Unmanned Aerial Vehicle Structure Research Articles

State of the art in mechanical vibration analysis for UAV´s

This article focuses on the study of the state of the art of theoretical and experimental investigations that carried out vibration analysis in an unmanned aerial vehicle (UAV). According to the review of literature, modal analysis is fundamental to determine the dynamic behaviour of the structure since it studies the nature of the vibrations and the behaviour of the UAV structure under different excitations. One unwanted vibration is flutter, which is a phenomenon that causes structural failure, performance degradation and in the worst case, partial or complete loss of the aircraft structure. The fundamental tool for the analysis of vibrations including flutter is the Fast Fourier Transform. As a result of the analysis of the studies reported in this work, some important points to be considered when implementing modal analysis in UAVs are listed.

Development of Bamboo Composite: Application in UAV Structure

Due to the constant rise in the usage of non-renewable resources, composite materials made of natural fibres and plastic are now receiving a lot of attention. To maximise their performance and viability, bamboo composites for use in UAV (Unmanned Aerial Vehicle) plate constructions confront several obstacles that must be overcome. This study was conducted to fabricate the UAV plate structure by using bamboo composite. Bamboo fibre, epoxy resin, epoxy hardener, and fibre mesh are the materials requiredto produce the sample. The tensile strength of the bamboo fibre evaluated by using Universal Testing Machine followed by ASTM standard D638. The density test of the bamboo fibre measured by using Mettler Toledo mode XS64. As the result of the study, the bamboo composite that weighted gram per area 15 g/150cm2has the highest tensile strength, which is 20.075 MPa and for the density test, bamboo composite that weighted per area 5 g/150cm2is denser than other conditions which is 1.115 g/cm3.

A low-profile ultra-wideband hexagonal-patch antenna for UAV-based applications

ABSTRACT A compact, low-profile, and ultra-wideband antenna is always desirable for airborne Unmanned Aerial Vehicles (UAV) applications. In this paper, an ultra-wideband monopole antenna is presented with the benefit of a compact and low-profile design structure. The antenna design enables efficient communication without interfering with the UAV’s other components. The antenna is designed using a slotted-hexagonal-patch fed from a co-planar microstrip feed. The bow-tie-shaped slot along with a few other rectangular-shaped parasitic slots are loaded in the hexagonal patch for bandwidth enhancement. The coplanar feed with top ground offers minimum interference with the UAV structure. The non-conducting bottom plane allows seamless integration without causing any interference to the antenna. An equivalent circuit model (ECM) of the proposed antenna is also developed. The antenna structure is fabricated and the measured results are validated. A good agreement between the measured and simulated results is observed. The antenna covers an ultra-wideband ranging from 3 GHz to 13.5 GHz with a maximum gain of 4.0dBi. The size of the antenna is 0.24λ0 ×0.29λ0 ×0.01λ0. The developed antenna, therefore, demonstrates that it is a strong contender for ultra-wideband UAV-based applications.

Quantifying the Reliability of Volumetric and Areal Calculation with UAV-Generated DEMs: A Comparative Study with Ground Truth Data

For performing an assessment of the volume estimation accuracy using Digital Elevation Models (DEMs) generated by Unmanned Aerial Vehicles (UAVs), an evaluation of suitability has been made. The study was operated at Tikrit University, on a man-made topographic depression in the form of fishponds. The generated DEM by using the images of the UAV followed by accuracy assessment using Ground Control Points (GCPs), the points distributed evenly throughout the pond. The results showed that the Root Mean Square Error (RMSE) calculated for the DEM at the optimum flight plane ranged between 0.14 to 0.45. Comparing the pond's predicted volume utilizing UAV DEMs to the ground truth volume obtained using GNSS RTK surveying, it was discovered that the UAV DEM calculation was 97% accurate. The study came to the conclusion that the UAV Structure from Motion (SFM) method and the generated DEMs are appropriate for precisely surveying the volumes utilizing the appropriate range of flying parameters based on prior knowledge.

Integrated Design and Flight Validation of Solar-Powered Unmanned Aerial Vehicle (UAV) Structure and Propulsion System

The development of solar-powered unmanned aerial vehicles (UAVs) primarily focuses on enhancing the efficiency of the propulsion system to minimize energy consumption during the conversion of valuable solar energy. However, due to the unique nature of UAV propulsion systems, there is limited cross-reference ability among existing solar-powered UAV systems. This paper proposes an integrated design approach for the propulsion system and UAV structure. Based on this approach, an overall design is conducted for the solar-powered UAV, including initial design goals and performance parameters. Aerodynamic layout design and performance estimation are carried out, and a prototype is fabricated and assembled for flight testing validation. The results demonstrate the significant importance of this approach in improving the efficiency of the solar-powered UAV propulsion system.

Individualized Indicators and Estimation Methods for Tiger Nut (Cyperus esculentus L.) Tubers Yield Using Light Multispectral UAV and Lightweight CNN Structure

Tiger nuts are a non-genetically modified organism crop with high adaptability and economic value, and they are being widely promoted for cultivation in China. This study proposed a new yield-estimation method based on a lightweight convolutional neural network (CNN) named Squeeze Net to provide accurate production forecasts for tiger nut tubers. The multispectral unmanned aerial vehicle (UAV) images were used to establish phenotypic datasets of tiger nuts, comprising vegetation indices (VIs) and plant phenotypic indices. The Squeeze Net model with a lightweight CNN structure was constructed to fully explore the explanatory power of the spectral UAV-derived information and compare the differences between the parametric and nonparametric models applied in tiger nut yield predictions. Compared with stepwise multiple linear regression (SMLR), both algorithms achieved good yield prediction performances. The highest obtained accuracies reflected an R2 value of 0.775 and a root-mean-square error (RMSE) value of 688.356 kg/ha with SMLR, and R2 = 0.780 and RMSE = 716.625 kg/ha with Squeeze Net. This study demonstrated that Squeeze Net can efficiently process UAV multispectral images and improve the resolution and accuracy of the yield prediction results. Our study demonstrated the enormous potential of artificial intelligence (AI) algorithms in the precise crop management of tiger nuts in the arid sandy lands of northwest China by exploring the interactions between various intensive phenotypic traits and productivity.

3D geometric survey of cultural heritage by UAV in inaccessible coastal or shallow aquatic environments

AbstractCultural heritage in coastal or shallow aquatic environments is often located in areas where access is difficult or where accurate survey and documentation may not always be possible with terrestrial or aquatic equipment. The combination of photogrammetry and unmanned aerial vehicles (UAVs) generates a range of possibilities across multiple sectors, including history, ethnography and cultural heritage studies. Additionally, these methods can be used to prospect new archaeological sites. This article presents three case studies that use UAV techniques and Structure from Motion and Multiview Stereo (SfM‐MVS) photogrammetry to conduct topographic and geometric registrations of archaeological, historical and ethnographic sites (some of which are classified as cultural heritage sites). These examples are located in coastal or shallow aquatic environments that are difficult to survey with traditional methods. The results show that it is possible to carry out detailed geometric registration and heritage prospection over large coastal or shallow aquatic environments using a low‐cost UAV. Furthermore, the results of this work show great advantages in terms of cost and quality, even in cases where the seabed is below a shallow water column. Other particularities of SfM‐MVS application in aquatic environments are discussed. From an interdisciplinary perspective, this methodology will offer new possibilities for the study, restoration and conservation of archaeological, historical and ethnographic monuments.

Topology optimization of UAV structure based on homogenization of honeycomb core

In this paper, a new optimization design strategy is explored to achieve a rapid design of lightweight structures for unmanned aerial vehicles (UAVs) using honeycomb sandwich panels. Based on the existing performance equivalent method of honeycomb core material, an effective mechanical property homogenization model of honeycomb material is proposed and applied to finite element analysis. A finite element model is established to analyze the mechanical response of honeycomb sandwich panels under different bending loads, and the rationality of the model is verified by mechanical tests. The error between the simulation results and the experimental results is about 10%. Based on the variable density method and rational approximation of material properties interpolation method, a topology optimization model with the minimum structural flexibility as the objective is constructed. By comparing with the experiment-based design scheme, it is found that the optimized design scheme has a more reasonable structure and better performance. The optimized design can effectively reduce the structural weight and improve the load transfer path while ensuring structural stiffness. In this way, the influence of stress concentration and large deformation on the structure is avoided, which is of great significance for the design of UAV structures.

Solar UAVs—More Aerodynamic Efficiency or More Electrical Power?

Solar UAVs (unmanned aerial vehicles) have experienced important development in recent years. The use of solar free energy is not neglected in the present energy crisis, with the intention to move toward green energies. However, an important problem arises concerning the limited amount of solar energy available on board UAVs. Until now, high-aerodynamic-efficiency configurations have been used. These configurations use high-aspect-ratio wings. However, high-aspect-ratio wings have some disadvantages regarding their excessive elasticity and weak bending resistance in the housing section. Additionally, the aircraft maneuverability is reduced. In this work, a study is proposed on a solar UAV configuration that sacrifices high aerodynamic efficiency for a higher surface area available for solar cells. In this manner, the amount of energy available on board the UAV is increased, and the UAV structure becomes more rigid and robust. The presented UAV fits better with more complex evolutions, is more maneuverable and the wingspan is much reduced. This UAV is more compact, can maneuver better in the take-off and landing phases, and the necessary storage space is considerably reduced. This paper highlights the performances that can be achieved using this kind of UAV and explores whether these performances are enough for some applications. Using an on-board energy balance, the possible performances of this new configuration is studied. As this is a preliminary study, the precision level is not very high, but it offers an image concerning the possibilities of this new configuration.

Impact Analysis of Solar Cells on Vertical Take-Off and Landing (VTOL) Fixed-Wing UAV

A vertical take-off and landing (VTOL) is a type of unmanned aerial vehicle (UAV) that allows for flight in harsh weather for surveillance and access to remote areas. VTOL can be performed without a runway. As such, VOTL UAVs are used in areas where there is limited space and in urban locations. The structural endurance of VTOL UAVs is limited and is further reduced in the case of fixed-wing UAVs. Long-endurance aerial vehicles allow for continuous flight, but their power supply systems must be able to harvest energy from external sources in order to meet the guidelines. The wings of these UAVs are often covered with solar cells. This article presents the extended range and flight time of a tail-sitter VTOL that incorporates solar cells on the UAV structure. A VTOL powered by solar cells can perform aviation missions with fewer landings, allowing for the performance of such UAVs to be increased and for their flight time to be extended several times over those without solar cells. Simulations accounting for the use of PV panels on the UAV structure show that depending on the scenario and flight date, VTOLs can double the flight time on the spring equinox and increase the flight time by more than six times on the summer solstice.

CFD-based pesticide selection for a nozzle used in a six-rotor UAV in hover mode for tea spraying.

A key challenge for unmanned aerial vehicle (UAV) spraying sometimes used in tea plantations is the downwash flow structure there stronger than in crops. In addition, the UAV spray is affected by the relationship between the nozzle design and the pesticide. However, there is little current research on this aspect. As a preliminary step this study focuses on the most appropriate pesticide for a designated nozzle in a six-rotor UAV according to the nozzle-pesticide relationship using a three-dimensional computational fluid dynamics model. This model considers the downwash flow structure effect and nozzle spray performance in hover. Nozzle FVP110-02, widely used in six-rotor UAVs, is used as a representative nozzle and bifenthrin and tea saponin water, commonly used in tea plantations, are used as the pesticides. The downwash flow structure of the six-rotor UAV in hover was conveniently controlled by the flight height and rotational speed, thereby causing the turbulence to be more stable. For nozzle FVP110-02, bifenthrin was more appropriate than tea saponin water at the same concentration, whilst bifenthrin and tea saponin water at a concentration of 1:1000 showed the best performance under identical working conditions. The numerical model developed here was shown to be effective for investigating the relationship between nozzle and pesticide. Our findings will help to not only improve UAV spraying for tea cultivation but also provide guidelines for pesticide selection in crops. Further work will address the comparison of the rigorous qualification of the numerical simulations with the measurements by the field test. © 2023 Society of Chemical Industry.

A Misalignment Tolerance and Lightweight Wireless Charging System via Reconfigurable Capacitive Coupling for Unmanned Aerial Vehicle Applications

This letter proposes a novel capacitive power transfer system for the unmanned aerial vehicles (UAVs) with lightweight in receivers and robust misalignment tolerance characteristics. The design and analysis of a reconfigurable capacitive coupler are presented at first. The transmitter adopts four rectangular copper plates, which can be reconfigured according to the landing positions and directions. The receiver includes two hollow cylindrical copper-foil sleeves wrapped at the bottom of UAV landing gears, thereby leading to the adaption of the UAV's structure and the tolerance to lateral-, longitudinal-, and angular misalignment. Then, simulation and prototype experiments are carried out to verify the proposal. The results show that the prototype has low leakage electric flux at the UAV fuselage and can deliver 212.1 W with a dc-to-dc efficiency of 82.5%. Especially the proposal allows UAV to be powered within ±0.76 A of output current variation for different landing positions and directions, and the system dc-to-dc efficiency variation is within ±1%.

Computer simulation of the deformed state and strength of the frame structure of a heavy uav

COMPUTER SIMULATION OF THE DEFORMED STATE AND STRENGTH OF THE FRAME STRUCTURE OF A HEAVY UAV
 The paper is devoted to the presentation of the computer simulation results of the stress-strain state and evaluation of the low-cycle strength of the frame structure of a heavy unmanned aerial vehicle (UAV) of the quadcopter type, the prototype of which was developed at NTU "KhPI". The quadcopter is built on the basis of the use of four internal combustion engines (ICE), which provide lifting power. The cycle of calculations of the stress-strain state of the frame structure, carried out to select the option required from the point of view of ensuring the required stiffness and strength, was performed using the Finite Element Method (FEM). A macro has been developed for use in the engineering software, which allows you to select the best parameters from the point of view of the required stiffness and strength of the frame. The results of calculations of the variant implemented in the design of the created UAV prototype are presented. According to the results of the analysis of the stress-strain state, it is shown that the structural parameters chosen for the steel frame of the prototype fully satisfy the conditions of minimal deformation and short-term strength. The constants included in the evolution equation for the damage parameter describing low-cycle loading, which occurs during UAV abnormal landings, have been determined. An elastic-plastic analysis of the frame structure deformation for the conditions of dangerous landing of the quadcopter was performed. Calculations of hidden damage accumulation processes in the most heavily loaded frame beams have been carried out. For the steel frame implemented in the created prototype, the possibility of satisfactory resistance to low-cycle deformation under abnormal landing conditions is demonstrated. It is shown that due to the insufficient low-cycle strength of the aluminum alloy structure, it is necessary to make structural changes in the new version of the UAV frame that is being designed.
 Key words: computer simulation, quadcopter, macro, beam structure, stress-strain state, FEM, low-cycle strength, damage.

Analysis of Crash Characteristics of Hydrogen Storage Structure of Hydrogen Powered UAV

In the context of green aviation, as an internationally recognized solution, hydrogen energy is lauded as the “ultimate energy source of the 21st century”, with zero emissions at the source. Developed economies with aviation industries, such as Europe and the United States, have announced hydrogen energy aviation development plans successively. The study and development of high-energy hydrogen fuel cells and hydrogen energy power systems have become some of the future aviation research focal points. As a crucial component of hydrogen energy storage and delivery, the design and development of a safe, lightweight, and efficient hydrogen storage structure have drawn increasing consideration. Using a hydrogen-powered Unmanned Aerial Vehicle (UAV) as the subject of this article, the crash characteristics of the UAV’s hydrogen storage structure are investigated in detail. The main research findings are summarized as follows: (1) A series of crash characteristics analyses of the hydrogen storage structure of a hydrogen-powered UAV were conducted, and the Finite Element Analysis (FEA) response of the structure under different impact angles, internal pressures, and impact speeds was obtained and analyzed. (2) When the deformation of the hydrogen storage structure exceeds 50 mm, and the strain exceeds 0.8, an initial crack will appear at this part of the hydrogen storage structure. The emergency release valve should respond immediately to release the gas inside the tank to avoid further damage. (3) Impact angle and initial internal pressure are the main factors affecting the formation of initial cracks.

A New Magnetic Coupler With High Rotational Misalignment Tolerance for Unmanned Aerial Vehicles Wireless Charging

In this letter, a novel magnetic coupler with high rotational misalignment tolerance is proposed for unmanned aerial vehicles (UAVs) wireless charging. The transmitting coil consists of an inner coil and an outer circular coil connected in series, which generates a horizontal magnetic field from the center to all periphery directions within the charging range. Then, to receive the flux effectively and to adapt to UAV's structure, a vertical square air-core coil is designed and attached to the landing gear. The proposed magnetic coupler can reduce significantly the magnetic fluxes penetrating through the body of the UAV, therefore mitigating electromagnetic interference with onboard devices. Simulation based on ANSYS Maxwell and experiments based on a laboratory prototype are carried out to validate the proposal. The results show that, the proposed magnetic coupler can transmit power in all 360° of rotational misalignments, and the output voltage of the wireless charging system fluctuates only 2% around the 48 V reference.

Experimental and numerical study of novel Coanda-based unmanned aerial vehicle

Conventional Coanda-based unmanned aerial vehicles (UAV) experience thrust losses in the radial direction. To address these losses, a rectangular, linear arrangement of the Coanda surface was adopted in the proposed novel design. This arrangement minimizes the area change in the radial direction to recover such thrust losses. A prototype of the proposed UAV structure was 3D printed and assembled with a single 9-inch propeller. Performance characteristics of the UAV were evaluated through static testing on a dynamometer under different loading conditions. Experimental results were validated through computational fluid dynamics (CFD) simulations, using the k-ε realizable turbulence model, while the multiple reference frame (MRF) approach was applied in steady state. CFD simulations provided good overall agreement with experimental results having errors less than 8%. Numerical comparison between the novel Coanda design and a conventional Coanda design, having similar radial dimensions, showed that the novel design offered an overall 17% improvement in thrust per the side surface area, thus demonstrating an effective reduction of thrust losses in the radial direction.

Stability Parameter Range of a Tethered Unmanned Aerial Vehicle

In this study, the stability parameter range of a tethered quadrotor unmanned aerial vehicle (UAV) under the action of the transient wind field is numerically analyzed, which can provide a theoretical basis for the design and application of such systems. Three factors affecting the stability of tethered UAV system are determined, namely, cable tension, cable elongation, and UAV vibration velocity, and the corresponding judgment criteria are obtained. Specifically, the priority of the three criteria sequentially decreases. According to these criteria, the stability parameter range of the tethered UAV is examined under the cable parameters such as length, diameter, and elastic modulus and the environmental parameters such as the amplitude and period of the wind field. The results show that for designing the tethered UAV structure, by reducing the length of the tethered cable and increasing its diameter and elastic modulus, the working stability of tethered UAV system can be improved.

Important Parameters and Settings in Unmanned Aerial Vehicles

Aim: The article presents a set of parameters and settings for unmanned aerial vehicles (UAV), which is crucial in the operational work of the fire brigade and its importance for the quality of the final material obtained from an RGB camera or a thermal imaging camera. Introduction: Unmanned aerial vehicles (UAVs) are more often and more boldly used by various uniformed formations, including pilots of the State Fire Service and Volunteer Fire Brigades. Currently, they are used to perform recognition of situations and coordination of activities with the use of RGB and thermal imaging cameras. There are also other applications of UAV, including firefighting, but at the moment they are only conceptual solutions, as they have not been tested during an actual firefighting operation. According to the authors, a drone is currently only a carrier of additional devices and its functionality during the operation depends largely on certainty and reliability of a given UAV structure, as well as on the type and quality of the elements and sensors mounted on it. Methodology: A review of literature and press reports, as well as the authors’ experience in working with UAVs and the results of their research were used to analyse the topic. Conclusions: Indicating a set of key parameters for the UAVs used by fire brigade users is only possible to define its application. Therefore, in this study, the authors presented the most common use of unmanned aerial vehicles, for which key parameters were indicated and the impact of these factors on the obtained results of drones was described. Due to the frequent neglect of camera operation and the importance of their parameters, the authors described the most frequently set parameters of photographs and their impact on the final result, which is of key importance for the usefulness of the collected material.

The Feasibility of Using UAV Structure from Motion Photogrammetry to Extract HBIM of the Great Ziggurat of UR

Culture heritage reflects nation’s legacy and therefore should be protected from damage in order to pass it to future generations. Recently, such protection can be applied by 3D digitization techniques such as conservation, restoration, documentation, etc. The 3D digitalization of heritage assets has encountered numerous focus in the last two decades due to the development in data capturing techniques and technological advancement in 3D remote sensing (RS) approaches such as photogrammetry and laser scanning. However, the abundance of 3D information resources and spatial data modelling and analysis methods have urged stakeholders to adopt intelligent 3D data management system so called Building Information Modelling (BIM) to facilitate data approaching and management. Historic Building Information Model (HBIM) is a special case of the BIM system, however it reflects the possibility to apply the BIM technology to the historical and heritage buildings. In this research, Structure from motion (SfM) photogrammetric routine based on Unmanned Aerial Vehicles (UAVs) images was applied to build HBIM of the Great Ziggurat of Ur in the south of Iraq. Based on the 3D geometric and texturing information extracted through photogrammetry and the historical information provided, virtual reconstruction has been carried out using (HBIM) technology. This was achieved by applying realistic materials and texturing information in order to document the building, which is directed to investigate the feasibility of implementing image-based 3D modelling within HBIM environments. Restoring the missing parts of the Ziggurat temple was also a focus of this research by implementing reverse engineering methodologies based on available information considered within the extracted HBIM. This can successfully represent a complete virtual reality model and a management information system of the Ziggurat building to be passed to future generations. The work also includes data assessment and validation with the as-built model generated from reference measurements within Computer Aided Design (CAD) environment.

Vineyard Pruning Weight Prediction Using 3D Point Clouds Generated from UAV Imagery and Structure from Motion Photogrammetry

In viticulture, information about vine vigour is a key input for decision-making in connection with production targets. Pruning weight (PW), a quantitative variable used as indicator of vegetative vigour, is associated with the quantity and quality of the grapes. Interest has been growing in recent years around the use of unmanned aerial vehicles (UAVs) or drones fitted with remote sensing facilities for more efficient crop management and the production of higher quality wine. Current research has shown that grape production, leaf area index, biomass, and other viticulture variables can be estimated by UAV imagery analysis. Although SfM lowers costs, saves time, and reduces the amount and type of resources needed, a review of the literature revealed no studies on its use to determine vineyard pruning weight. The main objective of this study was to predict PW in vineyards from a 3D point cloud generated with RGB images captured by a standard drone and processed by SfM. In this work, vertical and oblique aerial images were taken in two vineyards of Godello and Mencía varieties during the 2019 and 2020 seasons using a conventional Phantom 4 Pro drone. Pruning weight was measured on sampling grids comprising 28 calibration cells for Godello and 59 total cells for Mencía (39 calibration cells and 20 independent validation). The volume of vegetation (V) was estimated from the generated 3D point cloud and PW was estimated by linear regression analysis taking V as predictor variable. When the results were leave-one-out cross-validated (LOOCV), the R2 was found to be 0.71 and the RMSE 224.5 (g) for the PW estimate in Mencía 2020, calculated for the 39 calibration cells on the grounds of oblique images. The regression analysis results for the 20 validation samples taken independently of the rest (R2 = 0.62; RMSE = 249.3 g) confirmed the viability of using the SfM as a fast, non-destructive, low-cost procedure for estimating pruning weight.