High-speed Flight Research Articles

Aeroacoustic Analysis of a Lift-Offset Coaxial Rotor Using High-Fidelity CFD/CSD Loose Coupling Simulation

This paper presents the aeroacoustic analysis of a lift-offset coaxial rotor in high-speed forward flight using the high-fidelity computational fluid dynamics/computational structural dynamics (CFD/CSD) loose coupling software Helios. Acoustic simulations are performed using the software PSU-WOPWOP at eight microphones positioned below the coaxial rotor. The total power of the three speed cases—100, 150, and 200 kt—is validated against flight-test data and shows good agreement. A series of parametric studies is also conducted to investigate the effect of lift offset, flight speed, and rotor-to-rotor separation distance on acoustics of the coaxial rotor. Strong blade-crossover and self-blade–vortex interaction events of the coaxial rotor, which are major sources of loading noise, are captured via high-fidelity CFD simulations in all speed cases. Highly impulsive acoustic pressure signals are identified in all simulation cases, and the magnitude of mid-frequency sound pressure level (SPL) increases significantly with increasing flight speed and lift offset. The strength of mid-frequency SPL, on the other hand, is reduced significantly as the rotor-to-rotor separation distance increases at 100 kt. However, the higher speed cases do not show a significant reduction in mid-frequency SPL with increasing separation distance.

Autonomous Flight

This review surveys the current state of the art in the development of unmanned aerial vehicles, focusing on algorithms for quadrotors. Tremendous progress has been made across both industry and academia, and full vehicle autonomy is now well within reach. We begin by presenting recent successes in control, estimation, and trajectory planning that have enabled agile, high-speed flight using low-cost onboard sensors. We then examine new research trends in learning and multirobot systems and conclude with a discussion of open challenges and directions for future research.

The complexity and mechanism of transonic aeroelastic problems

In high-speed flight, transonic aeroelasticity commonly exists, leading to various problems such as transonic flutter, buffeting and buzz. To solve these problems, a good understanding of the underlying physical mechanisms is critical. Unfortunately, the nonlinearity and unsteadiness of airflows complicate this process and consequently impede the development of aircraft design. Here using the numerical simulation and modeling of complex transonic flows, we developed a unified analytical method for aeroelastic stability and response problems. We then explored the mechanisms of three complex aeroelastic phenomena and unveiled their relationships. (1) Transonic buzz is in essence a single degree of freedom (SDOF) flutter caused by the coupling between the most unstable fluid mode and structural mode. SDOF flutter of this kind is possible only when the fluid exhibits sufficiently low damping; that is, the freestream flow is near the buffet boundary or in the low supersonic zone. Besides, the unstable frequency boundaries are determined by zero and pole of the open loop system. (2) The frequency lock-in phenomenon in the transonic buffeting flow is caused by the SDOF flutter in the unstably separated flow rather than resonance. In this process, the response undergoes a transition from the forced vibration to the self-excited flutter, which results in a deviation of the lock-in region from the resonance point. By contrast, the traditional uncoupled method misestimates the risk range and underestimates the amplitude of a vibration. (3) When the pitching degree of freedom is released, transonic buffet will be induced at a lower angle of attack, indicative of the drawbacks of predicting buffet onset by a rigid model. Actually, the elastic characteristic plays a key role in predicting buffet onset in aircraft design. Collectively, the large-amplitude structural vibration in the transonic flow is closely related to aeroelastic instability. The dynamic linear model can correctly predict the boundary of some complex dynamic phenomena. The complexity of transonic aeroelasticity mainly arises from the stability reduction of airflows. Different from classical aeroelastic problems, the new fluid mode is derived from the reduction of fluid stability. The coupling between such a fluid mode and structural mode often leads to complex phenomena in transonic flows.

Study on the High-Speed and Close of the UVA Cooperative Formation Controller Design

Aiming at the high-speed flight of the UAVs cooperative formation, when a single UAV has occurred, need to exit the formation flight and be close or super close to form of the formation quickly. A fast close cooperative formation controller design method is proposed to make up for low the fighting robustness, and be shortcomings of timeliness poorly and analyze the dynamic characteristic of UAV formation flight. Taking the external factors known into consideration, setting up for the longitude maneuver of nonlinear thrust vector and unsteady aerodynamic model, according to the formation velocity, flat tail rudder angle and thrust vector and pitch angle velocity for corresponding input commend signals for the controller to research the dynamic characteristic of UAV formation flight. Meanwhile, the formation flight distance error is the convergence to a fixed value, and the stability of the cooperative formation flight is good. The simulation of results show that the controller can effectively improve the speed of the close or super close to formation, and maintain the stability of the formation flight, which provides a method of the close or super close formation flight controller design.

Review of vertical take-off and landing fixed-wing UAV and its application prospect in precision agriculture

Abstract: Compared with multi-rotor unmanned aerial vehicle (UAV) and fixed-wing UAV, vertical take-off and landing (VTOL) fixed-wing UAV has advantages such as good aerodynamic efficiency, high cruise speed and long flight duration, and has low requirements for the flatness and area of the landing site. Therefore, VTOL fixed-wing UAVs are widely used in many fields such as remote sensing, power line inspection, geological mapping and urban comprehensive patrol. However, there are still some problems in VTOL fixed-wing UAV, such as large aerodynamic interference of tilting propeller, high requirement of design strength of VTOL structure and complex power system matching in various flight states, which make it impossible to have a complete and reliable safety evaluation to guarantee the healthy development of VTOL fixed-wing UAV. In this paper, different types of VTOL fixed-wing UAVs are classified from aerodynamic layout and take-off mode, such as rotor, tilt-wing and tail-seat type. At the same time, the data of VTOL fixed-wing UAVs with certain representative design are compared, including maximum flight speed, maximum takeoff weight, range and flight time, etc. In addition, this paper also puts forward the application method and current situation of VTOL in precision agriculture. Finally, the future research direction and development suggestions are put forward to provide references for the performance improvement and further research of new VTOL fixed-wing UAV in the future. Keywords: vertical take-off and landing, fixed-wing, unmanned aerial vehicle, tilt rotor, composite wing DOI: 10.33440/j.ijpaa.20200304.130 Citation: Zhou M J, Zhou Z Y, Liu L H, Huang J, Lyu Z C. Review of vertical take-off and landing fixed-wing UAV and its application prospect in precision agriculture. Int J Precis Agric Aviat, 2020; 3(4): 8–17.

Research on Thermal Insulation Performance of Lightweight Thermal Protection Materials for High Speed Aircraft under Different Boundary Conditions

The determination of thermal insulation performance of thermal protection materials or structures is an indispensable and important step in the safety design of high speed flight vehicles. To obtain the temperature difference of the radiating surface for plate specimens under three different boundary conditions in heat insulation experiments (the specimens were placed either vertically or horizontally with the radiating surface facing down or horizontally with the radiating surface facing up), three thermal test setups were established to test the thermal insulation performance of light-weight ceramic specimens at different temperatures. The results show that the radiating surface temperature was the highest when the specimen was placed horizontally with the radiating surface facing down, while it was the lowest when the specimen was placed horizontally with the radiating surface facing up.The numerical calculation results agreed very well with the experimental ones, confirming the credibility and accuracy of the experimental results. The different thermal insulation performances of the plate specimens obtained under three different boundary conditions will provide important guidance for designers in the design of thermal protection systems for large cabins of high speed flight vehicles.

Nonlinear Random Response Analyses of Panels Considering Transverse Shear Deformations under Combined Thermal and Acoustic Loads

The panel structures of flight vehicles at supersonic or hypersonic speeds are subjected to combined thermal, acoustic, and aerodynamic loads. Because of the combined thermal and acoustic loads, the panel structure may exhibit nonlinear random vibration responses, such as the snap‐through phenomenon and random vibrations. These unique dynamic behaviors of the panel structure under combined thermal and acoustic loads can result in serious damage or fatigue failure of the panel structures of high‐speed flight vehicles. This study investigates the nonlinear random responses of thin and thick panels under combined thermal and acoustic loads. The panels are modeled based on the first‐order shear deformation theory (FSDT) to account for transverse shear deformations. The von‐Karman nonlinear strain–displacement relationship is used for geometric nonlinearity in the out‐of‐plane direction of the panel. The thermal load distribution is assumed to be constant in the thickness direction of the panel. The random acoustic load is represented as stationary White–Gaussian random pressure with zero mean and uniform magnitude over the panels. Static and dynamic equations are derived using the principle of virtual work and the nonlinear finite element method. A thermal postbuckling analysis is conducted using the Newton–Raphson method, and the dynamic nonlinear equations are solved using the Newmark‐β time integration method. In the present numerical analyses, the snap‐through responses for both the thin and thick panels are investigated, and the results indicate that the loading conditions that cause snap‐through are different for thin and thick panels.

Analysis of Infrared Radiation Characteristics and Penetration Measures of Warhead

Ballistic target recognition occupies a unique and important position in many application fields of target recognition because of its challenge and important position of ballistic missile defense in national security; recognition time of defense system becomes very limited because of ballistic missile high-speed flight; recognition distance of defense system is also due to stealth technology. The integrated application of active jamming and passive decoy greatly increases the difficulty of identification of defense system. Because of its special status and challenge, ballistic target recognition has attracted wide attention of researchers at home and abroad, making it one of the most important issues in infrared target recognition research at home and abroad. In this paper, the infrared characteristics of a ballistic missile warhead target/decoy are analyzed, and the corresponding penetration measures are put forward according to the analysis results.

Collision Avoidance in Fixed-Wing UAV Formation Flight Based on a Consensus Control Algorithm

This paper addresses a collision avoidance problem for multiple unmanned aerial vehicles (UAVs) in the process of high-speed flight, thereby enabling UAV cooperative formation flight and effective mission completion. The main contribution is to propose a collision avoidance control algorithm for a multi-UAV system based on a bi-directional network connection structure. To effectively avoid collisions between UAVs and between UAVs and obstacles, the proposed consensus-based algorithm and a “leader-follower” control strategy are simultaneously applied for UAV formation control to ensure the convergence of the formation. Each of the UAVs has the same forward velocity and heading angle in the horizontal plane, and they maintain a constant relative distance in the vertical direction. This paper proposes a consensus-based collision avoidance algorithm for multiple UAVs based on an improved artificial potential field method. Simulation tests involving multiple UAVs were performed to validate the proposed control algorithm and to provide a reference for engineering applications.

In-flight tracking and vibration control using the DLR’s multiple Swashplate system

This paper discusses the design, integration, and test of a higher harmonic control algorithm capable of both vibration control and in-flight blade tracking in conjunction with DLR’s multiple swashplate control system (META), while honoring predefined limits in usable control authority. The design of the control algorithm is described in detail and the results of coupled numerical investigations with both a general purpose multibody code by Politecnico di Milano and DLR’s comprehensive rotor code to determine the algorithm’s performance are presented. The integration of the control algorithm into the real-time control software is shown for the META system, where, for safety reasons, a semi-open loop approach was implemented. First tests of the controllers in-flight tracking mode to reduce 1/rev loads during hover yielded an almost complete reduction in 1/rev vibratory loads while maintaining constant rotor thrust. Following the experiments at DLR’s own facility, extensive wind-tunnel tests were performed in 2016 with META and a 5-bladed rotor system at the large low-speed facility of the German Dutch Wind Tunnels. The control algorithm was adapted to the 5-bladed rotor and successfully applied for in-flight blade tracking as well as the reduction of 5/rev hub loads using multi-harmonic pitch inputs with frequencies from 4/rev to 6/rev in cruise and high-speed flight condition. In both cases, the controller showed excellent performance and yielded satisfactory reductions of 1/rev rotor imbalances as well as a reduction of 5/rev hub vibrations by more than $$80\%$$ , while, at the same time, adhering to user set limits for the higher harmonic control amplitudes.

From Aeroacoustics Basic Research to a Modern Low-Noise Rotor Blade

In 2015, Airbus Helicopters unveiled the secrecy around its Dauphin successor and presented the all new H160 helicopter. A special feature that immediately attracted attention was its unusual and revolutionary fore-aft swept main rotor blade. This design, which aims to significantly reduce the blade-vortex interaction noise signature and reduce high-speed forward flight power requirements, was first investigated in the 1990s. At this time, DLR and ONERA formed a joint team to acoustically optimize a rectangular reference rotor blade. Based on state-of-the-art comprehensive rotor codes and a 50/50 work share, the ERATO rotor blade design was developed, patented worldwide and tested on both a rotor test rig and in the wind tunnel. Airbus Helicopters (then: Eurocopter) adopted the design, optimized hover and forward flight high lift performance and prepared it for serial production, as the Blue EdgeTM rotor blade on the Airbus Helicopters H160 helicopter.

Visualization of Projectile Flying at High Speed in Dusty Atmosphere

Considering a spacecraft that encounters particle-laden environment, such as dust particles flying up over the regolith by the jet of the landing thruster, high-speed flight of a projectile in such environment was experimentally simulated by using the ballistic range. At high-speed collision of particles on the projectile surface, they may be reflected with cracking into smaller pieces. On the other hand, the projectile surface will be damaged by the collision. To obtain the fundamental characteristics of such complicated phenomena, a projectile was launched at the velocity up to 400 m/s and the collective behaviour of particles around projectile was observed by the high-speed camera. To eliminate the effect of the gas-particle interaction and to focus on only the effect of the interaction between the particles and the projectile's surface, the test chamber pressure was evacuated down to 30 Pa. The particles about 400μm diameter were scattered and formed a sheet of particles in the test chamber by using two-dimensional funnel with a narrow slit. The projectile was launched into the particle sheet in the tangential direction, and the high-speed camera captured both projectile and particle motions. From the movie, the interaction between the projectile and particle sheet was clarified.

Interplay of Surface Deformation and Shock-Induced Separation in Shock/Boundary-Layer Interactions

Shock wave/boundary-layer interactions represent an extreme loading condition in the structural design of high-speed flight vehicles. However, the complexity of this problem has primarily restricted study to rigid, undeformed surfaces. As such, little is known regarding the impact surface deformation, due to structural compliance, has on the induced loads. The current study focuses on this problem by comparing changes in the shock-induced separation bubble length, as well as the resulting changes in the surface pressure distribution and integrated structural loads, for both static and unsteady surface deformation using a Reynolds-averaged Navier–Stokes flow solution. Results examine the differences due to the frequency of deformation, relative length between the separation bubble and deformation, and the deformation mode. Surprisingly, unsteady surface deformation is found to result in only marginal changes to the separation bubble length, whereas static surface deformation can result in both significant increases and decreases. This is linked, through analytical and numerical studies, to dependency of separation onset on both surface curvature and the spatial gradient of surface velocity, which have competing effects for harmonic motion. These findings provide important insight into the potential impact of surface compliance on shock-induced structural loads and guide the development of model reduction strategies for shock-dominated flows.

HiQuadLoc: A RSS Fingerprinting Based Indoor Localization System for Quadrotors

Indoor localization for quadrotors has attracted much attention recently. While efforts have been made to perform location estimation of quadrotors leveraging dedicated indoor infrastructures, the low-cost and commonly used RSS fingerprinting based approach utilizing existing Wi-Fi APs has yet to be applied. The challenge is that the high-speed flight reduces the RSS measuring opportunities for fingerprints comparison; moreover, the 3D space fingerprints collection incurs more overhead than in the traditional 2D case. In this paper, we present HiQuadLoc, a RSS fingerprinting based indoor localization system for quadrotors. We propose a series of mechanisms including path estimation, path fitting, and location prediction to deal with the negative influence incurred by the high-speed flight; moreover, we develop a 4D RSS interpolation algorithm to reduce the site survey overhead, where 3D is for the indoor physical space and 1D is for the RSS sample space. Experimental results demonstrate that HiQuadLoc reduces the average location error by more than 50 percent compared with simply applying the RSS fingerprinting based approach for 2D localization, and the overhead of RSS training data collection is reduced by more than 80 percent.

Development of an Unmanned Hybrid Vehicle Using Artificial Pectoral Fins

AbstractAn unmanned vehicle has been developed for dual use as both an aircraft and a submersible. To achieve long-range emplacement of a highly maneuverable underwater asset to a target environment, the Flimmer (Flying-Swimmer) vehicle is designed for both high-speed flight and low-speed swimming. Building on previous research in bioinspired propulsion and control systems, the vehicle employs a unique set of artificial flapping fins for underwater maneuvering, which must be considered when evaluating the flight and water landing capabilities. This paper describes the computational analysis and experimental results for all three phases of vehicle operation—flight, landing, and swimming. Computational fluid dynamics simulation results predict aero- and hydrodynamic characteristics and demonstrate landing loads on and trajectory of the vehicle. Experimental data demonstrate flight and swimming performance and validate the computational results, and experimental testing of water landing provides a comparison with computations. Results and analyses of the Flimmer vehicle performance demonstrate the operational capabilities of an unmanned hybrid vehicle for long-range flight and low-speed swimming.

Minimum Induced Power for a Rotor with a Finite Number of Blades

A dynamic inflow model is used to calculate minimum induced power for a helicopter in high-speed forward flight with a finite number of blades. Comparisons between analytical and numerical results are shown, and they show good agreement. Different flow conditions (such as with and without reverse flow or inflow feedback) are used to show how each condition affects optimum induced power. Several results confirm the findings of earlier investigations, such as a singularity in rotor power in reverse flow and induced power reduction with an increase in blade number. One of the new findings is that the induced power is affected by rotor solidity (in the absence of inflow feedback) and the effect of inflow feedback does not always reduce the induced power. Results obtained using higher harmonic blade pitch control show that induced power can thereby be reduced for all conditions with a finite number of blades.

Aerostructural optimization of a morphing wing for airborne wind energy applications

Airborne wind energy (AWE) vehicles maximize energy production by constantly operating at extreme wing loading, permitted by high flight speeds. Additionally, the wide range of wind speeds and the presence of flow inhomogeneities and gusts create a complex and demanding flight environment for AWE systems. Adaptation to different flow conditions is normally achieved by conventional wing control surfaces and, in case of ground generator-based systems, by varying the reel-out speed. These control degrees of freedom enable to remain within the operational envelope, but cause significant penalties in terms of energy output. A significantly greater adaptability is offered by shape-morphing wings, which have the potential to achieve optimal performance at different flight conditions by tailoring their airfoil shape and lift distribution at different levels along the wingspan. Hence, the application of compliant structures for AWE wings is very promising. Furthermore, active gust load alleviation can be achieved through morphing, which leads to a lower weight and an expanded flight envelope, thus increasing the power production of the AWE system. This work presents a procedure to concurrently optimize the aerodynamic shape, compliant structure, and composite layup of a morphing wing for AWE applications. The morphing concept is based on distributed compliance ribs, actuated by electromechanical linear actuators, guiding the deformation of the flexible—yet load-carrying—composite skin. The goal of the aerostructural optimization is formulated as a high-level requirement, namely to maximize the average annual power production per wing area of an AWE system by tailoring the shape of the wing, and to extend the flight envelope of the wing by actively alleviating gust loads. The results of the concurrent multidisciplinary optimization show a 50.7% increase of extracted power with respect to a sequentially optimized design, highlighting the benefits of morphing and the potential of the proposed approach.

Planning Dynamically Feasible Trajectories for Quadrotors Using Safe Flight Corridors in 3-D Complex Environments

There is extensive literature on using convex optimization to derive piece-wise polynomial trajectories for controlling differential flat systems with applications to three-dimensional flight for Micro Aerial Vehicles. In this work, we propose a method to formulate trajectory generation as a quadratic program (QP) using the concept of a Safe Flight Corridor (SFC). The SFC is a collection of convex overlapping polyhedra that models free space and provides a connected path from the robot to the goal position. We derive an efficient convex decomposition method that builds the SFC from a piece-wise linear skeleton obtained using a fast graph search technique. The SFC provides a set of linear inequality constraints in the QP allowing real-time motion planning. Because the range and field of view of the robot's sensors are limited, we develop a framework of Receding Horizon Planning , which plans trajectories within a finite footprint in the local map, continuously updating the trajectory through a re-planning process. The re-planning process takes between 50 to 300 ms for a large and cluttered map. We show the feasibility of our approach, its completeness and performance, with applications to high-speed flight in both simulated and physical experiments using quadrotors.

VTOL 타입의 복합형 비행체에 적용가능한 다수 프로펠러 기반 자세제어기법의 개발

In recent decades, many researchers have been struggling to developing the compound aircraft that is capable high speed and VTOL flight. And in recent years, multi-copters are very popular because of having advantages of VTOL and easy handling, but they are lack of doing long-range mission. Therefore, we presents simple aircraft architecture which is equipped fixed wing, multi propellers and no control surfaces. In this paper, we designed the attitude control for the compound aircraft prototype and measured the attitude control performance with flight test for validating prototype’s performance. We analysed the attitude control test result comparing with similar size of a fixed wing aircraft. The performance was almost same as fixed wing aircraft.

Optimal deployment schedule of an active twist rotor for performance enhancement and vibration reduction in high-speed flights

The best active twist schedules exploiting various waveform types are sought taking advantage of the global search algorithm for the reduction of hub vibration and/or power required of a rotor in high-speed conditions. The active twist schedules include two non-harmonic inputs formed based on segmented step functions as well as the simple harmonic waveform input. An advanced Particle Swarm assisted Genetic Algorithm (PSGA) is employed for the optimizer. A rotorcraft Computational Structural Dynamics (CSD) code CAMRAD II is used to perform the rotor aeromechanics analysis. A Computation Fluid Dynamics (CFD) code is coupled with CSD for verification and some physical insights. The PSGA optimization results are verified against the parameter sweep study performed using the harmonic actuation. The optimum twist schedules according to the performance and/or vibration reduction strategy are obtained and their optimization gains are compared between the actuation cases. A two-phase non-harmonic actuation schedule demonstrates the best outcome in decreasing the power required while a four-phase non-harmonic schedule results in the best vibration reduction as well as the simultaneous reductions in the power required and vibration. The mechanism of reduction to the performance gains is identified illustrating the section airloads, angle-of-attack distribution, and elastic twist deformation predicted by the present approaches.