Micro Air Vehicles Research Articles

Effect of Gurney Flap on the Aerodynamic Performance of a Flapping Foil: Micro-Aerial Vehicle Application

Abstract Even with its small size, the Gurney flap (GF) can help considerably in increasing the lift of foils and wings. To exploit this feature, the objective of this research was to numerically study the effects of this flow control device on the aerodynamic performance of oscillating foils for micro-aerial vehicle (MAV) applications. Three sets of each important parameter were selected: the height (0.01c, 0.04c, and 0.16c), angle (45 deg, 90 deg, and 135 deg) and location from trailing edge (T.E, 0.05c and 0.1c). A two-dimensional laminar, incompressible Navier–Stokes equation solver was used to computationally investigate the effect of the Gurney flap on the aerodynamic performance of a flat plate (chord length = 10 mm and thickness = 0.03c). It was found that the best aerodynamic performance was obtained when the Gurney flap was installed at the trailing edge with a height of 0.04c and mount angle of 90 deg. The height of the Gurney flap had a major impact on aerodynamic performance. Results showed an increase of 23.5% in mean lift coefficient, 15.5% in maximum lift coefficient, and 5% in power economy as compared to flat plate, which is accredited to the increase in effective camber and the formation of counter-rotating vortices, decreasing the adverse pressure gradient. The weakening of counter-rotating vortices downstream of Gurney flap could also be the contributing factor to its good performance. The results suggest that the Gurney flap may be useful in enhancing the performance of wings for bio-inspired flapping wing MAVs.

Experimental Study on Airflow Upwash and Control of Thrust Increase Induced by Ceiling Effect on Microrotor

Abstract This study examines the mechanism of thrust increase due to the ceiling effect in near-ceiling flight, which is one of the concerns for the practical application of micro-air vehicles in indoor environments. We also proposed a novel rotor blade shape with a pressure recovery hole as one of the passive control techniques and investigated its effectiveness in controlling the thrust increase. The obtained results showed a rapid thrust increase, with an outward swirling flow between the rotor blade and ceiling, observed with a rectangular blade with an extremely small distance between the upper surface of the rotor blade and ceiling. The rapid thrust increase was caused by a pressure difference between the rotor blade and the lower surface of the ceiling in addition to the rotor blade rotation, which generated an outward swirling flow between the rotor blade and ceiling. In addition to the force generated by the pressure difference between the upper and lower blade surfaces due to their rotation, the negative pressure area created by the rotor blades generates the airflow upwash. For the thrust-control rotor blade, the ratio of the pressure recovery hole to the rotor blade radius was 0.5, which maintained approximately 90% or more of the thrust without the upper wall of the rectangular blade; moreover, the distance between the ceiling and the rotor blade at which the thrust began to increase rapidly became shorter, flight performance was generally maintained in the absence of the ceiling, and the thrust increase amplitude was suppressed.

Airfoil optimization for rotors operating in the ultra-low Reynolds number regime

Ultra-low Reynolds number aerodynamics has gained significant attention in recent times, primarily due to the proliferation of micro-aerial vehicles and their utilization in low-density environments such as the Martian atmosphere and high altitudes in Earth's atmosphere. The Martian atmosphere presents a unique combination of low Reynolds numbers and high subsonic Mach numbers, necessitating the use of unconventional airfoil designs in this regime. This study focuses on optimizing airfoils for rotors operating in the compressible ultra-low Reynolds number regime. We demonstrate the capability of XFOIL, a panel method code incorporating an integral boundary layer formulation, to accurately predict airfoil loads when the flow remains attached. We determine the optimal camber and maximum camber positions by analyzing the four digits, two percent thickness, National Advisory Committee for Aeronautics XX02 airfoil family using XFOIL. Subsequently, we employ a multi-objective optimization approach, utilizing a Class Shape Transformation airfoil parameterization to maximize lift and minimize drag. We select several points from the resulting Pareto front and evaluate their performance through unsteady compressible Navier–Stokes simulations. Our findings reveal that incorporating sharp leading-edge variations in these airfoil designs enhances the peak efficiency by over 10%, primarily attributable to the development of laminar separation bubbles on the suction side of the airfoils. Importantly, these modified airfoils maintain favorable performance at low angles of attack.

Autonomous Expansion of Grasshopper Wings Reveals External Forces Contribute to Final Adult Wing Shape.

Ecdysis, transformation from juvenile to adult form in insects, is time-consuming and leaves insects vulnerable to predation. For winged insects, the process of wing expansion during ecdysis, unfurling and expanding the wings, is a critical bottleneck in achieving sexual maturity. Internal and external forces play a role in wing expansion. Vigorous abdominal pumping during wing expansion allows insects to pressurize and inflate their wings, filling them with hemolymph. In addition, many insects adopt expansion-specific postures and, if inhibited, do not expand their wings normally, suggesting that external forces such as gravity may play a role. However, two previous studies over 40years ago, reported that the forewings of swarming locusts can expand autonomously when removed from the emerging insect and laid flat on a saline solution. Termed "autoexpansion," we replicated previous experiments of autoexpansion on flat liquid media, documenting changes in both wing length and area over time while also focusing on the role of gravity in autoexpansion. Using the North American bird grasshopper Schistocerca americana, we tested four autoexpansion treatments of varying surface tension and hydrophobicity (gravity, deionized water, buffer, and mineral oil) while simultaneously observing and measuring intact, normal wing expansion. Finally, we constructed a simple model of a viscoelastic expanding wing subjected to gravity, to determine whether it could capture aspects of wing expansion. Our data confirmed that wing autoexpansion does occur in S. americana, but autoexpanding wings, especially hindwings, failed to increase to the same final length and area as intact wings. We found that gravity plays an important role in wing expansion, early in the expansion process. Combined with the significant mass increase we documented in intact wings, it suggests that hydraulic pumping of hemolymph into the wings plays an important role in increasing the area of expanding wings, especially in driving expansion of the large, pleated hindwings. Autoexpansion in a non-swarming orthopteran suggests that local cues driving wing autoexpansion may serve a broader purpose, reducing total expansion time and costs by shifting some processes from central to local control. Documenting wing autoexpansion in a widely studied model organism and demonstrating a mathematical model provides a tractable new system for exploring higher level questions about the mechanisms of wing expansion and the implications of autoexpansion, as well as potential bioinspiration for future technologies applicable to micro-air vehicles, space exploration, or medical and prosthetic devices.

Numerical Analysis of Bioinspired Tails in a Fixed-Wing Micro Air Vehicle

Bird tails play a key role in aerodynamics and flight stability. They produce extra lift for takeoff and landing maneuvers, enhance wing functions and maintain stability during flight (keeping the bird from yawing, rolling and pitching, or otherwise losing control). This paper investigates the use of bioinspired horizontal stabilizers for Micro Air Vehicles (MAVs) involving a Zimmerman wing-body geometry. A selection of five tail shapes of the main types existing in nature is presented, and a parametric analysis is conducted looking into the influence of the most relevant tail geometric parameters to increase the longitudinal static stability of the vehicle. Based on the parametric study, a smaller subset of candidate tail designs are shortlisted to perform a detailed aerodynamic analysis. Then, steady RANS CFD simulations are conducted for a higher-fidelity study of these candidate tail designs to obtain an optimum of each tail type. The criterion for selection of the optimum tail configuration is the maximum aerodynamic efficiency, CLCD , as well as a high longitudinal static stability. The squared-fan tail provides the highest aerodynamic efficiency while maintaining a high longitudinal stability of the vehicle. In conclusion, this paper provides an innovative study of improving longitudinal stability and aerodynamics through the implementation of bioinspired horizontal stabilizers in vehicles with these characteristics.

Efficient Object Detection for Micro Aerial Vehicle Navigation Using Enhanced SSD-HOG Descriptors

Autonomous robots, such as micro aerial vehicles (MAVs), require object detection for navigation and inspection tasks. However, the limited computational resources and real-time constraints of MAVs make object detection challenging. To address this, we propose an efficient object detection method for MAVs using enhanced SSD-HOG descriptors. Our method combines HOG and SSD techniques to create enhanced descriptors that provide better object detection accuracy and efficiency than traditional HOG or SSD descriptors. We evaluate our method on an aerial image dataset and compare it with state-of-the-art methods like SSD and YOLO. Our experimental results demonstrate that our method achieves high accuracy and real-time performance while using limited computational resources. Our proposed method is ideal for MAV navigation applications that require real-time object detection with limited computational resources.

Vortex dynamics and on the mechanism of vertical force enhancement in inclined stroke flapping wings

The rigorous numerical investigation of the class of inclined stroke plane flapping wings has brought to light the vertical force enhancement due to dipole jet interaction. In this study, the class of inclined stroke plane flapping wings with varying inter-plane distances and stroke inclinations are studied for vertical force generation. The configurations with stroke plane angles of 60° and 75° are found to produce significantly high vertical forces. Further, wake structures of the tandem flapping wing in an inclined stroke plane show various types of dipole vortex shedding and dipole jet interactions. The identified types of dipole jet interactions help in the understanding of vertical force trends of the forewing, hindwing, and hence the tandem wing configuration as a whole. It reveals the potential dipole jet interaction producing the highest vertical force in tandem wing configurations. In all, the study can classify the flows of the tandem wing configurations based on vortex shedding during hovering conditions and understand its vertical force enhancements. This not only helps in getting physical insights into the vortex dynamics of inclined stroke plane flapping wings but also in the design optimisation of the tandem wing configurations in biomimetic Micro Air vehicles (MAVs).

Dynamic Analysis and Experiment of Multiple Variable Sweep Wings on a Tandem-Wing MAV

The current morphing technologies are mostly regarded as auxiliary tools, providing additional control torques to enhance the flight maneuverability of unmanned aerial vehicles (UAVs), and they cannot exist independently of the traditional control surfaces. In this paper, we propose a tandem-wing micro aerial vehicle (MAV) with multiple variable-sweep wings, which can reduce the additional inertia forces and moments and weaken the dynamic coupling between longitudinal and lateral motion while the MAV morphs symmetrically for pitch control or asymmetrically for roll control, thereby flying without the traditional aileron and elevator. First, load experiments were conducted on the MAV to verify the structural strength of the multiple variable sweep wings, and the control moments caused by the morphing of the MAV were presented through numerical simulations. Then, the effects caused by symmetric and asymmetric morphing were investigated via dynamic response simulations based on the Kane dynamic model of the MAV, and the generated additional inertia forces and moments were also analyzed during morphing. Finally, dynamic response experiments and open-loop flight experiments were conducted. The experimental results demonstrated that the morphing mode in this study could weaken the coupling between the longitudinal and lateral dynamics and that it was feasible for attitude control without the traditional aileron and elevator while flying.

Optimization of mechanical properties of bio-inspired Voronoi structures by genetic algorithm

Numerous patterns in nature comprise aesthetics structures and intriguing geometries. The geometric factors behind these patterns include area, aspect ratio, circularity, orientation, and regularity. In this study, the quasi-3D porous structures with varying regularities were systematically described and generated by the Voronoi segmentation. Then, genetic algorithms were applied to optimize the mechanical properties of these structures under tension condition. The image-based finite element simulation could calculate each structure's fitness value (strength). Afterward, structures with higher fitness values were selected and underwent the replication process, including crossover and mutation. New structures were generated based on the input structures. The above steps driven by the genetic algorithms were repeated 30 iterations until the tensile strength of the structures was improved and converged. Both simulation and tensile testing results as well as digital image correlation of specimens fabricated by stereolithography 3D printing showed that the Voronoi structures optimized by the genetic algorithm could enhance their stiffness, strength, and toughness values by ∼30%. This research has the potential to be applied in the fields of structural materials and biomimetic micro-aerial vehicles, which require lightweight, strength, and toughness simultaneously.

Piezoelectric Based Adaptive Wing for Flow Control

An attempt is made to develop a piezoelectric based adaptive wing concept that can be used in Micro Air Vehicle (MAV) and Unmanned Air Vehicle (UAV) applications. Laminar boundary layer separation is considered to be a significant problem in low Reynolds number vehicles like MAV. Recently, periodic excitation has gained much interest in the control of flow separation through energizing the boundary layer. In the present study, a piezoelectric bimorph actuator has been used to develop a mechanism that will deflect or vibrate a segment of the wing to produce the required thickness variation and achieve flow control. A Finite Element model of the adaptive wing is developed in ANSYS and numerical results are compared with experiments. A five percent increase in the camber has resulted in 4.39 percent additional lift at higher angles of attack.

Unsteady Aerodynamic Characteristics of The Flapping Wing Micro Air Vehicle (MAV)

Flapping wing propulsion is center of attention in the field of micro air vehicle research in recent years. Knowledge in unsteady aerodynamics is insufficient to design an efficient wing that produces the required lift and thrust force for the flapping wing micro air vehicle. The unsteady aerodynamic effects on aerodynamic wing design parameters were discussed in this paper. The effect of aspect ratio and the wing planform shape on lift and thrust force were determined experimentally. The experiments were conducted in an open jet low speed wind tunnel having test section 350 mmx350 mm. A rack and gear type new flapping mechanism was designed and fabricated and this was used as test bed for all the experiments. Also an experiment was conducted to determine the effect of torsional flexibility of the wing. The result shows that the chord wise flexible wing produces more lift and thrust than the other wing.

Micro Air Vehicles in India - An Overview

The genesis of the Micro Air Vehicles (MICAV) program and its progress in the last decade is discussed. Fixed, rotary and flapping wing MICAVs (300 mm class) have been successfully flown using remote control by Educational Institutions, Laboratories and enthusiastic entrepreneurs, transmitting video imagery. They are now moving on to autonomous control and its integration with navigation and guidance. The need for micro-sensors has led to work on convergence of micro, nano, bio, info and cogno technologies which are discussed. The need for simulation and cooperative flying of MICAVs with UGVs is stressed and finally the thrust areas for the next five years have been suggested for aerodynamics, structures, control, navigation, guidance, sensors (including bio sensors), power sources, simulation and operations involving brain-machine interface.

Trends in Uav Platforms and Technologies

Unmanned Air Vehicles (UAVs) have developed into indispensable war fighting assets of modern battlefield. UAVs play a pivotal role in sharing and disseminating strategic information in a Network-centric war scenario. The fusion of the data obtained from different sensors/ payloads further helps enhance situational awareness and enables quicker sensor-to-shooter reactions. There is a marked shift in the importance given to the Unmanned Air Systems development world-wide with significant increases in budget allocations. From Micro Air Vehicles (MAVs) to Unmanned Combat Aircraft, each type of UAV plays a distinct role at the same time presenting unique technological challenges. The paper discusses the present state of, and future trends in, various UAV platforms as well as sensor payloads carried by them and also highlights challenges in the related technologies. It also describes the ongoing UAV programs and technology competence built up by DRDO.

Grid Based Construction of a Composite Micro Air Vehicle Airframe

The development of a light-weight composite Micro Air Vehicle (MAV) airframe incorporating a novel foam grid based design into Glass prepreg composites is presented. Various combinations of advanced materials are judiciously utilized in different structural configurations to realize diverse requirements in the airframe. Advanced manufacturing techniques such as rapid prototyping, water jet cutting and autoclave curing ensure accurate conformance to the complex contour as well as precise tailored distribution of structural mass. The foam grid based design approach using Glass composites ensures low mass, provides stiffness in multiple directions, imparts crashworthy features and eliminates the problem of electrical leakages. Good correlation between analysis and static test measurement of wing deformation is found. Various issues in the development of a lightweight MAV airframe pertaining to design, tooling, fabrication, testing and analysis are discussed.

Numerical Validation of Low Reynolds Number Airfoil for Uav

Low Reynolds flow continues to get researchers attention for its relevance in a number of areas including the flight regimes of Micro Air Vehicle (MAV). The Laminar Separation Bubble (LSB) which is largely responsible for the non-linear aerodynamic characteristics have been studied both numerically and experimentally. In this work, a high fidelity computation was carried out to study LSB as well as the dependence of aerodynamic characteristics on a symmetrical thick airfoil at low Reynolds numbers. Flow structure in the bubble is clearly brought out with the appropriate selection of turbulence model in the solver. Also, the extent of LSB was well predicted by transition SST turbulence model in the solver. A novel approach has been made to obtain the location and length of LSB using the values of wall shear stress and the results are encouraging. Also, a comparative analysis of flow transition and surface pressure distribution was made by four different turbulence models and the results are discussed.

Airfoil Optimisation at Ultra-Low Reynolds Number

In recent times micro-air vehicles are utilised for various civilian and military applications. Because of their small size and low flight speeds they fly in the low Reynolds number (Re) regime. At such low Re the aerodynamic characteristics, most importantly the lift-to-drag ratio (Cl/Cd), are deteriorated. An improvement in (Cl/Cd) can lead to enhanced vehicle performance. The aerodynamic improvements can be achieved by optimising the wing profile geometry. In the present paper, optimisation of an airfoil is done to maximise (Cl/Cd) at Re = 1000. The upper and lower surfaces of a modified NACA 2408 airfoil are parametrised using the Akima interpolation method. The fixed-direction set method is used in optimising the airfoil aerodynamic performance with six control points each on the upper and lower surfaces. A corrugated airfoil, with a trapped vortex within it, is obtained as a result of this optimisation. Further, optimisation is carried out only on the upper surface of the corrugated airfoil with flat lower surface by increasing the number of control points from six to ten. (Cl/Cd)max is 3.718 for the baseline airfoil, while the final optimal airfoil resulted in (Cl/Cd)max of 4.670, a 25.6 % increase.

The Lift Effects of Chordwise Wing Deformation and Body Angle on Low-Speed Flying Butterflies.

This work investigates the effects of body angle and wing deformation on the lift of free-flying butterflies. The flight kinematics were recorded using three high-speed cameras, and particle-image velocimetry (PIV) was used to analyze the transient flow field around the butterfly. Parametric studies via numerical simulations were also conducted to examine the force generation of the wing by fixing different body angles and amplifying the chordwise deformation. The results show that appropriately amplifying chordwise deformation enhances wing performance due to an increase in the strength of the vortex and a more stabilized attached vortex. The wing undergoes a significant chordwise deformation, which can generate a larger lift coefficient than that with a higher body angle, resulting in a 14% increase compared to a lower chordwise deformation and body angle. This effect is due to the leading-edge vortex attached to the curved wing, which alters the force from horizontal to vertical. It, therefore, produces more efficient lift during flight. These findings reveal that the chordwise deformation of the wing and the body angle could increase the lift of the butterfly. This work was inspired by real butterfly flight, and the results could provide valuable knowledge about lift generation for designing microaerial vehicles.

Decoupling wing-shape effects of wing-swept angle and aspect ratio on a forward-flying butterfly.

The effect of wing shape on a forward-flying butterfly via decoupled factors of the wing-swept angle and the aspect ratio (AR) was investigated numerically. The wing-shape effect is a major concern in the design of a microaerial vehicle (MAV). In nature, the wing of a butterfly consists of partially overlapping forewing and hindwing; when the forewing sweeps forward or backward relative to the hindwing, the wing-swept angle and the AR of the entire wing simultaneously change. The effects of the wing-swept angle and AR on aerodynamics are coupled. To decouple their effects, we established wing-shape models with varied combinations of the wing-swept angle and AR based on the experimental measurement of two butterfly species (Papilio polytes and Kallima inachus) and developed a numerical simulation for analysis. In each model, the forewing and hindwing overlapped partially, constructing a single wing. Across the models, the wing-swept angle and AR of these single wings varied sequentially. The results show that, through our models, the effects of the wing-swept angle and AR were decoupled; both have distinct flow mechanisms and aerodynamic force trends and are consistent in the two butterfly species. For a fixed AR, a backward-swept wing increases lift and drag because of the enhanced attachment of the leading-edge vortex with increased strength of the wingtip vortex and the spanwise flow. For a fixed wing-swept angle, a small AR wing increases lift and decreases drag because of the large region of low pressure downstream and the wake-capture effect. Coupling these effects, the largest lift-to-drag ratio occurs for a forward-swept wing with the smallest AR. These results indicate that, in a flapping forward flight, sweeping a forewing forward relative to a hindwing is suitable for cruising. The flow mechanisms and decoupled and coupled effects of the wing-swept angle and the AR presented in this paper provide insight into the flight of a butterfly and the design of a MAV.

REF: A Rapid Exploration Framework for Deploying Autonomous MAVs in Unknown Environments

Exploration and mapping of unknown environments is a fundamental task in applications for autonomous robots. In this article, we present a complete framework for deploying Micro Aerial Vehicles (MAVs) in autonomous exploration missions in unknown subterranean areas. The main motive of exploration algorithms is to depict the next best frontier for the MAV such that new ground can be covered in a fast, safe yet efficient manner. The proposed framework uses a novel frontier selection method that also contributes to the safe navigation of autonomous MAVs in obstructed areas such as subterranean caves, mines, and urban areas. The framework presented in this work bifurcates the exploration problem in local and global exploration. The proposed exploration framework is also adaptable according to computational resources available onboard the MAV which means the trade-off between the speed of exploration and the quality of the map can be made. Such capability allows the proposed framework to be deployed in subterranean exploration and mapping as well as in fast search and rescue scenarios. The performance of the proposed framework is evaluated in detailed simulation studies with comparisons made against a high-level exploration-planning framework developed for the DARPA Sub-T challenge as it will be presented in this article.

Swallow-tailed separation bubble on a low-aspect-ratio trapezoidal plate: effects of near-wall spanwise flow

The three-dimensional flow over a low-aspect-ratio (low- $A\!R$ ) trapezoidal plate is investigated experimentally with a focus on how the tip effects impact the structure and dynamics of the separation bubble. The chord-based Reynolds number is $5800$ , and the angle of attack varies from $4^\circ$ to $10^\circ$ . Once the flow separates, the separation bubble emerges and features a swallow-tailed structure that shrinks near the midspan, which is first found for the flows over low- $A\!R$ plates. This structure develops into the conventional single-tailed structure as the angle of attack increases. Moreover, the vortex shedding within the swallow-tailed separation bubble is restored from multiple asynchronously measured local velocity fields. It is revealed that the leading-edge vortex undergoes the novel transformation from a C-shape vortex into an M-shape vortex. This vortex transformation stems from the mass transport of the near-wall spanwise flow, which affects the fluid motion on the windward side of the C-shape vortex head, strengthening and accelerating the vortex head. The strengthened vortex head facilitates the entrainment of high-momentum fluid from the outer flow. This is responsible for the formation of the swallow-tailed structure. These findings help to fill the gaps left by the downwash at low angles of attack for low- $A\!R$ wings, and are of value in improving the cruising and gliding performance of micro-air vehicles.