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The attitude control of fixed-wing MAVS in turbulent environments
Abstract and Figures
The small scale and portability of fixed-wing Micro Aerial Vehicles lend them to many unique applications, however their utility is often limited by ineffective attitude control in turbulent environments. The performance of attitude control systems themselves are affected by a variety of factors. Assessment of this system’s performance needs to be viewed in relation to the MAVs’ unique constraints. Certain aspects and limitations of MAV attitude control related issues are addressed in the literature, but to fully address the degradation of utility, the entire system must be examined. These issues can only be fully addressed when considering them concurrently. There is no framework for defining the attitude control problem explicitly for MAVs. This paper attempts to (1) Define the MAV attitude control problem with respect to the unique constraints imposed by this class of Unmanned Aircraft; (2) Review current design trends of MAVs with respect to vulnerability to atmospheric turbulence.
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... The aircraft are equipped with cameras and sensors to obtain live and recorded aerial images as well as corresponding data based on the mission requirements. In order to obtain clear and reliable recordings, low speed flight of the small UAVs is desired due to low altitude flight and limitations in cameras frame rate and sensitivity of different sensors; thermal, humidity, gas, noise and motion sensors namely [2]. Various small UAVs such as Black Widow, SENDER, MITE, etc., have been built and flown throughout the past decade. ...
... The mean aerodynamic chord corresponds to chord of the wing for rectangular wings while for non-rectangular wings it is calculated using Eq. (2). ...
The application range of small UAVs for civil purposes such as aerial imaging, agricultural pest control and, corps management as well as drone delivery has been increasing throughout recent years. Promising results are observed from recent studies which support the effectiveness of the bio-inspired approach in improving the aerodynamic efficiency of small UAVs. However, very limited studies have investigated the effect of Alula on the aerodynamic performance of wing geometries. This research investigates the effects of employing Alula on the aerodynamic efficiency of the wing to overcome the challenge of small UAVs efficiency degradation. Numerical and experimental investigations are conducted on 6 different Alula configurations. The findings of the investigations shows a reduction in drag for wings and an increase in aerodynamic efficiency by 9% for wings with Alula compared to the conventional wing. It is observed that using Alula configuration helps in delaying flow separation by adding momentum into the flow which resulted in total drag reduction without having a significant effect on the lift. Consequently, the aerodynamic efficiency is increased, and this can be utilized in drone industry to increase the endurance of the flight.
... Since the AAM vehicles are capable of rotor-borne and wing-borne flight, the operation of these vehicles can be categorized into hover, transition, and cruise [1]. Hover and cruise are relatively simple to plan for trajectory of flight and control tasks owing to the knowledge and experience documented in the vast literature for multirotor [2][3][4][5][6] and fixed-wing vehicles [7][8][9], respectively. However, the transition phase poses challenges from two perspectives: trajectory generation and tracking control. ...
... where A| I is measured with respect to or from the inertial frame and A| B is measured with respect to or from the body frame. All terms must be expressed in the same coordinates or along the same unit vector (as discussed in [46]). If not, use the direction cosine rotation matrix to bring it along the same unit vectors as LHS. ...
In this research, a wide-body aircraft was analyzed with critical monitoring of its states, a function of several control inputs (wind gust, turbulence, and microburst). The aerodynamic and stability coefficients of a Boeing 747-200 were obtained from previously published works and 6- DOF equations were formulated. Simulations were conducted for various control inputs to determine the aircraft’s free response, as well as the forced response. In order to understand the nature of the atmosphere, three different models were incorporated, including (i) the Dryden Model, (ii) wind gust, and (iii) microburst. The aircraft was found to be stable in the longitudinal and lateral flight modes, with trim conditions agreeing with published data. For a vertical wind gust of −10 ft/s, the AoA and pitch rate were observed to oscillate sinusoidally and became stable with new trim conditions. These states were found to regain trim conditions once the gust was removed. In the case of 3D gust, it was found that the longitudinal modes achieved a new trim condition through Phugoid oscillations, whereas the lateral modes underwent short-period oscillations. For the case of turbulence, random fluctuations were observed for trim conditions with no unstable behavior. When considering the microburst case, it was found that the aircraft initially gained altitude in the region of the headwind; this was followed by a sharp descent under the influence of a vertical velocity component.
... Birds have evolved for billions of years to develop excellent bone structure and feather distribution, which allow them to realize their agile flight. 1 Their wings can transfer plane shape and flapping mode to obtain high maneuverability in comparison to current fixed-wing or rotary-wing aircraft. 2 Wingtip slots are essential structures to help birds achieve efficient flight, and they are commonly observed on various kinds of birds. As illustrated in Fig. 1, wingtip slots appear as outer parts of the wings, split apart, and present nonplanar extensions. ...
Bird wings have split primary feathers that extend out from the wing surface. This structure is called the wingtip slot, which is recognized as a product of bird evolution to improve flight performance. In this paper, numerical simulations based on RANS (Reynolds-averaged Navier–Stokes) equations are conducted to examine and understand the influence of wingtip slots on six wings at Re = 100 000. The overlapping grid method, driven by an in-house UDF (User Defined Function), is used to model the motion of the bionic slotted wings. The motion law of the winglets is improved based on the law extracted from a level-flying bald eagle. Then the aerodynamic force, pressure distribution, vorticity contours, wake stream, and other flow structures of the slotted wings with different layouts were compared and analyzed. The results show a significant increase in aerodynamic force when the slotted wingtips are employed. The maximum lift-to-drag ratio is also improved in our designed wing model with a non-planar wingtip by a maximum of 34% from the base wing. Each winglet works as a single wing due to the existence of slots, with a chordwise pressure distribution similar to that of the main wing. The vortex structures of slotted wings show expressive changes in the tip vortex as compared with the base wing. Additionally, an innovative bionic slotted wing is proposed with a dynamic wingtip that forms varying gaps between winglets. Due to the collective mechanism of aerodynamic interaction among multiple winglets for the innovative wing, it acquires the optimal time-averaged force during a flapping period. As expected, the slotted wingtip reduces the main wingtip vortex intensity and creates weaker vortices. The non-planarity and relative motion of the wingtip strengthen its weakening effect on the wingtip vortex and wake.
... Details of flow control strategies can be found in Gad-el-Hak (2001, Haritonidis (1989). Figure 1, illustrates a typical turbulence control and mitigation system (Mohamed et al. 2014b;Robin Murphy and Robin 2000;Panta et al. 2018). ...
Flow separation is a critical aerodynamic phenomenon impacting flight envelope of various flying objects. Its efficient detection and subsequent control are vital for enhanced manoeuvring, better fuel economy, superior flight controls, and even for the survival of small air vehicles such as MAVs and UAVs. Over the past 3 decades, MEMS technology has enabled a paradigm shift from macro-level conventional sensing techniques to micro-scale sensors. Critical aerodynamics phenomenon such as flow separation, boundary-layer transitioning, shock, local flow and vortex dynamics, etc. can now be captured with better spatial as well as temporal resolution. Analysis spectrum has also been stretched by compensating the limitations of conventional techniques. This review paper is focused on the use of MEMS technology in the field of flow separation detection. We first briefly introduce the flow separation phenomenon and the limitations of conventional sensing techniques. Then, the application of MEMS to flow separation detection will be discussed in detail with emphasis on the prominent research work and performance review of various MEMS sensors employed in recent past.
... Small fixed-wing drones are significantly challenged by disturbances from turbulence in the atmosphere or from the rapid changes in the angle of attack experienced when flying through the shear layers associated with flows around obstacles [1][2][3][4]. Existing turbulence methods were found to be limited by the response rate and authority of control actuators [5][6][7][8][9][10][11][12]; however, these studies investigated only with conventional trailingedge control surfaces as opposed to leading-edge control surfaces (hereafter called TECS and LECS). ...
This study investigates the use of rapidly actuated leading-edge and trailing-edge control surfaces to improve the control authority of small fixed-wing drones. Static and dynamic characteristics were investigated and presented in two separate papers. In this paper, the focus is on the dynamic effects observed from rapidly actuated 30% chord leading- or trailing-edge hinged control surfaces affixed to two flat-plate airfoils. Forces were resolved from surface pressure measurements and are augmented by PIV measurements, smoke flow visualization and analyses. The static study revealed that trailing-edge control surfaces exhibited higher effectiveness in producing time-averaged CL compared to leading-edge control surfaces. However, leading-edge control surfaces exhibit significantly less fluctuation in pressure and lift coefficients at fixed angles of attack and control surface deflections, indicating better stability. Unsteady aerodynamic effects of the airfoil at α=0∘ and “ramp” deflections of trailing- and leading-edge control surfaces from 0∘ to 40∘ with variations in actuation rates showed that CL peaks are approximately three to four times greater than static values for the case of the leading-edge control surface. This has significant implications for fixed-wing drone maneuverability and countering the effects of atmospheric turbulence.
... UAV's flight stability is endangered by the complex ground environment and random air flows. Its repeated crash incidents show that the customary flight control strategies intended for a conventional aircraft are not appropriate for UAV's flight control because of its light weight, low speed, small size and low cost actuators [4,5]. ...
Flapping wings provide a theoretically energy efficient mode of lift and maneuverability in Unmanned Air Vehicles (UAVs) for a wide variety of applications. However, their performance is significantly degraded in presence of turbulence. In order to address this problem of turbulence, motivated by the covert feathers present in birds, this paper presents a multibody Bond Graph Model (BGM) of a biologically inspired Flapping Wing UAV (FUAV) capable of flight in turbulent airflows. To further simplify computations and carry out stability analyses, Reduced Order Modeling (ROM) of the multibody model is offered. For the proposed multibody FUAV, a biomimetic closed loop Linear Quadratic Regulator (LQR) based flight controller is created in order to achieve stable flight during gusts. A successful gust mitigation of up to 32% is shown in simulations of the optimized model and proposed control approach, which consistently show stable behavior under turbulent flight. Furthermore, the accuracy and effectiveness of the suggested control approach is confirmed by the strong agreement between previously published experimental data and the present findings.
Insects, bats, and small birds show outstanding flight performance even under complex atmospheric conditions, which is partially due to the ability of these natural fliers to sense and react to disturbances quickly. These biological systems often use large numbers of sensors arrayed across their bodies to detect disturbances, but previous efforts to use large arrays of sensors in engineered fliers have typically resulted in slow responses due to the need to scan and process data from the large number of sensors. To address the challenges of capturing disturbances in a large sensing array with low latency, this work proposes and demonstrates a modular soft sensing system to quickly detect disturbances in small unmanned aerial vehicles. A large array of soft strain sensors with high sensing resolution covers the entire wingspan, providing rich information on wing deformation. Owing to the modular design, decentralized computation enables the sensing system to efficiently manage sensor data, resulting in sufficiently fast sampling to capture wing dynamics while all 32 sensors embedded in the modular soft sensing skin are used. This hardware architecture also results in significantly reduced noise in the sensing system, leading to a high signal-to-noise ratio. These methods can ultimately enable fast and reliable control of both soft and rigid robotic systems using large arrays of soft sensors.
Birds are notable for their ability to seamlessly transition between different locomotory functions by dynamically leveraging their shape-shifting morphology. In contrast, the performance of aerial vehicles is constrained to a narrow flight envelope. To understand which functional morphological principles enable birds to successfully adapt to complex environments on the wing, engineers have started to develop biomimetic models of bird morphing flight, perching, aerial grasping and dynamic pursuit. These studies show how avian morphological capabilities are enabled by the biomaterial properties that make up their multifunctional biomechanical structures. The hierarchical structural design includes concepts like lightweight skeletons actuated by distributed muscles that shapeshift the body, informed by embedded sensing, combined with a soft streamlined external surface composed of thousands of overlapping feathers. In aerospace engineering, these functions are best replicated by smart materials, including composites, that incorporate sensing, actuation, communication, and computation. Here we provide a review of recently developed biohybrid, biomimetic, and bioinspired robot structural design principles. To inspire integrative smart material design, we first synthesize the new principles into an aerial robot concept to translate it into its aircraft equivalent. Promising aerospace applications include multifunctional morphing wing structures composed out of smart composites with embedded sensing, artificial muscles for robotic actuation, and fast actuating compliant structures with integrated sensors. The potential benefits of developing and mass-manufacturing these materials for future aerial robots and aircraft include improving flight performance, mission scope, and environmental resilience.
This paper aims to achieve a study for increasing the level of confidence in the results concerning the lateral-directional stability of a real aircraft in the design stage, using the results from a flying scale model. Similarity coherent criteria are proposed for the dimensions, mass, inertia and cinematic characteristics between the real aircraft and the scale mockup model. Comparison between the real aircraft and the scale model plane show the same values for the damping factor in "Dutch roll". Values factored with a constant are obtained for the time characteristics in "Dutch roll", "Roll" and "Spiral" modes.
This paper provides a comprehensive, critical review of turbulence observations over cities. More than fifty studies are analysed with their experimental conditions summarized in an appendix. The main results are based on 14 high-quality experiments which met criteria based on stringent experimental requirements. The observations are presented as nan-dimensional statistics to facilitate comparison between urban studies and work conducted over other rough, inhomogeneous surfaces. Wake production associated with bluff bodies, and the inhomogeneous distribution of sources and sinks of scalars, result in a roughness sub-layer which for the studies reviewed extends to about 2.5 to 3 times the height of the buildings. It is shown that within this region the basis of several traditional micrometeorological approaches to describe the turbulent exchange is in doubt. There are strong similarities to flow over plant canopies, and many of the turbulence characteristics can be interpreted in the framework of a plane mixing layer. Future field observations should concentrate on the turbulent exchange near the top and within the urban canopy as well as within the urban boundary layer.
This paper introduces a new biomimetic concept whose purpose enhances aircraft stability and maneuverability during flight. The research consists of localized control loop systems being integrated throughout the wing that sense the forces being experienced from turbulent airflow or wind gusts during flight. The design consists of feather-like components installed on both the upper and lower wing surfaces and extending beyond the wing's trailing edge. These structures contain piezoelectric elements that act as sensors, actuators, and load-bearing members. Their purpose is to operate as the aircraft's flaps and ailerons and additionally to sense and react to the forces experienced by the wing surface. Computational Fluid Dynamics studies using k-ε turbulence models with Reynolds Averaged Navier Stokes equations are performed using the multi-physics software COMSOL. The combination of structure mechanics, CFD, piezoelectricity, and fluid-body interactions are assessed simultaneously. In the present study, several standard airfoil configurations with movable trailing-edge flaps are simulated in order to create a baseline of comparison with the proposed feathered wing system. This research is an initial step into a novel morphing wing design that enables aircraft to operate in a manner more closely inspired by their natural counterparts. © (2010) by the International Conference on Adaptive Structures and Technologies (ICAST).
Achieving useful endurance with Micro Air Vehicles (MAVs) using on-board electric powerplants remains challenging. This paper experimentally examined the feasibility of using orographic 'slope' lift in an urban built environment to increase the endurance of MAV platforms. The glide polar of a soaring MAV was measured in a wind-tunnel and validated through flight-testing, then compared with the velocity field immediately upwind of a representative urban building. The velocity field was mapped using a 1: 100 scale model of the building in a wind-tunnel with a scaled atmospheric boundary layer. The vertical velocity component was found to be in the order of 15% to 50% of the mean wind velocity at building height. These results were compared with data measured on the full-size building and found to agree well. As the sink rate of the MAV was less than the available vertical velocity component for a wide flight speed range, it was concluded that it is possible to 'soar' immediately upwind of urban buildings to increase endurance. However, considerable control challenges are thought to exist since the full-scale data demonstrated that the flow exhibited high turbulence intensities.
Major challenges to low speed micro flight are the transient and time-averaged velocities arising from the Atmospheric Boundary Layer (ABL), particularly turbulence a few metres above the ground. In this paper, existing data from meteorologists and wind engineers are reviewed and measurements dedicated to understanding the spatial and temporal velocity fields that MAVs experience are briefly described. Data from a wide variety of terrains are analysed, with the majority of data obtained in relatively well mixed turbulent flow (i.e. away from local effects such as buildings) and for conditions of nominally neutral stability. Spectra for data well removed from local effects exhibited the expected 5/3rds Kolmogorov law. Transient flow pitch angles were investigated (obtained from four small laterally displaced probes), in order to understand the possible roll and pitch inputs to MAVs. It was noted that for all data obtained the variation with lateral separation decreased relatively slowly with reducing separation down to the closest inter-probe spacing of 14mm. This effect is thought to explain the increasing piloting difficulties experienced in maintaining good roll control for decreasing scales of craft when any appreciable atmospheric winds are present.