CERES :: Browsing by Author "Lu, Linghai"

Browsing by Author "Lu, Linghai"

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Open Access
Advanced pilot modeling for prediction of rotorcraft handling qualities in turbulent wind
(Elsevier, 2022-03-18) Ji, Honglei; Lu, Linghai; White, Mark D.; Chen, Renliang
Turbulent wind is known for potentially deteriorating rotorcraft performance and handling qualities when fulfilling their operation and duties. However, related research is rarely reported and could be possibly attributed to the absence of an effective analytical tool. This paper proposes a pilot model to explore the effect of turbulent wind on rotorcraft handling qualities. It consists of three components: a stabilization control component, a trajectory planning component, and a trajectory tracking component. A slalom task in the turbulent wind is used for the validation of this new pilot model. The simulation results have demonstrated that the new pilot model can not only plan and track a desired trajectory by determining a pilot's guidance strategy, but also predict handling qualities through modeling the effect of the vestibular system. The new pilot model predicts the effect of steady wind component on a pilot's maneuvering aggressiveness and yaw control performance for various ground speeds. It is also capable of predicting the handling qualities in turbulent wind due to increased turbulence intensity and reduced flight control authority.
Open Access
Appraisal of handling qualities standards for rotorcraft lateral-directional dynamics
(European Rotorcraft Forum, 2021-01-04) Cameron, Neil; White, Mark D.; Padfield, Gareth D.; Lu, Linghai
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.
Open Access
Case studies to illustrate the rotorcraft certification by simulation process; CS 29/27 category A rejected take-off, confined area
(German Society for Aeronautics and Astronautics (DGLR: Deutsche Gesellschaft für Luft- und Raumfahrt), 2023-09-07) White, Mark; Dadswell, Christopher; Padfield, Gareth; van’t Hoff, Stefan; Bakker, Richard; Lu, Linghai; Quaranta, Giuseppe; Podzus, Philipp
This paper is one of a set presented at the 49th European Rotorcraft Forum discussing results from the EU Clean Sky 2 project, Rotorcraft Certification by Simulation (RoCS). The process developed by the RoCS team provides guidance on the use of flight simulation in certification and features four case studies that illustrate aspects of the process applied using flight simulation models and flight test data provided by Leonardo Helicopters. This paper presents the case study for Rejected Take-Off (RTO): Category A in a Confined Area, for the relevant certification paragraphs in the EASA Certification Specifications CS-27 and CS-29. The relevant paragraphs from the Specifications are described and results from simulation model fidelity assessment, and updating compared with test data, are presented for a reference flight condition. Results from piloted simulation trials, with a ‘new’ Flight Test Manoeuvre (FTM), are included to illustrate flight simulator fidelity assessment methods and to illustrate how the Rotorcraft Certification by Simulation process can be achieved.
Open Access
Case studies to illustrate the rotorcraft certification by simulation process; CS 29/27 dynamic stability requirements
(German Society for Aeronautics and Astronautics (DGLR: Deutsche Gesellschaft für Luft- und Raumfahrt), 2023-09-07) Lu, Linghai; Padfield, Gareth; White, Mark; Dadswell, Christopher; Quaranta, Giuseppe; van’t Hoff, Stefan; Podzus, Philipp
This paper is one of a set presented at the 49th European Rotorcraft Forum displaying results from the EU Clean Sky 2 project, Rotorcraft Certification by Simulation (RoCS). The process developed by the RoCS team provides guidance on the requirements for the use of simulation in certification and features four case studies that illustrate aspects of the process applied using flight simulation models and flight test data provided by Leonardo Helicopters. This paper presents the case study on Dynamic Stability, for the relevant certification paragraphs in the EASA Certification Specifications CS-27 and CS-29. The Dynamic Stability paragraphs from the Specifications are described and results from simulation model fidelity assessment, and updating compared with test data, are presented for a reference flight condition. The credibility of extrapolations of the flight simulation model results to conditions at higher altitude, different airspeeds and vertical rates of climb are then discussed. Preliminary results from piloted simulation trials, with a ‘new’ flight test manoeuvre, are included to illustrate flight simulator fidelity assessment methods and to explore the veracity of the stability margins set by the Certification Specifications.
Open Access
Case studies to illustrate the rotorcraft certification by simulation process; CS 29/27 low-speed controllability
(German Society for Aeronautics and Astronautics (DGLR: Deutsche Gesellschaft für Luft- und Raumfahrt), 2023-09-07) van’t Hoff, Stefan; White, Mark; Dadswell, Christopher; Lu, Linghai; Padfield, Gareth; Quaranta, Giuseppe; Podzus, Philipp
This paper is one of a set presented at the 49th European Rotorcraft Forum discussing results from the EU Clean Sky 2 project, Rotorcraft Certification by Simulation (RoCS). The process developed by the RoCS team provides guidance on the use of flight simulation in certification and features four case studies that illustrate aspects of the process using flight simulation models and flight test data provided by Leonardo Helicopters. This paper presents the case study for the low-speed controllability requirements from the relevant certification paragraphs in the EASA Certification Specifications CS-27 and CS-29. Following an introduction of the related specifications, and the motivation behind seeking compliance supported by simulation, the various phases of the RCbS process are explored in more detail. The intent is to exercise aspects of the RoCS guidance in a practical application to investigate the implementation, and the strengths and limitations, given real-world constraints. Emphasis is placed on the Validation & Verification as well as the Credibility Assessment, taking into account test and simulation uncertainties. Results from piloted simulation trials are included to illustrate possible flight simulator fidelity assessment methods.
Open Access
Control of an eVTOL using nonlinear dynamic inversion
(IEEE, 2022-05-27) Enenakpogbe, Emmanuel; Whidborne, James F.; Lu, Linghai
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.
Open Access
Control of an over-actuated fixed-wing vectored thrust eVTOL
(IEEE, 2024-05-22) Enenakpogbe, Emmanuel; Whidborne, James F.; Lu, Linghai
A novel full-envelope controller for an over-actuated fixed-wing vectored thrust eVTOL aircraft is presented. It proposes a generic control architecture applicable to piloted, semi-automatic and fully-automated flight consisting of an aircraft-level controller (high-level controller) and a control allocation scheme. The aircraft-level controller consists of a main inner-loop non-linear dynamic inversion controller and an outer-loop proportional-integral linear controller. The inner-loop classical non-linear dynamic inversion controller is used for forward cruise flight while the outer-loop proportional-integral linear controller is used for hover/low speed control and position control. The control allocation scheme uses a novel architecture which transfers the non-linearity in the vectored thrust effector model formulation to the computation of the actuator limits by converting the effector model from polar to rectangular form thus allowing the use of a linear optimisation technique. The linear optimisation technique is an Active Set Linear Quadratic Programming constrained optimisation algorithm with a weighted least squares formulation. The control allocation allocates the overall control demand (virtual controls) to individual redundant effectors while performing control error minimisation, control channel prioritization and control effort minimization. Simulation results shows forward transition from hover to cruise and clearly demonstrates that the controller can handle saturation (position or rate). The proposed controller can also handle actuator failures.
Open Access
The development of a pilot control adaptation metric for simulation perceptual fidelity assessment
(Vertical Flight Society, 2021-09-09) Memon, Wajih A.; Cameron, Neil; White, Mark D.; Padfield, Gareth D.; Lu, Linghai
This paper reports the use of a control compensation metric to examine pilot adaptation in the objective assessment of simulation perceptual fidelity. The utility of the proposed metric to quantify different levels of pilot control compensation, hence adaptation, whilst flying low and high aggression tasks is explored. The tasks were conducted by different test pilots using the Heliflight-R simulator to examine the effect of additional transport delays on overall simulation perceptual fidelity. A weighted adaptive control compensation metric shows strong correlation with (Cooper-Harper) Handling Qualities and Simulation Fidelity Ratings awarded for each of the tasks. Moreover, in combination with a time-varying frequency-domain exposure, the metric is shown to be insightful for understanding variations in the pilots’ assessment of simulation perceptual fidelity.
Open Access
Full envelope control of over-actuated fixed-wing vectored thrust eVTOL
(MDPI, 2024-11-27) Enenakpogbe, Emmanuel; Whidborne, James F.; Lu, Linghai
A novel full-envelope controller for an over-actuated fixed-wing vectored thrust eVTOL aircraft is presented. It proposes a generic control architecture, which is applicable to piloted, semi-automatic, and fully automated flight, consisting of an aircraft-level controller (high-level controller) and a control allocation scheme. The aircraft-level controller consists of a main inner loop classical nonlinear dynamic inversion controller and an outer loop proportional–integral linear controller. The inner loop nonlinear dynamic inversion controller is a velocity controller that cancels the nonlinear bare airframe dynamics, while the outer loop proportional–integral linear controller is an attitude and navigation position controller. Together, they are used for hover/low-speed control and forward flight. The control allocation scheme uses a novel architecture, which transfers the nonlinearity in the vectored thrust effector model formulation to the computation of the actuator limits by converting the effector model from polar to rectangular form, thus allowing the use of classical control allocation linear optimisation technique. The linear optimisation technique is an active set linear quadratic programming constrained optimisation algorithm with a weighted least squares formulation. The control allocation allocates the overall control demand (virtual controls) to individual redundant effectors while performing control error minimisation, control channel prioritisation and control effort minimisation. Simulation results show the transition from hover to cruise, climb and descent, and coordinated turn clearly demonstrate that the controller can handle actuator saturation (position or rate).
Open Access
Helicopter handling qualities: a study in pilot control compensation
(Cambridge University Press, 2021-11-16) Memon, W. A.; White, Mark D.; Padfield, Gareth D.; Cameron, N.; Lu, Linghai
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.
Open Access
Investigating pilots’ operational behaviours while interacting with different types of inceptors
(Springer, 2022-06-16) Korek, Wojciech Tomasz; Li, Wen-Chin; Lu, Linghai; Lone, Mudassir
There are different designs of inceptors applied in the modern flight deck. How do pilots define how to precisely control the aircraft as their intention? Ten pilots have been asked to take part in the flight simulation trials. They were given tasks to execute using sidestick, Xbox gamepad and touchscreen controller and provide feedback using the System Usability Scale. The aim was to investigate the feasibility of replacing conventional inceptors in aircraft. The results have shown that there is a potential in introducing alternate Human-Computer Interaction (HCI) methods in the flight deck, especially in terms of learnability, however there is still a lot of work before it happens. This paper summarises the ‘pilot study’ results and shows the potential for further research.
Open Access
Modelling and simulation of a novel bioinspired flapping-wing rotary MAV
(German Society for Aeronautics and Astronautics (DGLR: Deutsche Gesellschaft für Luft- und Raumfahrt), 2023-09-07) Huang, Xun; Lu, Linghai; Whidborne, James F.; Guo, Shijun
Achieving 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.
Open Access
A new heuristic approach to rotorcraft system identification
(Vertical Flight Society, 2022-11-03) Lu, Linghai; Agarwal, Dheeraj; Padfield, Gareth D.; White, Mark D.; Cameron, Neil
High-fidelity rotorcraft flight simulation relies on the availability of a quality flight model that further demands a good level of understanding of the complexities arising from aerodynamic couplings and interference effects. This paper explores rotorcraft flight dynamics in the low-speed regime where such complexities abound and presents a new heuristic approach in the time domain to aid identification of nonlinear dynamics and fidelity assessment. The approach identifies flight model parameters “additively,” based on their contribution to the local dynamic response of the system, in contrast with conventional approaches where parameter values are identified to minimize errors over a whole maneuver. In these early investigations, identified low-order, rigid-body, linear models show good comparison with flight-test data. The approach is extended to explore nonlinearities attributed to the so-called maneuver wake distortion and wake skew effects emerging in larger maneuvers. The results show a good correlation for the proposed nonlinear model structure, demonstrated by its capability to capture the time response and variations of the stability and control derivatives with response magnitude.
Open Access
Performance evaluation of UAM configurations cost estimation for the eVTOLs.
(Cranfield University, 2021-08) Wu, Xiaolong; Lu, Linghai
With the development and progress of society, the pressure on road traffic is increasing daily, causing enormous time and economic losses to the public. The concept of electric vertical take-off and landing vehicles (eVTOLs) was proposed in order to identify a speedy and effective way of commuting in congested cities. There are many different categories of eVTOLs on the market and many countries and companies are developing it, however the various types of vehicles have different target scenarios and will incur different operating costs. This research project will identify the most suitable configuration of eVTOL in the given scenario, predict its cost and present the lowest cost optimisation solution. The research project started with a reference and evaluation of the similarities and differences between the cost structure of electric general aviation and eVTOL. Top-down, bottom-up and parametric methods were then used to initially determine the cost structure and cost formula. Furthermore, the Lilium five-seater configuration was identified as the target for this project based on the requirements of the specified scenario. Finally, the operating cost per seat-mile, operating cost per hour, annual operating cost and the optimal trajectory schedule were obtained for the specified scenario.
Open Access
The potential impact of adverse aircraft-pilot couplings on the safety of tilt-rotor operations
(Cambridge University Press, 2022-03-17) Padfield, Gareth D.; Lu, Linghai
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.
Open Access
Preliminary guidelines for a requirements-based approach to certification by simulation for rotorcraft
(2022-09-08) van ’t Hoff, Stefan; Lu, Linghai; Padfield, Gareth D.; Podzus, Philipp; White, Mark D.; Quaranta, Giuseppe
The paper presents an introduction to the preliminary guidelines for rotorcraft certification by simulation developed by the partners of the Clean Sky 2 project Rotorcraft Certification by Simulation – RoCS . The guidelines are primarily aimed at the application of (rotorcraft) flight modelling and simulation in support of certification for compliance with standards CS-27 and CS-29, PART B (Flight) and other Flight-related aspects (e.g. CS-29, Appendix B, Airworthiness Criteria for Helicopter Instrument Flight). However, the guidelines are also applicable, in principle, to the certification of other types of rotorcraft, including tilt-rotors and e-VTOL configurations. A requirements-based approach is advocated and outlined, acknowledging the profound importance of assembling preliminary requirements, as complete as possible, before embarking on simulation development processes. The proposed approach presents examples of metrics for quantifying the fidelity that is ‘sufficient’ for application to relevant Applicable Certification Requirements (ACRs). The concept of ‘adaptive fidelity’ is introduced in this Guidance to emphasise that what might be sufficient is task-specific, and therefore ACR-specific. The paper introduces the structure of the proposed Rotorcraft Certification by Simulation process, together with the main concepts that guide applicants to the development of simulations that can be effectively employed to reduce the cost, timescales, complexity and risks that may be associated with certification performed solely though flight tests.
Open Access
Preliminary guidelines for the rotorcraft certification by simulation process: update no. 1, March 2023
(Rotorcraft Certification by Simulation (RoCS), 2023-03) Padfield, Gareth D.; vant' Hoff, Stefan; Lu, Linghai; Podzus, Philip; White, Mark; Quaranta, Giuseppe
This document presents preliminary Guidance for the application of (rotorcraft) flight modelling and simulation in support of certification for compliance with standards CS-27 and CS-29, PART B (Flight) and other Flight related aspects (e.g. CS-29, Appendix B, Airworthiness Criteria for Helicopter Instrument Flight). The Guidance is presented in the form of a structured process, starting from the relevant paragraphs in the Certification Specifications, through a comprehensive description of the assembly of flight simulation requirements, informed by judgements on Influence, Predictability and Credibility, and on into the detailed building of the three major elements of the process: • the flight simulation model (FSM), • the flight simulator (FS), and • the flight test measurement system (FTMS). The FTMS feeds both the flight model and simulator development with real-world test data to support validation and fidelity assessment. A structured and systematic approach to data/configuration management and documentation is recommended, aided by the creation of the Rotorcraft Certification by Simulation (RCbS) project management plan. This is the first update of the RCbS Guidelines and includes modifications based on the first round of feedback received before and after the European Rotors RoCS workshop held in Cologne on November 9th 2022. The Guidelines will continue to be updated, as appropriate, with the next major revision to include exercising the process in case studies based on applicable certification requirements from EASA CS-27 and CS-29 (to appear in Section 10). In the current update, the RoCS team have also addressed the issue of resourcing the RCbS process (within Section 9) and suggested potential next steps for aspiring applicants (new Section 11).
Open Access
Review of flight simulation fidelity requirements to help reduce ‘rotorcraft loss of control in‑flight’ accident rates
(Springer, 2021-08-21) White, Mark D.; Padfield, Gareth D.; Lu, Linghai; Advani, Sunjoo; Potter, Mario
This paper examines the fidelity requirements for flight simulators to improve training and address the problems associated with rotorcraft loss of control in-flight (LOC-I). To set the context, trends in rotorcraft accident statistics are presented. The data show that, despite recent safety initiatives, LOC-I rotorcraft accidents have been identified as a significant and growing contribution to accident rates. In the late 1990s, the fixed-wing commercial aircraft community faced a similar situation relating to upset prevention and recovery, and through a coordinated international effort, developed a focussed training programme to reduce accident rates. Lessons learned from the fixed-wing programme are presented to highlight how improved rotorcraft modelling and simulation tools are required to reduce rotorcraft accidents through higher quality, simulator-based training programmes. Relevant flight simulator certification standards are reviewed, with an emphasis on flight-model fidelity and vestibular motion cueing requirements. The findings from rotorcraft modelling and motion cueing research, that highlight relevant fidelity issues, are presented to identify areas for further activities to enhance the fidelity of simulators standards for use in LOC-I prevention training.
Open Access
Rotorcraft flight simulation to support aircraft certification: a review of the state of the art with an eye to future applications
(German Society for Aeronautics and Astronautics (DGLR: Deutsche Gesellschaft für Luft- und Raumfahrt), 2023-09-07) Rylko, Agata; van’t Hoff, Stefan; Lu, Linghai; Padfield, Gareth; Podzus, Philipp; White, Mark; Quaranta, Giuseppe
This paper presents the approach for Rotorcraft Certification by Simulation proposed within the RoCS project. In particular, the aspects of model validation and credibility assessment through the usage of uncertainty quantification techniques are reviewed, and some lesson learned are presented. It is shown that the increase of effort required to thoroughly evaluate the capability of the simulation model is often counterbalanced by the advantages of the insight that can be obtained and possibly exploited also for design purposes. It is shown that the numerical approaches, and in some cases even the tools required to perform the necessary uncertainty analyses are publicly available and can be directly employed. This paper is one of a set presented at the 49th European Rotorcraft Forum discussing results from the EU Clean Sky 2 project, Rotorcraft Certification by Simulation (RoCS).
Open Access
Rotorcraft lateral-directional oscillations: the anatomy of a nuisance mode
(Vertical Flight Society, 2021-07-22) Agarwal, Dheeraj; Lu, Linghai; Padfield, Gareth D.; White, Mark D.; Cameron, Neil
High-fidelity rotorcraft flight simulation relies on the availability of a quality flight model that further demands a good level of understanding of the complexities arising from aerodynamic couplings and interference effects. One such example is the difficulty in the prediction of the characteristics of the rotorcraft lateral-directional oscillation (LDO) mode in simulation. Achieving an acceptable level of the damping of this mode is a design challenge requiring simulation models with sufficient fidelity that reveal sources of destabilizing effects. This paper is focused on using System Identification to highlight such fidelity issues using Liverpool's FLIGHTLAB Bell 412 simulation model and in-flight LDO measurements from the bare airframe National Research Council's (Canada) Advanced Systems Research Aircraft. The simulation model was renovated to improve the fidelity of the model. The results show a close match between the identified models and flight test for the LDO mode frequency and damping. Comparison of identified stability and control derivatives with those predicted by the simulation model highlight areas of good and poor fidelity