Impact case study database

Loss of control in flight (LOC-I) is the main cause of fatal accidents in civil aviation. Over the period 2009-2018, there were 64 LOC-I accidents, 94% of which involved fatalities (IATA 2019, https://www.iata.org/contentassets/b6eb2adc248c484192101edd1ed36015/loc\-i\_2019.pdf\). To tackle this problem, the European Union Aviation Safety Agency has recently consolidated the existing requirements for UPRT (Decision 2019/005/R). UPRT aims at providing pilots with skills to prevent and recover from situations in which an aeroplane unintentionally exceeds the parameters for line operation. An important part of the training is based on the use of flight simulators, which should be equipped with flight models that extend the flight envelope.

Prof. Goman and his team at DMU have made important contributions [R1, R3, R5, R6] to the principles of aerodynamic modelling for out-of-the-envelope scenarios characterised by aerodynamic stall and LOC-I. The aerodynamic modelling is complemented with an efficient computational methodology and software tools, which have been developed to investigate nonlinear aircraft dynamics, allowing an efficient validation of out-of-the-envelope flight simulation models and computational clearance of flight control laws [R2, R4].

(1) NONLINEAR AERODYNAMICS: COHESION OF EXPERIMENTAL WIND TUNNEL DATA AND CFD SIMULATIONS

The approach to aerodynamic modelling draws on the use of experimental data obtained in wind tunnels and complementary simulations using Computational Fluid Dynamics (CFD) methods. Such a blended approach allows improved fidelity of the aerodynamic model in the stall region due to the elimination of interference effects in the experiment and extrapolation of the results for higher Reynolds numbers typical for real flight conditions [R6]. The key features of this approach include an adequate phenomenological modelling of aerodynamic hysteresis in dynamic conditions based on the so-called ‘Goman-Khrabrov model’ and CFD predictions for the intensity of aerodynamic autorotation in the stall region, which is required for realistic simulation of lateral aircraft departures at high Reynolds numbers [R3, R6].

(2) INNOVATION IN WIND TUNNEL TESTING

The development of new experimental techniques is important for correct phenomenological modelling of aerodynamics in the stall region. The existing experimental rigs for static, rotary-balance and forced oscillation wind tunnel tests do not address the effect of transient motions and presence of interference from a support system. An innovative five degree-of-freedom wind tunnel manoeuvre rig proposed by Prof. Goman was built at the Aerospace Department of the University of Bristol in collaboration with Prof. Lowenberg [R5]. This virtual-flight-in-wind-tunnel experimental rig enables a large set of conventional and extreme aircraft manoeuvres to be performed in the controlled environment of a wind tunnel, allowing improved physical simulation of aircraft dynamics in the stall region. The obtained observations of the asymmetric aerodynamic loads and static aerodynamic hysteresis phenomenon made possible important improvements in the aerodynamic structure of the flight simulation models.

(3) NONLINEAR FLIGHT DYNAMICS: COMPUTATIONAL METHODOLOGY

To address the validation problem for flight models, an efficient computational methodology for investigation of nonlinear aircraft dynamics has been developed. The combination of nonlinear aerodynamic dependencies with nonlinear inertial coupling in rigid body dynamics, as well as nonlinearities and constraints in the control system, leads to the study of a nonlinear system of large dimensions. The methodology is based on the use of Attainable Equilibrium Sets (AES) and classical methods of nonlinear dynamical systems theory, including a standard linearisation procedure and local stability analysis, which are complemented with computation and analysis of two-dimensional cross-sections of regions of attraction against disturbances in state variables and control inputs [R2]. The key element of the proposed methodology is the sequential consideration of the open airframe and a closed-loop system with nonlinearities due to control constraints. The methodology was implemented as an interactive computational system in MATLAB/Simulink. This development has allowed bifurcation and continuation techniques that are popular in academia to move to a level where exhaustive qualitative and quantitative computational studies of nonlinear aircraft dynamics can be carried out without overly simplifying the flight simulation models used in flight simulators [R4].

(4) IMPLEMENTATION

The research results outlined in the previous sections have allowed the development of realistic flight simulation models tested and tuned by experienced test pilots for an extended flight envelope. DMU has successfully implemented [R3] the approach in the EU FP7 ‘Simulation of Upset Recovery in Aviation’ (SUPRA) project (Grant Agreement ID:233543) for a typical airliner, in the first commercial UPRT flight simulator (AMST-Systemtechnik’s Airfox UPRT, https://www.amst.co.at/en/aerospace\-medicine/training\-simulation\-products/airfox/airfox\-uprt/\) for a class-specific transport aircraft, and for a manoeuvrable generic tailless aircraft (GTA) [R4].

The overarching theme of the research is developing modelling methods to solve challenging problems for the water industry. The model reduction method [R1] is based on the variable elimination concept. The method reduces the simulation time of water distribution network models by several orders of magnitude and is more robust than other methods based on optimisation techniques [R1]. The model reduction method facilitated solving many optimisation problems for WDS, including pump scheduling. Network model reduction and pump scheduling were developed further in the EPSRC ‘Neptune’ project [G1]. An online version of the model reduction method was implemented, including a new feature of network nodes reordering that accelerated the calculations by several orders of magnitude [R1].

One of the results of the EPSRC project ‘Efficient Energy Management for Water Distribution Systems and Treatment Processes’ [G2] was a new pump station model that removed numerical singularities from the optimisation problems [R2]. For pump station modelling, the power characteristic was evaluated from hydraulic and efficiency curves, or the mechanical power was approximated directly by a cubic polynomial and scaled by pump speed and the number of pumps.

Reducing water losses (leakage) in WDS was another important focus of Ulanicki’s research. Leakage reduction can be achieved by a coordinated action of pressure control and burst detection. Ulanicki investigated pressure control in the ‘Optimised On-line Pressure Control for Networks having Multiple Pressure Reducing Valve Inputs’ EPSRC project [G3]. Solutions were provided for district metering areas (DMA) with many inlets in the form of time or flow modulation strategies. Even when the pressure controls are in place, bursts still happen and need to be located and repaired quickly. A burst detection method was developed in the ‘Reduction of Water Losses and Energy Consumption Using an Effective Process for Burst Detection’ EPSRC/STI project [G4]. The pressure control research was initiated in the EPSRC RAIS project [G5]. Pressure control and burst detection were developed further in the EPSRC ‘Neptune’ project to include dynamic pressure transient aspects. The burst detection method was enhanced by using an active identification procedure that exploits the difference of head loss between the monitored nodes [R3]. This indicator removed errors caused by inaccurate elevation information and logger offsets.

WDS are equipped with sophisticated instrumentation for monitoring and control purposes, including local and global control loops, which, if not designed properly, can lead to spurious dynamic behaviour and instabilities in the water distribution system. Ulanicki developed methods [R4] to analyse the dynamic behaviour of WDS equipped with control components. In [R5, R6], he explained the root cause of instabilities that tend to arise in PRVs under low-flow conditions. He found that the loss of stability in PRVs is a direct result of an increase in the static valve-network gain as the valve position gets smaller, thus making pressure changes more sensitive to valve position adjustments. If the valve controller is tuned at medium valve openings characteristic of normal operating conditions, the increased gain at low valve openings can cause the control system to be too aggressive in its valve position adjustments, leading to oscillations. In [R6], he derived the gain equation for a simplified pipe-PRV-pipe model. The gain equation curve was then used to derive the formula for a gain compensator whose purpose is to keep the static gain constant across an entire range of permitted valve openings. A network transient model was then used to show the remedial effects of the gain compensator.

Dr C.H. Oxley has been involved in the research and manufacture of electronic devices in industry and academia for nearly 50 years. At Plessey Research Caswell (between 1970 and 1986), he was chief physicist for gallium arsenide (GaAs) transistors, securing funding from MoD, ESA, INTELSAT and private venture. From 1986 to 1989, he was Semiconductor Manager for Plessey Microwaves. He joined academic staff at DMU in October 1999 and initiated research work on thermal optimisation of microwave semiconductor devices. In 2001, he won an EPSRC grant [G1] in collaboration with GEC Research (Caswell) and the MoD. The project improved understanding of the thermal behaviour of the saturation velocity in gallium nitride (GaN) high electron mobility transistors (HEMT) [R1, R2] and the active thermal impedance in monolithic microwave integrated circuits (MMIC) [R2]. EPSRC rated the work as ‘tending towards outstanding’. In parallel, Oxley established an industrial collaboration with e2v Ltd (Lincoln) to improve the thermal performance of millimetric wave (77 GHz) Gunn diodes for adaptive automotive cruise control. The research work comprised computation and thermal measurement techniques to optimise the design of the integrated heatsink and bonding configurations to improve the thermal handling capability of vertical millimetric GaAs Gunn diodes. This work was commercially sensitive so little was published [R3]. The thermal work with e2v, MoD and GEC Research led to an EPSRC award in 2005 [G2]. The research outcomes included making more accurate IR temperature measurements on transparent semiconductors and metal contacts. Both a numerical computation method [R4], as well as a novel high emissivity MPIRS [R5] were developed. The novel MPIRS is very versatile, enabling a more accurate IR surface thermal measurement on materials with low surface emissivity, for example gold metal contacts, and on materials transparent to IR radiation, for example multilayer semiconductor devices [R5]. This approach totally eliminated the conventional method of coating semiconductor devices with a high emissivity paint resulting in lateral heat reducing the thermal spatial resolution and damaging the device. The MPIRS consists of a 3–20 μm diameter sphere fabricated from a high emissivity material placed in iso-thermo contact with the device surface, at the point the temperature is to be measured. By eliminating the high emissivity coating, the thermal spatial resolution is much improved (approximately 0.3 microns) without damaging the device, thereby adding an element of environmental friendliness. The research on enhancing thermal device design and the novel MPIRS technology secured a number of successful industrial and university collaborations. For example, in 2009, it led to a large collaborative EPSRC project [G3], partnering with Bristol (EP/H011366/1), Glasgow (EP/H011862/1) and Aberdeen (EP/H012532/1) universities. The project initiated groundbreaking research [R6] in the design and performance of GaAs and indium phosphide (InP)-based planar Gunn diodes using novel thermal management technologies, for example aluminium gallium arsenide / gallium arsenide (AlGaAs/​GaAs) integrated micro-coolers.