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Aerospace, Volume 8, Issue 9 (September 2021) – 42 articles

Cover Story (view full-size image): Hybrid rockets are an attractive technology for the future of commercial transportation in space due to their inherent safety and relatively low development cost. This paper reports on the change in velocity capability of a subcategory of hybrid rocket-based in-space transportation vehicles referred to as “post-boost stages” or “kick motors”. The impact of oxidizer selection, envelope shape, O/F-shift, and nozzle throat erosion on change in velocity are examined in detail. Hydrogen-peroxide-based motors are found to be most suitable when space is limited but propellant system mass is not, while nitrous oxide-based motors are found to be most suitable in the opposite case. View this paper
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Article
Multisatellite Flyby Inspection Trajectory Optimization Based on Constraint Repairing
Aerospace 2021, 8(9), 274; https://doi.org/10.3390/aerospace8090274 - 21 Sep 2021
Viewed by 551
Abstract
With the rapid development of on-orbit services and space situational awareness, there is an urgent demand for multisatellite flyby inspection (MSFI) that can obtain information about a large number of space targets with little fuel consumption in a short time. There are two [...] Read more.
With the rapid development of on-orbit services and space situational awareness, there is an urgent demand for multisatellite flyby inspection (MSFI) that can obtain information about a large number of space targets with little fuel consumption in a short time. There are two kinds of constraints, namely inspection constraints (ICs) at each flyby point and transfer process constraints (TPCs) in the actual mission. Further considering the influence of discrete and continuous variables such as inspection sequence, time, and maneuver scheme, it is complex and difficult to solve MSFI. To optimize it efficiently, the task flow and the problem model are defined firstly. Then, the algorithm framework based on constraint repairing is given, which contains repair methods of the ICs and the TPCs. Finally, the proposed method is compared with the nonrepair optimization method in two numerical examples. The results indicate that when the constraints are hard to meet, it is better and more efficient than the nonrepair method. Full article
(This article belongs to the Special Issue Spacecraft Dynamics and Control)
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Article
A Gimballed Control Moment Gyroscope Cluster Design for Spacecraft Attitude Control
Aerospace 2021, 8(9), 273; https://doi.org/10.3390/aerospace8090273 - 21 Sep 2021
Cited by 1 | Viewed by 925
Abstract
This paper addresses the problem of singularity avoidance in a cluster of four Single-Gimbal Control Moment Gyroscopes (SGCMGs) in a pyramid configuration when used for the attitude control of a satellite by introducing a new gimballed control moment gyroscope (GCMG) cluster scheme. Four [...] Read more.
This paper addresses the problem of singularity avoidance in a cluster of four Single-Gimbal Control Moment Gyroscopes (SGCMGs) in a pyramid configuration when used for the attitude control of a satellite by introducing a new gimballed control moment gyroscope (GCMG) cluster scheme. Four SGCMGs were used in a pyramid configuration, along with an additional small and simple stepper motor that was used to gimbal the full cluster around its vertical (z) axis. Contrary to the use of four variable-speed control moment gyroscopes (VSCMGs), where eight degrees of freedom are available for singularity avoidance, the proposed GCMG design uses only five degrees of freedom (DoFs), and a modified steering law was designed for the new setup. The proposed design offers the advantages of SGCMGs, such as a low weight, size, and reduced complexity, with the additional benefit of overcoming the internal elliptic singularities, which create a minor attitude error. A comparison with the four-VSCMG cluster was conducted through numerical simulations, and the results indicated that the GCMG design was considerably more efficient in terms of power while achieving a better gimbal configuration at the end of the simulation, which is essential when it is desired for different manoeuvres to be consecutively executed. Additionally, for a nano-satellite of a few kilograms, the results prove that it is feasible to manufacture the GCMG concept by using affordable and lightweight commercial off-the-shelf (COTS) stepper motors. Full article
(This article belongs to the Special Issue Spacecraft Dynamics and Control)
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Article
Experimental Validation for the Performance of MR Damper Aircraft Landing Gear
Aerospace 2021, 8(9), 272; https://doi.org/10.3390/aerospace8090272 - 19 Sep 2021
Cited by 3 | Viewed by 919
Abstract
The landing gear of an aircraft serves to mitigate the vibration and impact forces transmitted from the ground to the fuselage. This paper addresses magneto-rheological (MR) damper landing gear, which provides high shock absorption efficiency and excellent stability in various landing conditions by [...] Read more.
The landing gear of an aircraft serves to mitigate the vibration and impact forces transmitted from the ground to the fuselage. This paper addresses magneto-rheological (MR) damper landing gear, which provides high shock absorption efficiency and excellent stability in various landing conditions by adjusting the damping force using external magnetic field intensity. The performance and stability of an MR damper was verified through numerical simulations and drop tests that satisfied aviation regulations for aircraft landing gear. In this study, a prototype MR damper landing gear, a drop test jig, and a two-degree-of-freedom model were developed to verify the performance of the MR damper, with real-time control, for light aircraft landing gear. Two semi-active control algorithms, skyhook control and hybrid control, were applied to the MR damper landing gear. The drop tests were carried out under multiple conditions, and the results were compared with numerical simulations based on the mathematical model. It was experimentally verified that as the shock absorption efficiency increased, the landing gear’s cushioning performance significantly improved by 17.9% over the efficiency achieved with existing passive damping. Full article
(This article belongs to the Special Issue Vibration Control for Space Application)
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Article
A Parametric Blade Design Method for High-Speed Axial Compressor
Aerospace 2021, 8(9), 271; https://doi.org/10.3390/aerospace8090271 - 18 Sep 2021
Cited by 1 | Viewed by 887
Abstract
The blade geometry design method is an important tool to design high performance axial compressors, expected to have large design space while limiting the quantity of design variables to a suitable level for usability. However, the large design space tends to increase the [...] Read more.
The blade geometry design method is an important tool to design high performance axial compressors, expected to have large design space while limiting the quantity of design variables to a suitable level for usability. However, the large design space tends to increase the quantity of the design variables. To solve this problem, this paper utilizes the normalization and subsection techniques to develop a geometry design method featuring flexibility and local adjustability with limited design variables for usability. Firstly, the blade geometry parameters are defined by using the normalization technique. Then, the normalized camber angle f1(x) and thickness f2(x) functions are proposed with subsection techniques used to improve the design flexibility. The setting of adjustable coefficients acquires the local adjustability of blade geometry. Considering the usability, most of the design parameters have clear, intuitive meanings to make the method easy to use. To test this developed geometry design method, it is applied in the design of a transonic, two flow-path axial fan component for an aero engine. Numerical simulations indicate that the designed transonic axial fan system achieves good efficiency above 0.90 for the entire main-flow characteristic and above 0.865 for the bypass flow characteristic, while possessing a sufficiently stable operation range. This indicates that the developed design method has a large design space for containing the good performance compressor blade of different inflow Mach numbers, which is a useful platform for axial-flow compressor blade design. Full article
(This article belongs to the Special Issue Fluid Flow Mechanics)
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Article
Description of a Eulerian–Lagrangian Approach for the Modeling of Cooling Water Droplets
Aerospace 2021, 8(9), 270; https://doi.org/10.3390/aerospace8090270 - 18 Sep 2021
Viewed by 696
Abstract
The present paper describes a tool developed in-house for the modeling of free-falling water droplet cooling processes. A two-way coupling model is employed to account for the interactions between the droplets and the carrier fluid, following a Eulerian–Lagrangian approach. In addition, a stochastic [...] Read more.
The present paper describes a tool developed in-house for the modeling of free-falling water droplet cooling processes. A two-way coupling model is employed to account for the interactions between the droplets and the carrier fluid, following a Eulerian–Lagrangian approach. In addition, a stochastic separated flow technique is employed, involving random sampling of the fluctuating fluid velocity. In physical modeling, two empirical correlations are considered for determining the heat and mass transfer coefficients, with the possibility of accounting for vibrations. The numerical results indicate the preponderance of the interactions between droplet and carrier fluid at various humidity ratios. Full article
(This article belongs to the Special Issue Deicing and Anti-Icing of Aircraft (Volume II))
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Article
An Effective Form Analysis Approach for Designing and Optimizing a Cable-Net Structure of a Giant Active Reflector
Aerospace 2021, 8(9), 269; https://doi.org/10.3390/aerospace8090269 - 18 Sep 2021
Viewed by 489
Abstract
The cable-net structure of a giant active reflector is a structure with large deformations. The effective form analysis approach can help the active reflector to achieve high surface accuracy. In this paper, a new nonlinear numerical form analysis approach is proposed for the [...] Read more.
The cable-net structure of a giant active reflector is a structure with large deformations. The effective form analysis approach can help the active reflector to achieve high surface accuracy. In this paper, a new nonlinear numerical form analysis approach is proposed for the active reflector cable-net structure. The basic principle and calculation flow of the static equilibrium are provided and the calculation program is compiled. This approach does not only achieve a uniform cable tension design based on the idea of “equal-tension replacement” but also simulates the change process from the reference state to the target working state. The obtained final form conforms to the target configuration and achieves the force balance. This, in turn, proves the adaptability of the proposed approach for the design and optimization of active reflector cable-net structure. By employing the form analysis, the length change of actuators and force distribution of all cables can also be simultaneously obtained. This provides an important basis for the actual operation and control optimization of similar active reflectors. Full article
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Article
Micro-Vortex Generators on Transonic Convex-Corner Flow
Aerospace 2021, 8(9), 268; https://doi.org/10.3390/aerospace8090268 - 17 Sep 2021
Viewed by 717
Abstract
A convex corner models the upper surface of a deflected flap and shock-induced boundary layer separation occurs at transonic speeds. This study uses micro-vortex generators (MVGs) for flow control. An array of MVGs (counter-rotating vane type, ramp type and co-rotating vane type) with [...] Read more.
A convex corner models the upper surface of a deflected flap and shock-induced boundary layer separation occurs at transonic speeds. This study uses micro-vortex generators (MVGs) for flow control. An array of MVGs (counter-rotating vane type, ramp type and co-rotating vane type) with a height of 20% of the thickness of the incoming boundary layer is installed upstream of a convex corner. The surface pressure distributions are similar regardless of the presence of MVGs. They show mild upstream expansion, a strong favorable pressure gradient near the corner’s apex and downstream compression. A corrugated surface oil flow pattern is observed in the presence of MVGs and there is an onset of compression moving downstream. The counter-rotating vane type MVGs produce a greater reduction in peak pressure fluctuations and the ramp type decreases the separation length. The presence of MVGs stabilizes the shock and shock oscillation is damped. Full article
(This article belongs to the Section Aeronautics)
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Article
Neuroevolutionary Control for Autonomous Soaring
Aerospace 2021, 8(9), 267; https://doi.org/10.3390/aerospace8090267 - 17 Sep 2021
Cited by 2 | Viewed by 755
Abstract
The energy efficiency and flight endurance of small unmanned aerial vehicles (SUAVs) can be improved through the implementation of autonomous soaring strategies. Biologically inspired flight techniques such as dynamic and thermal soaring offer significant energy savings through the exploitation of naturally occurring wind [...] Read more.
The energy efficiency and flight endurance of small unmanned aerial vehicles (SUAVs) can be improved through the implementation of autonomous soaring strategies. Biologically inspired flight techniques such as dynamic and thermal soaring offer significant energy savings through the exploitation of naturally occurring wind phenomena for thrustless flight. Recent interest in the application of artificial intelligence algorithms for autonomous soaring has been motivated by the pursuit of instilling generalized behavior in control systems, centered around the use of neural networks. However, the topology of such networks is usually predetermined, restricting the search space of potential solutions, while often resulting in complex neural networks that can pose implementation challenges for the limited hardware onboard small-scale autonomous vehicles. In exploring a novel method of generating neurocontrollers, this paper presents a neural network-based soaring strategy to extend flight times and advance the potential operational capability of SUAVs. In this study, the Neuroevolution of Augmenting Topologies (NEAT) algorithm is used to train efficient and effective neurocontrollers that can control a simulated aircraft along sustained dynamic and thermal soaring trajectories. The proposed approach evolves interpretable neural networks in a way that preserves simplicity while maximizing performance without requiring extensive training datasets. As a result, the combined trajectory planning and aircraft control strategy is suitable for real-time implementation on SUAV platforms. Full article
(This article belongs to the Special Issue Energy Efficiency of Small-Scale UAVs)
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Article
Aircraft Trajectory Clustering in Terminal Airspace Based on Deep Autoencoder and Gaussian Mixture Model
Aerospace 2021, 8(9), 266; https://doi.org/10.3390/aerospace8090266 - 16 Sep 2021
Cited by 5 | Viewed by 883
Abstract
The aircraft trajectory clustering analysis in the terminal airspace is conducive to determining the representative route structure of the arrival and departure trajectory and extracting their typical patterns, which is important for air traffic management such as airspace structure optimization, trajectory planning, and [...] Read more.
The aircraft trajectory clustering analysis in the terminal airspace is conducive to determining the representative route structure of the arrival and departure trajectory and extracting their typical patterns, which is important for air traffic management such as airspace structure optimization, trajectory planning, and trajectory prediction. However, the current clustering methods perform poorly due to the large flight traffic, high density, and complex airspace structure in the terminal airspace. In recent years, the continuous development of Deep Learning has demonstrated its powerful ability to extract internal potential features of large dataset. Therefore, this paper mainly tries a deep trajectory clustering method based on deep autoencoder (DAE). To this end, this paper proposes a trajectory clustering method based on deep autoencoder (DAE) and Gaussian mixture model (GMM) to mine the prevailing traffic flow patterns in the terminal airspace. The DAE is trained to extract feature representations from historical high-dimensional trajectory data. Subsequently, the output of DAE is input into GMM for clustering. This paper takes the terminal airspace of Guangzhou Baiyun International Airport in China as a case to verify the proposed method. Through the direct visualization and dimensionality reduction visualization of the clustering results, it is found that the traffic flow patterns identified by the clustering method in this paper are intuitive and separable. Full article
(This article belongs to the Collection Air Transportation—Operations and Management)
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Article
The Rapid Data-Driven Prediction Method of Coupled Fluid–Thermal–Structure for Hypersonic Vehicles
Aerospace 2021, 8(9), 265; https://doi.org/10.3390/aerospace8090265 - 16 Sep 2021
Viewed by 669
Abstract
This work demonstrates the use of Latin Hypercube Sampling and Proper Orthogonal Decomposition in combination with a Radial Basis Function model to perform on vehicle prediction coupled fluid–thermal–structure. We regarded the Mach number, flight altitude and angle of attack as input parameters and [...] Read more.
This work demonstrates the use of Latin Hypercube Sampling and Proper Orthogonal Decomposition in combination with a Radial Basis Function model to perform on vehicle prediction coupled fluid–thermal–structure. We regarded the Mach number, flight altitude and angle of attack as input parameters and established a rapid prediction model. The basic process of numerical simulation of the hypersonic vehicle coupled fluid–thermal–structure was studied to obtain the database of pressure coefficient, heat flux, structural temperature and structural stress as the sample data to train this prediction method. The prediction error was analyzed. The prediction results showed that the data-driven method proposed in this paper based on proper orthogonal decomposition and radial basis function could be used for predicting vehicle coupled fluid–thermal–structure with good efficiency. Full article
(This article belongs to the Collection Hypersonics: Emerging Research)
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Article
Prediction of Aircraft Noise Impact with Application to Hong Kong International Airport
Aerospace 2021, 8(9), 264; https://doi.org/10.3390/aerospace8090264 - 15 Sep 2021
Viewed by 836
Abstract
As part of a collective research effort towards greener aviation, the present study focuses on the noise impact of aircraft operations around major airports. To this end, an aircraft noise prediction platform is developed, which relies on state-of-the-art functionalities as well as more [...] Read more.
As part of a collective research effort towards greener aviation, the present study focuses on the noise impact of aircraft operations around major airports. To this end, an aircraft noise prediction platform is developed, which relies on state-of-the-art functionalities as well as more specific, innovative features. Originally built upon the Aircraft Noise and Performance (ANP) database and its Noise–Power–Distance (NPD) table, the method is further refined to alleviate most of their inherent limitations (e.g., standardized and simplified aircraft noise scenarios). The resulting aircraft noise prediction platform is validated against benchmark cases of increasing complexity, being then applied to real-life situations involving actual aircraft operations around Hong Kong International Airport (HKIA). Specific comparative analyses are conducted, which allow highlighting the variability of the noise impact by aircraft, depending on their type (A330, B777) and/or operational conditions (power settings, meteorological conditions, routes, banks, etc.). The study delivers insightful outcomes, whether phenomenological (aircraft noise impact) or methodological (aircraft noise prediction). As a by-product, it illustrates how noise prediction methods/platforms such as the present one may help in guiding the further expansion of airport operations and/or infrastructures (as is currently the case with HKIA). Full article
(This article belongs to the Special Issue Aircraft Noise)
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Article
VHF Omnidirectional Range (VOR) Experimental Positioning for Stratospheric Vehicles
Aerospace 2021, 8(9), 263; https://doi.org/10.3390/aerospace8090263 - 15 Sep 2021
Cited by 1 | Viewed by 679
Abstract
The usage of aeronautical radio-frequency navigational aids can support the future stratospheric aviation as back-up positioning systems. Although GNSS has been extensively redundant in the last years of space operations, radio NavAids can still be supportive of navigation and tracking for novel mission [...] Read more.
The usage of aeronautical radio-frequency navigational aids can support the future stratospheric aviation as back-up positioning systems. Although GNSS has been extensively redundant in the last years of space operations, radio NavAids can still be supportive of navigation and tracking for novel mission profiles. As an example, in 2016, VHF Omnidirectional Range (VOR) has been proven to work well above its standard service volume limit on a stratospheric balloon flight with the STRATONAV experiment. While VOR provides the “radial” measurement, i.e., the angle between the Magnetic North and the line between the receiver and the transmitting ground station, the intersection of two or more radials at a time allows to perform ground track reconstruction for the vehicle to be tracked. This paper reports the results from the data re-processing from STRATONAV: the acquired radials have been intersected in order to achieve positioning. The radials interfacing method, the position calculation methodology, and the data acquisition strategies from STRATONAV are reported together with the data analysis results. Full article
(This article belongs to the Special Issue New Space: Advances in Space Science and Engineering)
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Article
Assignment of Natural Frequencies and Mode Shapes Based on FRFs
Aerospace 2021, 8(9), 262; https://doi.org/10.3390/aerospace8090262 - 14 Sep 2021
Viewed by 894
Abstract
This paper proposes a method of structural modification for the assignment of natural frequencies and mode shapes based on frequency response functions (FRFs). The method involves the addition of masses or stiffness (supporting stiffness or connection stiffness), the simultaneous addition of masses and [...] Read more.
This paper proposes a method of structural modification for the assignment of natural frequencies and mode shapes based on frequency response functions (FRFs). The method involves the addition of masses or stiffness (supporting stiffness or connection stiffness), the simultaneous addition of masses and stiffness, or the addition of mass-spring substructures to the original structure. Firstly, the proposed technique was formulated as an optimization problem based on the FRFs of the original structure and the masses or stiffness that needed to be added. Next, the required added masses and stiffness were obtained by solving the optimization problem using a genetic algorithm. Finally, numerical verification was performed for the different structural modification schemes. The results show that, compared to only adding either stiffness or masses, adding both simultaneously or adding spring-mass substructures obtained better optimization results. The advantage of this FRFs-based method is that the FRFs can be directly measured by modal testing, without knowledge of analytical or modal models. Furthermore, multiple structural modifications were considered in the assignment of natural frequencies and mode shapes, making the application of this method more applicable to engineering. Full article
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Article
High Temperature Magnetic Sensors for the Hot Section of Aeroengines
Aerospace 2021, 8(9), 261; https://doi.org/10.3390/aerospace8090261 - 13 Sep 2021
Cited by 5 | Viewed by 816
Abstract
Magnetic sensors are widely used in aeroengines and their health management systems, but they are rarely installed in the engine hot section due to the loss of magnetic properties by permanent magnets with increasing temperature. The paper presents and verifies models and design [...] Read more.
Magnetic sensors are widely used in aeroengines and their health management systems, but they are rarely installed in the engine hot section due to the loss of magnetic properties by permanent magnets with increasing temperature. The paper presents and verifies models and design solutions aimed at improving the performance of an inductive sensor for measuring the motion of blades operated at elevated temperatures (200–1000 °C) in high pressure compressors and turbines. The interaction of blades with the sensor was studied. A prototype of the sensor was made, and its tests were carried out on the RK-4 rotor rig for the speed of 7000 rpm, in which the temperature of the sensor head was gradually increased to 1100 °C. The sensor signal level was compared to that of an identical sensor operating at room temperature. The heated sensor works continuously producing the output signal whose level does not change significantly. Moreover, a set of six probes passed an initial engine test in an SO-3 turbojet. It was confirmed that the proposed design of the inductive sensor is suitable for blade health monitoring (BHM) of the last stages of compressors and gas turbines operating below 1000 °C, even without a dedicated cooling system. In real-engine applications, sensor performance will depend on how the sensor is installed and the available heat dissipation capability. The presented technology extends the operating temperature of permanent magnets and is not specific for blade vibration but can be adapted to other magnetic measurements in the hot section of the aircraft engine. Full article
(This article belongs to the Special Issue Technologies for Future Distributed Engine Control Systems)
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Article
The Impact of Automation on Air Traffic Controller’s Behaviors
Aerospace 2021, 8(9), 260; https://doi.org/10.3390/aerospace8090260 - 13 Sep 2021
Cited by 1 | Viewed by 826
Abstract
Air traffic controllers have to make quick decisions to keep air traffic safe. Their behaviors have a significant impact on the operation of the air traffic management (ATM) system. Automation tools have enhanced the ATM system’s capability by reducing the controller’s task-load. Much [...] Read more.
Air traffic controllers have to make quick decisions to keep air traffic safe. Their behaviors have a significant impact on the operation of the air traffic management (ATM) system. Automation tools have enhanced the ATM system’s capability by reducing the controller’s task-load. Much attention has been devoted to developing advanced automation in the last decade. However, less is known about the impact of automation on the behaviors of air traffic controllers. Here, we empirically tested the effects of three levels of automation—including manual, attention-guided, and automated—as well as varying traffic levels on eye movements, situation awareness and mental workload. The results showed that there are significant differences in the gaze and saccade behaviors between the attention-guided group and automated group. Traffic affected eye movements under the manual mode or under the attention-guided mode, but had no effect on eye movements under the automated mode. The results also supported the use of automation for enhancing situation awareness while reducing mental workload. Our work has potential implications for the design of automation and operation procedures. Full article
(This article belongs to the Collection Air Transportation—Operations and Management)
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Article
Parametric Study of a Composite Skin for a Twist-Morphing Wing
Aerospace 2021, 8(9), 259; https://doi.org/10.3390/aerospace8090259 - 13 Sep 2021
Cited by 3 | Viewed by 857
Abstract
Although the benefits of morphing wings have been proven in many studies in the last few decades, the wing skin design remains one of the challenges to advancing and implementing the morphing technology. This is due to the conflicting design requirements of high [...] Read more.
Although the benefits of morphing wings have been proven in many studies in the last few decades, the wing skin design remains one of the challenges to advancing and implementing the morphing technology. This is due to the conflicting design requirements of high out-of-plane stiffness to withstand aerodynamic loads and low in-plane stiffness to allow morphing with the available actuation forces. Advancements in the design of hybrid and flexible composites might allow for design solutions that feature this balance in stiffness required for this application. These composites offer new design parameters, such as the number of plies, the fiber-orientation angle of each ply in the skin laminate, and the spatial distribution of the plies on the skin surface. This paper presents a parametric study of a composite skin for a twist-morphing wing. The skin is made of periodic laminated composite sections, called “Twistkins”, integrated in an elastomeric outer skin. The twisting deformation is localized in the elastomeric sections between the Twistkins. The design parameters considered are the number of plies in the composite Twistkins, the fiber-orientation angle of the plies, the torsional rigidity of the elastomer, the width ratio, and the number of elastomeric sections. The computational analysis results showed that the torsional compliance can be increased by increasing the width ratio, decreasing the number of elastomeric sections, number of composite plies and the elastomer’s torsional rigidity. However, this would also lead to a decrease in the out-of-plane stiffness. The nonlinearity and rates at which these parameters affect the skin’s behavior are highlighted, including the effect of the fiber-orientation angle of the laminate plies. Hence, the study guides the design process of this twist-morphing skin. Full article
(This article belongs to the Special Issue Advances in Aerospace Sciences and Technology II)
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Article
Unmanned Aerial Vehicle Pitch Control under Delay Using Deep Reinforcement Learning with Continuous Action in Wind Tunnel Test
Aerospace 2021, 8(9), 258; https://doi.org/10.3390/aerospace8090258 - 11 Sep 2021
Cited by 1 | Viewed by 802
Abstract
Nonlinear flight controllers for fixed-wing unmanned aerial vehicles (UAVs) can potentially be developed using deep reinforcement learning. However, there is often a reality gap between the simulation models used to train these controllers and the real world. This study experimentally investigated the application [...] Read more.
Nonlinear flight controllers for fixed-wing unmanned aerial vehicles (UAVs) can potentially be developed using deep reinforcement learning. However, there is often a reality gap between the simulation models used to train these controllers and the real world. This study experimentally investigated the application of deep reinforcement learning to the pitch control of a UAV in wind tunnel tests, with a particular focus of investigating the effect of time delays on flight controller performance. Multiple neural networks were trained in simulation with different assumed time delays and then wind tunnel tested. The neural networks trained with shorter delays tended to be susceptible to delay in the real tests and produce fluctuating behaviour. The neural networks trained with longer delays behaved more conservatively and did not produce oscillations but suffered steady state errors under some conditions due to unmodeled frictional effects. These results highlight the importance of performing physical experiments to validate controller performance and how the training approach used with reinforcement learning needs to be robust to reality gaps between simulation and the real world. Full article
(This article belongs to the Special Issue AI/Machine Learning in Aerospace Autonomy)
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Article
Active Flutter Suppression of Smart-Skin Antenna Structures with Piezoelectric Sensors and Actuators
Aerospace 2021, 8(9), 257; https://doi.org/10.3390/aerospace8090257 - 11 Sep 2021
Cited by 1 | Viewed by 666
Abstract
A smart-skin antenna structure is investigated for active flutter control with piezoelectric sensors and actuators. The skin antenna is designed as a multilayer sandwich structure with a dielectric polymer to perform the role of antenna or radar structures. The governing equations are developed [...] Read more.
A smart-skin antenna structure is investigated for active flutter control with piezoelectric sensors and actuators. The skin antenna is designed as a multilayer sandwich structure with a dielectric polymer to perform the role of antenna or radar structures. The governing equations are developed according to the first-order shear deformation theory, and von Karman strain–displacement relationships are used for the moderate geometrical nonlinearity. To consider the supersonic airflow, first-order piston theory is performed for the aerodynamic pressures. The linear quadratic regulator (LQR) method is applied as a control algorithm, and Newmark’s method is studied to obtain the numerical results. In the present study, the effects of placements and shape of piezoelectric patches are discussed on the flutter control of the model in detail. In addition, the numerical results show that the skin antenna model can effectively suppress the panel flutter behaviors of the model, optimal conditions of piezoelectric patches are obtained for skin antenna structures. Full article
(This article belongs to the Special Issue Vibration Control for Space Application)
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Evaluation and Comparison of Hybrid Wing VTOL UAV with Four Different Electric Propulsion Systems
Aerospace 2021, 8(9), 256; https://doi.org/10.3390/aerospace8090256 - 09 Sep 2021
Cited by 1 | Viewed by 1151
Abstract
Electric propulsion technology has attracted much attention in the aviation industry at present. It has the advantages of environmental protection, safety, low noise, and high design freedom. An important research branch of electric propulsion aircraft is electric vertical takeoff and landing (VTOL) aircraft, [...] Read more.
Electric propulsion technology has attracted much attention in the aviation industry at present. It has the advantages of environmental protection, safety, low noise, and high design freedom. An important research branch of electric propulsion aircraft is electric vertical takeoff and landing (VTOL) aircraft, which is expected to play an important role in urban traffic in the future. Limited by battery energy density, all electric unmanned aerial vehicles (UAVs) are unable to meet the longer voyage. Series/parallel hybrid-electric propulsion and turboelectric propulsion are considered to be applied to VTOL UAVs to improve performances. In this paper, the potential of these four configurations of electric propulsion systems for small VTOL UAVs are evaluated and compared. The main purpose is to analyze the maximum takeoff mass and fuel consumption of VTOL UAVs with different propulsion systems that meet the same performance requirements and designed mission profiles. The differences and advantages of the four types propulsion VTOL UAV in the maximum takeoff mass and fuel consumption are obtained, which provides a basis for the design and configuration selection of VTOL UAV propulsion system. Full article
(This article belongs to the Special Issue UAV of Dual Mode Operation and Hybrid Power)
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Article
The Impact of Steady Blowing from the Leading Edge of an Open Cavity Flow
Aerospace 2021, 8(9), 255; https://doi.org/10.3390/aerospace8090255 - 09 Sep 2021
Viewed by 667
Abstract
Cavity flows occur in a wide range of low-speed applications (Mach number 0.3), such as aircraft wheel wells, ground transportation, and pipelines. In the current study, a steady jet is forced from a cavity leading edge at different momentum fluxes (0.11 [...] Read more.
Cavity flows occur in a wide range of low-speed applications (Mach number 0.3), such as aircraft wheel wells, ground transportation, and pipelines. In the current study, a steady jet is forced from a cavity leading edge at different momentum fluxes (0.11 kg/ms2, 0.44 kg/m·s2, and 0.96 kg/m·s2). The investigation was performed for an open cavity with length to depth ratio of 4 at the Reynolds number based on a cavity depth of approximately 50,000. Particle image velocimetry, surface oil flow visualisation, constant temperature anemometry, and pressure measurements were performed in this investigation. The aim of the jet blowing is to separate the cavity separated shear layer from the recirculation zone to reduce the cavity return flow, and hence stabilise the cavity separated shear layer. It was found that increasing the jet momentum flux causes an increase in the cavity return flow due to the increase in the thickness of the cavity separated shear layer. The study also found that the jet populates the separated shear layer with a large number of small-scale disturbances. These disturbances increase the broad band level of the pressure power spectra and Reynolds shear stress in the cavity separated shear layer. On the other hand, the jet disturbances make the shedding of the large vortical structures more intermittent. Full article
(This article belongs to the Special Issue Advances in Aerospace Sciences and Technology II)
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Article
Development of the EIRSAT-1 CubeSat through Functional Verification of the Engineering Qualification Model
Aerospace 2021, 8(9), 254; https://doi.org/10.3390/aerospace8090254 - 08 Sep 2021
Cited by 4 | Viewed by 1203
Abstract
The Educational Irish Research Satellite (EIRSAT-1) is a 2U CubeSat developed at University College Dublin. The project aims to build, test, launch, and operate Ireland’s first satellite and to perform in-orbit demonstrations of three novel payloads developed in-house. To reduce risk within the [...] Read more.
The Educational Irish Research Satellite (EIRSAT-1) is a 2U CubeSat developed at University College Dublin. The project aims to build, test, launch, and operate Ireland’s first satellite and to perform in-orbit demonstrations of three novel payloads developed in-house. To reduce risk within the mission, the project employs a prototype model philosophy in which two models of the spacecraft exist: an engineering qualification model (EQM) and a flight model (FM). This paper presents the verification approach of the functional tests implemented for the EIRSAT-1 project. The activities of the FlatSat and system level full functional tests of the EQM are presented and the results obtained during the test campaigns are discussed. Four test anomalies were encountered during the full functional test campaign resulting in two minor redesigns, and subsequent reassembly, of the CubeSat. The functional test campaigns highlighted the importance of FlatSat level testing of CubeSats to ensure compatibility of all subsystems prior to assembly and of thorough documentation to diagnose any unexpected behaviour of the hardware efficiently. The functional verification of the EQM proved that the system conformed to its design, verifying 57 mission requirements, and is a crucial step towards the development of the EIRSAT-1 FM. Full article
(This article belongs to the Special Issue Small Satellite Technologies and Mission Concepts)
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Article
Hybrid Rockets as Post-Boost Stages and Kick Motors
Aerospace 2021, 8(9), 253; https://doi.org/10.3390/aerospace8090253 - 07 Sep 2021
Cited by 3 | Viewed by 878
Abstract
Hybrid rockets are attractive as post-boost stages and kick motors due to their inherent safety and low cost, but it is not clear from previous research which oxidizer is most suitable for maximizing ΔV within a fixed envelope size, or what impact O [...] Read more.
Hybrid rockets are attractive as post-boost stages and kick motors due to their inherent safety and low cost, but it is not clear from previous research which oxidizer is most suitable for maximizing ΔV within a fixed envelope size, or what impact O/F shift and nozzle erosion will have on ΔV. A standard hybrid rocket design is proposed and used to clarify the impact of component masses on ΔV within three 1 m3 envelopes of varying height-to-base ratios. Theoretical maximum ΔV are evaluated first, assuming constant O/F and no nozzle erosion. Of the four common liquid oxidizers: H2O2 85 wt%, N2O, N2O4, and LOX, H2O2 85 wt% is shown to result in the highest ΔV, and N2O is shown to result in the highest density ΔV, which is the ΔV normalized for motor density. When O/F shift is considered, the ΔV decreases by 9% for the N2O motor and 12% for the H2O2 85 wt% motor. When nozzle erosion is also considered, the ΔV decreases by another 7% for the H2O2 85 wt% motor and 4% for the N2O motor. Even with O/F shift and nozzle erosion, the H2O2 85 wt% motor can accelerate itself (916 kg) upwards of 4000 m/s, and the N2O motor (456 kg) 3550 m/s. Full article
(This article belongs to the Special Issue Hybrid Rocket(Volume II))
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Article
Effect of Reaction Wheel Imbalances on Attitude and Stabilization Accuracy
Aerospace 2021, 8(9), 252; https://doi.org/10.3390/aerospace8090252 - 06 Sep 2021
Cited by 2 | Viewed by 713
Abstract
In this paper, the study of stabilization accuracy of a satellite equipped with a set of reaction wheels (RW) is presented. The model of motion takes into account possible static and dynamic reaction wheel imbalances. Due to the complexity of the model, which [...] Read more.
In this paper, the study of stabilization accuracy of a satellite equipped with a set of reaction wheels (RW) is presented. The model of motion takes into account possible static and dynamic reaction wheel imbalances. Due to the complexity of the model, which leads to the numerical issues, the effects of dynamic and static imbalances on inertial stabilization are studied analytically. As a result, estimations of the attitude and stabilization accuracy are presented in closed form. Full article
(This article belongs to the Special Issue Spacecraft Dynamics and Control)
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Article
Field-of-View Constrained Impact Time Control Guidance via Time-Varying Sliding Mode Control
Aerospace 2021, 8(9), 251; https://doi.org/10.3390/aerospace8090251 - 06 Sep 2021
Cited by 2 | Viewed by 614
Abstract
The problem of impact time control guidance with field-of-view constraint is addressed based on time-varying sliding mode control. The kinematic conditions that satisfy the impact time control with field-of-view constraint are defined, and then a novel time-varying sliding surface is constructed to achieve [...] Read more.
The problem of impact time control guidance with field-of-view constraint is addressed based on time-varying sliding mode control. The kinematic conditions that satisfy the impact time control with field-of-view constraint are defined, and then a novel time-varying sliding surface is constructed to achieve the defined conditions. The sliding surface contains two unknown coefficients: one is tuned to achieve the global sliding surface to satisfy the impact time constraint and zero miss distance, and the other is tuned to guarantee the field-of-view constraint. The guidance law is designed to ensure the realization of the global sliding mode. On this basis, the guidance law is modified to a closed-loop structure, and the maximum detection capability of the seeker is utilized to a greater extent. Under the proposed guidance law, neither the small angle assumption nor time-to-go estimation is needed. The guidance command is continuous and converges to 0 at the desired impact time. Simulation results demonstrate the effectiveness and superiority of the proposed guidance law. Full article
(This article belongs to the Section Aeronautics)
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Article
A Hybrid Real/Ideal Gas Mixture Computational Framework to Capture Wave Propagation in Liquid Rocket Combustion Chamber Conditions
Aerospace 2021, 8(9), 250; https://doi.org/10.3390/aerospace8090250 - 04 Sep 2021
Viewed by 571
Abstract
The present work focuses on the development of new mathematical and numerical tools to deal with wave propagation problems in a realistic liquid rocket chamber environment. A simplified real fluid equation of state is here derived, starting from the literature. An approximate Riemann [...] Read more.
The present work focuses on the development of new mathematical and numerical tools to deal with wave propagation problems in a realistic liquid rocket chamber environment. A simplified real fluid equation of state is here derived, starting from the literature. An approximate Riemann solver is then specifically derived for the selected conservation laws and primitive variables. Both the new equation of state and the new Riemann solver are embedded into an in-house one-dimensional CFD solver. The verification and validation of the new code against wave propagation problems are then performed, showing good behavior. Although such problems might be of interest for different applications, the present study is specifically oriented to the low order modeling of high-frequency combustion instability in liquid-propellant rocket engines. Full article
(This article belongs to the Special Issue Advances in Computational Methodologies for Aerospace Propulsion)
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Article
d2 Law and Penetration Length of Jatropha and Camelina Bio-Synthetic Paraffinic Kerosene Spray Characteristics at Take-Off, Top of Climb and Cruise
Aerospace 2021, 8(9), 249; https://doi.org/10.3390/aerospace8090249 - 04 Sep 2021
Viewed by 642
Abstract
A comparison of d2 law and penetration length of biofuels with Jet–A through the incorporation of fuel properties and actual combustor inlet data at various flight trajectories is presented. This study aims to identify fuel properties and flight operating conditions that most [...] Read more.
A comparison of d2 law and penetration length of biofuels with Jet–A through the incorporation of fuel properties and actual combustor inlet data at various flight trajectories is presented. This study aims to identify fuel properties and flight operating conditions that most influence droplet characteristics accurately. The study comprises two phases involving a simulation using GSP to predict combustor inlet data for the respective flight operating conditions and a simulation using ANSYS Fluent V18.1 to obtain combustion characteristics of biofuels and Jet–A. The biofuels chosen in this study are Jatropha Bio-synthetic Paraffinic Kerosene (JSPK) and Camelina Bio-synthetic Paraffinic Kerosene (CSPK), evaluated as pure (100%) and blend (50%) with Jet–A. Thrust specific fuel consumption (TSFC) of biofuels is improved due to lower fuel consumed by the engine. The d2 law curve shows a heat-up period that takes place at the early stage of the combustion process. The penetration length of the fuels is shorter at take-off. Combusting biofuels reduce combustion temperature and the penetration length of the droplet. Full article
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Article
Trajectory-Tracking Controller Design of Rotorcraft Using an Adaptive Incremental-Backstepping Approach
Aerospace 2021, 8(9), 248; https://doi.org/10.3390/aerospace8090248 - 04 Sep 2021
Viewed by 633
Abstract
This paper treats a robust adaptive trajectory-tracking control design for a rotorcraft using a high-fidelity math model subject to model uncertainties. In order to control the nonlinear rotorcraft model which shows strong inter-axis coupling and high nonlinearity, incremental backstepping approach with state-dependent control [...] Read more.
This paper treats a robust adaptive trajectory-tracking control design for a rotorcraft using a high-fidelity math model subject to model uncertainties. In order to control the nonlinear rotorcraft model which shows strong inter-axis coupling and high nonlinearity, incremental backstepping approach with state-dependent control effectiveness matrix is utilized. Since the incremental backstepping control suffers from performance degradation in the presence of control matrix uncertainties due to change of flight conditions, control system robustness is improved by combining the least squares parameter estimator to estimate time varying uncertainties contained in the control effectiveness matrix. Also, by selecting a suitable gain set by investigating the error dynamics, a uniform trajectory-tracking performance over operational flight envelope of the rotorcraft is ensured without resorting to the conventional gain scheduling method. To evaluate the proposed controller, comparative results between IBSC and Adaptive IBSC are provided in this paper with sequential maneuvers from the ADS-33E-PRF. The proposed method shows improved tracking performance under variations in control effective matrix in the flight simulation. Robust and stable parameter estimation is also guaranteed due to the implementation of the DF-RLS algorithm for the least squares estimator. Full article
(This article belongs to the Section Aeronautics)
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Article
A Complete Mission Concept Design and Analysis of the Student-Led CubeSat Project: Light-1
Aerospace 2021, 8(9), 247; https://doi.org/10.3390/aerospace8090247 - 04 Sep 2021
Cited by 2 | Viewed by 963
Abstract
Terrestrial gamma ray flashes (TGF) are intense and prompt bursts of X- and gamma-rays of up to 100 MeV of energy. Typically associated with thunderstorm activity, TGFs are produced by bremsstrahlung effects of electrons accelerated in strong electric fields generated by lightning. TGFs [...] Read more.
Terrestrial gamma ray flashes (TGF) are intense and prompt bursts of X- and gamma-rays of up to 100 MeV of energy. Typically associated with thunderstorm activity, TGFs are produced by bremsstrahlung effects of electrons accelerated in strong electric fields generated by lightning. TGFs can be effectively targeted by gamma detectors with enhanced time stamping capabilities onboard of satellites operating at near-Earth low obits (LEO). Light-1 is a miniature satellite, a 3U CubeSat designed to detect, monitor and study terrestrial gamma ray flashes in low Earth orbit. The two payload detectors are composed of a photomultiplier tube and silicon photomultipliers. The two detectors are mounted at two ends of the CubeSat and the proposed orientation of the CubeSat will ensure maximum TGF detection probability. To allow an increased frequency of data downlink, Khalifa University has collaborated with NanoAvionics Corp, and hence Light-1 has access to three ground stations situated across the map, Abu Dhabi in United Arab Emirates, Vilnius in Lithuania, and Aalborg in Denmark. The satellite expected to launch in late-2021 is currently in its assembly and integration phase. This paper describes mission, concept, objectives, success criteria, design, analysis, status, and the future plans of Light-1 satellite. Full article
(This article belongs to the Special Issue Small Satellite Technologies and Mission Concepts)
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Article
Variational Bayesian Iteration-Based Invariant Kalman Filter for Attitude Estimation on Matrix Lie Groups
Aerospace 2021, 8(9), 246; https://doi.org/10.3390/aerospace8090246 - 03 Sep 2021
Cited by 2 | Viewed by 641
Abstract
Motivated by the rapid progress of aerospace and robotics engineering, the navigation and control systems on matrix Lie groups have been actively studied in recent years. For rigid targets, the attitude estimation problem is a benchmark one with its states defined as rotation [...] Read more.
Motivated by the rapid progress of aerospace and robotics engineering, the navigation and control systems on matrix Lie groups have been actively studied in recent years. For rigid targets, the attitude estimation problem is a benchmark one with its states defined as rotation matrices on Lie groups. Based on the invariance properties of symmetry groups, the invariant Kalman filter (IKF) has been developed by researchers for matrix Lie group systems; however, the limitation of the IKF is that its estimation performance is prone to be degraded if the given knowledge of the noise statistics is not accurate. For the symmetry Lie group attitude estimation problem, this paper proposes a new variational Bayesian iteration-based adaptive invariant Kalman filter (VBIKF). In the proposed VBIKF, the a priori error covariance is not propagated by the conventional steps but directly calibrated in an iterative manner based on the posterior sequences. The main advantage of the VBIKF is that the statistics parameter of the system process noise is no longer required and so the IKF’s hard dependency on accurate process noise statistics can be reduced significantly. The mathematical foundation for the new VBIKF is presented and its superior performance in adaptability and simplicity is further demonstrated by numerical simulations. Full article
(This article belongs to the Special Issue Aircraft Modelling for Design, Simulation and Control)
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Article
Integrating Eye- and Mouse-Tracking with Assistant Based Speech Recognition for Interaction at Controller Working Positions
Aerospace 2021, 8(9), 245; https://doi.org/10.3390/aerospace8090245 - 03 Sep 2021
Viewed by 662
Abstract
Assistant based speech recognition (ABSR) prototypes for air traffic controllers have demonstrated to reduce controller workload and aircraft flight times as a result. However, two aspects of ABSR could enhance benefits, i.e., (1) the predicted controller commands that speech recognition engines use can [...] Read more.
Assistant based speech recognition (ABSR) prototypes for air traffic controllers have demonstrated to reduce controller workload and aircraft flight times as a result. However, two aspects of ABSR could enhance benefits, i.e., (1) the predicted controller commands that speech recognition engines use can be more accurate, and (2) the confirmation process of ABSR recognition output, such as callsigns, command types, and values by the controller, can be less intrusive. Both tasks can be supported by unobtrusive eye- and mouse-tracking when using operators’ gaze and interaction data. First, probabilities for predicted commands should consider controllers’ visual focus on the situation data display. Controllers will more likely give commands to aircraft that they focus on or where there was a mouse interaction on the display. Furthermore, they will more likely give certain command types depending on the characteristics of multiple aircraft being scanned. Second, it can be determined via eye-tracking instead of additional mouse clicks if the displayed ABSR output has been checked by the controller and remains uncorrected for a certain amount of time. Then, the output is assumed to be correct and is usable by other air traffic control systems, e.g., short-term conflict alert. If the ABSR output remains unchecked, an attention guidance functionality triggers different escalation levels to display visual cues. In a one-shot experimental case study with two controllers for the two implemented techniques, (1) command prediction probabilities improved by a factor of four, (2) prediction error rates based on an accuracy metric for three most-probable aircraft decreased by a factor of 25 when combining eye- and mouse-tracking data, and (3) visual confirmation of ABSR output promises to be an alternative for manual confirmation. Full article
(This article belongs to the Special Issue Aeronautical Informatics)
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