Vertical Take-off And Landing Unmanned Aerial Vehicles Research Articles

Low-cost development of a fully composite fixed-wing hybrid VTOL UAV

Fixed-wing hybrid vertical take-off and landing (VTOL) unmanned aerial vehicles (UAV) are popular due to their interoperability in the military and civilian domains, primarily where significant terrain difficulties exist for humans. Additionally, they can operate without requiring any runway infrastructure and have extended air endurance and efficiency. Since the hybrid UAV operates in distinct flight modes, viz., (a) VTOL and (b) fixed-wing cruise, carrying different payloads, the airframe structure requires careful design and manufacturing to realize sufficient strength. This experimental study aimed to identify the best combinations of various composite materials for manufacturing a lightweight, low-altitude long endurance (LALE) hybrid VTOL UAV. Primary materials include carbon fiber, Kevlar, fiber-reinforced plastic (FRP), resins, etc. Different rectangular test specimens of 120 × 5 mm size were made from ten different grades of carbon fiber, FRP, and resins using vacuum bagging. After properly curing these test specimens, we quantified their dynamic mechanical characteristics using various bending load experiments on a universal testing machine (UTM). An analysis of the experimental data facilitated the identification of the best composite combinations that provide maximum strength while reducing overall weight. Thus, we could understand the dynamic interplay between peak stress and test specimen weight. We also manufactured a UAV prototype using the identified combination and instrumented and flight-tested it to substantiate the experimental findings.

Enhanced robust adaptive flight control for a convertible VTOL UAV

In this paper, we propose less conservative formulations for candidate controllers of robust adaptive mixing control (RAMC) strategies. The RAMCs are employed to solve the trajectory tracking problem throughout the full flight envelope, with guaranteed stability, of a convertible plane (CP) vertical take-off and landing (VTOL) unmanned aerial vehicle (UAV). Initially, the multi-body nonlinear dynamic model of the CP VTOL UAV is obtained using the Euler–Lagrange formalism, from which a linear parameter-varying (LPV) model is derived to be used in the design of the RAMC candidate controllers. The new formulations of the candidate controllers are proposed considering two approaches: a (i) Parallel Distributed Compensation (PDC); and (ii) a parameter-dependent Lyapunov function. Hardware-In-the-Loop (HIL) experiments are performed in a high-fidelity flight simulator to verify the fulfillment of real-time constraints, ensuring a computationally lightweight control strategy ready for implementation in a low-cost embedded computational system.

Investigation of Empennage Location Effect to the Unmanned Aerial Vehicle (UAV) Structure Characteristic

Over the past few decades, unmanned aerial vehicles, commonly known as UAVs, have been widely used in a number of consumer and military applications, such as surveillance, tracking, monitoring, and aerial photography. Fixed-wing and rotary UAVs are the two primary categories in UAV. Interestingly, the hybridization of fixed-wing and rotary UAV gives better performance in terms of energy consumption and the needs of runaway. Designing new hybrid fixed wing-rotary UAV or hybrid vertical take-off and landing (VTOL) is challenging especially to identify the critical location in the UAV and material selection. Therefore, the objective of this research is to study the effect of empennage location and material selection on the structural strength of a hybrid VTOL UAV. The SolidWorks software was employed to design a 3D model of the UAV with different empennage locations, as well as perform a simulation of the structural strength of fibre glass, carbon fibre and kenaf for the hybrid VTOL UAV. The simulation analysis presents stress (Von Mises). The results show that the fibre glass (4.342 N/m2) at top empennage gives the best performance as compared to other parameters. In conclusion, this study is necessary to give a better picture of structural strength of composite materials and best design location in hybrid VTOL UAV for future research.

Dynamic characteristics analysis and robust transition control of tail-sitter VTOL UAVs

The strong nonlinear dynamics of tail-sitter vertical takeoff and landing (VTOL) unmanned aerial vehicles (UAVs) have consistently constrained their applications. Particularly, the widely-varying flight speed and attitude during the transition flights pose substantial challenges for the controller design. In order to gain a clear understanding of the nonlinear transition flights, the long- and short- period dynamic characteristics of tail-sitter UAVs are investigated for the first time. The results reveal that the short-period mode exhibits either sluggish response or excessive overshoot in different transition stages, primarily due to the widely-varying damping ratio and natural angular frequency. Even worse, the long-period mode can diverge due to positive poles. To address these issues, a systematic controller is proposed. First, the transition trajectory is optimized and the optimal control inputs are employed in a feedforward manner to trim the nominal forces and moments. On this basis, a discrete-time linear quadratic regulation (LQR) controller with a predefined decay rate is developed to position the desired poles, and a novel angular acceleration estimation method is introduced to compensate for unmodeled dynamics. Simulations under different uncertainties indicate that the proposed control method performs better in both transition trajectory tracking and uncertainty suppression, compared to the PID and incremental nonlinear dynamic inversion (INDI) controllers.

Development of Adaptive Control System for Aerial Vehicles

This article represents and compares two control systems for a vertical takeoff and landing (VTOL) unmanned aerial vehicle (UAV): a sliding proportional–integral–derivative (PID) controller and an adaptive L1 controller. The goal is to design a high-performing and stable control system for a specific VTOL drone. The mathematical model of the unique VTOL drone is presented as a control object. The sliding PID and adaptive L1 controllers are then developed and simulated, and their performance is compared. Simulation results demonstrate that both control systems achieve stable and accurate flight of the VTOL drone, but the adaptive L1 controller outperforms the sliding PID controller in terms of robustness and adaptation to changing conditions. This research contributes to ongoing work on adaptive control systems for VTOL UAVs and highlights the potential benefits of using L1 adaptive control for this application.

Development of Multimode Flight Transition Strategy for Tilt-Rotor VTOL UAVs

The purpose of this paper is to establish a transition strategy for tilt-rotor vertical takeoff and landing (VTOL) unmanned aerial vehicles (UAVs) based on an optimal design method. Firstly, The flyable transition corridor was calculated based on both the UAV’s dynamic equations and its aerodynamic and dynamic characteristics. The dynamic equations of the UAV were organized into state equation forms. The initial and final value constraints of the control and state variables in the transition process were recorded, as were the constraints of the transition process. The transition strategy design problem was transformed into an optimal control problem with constraints, while the Gauss pseudospectral method (GPM) was employed to transform and solve the problem. In addition, performance indicators were designed based on the transition quality requirements for transition time, attitude stability, and control continuity. Sensitivity was analyzed according to different index terms with different dimensions and effects. Finally, the rationality of the transition strategy designed in this paper was verified according to different simulation scenarios.

Validation of Quad Tail-sitter VTOL UAV Model in Fixed Wing Mode

Vertical take-off and landing (VTOL) is a type of unmanned aerial vehicle (UAV) that is growing rapidly because its ability to take off and land anywhere in tight spaces. One type of VTOL UAV, the tail-sitter, has the best efficiency. However, besides the efficiency offered, some challenges must still be overcome, including the complexity of combining the ability to hover like a helicopter and fly horizontally like a fixed-wing aircraft. This research has two contributions: in the form of how the analytical model is generated and the tools used (specifically for the small VTOL quad tail-sitter UAV) and how to utilize off-the-shelf components for UAV empirical modeling. This research focuses on increasing the speed and accuracy of the UAV VTOL control design in fixed-wing mode. The first step is to carry out analysis and simulation. The model is analytically obtained using OpenVSP in longitudinal and lateral modes. The next step is to realize this analytical model for both the aircraft and the controls. The third step is to measure the flight characteristics of the aircraft. Based on the data recorded during flights, an empirical model is made using system identification technique. The final step is to vali-date the analytical model with the empirical model. The results show that the characteristics of the analytical mode fulfill the specified requirements and are close to the empirical model. Thus, it can be concluded that the analytical model can be implemented directly, and consequently, the VTOL UAV design and development process has been shortened.

Intelligent Vision-based Autonomous Ship Landing Of VTOL UAVs

The paper discusses an intelligent vision-based control solution for autonomous tracking and landing of vertical take-off and landing (VTOL) capable unmanned aerial vehicles (UAVs) on ships without utilizing GPS signals. The central idea involves automating the Navy helicopter ship landing procedure where the pilot utilizes the ship as the visual reference for long-range tracking; however, it refers to a standardized visual cue installed on most Navy ships called the "horizon bar" for the final approach and landing phases. This idea is implemented using a uniquely designed nonlinear controller integrated with machine vision. The vision system utilizes machine learning based object detection for long-range ship tracking and classical computer vision for the estimation of aircraft relative position and orientation utilizing the horizon bar during the final approach and landing phases. The nonlinear controller operates based on the information estimated by the vision system and has demonstrated robust tracking performance even in the presence of uncertainties. The developed autonomous ship landing system was implemented on a quad-rotor UAV equipped with an onboard camera, and approach and landing were successfully demonstrated on a moving deck, which imitates realistic ship deck motions. Extensive simulations and flight tests were conducted to demonstrate vertical landing safety, tracking capability, and landing accuracy.

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.

A Computational Investigation of the Hover Mechanism of an Innovated Disc-Shaped VTOL UAV

Inventive approaches are constantly being revealed in the field of vertical take-off and landing (VTOL) unmanned aerial vehicle (UAV) configuration concepts and designs. To date, a body-associated configuration of UAVs for augmented lift remains unclear among other approached designs. The current paper investigates the mechanism of a high-lift ducted fan mounted in the central body for VTOL UAV designs. We report an unresolved design of a disc-shaped UAV with a single rotor that aims to enhance the cost-effectiveness of fuel consumption with a substantial contribution of body lift to hover thrust. The convex upper surface curvature was applied to generate a significant lift contribution from the body during hover. The computational fluid dynamics (CFD) approach based on unstructured discretization followed by three-dimensional steady Reynolds-averaged Navier–Stokes (RANS) flow was applied in ANSYS CFX to mechanistically investigate the underlying design considerations. The disc-shaped UAV uses the lip curvature on the duct inlet to generate a vertical force that demonstrates a significant contribution of 95% of the rotor thrust during hovering. The UAV’s upper surface generates prolonged flow entrainment free from momentum losses in swirling flows. This phenomenon is followed by reduced power consumption in hovering and vertical flight, making the UAV aerodynamically stable and environmentally safe.

The MPU RX-4 Project: Design, Electronics, and Software Development of a Geofence Protection System for a Fixed-Wing VTOL UAV

Unmanned aerial vehicles (UAVs) are at the forefront of this century’s technological shift, becoming ubiquitous in research and market areas. However, as a UAV navigates autonomously, there are unanticipated occasions, e.g., collisions with dynamic obstacles or loss of data provided by the global navigation satellite system (GNSS), where the aircraft has to change its mission plans. In particular, to be protected from possible accidents, the platform’s geofence protection system should adjust the trajectory appropriately when obstacles are detected and select the proper ground surface when emergency landing is prompted. As these processes require fast reaction times, utilizing low latency sensors and algorithms is necessary. This article proposes a complete and low-complexity geofence protection system for recognizing moving objects and assessing the ground surface’s suitability using onboard sensing and processing modules. The proposed system is implemented on a novel fixed-wing UAV, designated as MPU RX-4, which features an unconventional flying wing layout and vertical take-off and landing (VTOL) capabilities. Our system is based on a forward-facing laser imaging, detection, and ranging (LIDAR) sensor and three downward-facing laser rangefinders. We take advantage of the high-precision distance measurements and operational speed to identify moving obstacles using the LIDAR module, while the ground’s slope and the existence of any obstacle therein are computed through the rangefinders. First, the article describes the UAV design procedure and its aerodynamic performance characteristics, which allowed us to evaluate our approach on a testbed aircraft. Then, the evaluation protocol shows that our system can perform robustly and under real-time constraints reaching an overall latency of only 165.5 ms, sufficient for reliable detection and avoidance of moving obstacles.

Landmark-Based Scale Estimation and Correction of Visual Inertial Odometry for VTOL UAVs in a GPS-Denied Environment.

With the rapid development of technology, unmanned aerial vehicles (UAVs) have become more popular and are applied in many areas. However, there are some environments where the Global Positioning System (GPS) is unavailable or has the problem of GPS signal outages, such as indoor and bridge inspections. Visual inertial odometry (VIO) is a popular research solution for non-GPS navigation. However, VIO has problems of scale errors and long-term drift. This study proposes a method to correct the position errors of VIO without the help of GPS information for vertical takeoff and landing (VTOL) UAVs. In the initial process, artificial landmarks are utilized to improve the positioning results of VIO by the known landmark information. The position of the UAV is estimated by VIO. Then, the accurate position is estimated by the extended Kalman filter (EKF) with the known landmark, which is used to obtain the scale correction using the least squares method. The Inertial Measurement Unit (IMU) data are used for integration in the time-update process. The EKF can be updated with two measurements. One is the visual odometry (VO) estimated directly by a landmark. The other is the VIO with scale correction. When the landmark is detected during takeoff phase, or the UAV is returning to the takeoff location during landing phase, the trajectory estimated by the landmark is used to update the scale correction. At the beginning of the experiments, preliminary verification was conducted on the ground. A self-developed UAV equipped with a visual-inertial sensor to collect data and a high-precision real time kinematic (RTK) to verify trajectory are applied to flight tests. The experimental results show that the method proposed in this research effectively solves the problems of scale and the long-term drift of VIO.

VTOL UAV digital twin for take-off, hovering and landing in different wind conditions

With UAVs becoming increasingly popular in the industry, vertical take-off and landing (VTOL) convertiplanes are emerging as a compromise between the advantages of planes and multicopters. Due to their large wing surface area, VTOL convertiplanes are subject to a strong wind dependence on critical phases such as take-off, landing, and hovering. Developing a new and improved unmanned aerial vehicle (UAV) is often expensive and associated with failures and accidents. This paper proposes the dynamic characterization of a commercial VTOL convertiplane UAV in copter mode and provides a novel method to estimate the aerodynamic forces and moments for any possible wind speed and direction. Starting from Euler’s equations of rigid body dynamics, we have derived the mathematical formulation to precisely consider aerodynamic forces and moments caused by any wind speed and direction. This unique approach will allow for VTOL convertiplane UAVs to be trained and tested digitally in take-off, hovering, and landing maneuvers without the cost and hassle of physical testing, and the dependence on existing wind conditions. A digital twin of a VTOL convertiplane UAV in copter mode has been modeled and tested in the Gazebo robotics simulator. Take-off, hovering and landing maneuvers have been compared with and without the wind physics model. Finally, the simulator has been tested against real flight conditions (reproducing the mean wind speed and direction only), showing a natural and realistic behavior.

Real-Time Object Detection and Recognition Using Fixed-Wing LALE VTOL UAV

This research aimed to identify objects from a real-time video (30 Hz) stream transmitted from a low-altitude long-endurance (LALE) fixed-wing hybrid vertical takeoff and landing (VTOL) unmanned aerial vehicle (UAV) designed for asset monitoring. Illumination, rotation, and scale variations complicate the video frames created by UAVs’ optical payload, requiring preprocessing. Detection, identification, and classification of different objects (or targets) from a real-time video feed require fast and reliable detection algorithms, models, and computation. The weight restrictions and the onboard power supply only allowed for a Raspberry Pi onboard the UAV, thereby seriously limiting computational power. Image frames were created from the video data after enhancing its quality using image processing algorithms. Then, anchor-based object detection algorithms were applied to the processed and enhanced image frames. The Raspberry Pi at airspeeds of 16–18 m/s requires fast object detection algorithms having minimal computational overhead. A supervised learning technique using a supporting library further augmented the real-time detection at the ground control station (GCS). Four scenarios trained the candidate algorithms, namely: 1) helipad; 2) green area; 3) burned area; and 4) under construction area. The single-shot multibox detector (SSD) aided the supervised learning process during trials utilizing the labeled image datasets. The comparison of detection and identification performance of the SSD with ResNet-50 and ResNet-101 backbones with other methods, such as Faster-RCNN with four backbone architectures, namely: 1) ResNet-101; 2) ResNet-50; 3) deformable ResNet; and 4) FPN in four detection scenarios indicated better precision and reduced detection delays.

수직이착륙 무인기를 위한 의미론적 영상 분할 기반 안전 착륙 지점 판단 기법

This paper presents a safe landing point determination method for vertical take-off and landing (VTOL) unmanned aerial vehicles (UAVs) with a downward-looking camera. The semantic image segmentation method was applied to determine whether the VTOL UAV could land safely at the designated landing site during the final stage of landing. A deep neural network with a U-Net structure was constructed for the semantic image segmentation, and a deep neural network was trained to recognize the landing site and any obstacles. The dataset used for training the deep neural network was collected through efficient labeling rules. The deep neural network of the U-Net structure outputs the area of the landing field and the area of the obstacle in units of pixels. Based on this output data, the area occupied by the obstacle in the area of the landing site is calculated to determine whether to land. Finally, the performance of the safe landing point determination method was verified using a landing pad image taken from the downward-looking camera.

Back-Transition Control With Large Deceleration for a Dual Propulsion VTOL UAV Based on Its Maneuverability

This letter analyzes the maneuverability of a fixed-wing hybrid Vertical Take-Off and Landing (VTOL) Unmanned Aerial Vehicle (UAV), and proposes a control strategy for back-transitions from fixed-wing mode to rotary-wing mode based on it. Longitudinal aerodynamic forces and moment are modeled to derive a dynamic model of the UAV, and the translational and rotational accelerations that can be generated for the UAV are examined based on the model. To achieve a back-transition with the largest deceleration while maintaining level flight, we search for the best choice of pitch angle and thrust forces for each forward speed by numerical optimization. Flight experiments demonstrate that it is possible to perform a horizontal back-transition while generating a large deceleration with the chosen pitch angles.

Practical Distributed Control for VTOL UAVs to Pass a Virtual Tube

Unmanned Aerial Vehicles (UAVs) are now becoming increasingly accessible to amateur and commercial users alike. An air traffic management (ATM) system is needed to help ensure that this newest entrant into the skies does not collide with others. In an ATM, airspace can be composed of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">airways, intersections</i> and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nodes.</i> In this paper, for simplicity, distributed coordinating the motions of Vertical TakeOff and Landing (VTOL) UAVs to pass an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">airway</i> is focused. This is formulated as a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">tube passing problem</i> , which includes passing a virtual tube, inter-agent collision avoidance and keeping within the virtual tube. Lyapunov-like functions are designed elaborately, and formal analysis based on <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">invariant set theorem</i> is made to show that all UAVs can pass the virtual tube without getting trapped, avoid collision and keep within the virtual tube. What is more, by the proposed distributed control, a VTOL UAV can keep away from another VTOL UAV or return back to the virtual tube as soon as possible, once it enters into the safety area of another or has a collision with the virtual tube during it is passing the virtual tube. Simulations and experiments are carried out to show the effectiveness of the proposed method and the comparison with other methods.

Design of Ducted Fan Based on Optimal Circulation Distribution

The ducted fan lift system has been widely used in vertical takeoff and landing (VTOL) unmanned aerial vehicle (UAV). According to the overall design requirements of a VTOL UAV, based on the optimal circulation distribution condition without blade element drag and combined with strip theory, the blade torsion angle distribution and chord length distribution are given. The parameters of ducted fan are designed, and the preliminary design of ducted fan lift system of this UAV is completed. The aerodynamic characteristics of the isolated fan and the ducted fan are compared by numerical simulation method, and the thrust characteristics of the ducted fan at different flight speeds are calculated. The results show that the duct increases the induced velocity at the propeller. Compared with the isolated fan, the power load and hover efficiency of the ducted fan are increased by 36.8% and 36.3% respectively. The hovering efficiency of the ducted fan designed by this method reaches 83%, which meets the design requirements and proves the effectiveness of the design method.

Homography-Based Visual Servoing for Underactuated VTOL UAVs Tracking a 6-DOF Moving Ship

This paper develops a novel homography-based visual servo control method for the vertical take-off and landing (VTOL) unmanned aerial vehicle (UAV) tracking the trajectory of a 6 degrees of freedom (6-DOF) moving ship. Different from the classical homography-based visual servoing which is only suitable for the trajectory tracking of a planar moving target, an extended homography-based visual servoing framework is proposed to track the trajectory of a 6-DOF moving target regardless of its time-varying rotational motion. Taking entries in the homography matrix as feedback, the visual dynamics of UAV is established and decoupled into a translational motion sub-dynamics and an attitude sub-dynamics based on the hierarchical control strategy. In the translational motion subsystem, the robust controller is designed by embedding the nonlinear differentiator, the adaptive method, and a smooth saturation model in the backstepping control scheme. The saturation model is introduced to generate the constrained control inputs to ensure the nonsingular attitude extraction and help the visual points remain in the camera’s field of view (FoV) simultaneously. In the attitude subsystem, a robust state-constrained control method is proposed by utilizing a robust control barrier function (RCBF), the quadratic programming (QP) and a saturation model to guarantee the visibility, where RCBF can accommodate unknown dynamics. The theoretical analysis demonstrates the asymptotic stability of the closed-loop system. Simulations are carried out to further validate the controller performance.

Adaptive Fault-Tolerant Control of a Hybrid VTOL UAV against Actuator Faults and Model Uncertainties under Fixed-Wing Mode

This paper proposes an adaptive fault-tolerant control strategy for a hybrid vertical take-off and landing (VTOL) unmanned aerial vehicle (UAV) to simultaneously compensate actuator faults and model uncertainties. With the proposed adaptive control schemes, both actuator faults and model uncertainties can be accommodated without the knowledge of fault information and uncertainty bounds. The proposed control scheme is constructed with two separate control modules. The low-level control allocation module is used to distribute the virtual control signals among the available redundant actuators. The high-level control module is constructed with an adaptive sliding mode controller, which is employed to maintain the overall system tracking performance in both faulty and uncertain conditions. In the case of actuator faults and model uncertainties, the adaptive scheme will be triggered to generate more virtual control signals to compensate the virtual control error and maintain the desired system tracking performance. The effectiveness of the proposed control strategy is validated through comparative simulation tests under different faulty and uncertain scenarios.