clearance of flight control laws using optimisation
Overview
Background & policy context:
Proving to the certification authorities that an aircraft is safe to fly is a long and complicated process. It is the responsibility of the manufacturer to show that the aircraft complies with the certification specifications, and especially the so-called airworthiness code. This code contains a huge amount of different criteria that have to be met. Before manned flights are performed to show that an aircraft meets all the clearance criteria, simulations and computer computations are performed.
This project focussed on the computer computations in the certification process. If the computations can be made faster, time is saved which will reduce time to market new products and will also allow rapid prototyping. Moreover, it is also desirable to make the computations more detailed and accurate which would improve the quality of the certification process, and thus increase the safety of aircraft.
Objectives:
It is important to keep in mind that the questions addressed in this project are not purely technical, since industry is already technically able to successfully clear flight control laws (CFCL). The main industrial benefits of the new methods should be related to reducing the involved effort and cost, whilst getting sufficiently reliable results, or increasing the reliability of the analysis results with a reasonable amount of effort. Therefore a benchmark problem has been defined according to current industrial standards and the results obtained from optimisation-based clearance have been compared with a baseline traditional solution based on gridding the parameter space and testing the flight control laws for a finite number of manoeuvres. The clearance criteria have been selected so that their successful implemntation, in conjunction with optimisation-based CFCL will result in fewer off-line and manned simulations.
Optimisation-based CFCL will not only increase safety but it will also simplify the whole certification and qualification process, thus reduce costs. The speedup achieved by using the new optimisation-based approach will also support rapid modelling and prototyping and reduce 'time to market'. The project addressed the two top-level objectives of the Work Programme:
- To meet society's needs for a more efficient, safer and environmentally friendly air transport.
- To win global leadership for European aeronautics, with a competitive supply chain, including small and medium size enterprises.
Methodology:
For civil aircraft, dynamics related to the flexible structure require different, more detailed and thus larger models than necessary for military aircraft. Therefore new, integrated models were developed and special attention paid to the fast trimming and linearisation of these models. Also the question of how to obtain rational approximations of the state space matrices of the linear parameter-varying systems resulting from the linearisation was addressed. This was essential in order to build linear fractional transformation-based parametric models, which were the State-of-the-Art model representations used in robustness and stability analysis of control systems.
In addition to this, the optimisation problem for CFCL is, in some cases, non-convex, hence there are local optima. This means that many optimisation methods could not find the global worst-case parameter combination, which for the CFCL might result in the wrong conclusions. Moreover, optimisation algorithms for non-convex problems often have tuning parameters which for the ordinary engineer might be difficult to understand. Also some optimisation problems might have such a large dimension, or the number of problems to be solved might be so large, that answers might not be found in reasonable time. Thus there was a need for more research in optimisation algorithms dedicated to CFCL in order to overcome the above-mentioned obstacles.
The project was structured into three work packages, namely modelling, optimisation, evaluation.
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