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COFCLUO
Clearance of Flight Control Laws using Optimisation

Tags: Air

Background

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 has 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 will focus on the computer computations in the certification process. If the computations can be made faster, time is saved which will reduce time to market for new products and will also allow for 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. The main industrial benefits of the new methods should be related to reducing the involved effort and cost, while getting sufficiently reliable results, or increasing the reliability of the analysis results with a reasonable amount of effort. Therefore a benchmark problem will be defined according to current industrial standards and the results obtained from optimisation-based clearance will be compared with a baseline traditional solution based on gridding the parameter space and testing the flight control laws for a finite number of manoeuvres.

More specifically the following objectives will be demonstrated:

  1. a higher reliability of optimisation-based clearance of flight control laws (CFCL) in detecting safe and unsafe flight conditions as compared to the current industrial standard
  2. a reduction in effort and cost in terms of simulations and flights when optimisation-based CFCL is used prior to final in-flight validation in order to define the test campaign
  3. a significant increase in safety through better quality and confidence in the clearance process when optimisation-based CFCL is used prior to final in-flight validation in order to define the test campaign.

Description of work

The clearance criteria will be selected so that the successful use of them in conjunction with optimisation-based CFCL will result in fewer off-line and manned simulations. For civil aircraft, dynamics related to the flexible structure require different, more detailed and thus larger models than what is necessary for military aircraft. Therefore new, integrated models will be developed and special attention will be 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 will be addressed. This will be essential in order to build so-called linear fractional transformation-based parametric models, which are 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 will 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 is a need for more research in optimisation algorithms dedicated to CFCL in order to overcome the above-mentioned obstacles.

Results

When the methods and tools developed in the frame of COFCLUO prove to be reliable, accurate and relevant in the validation and clearance process performed by design engineers, they are expected to become very soon part of the internal Airbus flight control laws validation process. Later on when confidence has been gained internally on such a new process involving optimisation-based methods, Airbus could propose to airworthiness authorities that they include the methods in the official clearance process. Some of the results of the project will be developed into production quality clearance tools. These tools will either be sold or licensed, and used in-house or for consulting services.

The results from the project are useful not only for clearance of flight control laws for civil aircraft but also for military aircraft. Many of the results obtained are general and can be adapted for clearance of control laws for vehicles other than aeroplanes, such as unmanned aerial vehicles, cars and trucks. Flight clearance for unmanned aerial vehicles is expected to be even more important than for manned aircraft. For the car industry, one application of optimisation-based clearance of control laws could be to improve the reliability of existing systems, such as vehicle stability control and traction control. Another application in future control systems development is automatic obstacle avoidance. The results obtained can also be used in the connection of validation of many other different types of systems, and thus the results will strengthen the ability of European industry to validate safety-critical systems in general.

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