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Aerospace, Volume 8, Issue 3 (March 2021) – 32 articles

Cover Story (view full-size image): The development of future helicopters is subject to the reduction of the environmental impact and the expansion of the flight envelope toward higher cruising speeds. The speed limitation is tackled by the compound helicopter RACER. Due to the high cruise speed, drag mitigation is an important topic. The rotor head represents one of the main drag sources, and the application of rotor head fairings can be an effective drag reduction measure. As part of the RACER full-fairing concept, a new blade-sleeve fairing was aerodynamically optimized for cruise flight. Within the present study, this newly developed blade-sleeve fairing is investigated by numerical flow simulations of an isolated, five-bladed rotor head featuring cyclic pitch movement. View this paper.
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Article
Transient Dynamic System Behavior of Pressure Actuated Cellular Structures in a Morphing Wing
Aerospace 2021, 8(3), 89; https://doi.org/10.3390/aerospace8030089 - 20 Mar 2021
Cited by 4 | Viewed by 1381
Abstract
High aspect ratio aircraft have a significantly reduced induced drag, but have only limited installation space for control surfaces near the wingtip. This paper describes a multidisciplinary design methodology for a morphing aileron that is based on pressure-actuated cellular structures (PACS). The focus [...] Read more.
High aspect ratio aircraft have a significantly reduced induced drag, but have only limited installation space for control surfaces near the wingtip. This paper describes a multidisciplinary design methodology for a morphing aileron that is based on pressure-actuated cellular structures (PACS). The focus of this work is on the transient dynamic system behavior of the multi-functional aileron. Decisive design aspects are the actuation speed, the resistance against external loads, and constraints preparing for a future wind tunnel test. The structural stiffness under varying aerodynamic loads is examined while using a reduced-order truss model and a high-fidelity finite element analysis. The simulations of the internal flow investigate the transient pressurization process that limits the dynamic actuator response. The authors present a reduced-order model based on the Pseudo Bond Graph methodology enabling time-efficient flow simulation and compare the results to computational fluid dynamic simulations. The findings of this work demonstrate high structural resistance against external forces and the feasibility of high actuation speeds over the entire operating envelope. Future research will incorporate the fluid–structure interaction and the assessment of load alleviation capability. Full article
(This article belongs to the Special Issue Adaptive/Smart Structures and Multifunctional Materials in Aerospace)
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Review
A Survey on Low-Thrust Trajectory Optimization Approaches
Aerospace 2021, 8(3), 88; https://doi.org/10.3390/aerospace8030088 - 19 Mar 2021
Cited by 7 | Viewed by 1978
Abstract
In this paper, we provide a survey on available numerical approaches for solving low-thrust trajectory optimization problems. First, a general mathematical framework based on hybrid optimal control will be presented. This formulation and their elements, namely objective function, continuous and discrete state and [...] Read more.
In this paper, we provide a survey on available numerical approaches for solving low-thrust trajectory optimization problems. First, a general mathematical framework based on hybrid optimal control will be presented. This formulation and their elements, namely objective function, continuous and discrete state and controls, and discrete and continuous dynamics, will serve as a basis for discussion throughout the whole manuscript. Thereafter, solution approaches for classical continuous optimal control problems will be briefly introduced and their application to low-thrust trajectory optimization will be discussed. A special emphasis will be placed on the extension of the classical techniques to solve hybrid optimal control problems. Finally, an extensive review of traditional and state-of-the art methodologies and tools will be presented. They will be categorized regarding their solution approach, the objective function, the state variables, the dynamical model, and their application to planetocentric or interplanetary transfers. Full article
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Article
Influence of the Apron Parking Stand Management Policy on Aircraft and Ground Support Equipment (GSE) Gaseous Emissions at Airports
Aerospace 2021, 8(3), 87; https://doi.org/10.3390/aerospace8030087 - 19 Mar 2021
Cited by 1 | Viewed by 963
Abstract
The purpose of this study is to analyze the concept of a hybrid apron with a fixed number of parking positions considering the management model influence for the average delay per aircraft and the gaseous emissions generated by aircraft and ground support equipment [...] Read more.
The purpose of this study is to analyze the concept of a hybrid apron with a fixed number of parking positions considering the management model influence for the average delay per aircraft and the gaseous emissions generated by aircraft and ground support equipment (GSE) altogether. The apron is studied based on two gate management models: in the first model, the aircraft are allocated in each gate due to operational factors only; in the second model, the rules of exclusive use of each gate according to the airline are included. The emissions generated by aircraft operations and that of their GSE (produced by the service and movements on the apron) are quantified and compared in the two gate management models: operation in the standard LTO cycle of the studied aircraft, GSE emissions have a similar relation with the compared gasses (NOx and CO), ranging between 1% and 3%. Further, if it compares the emissions between support vehicles and aircraft taking only into account the in-out taxiway, the relation between both CO sources shows similar values to those of the previous comparison, whereas NOx emissions produced by GSE reach an approximately 20%. The study considers different demand conditions obtained from the average day of the peak month of Aeroparque Jorge Newbery airport. Subsequently, through the SIMMOD PLUS software, the aircraft operations are simulated. The gates assignment and the arrival timetables are used as inputs for the GSE study due to an analytical model developed by us. Once the operational dimension is characterized and evaluated, the necessary data to quantify the gaseous emissions from the sources (Aircraft-GSE), based on the International Civil Aviation Organization (ICAO) guidelines, is obtained. Full article
(This article belongs to the Collection Air Transportation—Operations and Management)
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Article
Recent Advances of the BIRALET System about Space Debris Detection
Aerospace 2021, 8(3), 86; https://doi.org/10.3390/aerospace8030086 - 19 Mar 2021
Cited by 3 | Viewed by 992
Abstract
Space debris is internationally recognized as a planetary threat. Efforts to enhance the worldwide radar monitoring networks have been intensified in the last years. Among the new radars employed for the observations, one of the most promising is the Bistatic Radar for Low [...] Read more.
Space debris is internationally recognized as a planetary threat. Efforts to enhance the worldwide radar monitoring networks have been intensified in the last years. Among the new radars employed for the observations, one of the most promising is the Bistatic Radar for Low Earth Orbit (LEO) Tracking (BIRALET), which employs the Sardinia Radio Telescope as a receiving segment. The Sardinia Radio Telescope (SRT) has recently been proven to be a reliable instrument for space debris monitoring and, for this purpose, over the years has undergone some substantial modifications in order to be able to rise to the status of a fully functional radar receiver. However, an extensive measurement campaign, in order to assess the real potential of the radar, has never been done before. In this paper, the authors present the first real space debris measurement campaign of the SRT, made between December 2018 and October 2019 using the new dedicated channel of the P-band receiver. A total of 27 objects were correctly detected during this campaign, characterized by a radar cross section (RCS) interval between 0.13 and 13.4 m2 and a range interval between 459 and 1224 km. Full article
(This article belongs to the Special Issue New Space: Advances in Space Science and Engineering)
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Article
Active Energy Management Based on Meta-Heuristic Algorithms of Fuel Cell/Battery/Supercapacitor Energy Storage System for Aircraft
Aerospace 2021, 8(3), 85; https://doi.org/10.3390/aerospace8030085 - 19 Mar 2021
Cited by 4 | Viewed by 1208
Abstract
This paper presents the application of an active energy management strategy to a hybrid system consisting of a proton exchange membrane fuel cell (PEMFC), battery, and supercapacitor. The purpose of energy management is to control the battery and supercapacitor states of charge (SOCs) [...] Read more.
This paper presents the application of an active energy management strategy to a hybrid system consisting of a proton exchange membrane fuel cell (PEMFC), battery, and supercapacitor. The purpose of energy management is to control the battery and supercapacitor states of charge (SOCs) as well as minimizing hydrogen consumption. Energy management should be applied to hybrid systems created in this way to increase efficiency and control working conditions. In this study, optimization of an existing model in the literature with different meta-heuristic methods was further examined and results similar to those in the literature were obtained. Ant lion optimizer (ALO), moth-flame optimization (MFO), dragonfly algorithm (DA), sine cosine algorithm (SCA), multi-verse optimizer (MVO), particle swarm optimization (PSO), and whale optimization algorithm (WOA) meta-heuristic algorithms were applied to control the flow of power between sources. The optimization methods were compared in terms of hydrogen consumption and calculation time. Simulation studies were conducted in Matlab/Simulink R2020b (academic license). The contribution of the study is that the optimization methods of ant lion algorithm, moth-flame algorithm, and sine cosine algorithm were applied to this system for the first time. It was concluded that the most effective method in terms of hydrogen consumption and computational burden was the sine cosine algorithm. In addition, the sine cosine algorithm provided better results than similar meta-heuristic algorithms in the literature in terms of hydrogen consumption. At the same time, meta-heuristic optimization algorithms and equivalent consumption minimization strategy (ECMS) and classical proportional integral (PI) control strategy were compared as a benchmark study as done in the literature, and it was concluded that meta-heuristic algorithms were more effective in terms of hydrogen consumption and computational time. Full article
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Article
Sensitivity Study of Ice Accretion Simulation to Roughness Thermal Correction Model
Aerospace 2021, 8(3), 84; https://doi.org/10.3390/aerospace8030084 - 19 Mar 2021
Cited by 2 | Viewed by 886
Abstract
The effects of atmospheric icing can be anticipated by Computational Fluid Dynamics (CFD). Past studies show that the convective heat transfer influences the ice accretion and is itself a function of surface roughness. Uncertainty quantification (UQ) could help quantify the impact of surface [...] Read more.
The effects of atmospheric icing can be anticipated by Computational Fluid Dynamics (CFD). Past studies show that the convective heat transfer influences the ice accretion and is itself a function of surface roughness. Uncertainty quantification (UQ) could help quantify the impact of surface roughness parameters on the reliability of ice accretion prediction. This paper aims to quantify ice accretion uncertainties and identify the key surface roughness correction parameters contributing the most to the uncertainties in a Reynolds-Averaged Navier-Stokes (RANS) formulation. Ice accretion simulations over a rough flat plate using two thermal correction models are used to construct a RANS database. Non-Intrusive Polynomial Chaos Expansion (NIPCE) metamodels are developed to predict the convective heat transfer and icing characteristics of the RANS database. The metamodels allow for the computation of the 95% confidence intervals of the output probability distribution (PDF) and of the sensitivity indexes of the roughness parameters according to their level of influence on the outputs. For one of the thermal correction models, the most influential parameter is the roughness height, whereas for the second model it is the surface correction coefficient. In addition, the uncertainty on the freestream temperature has a minor impact on the ice accretion sensitivity compared to the uncertainty on the roughness parameters. Full article
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Article
Experimental Heat Loads for Electrothermal Anti-Icing and De-Icing on UAVs
Aerospace 2021, 8(3), 83; https://doi.org/10.3390/aerospace8030083 - 18 Mar 2021
Cited by 10 | Viewed by 2106
Abstract
Atmospheric in-flight icing on unmanned aerial vehicles (UAVs) is a significant hazard. UAVs that are not equipped with ice protection systems are usually limited to operations within visual line of sight or to weather conditions without icing risk. As many military and commercial [...] Read more.
Atmospheric in-flight icing on unmanned aerial vehicles (UAVs) is a significant hazard. UAVs that are not equipped with ice protection systems are usually limited to operations within visual line of sight or to weather conditions without icing risk. As many military and commercial UAV missions require flights beyond visual line of sight and into adverse weather conditions, energy-efficient ice protection systems are required. In this experimental study, two electro-thermal ice protection systems for fixed-wing UAVs were tested. One system was operated in anti-icing and de-icing mode, and the other system was designed as a parting strip de-icing system. Experiments were conducted in an icing wind tunnel facility for varying icing conditions at low Reynolds numbers. A parametric study over the ice shedding time was used to identify the most energy-efficient operation mode. The results showed that longer intercycle durations led to higher efficiencies and that de-icing with a parting strip was superior compared to anti-icing and de-icing without a parting strip. These findings are relevant for the development of energy-efficient systems in the future. Full article
(This article belongs to the Special Issue Deicing and Anti-Icing of Aircraft (Volume II))
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Article
Modular Line-Focused Space Solar Power Satellite
Aerospace 2021, 8(3), 82; https://doi.org/10.3390/aerospace8030082 - 18 Mar 2021
Cited by 1 | Viewed by 838
Abstract
The Space Solar Power Satellite is an ultra-large space structure, which collects sunlight directly in space and then transmits it into the ground. Since the idea was invented in 1968, scientists around the world have proposed several typical conceptual design models. Nevertheless, the [...] Read more.
The Space Solar Power Satellite is an ultra-large space structure, which collects sunlight directly in space and then transmits it into the ground. Since the idea was invented in 1968, scientists around the world have proposed several typical conceptual design models. Nevertheless, the conceptual models have not been implemented for technological, manufacturing, and cost reasons. This paper presents a novel Space Solar Power Satellite scheme with modular line-focused concentrators and low concentration photovoltaic modules. First, the line-focused mode is analyzed and the optical performance of the circular trough concentrator is evaluated via ray-trace technique. Then, shape optimization for the cell array based on the Bézier curve is carried out to improve the optical property. Numerical examples indicate that the optimized cell array could obtain high power collection efficiency and suitable energy distribution. Moreover, the area of the photovoltaic cell array is reduced, which is conducive to cost reduction. Furthermore, modular design is conducted on the circular trough concentrator. Finally, the primary scheme of the novel Space Solar Power Satellite is designed with the previous modular concentrator and optimized cell array. Full article
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Article
Dust Ingestion in a Rotorcraft Engine Compressor: Experimental and Numerical Study of the Fouling Rate
Aerospace 2021, 8(3), 81; https://doi.org/10.3390/aerospace8030081 - 18 Mar 2021
Cited by 4 | Viewed by 904
Abstract
Helicopters often operate in dusty sites, ingesting huge amounts of contaminants during landing, take-off, hover-taxi, and ground operations. In specific locations, the downwash of the rotor may spread soil particles from the ground into the environment and, once ingested by the engine, may [...] Read more.
Helicopters often operate in dusty sites, ingesting huge amounts of contaminants during landing, take-off, hover-taxi, and ground operations. In specific locations, the downwash of the rotor may spread soil particles from the ground into the environment and, once ingested by the engine, may stick to the compressor airfoils. In the present work, the Allison 250 C18 engine’s multistage axial-flow compressor is employed to study the fouling rate on rotor blades and stator vanes from both numerical and experimental standpoints. The compressor is operated in a typical ground-idle operation, in terms of the rotational regime and contaminant concentration, in laboratory-controlled conditions. The mass of deposits is collected from the airfoil surfaces at the end of the test and compared to that estimated through the numerical model. The experimental test shows that the airfoils collect almost 1.6% of the engine’s total mass ingested during a ground-idle operation. The capability of numerical methods to predict the fouling rate on the rotating and stationary airfoils of a multistage compressor is tested through the implementation of literature based deposition models. Sticking models show a good agreement in terms of the relative results; nevertheless, an overestimation of the deposited mass predicted is observed. Full article
(This article belongs to the Special Issue Life Cycle Modeling of Aircraft Propulsion Systems)
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Article
Strength Analysis of Alternative Airframe Layouts of Regional Aircraft on the Basis of Automated Parametrical Models
Aerospace 2021, 8(3), 80; https://doi.org/10.3390/aerospace8030080 - 17 Mar 2021
Cited by 2 | Viewed by 986
Abstract
This publication presents the results of complex parametrical strength investigations of typical wings for regional aircrafts obtained by means of the new version of the four-level algorithm (FLA) with the modified module responsible for the analysis of aerodynamic loading. This version of FLA, [...] Read more.
This publication presents the results of complex parametrical strength investigations of typical wings for regional aircrafts obtained by means of the new version of the four-level algorithm (FLA) with the modified module responsible for the analysis of aerodynamic loading. This version of FLA, as well as a base one, is focused on significant decreasing time and labor input of a complex strength analysis of airframes by using simultaneously different principles of decomposition. The base version includes four-level decomposition of airframe and decomposition of strength tasks. The new one realizes additional decomposition of alternative variants of load cases during the process of determination of critical load cases. Such an algorithm is very suitable for strength analysis and designing airframes of regional aircrafts having a wide range of aerodynamic concepts. Results of validation of the new version of FLA for a high-aspect-ratio wing obtained in this work confirmed high performance of the algorithm in decreasing time and labor input of strength analysis of airframes at the preliminary stages of designing. During parametrical design investigation, some interesting results for strut-braced wings having high aspect ratios were obtained. Full article
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Article
Unmanned Aerial Vehicle Operating Mode Classification Using Deep Residual Learning Feature Extraction
Aerospace 2021, 8(3), 79; https://doi.org/10.3390/aerospace8030079 - 16 Mar 2021
Cited by 7 | Viewed by 1176
Abstract
Unmanned Aerial Vehicles (UAVs) undoubtedly pose many security challenges. We need only look to the December 2018 Gatwick Airport incident for an example of the disruption UAVs can cause. In total, 1000 flights were grounded for 36 h over the Christmas period which [...] Read more.
Unmanned Aerial Vehicles (UAVs) undoubtedly pose many security challenges. We need only look to the December 2018 Gatwick Airport incident for an example of the disruption UAVs can cause. In total, 1000 flights were grounded for 36 h over the Christmas period which was estimated to cost over 50 million pounds. In this paper, we introduce a novel approach which considers UAV detection as an imagery classification problem. We consider signal representations Power Spectral Density (PSD); Spectrogram, Histogram and raw IQ constellation as graphical images presented to a deep Convolution Neural Network (CNN) ResNet50 for feature extraction. Pre-trained on ImageNet, transfer learning is utilised to mitigate the requirement for a large signal dataset. We evaluate performance through machine learning classifier Logistic Regression. Three popular UAVs are classified in different modes; switched on; hovering; flying; flying with video; and no UAV present, creating a total of 10 classes. Our results, validated with 5-fold cross validation and an independent dataset, show PSD representation to produce over 91% accuracy for 10 classifications. Our paper treats UAV detection as an imagery classification problem by presenting signal representations as images to a ResNet50, utilising the benefits of transfer learning and outperforming previous work in the field. Full article
(This article belongs to the Collection Unmanned Aerial Systems)
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Article
Aspect Ratio Driven Relationship between Nozzle Internal Flow and Supersonic Jet Mixing
Aerospace 2021, 8(3), 78; https://doi.org/10.3390/aerospace8030078 - 16 Mar 2021
Cited by 1 | Viewed by 1139
Abstract
This work attempts to connect internal flow to the exit flow and supersonic jet mixing in rectangular nozzles with low to high aspect ratios (AR). A series of low and high aspect ratio rectangular nozzles (design Mach number = 1.5) with sharp throats [...] Read more.
This work attempts to connect internal flow to the exit flow and supersonic jet mixing in rectangular nozzles with low to high aspect ratios (AR). A series of low and high aspect ratio rectangular nozzles (design Mach number = 1.5) with sharp throats are numerically investigated using steady state Reynolds-averaged Navier−Stokes (RANS) computational fluid dynamics (CFD) with k-omega shear stress transport (SST) turbulence model. The numerical shadowgraph reveals stronger shocks at low ARs which become weaker with increasing AR due to less flow turning at the throat. Stronger shocks cause more aggressive gradients in the boundary layer resulting in higher wall shear stresses at the throat for low ARs. The boundary layer becomes thick at low ARs creating more aerodynamic blockage. The boundary layer exiting the nozzle transforms into a shear layer and grows thicker in the high AR nozzle with a smaller potential core length. The variation in the boundary layer growth on the minor and major axis is explained and its growth downstream the throat has a significant role in nozzle exit flow characteristics. The loss mechanism throughout the flow is shown as the entropy generated due to viscous dissipation and accounts for supersonic jet mixing. Axis switching phenomenon is also addressed by analyzing the streamwise vorticity fields at various locations downstream from the nozzle exit. Full article
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Article
On the Handling Qualities of Two Flying Wing Aircraft Configurations
Aerospace 2021, 8(3), 77; https://doi.org/10.3390/aerospace8030077 - 16 Mar 2021
Cited by 2 | Viewed by 981
Abstract
The coupling of the longitudinal and lateral stability modes of an aeroplane is considered in two cases: (i) weak coupling, when the changes in the frequency and damping of the phugoid, short period, dutch roll, and helical modes are small, i.e., the square [...] Read more.
The coupling of the longitudinal and lateral stability modes of an aeroplane is considered in two cases: (i) weak coupling, when the changes in the frequency and damping of the phugoid, short period, dutch roll, and helical modes are small, i.e., the square of the deviation is negligible compared to the square of the uncoupled value; (ii) strong coupling, when the coupled values may differ significantly from the uncoupled values. This allows a comparison of three values for the frequency and damping of each mode: (i) exact, i.e., fully coupled; (ii) with the approximation of weak coupling; (iii) with the assumption of decoupling. The comparison of these three values allows an assessment of the importance of coupling effects. The method is applied to two flying wing designs, concerning all modes in a total of eighteen flight conditions. It turns out that lateral-longitudinal coupling is small in all cases, and thus classical handling qualities criteria can be applied. The handling qualities are considered for all modes, namely the phugoid, short period, dutch roll, spiral, and roll modes. Additional focus is given to the pitch axis, considering the control anticipation parameter (CAP). The latter relates to the two kinds of manouever points, where damping vanishes, that are calculated for minimum speed, take-off, and initial and final cruise conditions. The conclusion compares two flying wings designs (the “long narrow” and “short wide” fuselage concepts) not only from the point of view of flight stability, but also from other viewpoints. Full article
(This article belongs to the Special Issue Advances in Aerospace Sciences and Technology)
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Article
Remote Sensing Image Super-Resolution for the Visual System of a Flight Simulator: Dataset and Baseline
Aerospace 2021, 8(3), 76; https://doi.org/10.3390/aerospace8030076 - 15 Mar 2021
Cited by 1 | Viewed by 980
Abstract
High-resolution remote sensing images are the key data source for the visual system of a flight simulator for training a qualified pilot. However, due to hardware limitations, it is an expensive task to collect spectral and spatial images at very high resolutions. In [...] Read more.
High-resolution remote sensing images are the key data source for the visual system of a flight simulator for training a qualified pilot. However, due to hardware limitations, it is an expensive task to collect spectral and spatial images at very high resolutions. In this work, we try to tackle this issue with another perspective based on image super-resolution (SR) technology. First, we present a new ultra-high-resolution remote sensing image dataset named Airport80, which is captured from the airspace near various airports. Second, a deep learning baseline is proposed by applying the generative and adversarial mechanism, which is able to reconstruct a high-resolution image during a single image super-resolution. Experimental results for our benchmark demonstrate the effectiveness of the proposed network and show it has reached satisfactory performances. Full article
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Article
Tribology in Space Robotic Actuators: Experimental Method for Evaluation and Analysis of Gearboxes
Aerospace 2021, 8(3), 75; https://doi.org/10.3390/aerospace8030075 - 13 Mar 2021
Cited by 2 | Viewed by 1252
Abstract
Liquid lubricants are critical to enable long-life operation of high-performance machinery, such as geared actuators employed in robotics. In space applications, actuator gearboxes must operate in low temperatures, where liquid lubricants face inherent problems related to low temperature rheology. Heaters are relied upon [...] Read more.
Liquid lubricants are critical to enable long-life operation of high-performance machinery, such as geared actuators employed in robotics. In space applications, actuator gearboxes must operate in low temperatures, where liquid lubricants face inherent problems related to low temperature rheology. Heaters are relied upon to provide acceptable gearbox temperatures. Unfortunately, heating is energy-intense and does not scale well with increasing mechanism mass and performance. Effective boundary lubrication (BL), on the other hand, can minimize problems of low temperature rheology. BL relies on tribofilm formation over conventional fluid film separation. Effective space grade boundary lubricants can potentially allow for drastically reduced amounts of oil and the accompanying rheological problems. In this work, we describe the design of a methodology to evaluate and analyze tribology of actuator gearboxes operated under cryogenic oil-starved conditions in N2 atmosphere. The devised methodology enables research pertinent to space actuator tribology by accelerated testing and advanced analysis, as demonstrated by a lubricant candidate case study. Complementary microscopy techniques are discussed, and a novel methodology devised for gear internal microstructure analysis by X-ray microtomography (XMT) is presented. Full article
(This article belongs to the Special Issue Adaptive/Smart Structures and Multifunctional Materials in Aerospace)
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Article
A Review of Current Research in Subscale Flight Testing and Analysis of Its Main Practical Challenges
Aerospace 2021, 8(3), 74; https://doi.org/10.3390/aerospace8030074 - 12 Mar 2021
Cited by 6 | Viewed by 1118
Abstract
Testing of untethered subscale models, often referred to as subscale flight testing, has traditionally had a relatively minor, yet relevant use in aeronautical research and development. As recent advances in electronics, rapid prototyping and unmanned-vehicle technologies expand its capabilities and lower its cost, [...] Read more.
Testing of untethered subscale models, often referred to as subscale flight testing, has traditionally had a relatively minor, yet relevant use in aeronautical research and development. As recent advances in electronics, rapid prototyping and unmanned-vehicle technologies expand its capabilities and lower its cost, this experimental method is seeing growing interest across academia and the industry. However, subscale models cannot meet all similarity conditions required for simulating full-scale flight. This leads to a variety of approaches to scaling and to other alternative applications. Through a literature review and analysis of different scaling strategies, this study presents an overall picture of how subscale flight testing has been used in recent years and synthesises its main issues and practical limitations. Results show that, while the estimation of full-scale characteristics is still an interesting application within certain flight conditions, subscale models are progressively taking a broader role as low-cost technology-testing platforms with relaxed similarity constraints. Different approaches to tackle the identified practical challenges, implemented both by the authors and by other organisations, are discussed and evaluated through flight experiments. Full article
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Article
Task Planning for Multiple-Satellite Space-Situational-Awareness Systems
Aerospace 2021, 8(3), 73; https://doi.org/10.3390/aerospace8030073 - 12 Mar 2021
Cited by 3 | Viewed by 1034
Abstract
Space situational awareness (SSA) plays an important role in maintaining space advantages. Task planning is one of the key technologies in SSA to allocate multiple tasks to multiple satellites, so that a satellite may be allocated to supervise multiple space objects, and a [...] Read more.
Space situational awareness (SSA) plays an important role in maintaining space advantages. Task planning is one of the key technologies in SSA to allocate multiple tasks to multiple satellites, so that a satellite may be allocated to supervise multiple space objects, and a space object may be supervised by multiple satellites. This paper proposes a hierarchical and distributed task-planning framework for SSA systems with focus on fast and effective task planning customized for SSA. In the framework, a global task-planner layer performs satellite and object clustering, so that satellites are clustered into multiple unique clusters on the basis of their positions, while objects are clustered into multiple possibly intersecting clusters, hence allowing for a single object to be supervised by multiple satellites. In each satellite cluster, a local task planner performs distributed task planning using the contract-net protocol (CNP) on the basis of the position and velocity of satellites and objects. In addition, a customized discrete particle swarm optimization (DPSO) algorithm was developed to search for the optimal task-planning result in the CNP. Simulation results showed that the proposed framework can effectively achieve task planning among multiple satellites and space objects. The efficiency and scalability of the proposed framework are demonstrated through static and dynamic orbital simulations. Full article
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Editorial
Special Issue: Deicing and Anti-Icing of Aircrafts
Aerospace 2021, 8(3), 72; https://doi.org/10.3390/aerospace8030072 - 10 Mar 2021
Viewed by 1016
Abstract
In-flight icing for aircrafts is a large concern for all those involved in aircraft operations [...] Full article
(This article belongs to the Special Issue Deicing and Anti-Icing of Aircraft)
Article
Human–Machine Interface Design for Monitoring Safety Risks Associated with Operating Small Unmanned Aircraft Systems in Urban Areas
Aerospace 2021, 8(3), 71; https://doi.org/10.3390/aerospace8030071 - 10 Mar 2021
Cited by 6 | Viewed by 1326
Abstract
The envisioned introduction of autonomous Small Unmanned Aircraft Systems (sUAS) into low-altitude urban airspace necessitates high levels of system safety. Despite increased system autonomy, humans will most likely remain an essential component in assuring safety. This paper derives, applies, and evaluates a display [...] Read more.
The envisioned introduction of autonomous Small Unmanned Aircraft Systems (sUAS) into low-altitude urban airspace necessitates high levels of system safety. Despite increased system autonomy, humans will most likely remain an essential component in assuring safety. This paper derives, applies, and evaluates a display design concept that aims to support safety risk monitoring of multiple sUAS by a human operator. The concept comprises of five design principles. The core idea of the concept is to limit display complexity despite increasing the number of sUAS monitored by primarily visualizing highly abstracted information while hiding detailed information of lower abstraction, unless specifically requested by the human operator. States of highly abstracted functions are visualized by function-specific icons that change hue in accordance to specified system states. Simultaneously, the design concept aims to support the human operator in identifying off-nominal situations by implementing design properties that guide visual attention. The display was evaluated in a study with seven subject matter experts. Although preliminary, the results clearly favor the proposed display design concept. The advantages of the proposed design concept are demonstrated, and the next steps for further exploring the proposed display design concept are outlined. Full article
(This article belongs to the Collection Unmanned Aerial Systems)
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Article
Phasing Maneuver Analysis from a Low Lunar Orbit to a Near Rectilinear Halo Orbit
Aerospace 2021, 8(3), 70; https://doi.org/10.3390/aerospace8030070 - 09 Mar 2021
Cited by 1 | Viewed by 906
Abstract
The paper describes the preliminary design of a phasing trajectory in a cislunar environment, where the third body perturbation is considered non-negligible. The working framework is the one proposed by the ESA’s Heracles mission in which a passive target station is in a [...] Read more.
The paper describes the preliminary design of a phasing trajectory in a cislunar environment, where the third body perturbation is considered non-negligible. The working framework is the one proposed by the ESA’s Heracles mission in which a passive target station is in a Near Rectilinear Halo Orbit and an active vehicle must reach that orbit to start a rendezvous procedure. In this scenario the authors examine three different ways to design such phasing maneuver under the circular restricted three-body problem hypotheses: Lambert/differential correction, Hohmann/differential correction and optimization. The three approaches are compared in terms of ΔV consumption, accuracy and time of flight. The selected solution is also validated under the more accurate restricted elliptic three-body problem hypothesis. Full article
(This article belongs to the Special Issue Advances in Aerospace Sciences and Technology)
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Article
Background Pressure Effects on the Performance of a 20 kW Magnetically Shielded Hall Thruster Operating in Various Configurations
Aerospace 2021, 8(3), 69; https://doi.org/10.3390/aerospace8030069 - 09 Mar 2021
Cited by 2 | Viewed by 946
Abstract
The paper reports the characterization results of a 20 kW-class magnetically shielded Hall thruster in three different configurations and operating with a centrally mounted cathode. The characterization was carried out at two different pumping speeds in SITAEL’s IV10 vacuum chamber, resulting in two [...] Read more.
The paper reports the characterization results of a 20 kW-class magnetically shielded Hall thruster in three different configurations and operating with a centrally mounted cathode. The characterization was carried out at two different pumping speeds in SITAEL’s IV10 vacuum chamber, resulting in two different background pressure levels for each tested operating point. A linear behavior of discharge current and thrust values versus the anode mass flow rate was noticed for both pumping speeds levels and for all the three configurations. In addition, the thrust and discharge current values were always found to be lower at lower background pressure levels. From the performance levels, a preliminary estimate of the ingested mass flow rates was performed, and the values were then compared to a recently developed background flow model. The results suggested that, for this thruster and in the tested operating regimes, the change in performance due to background pressure could be ascribed not only to the ingestion of external mass flow coming from the chamber but also to other physical processes caused by the flux of residual background neutrals. Full article
(This article belongs to the Special Issue Electric Propulsion)
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Article
Rendezvous in Cis-Lunar Space near Rectilinear Halo Orbit: Dynamics and Control Issues
Aerospace 2021, 8(3), 68; https://doi.org/10.3390/aerospace8030068 - 08 Mar 2021
Cited by 2 | Viewed by 836
Abstract
The paper presents the development of a fully-safe, automatic rendezvous strategy between a passive vehicle and an active one orbiting around the Earth–Moon L2 Lagrangian point. This is one of the critical phases of future missions to permanently return to the Moon, which [...] Read more.
The paper presents the development of a fully-safe, automatic rendezvous strategy between a passive vehicle and an active one orbiting around the Earth–Moon L2 Lagrangian point. This is one of the critical phases of future missions to permanently return to the Moon, which are of interest to the majority of space organizations. The first step in the study is the derivation of a suitable full 6-DOF relative motion model in the Local Vertical Local Horizontal reference frame, most suitable for the design of the guidance. The main dynamic model is approximated using both the elliptic and circular three-body motion, due to the contribution of Earth and Moon gravity. A rather detailed set of sensors and actuator dynamics was also implemented in order to ensure the reliability of the guidance algorithms. The selection of guidance and control is presented, and evaluated using a sample scenario as described by ESA’s HERACLES program. The safety, in particular the passive safety, concept is introduced and different techniques to guarantee it are discussed that exploit the ideas of stable and unstable manifolds to intrinsically guarantee some properties at each hold-point, in which the rendezvous trajectory is divided. Finally, the rendezvous dynamics are validated using available Ephemeris models in order to verify the validity of the results and their limitations for future more detailed design. Full article
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Article
Fuel Planning Strategies Considering Operational Uncertainties of Aerodynamic Formation Flight
Aerospace 2021, 8(3), 67; https://doi.org/10.3390/aerospace8030067 - 07 Mar 2021
Cited by 2 | Viewed by 1112
Abstract
The operational concept of aerodynamic formation flight, also referred to as aircraft wake-surfing for efficiency (AWSE), has high potential in terms of fuel savings and climate impact mitigation. In order to implement this concept, many technological and operational challenges have to be coped [...] Read more.
The operational concept of aerodynamic formation flight, also referred to as aircraft wake-surfing for efficiency (AWSE), has high potential in terms of fuel savings and climate impact mitigation. In order to implement this concept, many technological and operational challenges have to be coped with. As the fuel consumption during a mission strongly depends on a successful execution of AWSE, the existing uncertainties regarding flight planning increase. While a conservative fuel planning ensures a follower to complete the mission even in the case of a formation failure, it might result in high amounts of excess fuel and, therefore, additional fuel consumption. In this study, this issue is addressed by the adaptation of flight planning procedures to the requirements of AWSE focusing on fuel planning in particular, considered from the perspective of a designated follower aircraft of a two-aircraft formation. This trade-off is modeled as an n-action two-event decision-making problem. Each of the possible actions is represented by a combination of mission routing and a corresponding diversion airport, taking atmospheric effects (e.g., wind) into account in order to determine the resulting amount of trip fuel. The two events under consideration are a total formation failure in contrast to a complete success. Based on a scenario with a set of double origin destination pairs characterizing the formations and representative weather patterns for the North Atlantic region, each action is analyzed with regard to the expected fuel consumption and expense. Based on a set of assumed formation success probabilities, we find that the proposed method holds a savings potential to reduce the follower’s fuel consumption by 4.8% and its monetary expenses by 1.2% compared with a conventional flight planning. In order to gain a monetary profit margin applying this method, the required formation success probability is shown to vary between 92% and 96%, depending on the assumed fuel price. Full article
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Article
Numerical Investigation of an Optimized Rotor Head Fairing for the RACER Compound Helicopter in Cruise Flight
Aerospace 2021, 8(3), 66; https://doi.org/10.3390/aerospace8030066 - 05 Mar 2021
Cited by 3 | Viewed by 1202
Abstract
The present work is part of the Clean Sky 2 project Full-Fairing Rotor Head Aerodynamic Design Optimization (FURADO), which deals with the aerodynamic design optimization of a full-fairing rotor head for the Rapid And Cost-Effective Rotorcraft (RACER) compound helicopter. The rotor head is [...] Read more.
The present work is part of the Clean Sky 2 project Full-Fairing Rotor Head Aerodynamic Design Optimization (FURADO), which deals with the aerodynamic design optimization of a full-fairing rotor head for the Rapid And Cost-Effective Rotorcraft (RACER) compound helicopter. The rotor head is a major drag source and previous investigations have revealed that the application of rotor head fairings can be an effective drag reduction measure. As part of the full-fairing concept, a new blade-sleeve fairing was aerodynamically optimized for cruise flight. Within this publication, the newly developed blade-sleeve fairing is put to test on an isolated, five-bladed rotor head and compared to an already existing reference blade-sleeve fairing, which was developed at Airbus Helicopters. Numerical flow simulations are performed with ANSYS Fluent 2019 R2 considering a rotating rotor head with cyclic pitch movement. The aerodynamic forces of the isolated rotor head are analyzed to determine the performance benefit of the newly developed blade-sleeve fairing. A drag reduction of 4.7% and a lift increase of 20% are obtained in comparison to the Airbus Helicopters reference configuration. Furthermore, selected surface and flow field quantities are presented to give an overview on the occurring flow phenomena. Full article
(This article belongs to the Special Issue Helicopter Aerodynamics)
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Review
Spoken Instruction Understanding in Air Traffic Control: Challenge, Technique, and Application
by
Aerospace 2021, 8(3), 65; https://doi.org/10.3390/aerospace8030065 - 05 Mar 2021
Cited by 12 | Viewed by 1487
Abstract
In air traffic control (ATC), speech communication with radio transmission is the primary way to exchange information between the controller and aircrew. A wealth of contextual situational dynamics is embedded implicitly; thus, understanding the spoken instruction is particularly significant to the ATC research. [...] Read more.
In air traffic control (ATC), speech communication with radio transmission is the primary way to exchange information between the controller and aircrew. A wealth of contextual situational dynamics is embedded implicitly; thus, understanding the spoken instruction is particularly significant to the ATC research. In this paper, a comprehensive review related to spoken instruction understanding (SIU) in the ATC domain is provided from the perspective of the challenges, techniques, and applications. Firstly, a full pipeline is represented to achieve the SIU task, including automatic speech recognition, language understanding, and voiceprint recognition. A total of 10 technique challenges are analyzed based on the ATC task specificities. In succession, the common techniques for SIU tasks are categorized from common applications, and extensive works in the ATC domain are also reviewed. Finally, a series of future research topics are also prospected based on the corresponding challenges. The author sincerely hopes that this work is able to provide a clear technical roadmap for the SIU tasks in the ATC domain and further make contributions to the research community. Full article
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Article
Development of a Novel Deployable Solar Panel and Mechanism for 6U CubeSat of STEP Cube Lab-II
Aerospace 2021, 8(3), 64; https://doi.org/10.3390/aerospace8030064 - 05 Mar 2021
Cited by 3 | Viewed by 1586
Abstract
The structural safety of solar cells mounted on deployable solar panels in the launch vibration environment is a significant aspect of a successful CubeSat mission. This paper presents a novel highly damped deployable solar panel module that is effective in ensuring structural protection [...] Read more.
The structural safety of solar cells mounted on deployable solar panels in the launch vibration environment is a significant aspect of a successful CubeSat mission. This paper presents a novel highly damped deployable solar panel module that is effective in ensuring structural protection of solar cells under the launch environment by rapidly suppressing the vibrations transmitting through the solar panel by constrained layer damping achieved using printed circuit board (PCB)-based multilayered thin stiffeners with double-sided viscoelastic tapes. A high-damping solar panel demonstration model with a three-pogo pin-based burn wire release mechanism was fabricated and tested for application in the 6U CubeSat “STEP Cube Lab-II” developed by Chosun University, South Korea. The reliable release function and radiation hardness assurance of the mechanism in an in-orbit environment were confirmed by performing solar panel deployment tests and radiation tests, respectively. The design effectiveness and structural safety of the proposed solar panel module were validated by launch vibration and in-orbit environment tests at the qualification level. Full article
(This article belongs to the Special Issue Vibration Control for Space Application)
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Article
A Numerical Investigation on Stress Modal Analysis of Composite Laminated Thin Plates
Aerospace 2021, 8(3), 63; https://doi.org/10.3390/aerospace8030063 - 04 Mar 2021
Cited by 1 | Viewed by 922
Abstract
Because of the light weight and high strength, composite laminates have many advantages in aircraft structures; however, they are frequently subjected to severe dynamic loadings during flight. To understand the dynamic properties of composite laminated thin plates at the stress scale, this paper [...] Read more.
Because of the light weight and high strength, composite laminates have many advantages in aircraft structures; however, they are frequently subjected to severe dynamic loadings during flight. To understand the dynamic properties of composite laminated thin plates at the stress scale, this paper studies the stress modal analysis (SMA) of composite laminated thin plates by finite element method (FEM). Firstly, the basic theory on SMA of composite laminates was given from the classical displacement modal analysis. Secondly, a square laminated thin plate was numerically studied to obtain some distribution laws of the stress mode shapes (SMSs) from the layup and stress component perspectives. Then, based on the characteristics of SMSs in different plies, a modified layup configuration was conducted for possible lower magnitude and more uniform distributions of SMSs. Results indicate that ±45° layups can improve the performance of SMSs of the square plate, without excessively decreasing the modal frequencies. Such fact manifests that ±45° layups are critically vital for the dynamic stress reduction of the square composite laminated plates. Modal participation factor and strain energy were evaluated to assist the determination of critical modes. Lastly, the aspect ratio of the composite plate on layup design was considered. Numerical investigation in this study can serve as a preliminary step of SMSs perspective for the analysis and optimization of dynamic composite laminates. Full article
(This article belongs to the Special Issue Crashworthiness Design for Aviation Safety)
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Article
Analysis of the Cosmic Ray Effects on Sentinel-1 SAR Satellite Data
Aerospace 2021, 8(3), 62; https://doi.org/10.3390/aerospace8030062 - 03 Mar 2021
Cited by 1 | Viewed by 1105
Abstract
Ionizing radiation sources such as Solar Energetic Particles and Galactic Cosmic Radiation may cause unexpected errors in imaging and communication systems of satellites in the Space environment, as reported in the previous literature. In this study, the temporal variation of the speckle values [...] Read more.
Ionizing radiation sources such as Solar Energetic Particles and Galactic Cosmic Radiation may cause unexpected errors in imaging and communication systems of satellites in the Space environment, as reported in the previous literature. In this study, the temporal variation of the speckle values on Sentinel 1 satellite images were compared with the cosmic ray intensity/count data, to analyze the effects which may occur in the electromagnetic wave signals or electronic system. Sentinel 1 Synthetic Aperture Radar (SAR) images nearby to the cosmic ray stations and acquired between January 2015 and December 2019 were processed. The median values of the differences between speckle filtered and original image were calculated on Google Earth Engine Platform per month. The monthly median “noise” values were compared with the cosmic ray intensity/count data acquired from the stations. Eight selected stations’ data show that there are significant correlations between cosmic ray intensities and the speckle amounts. The Pearson correlation values vary between 0.62 and 0.78 for the relevant stations. Full article
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Article
Aircraft Requirements for Sustainable Regional Aviation
Aerospace 2021, 8(3), 61; https://doi.org/10.3390/aerospace8030061 - 26 Feb 2021
Cited by 6 | Viewed by 2342
Abstract
Recently, the new Green Deal policy initiative was presented by the European Union. The EU aims to achieve a sustainable future and be the first climate-neutral continent by 2050. It targets all of the continent’s industries, meaning aviation must contribute to these changes [...] Read more.
Recently, the new Green Deal policy initiative was presented by the European Union. The EU aims to achieve a sustainable future and be the first climate-neutral continent by 2050. It targets all of the continent’s industries, meaning aviation must contribute to these changes as well. By employing a systems engineering approach, this high-level task can be split into different levels to get from the vision to the relevant system or product itself. Part of this iterative process involves the aircraft requirements, which make the goals more achievable on the system level and allow validation of whether the designed systems fulfill these requirements. Within this work, the top-level aircraft requirements (TLARs) for a hybrid-electric regional aircraft for up to 50 passengers are presented. Apart from performance requirements, other requirements, like environmental ones, are also included. To check whether these requirements are fulfilled, different reference missions were defined which challenge various extremes within the requirements. Furthermore, figures of merit are established, providing a way of validating and comparing different aircraft designs. The modular structure of these aircraft designs ensures the possibility of evaluating different architectures and adapting these figures if necessary. Moreover, different criteria can be accounted for, or their calculation methods or weighting can be changed. Full article
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Article
Generation and Mitigation Mechanism Studies of Nonlinear Thermoacoustic Instability in a Modelled Swirling Combustor with a Heat Exchanger
Aerospace 2021, 8(3), 60; https://doi.org/10.3390/aerospace8030060 - 26 Feb 2021
Cited by 4 | Viewed by 1012
Abstract
In the present work, 3D Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations are performed to investigate the generation and mitigation mechanism of combustion-sustained thermoacoustic instabilities in a modelled swirl combustor. The effects of (1) swirling number SN, (2) inlet air flow rate V [...] Read more.
In the present work, 3D Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations are performed to investigate the generation and mitigation mechanism of combustion-sustained thermoacoustic instabilities in a modelled swirl combustor. The effects of (1) swirling number SN, (2) inlet air flow rate Va and (3) inlet temperature Ti on the amplitudes and frequencies of swirling combustion-excited limit cycle oscillations are examined. It is found that the amplitude of acoustic fluctuations is increased with increasing SN and Va and decreased with the increase of Ti. The dominant frequency of oscillations is also found to increases with the increase of SN and Va. However, increasing Ti leads to the dominant frequency being decreased first and then increased. An alternative passive control method of installing an adjustable temperature heat exchanger on the combustion chamber wall is then proposed. Numerical results show that thermoacoustic oscillations could be excited and mitigated by setting the heat exchanger temperature to TH. Global and local Rayleigh indexes are applied to further reveal the excitation and attenuation effects on mechanisms. The present study is conducive to developing a simulation platform for thermoacoustic instabilities in swirling combustors. It also provides an alternative method to amplify or mitigate thermoacoustic oscillations. Full article
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