Vertical Flight Research Articles

The Design of Improved Series Hybrid Power System Based on Compound-Wing VTOL

Hybrid power systems are now widely utilized in a variety of vehicle platforms due to their efficacy in reducing pollution and enhancing energy utilization efficiency. Nevertheless, the existing vehicle hybrid systems are of a considerable size and weight, rendering them unsuitable for integration into 25 kg compound-wing UAVs. This study presents a design solution for a compound-wing vertical takeoff and landing unmanned aerial vehicle (VTOL) equipped with an improved series hybrid power system. The system comprises a 48 V lithium polymer battery(Li-Po battery), a 60cc internal combustion engine (ICE), a converter, and a dedicated permanent magnet synchronous machine (PMSM) with four motors, which collectively facilitate dual-directional energy flow. The four motors serve as a load and lift assembly, providing the requisite lift during the take-off, landing, and hovering phases, and in the event of the ICE thrust insufficiency, as well as forward thrust during the level cruise phase by mounting the variable pitch propeller directly on the ICE. The entire hybrid power system of the UAV undergoes numerical modeling and experimental simulation to validate the feasibility of the complete hybrid power configuration. The validation is achieved by comparing and analyzing the results of the numerical simulations with ground tests. Moreover, the effectiveness of this hybrid power system is validated through the successful completion of flight test experiments. The hybrid power system has been demonstrated to significantly enhance the endurance of vertical flight for a compound-wing VTOL by more than 25 min, thereby establishing a solid foundation for future compound-wing VTOLs to enable multi-destination flights and multiple takeoffs and landings.

Nanostructural Influence on Optical and Thermal Properties of Butterfly Wing Scales Across Forest Vertical Strata.

Butterfly wing scales feature complex nanostructures that influence wing coloration and various mechanical and optical properties. This configuration plays a key role in ecological interactions, flight conditions, and thermoregulation, facilitated by interactions with environmental electromagnetic energy. In tropical forests, butterflies occupy distinct vertical habitats, experiencing significant light and temperature variations. While wing nanostructures have been widely studied, their variation across different vertical flight preferences remains underexplored. This study investigates the wing nanostructures of 12 tropical butterfly species from the Nymphalidae family, focusing on their optical, morphological, and thermal properties across different forest strata. We analyzed the optical response through diffuse reflectance in the UV, Vis, and NIR ranges, correlating these findings with nanostructural configuration and thermal stability using thermogravimetric analysis (TGA). Our results reveal a significant correlation between flight stratification and wing optical responses, alongside distinct nanostructural features within each stratum. This study demonstrates the variability in butterfly wing nanostructures along the vertical stratification of the forest to cope with environmental conditions, raising new questions for future research on eco-evolutionary flight and thermal adaptations. Additionally, this underscores the importance of understanding how these structural adaptations influence butterfly interactions with their environment and their evolutionary success across different forest strata.

Vortex safety when flying at an assigned flight level

Air traffic intensity between countries and within individual countries is increasing year by year. As a rule, airways follow the same routes. As a result, so-called “roads in the sky” are formed. And where there are roads, there are bumps by the time, in the form of CAT, updraughts and downdraughts and increased turbulence. Horizontal and vertical separation plays an important role in ensuring flight safety on route. Currently, a variety of regulatory documents, defining safe separation at the flight level, has been adopted. Thus, provided there is turbulence in the vortex wake, longitudinal separation is based on the arrangement of aircraft types by three categories according to their maximum certified take-off weight. Since November 2011, the Western standard of vertical separation RVSM (Reduced Vertical Separation Minimum) has been introduced in Russia. Vertical separation is the distance between the vertical flight levels on route. Previously, this distance amounted to 600 m (2000 ft), but due to the increasing intensity of air traffic, it was decided to reduce the vertical separation to 300 m (1000 ft). Hence, at the most common flight level, the vertical separation is 300 m. The question arises if this separation ensures the safety of air transportation? The fact is that the altitude of the flight level does not necessarily coincide with the actual aircraft height. Aircraft altimeters are, inherently, calibrated barometers, that calculate the altitude by the difference in pressure on the ground and in the air. To calculate the height above ground, it would be necessary to constantly input atmospheric pressure data to altimeters at each waypoint and take into consideration the waypoint altitude above the sea level. Consequently, it is customary to use standard pressure. If the same pressure values are set on the altimeter on all aircraft, then, altitude readings on the instrument at an assigned point of airspace will be similar. Therefore, from a certain moment during the climb (transition level) to a certain moment during the descent (transition level), the aircraft height is calculated according to the standard pressure. The value of the standard pressure (QNE) is the same all over the world and amounts to 760 mmHg (1013.2 hectopascals). Thus, the flight on route is controlled by an altimeter, a barometric altimeter, which is comprised into the integrated flight and navigation system. An analysis of the instrument accuracy shows that when atmospheric pressure drops, altimeter readings may differ from true reading by ±100 m. It is known that a trailing vortex forms behind a flying plane. By the time, the trailing vortex descends and may be found at another flight level. May this cause air bump at the flight level? To answer this question, the A-380 aircraft was chosen as the object of research. This is one of the largest aircraft in the world. Therefore, the study of a trailing vortex behind the A-380 at the flight level, as the most dangerous in terms of the impact of its trailing vortex on other aircraft, will allow us to understand how safe and reasonable the accepted vertical and horizontal separation is. For the study, the special computational software system, based on the discrete vortex method, was used. This complex has passed the evaluation test and the state registration.

Full Envelope Flight Control System Design and Optimization for a Tilt-Wing Aircraft

Novel vertical take-off and landing advanced air mobility aircraft which are overactuated and transition between vertical and forward flight modes pose unique challenges for the design of safe and robust full‐envelope fly‐by‐wire flight control systems. This paper presents a methodology for designing and optimizing a control system architecture for a tilt-wing urban air mobility concept. It features the total energy control system algorithm, which is extended to be applicable to hover and transitioning flight and explicit model following inner loops. Control system parameters are optimized using a genetic algorithm optimization scheme, subject to constraints on closed-loop dynamic stability and control response characteristics. Control system performance is demonstrated by simulation of lateral, longitudinal, and directional maneuvering for hover, transition, and forward flight conditions, followed by simulation of departure and arrival transitions while tracking a prescribed flightpath and speed profile in calm and turbulent air. The results demonstrate the effectiveness of the proposed control system architecture and the demonstrated optimization methodology.

Motion Equations and Attitude Control in the Vertical Flight of a VTOL Bi-Rotor UAV: Part 2

This paper gathers the dynamical modeling of an unmanned aircraft and the design and simulation of the control system, allowing it to perform a Vertical Take-Off (VTOL) maneuver, a fixed-wing (FW) flight and a transition between the two configurations using two tilting rotors (Bi-Tilt). These Unmanned Aerial Vehicles (UAVs) operating in this configuration are categorized as Hybrid UAVs, for their capability of having a dual flight envelope: flying like a multi-rotor and navigating like a traditional fixed-wing aircraft, allowing the drone to perform complex missions where these two flight configurations are essential. This work exhibits the Bi-Rotor non-linear dynamics, valid for both flight configurations, the design of the control algorithm for stability and navigation, and a simulation of a complete flight mission.

Incremental Nonlinear Dynamics Inversion and Incremental Backstepping: Experimental Attitude Control of a Tail-Sitter UAV

Incremental control strategies such as Incremental Nonlinear Dynamics Inversion (INDI) and Incremental Backstepping (IBKS) provide undeniable advantages for controlling Uncrewed Aerial Vehicles (UAVs) due to their reduced model dependency and accurate tracking capacities, which is of particular relevance for tail-sitters as these perform complex, hard to model manoeuvres when transitioning to and from aerodynamic flight. In this research article, a quaternion-based form of IBKS is originally deduced and applied to the stabilization of a tail-sitter in vertical flight, which is then implemented in a flight controller and validated in a Hardware-in-the-Loop simulation, which is also made for the INDI controller. Experimental validation with indoor flight tests of both INDI and IBKS controllers follows, evaluating their performance in stabilizing the tail-sitter prototype in vertical flight. Lastly, the tracking results obtained from the experimental trials are analysed, allowing an objective comparison to be drawn between these controllers, evaluating their respective advantages and limitations. From the successfully conducted flight tests, it was found that both incremental solutions are suited to control a tail-sitter in vertical flight, providing accurate tracking capabilities with smooth actuation, and only requiring the actuation model. Furthermore, it was found that the IBKS is significantly more computationally demanding than the INDI, although having a global proof of stability that is of interest in aircraft control.

Energy supply during nocturnal endurance flight of migrant birds: effect of energy stores and flight behaviour

BackgroundMigrating birds fly non-stop for hours or even for days. They rely mainly on fat as fuel complemented by a certain amount of protein. Studies on homing pigeons and birds flying in a wind-tunnel suggest that the shares of fat and protein on total energy expenditure vary with flight duration and body fat stores. Also, flight behaviour, such as descending flight, is expected to affect metabolism. However, studies on free flying migrant birds under natural conditions are lacking.MethodsOn a Swiss Alpine pass, we caught three species of nocturnal migrant passerines out of their natural migratory flight. Since most night migrants start soon after dusk, we used time since dusk as a measure of flight duration. We used plasma concentrations of metabolites of the fat, protein, and carbohydrate metabolism as indicators of relative fuel use. We used flight altitudes of birds tracked with radar and with atmospheric pressure loggers to characterize flight behaviour.ResultsThe indicators of fat catabolism (triglycerides, very low-density lipoproteins, glycerol) were positively correlated with body energy stores, supporting earlier findings that birds with high fat stores have a higher fat catabolism. As expected, plasma levels of triglycerides, very low-density lipoproteins, glycerol and ß-hydroxy-butyrate increased at the beginning of the night, indicating that nocturnal migrants increased their fat metabolism directly after take-off. Surprisingly, fat catabolism as well as glucose levels decreased in the second half of the night. Data from radar observations showed that the number of birds aloft, their mean height above ground and vertical flight speed decreased after midnight. Together with the findings from atmospheric pressure-loggers put on three species, this shows that nocturnal migrants migrating over continental Europe descend slowly during about 1.5 h before final landfall at night, which results in 11–30% energy savings according to current flight models.ConclusionsWe suggest that this slow descent reduces energy demands to an extent which is noticeable in the plasma concentration of lipid, protein, and carbohydrate metabolites. The slow descent may facilitate the search for a suitable resting habitat and serve to refill glycogen stores needed for foraging and predator escape when landed.

Vertical and temporal flight patterns of coffee berry borer (Coleoptera: Curculionidae) in Hawaii.

Coffee berry borer (Hypothenemus hampei Ferrari) (Coleoptera: Curculionidae) is the most damaging insect pest of coffee worldwide, causing significant losses in coffee yields and quality. Knowledge of vertical and temporal flight patterns in coffee berry borer could be used to optimize spray timing and precision targeting of areas within the coffee tree, which may be more susceptible. In the present study, we estimated the vertical distribution of coffee berry borer females using traps set at 1-m intervals up to 5 m in height. We also quantified coffee berry borer infestation in the low, mid, and high canopy and documented fruit availability. Temporal flight patterns were estimated using timer traps, and correlation analyses were conducted to determine the relationship between the timing of daily flight and weather variables. Across the 4 study sites, we observed that 77%-84% of the trap catch was at 1 m, 11%-20% was at 2 m, and 1%-4% was at 3-5 m in height. Fruit infestation was significantly higher in the low branches (35%) relative to the high branches (17%). Flight height remained the same year-round, regardless of fruit availability. Coffee berry borer flew in low numbers during the day and night but peaked from 12 to 4 PM. Daily flight was positively correlated with an increase in air temperature and wind speed and negatively correlated with relative humidity. Findings from this study suggest that pesticide sprays should target low- to mid-level branches at 1-2 m in height and aim to be conducted in the early afternoon when coffee berry borer are actively flying and most vulnerable to chemical controls.

Extended state observer-based continuous finite time control for a fixed-wing vertical take-off and landing unmanned aerial vehicle

Dual system Vertical Take-Off and Landing (VTOL) fixed-wing Unmanned Aerial Vehicles (UAVs) combine the advantages of both the rotorcraft and fixed-wing UAV, characterizing horizontal and vertical flight capabilities. However, they face challenges such as aerodynamic coupling between propulsion systems, model uncertainty, and external disturbances, significantly deteriorating the tracking accuracy and robustness. This paper proposes an Extended State Observer based Continuous Finite-Time Control (ESO-CFTC) scheme to solve the attitude and trajectory tracking problem of such a VTOL fixed-wing UAV. Initially, the Nonsingular Fast Terminal Sliding Mode (NFTSM) controller guarantees the tracking errors reach a prescribed manifold in finite time with a continuous reaching law, then converge to the origin in finite time while avoiding singularities. To suppress the inherent chattering problem of the sliding mode control, an ESO technique is incorporated into the ESO-CFTC scheme, which is completely decoupled with the basic NFTSM controller. The supremum of the estimation residual set is derivated theoretically. The scheme forces the tracking errors to converge into the neighborhood of the origin in finite time without requiring prior knowledge of lumped disturbance supremum values. Finally, the numerical simulation results in a Matlab/Simulink environment demonstrate that the presented method outperforms existing benchmark in terms of disturbance estimation, tracking accuracy, convergence rate, and robustness.

Diversity and Flight Patterns of Caddisflies (Trichoptera) in an Atlantic Forest Fragment: Implications for Species Conservation in Threatened Ecosystems.

Anthropogenic activities have decimated the Atlantic Forest domain (AF) and increased the pressure on freshwater biota, such as Trichoptera, which is the most affected order by the current insect decline. Adult mobility is crucial for the colonisation of new environments unconnected by water sources. In this article, we describe the assemblage of caddisflies in a preserved AF fragment related to their functional feeding group and provide empirical data on the patterns of horizontal and vertical flight. Adults were collected using white sheet and light attraction traps, placed at different distances and heights from a stream in Pernambuco, Brazil. A total of 2934 specimens of 15 species from five families were collected, mostly collector-filterers. Horizontal flight was limited, with 80% of the abundance concentrated up to 20m. Vertical stratification was also concentrated at lower heights. A female-biased proportion was observed at higher strata. The richness and abundance of species decreased with increasing distances and heights from the stream. Overall, Chimarra sp. and Macrostemum scharfi were the dominant species. Trichoptera is a key taxon used as a biological indicator of water quality, and here, knowledge on the diversity and flight patterns of adults is expanded. At the risk of intensive pollution of rivers in Atlantic forests, data on the adult dispersal can be incorporated in the assessment of endangerment status and in conservation strategies.

The History of Vertical Flight and Air Medical Transport.

Vertical flight was sought-after by scientists for centuries, finding early beginnings in children's toys and slowly developing into the modern helicopter. For centuries, true success was elusive, and many machines failed to achieve safe, controlled flight. Despite slow progress, vertical flight enthusiasts were unrelenting and continued trialing new concepts until one finally rose above the rest. As technology advanced, the critical role of helicopters in medical evacuation became apparent. Throughout recent history, continuous progress in vertical flight technology has led to widespread and multifaceted use of helicopters in civilian and military medical operations.

Numerical comparison between symmetric and asymmetric flapping wing in tandem configuration

Dragonflies show impressive flight performance due to their unique tandem flapping wing configuration. While previous studies focused on forewing-hindwing interference in dragonfly-like flapping wings, few have explored the role of asymmetric pitching angle in tandem flapping wings. This paper compares the aerodynamic performance of asymmetric dragonfly-like wings with symmetric hummingbird-like wings, both arranged in tandem. Using a three-dimensional numerical model, we analyzed wing configurations with single/tandem wings, advance ratios (J) from 0 to 0.45, and forewing-hindwing phase differences (ϕ) from 0° to 180° at a Reynolds number of 7000. Results show that asymmetric flapping wings exhibit higher vertical force and flight efficiency in both single and tandem wing configurations. Increasing the phase difference (ϕ) improves flight efficiency with minimal loss of vertical force in the asymmetric flapping mode, while the symmetrical flapping mode significantly reduces vertical force at a 180° phase difference. Additionally, symmetric tandem flapping wings unexpectedly gain extra vertical force during in-phase flapping. This study uncovers the flow characteristics of dragonfly-like tandem flapping wings, providing a theoretical basis for the design of tandem flapping wing robots.

Total Energy-Based Control Design and Optimization for Lift-Plus-Cruise Aircraft

This paper presents a methodology for optimizing a Total Energy–based control system architecture for a lift-plus-cruise vertical takeoff and landing urban air mobility concept. The Total Energy Control System algorithm, which was originally developed for fixed-wing applications, is extended to also be applicable to hovering and transitioning flight. Control system parameters are optimized using a genetic algorithm optimization scheme, subject to constraints on dynamic stability and control response characteristics. Control system optimization and flight simulations are performed using the Modular Aircraft Dynamics and Control Algorithm Simulation Platform. Results presented include simulations of the optimized flight control system operation for maneuvering scenarios representative of hover, transition, and forward flight conditions as well as during transitions between vertical and forward flight modes. The presented results demonstrate the effectiveness of the proposed control system architecture and the demonstrated optimization methodology.

A new propulsion system GUI based control amenable model development for high‐power rockets

SummaryThis paper proposes a new algorithm to model and characterize an autonomous high‐power rocket using an indigenously developed graphical user interface (GUI) platform. This platform features a newly devised app, termed as THIEC Rocketry App which embeds the simulation based analysis to determine the design parameters of the rocket, required for a vertical flight. A solid propellant using potassium nitrate and sucrose, also known as rocket‐candy, is considered for the GUI development. The GUI facilitates the designer to specify the desired flight parameters for the rocket propulsion system. Various characteristic plots for visualization and analysis are made available in GUI. The obtained parameters from the GUI are then utilized in computer‐aided designing (CAD) for further identification of geometrical parameters like inertia tensor, center of gravity (CG) and center of pressure (CP). The mathematical control amenable model of the rocket is then developed using first principles so as to achieve an altitude up to 3 km. The overall system represents a complex nonlinear multi‐input multi‐output (MIMO) dynamics, having six degrees of freedom. The Newton‐Euler formulation is employed to develop the equations of motion. The attitude control using canards is analyzed via simulations for the complete flight path ‐ the boost and coast flights. Finally, the developed GUI based model is validated by practically manufacturing the components of the propulsion system for the small‐scale high‐power rocket. The proposed model will create the pathway for the development of some robust model‐based control schemes for such autonomous rockets in future.

Vertical profiles of temperature, wind, and turbulent fluxes across a deciduous forest over a slope observed with a UAV

To contribute to a better understanding of the dynamics of the atmosphere inside and above a forest, vertical profiles are flown with a remotely-controlled multicopter in the Steinkrug forest. This area is located over a slope in the Solling natural area in Lower Saxony (Germany), composed mostly of deciduous trees about 30 m tall. Fifteen vertical flights made near sunset between summer 2019 and spring 2020 were inspected from the surface to 100 m above ground level. These measurements provide information on the vertical structures of wind and temperature within and above the canopy, including the effects of shallow slope flows near the ground. Contrasting measurements downhill outside the forest were also made. The gathered data allow estimated profiles of the turbulent fluxes of sensible heat and momentum to be obtained by computing averages and fluctuations for layers of 5 m depth. A leaf area density profile in both leafy and leafless conditions could also be produced. The presence of a slope flow is inspected at both sites, and the applicability of existing theories is explored.

Experimental Nonlinear and Incremental Control Stabilization of a Tail-Sitter UAV with Hardware-in-the-Loop Validation

Tail-sitters aim to combine the advantages of fixed-wing aircraft and rotorcraft but require a robust and fast stabilization strategy to perform vertical maneuvers and transitions to and from aerodynamic flight. The research conducted in this work explores different nonlinear control solutions for the problem of stabilizing a tail-sitter when hovering. For this purpose, the first controller is an existing strategy for tail-sitter control obtained from the literature, the second is an application of Nonlinear Dynamic Inversion (NDI), and the last one is its incremental version, INDI. These controllers were implemented and tuned in a simulation in order to stabilize a model of the tail-sitter, complemented by estimation methods that allow the feedback of the necessary variables. These estimators and controllers were then implemented in a microcontroller and validated in a Hardware-in-the-Loop (HITL) scenario with simple maneuvers in vertical flight. Lastly, the developed control solutions were used to stabilize the aircraft in experimental flight while being monitored by a motion capture system. The experimental results allow the validation of the model of the X-Vert and provide a comparison of the performance of the different control solutions, where the INDI presents itself as a robust control strategy with accurate tracking capabilities and less actuator demand.

A novel dragonfly dual-wing hovering flight model

During the hovering flight of dragonflies, the coupling interaction between the forewings and hindwings leads to a reduction in the lift of each wing. Numerous scholars have reached a unanimous conclusion that under the coupling effect, the lift of the hindwings is significantly decreased. Meanwhile, the coupling of the forewings and hindwings enhances the controllability of dragonfly flight. In this article, a novel hovering flight model termed the partial advanced dual-wing model (PADM) is proposed. This model is capable of increasing the lift of both the forewings and hindwings. The maximum average lift of the forewings is increased by 18.09%, and the maximum average lift of the hindwings is increased by 41.58%. In addition to the shared advantage of enhanced positive pressure on the rear half of the wing surface due to the advanced rotation, the superior performance of the hindwings compared to the forewings is attributed to the hindwings cutting off the trailing-edge vortex ring formed by the coupling of the fore and hind wings during the downstroke phase. The vertical force and energy consumption exhibit a linear relationship with the partially advanced time, independent of the coupled aerodynamic effects. The PADM model not only sustains the weight of the dragonfly but also plays a controlling role in transitioning from a hovering flight model to a vertical leap flight model. Furthermore, it enables dragonflies and micro air vehicles to maintain hovering flight while carrying additional loads.

Development of a tube-launched tail-sitter unmanned aerial vehicle

This paper describes the development and flight testing of a tail-sitter unmanned aerial vehicle (UAV) platform that has the potential to be tube-launched. Integrated with vertical take-off and landing (VTOL) capability, the platform can perform high-endurance loiter tasks along with low-speed and hovering flights, and land vertically in limited spaces. The current design utilizes a thrust-vectored counter-rotating coaxial propeller combined with a foldable conventional fixed-wing to accomplish both vertical and horizontal flights. A feedback control strategy capable of stabilizing and controlling the vehicle in both vertical and horizontal flight was designed and implemented on a custom-designed 1.7-gram autopilot. Several tests were conducted in hover and edgewise flight with aggressive pilot inputs to demonstrate the controllability of the aircraft even in the presence of moderate wind gusts. Additionally, transition flight tests were also performed, which demonstrated the vehicle’s capability of transitioning from vertical to fixed-wing horizontal flight using thrust-vectoring-based pitch control.

Analysis of vertical flight efficiency in European countries with extended intuitionistic fuzzy TOPSIS method

Analysis of vertical flight efficiency in European countries with extended intuitionistic fuzzy TOPSIS method

Analysis of vertical flight efficiency in European countries with extended intuitionistic fuzzy TOPSIS method

Analysis of vertical flight efficiency in European countries with extended intuitionistic fuzzy TOPSIS method