Vertical Take-off And Landing Research Articles

Progress in Rotorcraft Design: 1974–2024

Fundamental advancements in aircraft design over the past 50 years have enabled a range of vertical takeoff and landing (VTOL) air vehicle configurations and have significantly enhanced aircraft performance, safety, and reliability. This summary paper chronicles the evolution of rotorcraft design from 1974 to 2024. It is segregated into three key pillars of aircraft design preceded by an aircraft design overview. The three pillars are processes and tools, technology, and air vehicle configuration. The first pillar, on design processes and tools, describes advancements in technology, methodologies, and computational capabilities such as the transition from design solely by wind tunnel testing, physical models, and hand calculations to computer-aided design/synthesis and simulation in a model-based engineering digital-twin environment. The second pillar, on technology, focuses on advances in disciplinary technologies and how they have been incorporated into aircraft design. Technologies discussed include composite materials; rotor systems; propulsion systems, ranging from advancements in turbine engines to all- and more-electric propulsion technologies utilizing various energy storage systems to convertible engines; fly-by-wire systems; avionics and cockpit architectures, including digital displays, navigation aids, and communication equipment; and autonomous and semiautonomous systems. The third pillar, on air vehicle configuration, focuses on platform architectures. Major architectures discussed are high-speed VTOLs such as the thrust-vectored aircraft, tiltrotors, lift/thrust compounded helicopters, and all- and more-electric aircraft.

Design of Bionic Landing Gear with Multi-Link Mechanism for UAVs in Desert Terrain

The terrain in desert areas is rugged and uneven, lacking suitable landing points for unmanned aerial vehicles (UAVs). In addition, sand and dust in the desert can easily damage the internal circuits of propellers through vortexes. To address the high demands for hardness and smoothness of landing sites for vertical takeoff and landing (VTOL) aircraft, as well as the low level of intelligence of traditional landing gear, an optimized biomimetic leg-type terrain-adaptive landing gear based on a multi-link hybrid mechanism was developed for landing in desert areas. First, the design configuration of a single leg was optimized, and the length of each link was calculated accordingly, followed by motion simulations. Secondly, the overall design was developed, and slope landing simulations were conducted on the four-leg mechanism. Research results showed that the optimized multi-link hybrid mechanism terrain-adaptive landing gear was better suited for adapting to desert terrain and could land on sand dune slopes, effectively improving the adaptability of VTOL aircraft in desert areas.

Acoustic measurements in single-rotor/wing interaction at low disk loading and Reynolds number

The tiltrotor design is favored for urban air mobility (UAM) prototypes due to the combination of vertical takeoff and landing (VTOL) capability and efficient forward flight. With rising UAM air traffic at low altitudes, noise from these aircraft is a crucial design factor. Most tiltrotor noise research focuses on high disk loading and Reynolds number setups, leaving smaller aircraft configurations less explored. This study investigates aero-acoustic trends from rotor-wing interaction at low disk loading ([Formula: see text]100 N/m2) and Reynolds number (Re < 100,000). While prior literature suggests lowering disk loading and reducing rotor wake interference can mitigate rotor noise, such ideas lack empirical validation. The setup involves an anechoic chamber housing a two-blade rotor, along with flat and NACA 0012 airfoil wings. Microphones and a rotation stage capture acoustic data for analysis. Factors like flow recirculation, isolated rotor noise, rotor height, rotation direction/rate, and wing curvature are assessed for impact on noise signature. It is found that the deflected rotor wake in rotor-wing interaction significantly increases low-frequency broadband noise and overall sound pressure level (OASPL), compared to an isolated rotor. Dominant tonal noise diminishes based on the strength of the deflected rotor wake. These findings offer insights into reducing noise from rotor wake impingement on the wing.

Multi-Phase Vertical Take-Off and Landing Trajectory Optimization with Feasible Initial Guesses

The advancement of electric vertical take-off and landing (eVTOL) aircraft has expanded the horizon of urban air mobility. However, the challenge of generating precise vertical take-off and landing (VTOL) trajectories that comply with airworthiness requirements remains. This paper presents an approach for optimizing VTOL trajectories considering six degrees of freedom (6DOF) dynamics and operational constraints. Multi-phase optimal control problems are formulated to address specific constraints in various flight stages. The incremental nonlinear dynamic inversion (INDI) controller is employed to execute the flight mission in each phase. Controlled flight simulations yield dynamically feasible trajectories that serve as initial guesses for generating sub-optimal trajectories within individual phases. A feasible and sub-optimal initial guess for the holistic multi-phase problem is established by concatenating these single-phase trajectories. Focusing on a tilt-wing eVTOL aircraft, this paper computes VTOL trajectories leveraging the proposed initial guess generation procedure. These trajectories account for complex flight dynamics, align with various operation constraints, and minimize electric energy consumption.

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.

Design and manufacture of a UAV using additive technologies and composite materials

The aim of the study is the development of a VTOL (Vertical Takeoff and Landing) bicopter, utilizing Autodesk Inventor and Ultimaker Cura software. Design models were inspired by the V22-Osprey and Samson SA-2 from the Avatar movie. Composite materials (carbon fiber and PLA for 3D printing) were chosen due to their high-performance capabilities, as aerospace technology is in an ongoing development and evolution. The construction of the bicopter employed the FAAT method, and for control, the MATEK F411-WSE was utilized to process data from sensors, with specific firmware and the BetaFLIGHT interface for optimizing drone performance.

Vertical takeoff and landing aircraft: Categories, applications, and technology

A Vertical Take-Off and Landing (VTOL) aircraft is an aircraft with the ability to take off and land vertically. The ability of removing the need for a runway allows VTOL aircraft to be used for applications that standard aircraft cannot be used for. These applications include military, firefighting, and transportation applications. For the purposes of this paper, helicopters will be considered to be VTOL aircraft, as they fit the general criteria and are among the first to be widely used. VTOL aircraft can be split into several categories, including hybrid VTOL aircraft, VTOL Unmanned Aerial Vehicles, and electric VTOL aircraft. These categories of VTOL aircraft are compared. Famous VTOL aircrafts like Osprey and Harrier are introduced. The technology behind VTOL aircraft is explained, and the ways in which VTOL aircraft can be made useful are reviewed. In addition, the prospect of VTOL aircraft has been made. This paper may offer a reference for the future works about VTOL.

Development of a Mission-Tailored Tail-Sitter MAV

Vertical takeoff and landing (VTOL) vehicles are among the most versatile UAVs, appropriate for various missions. Given that there are still open challenges regarding the VTOL design, this paper presents the full development and test cycle of a tail-sitter. IMAV 2022 competition rules were used to define the mission. A multidisciplinary design and optimization strategy was defined with the goal of maximizing competition score considering design, manufacturing, and competition constraints. The resulting vehicle was designed to fly at 18[Formula: see text]m/s while carrying 200[Formula: see text]g of payload with a total weight of approximately 720[Formula: see text]g. It flew for roughly 13 min at IMAV2022, helping its team to achieve 1st place at the “Package delivery challenge”. Further flight tests revealed the ultimate endurance performance as 18[Formula: see text]min.

Adaptive control of a VTOL uncrewed aerial vehicle for high-performance aerobatic flight

We present two adaptive control methods conceived to enable a vertical take-off and landing (VTOL) uncrewed aerial vehicle (UAV) to perform a class of aerobatic maneuvers in the presence of aerodynamic-coefficient and torque-latency variations induced by changes of the local flow fields during high-speed aerobatic flight. First, we introduce a linear time-varying (LTV) dynamical model, which is assumed to have unknown time-dependent parameters, to describe the aerodynamic effects acting on the actuation dynamics of the system. Then, we present the design of a Lyapunov-based adaptive control scheme aimed to compensate for undesired behavior of the LTV actuator dynamics, according to which we derive the control and adaptation laws from a single Lyapunov candidate function. Next, we propose a modular adaptive control scheme to address the same problem, but in which the adaptation law is specified separately from that of the controller. We use modern nonlinear theory to deduce and analyze the conditions that guarantee the global asymptotic stability of both adaptive control strategies. In order to exemplify the controller synthesis procedures, we implemented both adaptive control methods on a quadrotor UAV to perform three different types of aerobatic-flight maneuvers—namely, triple flips, Pugachev’s cobras, and mixed flips. The obtained experimental results provide compelling evidence of the effectiveness of the two proposed methods to compensate for the undesired effects induced by aerodynamic-coefficient and torque-latency variations. Furthermore, the experimental data demonstrate that both adaptive schemes significantly improve the flight performance of the quadrotor UAV during the execution of aerobatic maneuvers, compared to those achieved with a controller that disregards the LTV actuator dynamics induced by high-speed aerodynamic effects. The suitability of the time-varying approach used to model the influence of high-speed local flow fields on the dynamics of the controlled UAV was indirectly validated through data obtained during the aerobatic-flight experiments presented here.

A new efficient algorithm for short path planning of the vertical take-off and landing air-ground integrated vehicle

With excellent air-ground multi-mode movements, the vertical take-off and landing (VTOL) air-ground integrated vehicle can easily traverse complex terrains and maintain high energy efficiency. During movement, path planning plays an important role in achieving the autonomous operation of the vehicle, which faces the following difficulty. A short air-ground multi-mode path requires efficient planning, with proper judgment of the timing and position for mode switching. To address this difficulty, we propose a new path planning algorithm, named Dynamically Directed Graph Algorithm (DDGA). It can realize short path search in limited search nodes via dynamically extracting key search nodes in maps and forming a dynamically directed graph. To be specific, adjacent nodes of the first obstacle traversed by the connection line from the current search node to the destination are defined as key search nodes. As the current node changes, key search nodes are dynamically updated. The above key search nodes and the directed paths between them form a dynamically directed graph. Considering the air-ground movement capability, obstacle areas below maximum flight altitudes in maps are defined as pending flight areas. The directed paths traversing these areas are considered in the above graph. Besides the two-dimensional distance cost, flight altitude cost is added to the cost values of different directed paths. This cost contributes to judging the proper switching timing and position. Compared to other algorithms, DDGA can find short paths with fewer search nodes in multiple obstacle maps. It efficiently plans a short air-ground multi-mode path for the VTOL air-ground integrated vehicle.

A Y-Shaped Tilting Wing UAV for Vertical Take-off and Landing Cargo

To address the growing demand for express cargo transportation within and between cities, this study presents a conceptual design of a Y-shaped tilting wing unmanned aerial vehicle (UAV) capable of vertical take-off and landing (VTOL). The design aims to overcome the limitations of existing UAV designs and combine the advantages of multi-rotor and fixed-wing aircraft. By incorporating fixed-wing features, the UAV offers an extended battery life and range, making it an ideal choice for urban express cargo transportation. Firstly, a master geometry model of the UAV is established. Subsequently, dynamic simulations are conducted to evaluate the UAV’s performance under different trajectories. The analysis of the research results verifies that the UAV can successfully complete its flight missions along predetermined paths.

Wireless-Enabled Asynchronous Federated Fourier Neural Network for Turbulence Prediction in Urban Air Mobility (UAM)

To meet the growing mobility needs in intra-city transportation, the concept of urban air mobility (UAM) has been proposed in which vertical takeoff and landing (VTOL) aircraft are used to provide a ride-hailing service. In UAM, aircraft can operate in designated air spaces known as <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">corridors</i> , that link the aerodromes, thus avoiding the use of complex routing strategies such as those of modern-day helicopters and alleviating the burden on the ground transportation system. For safety, a UAM aircraft must use air-to-ground communications to report flight plan, off-nominal events, and real-time movement to ground base stations (GBSs). A reliable communication network between GBSs and aircraft enables UAM to adequately utilize the airspace and create a fast, efficient, and safe transportation system. In this paper, to characterize the wireless connectivity performance for UAM, a suitable spatial model is proposed. For the considered setup, assuming that any given aircraft communicates with the closest GBS, the distribution of the distance between an arbitrarily selected GBS and its associated aircraft and the Laplace transform of the interference experienced by the GBS are derived. Using these results, the signal-to-interference ratio (SIR)-based connectivity probability is determined to capture the connectivity performance of the UAM aircraft-to-ground communication network. Then, leveraging these connectivity results, a wireless-enabled asynchronous federated learning (AFL) framework that uses a Fourier neural network is proposed to tackle the challenging problem of turbulence prediction during UAM operations. For this AFL scheme, a staleness-aware global aggregation scheme is introduced to expedite the convergence to the optimal turbulence prediction model used by UAM aircraft. Simulation results validate the theoretical derivations for the UAM wireless connectivity. The results also demonstrate that the proposed AFL framework converges to the optimal turbulence prediction model faster than the synchronous federated learning baselines and a staleness-free AFL approach. Furthermore, the results characterize the performance of wireless connectivity and convergence of the aircraft’s turbulence model under different parameter settings, offering useful UAM design guidelines.

Experimental Stability Analysis of Vertical Takeoff and Landing System Based on Robust Control Strategy

The Vertical Take-Off and Landing (VTOL) system is a multi-variable system subjected to harsh weather conditions, which creates challenges in proving the stability of the system before takeoff, which is essential for a flight dynamics system. The presented research work is based on the experimental results of the VTOL system to investigate and prove the stability using Lyapunov theory. This is achieved by tracking the pitch along the x-axis using cascaded control and integral super twisting sliding mode control (ISTSMC) algorithms. The motor current of the propeller assembly is regulated based on proportional integral (PI) and proportional integral derivative (PID) controllers. The cascaded control shows the maximum tracking error due to high-frequency fluctuations in the controller input signal, which lead to expensive mechanical losses for the actuators. The comparison of the results shows that ISTSMC outperforms the cascaded control strategy by reducing the tracking error to less than 1% percent and reducing the high-frequency fluctuations in the controller input signal. The hardware results show a minor delay in the transient response during vertical takeoff due to the inertia of the system and the tracking error due to air friction, etc., of the external environment, compared to the simulation results obtained in MATLAB.

Improving EMS response times for out-of-hospital cardiac arrest in urban areas using drone-like vertical take-off and landing air ambulances: An international, simulation-based cohort study

BackgroundAdvances in vertical take-off and landing (VTOL) technologies may enable drone-like crewed air ambulances to rapidly respond to out-of-hospital cardiac arrest (OHCA) in urban areas. We estimated the impact of incorporating VTOL air ambulances on OHCA response intervals in two large urban centres in France and Canada. MethodsWe included adult OHCAs occurring between Jan. 2017–Dec. 2018 within Greater Paris in France and Metro Vancouver in Canada. Both regions utilize tiered OHCA response with basic (BLS)- and advanced life support (ALS)-capable units. We simulated incorporating 1–2 ALS-capable VTOL air ambulances dedicated to OHCA response in each study region, and computed time intervals from call reception by emergency medical services (EMS) to arrival of the: (1) first ALS unit (“call-to-ALS arrival interval”); and (2) first EMS unit (“call-to-first EMS arrival interval”). ResultsThere were 6,217 OHCAs included during the study period (3,760 in Greater Paris and 2,457 in Metro Vancouver). Historical median call-to-ALS arrival intervals were 21 min [IQR 16–29] in Greater Paris and 12 min [IQR 9–17] in Metro Vancouver, while median call-to-first EMS arrival intervals were 11 min [IQR 8–14] and 7 min [IQR 5–8] respectively. Incorporating 1–2 VTOL air ambulances improved median call-to-ALS arrival intervals to 7–9 min and call-to-first EMS arrival intervals to 6–8 min in both study regions (all P < 0.001). ConclusionVTOL air ambulances dedicated to OHCA response may improve EMS response intervals, with substantial improvements in ALS response metrics.

Large Eddy Simulation for Empirical Modeling of the Wake of Three Urban Air Mobility Vehicles

Recent advances in urban air mobility have driven the development of many new vertical take-off and landing (VTOL) concepts. These vehicles often feature original designs departing from the conventional helicopter configuration. Due to their novelty, the characteristics of the supervortices forming in the wake of such aircraft are unknown. However, these vortices may endanger any other vehicle evolving in their close proximity, owing to potentially large induced velocities. Therefore, improved knowledge about the wakes of VTOL vehicles is needed to guarantee safe urban air mobility operations. In this work, we study the wake of three VTOL aircraft in cruise by means of large eddy simulation. We present a two-stage numerical procedure that enables the simulation of long wake ages at a limited computational cost. Our simulations reveal that the wakes of rotary vehicles (quadcopter and side-by-side helicopter) feature larger wake vortex cores than an isolated wing. Their decay is also accelerated due to self-induced turbulence generated during the wake roll-up. A tilt-wing wake, on the other hand, is moderately turbulent and has smaller vortex cores than the wing. Finally, we introduce an empirical model of the vortex circulation distribution that enables fast prediction of wake-induced velocities, within a 2% error of the simulation results on average.

Design of a landing gear system for UAV in complex ground environments

The current UAV technology is developing rapidly and there are various UAV concepts such as hybrid vertical take-off and landing (VTOL) UAV in addition to traditional UAV. The main solution direction of these UAV concepts lies in the compatibility of vertical take-off and landing and long endurance and high load. However, in some complex environments, such as post-earthquake debris, or uneven mountainous terrain, UAV take-off and landing still has great difficulties. Therefore, this paper designs a landing gear that can adapt to complex ground conditions and its corresponding landing algorithm based on DJI's publicly available UAV model. First, a movable landing gear controlled by a linear actuator is used to adapt to a variety of complex terrains. Then, we designed a UAV foot similar to a 3D carved needle with oil-hydraulic cushioning to make it fit more to the complex ground and stand stably on the ground. Then, we obtained the mathematical relationship between the linear actuator and the landing gear moving end based on the simplified mathematical model of the UAV model, and applied it in the subsequent landing algorithm. And then, in detecting the landing point, we use the column coordinate system for coordinate simulation while the algorithm written in C language to get the most suitable UAV landing point. Finally, we have simulated the UAV landing using MATLAB and the results show the high engineering value.

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.

Modeling and Passivity-Based Control for a convertible fixed-wing VTOL

This article presents a mathematical model and a controller for a convertible fixed-wing Vertical Take-Off and Landing (VTOL). The mathematical model considers the aerodynamic forces generated by the motors. The developed Passivity-Based Control (PBC) law stabilizes the rotational and translational dynamics of a convertible Unmanned Aerial Vehicle (UAV) in the transition stages of cruise-stationary flight. The control objective is to allow the realization of the two flight regimes of a convertible VTOL along a trajectory. A control assignment technique is also presented that allows the decoupling of the angles of the front motors so that they can have different positions. Finally, numerical simulations are carried out to validate the performance of the presented algorithm. The results indicate that this controller can provide enough maneuverability to track different trajectories with good performance.

Flow physics and loss mechanisms of tip leakage flow in variable-blade-pitch-angle ducted fans

This paper investigates a ducted fan, which is able to adjust the output characteristics to meet different requirements of VTOL (Vertical Take-Off and Landing) aircrafts at various states, e.g., take-off, landing and high-speed cruise. The variable-blade-pitch-angle method is employed for the ducted fan, so that the fan can present more various performance. The loss mechanisms of the ducted fan are analyzed at seven different blade pitch angles ranging from −15 degrees to 15 degrees, and the importance of the tip leakage flow on the aerodynamic performance of the fan is highlighted. Results show that there exists a critical value for the blade pitch angle, and below the critical value, the tip leakage vortex sweeps to the adjacent blade tip and simultaneously causes the double tip leakage, which results in the high relative-total-pressure loss and a severe deterioration of rotor efficiency. The flow physics and loss mechanisms are discussed in detail. In addition, an analytical model of the tip leakage flow is established, and the effects of blade solidity on the critical blade pitch angle are discussed. The results provide useful guidelines on the design and the control of the variable-blade-pitch-angle fans.

Validation of the Flight Dynamics Engine of the X-Plane Simulator in Comparison with the Real Flight Data of the Quadrotor UAV Using CIFER

The vertical take-off and landing (VTOL) of unmanned aerial vehicles (UAVs) is extensively employed in various sectors. To ensure adherence to design specifications and mission requirements, it is vital to verify flight control and system performance using an accurate dynamic model specific to UAV configuration. Traditionally, engineers follow a sequential approach in UAV design, which involves multiple design iterations comprising CAD drawings, material collection, fabrication, flight tests, system identification, modifications, dynamic model extraction, checking if the results meet requirements, and then repeating the process. However, as UAVs become larger, heavier, and more enduring to meet complex system demands, the costs and time associated with each design iteration of creating a new UAV escalate exponentially. The bare-airframe dynamics of the UAV are crucial for engineers to design a controller and validate handling quality and performance. This paper proposes a novel method to accurately predict the dynamic model of the bare airframe for quadrotor UAVs without physically constructing them in the real world. The core concept revolves around converting the quadrotor UAV design from CAD software into a UAV model within an X-Plane simulator. Leveraging the CIFER software’s two key features—frequency domain system identification and parametric model fitting—the unstable bare-airframe dynamics are extracted for both the UAV model in X-Plane and a real-world DJI 450 UAV with the same physical configuration. This paper provides essential parameters and guidance for constructing a 92% high-fidelity dynamic model of the given UAV configuration in X-Plane. The flight test results demonstrate excellent alignment with the simulation outcomes, instilling confidence in the effectiveness of the proposed method for designing and validating new UAVs. Moreover, this approach significantly reduces the time and cost associated with the traditional design process, which requires an actual build of the UAV and many flight tests to verify the performance.