Nonlinear Flexibility Effects on Flight Dynamics and Control of Next-Generation Aircraft

Lead Research Organisation: Imperial College London
Department Name: Dept of Aeronautics

Abstract

This project will develop a systematic approach to flight control system (FCS) design for very flexible or very large aircraft, of the type being considered for low-environmental-impact air transport and for long-endurance unmanned operations. It will create a virtual flight test environment that will support the design of advanced nonlinear FCS that fully account for the vehicle structural flexibility. To model the flight dynamics of flexible aircraft, it is necessary to develop analytical methods for generating Reduced Order Models (ROMs) via reduction of the full-order nonlinear equations of motion, and to do this in such a way that the essential nonlinear behaviour is preserved. The key issues addressed by our approach are that:1. The usual separation of flight dynamics and aeroelasticity is not appropriate for flight control when very low structural frequencies (which are also often associated with large amplitude motions) are present. Modelling and design methods based on a fully coupled system analysis are therefore necessary.2. Large wing deformations bring nonlinear dynamic behaviour, but current model reduction methods assume linearity. The development of nonlinear ROMs is an area that urgently needs advances, in general, and is necessary for control applications of flexible aircraft, in particular.3. Standard linear control design methods are inadequate for highly flexible aircraft, since their dynamic behaviour is intrinsically nonlinear. Fresh approaches to nonlinear FCS design are then required to control these systems in a provably robust way.The technical and scientific challenges to be overcome then include the simulation of significant aerodynamic and structural nonlinearities in full aircraft dynamics through the systematic development of a hierarchy of fully coupled large-order models, the reduction of these models to small-order nonlinear systems suitable for control development, and the development of robust control laws based on these reduced nonlinear models for gust load alleviation, trajectory control and stability augmentation. These methods will be exemplified in next-generation aircraft concepts that will be defined in discussion with end users. In fact, the project will benefit from a strong collaboration with major UK industrial partners, which will provide substantial technical inputs and support to the planned research activities.
 
Description We have developed new strategies to support the design and analysis of aircraft with very high aspect ratio wings. They are known to have very high aerodynamic efficiency (very low drag with respect to the lift they generate) but span is currently limited to avoid very large wing deformations. In this project we have developed the most detailed numerical models to-date to quantify the response of such vehicles to a variety of situations (turbulent weather, thermals, encountering wakes of a previous vehicle, etc.) and have shown that the large deformations, when accounted for, can in fact be beneficial -- i.e., the aerodynamic loading on the vehicle reduces and the ride becomes smoother.

We have also developed the associated flight control strategies, using both offline and online control algorithms, to steer, stabilize and damp unwanted vibrations on those aircraft with very flexible wings. We have demonstrated a strategy to generate online controls (Model-predictive control) using a nonlinear description of the full flexible vehicle, in which the simulation can be run sufficiently fast to feed into a real-time optimizer that decides actions to best track a predetermined manoeuvre. We have also developed strategies for mitigate the effect of gust encounters based on multiple sensors and actuators distributed through the flexible vehicle. Overall computer simulations have showed that reductions of 25% in the design loads (which implies similar reductions in structural weight) are possible using our proposed model-based control architectures.

Finally, we have developed strategies to integrate our simulation approach, which deals with the full system nonlinearity, into industrial design methods, which are based on assumptions of linear behaviour. We have shown that, starting from a detailed aeroelastic (e.g. structural and aerodynamic) aircraft model, we can extract all the relevant information in a postprocessing step to construct a nonlinear flexible vehicle model to investigate both the flight mechanics performance and the response to atmospheric conditions. Importantly, we preserve the linear solutions exactly by using the normal modes of the original model to describe the nonlinear system dynamics.
Exploitation Route Our findings regarding the modelling features and the transient behaviour of vehicles with very flexible wings will be useful to aircraft manufacturers to seek reduction of structural weight in next-generation air vehicles with composite wings and advanced control systems. We have released our aircraft analysis software under a non-restrictive open-source license and it has been so far used, to our knowledge, to support the design of two solar-aircraft prototypes currently undergoing flight testing. In a follow-on project we are continuing to improve the predictive capabilities of this software to facilitate the nonlinear aeroelastic analysis of a variety of vehicles. The open-source license will also allow other academic groups to use the software to develop new ideas.
Sectors Aerospace, Defence and Marine,Manufacturing, including Industrial Biotechology

URL http://www.imperial.ac.uk/aeroelastics
 
Description Following the project, we supported the UK subsidiaries of two large technology companies (Arbus Defence and Space and Facebook Ltd) in the development of solar-powered aircraft. The tools and methods developed in this project have been used to predict the in-flight dynamic behaviour of their prototype aircraft and to improve their structural and aerodynamic design.
First Year Of Impact 2015
Sector Aerospace, Defence and Marine
Impact Types Economic

 
Description (ConFlex) - Control of flexible structures and fluid-structure interactions
Amount € 3,932,721 (EUR)
Funding ID 765579 
Organisation European Commission 
Sector Public
Country European Union (EU)
Start 10/2017 
End 09/2021
 
Description Airbus-sponsored research
Amount £156,000 (GBP)
Organisation Airbus Group 
Sector Academic/University
Country France
Start 09/2016 
End 08/2019
 
Description EPSRC Doctoral Price Award to Yinan Wang
Amount £50,427 (GBP)
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 10/2015 
End 09/2016
 
Description EPSRC-ICASE Airbus
Amount £81,000 (GBP)
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 10/2016 
End 09/2021
 
Description Robust- and sustainable-by-design ultra-high aspect ratio wing and airframe (RHEA)
Amount € 2,000,000 (EUR)
Funding ID 883670 
Organisation European Commission H2020 
Sector Public
Country Belgium
Start 06/2020 
End 05/2022
 
Description Collaboration in Aeroelasticity and Structural Dynamics with Technion 
Organisation Technion - Israel Institute of Technology
Country Israel 
Sector Academic/University 
PI Contribution A collaboration was started with Prof Moti Karpel, from Technion, on the development of an aeroservoelastic analysis methodology to introduce the effects of large wing deformations into the design of load control systems on very flexible (e.g. solar-powered) air vehicles. Prof Karpel visited Imperial in two occasions (in 2011 and 2012) and has been co-author of a conference paper, which is now being submitted to the AIAA Journal. Load control systems are essential in the successful design of very large aircraft, as they cut structural weight by reducing peak loads. Their design is done using aeroservoelastic models that account for structural dynamics, aerodynamics, sensor/actuators and control. For large aircraft, wings may experience large deformations under transient dynamic response. This collaboration is developing a full description of an intrinsic nonlinear theory for load analysis of very flexible wings.
Collaborator Contribution Prof Karpel gave a short course in aeroelasticity at Imperial and provide technical experience in the development of the theory.
Impact A conference paper and a software tool
Start Year 2011
 
Description Collaboration with University of Liverpool 
Organisation University of Liverpool
Department Institute of Integrative Biology
Country United Kingdom 
Sector Academic/University 
PI Contribution The CFD Lab at Liverpool were co-investigators in this project. We developed geometrically-nonlinear structural model suitable for high-fidelity flexible aircraft simulations, including interfacing strategies.
Collaborator Contribution Our colleagues at Liverpool (led by Prof Ken Badcock) integrated our models into their unsteady RANS fluid solver. As a result we were able to investigate full vehicle response and develop nonlinear reduced-order methods based on the full fidelity description.
Impact The main output is software and methods that are now in place at the CFD Lab in Liverpool. My colleagues there have long-standing interactions with Airbus and BAE Systems and this software is available for nonlinear simulations of aircraft response. We presented two joint papers at AIAA conferences with the details of the approach.
Start Year 2011
 
Title SHARPy: Simulation of High-Aspect-Ratio aircraft and wind turbines in Python 
Description Nonlinear and dynamically linearized models of very flexible aircraft dynamics for design, analysis, and control law synthesis. 
Type Of Technology Software 
Year Produced 2014 
Open Source License? Yes  
Impact It has been used in the aerodynamic and structural design of Facebook's Aquila, Airbus's Zephyr and Astigan Ltd's solar-powered aircraft. 
URL http://www.imperial.ac.uk/aeroelastics/sharpy
 
Description Interview at BBC World Service 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Interview to Dr Palacios at BBC Click
Year(s) Of Engagement Activity 2016
URL http://www.bbc.co.uk/programmes/p03jpn3t