Flapping-wing Vehicles Research Articles

Numerical investigation of wingtip aerodynamic interference of two flapping wings on opposite sides

Aerodynamic interference occurs at the wingtips when flying organisms fly in a V formation. In this paper, the wingtip aerodynamic interference of two flapping wings on opposite sides at low Reynolds numbers (Re) is numerically investigated. The effects of streamwise spacing (L1), spanwise spacing (L2), and phase angle (γ) on aerodynamic performance are considered. The results show that, compared to a single wing, a favorable combination of L1 and L2 can improve the overall thrust by 24% while keeping the overall lift essentially unchanged. In an unfavorable case, overall lift and thrust decrease by 18% and 20%, respectively. The overall aerodynamic forces are dominated by the rear wing. Analyzing the essential flow characteristics reveals the double-edged role of downwash and upwash in force generation. Moreover, it is found that the rear wing can realize the upwash/downwash exploitation by flap phasing, turning an unfavorable situation into a favorable one. The key flow physics behind this transformation lies in the relationship between the direction of wing motion and the direction of fluid velocity induced by vortices. These findings provide valuable insights into the understanding of biological phenomena and the design of new flapping wing vehicles.

Aerodynamic Effects of Time-Varying Corrugations on Dragonfly Wings in Flapping Flight.

The aerodynamic effects of wing corrugation on insect flight have received widespread attention. However, there has hardly been any specific focus on dynamic changes to corrugation angle in the models. The flexible vein joints containing resilin in the wings of dragonflies and damselflies enable the longitudinal veins to rotate and thereby change the corrugation angles throughout flapping cycles. Therefore, a two-dimensional corrugated airfoil with time-varying corrugation angles is proposed and the aerodynamic performance is evaluated in terms of aerodynamic force, power and efficiency. The results indicate that the airfoil with time-varying corrugations outperforms the rigid one in terms of enhancing thrust and reducing power consumption. The aerodynamic performance of time-varying corrugated airfoils is optimal when the angle varies in a specific range, and an excessively large angle variation may have negative effects. In addition, excessive height or a negative leading edge of the corrugation can lead to a reduction in the thrust. A design concept for the 2D airfoil with time-varying corrugations is provided and the findings are of significance for enhancing the aerodynamic performance of biomimetic flexible flapping-wing vehicles.

Design of bionic active folding flapping wing vehicle

Flapping wing vehicles mimic the wing flapping of flying creatures such as birds, bats and insects, and are characterized by simplicity, lightness, good concealment, high maneuverability and diversified flight attitudes. Currently, the development of wing-fluttering vehicles mainly focuses on wing-fluttering configurations, while there is little research on mimicking the large-scale active folding of wings of birds and bats. Here, we developed two types of large-scale folding wing vehicles with light mass and actively folded wings respectively, based on the property that the wings of flying organisms can be actively folded and contracted in a large scale, and tested their flight capabilities. The test results show that the latter vehicle, which combines the flapping wing mechanism and the active folding mechanism, has good flight performance and is able to accomplish the airborne folding of the wings on the basis of the flapping wing flight.

Measurement of wing motion, deformation, and inertial forces of a biomimetic butterfly.

This paper introduces a method for measuring wing motion, deformation, and inertial forces in bio-inspired aircraft research using a camera motion capture system. The method involves placing markers on the wing surface and fitting rigid planes to determine the wing's spatial axis. This allows for describing the wing's rigid motion and obtaining deformation characteristics, such as deflection, twist angle, and gap distance of the forewing and hindwing. An image-based method is proposed for determining wing mass distribution, mass blocks, and mass points for inertial force measurement. The study addresses wing motion, deformation, and inertial force measurement in a real butterfly-like flapping wing vehicle and demonstrates the effectiveness of the approach. The results reveal that inertial forces play a negligible role in the generation of lift peaks and contribute minimal lift during the entire flapping cycle. Furthermore, a transitional phase between downstroke and upstroke is found in flexible wing motion, which has high lift production. This measurement approach offers a rapid and effective solution to experimental challenges in bio-inspired aircraft design and optimization.

Effects of dynamical spanwise retraction and stretch on flapping-wing forward flights

Birds and bats retract and stretch their wings dynamically during each flap in level flights, implying intriguing mechanisms for the aerodynamic performance improvement of flapping wings. A numerical investigation into the aerodynamic effects of such bio-inspired concept in forward flights has been performed based on a three-dimensional wing in plunging motion and a two-section wing in flapping motion. The currently considered Reynolds number and Strouhal number are Re=1.5×105 and St=0.3, respectively. During the research, the mean angle of attack is varied in relatively wide ranges to achieve lift-thrust interconversion for the wings. The conclusive results show that dynamical spanwise retraction and stretch has induced three absolutely desirable scenarios for the oscillating wings in forward flights, namely producing more lift and consuming less power for a given thrust generation, producing more thrust and consuming less power for a given lift generation, and producing more lift and more thrust while consuming less power. Furthermore, the morphing wings have alleviated periodical aerodynamic load fluctuations compared with the non-morphing baseline. The mechanism of the aerodynamic effects of the bionic morphing mode is analyzed with the aid of field visualization. The current article is the first to reveal the absolute advantages of the bionic spanwise morphing. Hopefully, it may help comprehend the behaviors of natural fliers and provide inspirations for performance enhancement of micro artificial flapping-wing vehicles.

Implementation and Linearization of State-Space Free-Vortex Wake Models for Rotary- and Flapping-Wing Vehicles

This article describes the implementation and linearization of free-vortex wake models in state-variable form as applied to rotary- and flapping-wing vehicles. More specifically, the wake models are implemented and tested for a UH-60 rotor in forward flight and for a hovering insect representative of a hawk moth. A periodic solution to each wake model is found by time marching the coupled rotor/wing and vortex wake dynamics. Next, linearized harmonic decomposition models are obtained and validated against nonlinear simulations. Order reduction methods are explored to guide the development of linearized wake models that provide increased runtime performance compared to the nonlinear and linearized harmonic decomposition wake models while guaranteeing satisfactory prediction of the periodic response of the wake. This constitutes a first attempt to extend free-vortex wake methods in state-variable form, originally developed for rotary-wing applications, to flapping-wing flight.

Intelligent perception of tandem flapping wings' flow field environment with sparse detectors

Bionic flapping wing vehicles have great potential for civil and defense applications due to their flexibility and concealment at low Reynolds numbers. Since traditional flow field pattern recognition methods are difficult to identify effective information from the measured local flow field and deduce the state information of the moving body, this study uses an artificial intelligence method to establish the internal correlation between flow field pattern and state information. Specifically, a fully connected neural network is adopted to recognize the tandem flapping wings' flow field pattern by using different data acquisition methods and detector array distribution methods. Compared with the neural network based on time series data, the neural network based on spatial distribution data can realize the real-time judgment of flow field environment, which is closer to the real-time requirements in practical applications. In the paper, the intelligent perception of multi-flapping wings' flow field environment with sparse detectors is carried out and lays the theoretical foundation for autonomous navigation and obstacle avoidance of flapping wing aircrafts.

Review of the Flight Control Method of a Bird-like Flapping-Wing Air Vehicle.

The Bird-like Flapping-wing Air Vehicle (BFAV) is a robotic innovation that emulates the flight patterns of birds. In comparison to fixed-wing and rotary-wing air vehicles, the BFAV offers superior attributes such as stealth, enhanced maneuverability, strong adaptability, and low noise, which render the BFAV a promising prospect for numerous applications. Consequently, it represents a crucial direction of research in the field of air vehicles for the foreseeable future. However, the flapping-wing vehicle is a nonlinear and unsteady system, posing significant challenges for BFAV to achieve autonomous flying since it is difficult to analyze and characterize using traditional methods and aerodynamics. Hence, flight control as a major key for flapping-wing air vehicles to achieve autonomous flight garners considerable attention from scholars. This paper presents an exposition of the flight principles of BFAV, followed by a comprehensive analysis of various significant factors that impact bird flight. Subsequently, a review of the existing literature on flight control in BFAV is conducted, and the flight control of BFAV is categorized into three distinct components: position control, trajectory tracking control, and formation control. Additionally, the latest advancements in control algorithms for each component are deliberated and analyzed. Ultimately, a projection on forthcoming directions of research is presented.

Lift enhancement of a butterfly-like flapping wing vehicle by reinforcement learning algorithm

In order to enhance the take-off lift of a butterfly-like flapping wing vehicle (FWV), we implemented an integrated experimental platform and applied a reinforcement learning algorithm. The vehicle, which has a wingspan of 81 cm and is mounted on a stand with a force sensor, is driven by two servos that are powered and controlled wirelessly. To achieve the goal of enhancing take-off lift, we used a model-free, on-policy actor-critic proximal policy optimization algorithm. After 300 learning steps, the average aerodynamic lift force increased significantly from 0.044 N to 0.861 N. This enhanced lift force was sufficient to meet the take-off requirements of the vehicle without the need for any additional aids or airflow. Additionally, we observed a strong lift peak in the upstroke after analyzing the learning results. Further experiments showed that this lift peak is directly related to the elastic release of the wing twist and the opening and closing of the gap between the forewing and hindwing in the early stage of the upstroke. These findings were not easily predicted or discovered using traditional aerodynamic methods. This work provides valuable reinforcement learning experience for the future development of FWVs.

Aerodynamic Evaluation of Flapping Wings with Leading-Edge Twisting

The purpose of the current study is to emphasize the characteristics and phenomena of leading-edge twisting in flapping wing vehicles. A fused deposition modeling (FDM) 3D printing method is applied to develop the flapping mechanisms with bevel gears to achieve the leading-edge twisting. Three flapping mechanisms were developed, including simple flapping only (type-A1: normal servo mechanism), flapping with continuous leading-edge twisting (type-B: servo-bevel gear mechanism), and flapping with restricted leading-edge twisting via mechanical stoppers (type-B1: servo-bevel gear mechanism with adjustable mechanical stoppers). Utilizing a low-speed wind tunnel, the aerodynamic performances of these mechanisms are examined by extracting their lift and net thrust forces. The wind tunnel testing data showed that the flapping with restricted leading-edge twisting via mechanical stoppers (type-B1) showed better performance than the simple flapping (type-A1) by 32.9%, and also better performance than the flapping with continuous leading-edge twisting (type-B) by 64%. Next, MATLAB software was used to create the 3D wing surfaces from the instantaneous stereophotography Kwon3D trajectories to fully sketch the leading-edge twisting features. The 2D airfoil cut sections at the mean aerodynamic chord at different stroke moments depict the instantaneous angles of attack to justify the aforementioned wind tunnel testing data and it was verified using a theoretical trajectory model. This comprehensive study using the 3D-printed mechanisms is well suited for the quantitative evaluation of the lift contribution from leading-edge twisting.

How ornithopters can perch autonomously on a branch

Flapping wings produce lift and thrust in bio-inspired aerial robots, leading to quiet, safe and efficient flight. However, to extend their application scope, these robots must perch and land, a feat widely demonstrated by birds. Despite recent progress, flapping-wing vehicles, or ornithopters, are to this day unable to stop their flight. In this paper, we present a process to autonomously land an ornithopter on a branch. This method describes the joint operation of a pitch-yaw-altitude flapping flight controller, an optical close-range correction system and a bistable claw appendage design that can grasp a branch within 25 milliseconds and re-open. We validate this method with a 700 g robot and demonstrate the first autonomous perching flight of a flapping-wing robot on a branch, a result replicated with a second robot. This work paves the way towards the application of flapping-wing robots for long-range missions, bird observation, manipulation, and outdoor flight.

To tread or not to tread: comparison between water treading and conventional flapping wing kinematics

Hovering insects are limited by their physiology and need to rotate their wings at the end of each back-and-forth motion to keep the wing’s leading edge ahead of its trailing edge. The wing rotation at the end of each half-stroke pushes the leading edge vortex away from the wing which leads to a loss in the lift. Unlike biological fliers, human-engineered flapping wing micro air vehicles have different design limitations. They can be designed to avoid the end of stroke wing rotation and use so-called water-treading flapping kinematics. Flapping wings using conventional flapping kinematics have a designated leading and trailing edge. In the water-treading mode, the role of the leading and trailing edges are continuously alternated throughout the stroke. Here, we compare velocity field and force measurements for a rectangular flapping wing conducting normal hovering and water-treading kinematics to study the difference in fluid dynamic performance between the two types of flapping kinematics. We show that for similar power consumption, the water-treading mode produces more lift than the conventional hovering mode and is 50% more efficient for symmetric pitching kinematics. In the water-treading mode, the leading edge vortex from the previous stroke is not pushed away but is captured and keeps the newly formed leading edge vortex closer to the wing, leading to a more rapid increase of the lift coefficient which is sustained for longer. This makes the water-treading mode a promising alternative for human-engineered flapping wing vehicles.

Frequency-Modulated Control for Insect-Scale Flapping-Wing Vehicles

Many insects utilize both amplitude and frequency modulation of their wingstrokes to control flight, a combination with potential advantages for flapping-wing micro-aerial vehicle (FWMAV) flight control as we attempt to mimic the agility of small biological fliers. However, frequency-modulated control is uncommon in insect-scale FWMAVs, and these vehicles have not yet demonstrated graceful, mid-wingstroke frequency transitions in flight. We propose a method to allow active frequency control as a primary control variable, and describe a method to achieve smooth frequency transitions in flight. We demonstrate that frequency, in combination with other flapping parameters, may be used to generate arbitrary, time-varying forces and torques. Additionally, we explore the advantages of frequency modulation in FWMAV flight, applying a frequency-based method to control the Harvard RoboBee during hovering flight.

Optimized flapping flight in Venus surface atmospheric conditions

Three recently approved space missions are headed towards Venus, to help answer major questions about Venus atmosphere and geology. However, many existing questions cannot be properly addressed without direct in situ measurements from Venus surface or within the atmosphere. To this end, flapping wing vehicle concept is selected, optimized for Venus atmospheric flight, and evaluated using energy efficiency as performance criteria. Flapping wing vehicle computational model is derived based on discrete variational mechanics and quasi-steady aerodynamics, with all relevant aerodynamic phenomena included. Flapping wing vehicle computational model is then embedded within optimization algorithm, which is utilized to obtain energy efficient flapping patterns for forward flight in Venus surface atmospheric conditions. Numerical optimization is performed for different neutrally buoyant configurations, with wingspan ranging from 10 mm to 1 m. Different forward velocities are used as well, where maximum velocity is limited by an advance ratio of 0.5. Bumblebee and hummingbird-sized vehicles, with a wingspan of 30 mm and 30 cm, are selected as the most representative test cases and thoroughly studied. It is proved that flapping wing propulsion is a feasible and effective concept for Venus exploration purposes. Finally, based on a comparison of the selected test cases, general conclusions are drawn on the flapping wing dynamics and flight mechanics in the Venus atmosphere. Significant difference in the propulsion mechanism has been observed, based on the aerial vehicle size. In order to maximize propulsive efficiency, the smaller vehicle mostly exploited aerodynamic forces related to the leading edge vortex, while the larger vehicle relied more on added mass and rotational forces.

Aerodynamic and flight dynamic parametric studies of a flapping wing

PurposeFlapping-wing vehicles show various advantages as compared to fixed wing vehicles, making flapping-wing vehicles' study necessary in the current scenario. The present study aims to provide guidelines for fixing geometric parameters for an initial engineering design by a simple aerodynamic and flight dynamic parametric study.Design/methodology/approachA mathematical analysis was performed to understand the aerodynamics and flight dynamics of the micro-air vehicle (MAV). Only the forces due to the flapping wing were considered. The flapping motion was considered to be a combination of the pitching and plunging motion. The geometric parameters of the flapping wing were varied and the aerodynamic forces and power were observed. Attempts were then made to understand the flight stability envelope of the MAV in a forward horizontal motion in the vertical plane with similar parametric studies as those conducted in the case of aerodynamics.FindingsFrom the aerodynamic study, insights were obtained regarding the interaction of design parameters with the aerodynamics and feasible ranges of values for the parameters were identified. The flapping wing was found to have neutral static stability. The flight dynamic analysis revealed the presence of an unstable oscillatory mode, a stable fast subsidence mode and a neutral mode, in the forward flight of the MAV. The presence of unstable modes highlighted the need for active control to restore the MAV to equilibrium from its unstable state.Research limitations/implicationsThe study does not take into account the effects of control surfaces and tail on the aerodynamics and flight dynamics of the MAV. There is also a need to validate the results obtained in the study through experimental means which shall be taken up in the future.Practical implicationsThe parametric study helps us to understand the extent of the impact of the design parameters on the aerodynamics and stability of the MAV. The analysis of both aerodynamics and dynamic stability provides a holistic picture for the initial design. The study incorporates complex mathematical equations and simplifies such to understand the aerodynamics and flight stability of the MAV from an engineering perspective.Originality/valueThe study adds to already existing knowledge on the design procedures of a flapping wing.

Scalability of resonant motor-driven flapping wing propulsion systems

This work aims to develop an integrated conceptual design process to assess the scalability and performance of propulsion systems of resonant motor-driven flapping wing vehicles. The developed process allows designers to explore the interaction between electrical, mechanical and aerodynamic domains in a single transparent design environment. Wings are modelled based on a quasi-steady treatment that evaluates aerodynamics from geometry and kinematic information. System mechanics is modelled as a damped second-order dynamic system operating at resonance with nonlinear aerodynamic damping. Motors are modelled using standard equations that relate operational parameters and AC voltage input. Design scaling laws are developed using available data based on current levels of technology. The design method provides insights into the effects of changing core design variables such as the actuator size, actuator mass fraction and pitching kinematics on the overall design solution. It is shown that system efficiency achieves peak values of 30–36% at motor masses of 0.5–1 g when a constant angle of attack kinematics is employed. While sinusoidal angle of attack kinematics demands more aerodynamic and electric powers compared with the constant angle of attack case, sinusoidal angle of attack kinematics can lead to a maximum difference of around 15% in peak system efficiency.

Preliminary design of bionic flapping wing vehicle

This paper deals with the preliminary design of bionic flapping wing vehicle. The design is driven by its requirement and objective namely the maximum weight should be less than 0.5 kg with 1.5 m wingspan. In the conceptual design, the wing planform, wing structures and system of flapping mechanism will be considered to find the initial configuration and then continued the sizing of the flapping vehicle. The analysis of designed flapping vehicle is increasingly complex due to firstly the generation of time-dependent aerodynamic forces and moments from unsteady flow around the flapping wing, secondly, flexible wing structure which generates higher thrust may cause a structure failure and thirdly, a flapping mechanism system generating differential flapping motion for the vehicle maneuver. The aerodynamic analysis for given flapping motion model is carried out using Computational Fluid Dynamics method based the solution of unsteady Reynold Averaged Navier-Stokes equations and the structure analysis is conducted with the input of aerodynamic loads using Finite Element method to find critical stresses that may cause a structure failure. As a solution of differential flapping two controlled servos are used. The architecture of electrical system is made to analysis of the distribution of data signal and power. The flight maneuver is achieved by changes in flapping frequency and sweeping angle. The selection of system components is performed by considering weight constraint. The flight maneuver is achieved by changes in flapping frequency and sweeping angle. The wing platform has elliptical shape with airfoil maximum camber of 6% chord at a quarter chord. The outer body and wing are made of foam laminated by glass fibre with the foam density of 0.045 g/cm3. The horizontal tail is made of mylar film with density of 1.38 g/cm3 and its area of 168.3 cm2. As simulated results, the amount of total lift is 4.18 Newton generated at 6 Hz flapping frequency.

Design and analysis of an innovative flapping wing micro aerial vehicle with a figure eight wingtip trajectory

Abstract. A multi-mode flapping wing micro air vehicle (FWMAV) that uses a figure eight wingtip motion trajectory with wing flapping, rotation, and swing motion is presented in this paper. The flapping wing vehicle achieves three active degrees of freedom (DOF) wing movements only with one driving micromotor which has a good balance in the mechanism design (that is inspired by natural fliers) and total weight. Owing to these characteristics being integrated into the simple mechanism design, the aerodynamic force is improved. The aerodynamic performance of the thrust force is improved by 64.3 % compared to one that could only flap up and down with one active DOF under the condition of routine flapping frequency.

Optimal flapping wing shape and kinematics are different for different flight velocities: an analysis on local relative wind

Different species of insects and birds fly differently. Their wing forms and wing motion are different. Understanding the purpose of this difference will lead to successful development of flapping wing vehicles for different purposes. This paper discusses the influence of one of the important factors which affects the aerodynamic performance of a flapping wing: the local instantaneous relative wind. Its distribution along the span of a flapping wing has been analyzed for the practical flight range of advance ratios ranging from 0 to 1.5 and stroke plane orientations ranging from 0 to 90°. In this domain, the variation of spanwise distributions of magnitude and direction of relative wind are presented separately for downstroke and upstroke of a flapping cycle. Accordingly, qualitative suggestions are given for wing planform design, orientation and twist of the wing, and actuation of wing for effective utilization of the relative wind and obtaining the necessary force distributions for different flight phases. The formulation presented will be helpful for experimenting flapping wings in hovering and forward flight; especially for setting the wing twist for a required angle of attack distribution and designing the flapping kinematics in upstroke and downstroke.

Data-driven CFD scaling of bioinspired Mars flight vehicles for hover

One way to improve our model of Mars is through aerial sampling and surveillance, which could provide information to augment the observations made by ground-based exploration and satellite imagery. Flight in the challenging ultra-low-density Martian environment can be achieved with properly scaled bioinspired flapping wing vehicle configurations that utilize the same high lift producing mechanisms that are employed by insects on Earth. Through dynamic scaling of wings and kinematics, we investigate the ability to generate solutions for a broad range of flapping wing flight vehicles with masses ranging from insects O(10−3) kg to the Mars helicopter Ingenuity O(100) kg. A scaling method based on a neural-network trained on 3D Navier-Stokes solutions is proposed to determine approximate wing size and kinematic values that generate bioinspired hover solutions. We demonstrate that a family of solutions exists for designs that range from 1 to 1000 g, which are verified and examined using a 3D Navier-Stokes solver. Our results reveal that unsteady lift enhancement mechanisms, such as delayed stall and rotational lift, are present in the bioinspired solutions for the scaled vehicles hovering in Martian conditions. These hovering vehicles exhibit payloads of up to 1 kg and flight times on the order of 100 min when considering the respective limiting cases of the vehicle mass being comprised entirely of payload or entirely of a battery and neglecting any transmission inefficiencies. This method can help to develop a range of Martian flying vehicle designs with mission viable payloads, range, and endurance.