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.
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.
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.
Summary of the work done in the Work Packages:
This work package was dedicated to the development of suitable parametric models to serve for aircraft clearance purposes and related software tools to support various modelling activities. The basic aircraft models describe the dynamics of a generic two engines civil aircraft and serves primarily for simulation and structural modes analysis purposes. The provided flight controller covers both the normal as well as peripheral flight envelope. For implementing different clearance criteria for a range of optimisation-based approaches, different types of parametric models were needed to be employed. A non-linear dynamics aircraft model with explicit parametric dependencies has been developed together with appropriate flight control laws to be cleared. Also so-called integral linear models depending on relevant parameters have been provided to model flexible aircraft configurations. A criteria library has been defined and implemented starting from the specifications of the benchmark problem for both the integral linearised as well as non-linear closed-loop aircraft models. The trimming and linearisation tools have been also used to obtain parameter dependent linearised models (so-called LPV models), which can be alternatively described using linear fractional transformation (LFT) based representation of system matrices. LFT-based models for the closed-loop aircraft models (both nonlinear and integral models) have been generated to serve for analysis purposes. While most of the work has been carried out during the first year, the LPV- modelling and LFT-generation activities have been pursued practically during the whole project period by improving successively the quality of approximations, developing new LPV-approximation methods and generating LFT - models of lower complexity.
Several different techniques were developed to address clearance of flight control laws (CFCL). They can be grouped in two different categories:
- sufficient techniques based on solving convex optimisation problems and using LFR models of the aircraft;
- necessary techniques based on solving nonlinear optimisation problems and using standard nonlinear differential equation models of the aircraft.
In the first category LFT models had to be developed and then convex optimisation problems were solved. In case the method delivers a positive answer, it is for sure kn
Industry has made a huge step forward thanks to the COFCLUO project and a part of the developed methods will be certainly used in a development context within AIRBUS. Later on when confidence has been gained internally, can AIRBUS 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 CFCL for civil aircraft but also for military aircraft. Many of the results obtained are general and can be adapted for CFCL 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 different types of systems, and thus the results will strengthen the ability of European industry to validate safety-critical systems in general.