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Aerospace, Volume 8, Issue 4 (April 2021) – 30 articles

Cover Story (view full-size image): Aircraft maintenance is a relevant factor for the direct operating costs of an airline. Therefore, there is an increasing interest in planning maintenance more efficiently and increasing aircraft availability. This study presents a Reinforcement Learning approach to optimize the scheduling of long-term maintenance checks. This type of algorithms' learning potential and adaptability can be valuable for producing fast and efficient check schedules while dealing with operation disruptions or other unexpected events. Results show a reduction in the checks scheduled in the long term and a consequent increase in aircraft availability, which can have a significant financial impact on the airline. View this paper.
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Article
Exemplification of Detecting Gas Turbine Blade Structure Defects Using the X-ray Computed Tomography Method
Aerospace 2021, 8(4), 119; https://doi.org/10.3390/aerospace8040119 - 20 Apr 2021
Cited by 3 | Viewed by 890
Abstract
X-ray computed tomography is more often applied in non-destructive testing the quality of construction elements significantly crucial for reliability and safety of device elements, machines and complex industrial systems. This article describes the computed tomography (CT) system used to inspect the technical condition [...] Read more.
X-ray computed tomography is more often applied in non-destructive testing the quality of construction elements significantly crucial for reliability and safety of device elements, machines and complex industrial systems. This article describes the computed tomography (CT) system used to inspect the technical condition of turbine blades of the aircraft engine. The impact of the experimental conditions on the correctness of the obtained results was determined. The appropriate selection of parameters for the experiment was given, and the correct test results of gas turbine blades were presented. Failures, manufacturing defects, material deviations of nickel-cobalt alloyed blades were identified. The thickness of walls was measured in the selected cross-sections with the accuracy of 0.01 mm, and selected manufacturing defects of cooling passages were diagnosed. It was demonstrated that the application of the CT system allows for detailed non-destructive inspection of the technical condition of machine parts. The test results proved that the X-ray computed tomography could be applied in the production and repairs of machines. Full article
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Article
A Breakdown of System of Systems Needs Using Architecture Frameworks, Ontologies and Description Logic Reasoning
Aerospace 2021, 8(4), 118; https://doi.org/10.3390/aerospace8040118 - 20 Apr 2021
Cited by 2 | Viewed by 1147
Abstract
Aerospace systems are connected with the operational environment and other systems in general. The focus in aerospace product development is consequently shifting from a singular system perspective to a System-of-Systems (SoS) perspective. This increasing complexity gives rise to new levels of uncertainty that [...] Read more.
Aerospace systems are connected with the operational environment and other systems in general. The focus in aerospace product development is consequently shifting from a singular system perspective to a System-of-Systems (SoS) perspective. This increasing complexity gives rise to new levels of uncertainty that must be understood and managed to produce aerospace solutions for an ever-changing future. This paper presents an approach to using architecture frameworks, and ontologies with description logic reasoning capabilities, to break down SoS needs into required capabilities and functions. The intention of this approach is to provide a consistent way of obtaining the functions to be realized in order to meet the overarching capabilities and needs of an SoS. The breakdown with an architecture framework results in an initial design space representation of functions to be performed. The captured knowledge is then represented in an ontology with description logic reasoning capabilities, which provides a more flexible way to expand and process the initial design space representation obtained from the architecture framework. The proposed approach is ultimately tested in a search and rescue case study, partly based on the operations of the Swedish Maritime Administration. The results show that it is possible to break down SoS needs in a consistent way and that ontology with description logic reasoning can be used to process the captured knowledge to both expand and reduce an available design space representation. Full article
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Article
Methodology for Evaluating Risk of Visual Inspection Tasks of Aircraft Engine Blades
Aerospace 2021, 8(4), 117; https://doi.org/10.3390/aerospace8040117 - 19 Apr 2021
Cited by 5 | Viewed by 1349
Abstract
Risk assessment methods are widely used in aviation, but have not been demonstrated for visual inspection of aircraft engine components. The complexity in this field arises from the variety of defect types and the different manifestation thereof with each level of disassembly. A [...] Read more.
Risk assessment methods are widely used in aviation, but have not been demonstrated for visual inspection of aircraft engine components. The complexity in this field arises from the variety of defect types and the different manifestation thereof with each level of disassembly. A new risk framework was designed to include contextual factors. Those factors were identified using Bowtie analysis to be criticality, severity, and detectability. This framework yields a risk metric that describes the extent to which a defect might stay undetected during the inspection task, and result in adverse safety outcomes. A simplification of the framework provides a method for go/no-go decision-making. The results of the study reveal that the defect detectability is highly dependent on specific views of the blade, and the risk can be quantified. Defects that involve material separation or removal such as scratches, tip rub, nicks, tears, cracks, and breaking, are best shown in airfoil views. Defects that involve material deformation and change of shape, such as tip curl, dents on the leading edges, bents, and battered blades, have lower risk if edge views can be provided. This research proposes that many risk assessments may be reduced to three factors: consequence, likelihood, and a cofactor. The latter represents the industrial context, and can comprise multiple sub-factors that are application-specific. A method has been devised, including appropriate scales, for the inclusion of these into the risk assessment. Full article
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Article
Investigation of the Film-Cooling Performance of 2.5D Braided Ceramic Matrix Composite Plates with Preformed Hole
Aerospace 2021, 8(4), 116; https://doi.org/10.3390/aerospace8040116 - 19 Apr 2021
Cited by 1 | Viewed by 854
Abstract
The film-cooling performance of a 2.5D braided ceramic matrix composite (CMC) plate with preformed holes was numerically studied. Four numerical models containing braided structures were established: one model with film-cooling holes preformed through fiber extrusion deformation (EP-Hole), one model with film-cooling holes directly [...] Read more.
The film-cooling performance of a 2.5D braided ceramic matrix composite (CMC) plate with preformed holes was numerically studied. Four numerical models containing braided structures were established: one model with film-cooling holes preformed through fiber extrusion deformation (EP-Hole), one model with film-cooling holes directly woven through fibers (WP-Hole), and two models with directly drilled holes (DP-Hole1,2). Besides, the influence of the ratio between the equivalent thermal conductivities on the axial and radial directions of fiber Kr was investigated. The results show that the preformed holes have better performance in controlling the thermal gradient with the increase of Kr. The maximum thermal gradient around the DP-Hole is significantly higher than that of the WP-Hole and EP-Hole, and the maximum relative variation reaches 123.3%. With Kr increasing from 3.32 to 13.05, the overall cooling effectiveness on the hot-side wall decreases for all models, by about 10%. Compared with the traditional drill method, the new preformed film-cooling hole studied in this paper can reduce the temperature and the thermal gradient in the region around the holes. Full article
(This article belongs to the Special Issue Advances in Aerothermal Engineering)
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Article
Multi-Aircraft Trajectory Collaborative Prediction Based on Social Long Short-Term Memory Network
Aerospace 2021, 8(4), 115; https://doi.org/10.3390/aerospace8040115 - 19 Apr 2021
Cited by 11 | Viewed by 1239
Abstract
Aircraft trajectory prediction is the basis of approach and departure sequencing, conflict detection and resolution and other air traffic management technologies. Accurate trajectory prediction can help increase the airspace capacity and ensure the safe and orderly operation of aircraft. Current research focuses on [...] Read more.
Aircraft trajectory prediction is the basis of approach and departure sequencing, conflict detection and resolution and other air traffic management technologies. Accurate trajectory prediction can help increase the airspace capacity and ensure the safe and orderly operation of aircraft. Current research focuses on single aircraft trajectory prediction without considering the interaction between aircraft. Therefore, this paper proposes a model based on the Social Long Short-Term Memory (S-LSTM) network to realize the multi-aircraft trajectory collaborative prediction. This model establishes an LSTM network for each aircraft and a pooling layer to integrate the hidden states of the associated aircraft, which can effectively capture the interaction between them. This paper takes the aircraft trajectories in the Northern California terminal area as the experimental data. The results show that, compared with the mainstream trajectory prediction models, the S-LSTM model in this paper has smaller prediction errors, which proves the superiority of the model’s performance. Additionally, another comparative experiment is conducted on airspace scenes with aircraft interactions, and it is found that S-LSTM has a better prediction effect than LSTM, which proves the effectiveness of the former considering aircraft interaction. Full article
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Article
Self-Oscillations of The Free Turbine Speed in Testing Turboshaft Engine with Hydraulic Dynamometer
Aerospace 2021, 8(4), 114; https://doi.org/10.3390/aerospace8040114 - 17 Apr 2021
Cited by 1 | Viewed by 948
Abstract
Self-oscillations are one of the common problems in the complex automatic system, that can occur due to the features of the workflow and the design of the governor. The development of digital control systems has made it possible to damp self-oscillations by applying [...] Read more.
Self-oscillations are one of the common problems in the complex automatic system, that can occur due to the features of the workflow and the design of the governor. The development of digital control systems has made it possible to damp self-oscillations by applying complex control laws. However, for hydromechanical systems, such way is unacceptable due to the design complexity and the governor cost. The objective of this work is to determine the parameters of the hydromechanical free turbine speed controller, ensuring the absence of self-oscillations during ground tests of the turboshaft engine with a hydraulic dynamometer. The TV3-117VM engine (Ukraine) with the NR-3VM regulator pump (Ukraine) was selected as the object of the study. However, self-oscillations can also occur in any modifications of the TV3-117 engine with any NR-3 regulator pump. The results of the research may be of interest to engineers and scientists who investigate the dynamics of automatic control systems for similar engines. The paper analyses the nonlinear features of the empirical characteristics of the FTSC leading to self-oscillations of the engine speed. The authors propose the mathematical model of the automatic control system dynamics, which takes into account all the features of the engine and regulator pump. It is shown that the load characteristics of the water brake and the helicopter main rotor can differ significantly. Research of the dynamic characteristics of the TV3-117VM engine was carried out. The analysis showed a good agreement between the calculation results and the field test results, and made it possible to determine the parameters of the controller, which lead to self-oscillations during test. Two cases are considered. The first case includes ground tests of the engine with a water brake; the second case—flight tests of the engine as part of the helicopter’s power plant. The data obtained make it possible to develop recommendations for adjusting the hydromechanical governor without testing it on the engine. Full article
(This article belongs to the Special Issue Technologies for Future Distributed Engine Control Systems)
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Article
Aircraft Maintenance Check Scheduling Using Reinforcement Learning
Aerospace 2021, 8(4), 113; https://doi.org/10.3390/aerospace8040113 - 17 Apr 2021
Cited by 4 | Viewed by 1530
Abstract
This paper presents a Reinforcement Learning (RL) approach to optimize the long-term scheduling of maintenance for an aircraft fleet. The problem considers fleet status, maintenance capacity, and other maintenance constraints to schedule hangar checks for a specified time horizon. The checks are scheduled [...] Read more.
This paper presents a Reinforcement Learning (RL) approach to optimize the long-term scheduling of maintenance for an aircraft fleet. The problem considers fleet status, maintenance capacity, and other maintenance constraints to schedule hangar checks for a specified time horizon. The checks are scheduled within an interval, and the goal is to, schedule them as close as possible to their due date. In doing so, the number of checks is reduced, and the fleet availability increases. A Deep Q-learning algorithm is used to optimize the scheduling policy. The model is validated in a real scenario using maintenance data from 45 aircraft. The maintenance plan that is generated with our approach is compared with a previous study, which presented a Dynamic Programming (DP) based approach and airline estimations for the same period. The results show a reduction in the number of checks scheduled, which indicates the potential of RL in solving this problem. The adaptability of RL is also tested by introducing small disturbances in the initial conditions. After training the model with these simulated scenarios, the results show the robustness of the RL approach and its ability to generate efficient maintenance plans in only a few seconds. Full article
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Article
A Text-Driven Aircraft Fault Diagnosis Model Based on a Word2vec and Priori-Knowledge Convolutional Neural Network
Aerospace 2021, 8(4), 112; https://doi.org/10.3390/aerospace8040112 - 14 Apr 2021
Cited by 4 | Viewed by 1185
Abstract
In the process of aircraft maintenance and support, a large amount of fault description text data is recorded. However, most of the existing fault diagnosis models are based on structured data, which means they are not suitable for unstructured data such as text. [...] Read more.
In the process of aircraft maintenance and support, a large amount of fault description text data is recorded. However, most of the existing fault diagnosis models are based on structured data, which means they are not suitable for unstructured data such as text. Therefore, a text-driven aircraft fault diagnosis model is proposed in this paper based on Word to Vector (Word2vec) and prior-knowledge Convolutional Neural Network (CNN). The fault text first enters Word2vec to perform text feature extraction, and the extracted text feature vectors are then input into the proposed prior-knowledge CNN to train the fault classifier. The prior-knowledge CNN introduces expert fault knowledge through Cloud Similarity Measurement (CSM) to improve the performance of the fault classifier. Validation experiments on five-year maintenance log data of a civil aircraft were carried out to successfully verify the effectiveness of the proposed model. Full article
(This article belongs to the Special Issue Fault Detection and Prognostics in Aerospace Engineering)
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Editorial
Special Issue “10th EASN International Conference on Innovation in Aviation & Space to the Satisfaction of the European Citizens”
Aerospace 2021, 8(4), 111; https://doi.org/10.3390/aerospace8040111 - 14 Apr 2021
Viewed by 776
Abstract
This Special Issue contains selected papers from works presented at the 10th EASN International Conference on Innovation in Aviation & Space to the Satisfaction of the European Citizens, which was held successfully from the 2nd until the 4th of September, 2020 [...] Full article
Editorial
Special Issue “9th EASN International Conference on Innovation in Aviation & Space”
Aerospace 2021, 8(4), 110; https://doi.org/10.3390/aerospace8040110 - 14 Apr 2021
Viewed by 666
Abstract
This Special Issue contains selected papers from works presented at the 9th EASN International Conference on Innovation in Aviation & Space, which was successfully held in Athens, Greece, between the 3rd and 6th of September 2019 [...] Full article
(This article belongs to the Special Issue 9th EASN International Conference on Innovation in Aviation & Space)
Article
Sol–Gel Waveguide-Based Sensor for Structural Health Monitoring on Large Surfaces in Aerospace Domain
Aerospace 2021, 8(4), 109; https://doi.org/10.3390/aerospace8040109 - 14 Apr 2021
Cited by 5 | Viewed by 1567
Abstract
The potential of sol–gel-based optical sensors is investigated for applications in the aerospace domain. To this aim, a low-cost and non-intrusive sol–gel sensor based on waveguides, arranged as a 2D matrix structure, is fabricated by UV photolithography for delamination and damage detection. Two [...] Read more.
The potential of sol–gel-based optical sensors is investigated for applications in the aerospace domain. To this aim, a low-cost and non-intrusive sol–gel sensor based on waveguides, arranged as a 2D matrix structure, is fabricated by UV photolithography for delamination and damage detection. Two different organic–inorganic sol–gels were selected to fabricate the photonic device: TiO2–SiO2 and ZrO2–SiO2, acting as the waveguide core and the cladding, respectively. A systematic study was performed to determine the manufacturing parameters controlling their properties. The results show that large surfaces can be functionalized via sol–gel methods using the direct laser-writing approach. The structures are characterized in terms of refractive index, and the guiding properties were investigated through simulations and experiments, indicating an excellent behavior regarding the light guidance in a straight waveguide or in the 2D matrix structure grid. Additionally, preliminary tests show that the presence of impact can be easily detected after damage through the induced optical losses on large surfaces. This proof of concept sensor is a promising tool for structural health monitoring. To achieve the ultimate goal, the integration of this photonic sensor will be later performed on aircraft wings. Full article
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Article
Fault Diagnosis and Reconfigurable Control for Commercial Aircraft with Multiple Faults and Actuator Saturation
Aerospace 2021, 8(4), 108; https://doi.org/10.3390/aerospace8040108 - 14 Apr 2021
Cited by 2 | Viewed by 945
Abstract
Active fault-tolerant control systems perform fault diagnosis and reconfigurable control. There is a bidirectional uncertainty between them, and an integrated scheme is proposed here to account for that. The system considers both actuator and sensor faults, as well as the external disturbance. The [...] Read more.
Active fault-tolerant control systems perform fault diagnosis and reconfigurable control. There is a bidirectional uncertainty between them, and an integrated scheme is proposed here to account for that. The system considers both actuator and sensor faults, as well as the external disturbance. The diagnostic module is designed using an unknown input observer, and the controller is constructed on the basis of an adaptive method. The integrated strategy is presented, and the stability of the overall system is analyzed. Moreover, different kinds of anti-windup techniques are utilized to modify the original controllers, because of the different controller structures. A simulation of the integrated anti-windup fault-tolerant control method is demonstrated using a numerical model of Boeing 747. The results show that it can guarantee the stability of the post-fault aircraft and increase the control performance for the overall faulty system. Full article
(This article belongs to the Special Issue Fault Detection and Prognostics in Aerospace Engineering)
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Article
Radiation Risks in Cis-Lunar Space for a Solar Particle Event Similar to the February 1956 Event
Aerospace 2021, 8(4), 107; https://doi.org/10.3390/aerospace8040107 - 14 Apr 2021
Cited by 1 | Viewed by 919
Abstract
Solar particle events (SPEs) can pose serious threats for future crewed missions to the Moon. Historically, there have been several extreme SPEs that could have been dangerous for astronauts, and thus analyzing their potential risk on humans is an important step towards space [...] Read more.
Solar particle events (SPEs) can pose serious threats for future crewed missions to the Moon. Historically, there have been several extreme SPEs that could have been dangerous for astronauts, and thus analyzing their potential risk on humans is an important step towards space exploration. In this work, we study the effects of a well-known SPE that occurred on 23 February 1956 on a mission in cis-Lunar space. Estimates of the proton fluence spectra of the February 1956 event were obtained from three different parameterized models published within the past 12 years. The studied geometry consists of a female phantom in the center of spherical spacecraft shielded by aluminum area densities ranging from 0.4 to 40 g cm2. The effective dose, along with lens, skin, blood forming organs, heart, and central nervous system doses, were tallied using the On Line Tool for the Assessment of Radiation In Space (OLTARIS), which utilizes the High Z and Energy TRansport code (HZETRN), a deterministic radiation transport code. Based on the parameterized models, the results herein show that thicknesses comparable to a spacesuit might not protect against severe health consequences from a February 1956 category event. They also show that a minimum aluminum shielding of around 20 g cm2 is sufficient to keep the effective dose and critical organ doses below NASA’s permissible limits for such event. In addition, except for very thin shielding, the input models produced results that were within good agreement, where the doses obtained from the three proton fluence spectra tended to converge with slight differences as the shielding thickness increases. Full article
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Article
Global Visualization of Compressible Swept Convex-Corner Flow Using Pressure-Sensitive Paint
Aerospace 2021, 8(4), 106; https://doi.org/10.3390/aerospace8040106 - 11 Apr 2021
Cited by 2 | Viewed by 966
Abstract
This study used pressure-sensitive paint (PSP) and determined the surface pressure distributions for a compressible swept convex-corner flow. The freestream Mach numbers were 0.64 and 0.83. The convex-corner angle and swept angle were, respectively, 10–17° and 5–15°. Expansion and compression near the corner [...] Read more.
This study used pressure-sensitive paint (PSP) and determined the surface pressure distributions for a compressible swept convex-corner flow. The freestream Mach numbers were 0.64 and 0.83. The convex-corner angle and swept angle were, respectively, 10–17° and 5–15°. Expansion and compression near the corner apex were clearly visualized. For the test case of shock-induced boundary layer separation, there were greater spanwise pressure gradient and curved shocks. The acquired PSP data agree with the experimental data measured using the Kulite pressure transducers for a subsonic expansion flow. For a transonic expansion flow, the discrepancy was significant. The assumption of a constant recovery factor is not valid in the separation region, and temperature correction for PSP measurements is required. Full article
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Article
Bearing Strength and Failure Mechanisms of Riveted Woven Carbon Composite Joints
Aerospace 2021, 8(4), 105; https://doi.org/10.3390/aerospace8040105 - 09 Apr 2021
Cited by 3 | Viewed by 1038
Abstract
This research aimed to determine riveted carbon/epoxy composites’ mechanical performance when fabricated by resin transfer molding (RTM). As this manufacturing process is gaining importance in the aeronautics and automotive industries, assembly methods and their reliability must be studied in terms of their airworthiness [...] Read more.
This research aimed to determine riveted carbon/epoxy composites’ mechanical performance when fabricated by resin transfer molding (RTM). As this manufacturing process is gaining importance in the aeronautics and automotive industries, assembly methods and their reliability must be studied in terms of their airworthiness and transportation implementation. The study case resumes the determination of the bearing strength of RTM-woven carbon composites for different rivet joint diameters (1/8, 5/32 and 3/16 in). The joint shear strength was obtained following the ASTM D5961 instructions, and post-failure analysis was carried out by a computerized tomography scan. A residual strength curve is provided with the results to infer the bearing strength for the riveted composites as a function of the rivet width-to-diameter ratio. A discussion of the fracture mechanism and tensile strength is carried out to assess the understanding of the riveted woven composites. Full article
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Article
Numerical Evaluation of Structural Safety of Linear Actuator for Flap Control of Aircraft Based on Airworthiness Standard
Aerospace 2021, 8(4), 104; https://doi.org/10.3390/aerospace8040104 - 07 Apr 2021
Viewed by 1084
Abstract
Airworthiness standards of Korea recommend verifying structural safety by experimental tests and analytical methods, owing to the development of analysis technology. In this study, we propose a methodology to verify the structural safety of aircraft components based on airworthiness requirements using an analytical [...] Read more.
Airworthiness standards of Korea recommend verifying structural safety by experimental tests and analytical methods, owing to the development of analysis technology. In this study, we propose a methodology to verify the structural safety of aircraft components based on airworthiness requirements using an analytical method. The structural safety and fatigue integrity of a linear actuator for flap control of aircraft was evaluated through numerical analysis. The static and fatigue analyses for the given loads obtained from the multibody dynamics analysis were performed using the finite element method. Subsequently, the margin of safety and vulnerable area were acquired and the feasibility of the structural safety evaluation using the analytical method was confirmed. The proposed numerical analysis method in this study can be adopted as an analytical verification methodology for the airworthiness standards of civilian aircraft in Korea. Full article
(This article belongs to the Special Issue Vibration Control for Space Application)
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Article
Aircraft Fleet Health Monitoring with Anomaly Detection Techniques
Aerospace 2021, 8(4), 103; https://doi.org/10.3390/aerospace8040103 - 07 Apr 2021
Cited by 7 | Viewed by 2203
Abstract
Predictive maintenance has received considerable attention in the aviation industry where costs, system availability and reliability are major concerns. In spite of recent advances, effective health monitoring and prognostics for the scheduling of condition-based maintenance operations is still very challenging. The increasing availability [...] Read more.
Predictive maintenance has received considerable attention in the aviation industry where costs, system availability and reliability are major concerns. In spite of recent advances, effective health monitoring and prognostics for the scheduling of condition-based maintenance operations is still very challenging. The increasing availability of maintenance and operational data along with recent progress made in machine learning has boosted the development of data-driven prognostics and health management (PHM) models. In this paper, we describe the data workflow in place at an airline for the maintenance of an aircraft system and highlight the difficulties related to a proper labelling of the health status of such systems, resulting in a poor suitability of supervised learning techniques. We focus on investigating the feasibility and the potential of semi-supervised anomaly detection methods for the health monitoring of a real aircraft system. Proposed methods are evaluated on large volumes of real sensor data from a cooling unit system on a modern wide body aircraft from a major European airline. For the sake of confidentiality, data has been anonymized and only few technical and operational details about the system had been made available. We trained several deep neural network autoencoder architectures on nominal data and used the anomaly scores to calculate a health indicator. Results suggest that high anomaly scores are correlated with identified failures in the maintenance logs. Also, some situations see an increase in the anomaly score for several flights prior to the system’s failure, which paves a natural way for early fault identification. Full article
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Article
Online Identification-Verification-Prediction Method for Parallel System Control of UAVs
Aerospace 2021, 8(4), 99; https://doi.org/10.3390/aerospace8040099 - 02 Apr 2021
Cited by 1 | Viewed by 1013
Abstract
In order to solve the problem of how to efficiently control a large-scale swarm Unmanned Aerial Vehicle (UAV) system, which performs complex tasks with limited manpower in a non-ideal environment, this paper proposes a parallel UAV swarm control method. The key technology of [...] Read more.
In order to solve the problem of how to efficiently control a large-scale swarm Unmanned Aerial Vehicle (UAV) system, which performs complex tasks with limited manpower in a non-ideal environment, this paper proposes a parallel UAV swarm control method. The key technology of parallel control is to establish a one-to-one artificial UAV system corresponding to the aerial swarm UAV on the ground. This paper focuses on the computational experiments algorithm for artificial UAV system establishment, including data processing, model identification, model verification and state prediction. Furthermore, this paper performs a comprehensive flight mission with four common modes (climbing, level flighting, turning and descending) for verification. The results of the identification experiment present a good consistency between the outputs of the refined dynamics model and the real flight data. The prediction experiment results show that the prediction method in this paper can basically guarantee that the prediction states error is kept within 10% about 16 s. Full article
(This article belongs to the Collection Unmanned Aerial Systems)
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Article
Design and Development of an Experimental Setup of Electrically Powered Spinning Rotor Blades in Icing Wind Tunnel and Preliminary Testing with Surface Coatings as Hybrid Protection Solution
Aerospace 2021, 8(4), 98; https://doi.org/10.3390/aerospace8040098 - 02 Apr 2021
Cited by 4 | Viewed by 1227
Abstract
In order to study ice protection systems for rotating blades, a new experimental setup has been developed at the Anti-Icing Materials International Laboratory (AMIL). This system consists of two small-scale rotating blades in a refrigerated icing wind tunnel where atmospheric icing can be [...] Read more.
In order to study ice protection systems for rotating blades, a new experimental setup has been developed at the Anti-Icing Materials International Laboratory (AMIL). This system consists of two small-scale rotating blades in a refrigerated icing wind tunnel where atmospheric icing can be simulated. Power is brought to the blades through a slip ring, through which the signals of the different sensors that are installed on the blades also pass. As demonstrated by the literature review, this new setup will address the need of small-scale wind tunnel testing on electrically powered rotating blades. To test the newly designed apparatus, preliminary experimentation is done on a hybrid ice protection system. Electrothermal protection is combined with different surface coatings to measure the impact of those coatings on the power consumption of the system. In anti-icing mode, the coatings tested did not reduce the power consumption on the system required to prevent ice from accumulating on the leading edge. The coatings however, due to their hydrophobic/superhydrophobic nature, reduced the power required to prevent runback ice accumulation when the leading edge was protected. One of the coatings did not allow any runback accumulation, limiting the power to protect the whole blades to the power required to protect solely the leading edge, resulting in a potential 40% power reduction for the power consumption of the system. In de-icing mode, the results with all the substrates tested showed similar power to achieve ice shedding from the blade. Since the coatings tested have a low icephobicity, it would be interesting to perform additional testing with icephobic coatings. Also, a small unheated zone at the root of the blade prevented complete ice shedding from the blade. A small part of the ice layer was left on the blade after testing, meaning that a cohesive break had to occur within the ice layer, and therefore impacting the results. Improvements to the setup will be done to remedy the situation. Those preliminary testing performed with the newly developed test setup have demonstrated the potential of this new device which will now allow, among other things, to measure heat transfer, force magnitudes, ice nucleation, and thermal equilibrium during ice accretion, with different innovative thermal protection systems (conductive coating, carbon nanotubes, impulse, etc.) as well as mechanical systems. The next step, following the improvements, is to measure forced convection on a thermal ice protection system with and without precipitation and to test mechanical ice protection systems. Full article
(This article belongs to the Special Issue Deicing and Anti-Icing of Aircraft (Volume II))
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Article
Coupled Fluid–Solid Numerical Simulation for Flow Field Characteristics and Supporting Performance of Flexible Support Cylindrical Gas Film Seal
Aerospace 2021, 8(4), 97; https://doi.org/10.3390/aerospace8040097 - 02 Apr 2021
Cited by 2 | Viewed by 867
Abstract
A new type of cylindrical gas film seal (CGFS) with a flexible support is proposed according to the working characteristics of the fluid dynamic seal in high-rotational-speed fluid machinery, such as aero-engines and centrifuges. Compared with the CGFS without a flexible support, the [...] Read more.
A new type of cylindrical gas film seal (CGFS) with a flexible support is proposed according to the working characteristics of the fluid dynamic seal in high-rotational-speed fluid machinery, such as aero-engines and centrifuges. Compared with the CGFS without a flexible support, the CGFS with flexible support presents stronger radial floating characteristics since it absorbs vibration and reduces thermal deformation of the rotor system. Combined with the structural characteristics of a film seal, an analytical model of CGFS with a flexible wave foil is established. Based on the fluid-structure coupling analysis method, the three-dimensional flow field of a straight-groove CGFS model is simulated to study the effects of operating and structural parameters on the steady-state characteristics and the effects of gas film thickness, eccentricity, and the number of wave foils on the equivalent stress of the flexible support. Simulation results show that the film stiffness increases significantly when the depth of groove increases. When the gas film thickness increases, the average equivalent stress of the flexible support first decreases and then stabilizes. Furthermore, the number of wave foils affects the average foils thickness. Therefore, when selecting the number of wave foils, the support stiffness and buffer capacity should be considered simultaneously. Full article
(This article belongs to the Special Issue Secondary Air Systems in Gas Turbine Engines II)
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Article
Wing Structure of the Next-Generation Civil Tiltrotor: From Concept to Preliminary Design
Aerospace 2021, 8(4), 102; https://doi.org/10.3390/aerospace8040102 - 02 Apr 2021
Cited by 6 | Viewed by 1427
Abstract
The main objective of this paper is to describe a methodology to be applied in the preliminary design of a tiltrotor wing based on previously developed conceptual design methods. The reference vehicle is the Next-Generation Civil Tiltrotor Technology Demonstrator (NGCTR-TD) developed by Leonardo [...] Read more.
The main objective of this paper is to describe a methodology to be applied in the preliminary design of a tiltrotor wing based on previously developed conceptual design methods. The reference vehicle is the Next-Generation Civil Tiltrotor Technology Demonstrator (NGCTR-TD) developed by Leonardo Helicopters within the Clean Sky research program framework. In a previous work by the authors, based on the specific requirements (i.e., dynamics, strength, buckling, functional), the first iteration of design was aimed at finding a wing structure with a minimized structural weight but at the same time strong and stiff enough to comply with sizing loads and aeroelastic stability in the flight envelope. Now, the outcome from the first design loop is used to build a global Finite Element Model (FEM), to be used for a multi-objective optimization performed by using a commercial software environment. In other words, the design strategy, aimed at finding a first optimal solution in terms of the thickness of composite components, is based on a two-level optimization. The first-level optimization is performed with engineering models (non-FEA-based), and the second-level optimization, discussed in this paper, within an FEA environment. The latter is shown to provide satisfactory results in terms of overall wing weight, and a zonal optimization of the composite parts, which is the starting point of an engineered model and a detailed FEM (beyond the scope of the present work), which will also take into account manufacturing, assembly, installation, accessibility and maintenance constraints. Full article
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Editorial
Aerospace Best Paper Awards 2019
Aerospace 2021, 8(4), 101; https://doi.org/10.3390/aerospace8040101 - 02 Apr 2021
Viewed by 617
Abstract
Aerospace has launched annual awards to recognize outstanding papers published in the journal [...] Full article
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Article
Aeroelastic Stability Analysis of Electric Aircraft Wings with Distributed Electric Propulsors
Aerospace 2021, 8(4), 100; https://doi.org/10.3390/aerospace8040100 - 02 Apr 2021
Cited by 3 | Viewed by 1306
Abstract
In this paper, the effect of distributed electric propulsion on the aeroelastic stability of an electric aircraft wing was investigated. All the electric propulsors, which are of different properties, are attached to the wing of the aircraft in different positions. The wing structural [...] Read more.
In this paper, the effect of distributed electric propulsion on the aeroelastic stability of an electric aircraft wing was investigated. All the electric propulsors, which are of different properties, are attached to the wing of the aircraft in different positions. The wing structural dynamics was modelled by using geometrically exact beam equations, while the aerodynamic loads were simulated by using an unsteady aerodynamic theory. The electric propulsors were modelled by using a concentrated mass attached to the wing, and the motor’s thrust and angular momentum were taken into account. The thrust of each propulsor was modelled as a follower force acting exactly at the centre of gravity of the propulsor. The nonlinear aeroelastic governing equations were discretised using a time–space scheme, and the obtained results were verified against available results and very good agreement was observed. Two case studies were considered throughout the paper, resembling two flight conditions of the electric aircraft. The numerical results show that the tip propulsor thrust, mass, and angular momentum had the most impact on the aeroelastic stability of the wing. In addition, it was observed that the high-lift motors had a minimal effect on the aeroelastic stability of the wing. Full article
(This article belongs to the Special Issue Advances in Aerospace Sciences and Technology)
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Article
An Experimental Investigation of the Convective Heat Transfer on a Small Helicopter Rotor with Anti-Icing and De-Icing Test Setups
Aerospace 2021, 8(4), 96; https://doi.org/10.3390/aerospace8040096 - 01 Apr 2021
Cited by 3 | Viewed by 1218
Abstract
Successful icing/de-icing simulations for rotorcraft require a good prediction of the convective heat transfer on the blade’s surface. Rotorcraft icing is an unwanted phenomenon that is known to cause flight cancelations, loss of rotor performance and severe vibrations that may have disastrous and [...] Read more.
Successful icing/de-icing simulations for rotorcraft require a good prediction of the convective heat transfer on the blade’s surface. Rotorcraft icing is an unwanted phenomenon that is known to cause flight cancelations, loss of rotor performance and severe vibrations that may have disastrous and deadly consequences. Following a series of experiments carried out at the Anti-icing Materials International Laboratory (AMIL), this paper provides heat transfer measurements on heated rotor blades, under both the anti-icing and de-icing modes in terms of the Nusselt Number (Nu). The objective is to develop correlations for the Nu in the presence of (1) an ice layer on the blades (NuIce) and (2) liquid water content (LWC) in the freestream with no ice (NuWet). For the sake of comparison, the NuWet and the NuIce are compared to heat transfer values in dry runs (NuDry). Measurements are reported on the nose of the blade-leading edge, for three rotor speeds (Ω) = 500, 900 and 1000 RPM; a pitch angle (θ) = 6°; and three different radial positions (r/R), r/R = 0.6, 0.75 and 0.95. The de-icing tests are performed twice, once for a glaze ice accretion and another time for rime ice. Results indicate that the NuDry and the NuWet directly increased with V, r/R or Ω, mainly due to an increase in the Reynolds number (Re). Measurements indicate that the NuWet to NuDry ratio was always larger than 1 as a direct result of the water spray addition. NuIce behavior was different and was largely affected by the ice thickness (tice) on the blade. However, the ice acted as insulation on the blade surface and the NuIce to NuDry ratio was always less than 1, thus minimizing the effect of convection. Four correlations are then proposed for the NuDry, the NuWet and the NuIce, with an average error between 3.61% and 12.41%. The NuDry correlation satisfies what is expected from heat transfer near the leading edge of an airfoil, where the NuDry correlates well with Re0.52. Full article
(This article belongs to the Special Issue Deicing and Anti-Icing of Aircraft (Volume II))
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Article
On-Orbit Pulse Phase Estimation Based on CE-Adam Algorithm
Aerospace 2021, 8(4), 95; https://doi.org/10.3390/aerospace8040095 - 01 Apr 2021
Cited by 3 | Viewed by 703
Abstract
Pulse phase is the basic measurements of X-ray pulsar-based navigation, and thus how to estimate a pulse phase for an orbiting spacecraft is important. The current methods for on-orbit pulse phase estimation could provide an accurate estimation performance enhancing with the photon amount, [...] Read more.
Pulse phase is the basic measurements of X-ray pulsar-based navigation, and thus how to estimate a pulse phase for an orbiting spacecraft is important. The current methods for on-orbit pulse phase estimation could provide an accurate estimation performance enhancing with the photon amount, but its central processing unit (CPU) time cost also increases sharply with the increase of photon amount. In this paper, an on-orbit pulse phase estimation method based on the cross-entropy adaptive moment estimation (CE-Adam) algorithm is proposed to reduce the CPU time cost while retaining decent estimation accuracy. This method combines the CE and Adam algorithms, and is able to obtain a global optimum with low CPU time cost. The performance of the proposed algorithm is verified by simulation data and real data from the Neutron Star Internal Composition Detector (NICER). The results show that the proposed algorithm could greatly reduce the CPU time cost, which is about 1.5% of the CE algorithm, and retain similar estimation accuracy of pulse phase with CE algorithm. Full article
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Article
An Assembly Method for the Multistage Rotor of An Aero-Engine Based on the Dual Objective Synchronous Optimization for the Coaxality and Unbalance
Aerospace 2021, 8(4), 94; https://doi.org/10.3390/aerospace8040094 - 01 Apr 2021
Cited by 1 | Viewed by 860
Abstract
The assembly quality of an aero-engine directly determines its stability in high-speed operation. The coaxiality and unbalance out of tolerance caused by improper assembly may give rise to complicated vibration faults. To meet the requirements of the dual objective and reduce the test [...] Read more.
The assembly quality of an aero-engine directly determines its stability in high-speed operation. The coaxiality and unbalance out of tolerance caused by improper assembly may give rise to complicated vibration faults. To meet the requirements of the dual objective and reduce the test cost, it is necessary to predict the optimal assembly angles of the rotors at each stage during pre-assembly. In this study, we proposed an assembly optimization method for a multistage rotor of an aero-engine. Firstly, we developed a coordinate transmission model to calculate the coordinates of any point in the rotors at each stage during the assembly processes of a multistage rotor. Moreover, we proposed two different pieces of assembly optimization data for the coaxiality and unbalance, and established a dual objective evaluation function of that. Furthermore, we used the genetic algorithm to solve the optimal assembly angles of the rotors at each stage. Finally, the Monte Carlo simulation technique was used to investigate the effects of the geometric measured errors of each rotor on the proposed genetic algorithm. The simulation results show that the process of the dual objective optimization had good convergence, and the obtained optimal assembly angles of each rotor were not affected by the geometric measured errors. In addition, the dual objective optimization can ensure that both the coaxiality and unbalance can approach their respective optimal values to the most extent, and the experimental results also verified this conclusion. Therefore, the assembly optimization method proposed in this study can be used to guide the assembly processes of the multistage rotor of an aero-engine to achieve synchronous optimization for the coaxality and unbalance. Full article
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Article
Velocity Obstacle Based Conflict Avoidance in Urban Environment with Variable Speed Limit
Aerospace 2021, 8(4), 93; https://doi.org/10.3390/aerospace8040093 - 01 Apr 2021
Viewed by 1018
Abstract
Current investigations into urban aerial mobility, as well as the continuing growth of global air transportation, have renewed interest in conflict detection and resolution (CD&R) methods. The use of drones for applications such as package delivery, would result in traffic densities that are [...] Read more.
Current investigations into urban aerial mobility, as well as the continuing growth of global air transportation, have renewed interest in conflict detection and resolution (CD&R) methods. The use of drones for applications such as package delivery, would result in traffic densities that are orders of magnitude higher than those currently observed in manned aviation. Such densities do not only make automated conflict detection and resolution a necessity, but will also force a re-evaluation of aspects such as coordination vs. priority, or state vs. intent. This paper looks into enabling a safe introduction of drones into urban airspace by setting travelling rules in the operating airspace which benefit tactical conflict resolution. First, conflicts resulting from changes of direction are added to conflict resolution with intent trajectory propagation. Second, the likelihood of aircraft with opposing headings meeting in conflict is reduced by separating traffic into different layers per heading–altitude rules. Guidelines are set in place to make sure aircraft respect the heading ranges allowed at every crossed layer. Finally, we use a reinforcement learning agent to implement variable speed limits towards creating a more homogeneous traffic situation between cruising and climbing/descending aircraft. The effects of all of these variables were tested through fast-time simulations on an open source airspace simulation platform. Results showed that we were able to improve the operational safety of several scenarios. Full article
(This article belongs to the Special Issue Application of Data Science to Aviation)
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Article
Shape Optimization of Labyrinth Seals to Improve Sealing Performance
Aerospace 2021, 8(4), 92; https://doi.org/10.3390/aerospace8040092 - 01 Apr 2021
Cited by 3 | Viewed by 851
Abstract
To reduce gas leakage, shape optimization of a straight labyrinth seal was carried out. The six design parameters included seal clearance, fin width, fin height, fin pitch, fin backward, and forward expansion angle. The CFD (Computational Fluid Dynamics) model was solved to generate [...] Read more.
To reduce gas leakage, shape optimization of a straight labyrinth seal was carried out. The six design parameters included seal clearance, fin width, fin height, fin pitch, fin backward, and forward expansion angle. The CFD (Computational Fluid Dynamics) model was solved to generate the training and testing samples for the surrogate model, which was established by the least square support vector machine. A kind of chaotic optimization algorithm was used to determine the optimal design parameters of the labyrinth seal. As seal clearance, fin width, fin height, fin pitch, fin backward and forward expansion angles are 0.2 mm, 0.1 mm, 7 mm, 9 mm, 0°, and 15°, the discharge coefficient can reach its minimum value in the design space. The chaotic optimization algorithm coupled with least square support vector machine is a promising scheme for labyrinth seal optimization. Full article
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Article
Numerical Investigation of a Dynamic Stall on a Single Rotating Blade
Aerospace 2021, 8(4), 90; https://doi.org/10.3390/aerospace8040090 - 30 Mar 2021
Cited by 1 | Viewed by 747
Abstract
Dynamic stall is a phenomenon on the retreating blade of a helicopter which can lead to excessive control loads. In order to understand dynamic stall and fill the gap between the investigations on pitching wings and full helicopter rotor blades, a numerical investigation [...] Read more.
Dynamic stall is a phenomenon on the retreating blade of a helicopter which can lead to excessive control loads. In order to understand dynamic stall and fill the gap between the investigations on pitching wings and full helicopter rotor blades, a numerical investigation of a single rotating and pitching blade is carried out. The flow phenomena thereupon including the Ω-shaped dynamic stall vortex, the interaction of the leading edge vortex with the tip vortex, and a newly noticed vortex structure originating inboard are examined; they show similarities to pitching wings, while also possessing their unique features of a rotating system. The leading edge/tip vortex interaction dominates the post-stall stage. A newly noticed swell structure is observed to have a great impact on the load in the post-stall stage. With such a high Reynolds number, the Coriolis force exerted on the leading edge vortex is negligible compared to the pressure force. The force history/vortex structure of the slice r/R = 0.898 is compared with a 2D pitching airfoil with the same harmonic pitch motion, and the current simulation shows the important role played by the swell structure in the recovery stage. Full article
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Article
Two-Phase Flow Phenomena in Gas Turbine Compressors with a Focus on Experimental Investigation of Trailing Edge Disintegration
Aerospace 2021, 8(4), 91; https://doi.org/10.3390/aerospace8040091 - 26 Mar 2021
Viewed by 887
Abstract
Two-phase flow in gas turbine compressors occurs, for example, at heavy rain flight condition or at high-fogging in stationary gas turbines. The liquid dynamic processes are independent of the application. An overview on the processes and their approach in literature is given. The [...] Read more.
Two-phase flow in gas turbine compressors occurs, for example, at heavy rain flight condition or at high-fogging in stationary gas turbines. The liquid dynamic processes are independent of the application. An overview on the processes and their approach in literature is given. The focus of this study lies on the experimental investigation of the trailing edge disintegration. In the experiments, shadowgraphy is used to observe the disintegration of a single liquid rivulet with constant liquid mass flow rate at the edge of a thin plate at different air flow velocities. A two side view enables calculating droplet characteristics with high accuracy. The results show the asymptotic behavior of the ejected mean droplet diameters and the disintegration period. Furthermore, it gives a detailed insight into the droplet diameter distribution and the spreading of the droplets perpendicular to the air flow. Full article
(This article belongs to the Special Issue Life Cycle Modeling of Aircraft Propulsion Systems)
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