Hover Flight Research Articles

Winging it: hummingbirds alter flying kinematics during molt.

Hummingbirds are well known for their hovering flight, one of the most energetically expensive modes of locomotion among animals. Molt is a costly event in the annual cycle, in which birds replace their feathers, including all their primary feathers, which, in hummingbirds, comprise most of the area of the wing. Despite this, the effects of molt on hovering flight are not well known. Here, we examined high-speed videos (14 individuals of three species from the Colombian Andes recorded at 1200 frames per second) comparing molting and non-molting hummingbirds' wing kinematics and wingtip trajectories. We found that molting hummingbirds rotated their wings in more acute angles during both downstroke and upstroke compared to non-molting individuals (10° versus 20°, and 15° versus 29°, respectively), while other flight parameters remained unchanged. Our findings show that hummingbirds are capable of sustaining hovering flight and thereby maintaining their weight support even under impressive wing area reductions by adjusting their stroke amplitudes.

Machine Learning‐Based Wind Classification by Wing Deformation in Biomimetic Flapping Robots: Biomimetic Flexible Structures Improve Wind Sensing

Flying animals such as insects and birds have strain receptors on their wings. The functions and information processing capabilities of these strain receptors, however, are largely unknown. Herein, the potential ability of wing strain sensors to detect wind direction in flapping flight is experimentally explored. Seven strain gauges are attached to the biomimetic wing shafts of flexible wings. The wings flap through an electric mechanism emulating hovering flight in a wind tunnel with gentle wind, and a convolutional neural network model for wind direction classification is developed. The results show that with only one strain gauge, accurate wind classification is achieved using segmented data of ≈1 flapping cycle. Even with segmented data of only 0.2 flapping cycles, wind classification can be achieved with all seven strain gauges. Moreover, the use of wing shafts improves the classification accuracy, especially with shorter segmented data. Additional flapping phase information does not substantially improve classification performance. These results suggest that hovering animals determine the wind direction via strain sensing, which is useful for flight control. The implementation of wing strain sensors in flapping‐wing aerial robots may improve flight control ability.

The thrust balance model during the dragonfly hovering flight.

In recent years, the micro air vehicle (MAV) oscillations caused by thrust imbalances have received more attention. This paper proposes a dual-wing thrust balance model (DTBM) that can solve the above problem by iterating the modified rotation angle formula. The core control parameter of the DTBM model is the au angle, which refers to the angle between the wing surface and the stroke plane at the mid-stroke position during the upstroke. For each degree change in the au angle, the range of variation in the dimensionless average thrust coefficient is between 0.0225 and 0.0268. A thrust coefficient of 0.0225 causes the dragonfly to move forward by 9.037 cm in one second, which is equivalent to 1.29 times its body length. By using DTBM, the average thrust coefficient can be reduced to below 0.001 in just a few iterations. No matter how complex the motion pattern is, the DTBM can achieve thrust balance within 0.278 seconds. Through our research, when selecting the deviation angle motion of real dragonflies, the dual-wing au angles exhibit a highly linear correlation with wing spacing, called linear motion. In contrast, the nonlinear variation of the au angle appears in the hindwing of the no-deviation motion and the forewing of the elliptical deviation motion. All of the nonlinear changes are referred to as nonlinear motion. Nonlinear variation of the au angle arises from larger disturbances of the lateral force during the upstroke. The stronger lateral force is closely related to the flapping trajectory. When the flapping trajectory causes the dual-wing to closely approach each other in the mid-stroke, a continuous positive pressure zone forms between the dual-wing. The collision of the leading-edge vortex and the shedding of the trailing-edge vortex is the special flow field structure in the nonlinear motion. Guided by the DTBM, future designs of MAVs will be able to better achieve thrust balance during hovering flight, requiring only the embedding of the iteration algorithm and prediction function of the DTBM in the internal chip.

Hover Flight Improvement of a Quadrotor Unmanned Aerial Vehicle Using PID Controllers with an Integral Effect Based on the Riemann–Liouville Fractional-Order Operator: A Deterministic Approach

The hovering flight of a quadrotor Unmanned Aerial Vehicle (UAV) refers to maintaining the aircraft in a fixed position in the air, without lateral, vertical, or rotational movements, using only the vehicle’s control systems to maintain proper balance in all spatial dimensions. Algorithms and control systems have been developed to continuously adjust motor speeds to counteract deviations from the desired position and achieve effective hovering flight. This paper proposes a set of PID controllers with an integral effect based on the Riemann–Liouville fractional-order approach to improve the hovering flight of a quadrotor UAV. This research innovates by introducing a set of fractional-order PID controllers for UAV hover stability, which offer better adaptability to non-linear dynamics and robustness than traditional PID controllers. Also presented is the development of new performance metrics (MSE, BQC-LR), which allow for more comprehensive control system evaluations. A thorough comparative analysis with conventional control methods demonstrates the superior performance of fractional-order control in real-world simulations. The numerical simulation results show the effectiveness of the proposed Fractional Integral Action PID Controller in the control of UAV hovering flight, while comparative analyses against a classical controller emphasize the benefits of the fractional-order approach in terms of control accuracy.

Aeroacoustic analysis of fixed- and variable-pitch controlled multirotors and noise reduction via rotor phase control

This research conducts a comparative analysis of the aerodynamic and aeroacoustic characteristics of fixed-pitch and variable-pitch controlled multirotors (i.e., revolution per minute and collective pitch control). The study encompasses hovering and forward flight conditions, considering wind gust effects. Specifically, high-resolution time-frequency analysis is conducted to investigate the interference and modulation effects of multirotor noise. The analysis reveals significant variations in spectral characteristics depending on the control method. Notably, the frequencies of tonal noise from variable-pitch controlled multirotor do not exhibit short-periodic modulation, in contrast to those of fixed-pitch controlled multirotor. Considering the distinctive spectral characteristics of variable-pitch controlled multirotor, this study explores the effectiveness of noise reduction in variable-pitch controlled multirotor by implementing rotor phase control methods. To validate the noise reduction in time series simulations, a deep learning model is employed to determine the dynamically optimized rotor phases. A multilayer perceptron neural network was trained to derive the optimal rotor phase angles over time, considering the target observer points and flight conditions. The results demonstrate a substantial reduction in noise at the target observer points.

Aerodynamic analysis of rotor-to-rotor interactions in different octocopter configurations

Rotor-to-rotor interaction among neighboring rotors of a multirotor has great significance for aerodynamically efficient multirotor design. Current research is conducted to analyze aerodynamic performance of different octocopter configurations amid hover and forward flight. Conventional and coaxial configurations are studied and a hybrid configuration is also proposed to rectify the disadvantages associated with the earlier two. Comparison is carried out for the aforementioned configurations along with comparison of coaxial and hybrid octocopters with bigger diameter rotors in the same confined space for high thrust requirement missions. Vertical spacing of coaxial configuration is also studied. Virtual Blade Method (VBM) is considered herein due to its great computational efficiency. The results show that there are 11.89% and 14.22% loss in thrust for coaxial octocopter compared to conventional and hybrid configurations with normal size rotors and 15.61% loss compared to hybrid configuration with bigger rotors in hover, whereas coaxial square configuration performs the worst in forward flight with a lift loss of 9.1%, 14.77% and 18.8% compared to coaxial diamond, conventional and hybrid configurations with normal size rotors and 9.96% and 17.82% loss compared to coaxial diamond and hybrid configurations with bigger rotors. Combined FM shows that hybrid configuration outperforms other octocopter configurations in overall aerodynamic performance.

Numerical simulation investigation on aerodynamic characteristics of rotor in plateau environment

To investigate the impact of high-altitude environments on helicopter rotor aerodynamic characteristics, this study developed a computational model that integrates CFD, BP neural networks, and the free wake method. The accuracy of the model was validated through wind tunnel experiments and comparisons with CFD results. Using this model, the effects of altitude and temperature at high altitudes on rotor aerodynamic characteristics during hover and forward flight were studied. The results indicate that with constant temperature, rotor thrust significantly decreases as altitude increases, primarily due to reduced air density. As altitude rises with unchanged temperature, the decrease in air density directly affects the lift and thrust generated by the rotor, leading to a decline in hover performance. Furthermore, the study found that rotor aerodynamic coefficients do not vary significantly across different altitudes. Further analysis suggests this stability is due to the high Reynolds number in the calculated state. In the high Reynolds number range, changes in Reynolds number have minimal impact on aerodynamic coefficients, resulting in relatively stable aerodynamic performance across varying altitudes. At a constant altitude, lower temperatures lead to slight increases in rotor thrust and torque, while the thrust coefficient and power coefficient remain almost unaffected. This phenomenon is attributed to changes in air density and viscosity caused by the lower temperatures. The increase in air density positively impacts thrust and torque, while the reduction in air viscosity decreases the rotor's surface frictional resistance. This partially offsets the increase in lift and drag caused by the higher air density, leading to negligible changes in thrust and power coefficients.

Design and Performance of a Novel Tapered Wing Tiltrotor UAV for Hover and Cruise Missions

This research focuses on a novel convertible unmanned aerial vehicle (CUAV) featuring four rotors with tilting capabilities combined with a tapered form. This paper studies the transition motion between multirotor and fixed-wing modes based on the mechanical and aerodynamics design as well as the control strategy. The proposed CUAV involves information about design, manufacturing, operation, modeling, control strategy, and real-time experiments. The CUAV design considers a fixed-wing with tiltrotors and provides the maneuverability to perform take-off, hover flight, cruise flight, and landing, having the characteristics of a helicopter in hover flight and an aircraft in horizontal flight. The manufacturing is based on additive manufacturing, which facilitates the creation of a lattice structure within the wing. The modeling is obtained using the Newton–Euler equations, and the control strategy is a PID controller based on a geometric approach on SE(3). Finally, the real-time experiments validate the proposed design for the complete regime of flight, and the research meticulously evaluates the feasibility of the prototype and its potential to significantly enhance the mission versatility.

A novel efficient calculation method for rotor aeroacoustic characteristics based on multiple aerodynamic surfaces source model

Based on the simplified acoustic source model of multiple aerodynamic surfaces, an efficient calculation method for rotor aeroacoustic characteristics has been established. Firstly, the aerodynamic characteristics database of the airfoil is obtained based on the Computational Fluid Dynamics (CFD) method. The inflow environment of each blade element is calculated using the free wake method, and the distribution of chord-wise loading is obtained by combining the established database. Subsequently, based on the F1A equation, a simplified formula for loading noise with pressure difference as a parameter is derived. The thickness noise is obtained from the blade shape and operating conditions, and the blade surface mesh is constructed by using the adaptive curvature distribution strategy of airfoil points. The validation is carried out by comparing with experimental values in the hover and forward flight states. Then, A hybrid weighted interpolation scheme is proposed based on inverse distance weighted interpolation (IDW) and bidirectional linear interpolation method, which is suitable for capturing blade/vortex interaction (BVI) noise. A set of parameters suitable for engineering applications is obtained through the parametric sensitivity analysis. The computational time and memory usage are reduced to 1/47 and 1/4 of the original amount, respectively. Finally, Using the above method, the influence of rotational speeds (ω) on rotor aeroacoustic characteristics are studied. The results show that ω and sound pressure level (SPL) are linearly related and the SPL decreases by approximately 5 dB for every 0.1 Ω decrease in ω. The rotational speed ω≈90 %Ω can effectively suppress rotor aeroacoustic levels, benefiting the stealth design of helicopters.

ECR Spotlight – Mario Martinez Groves-Raines

ECR Spotlight is a series of interviews with early-career authors from a selection of papers published in Journal of Experimental Biology and aims to promote not only the diversity of early-career researchers (ECRs) working in experimental biology but also the huge variety of animals and physiological systems that are essential for the ‘comparative’ approach. Mario Martinez Groves-Raines is an author on ‘ Steady as they hover: kinematics of kestrel wing and tail morphing during hovering flights’, published in JEB. Mario is a PhD Student in the lab of Dr Shane Windsor (University of Bristol, UK), Dr Abdulghani Mohamed (RMIT, Australia) and Professor Simon Watkins (RMIT, Australia), investigating the flight of birds from an aerodynamics and flight control perspective, learning and inspiring new flight techniques for human-made aircraft.

Steady as they hover: kinematics of kestrel wing and tail morphing during hovering flights.

Wind-hovering birds exhibit remarkable steadiness in flight, achieved through the morphing of their wings and tail. We analysed the kinematics of two nankeen kestrels (Falco cenchroides) engaged in steady wind-hovering flights in a smooth flow wind tunnel. Motion-tracking cameras were used to capture the movements of the birds as they maintained their position. The motion of the birds' head and body, and the morphing motions of their wings and tail were tracked and analysed using correlation methods. The results revealed that wing sweep, representing the flexion/extension movement of the wing, played a significant role in wing motion. Additionally, correlations between different independent degrees of freedom (DoF), including wing and tail coupling, were observed. These kinematic couplings indicate balancing of forces and moments necessary for steady wind hovering. Variation in flight behaviour between the two birds highlighted the redundancy of DoF and the versatility of wing morphing in achieving control. This study provides insights into fixed-wing craft flight control from the avian world and may inspire novel flight control strategies for future fixed-wing aircraft.

Blade shape optimization in hover and forward flight

Blade shape optimization in hover and forward flight

Control of Helicopter Using Virtual Swashplate

This article presents a virtual swashplate mechanism for a mini helicopter in classic configuration. The propeller bases are part of a passive mechanism driven by main rotor torque modulaton, this mechanism generates a synchronous and opposite change in the propellers angle of attack, then the thrust vector tilts. This approach proposes to control the 6 degrees of freedom of the aircraft using two rotors. The main rotor controls vertical displacement and uses torque modulation and swing-hinged propellers to generate pitch and roll moments and the horizontal displacement while the yaw moment is controlled by the tail rotor. The dynamic model is obtained using the Newton-Euler approach and robust control algorithms are proposed. Experimental results are presented to show the performance of the proposed virtual swashplate in real-time outdoor hover flights.

Leading-edge curvature effect on aerodynamic performance of flapping wings in hover and forward flight

This study investigates the role of leading-edge (LE) curvature in flapping wing aerodynamics considering hovering and forward flight conditions. A scaled-up robotic model is towed along its longitudinal axis by a rack gear carriage system. The forward velocity of the robotic model is changed by varying the advance ratio J from 0 (hovering) to 1.0. The study reveals that the LE curvature has insignificant influence on the cycle-average aerodynamic lift and drag. However, the time-history lift coefficient shows that the curvature can enhance the lift around the middle of downstroke. This enhanced lift is reduced from 5% to 1.2% as J changed from 0 to 1.0. Further flow examinations reveal that the LE curvature is beneficial by enhancing circulation only at the outboard wing sections. The enhanced outboard circulation is found to emanate from the less stretched leading-edge vortices (LEVs), weakened trailing-edge vortices (TEVs), and the coherent merging of the tip vortices (TVs) with the minor LEVs as observed from the phase-lock planar digital particle image velocimetry measurements. The far-wake observation shows that the LE curvature enhances the vorticity within the TV, helping to reduce the overall flow fluctuations in the far field. These findings can be extended to explain the predominantly straight LE wing shape with a small amount of curvature only observed near the wing tip for flapping fliers with Re from 103 to 104.

Analysis of hovering flight stability of an insect-like flapping-wing robot in Martian condition

In efforts to search for life-form on a planet other than Earth, many projects have been launched to measure and analyze the surface and atmospheric conditions on Mars. To extend the exploration of Mars, scientists have attempted to perform flight in the extremely thin air environment of Mars. Inspired by the successful controlled flight of NASA's Helicopter on Mars, in this work, we investigate the six-degrees-of-freedom hovering flight dynamics of an insect-like flapping-wing robot, called KUBeetle. To hover the KUBeetle on Mars, using the scaling ratios of air density and gravity on Earth and Mars, we have found that the flapping frequency should be about 113.48 Hz, which is 4.93 times faster than that on Earth, and the required lift on Mars is about 38% of that on Earth. The computational fluid dynamics (CFD) analysis suggests that the lift produced by the flapping wings at the frequency on Mars is close to that predicted by the scaling ratios. A series of simulations are used to compute the stability derivatives to complete the equations of motion. The eigenvalue and eigenvector analyses have identified one fast subsidence mode, one slow subsidence mode, and one divergence oscillation mode in the longitudinal and lateral motions on Mars, which are identical to those on Earth. The dynamic responses of the robot under three initial disturbances in the translational speeds indicate that due to the 94% smaller real part of the complex roots in the divergence oscillation modes of the longitudinal and lateral motions on Mars, the growth of flight disturbances is much slower than on Earth. Therefore, the KUBeetle is capable of hovering flight on Mars, since it has enough time to use the feedback controls to stabilize the system. This work can be used to support the studies of the flight control of flapping wing robots in the Martian atmospheric condition, and to generalize such flight control for other non-terrestrial applications in the atmospheres of other planets and/or their moons.

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.

An evaluation of different measurement strategies to measure wind turbine near wake flow with small multicopter UAS

Wind turbine wake flow, especially in the near wake, that is up to one rotor diameter D downstream, is subject to interaction between tip vortices and ambient turbulence. These interactions are important to understand wake decay, but most difficult to measure with common instrumentation. Small uncrewed aerial systems (UAS) can help to measure at such locations where no masts can be installed. We contrast two measurement strategies, the hover flight with multiple UAS and cross-section flights with single UAS. We show that both strategies have advantages; the cross-section flights provide a full picture of the width and wind speed deficit across the rotor diameter whereas multi-UAS hover flights can provide more reliable turbulence intensity and turbulent flux measurements at specific locations. With both strategies, tip vortices can be detected and qualified to characterize the state of wake decay at different positions. A fit to the vortex models Lamb-Oseen and Burnham-Hallock allows to estimate circulation and core radius of the vortices. For best characterization of the wake, we recommend to combine the hover and cross-section flight strategies in future.

Experimental characterisation of rotor noise in tandem configuration

This paper presents a comprehensive investigation into the noise radiation characteristics of two rotors in a tandem configuration. Through an extensive experimental campaign, the study explores the effect of the separation distance between two rotors on the noise directivity patterns, spectral characteristics and temporal features of the radiated noise under both hovering and edge-wise flight conditions. The directivity patterns of the overall sound pressure level obtained from the polar and side arrays demonstrate a dependence on the separation distance and advance ratio. Spectral characteristics elucidate the influence of separation distance on tonal and broadband energy content. While the high-frequency broadband noise increases for both hovering and edge-wise flight similarly, the low-frequency broadband energy strongly depends on the advance ratio and the rotor separation distance. The results reveal a distinct envelope in the design space, where the radiated broadband noise for tandem configurations can be minimized, offering potential insights for noise reduction strategies. Additionally, the wavelet analysis for tandem configurations reveals that the emitted noise at the first and second blade passing frequency is strongly intermittent with amplitude modulation, which shall be considered for annoyance perception.

Acoustics and Forces from a Subscale Electric Vertical-Takeoff-and-Landing Rotor in Edgewise Flight

The far-field noise and forces of a subscale two-blade rotor were measured in an anechoic wind tunnel for both hover and edgewise flight conditions. The mean thrust and torque coefficients increased with the edgewise advance ratio in relation to the hover coefficients. The tonal and broadband noise contributions were separated and analyzed in both the time and frequency domains. The sound pressure level (SPL) at the blade pass frequency (BPF) had the greatest contribution to the overall SPL, and the dominant broadband noise source changed from high-frequency trailing edge noise to midfrequency blade wake interaction noise with an increasing edgewise advance ratio. The tip Mach number scaling of the noise sources was examined and found to be highly dependent on the oncoming freestream velocity. The BPF SPL directivity showed a dependency on the advance ratio, with a peak magnitude below and upstream of the rotor’s retreating side. The broadband noise had a dipole directivity with a minimum in the rotor plane. The midfrequency broadband noise had signatures indicative of blade wake interaction noise, and the high-frequency noise had signatures indicative of amplitude modulated trailing edge noise. The results demonstrate the importance of unsteady loading noise and broadband noise for subscale urban air mobility rotors.

Clap-and-Fling Mechanism of Climbing-Flight Coccinella Septempunctata.

Previous studies on the clap-fling mechanism have predominantly focused on the initial downward and forward phases of flight in miniature insects, either during hovering or forward flight. However, this study presents the first comprehensive kinematic data of Coccinella septempunctata during climbing flight. It reveals, for the first time, that a clap-and-fling mechanism occurs during the initial upward and backward phase of the hind wings' motion. This discovery addresses the previously limited understanding of the clap-and-fling mechanism by demonstrating that, during the clap motion, the leading edges of beetle's wings come into proximity to form a figure-eight shape before rotating around their trailing edge to open into a "V" shape. By employing numerical solutions to solve Navier-Stokes (N-S) equations, we simulated both single hind wings' and double hind wings' aerodynamic conditions. Our findings demonstrate that this fling mechanism not only significantly enhances the lift coefficient by approximately 9.65% but also reduces the drag coefficient by about 1.7%, indicating an extension of the applicability range of this clap-and-fling mechanism beyond minute insect flight. Consequently, these insights into insect flight mechanics deepen our understanding of their biological characteristics and inspire advancements in robotics and biomimetics.