Inverse modelling and inverse simulation for system engineering and control applications

Following extensive development over the past two decades, techniques of inverse simulation have led to a range of successful applications, mainly in the fields of helicopter flight mechanics, aircraft handling qualities and associated issues in terms of model validation. However, the available methods still have some well-known limitations. The traditional methods based on the Newton-Raphson algorithm suffer from numerical problems such as high-frequency oscillations and can have limitations in their applicability due to problems of input-output redundancy. The existing approaches may also show a phenomenon which has been termed “constraint oscillations” which leads to low-frequency oscillatory behaviour in the inverse solutions. Moreover, the need for derivative information may limit their applicability for situations involving manoeuvre discontinuities, model discontinuities or input constraints. Two new methods are developed to overcome these issues. The first one, based on sensitivity-analysis theory, allows the Jacobian matrix to be calculated by solving a sensitivity equation and also overcomes problems of input-output redundancy. In addition, it can improve the accuracy of results compared with conventional methods and can deal with the problem of high-frequency oscillations to some extent. The second one, based on a constrained Nelder-Mead search-based optimisation algorithm, is completely derivative-free algorithm for inverse simulation. This approach eliminates problems which make traditional inverse simulation techniques difficult to apply in control applications involving discontinuous issues such as actuator amplitude or rate limits. This thesis also offers new insight into the relationship between mathematically based techniques of model inversion and the inverse simulation approach. The similarities and shortcomings of both these methodologies are explored. The findings point to the possibility that inverse simulation can be used successfully within the control system design process for feedforward controllers for model-based output-tracking control system structures. This avoids the more complicated and relatively tedious techniques of model inversion which have been used in the past for feedforward controller design. The methods of inverse simulation presented in this thesis have been applied to a number of problems which are concerned mainly with helicopter and ship control problems and include cases involving systems having nonminimum-phase characteristics. The analysis of results for these practical applications shows that the approaches developed and presented in this thesis are of practical importance. It is believed that these developments form a useful step in moving inverse simulation methods from the status of an academic research topic to a practical and robust set of tools for engineering system design

The potential impact of adverse aircraft-pilot couplings on the safety of tilt-rotor operations

This paper addresses the potential impact of adverse aircraft-pilot couplings on tiltrotor safety, when a pilot or autopilot attempts to constrain flight dynamics with strong control. The work builds on previously published research on the theory and application of constrained flight to fixed- and rotary-wing aircraft. Tiltrotor aircraft feature characteristics from both types of aircraft and how these determine behaviour in a unique manner is investigated using a FLIGHTLAB simulation model of the XV-15 aircraft. Two different scenarios are explored in detail, using linearised models that reflect the flight-physics of stability for small deviations from trim. First, the control of vertical flight path with longitudinal cyclic pitch and elevator, and the consequences for the stability of the aircraft surge mode and short-period pitch-heave mode. The classical surge-mode instability for flight at speeds below minimum power is shown to apply to the tiltrotor in helicopter mode but alleviated in conversion and airplane modes. The impact on the short–period mode is seen to be a trade-off between the stabilising pitch attitude and destabilising incidence (angle-of-attack) contributions to the flight-path angle. The second example concerns strong control of roll attitude in the presence of adverse aileron-yaw. Here, the yaw-sway motion can be driven unstable, a problem encountered on fixed-wing aircraft with weak weathercock stability, but rare in the rotorcraft world. For both examples, the loss of stability is expressed as the change in sign of effective damping or stiffness stability derivatives. The explanatory theory for these non-oscillatory or low-frequency aircraft-pilot couplings is presented, along with interpretations in terms of handling qualities criteria. The paper also addresses the question of how to translate the findings into a form of aeronautical knowledge useful for the pilot training community.European Union fundin

Control of an eVTOL using nonlinear dynamic inversion

This paper presents a Nonlinear Dynamic Inversion (NDI) based flight controller using virtual controls, generalised forces and moments for the longitudinal motion control of a VTOL aircraft including transition manoeuvres. The control architecture is general for piloted, semi-automatic and fully-automated flight. It consists of a main inner-loop NDI controller that is used for forward cruise flight and an outer linear controller used for low speed and hover. Forward and backward transition manoeuvres are executed by switching between the NDI-based controller and position control loops. Simulation results show the control potential for both hover and cruise as well as over the vital transition flight phase

Sensitivity-analysis method for inverse simulation application

An important criticism of traditional methods of inverse simulation that are based on the Newton–Raphson algorithm is that they suffer from numerical problems. In this paper these problems are discussed and a new method based on sensitivity-analysis theory is developed and evaluated. The Jacobian matrix may be calculated by solving a sensitivity equation and this has advantages over the approximation methods that are usually applied when the derivatives of output variables with respect to inputs cannot be found analytically. The methodology also overcomes problems of input-output redundancy that arise in the traditional approaches to inverse simulation. The sensitivity- analysis approach makes full use of information within the time interval over which key quantities are compared, such as the difference between calculated values and the given ideal maneuver after each integration step. Applications to nonlinear HS125 aircraft and Lynx helicopter models show that, for this sensitivity-analysis method, more stable and accurate results are obtained than from use of the traditional Newton–Raphson approach

Scheduled flight control system of tilt-rotor VTOL PAV

This paper develops the longitudinal flight control scheme for the tilt-rotor VTOL Aston Martin Volante Vision. It is envisaged that the aircraft will be flown by a "flight-naive pilot" who is less well-trained than normal. The conversion corridor is developed and the optimum tilt schedule is determined using the minimum power curve and numerical optimisation to encompass hover, conversion and cruise flight without manipulating the rotor tilt angle. The methodology is found to be unique because two conversion schedules were integrated to reflect critical factors for conversion and stationary configurations. A proportional-integral-derivative (PID) controller has been designed with a suitable inceptor response and control allocation for a flight-naïve pilot while a blending schedule has unified different response types at the low and high speed into a single control system. The PID controllers are evaluated using design guidelines of ADS-33E-PRF and MIL-STD-1797A. The conversion flight simulation shows that the suggested longitudinal stability and control augmentation system (SCAS) are expected to reduce the pilot workload and improve handling qualities making performance and handling more suitable for a flight-naive pilot

Rotorcraft Loss of Control In-Flight – The need for research to support increased fidelity in flight training devices, including analogies with upset recovery for fixed-wing aircraft

A review of the worldwide commercial jet fleet accident data, 2001 – 2010, showed that the largest single factor leading to fatalities was Loss of Control In-Flight (LOC-I). 20 such accidents occurred during this timeframe with over 1800 fatalities [1], highlighting the need for research to investigate the causes of this problem and to develop new regulations and training programmes to improve flight safety. For civil helicopter operations, the need to significantly reduce accident rates has been the focus of the International Helicopter Safety Team (IHST), which was formed in 2005 to address factors affecting the “unacceptable” helicopter accident rate. The Team’s mission was to facilitate an 80% reduction in accident rates by 2016. From 2006 to 2011, a team completed a review of 523 U.S. helicopter accidents, from which LOC-I was cited as the main factor in accidents; LOC-I was evident in 217 (41%) of the accidents [2]. Addressing LOC-I for fixed-wing aircraft, the Royal Aeronautical Society’s Flight Simulation Group (FSG) 2009 Spring Conference was entitled: ‘Flight Simulation: Towards the Edge of the Envelope’, during which Upset Prevention and Recovery Training (UPRT) was highlighted as a major potential contributor to enhanced aviation safety. During the FSG conference, the International Committee for Aviation Training in Extended Envelopes (ICATEE) was formed to deliver a long-term strategy for reducing the rate of LOC-I accidents and incidents through enhanced UPRT [3]. To achieve this, ICATEE created two streams: the Training and Regulations Stream addressing the development of a UPRT training requirements matrix, and the Research and Technology Stream performing a thorough analysis of the technological requirements for UPRT. Key recommendations from the ICATEE work included better use of existing simulators for training, and aerodynamic enhancements to simulators to include stall characteristics. The impact of the ICATEE work is that their recommendations resulted in a new ICAO publication, “Manual on Aeroplane Upset Prevention and Recovery Training” [4]. National Authority regulations have also been impacted, with EASA UPRT requirements expected to be complete by May 2019 and the FAA requiring all Part 121 pilots to be UPRT-trained by March 2020. For the rotorcraft community, an equivalent safety initiative has recently been established. In 2016, the US Helicopter Safety Team (USHST) began the analysis of 104 fatal helicopter accidents (2009–2013) to develop intervention strategies and produce Helicopter Safety Enhancements (H-SE) that would further reduce rotorcraft accident rates. The USHST analysed accidents where LOC-I occurred during basic manoeuvres (e.g., hover, quick stop) and during unsuccessful attempted recoveries from potentially unsafe conditions (e.g., loss of tail rotor effectiveness, settling with insufficient power). Helicopter Safety Enhancement (H-SE) 81 titled, “Improve Simulator Modeling for Outside-the-Envelope Flight Conditions” [5] was established to “improve the accuracy of full flight simulators (FFS)/flight training devices by providing recommendations for developing better mathematical/physics-based models for helicopter flight dynamics”. The goal is to “achieve more realistic, higher-fidelity simulations of outside-the-envelope flight conditions” and to examine the “possible use of simulation for purposes of preventing, recognizing, and recovering from spatial disorientation”. Complementing the H-SE 81 initiative, a rotorcraft simulation fidelity research activity is underway at the University of Liverpool and Liverpool John Moores University [6]. The goal of this work is to establish a rational and systematic engineering approach to flight simulation fidelity enhancement, using physics-based models, linking in with goals of H-SE 81. Whilst rotorcraft operations pose different challenges to fixed-wing operations, drawing on the best practices developed by the fixed-wing safety community could benefit the rotorcraft community by reducing the time to implement new safety regulations and develop new training programmes. The presentation will provide an overview of the critical success factors of the ICATEE work, will report on the rotorcraft fidelity research ongoing in Liverpool, highlighting challenges and opportunities involved in developing simulator-based training for rotorcraft LOC-I scenarios

Modelling and simulation of a novel bioinspired flapping-wing rotary MAV

© The AuthosAchieving high lift efficiency represents a major research focus in the Micro Air Vehicle (MAV) domain due to stringent size and payload constraints. The Cranfield research team presents a novel semi-biomimetic design called the Flapping Wing Rotor (FWR) to address this challenge. This innovative concept combines a bioinspired flapping wing mechanism with passive rotor rotation, leveraging unsteady aerodynamic principles analogous to insect flight. The research aims to highlight a promising biomimetic flapping-rotor MAV enabled through advanced modeling to unlock the benefits of bio-inspired unsteady aerodynamics. To demonstrate this approach, a 60g proof-of-concept prototype was developed alongside a digital twin methodology for modeling, simulation, and control. A mathematical model has been formulated to analyze FWR's lift generation performance and enable flight control system design for stabilization and controllability. This work concentrates on enhancing the physical modeling process. The model is refined by tuning two key aerodynamic coefficients to account for nonlinearities from unsteady aerodynamics, flexible structures, and low Reynolds number flow inherent in MAV flight. This improved model achieves superior lift prediction accuracy versus real flight test data. Ongoing efforts focus on optimizing control torque, load distribution, and stability to further augment FWR's flight capabilities

Helicopter handling qualities: a study in pilot control compensation

The research reported in this paper is aimed at the development of a metric to quantify and predict the extent of pilot control compensation required to fly a wide range of mission task elements. To do this, the utility of a range of time- and frequency-domain measures to examine pilot control activity whilst flying hover/low-speed and forward flight tasks are explored. The tasks were performed by two test pilots using both the National Research Council (Canada)’s Bell 412 Advanced Systems Research Aircraft and the University of Liverpool’s HELIFLIGHT-R simulator. Handling qualities ratings were awarded for each of the tasks and compared with a newly developed weighted adaptive control compensation metric based on discrete pilot inputs, showing good correlation. Moreover, in combination with a time-varying frequency-domain exposure, the proposed metric is shown to be useful for understanding the relationship between the pilot’s subjective assessment, measured control activity and task performance. By collating the results from the subjective and objective metrics for a range of different mission task elements, compensation boundaries are proposed to predict and verify the subjective assessments from the Cooper-Harper Handling Qualities Rating scale.Engineering and Physical Sciences Research Council (EP/P031277/1 and EP/P030009/1

Appraisal of handling qualities standards for rotorcraft lateral-directional dynamics

The coupled vehicle roll-yaw-sway motion of Lateral-Directional Oscillations is often a contributor to rotorcraft Handling Qualities deficiencies. The extent of the deficiencies, and the required pilot control compensation to mitigate their effects, depend critically on the LDO damping and frequency and relative contributions from the roll, yaw and sway motions. Current rotorcraft performance/certification standards (e.g. ADS-33E-PRF/CS-29) for LDO stability have been developed from standards that date from the 1950s or from fixed-wing requirements; there has been limited flight test to support their validation. This paper builds on previous work examining the suitability of these LDO stability criteria to modern rotorcraft operations through ground-based simulation assessment covering a range of HQs, selected based on a frequency of 2.5 rad/s with varying damping and roll-yaw ratio. The underlying simulation model is a FLIGHTLAB Bell 412 model, augmented to ensure that the non-LDO HQs are Level 1. The LDO test configurations have been developed with delta-derivatives added to the nonlinear model to change the LDO frequency, damping and the magnitude ratio of the roll/yaw motion, whilst preserving yaw control sensitivity. The preliminary results demonstrate Handling Qualities generally degrade as the amount of roll in the LDO increased with a p/r = 1.5 giving a reasonable match with the military standards. If the ratio is reduced, Level 1 ratings were awarded with a lower damping. Conversely, no Level 1 ratings were returned for p/r = 2 when the LDO was triggered in the closed loop task