In-flight Simulation Research Articles

Design and development of an in-flight simulator for flooding and drawdown

Geotechnical structures such as levees, dykes, canals and tailing dams, when subjected to extreme climatic conditions such as flooding and drawdown, are prone to failure on the upstream and downstream sides due to transient seepage conditions. Therefore, it is imperative to physically replicate such conditions in the actual stress state. Hence, the objective of this paper is to present the performance of an in-flight simulator developed to simulate flooding and drawdown (ISFD) events at enhanced gravities. The working principle, design details, various components of the developed simulator and calibration at normal (1g) and high gravities are discussed. In this study, the calibration and performance of the ISFD set-up were demonstrated on a model levee section in a 4.5 m radius large beam geotechnical centrifuge facility at the Indian Institute of Technology Bombay, India. In total, three centrifuge model tests were conducted on a scaled-down levee section (with and without internal drainage layers) constructed with silty sand-type material to validate the ISFD capabilities. In addition, seepage and slope stability analysis for the centrifuge models was carried out using Plaxis-2D geotechnical software, which compared favourably with physically observed tests results.

Level-Set Mass-Conservative Front-Tracking Technique for Multistep Simulations of In-Flight Ice Accretion

This paper presents a novel level-set-based approach to model evolving boundary problems for in-flight ice accretion. No partial differential equations are solved as in the standard level-set formulation, but simple geometrical quantities are employed to provide an implicit discretization of the updated boundary. This method avoids mesh entanglements and grid intersections typical of algebraic and mesh deforming techniques, making it suitable for generating a body-fitted discretization of arbitrarily complex geometries as in-flight ice shapes, including the collision of separate ice fronts. Moreover, this paper presents a local ice thickness correction, which accounts for the body’s curvature, to conserve the prescribed iced mass locally. The verification includes ice accretion over an ellipse and a manufactured example to show the proposed strategy’s advantages and robustness compared to standard algebraic methods. Finally, the method is applied to ice accretion problems. A temporal and grid convergence study is presented for automatic multistep in-flight simulations over a NACA0012 airfoil in rime, glaze, and mixed ice conditions.

Design and Development of Tilt Table-Based In-Flight Simulator for Pseudostatic Loading in a Geotechnical Centrifuge

Abstract In the recent past, failures of many geotechnical structures storing enormous volumes of water and tailings material have been witnessed. These retention type earthen dams and embankments have often been designed for inertial loading considering the pseudostatic method. This paper aims to present the performance of a tilt table-based in-flight simulator for pseudostatic loading (ISPL) in the horizontal direction at enhanced gravity in a geotechnical centrifuge. The developed tilt table-based in-flight simulator consists of a screw jack gearbox-based mechanism with a pivotal hinge to produce a maximum tilting angle of 20° (equivalent to horizontal seismic coefficient of 0.36) with tilting rates varying from 0.2°/min to 0.8°/min at enhanced gravity levels. In the present study, the performance of the ISPL setup was demonstrated on an earthen dam section subjected to upstream water rising and pseudostatic loading at 30 times gravity in a 4.5-m-radius large-beam geotechnical centrifuge facility available at the Indian Institute of Technology (Bombay, India). The data were recorded by pore water pressure transducers and linear variable differential transformers, coupled with front elevations of model earthen dams captured using an onboard digital camera, and analyzed. The results are presented in terms of the development of phreatic surface, pore water pressure, slope face movement, and crest surface settlement before and after earthen dam sections were subjected to hybrid loading (upstream filling and tilting) conditions. In addition, seepage and slope stability analyses of prototype centrifuge models were carried out under hybrid loading conditions. The seepage and slope stability analyses’ results were found to corroborate well with physically observed centrifuge test results.

Averaged sub-gradient integral sliding mode control design for cueing end-effector acceleration of a two-link robotic arm

Acceleration tracking is a significant problem in aeronautics, automotive, and biomedical technical areas because its solution may yield effective simulation of motion cues. In the case of aeronautics, the proper solution for the tracking problem improves the in-flight simulations for the training of plane pilots. These simulators can be set up using robotic devices that develop controlled motions with the end-effector following the required three-dimensional reference accelerations robustly. Hence, the primary goal of this study is the effective application of the integral sliding mode controller to solve the acceleration tracking problem for the end-effector of a two-link robotic arm. The control design problem is formulated as an optimization of a convex (non-strict) performance functional depending on the difference between the acceleration of the robotic arm and the desired acceleration using the averaged sub-gradient (ASG) descendant method. A novel sliding surface considers the sensitiveness threshold for acceleration dynamics, inspired by the limit of detection in the pilot vestibular apparatus. The proposed controller was analyzed in terms of the finite-time convergence of the sliding surface and the practical stability analysis for the tracking error dynamics. Our main contribution is the design of the online averaged sub-gradient optimization controller based on integral SMCs. The controller solves the end-effector acceleration tracking for a two-link robotic arm, which implements a simplified version of a flight simulator that is considered to be operated under uncertain scenarios and assumes the presence of perturbations and modeling errors. The controller considers the case of incomplete knowledge of the robotic arm model, which adds an extra degree of robustness to the control design. The numerical evaluations demonstrate the attributes of the ASG formulation compared to traditional state feedback control, using the performance functional, the norm of the acceleration tracking error, and the control input variation

Configuration system design and longitudinal flight simulation verification of variable stability helicopter

For in-flight simulation of variable stability helicopter, how to have the ability to change a wide range of response characteristics and ensure flight safety is an important design problem. This paper proposes configuration design technology of the variable stability helicopter, which purpose is to safely change steady-state and dynamic response characteristics of the variable stability helicopter, and make the pilot feel good or poor helicopter response characteristics. The configuration control system of the variable stability helicopter is designed theoretically, and the configuration switch logic is studied from the perspective of flight safety. The numerical simulation test of the longitudinal configuration control law was carried out, and the configuration control law and configuration switch logic were added to the ground closed-loop test to verify. The results show that the expected response characteristics of the helicopter are obtained by applying the configuration system design technology.

Use of Centrifuge-Based Physical Modelling for Understanding the Performance of Geostructures

Centrifuge-based physical modelling is widely adopted for understanding the performance of geostructures, like levees subjected to flooding and drawdown and geogrid-reinforced soil walls subjected to seepage. In this paper, an attempt has been made to bring out the advantage of centrifuge-based physical modelling to understand (i) the performance of levees subjected to flooding using a custom-designed and developed in-flight flood simulator at 30 gravities with and without chimney drain and (ii) the performance of geogrid-reinforced soil walls with and without chimney drain subjected to seepage at 40 gravities. In both the cases, silty sand was used to model soil and fine sand was used in chimney drain. All centrifuge model tests were performed using the 4.5-m-radius large-beam centrifuge facility available at IIT Bombay. Models were instrumented with linearly variable differential transformers for measuring surface settlements and pore pressure transducers to measure raise in pore water pressure within the soil at the onset of flooding for levees and at the onset of seepage for geogrid-reinforced soil walls. Additionally, digital image analyses of photographs of front elevation of levee models and geogrid-reinforced soil wall models were carried out to obtain face movements, movements of markers embedded within the levee, markers stuck to geogrid layers of reinforced soil walls at the onset of flooding and seepage. The developed in-flight flood simulator was found to be capable of generating the flood rate ranging from 2.2 to 7 m/day. Further, results of centrifuge model tests conducted on levees without any chimney drain were noticed to undergo catastrophic failure within 4.25 days of flooding-induced seepage, whereas a levee with chimney drain was found to sustain flooding-induced seepage of 37.5 days. Geogrid-reinforced soil wall was constructed with silty sand as a structural as well as backfill, without any drainage system experienced catastrophic failure. Contrary to this, geogrid-reinforced soil walls with chimney drain as an external drainage system helped in averting catastrophic failure. However, probability of piping failure near the toe region of the wall cannot be ruled out. Further, the use of geocomposite layers as an internal drainage system within the reinforced zone also explored and the placement of geocomposite layers at one-third portion of height from bottom was found to be effective.

A Novel Hybrid Robust Control Design Method for F-16 Aircraft Longitudinal Dynamics

This paper presents a hybrid robust control design method for a third-order lower-triangular model of nonlinear dynamic systems in the presence of disturbance. In this paper, a novel control design is presented systematically to synthesize a robust nonlinear feedback controller, called backstepping sliding mode control (BSMC), for the proposed system by a combined approach of backstepping design and sliding mode control. In this approach, a family of the “sliding surface” is introduced in state transformations. Then, a smooth switching function of the sliding surface is introduced and enforced to include in virtual feedbacks and a real control law from the control selection phrases of the backstepping design loop. The achieved control method proves a well-tracking command with asymptotic stability, provides a robustness in the presence of uncertainties, and eliminates completely a chattering phenomenon. The application of flight-path angle control corresponding to the longitudinal dynamics of a high-performance F-16 aircraft simulation model is implemented. Under some assumptions, full nonlinear longitudinal dynamics is reformed into a lower-triangular system for a direct application to formulate a control law. A closed-loop system is achieved for in-flight simulation with different flight profiles for a comparison of the existing methods. Also, an external disturbance on different loading/unloading conditions in flight is applied to verify and validate robustness of the proposed control method.

Variable Stability In-Flight Simulation System Based on Existing Autopilot Hardware

A variable stability in-flight simulator has the capabilities to change the response of an aircraft in-flight, often without changing the physical properties of the aircraft. The ability to adjust the aircraft response characteristics and handling qualities has various purposes, such as pilot training, control system development, and handling quality research. A variable stability control system is designed for a medium-range business jet using incremental nonlinear dynamic inversion. The performance of the in-flight simulator is verified by two experiments, one conducted in a fixed-base flight simulator and one in a Cessna Citation II laboratory aircraft. The fly-by-wire actuation system in the Cessna Citation II is based on its existing autopilot, inheriting the limited performance and safety protections. The simulator experiment shows differences between the experienced handling qualities for a reference model and the designed controller combined with aircraft dynamics. These differences mainly arise due to actuator saturation for specific handling quality settings. The in-flight experiment supports the simulator findings but also reveals how the available control authority around the initial condition is limited due to constraints of the fly-by-wire system.

In-flight icing simulation for two-dimensional configurations

This paper presents a comprehensive aircraft icing simulation tool implemented in an in-house Navier–Stokes parallel multi-block solver. In detail, the droplet flow field is solved by Eulerian approach, and a Partial Differential Equation (PDE)-based ice accretion model is adopted to determine the runback water flow and icing rate. Numerical validations are performed on the two-dimensional (2D) NACA 0012 airfoil, where good agreements with the literature are observed. Additionally, the paper investigates the influence of droplet size on the final ice shape. Results show that droplets with greater Median Volume Diameter (MVD) are more likely to impact on the wall, which results in larger droplet impingement limit and icing limit.

Design and Flight Evaluation of Primary Control System for Learjet-25B Aircraft

In this paper, a multi-input/multi-output servomechanism primary flight controller is presented. Also discussed are flight data related to the system performance and pilot assessments of handling qualities. The data were collected during flight-test experiments (March 2018) on the Calspan Variable Stability In-Flight Simulator Learjet-25B aircraft. Aircraft body angular rates and linear accelerations are used to enable decoupled command tracking in pitch, roll, and yaw axes. Angle of attack and angle of sideslip measurements are not required for the system to operate. The developed primary flight controller translates pilot controls into the corresponding pitch, roll, and yaw commands for the aircraft to track. The system consists of a gain-scheduled robust linear baseline augmented with a direct adaptive model reference output feedback. The baseline is designed via the Observer-Based Loop Transfer Recovery method. The adaptive component incorporates the closed-loop reference model, whereby a state observer from the baseline Observer-Based Loop Transfer Recovery system also acts as the reference model for the adaptive augmentation. In March 2018, the Observer-Based Loop Transfer Recovery controller was successfully flight verified on the Calspan Learjet-25B aircraft, at the U.S. Air Force Test Pilot School, Edwards Air Force Base, California. Flight-test results are discussed, and a brief summary is given at the end.

Robust Gain-Scheduled Flight Controller via A New Formulation for Over-Bounding Scheduling Parameter Errors

Abstract This note addresses Gain-Scheduled (GS) flight controller design for the lateral-directional motions of an In-Flight Simulator (IFS) in a certain speed range. The design requirements for the GS flight controller are twofold: Model-matching and gust alleviation under some plant uncertainties as well as bounded errors in the provided scheduling parameters. We design a GS Output Feedback (GSOF) controller which meets the afore-mentioned design requirements via a new formulation for over-bounding the errors between actual scheduling parameters and inexactly provided ones. Control performance comparison between our new controller and the controller designed with an existing method in literature confirms that the former has less conservatism than the latter in synthesis as well as a posteriori analysis.

Dynamic Turbine Clearance Simulation Considering the Influence of Temperature on Mechanical Load–Induced Displacements

AbstractThis paper addresses the problem of precise in-flight simulation of all dynamic engine operational modes, considering the specific features of the control in these modes. One of the dynamic...

In-Flight Simulation of the Excavation of Foundation Pit in Centrifuge Model Tests

Abstract It is of great importance to simulate the excavation of foundation pits in centrifuge model tests. A new device was designed with new principles based on the understanding of the stress field in the soil base before and after excavation. A loading system was used to replace the excavated soil so that the appropriate self-weight stress state could be achieved with an increase of centrifugal acceleration before excavation. The compensation unit and the confining structure were simultaneously removed using the loading unit to simulate the excavation with accuracy. The device was made using a series of new structures for a high degree of automation and good accuracy in simulating the excavation of foundation pits. This device can be used to test a wide range of pit foundations and supporting structures. All of the components of the device were carefully designed to minimize their size and mass with a good control on the deformation of themselves. The device was confirmed to be effective in simulating the excavation of foundation pits at the 50-g level by a series of centrifuge model tests. The true paths of the stress and deformation of the foundation pit could be reasonably simulated in the centrifuge model tests, which cannot be captured by the traditional method in which the excavation was conducted previously at the 1-g level. The traditional method underestimates the earth pressures and the deformation of the soil due to the excavation of the foundation pit according to the comparison tests.

Flight-Test Evaluation of a Loss-of-Control Mitigation System

For more than a decade, researchers have been developing novel adaptive flight control systems that provide a means to safely operate an air vehicle in the presence of damage or failures. Although these systems show great promise, integration of exemplar systems in flight-test aircraft at NASA has revealed the potential for unfavorable pilot–vehicle coupling, including pilot-induced oscillations. To address this issue directly, the Smart Adaptive Flight Effective Cue system was developed that features an adaptive command path gain to mitigate oscillation tendencies and an inceptor force-feedback cue to alert the pilot that the system is active. To evaluate the system, a representative adaptive controller that had been successfully evaluated in previous flight-test programs and the Smart Adaptive Flight Effective Cue system were implemented in a Calspan Learjet in-flight simulator. Flight-test evaluations consisted of three test pilots conducting two sorties each to evaluate the effectiveness of the combined system in the presence of failures. Pilot opinion ratings and quantitative assessments of performance indicated that the Smart Adaptive Flight Effective Cue system successfully eliminated the oscillation tendencies that can lead to loss of control. Furthermore, the system restored a more linear vehicle response that allowed the pilots to successfully complete the evaluation tasks with near baseline aircraft performance.

Variable stability system control law development for in-flight simulation of pitch/roll/yaw rate and normal load

This paper describes the development of variable stability system (VSS) control laws for the KFA-i to simulate the dynamics of KFA-m aircraft. The KFA-i is a single engine, Class IV aircraft and was selected as an in-flight simulator (IFS) aircraft, whereas the KFA-m is a simulated aircraft that is based on the F-16 aircraft. A 6-DoF math model of KFA-i aircraft was developed, linearized, and separated into longitudinal and lateral motion for VSS control law synthesis. The KFA-i aircraft has five primary control surfaces: two flaperons, two all movable horizontal tails, and one rudder. Flaperons are used for load control, the horizontal tails are used for pitch and roll rate control, and the rudder is used for yaw rate control. The developed VSS control law can simulate four parameters of the KFA-m aircraft simultaneously, such as pitch, roll, yaw rates, and load. The simulation results show that KFA-i follows the responses of KFA-m with high accuracy.

In-flight Simulation of Pile Installation in Slopes in Centrifuge Model Tests

A new device was developed to simulate the in-flight installation of jacked displacement piles in slopes using centrifuge model tests. In the modularization-based design of this device, the pile installation process is divided into combinations of five-axis movements, which are realized by corresponding electric motors. New structures consisting of combined cubes and cylinders are proposed for the pile top and the picking-up unit. In addition, this device utilizes a series of techniques, such as the use of linear guideways, to increase digital control and the local stiffness of key structures. These measures provide a high degree of automation with good accuracy in determining the locations and vertical extents of piles during installation. The structures and electric motors in the device were optimized to provide high capacities for the load (10 kN) and displacement (260 mm). This device was designed with hollow components to control the size and mass of the entity (only 50 kg) and to satisfy rigorous requirements regarding the allowable deformation and stability levels during centrifuge model tests. The effectiveness of the device was verified using pile installation tests under various conditions. The test results were compared with results that were obtained using pre-flight pile installation. Overall, the pile installation method influenced the responses of the pile-reinforced slopes. For example, the loading-induced displacement of the slope with piles installed in-flight was considerably greater than that in the slope with piles installed at the 1g level when the load was near the limit.

A rating scale for the subjective assessment of simulation fidelity

AbstractA new rating scale for capturing pilot subjective assessment of simulation fidelity is described in this paper. The scale has been developed through a series of flight and simulation trials using six test pilots from a variety of backgrounds, and is based on the methodology utilised with the Cooper-Harper Handling Qualities Rating scale and the concepts of transfer of training, comparative task performance and task strategy adaptation. The development of the new rating scale has been undertaken using simulations of rotary-wing aircraft on the University of Liverpool’s HELIFLIGHT-R research simulator, in conjunction with the Canadian Flight Research Laboratory’s Bell 412 ASRA in-flight simulator. The utility of the scale applied to locating fidelity boundaries for quantitative metrics is illustrated for an inter-axis coupling criterion. The work described in this paper is preliminary in nature, and research activities are on-going to continue the validation of the fidelity rating scale.

Evaluation of Airplane Upset Recovery Training

Loss of control is one of the leading factors in hull losses and fatalities in airline aircraft. To reduce the risk of this type of accident, four types of airplane upset recovery training have been developed (ground-based flight simulation, aerobatic flight, ground-based flight simulation with aerobatic flight, and in-flight simulation). These were evaluated during in-flight reenactments of fatal, hull loss airline airplane accidents. A between-subjects design, with five groups of eight nonmilitary pilots flying in their probationary year for airlines, was used to evaluate these types of training. Each group had received a different type of training, including one group that had received no training. Each pilot completed a 1.4-hr evaluation flight in which eight airplane upsets were introduced. For some scenarios, training clearly works – specifically, 39 evaluation pilots recovered from the wind shear upset. But few evaluation pilots used bank to change the direction of the lift vector to recover from nose-high upsets. Further, very few used differential thrust to recover from rudder- or aileron-induced roll upsets (use of alternate controls). Finally, recovery attempts from icing-induced stalls were generally inadequate.

In-flight simulation of wake encounters using deformed vortices

AbstractDuring the decay process the shape of wake vortices changes significantly which has an influence on the encounter characteristics, hence on the encounter hazard. In order to evaluate the influence of vortex deformation on the wake encounter hazard, in-flight simulations with the DLR research aircraft ATTAS were carried out. For a realistic analysis of wake encounters flow fields of matured vortices were generated with large eddy simulations. These flow fields were used for the determination of histories of induced forces and moments acting on the wake encountering aircraft. The force and moment histories were then fed into the equations of motions of the non-linear six degree-of-freedom in-flight simulation of the DLR research aircraft ATTAS. In order to compare different stages of vortex deformation, encounters were simulated in flight with wavy vortices and vortex rings. The most important benefit of the in-flight-simulation is the realistic environment, which enables a realistic assessment of pilots’ encounter acceptance. The analysed scenario was of a wake encounter during final approach. The encounter conditions correspond to separation distances of about 4nm and 5nm behind an aircraft of the ‘heavy’ category. During the encounters the ATTAS was flown under manual control. Altogether 31 encounters were simulated in flight, 9 with wavy vortices and 22 with vortex rings.

Tau Guidance in Boundary-Avoidance Tracking - New Perspectives on Pilot-Induced Oscillations

DOI: 10.2514/1.54065 Tau theory, introduced to the flight control discipline as a model for natural guidance, is shown to provide an approach to predicting a class of adverse aircraft-pilot couplings described as boundary-avoidance tracking events and pilot-induced oscillations. These have previously been modeled a posterior as discrete events using timedependent feedback gains. Drawing on the prospective nature of the time-to-contact variable optical tau � , a new method is proposed for modeling such phenomenon and also for determining the critical incipience for this class of aircraft-pilotcoupling.Inthepresentstudy,theapproachhasbeenappliedtotauguidanceinarotorcrafttrajectory tracking maneuver, to predict the conditions under which aircraft-pilot couplings may occur. In addition, a strong correlation between motion and control activity and the derivatives of tau adds substance to the hypothesis that the pilot’sperceptualsystemworksdirectlywithinvariantsintheoptical flowduringvisualguidance.Resultsfrom flight simulation tests conducted at the University of Liverpool and complementary flight tests carried out with the National Research Council (Canada) advanced systems research aircraft in-flight simulator support the tau control hypothesis. The theory suggests ways that pilots could be alerted to the impending threat of such adverse aircraftpilot couplings.