Micro Aerial Vehicle Design Research Articles

The spatial-temporal effects of wing flexibility on aerodynamic performance of the flapping wing

In this paper, three-dimensional fluid–structure interaction simulation of flapping of a flexible wing is carried out. The aerodynamic effect of the flexible wing can be explained by analyzing the spatial and temporal effects of wing flexibility on aerodynamic performance. It is concluded that the flexible wing can increase the average lift and the aerodynamic efficiency. The spatial influence of flexible deformation mainly comes from the contribution of camber. In the mid-downstroke, wing flexibility results in significant camber near the wingtip, which is conducive to the attachment of the leading-edge vortex to the wing surface, thus enhancing the ability of the wingtip to generate lift. The temporal influence of flexible deformation mainly comes from the contribution of twist and bend. The fast pitching-down rotation due to the wing twist in the early downstroke is conducive to the accumulation of vorticity. The spanwise bend of the flexible wing due to the aerodynamic force and inertia can increase the flapping amplitude, which accounts for the lift increase. The above spatial-temporal effects make the flexible wing have better performance in generating lift and aerodynamic efficiency. The results are beneficial to systematically understand the aerodynamic effects of insect wing deformation and can provide guidance for the wing design of micro aerial vehicles.

Can the ground enhance vertical force for inclined stroke plane flapping wing?

The numerical investigation of 2D insect wing kinematics in an inclined stroke plane is carried out using an immersed boundary solver. The effect of vortex shedding and dipole jet on the vertical force generation by the flapping wing due to change in the stroke plane angle is investigated in the vicinity of the ground. The results of instantaneous force and vorticity contours reveal the underlying lift enhancement mechanisms for the inclined stroke plane flapping wing. Moreover, they aid in the understanding of the wake-ground interaction and the associated shear layers. The calculated average vertical force delineates different force trends for the inclined stroke plane flapping near the ground. Furthermore, the dipole jet patterns are analyzed for different heights. These patterns are found to be a better tool to assess the kinematics for the vertical force enhancement and reduction, especially at intermediate heights. Vertical force enhancement is the critical parameter in the design of the micro aerial vehicle (MAV). Through this study, it is certain that the dipole jet has the potential to be used as a lift modification mechanism in MAVs. In summary, the study gives a holistic view of the physics of the inclined plane kinematics near the ground and serves as the basis for the design of MAVs.

Enhanced lift and thrust via the translational motion between the thorax-abdomen node and the center of mass of a butterfly with a constructive abdominal oscillation.

Butterflies fly with an abdomen oscillating relative to the thorax; the abdominal oscillation causes body parts to undulate translationally relative to the center of mass of a butterfly, which could generate a significant effect on flight. Based on experimental measurements, we created a numerical model to investigate this effect in a free-flying butterfly (Idea leuconoe). We fixed the motions of wing-flapping and thorax-pitching, and parametrized the abdominal oscillation by varied oscillating phase. To concentrate the analysis on translational dynamics, we used a motion of a thorax-abdomen node, a joint that the thorax and the abdomen rotate about, to express the translational motion of body parts relative to the center of mass. The results show that the abdominal oscillation enhances lift and thrust via the translational motion of the thorax-abdomen node relative to the center of mass. With the abdominal oscillating phase recorded from real butterflies, the abdominal oscillation causes the thorax-abdomen node to move downward relative to the center of mass in downstroke and move upward relative to the center of mass in upstroke. This constructive movement amplifies the wing-flapping speed relative to the center of mass, which enhances the angle of attack and the strength of leading- and trailing-edge vortices on the wings. The wings thereby generate increased values of instantaneous lift and thrust by 50.32% and 32.57% compared to the case of no abdominal oscillation. Natural butterflies are stated to utilize a particular phase offset of abdominal oscillation to fly. With comparing varied oscillating phases, only the abdominal oscillating phase recorded from natural butterflies produces the best constructive effect on the translational motion of thorax-abdomen node, which maximizes the lift and thrust generated on the wings. It clarifies that butterflies use this specific range of abdominal oscillating phase to regulate the translational motion between the thorax-abdomen node and the center of mass to enhance flight. Our work reveals the translational mechanism of the abdominal oscillation, which is as important as the thorax-pitching effect. The findings in this work provide insight into the flight of butterflies and the design of micro aerial vehicles.

Recent advancements in flapping mechanism and wing design of micro aerial vehicles

Recent studies on understanding of natural flyers have encouraged researchers in development of micro aerial vehicles mimicking birds and insects such as hummingbirds, dragonflies, bats and many more. The vehicles find their applications in reconnaissance and situational awareness in combat field, search and rescue operations, biological and nuclear compromised sites and broadcasting and sports. The focus of this review is to assess recent progress in sub systems of these vehicles including drive mechanisms, actuation mechanisms and wing designs that define the aerodynamics, propulsion, stability, and control of the vehicles. Limited research has been carried out on drive mechanisms capable of producing figure-of-eight wingtip motion contrary to conventional four and five-bar linkage mechanisms along with modified planar and spherical attachments. Motor and piezoelectric actuation mechanisms are being used extensively in these vehicles due to lightweight and power efficiency as compared to non-conventional power sources. Wing shape and rigidity plays a key role in determining the required lift and thrust along with frequency limitations and material constraints. A relatively new field of structural and kinematic optimization for the development of a lightweight flapping vehicle with high endurance capability is also a part of this review. This review has pointed out the research gaps including 3-DoF piezoelectric kinematics, under-actuated mechanisms, structural contact analysis, limited static and dynamic structural analysis, limited fatigue analysis and development of optimization techniques.

Locomotion of a self-propulsive pitching plate in a quiescent viscous fluid

The locomotion of a flexible plate pitching in a quiescent viscous fluid is numerically studied by using the lattice Boltzmann method (LBM) for the fluid and a finite element method (FEM) for the plate, with an immersed boundary (IB) method for the fluid–structure interaction (FSI). In the simulation, the leading edge of the plate undergoes a prescribed pitching motion, and the entire plate moves freely due to the fluid–plate interaction. The effects of the pitching amplitude, bending rigidity, plate-to-fluid mass ratio and Reynolds number on the propulsive performance of the flexible plate are examined in a range of parameters. The numerical results show that a certain flexibility can remarkably improve the propulsive speed and efficiency. The optimal parameters for the pitching plate are obtained, i.e. [Formula: see text] ([Formula: see text] is a non-dimensional frequency, with [Formula: see text] means rigid plate and larger [Formula: see text] means more flexible) and 20° ≤ α0 ≤ 25° ( α0 is the pitching amplitude). The comparisons of three plate-to-fluid mass ratios (1.0, 2.5 and 5.0) show that the mass of the plate decreases the propulsive speed, but contrarily increases the efficiency. The results obtained in the present study provide an insight into the understanding of the performance of self-propulsive plate in pitching motion and can further guide the engineering design of micro aerial vehicles.

Variable-Fidelity Lower Confidence Bounding Approach for Engineering Optimization Problems with Expensive Simulations

Engineering design optimization with expensive simulations is usually a computationally prohibitive process. As one of the most famous efficient global optimization approaches, the lower confidence bounding (LCB) approach has been widely applied to relieve this computational burden. However, the LCB approach can be used only for design optimization problems with the single-fidelity level. In this paper, a variable-fidelity lower confidence bounding (VF-LCB) approach is developed to extend the LCB approach to engineering problems with multifidelity levels. First, a VF-LCB function is analytically derived to adaptively select LF or HF samples according to the predicted values and uncertainty from the VF metamodel. Then the coefficient of variations (CoV) of the predicted values and uncertainty of the VF model, which can reflect the degree of dispersion of the prediction values and uncertainty, respectively, are introduced to balance the global exploration and local exploitation objectively. To illustrate the effectiveness and merits of the proposed VF-LCB approach, eight numerical examples, and the design of micro-aerial vehicle (MAV) fuselage are tested. Comparative results between the proposed approach and the other five existing methods show that the average cost savings are about 25% for eight numerical examples and about 45% for the MAV problem compared with the other five existing methods.

Numerical investigation of the aerodynamic performance of dragonfly-like flapping foil in take-off flight

In this study, the aerodynamic performance of a dragonfly-like flapping foil is investigated in take-off flight using two-dimensional numerical simulations. Two parameters, foil spacing L * ( L/c) and the phase shift between forefoil and hindfoil ψ, which characterize the flow in the tandem configuration, are varied to explore their impact on the resulting aerodynamic performance. Both the vertical and thrust forces generated in one cycle are found to be strongly dependent on ψ and relatively weakly dependent on L *. The tandem configuration is beneficial for the vertical force generation in the range of 0° ≤ ψ ≤ 30°, and further, it aids the thrust force generation for 40° ≤ ψ ≤ 110°. The flow interactions between the forefoil and hindfoil can increase the vertical force generated in a cycle by a maximum of ∼1.2 times when ψ is close to 0° relative to the no foil–foil interaction case (additive effect of two single foils). These interactions can also increase the thrust force generated in a cycle by a maximum of ∼2.8 times when ψ is close to 80°. The vortex structures reveal that the enhancement in thrust force mainly depends on the timing of interactions and how the hindfoil utilizes the vortex detached from the forefoil to benefit from it. Further, we find that dragonflies use in-phase stroking pattern ( ψ = 0 °) with a large downstroke angle of attack, αD( αD = 86 °), in the initial phase of take-off as it aids in generating additional vertical force. Our findings can significantly contribute to the design of micro aerial vehicles.

Ground effects on the stability of separated flow around a NACA 4415 airfoil at low Reynolds numbers

We perform a linear BiGlobal modal stability analysis on the separated flow around a NACA 4415 airfoil at low Reynolds numbers (Re=300–1000) and a high angle of attack (α=20°), with a focus on the effect of the airfoil's proximity to two different types of ground: a stationary ground and a moving ground. The results show that the most dominant perturbation is a Kelvin–Helmholtz mode, which gives rise to a supercritical Hopf bifurcation to a global mode, leading to large-scale vortex shedding at a periodic limit cycle. As the airfoil approaches the ground, this mode can become more unstable or less unstable, depending on the specific type of ground: introducing a stationary ground to an otherwise groundless system is destabilizing but introducing a moving ground is stabilizing, although both effects weaken with increasing Re. By performing a Floquet analysis, we find that short-wavelength secondary instabilities are damped by a moving ground but are amplified by a stationary ground. By contrast, long-wavelength secondary instabilities are relatively insensitive to ground type. This numerical–theoretical study shows that the ground can have an elaborate influence on the primary and secondary instabilities of the separated flow around an airfoil at low Re. These findings could be useful for the design of micro aerial vehicles and for improving our understanding of natural flyers such as insects and birds.

Visual–Inertial Navigation Systems for Aerial Robotics: Sensor Fusion and Technology

In this paper, we comprehensively discuss the current progress of visual–inertial (VI) navigation systems and sensor fusion research with a particular focus on small unmanned aerial vehicles, known as microaerial vehicles (MAVs). Such fusion has become very topical due to the complementary characteristics of the two sensing modalities. We discuss the pros and cons of the most widely implemented VI systems against the navigational and maneuvering capabilities of MAVs. Considering the issue of optimum data fusion from multiple heterogeneous sensors, we examine the potential of the most widely used advanced state estimation techniques (both linear and nonlinear as well as Bayesian and non-Bayesian) against various MAV design considerations. Finally, we highlight several research opportunities and potential challenges associated with each technique.

MEMS Based Micro Aerial Vehicles

Designing a flapping wing insect robot requires understanding of insect flight mechanisms, wing kinematics and aerodynamic forces. These subsystems are interconnected and their dependence on one another affects the overall performance. Additionally it requires an artificial muscle like actuator and transmission to power the wings. Several kinds of actuators and mechanisms are candidates for this application with their own strengths and weaknesses. This article provides an overview of the insect scaled flight mechanism along with discussion of various methods to achieve the Micro Aerial Vehicle (MAV) flight. Ongoing projects in Chalmers is aimed at developing a low cost and low manufacturing time MAV. The MAV design considerations and design specifications are mentioned. The wings are manufactured using 3D printed carbon fiber and are under experimental study.

Wind alters landing dynamics in bumblebees.

Landing is an important but understudied behavior that flying animals must perform constantly. In still air, insects decelerate smoothly prior to landing by employing the relatively simple strategy of maintaining a constant rate of image expansion during their approach. However, it is unclear whether insects employ this strategy when faced with challenging flight environments. Here, we tested the effects of wind on bumblebees (Bombus impatiens) landing on flowers. We find that bees' approach paths to flowers shift from multidirectional in still air to unidirectional in wind, regardless of flower orientation. In addition, bees landing in a 3.5 m s-1 headwind do not decelerate smoothly, but rather maintain a high flight speed until contact, resulting in higher peak decelerations upon impact. These findings suggest that wind has a strong influence on insect landing behavior and performance, with important implications for the design of micro aerial vehicles and the ecomechanics of insect flight.

A Near-Hover Adaptive Attitude Control Strategy of a Ducted Fan Micro Aerial Vehicle with Actuator Dynamics

The aerodynamic parameters of ducted fan micro aerial vehicles (MAVs) are difficult and expensive to precisely measure and are, therefore, not available in most cases. Furthermore, the actuator dynamics with risks of potentially destabilizing the overall system are important but often neglected consideration factors in the control system design of ducted fan MAVs. This paper presents a near-hover adaptive attitude control strategy of a prototype ducted fan MAV with actuator dynamics and without any prior information about the behavior of the MAV. The proposed strategy consists of an online parameter estimation algorithm and an adaptive gain scheduling algorithm, with the former accommodating parametric uncertainties, and the latter approximately eliminating the coupling among axes and guaranteeing the control quality of the MAV. The effectiveness of the proposed strategy is verified numerically and experimentally.

Design and Manufacturing of Biologically Inspired Micro Aerial Vehicle Wings Using Rapid Prototyping

Micro Aerial Vehicles (MAVs) are an emerging class of uninhabited aerial vehicle (UAV). Their reduced scale (maximum dimension of approximately 150 mm) provides advantages in terms of advanced mission capabilities, such as wildlife monitoring and urban search and rescue. This introduces the design challenge of flying efficiently at very low Reynolds numbers (e.g. Re<10000). To date, three basic MAV design concepts have been developed: fixed, rotary and flapping wings. Each approach has been met with limited success due to gust stability, flight control and propulsive efficiency. The design of both fixed and rotary wing aircraft is relatively mature, whereas flapping wing design is in its infancy and therefore its viability cannot yet be assessed. Nonetheless flapping wing MAVs have the potential to offer advantages such as stealth, manoeuvrability, and improved propulsive efficiencies. This paper focuses on the challenging problem of the manufacture and testing of flapping wings for MAVs. A review of the current state of flapping wing aerodynamics, manufacturing, and wing structures is provided. A detailed assessment of the aerodynamic performance of flexible MAV-scale wings was carried out. Aerodynamic force measurements were collected using a spin rig to assess the effect of design details on lift generation. It was found that a simple three-vein wing structure manufactured using a fused filament fabrication 3D printer could produce lift forces close to those of natural insect wings. The lift and stall performance was found to be sensitive to chordwise stiffness by testing wings without veins. These results demonstrate that it is possible to produce low cost biologically inspired wings with aerodynamic performance equal to or better than natural wings – a critical step on the path to a functional and practical flapping wing MAV.

Study of asymmetric hovering in flapping flight

Study of the unsteady aerodynamics of flapping airfoils has received a major boost in the recent past for its application in the field of MAV design. The present study investigates the unsteady flow-field over a flapping airfoil with asymmetric kinematics. Different asymmetry mechanisms are investigated for their lift enhancement capabilities during hover. Three asymmetries have been tried: amplitude, frequency and unsteady free-stream. Average aerodynamic loads are compared for all the cases. Flow field vortices are also compared. For the amplitude asymmetry case, pitch angles are varied between the strokes. Both normal and sinusoidal hover are considered. The normal hover case shows a significant increase in the average lift value compared to its sinusoidal counterpart. Subsequently, the frequency asymmetry mechanism has shown a significant increase in load for the sinusoidal hover case. Frequency asymmetry kinematics is achieved by using a faster stroke followed by a slower stroke. Depending on the level of asymmetry, a large amount of lift can be generated during the faster stroke. A third mechanism of sinusoidally varying oncoming flow with mean zero is also investigated. The flow-field is simulated with a Lagrangian particle based technique. In this algorithm, field property vorticity is represented by discrete particles. The flow-field is simulated at a Reynolds number around 3000 which is much higher than similar studies reported earlier. The Reynolds number range considered here is applicable to various man-made flapping devices like Micro Aerial Vehicles (MAVs). Crucial flow features like the dynamic stall vortex and its interaction with the trailing edge structures are studied for different asymmetry factors in the flow-field.

A Biologically-Inspired Micro Aerial Vehicle

This paper introduces a novel framework for the design, modeling and control of a Micro Aerial Vehicle (MAV). The vehicle's conceptual design is based on biologically-inspired principles and emulates a dragonfly (Odonata---Anisoptera). We have taken inspiration from the flight mechanism features of the dragonfly and have developed indigenous designs in creating a novel version of a Flapping Wing MAV (FWMAV). The MAV design incorporates a complex mechanical construction and a sophisticated multi-layered, hybrid, linear/non-linear controller to achieve extended flight times and improved agility compared to other rotary wing and FWMAV Vertical Take Off and Landing (VTOL) designs. The first MAV prototype will have a ballpark weight including sensor payload of around 30 g. The targeted lifting capability is about twice the weight. The MAV features state of the art sensing and instrumentation payload, which includes integrated high-power on-board processors, 6DoF inertial sensors, 3DoF compasses, GPS, embedded camera and long-range telemetry capability. A 3-layer control mechanism has been developed to harness the dynamics and attain complete navigational control of the MAV. The inner-layer is composed of a `quad hybrid-energy controller' and two higher layers are at present, implementing a linear controller; the latter will be replaced eventually with a dynamic adaptive non-linear controller. The advantages of the proposed design compared to other similar ones include higher energy efficiency and extended flight endurance. The design features elastic storage and re-use of propulsion energy favoring energy conservation during flight. The design/modeling of the MAV and its kinematics & dynamics have been tested under simulation to achieve desired performance. The potential applications for such a high endurance vehicle are numerous, including air-deployable mass surveillance and reconnaissance in cluster and swarm formations. The efficacy of the design is demonstrated through a simulation environment. The dynamics are verified through simulations and a general linear controller coupled with an energy based non-linear controller is shown to operate the vehicle in a stable regime. In accordance with specified objectives a prototype is being developed for flight-testing and demonstration purposes.

The Low Reynolds Number Wing Aerodynamics and Micro Aerial Vehicles Development

Micro Aerial Vehicle (MAV) has emerged as an attractive and significant research topic in aeronautical academia in last decade because of its profound applications and complicated aerodyanmics. This paper reports a summary of review about the MAV development and its low Reynolds number wing aerodynamic characteristics (such as instability and evolutional behavior) and vehicle design considerations made in past years in our lab. The most recent progress on optimization of single-winged and cascade-winged MAV design and flight tests is also discussed.

Low Reynolds Number Aerodynamics of Low-Aspect-Ratio, Thin/Flat/Cambered-Plate Wings

The design of micro aerial vehicles requires a better understanding of the aerodynamics of small low-aspect-ratio wings. An experimental investigation has focused on measuring the lift, drag, and pitching moment about the quarter chord on a series of thin flat plates and cambered plates at chord Reynolds numbers varying between 60,000 and 200,000. Results show that the cambered plates offer better aerodynamic characteristics and performance. It also appears that the trailing-edge geometry of the wings and the turbulence intensity in the wind tunnel do not have a strong effect on the lift and drag for thin wings at low Reynolds numbers. Moreover, the results did not show the presence of any hysteresis, which is usually observed with thick airfoils/wings