Low Aspect Ratio Wings Research Articles

Membrane Micro Air Vehicles with Adaptive Aerodynamic Twist: Numerical Modeling

Micro air vehicles are typically characterized by a low aspect ratio wing operating at low Reynolds numbers (104): aerodynamics involve a three-dimensional flow field with numerous regions of separated flow. Furthermore, aerodynamic twist can be built into the wing through the use of a thin membrane skin, to adaptively increase the wing camber. This work formulates a static aeroelastic model of such a wing, by coupling a linear membrane model to a well-validated steady laminar Navier–Stokes solver. The membrane deformation causes a significant pressure redistribution which increases lift and longitudinal static stability, though a drag penalty also develops. The efficiency of a rigid wing increases with Reynolds number, but decreases for a membrane wing, as the deformation generally provides a nonoptimal airfoil shape. Membrane deformation leads to larger separation bubbles, and acts as a barrier to the tip vortex formation. At high angles of attack, the aerodynamic twist causes a direct interaction between the recirculating flow and the tip vortices, indicating potential roll instabilities.

Multi-Objective Topology Optimization of Wing Skeletons for Aeroelastic Membrane Structures

This work considers the multi-objective aeroelastic optimization of a membrane micro air vehicle wing through topology optimization. The low aspect ratio wing is discretized into panels: a two material formulation on the wetted surface is used, where each panel can be membrane (wing skin) or carbon fiber (laminate reinforcement). An analytical sensitivity analysis of the aeroelastic system is used for the gradient-based optimization of aerodynamic objective functions. An explicit penalty is added, as needed, to force the structure to a 0–1 distribution. Pareto trade-off curves are constructed by considering convex combinations of two disparate lift, drag, or pitching moment-based objective functions. The general relationship between spatial stiffness distribution (wing topology) and aerodynamic performance is discussed, followed by the Pareto optimality of the computed designs over a series of baseline wing structures. The work concludes with an experimental validation of the superiority of select optimal designs.

Dynamic Aeroelasticity of Structurally Nonlinear Configurations Using Linear Modally Reduced Aerodynamic Generalized Forces

Steady state and time domain methods for integrating commonly used frequency-domain unsteady aerodynamic modeling based on a modal approach with full order flnite element models for structures with geometric nonlinearities are introduced. The methods are aimed at airplane conflgurations where geometric stifiness efiects are important but where deformations are moderate, ∞ow is attached, and linear unsteady aerodynamic modeling is adequate, such as joined-wing and strut-braced wings at small to moderate angles of attack or low aspect ratio wing. Results obtained using full order nonlinear structural modeling and full order aerodynamic modeling (including aerodynamic in∞uence coe‐cients for all aerodynamic panels) are compared to results obtained using full order structural modeling with generalized aerodynamic matrices obtained using a modal approach.

비선형 와류격자법을 이용한 낮은 종횡비 날개의 공력특성 계산

본 논문에서는 새로운 비선형 와류격자법 계산 과정이 제안된다. 기존의 계산 과정은 자유와의 형태 계산을 위해 내부 반복계산 및 하향이완법을 포함한다. 하지만 본 논문에서는 유사 정상 개념에 기초한 새로운 수식을 제안하여 자유와의 형태를 계산함으로써, 계산 과정에서 내부 반복계산 및 하향이완법을 생략한다. 또한 반복계산이 진행됨에 따라 각 분절에 유도되는 유속도를 적절히 평균해 줌으로써 알고리듬의 수치적 안정성을 향상시킨다. 그리고 낮은 종횡비 날개에 대한 수치실험을 수행하여 분절의 길이, 와류중심반경, 후류영역 계산범위 등과 같은 중요 인자들의 적절한 기준을 경험적으로 결정한다. new computational procedure for the Non-Linear Vortex Lattice Method (NLVLM) is suggested in this work. Conventional procedures suggested so far usually involves inner iteration loop to update free vortex shape and an under-relaxation based iteration loop to determine the free vortex shape. In this present work, we suggest a new formula based on quasi-steady concept to fix free vortex shape which eliminates the need for inner iteration loop. Further, the ensemble averaging of the induced velocities for a given free vortex segment evaluated at each iteration significantly improves the convergence property of the algorithm without resorting to the under-relaxation technique. Numerical experiments over several low aspect ratio wings are carried out to obtain optimal empirical parameters such as the length of the free vortex segment, the vortex core radius, and the rolled-up wake length.

Aeroelastic topology optimization of membrane structures for micro air vehicles

This work considers the aeroelastic optimization of a membrane micro air vehicle wing through topology optimization. The low aspect ratio wing is discretized into panels: a two material formulation on the wetted surface is used, where each panel can be membrane (wing skin) or carbon fiber (laminate reinforcement). An analytical sensitivity analysis of the aeroelastic system is used for the gradient-based optimization of aerodynamic objective functions. An explicit penalty is added, as needed, to force the structure to a 0–1 distribution. The dependence of the solution upon initial design, angle of attack, mesh density, and objective function are presented. Deformation and pressure distributions along the wing are studied for various load-augmenting and load-alleviating designs (both baseline and optimized), in order to establish a link between stiffness distribution and aerodynamic performance of membrane micro air vehicle wings. The work concludes with an experimental validation of the superiority of select optimal designs.

Dual Location Separation Control on a Semispan Wing

Separation is controlled on a semispan wing via nominally two-dimensional zero mass-flux perturbations that are introduced at 1) the leading edge, 2) the shoulder of deflected flaps (20-40 deg deflection), and 3) both locations simultaneously. Leading-edge perturbations mimic the effect of a leading-edge device and flap-shoulder perturbations simulate additional flap deflection while simultaneously reducing drag. When perturbations are introduced simultaneously, at reduced frequencies of O(1) at poststall angles of attack, their combined effect on the wing's lift exceeds each individual contribution. When the physical frequencies are the same, lift is strongly dependent on the difference in phase between the two perturbations and an optimum is reached when the suction portion of flap-shoulder control coincides with the minimum shear layer proximity to the flap surface. At the optimum phase difference, increases in lift are observed over the entire semispan, resulting in an overall increase in the wing maximum lift coefficient between 0.24 and 0.44 depending on the flap deflection configuration and angle. Flap-shoulder control effectiveness does not diminish at the extremities of the finite inboard flap; neither inboard in the vicinity of the junction vortex, nor outboard in the vicinity of the flap-edge vortex. With a large flap deflection across the entire span, flap-shoulder control perturbations produce a powerful tip vortex that may be exploited for lift enhancement on low aspect ratio wings. The sensitivity to perturbation phase difference, however, diminished significantly toward the wing tip.

Predicting Maximum Lift Coefficient for Twisted Wings Using Computational Fluid Dynamics

*† ‡ A method is presented that allows one to predict the maximum lift coefficient for a finite wing from knowledge of wing geometry and maximum airfoil section lift coefficient. This approach applies to wings of arbitrary planform and includes the effects of twist and sweep. The method utilizes a correlation obtained from grid-resolved computational fluid dynamics solutions for 25 different wing geometries. These wings had aspect ratios ranging from 4 to 20, taper ratios from 0.5 to 1.0, quarter-chord sweep angles from 0 to 30 degrees, and linear geometric washout ranging from 0 to 8 degrees. For this range of parameters, the ratio of maximum wing lift coefficient to maximum airfoil section lift coefficient varied from about 0.70 to 0.98, with high-aspect-ratio tapered wings producing the highest values and lowaspect-ratio wings with washout and sweep producing the lowest values.

Aerodynamic Coefficients and Deformation Measurements on Flexible Micro Air Vehicle Wings

This paper documents the elastic deformations and corresponding aerodynamic coefficients of flexible wings used for micro air vehicles (MAVs). These low-aspect ratio wings, developed and fabricated at the University of Florida, incorporate an elastic latex membrane skin covering a thin carbon fiber skeleton. The wings were tested in a unique low-speed wind tunnel facility integrating a visual image correlation (VIC) system with a six-component strain gauge sting balance. Model characteristics are presented, along with the appropriate specimen preparation techniques and wind tunnel instrumentation. The static response characteristics, including full-field displacements and plane strain measurements, for three distinct MAV wing designs are presented. The full-field deformation results show how passive wing flexibility preferably affects aerodynamic performance when compared to a rigid model of similar geometry.

Wake Vorticity Measurements for Low Aspect Ratio Wings at Low Reynolds Number

Trailing vortex structure of low aspect ratio wings was studied in a water tunnel at Reynolds numbers of 8000 and 24,000 using dye injection and digital particle image velocimetry in cross-flow planes in the near wake, for rectangular, semi-elliptical, and delta-wing planforms. The velocity data were used to calculate lift via circulation and effective span, and the results were compared with force balance measurements and classical inviscid theory. The objectives of the study were to assess how low-Reynolds number effects might affect the measurement of lift coefficient from tip-vortex circulation, how well the measurements fit the various theoretical models of lift curve slope for low aspect ratio wings, and the extent to which planform shape affects lift coefficient while aspect ratio and planform area are kept constant. All models were thin flat plates with square edges

Numerical Investigation of Low-Aspect-Ratio Wings at Low Reynolds Numbers

A numerical analysis is performed to study the flow around low-aspect-ratio (LAR) wings and more particularly the resulting lift-and-drag force. The research is focused on low-Reynolds-number aerodynamics, as LAR wings are crucial for the development of microair vehicles (MAVs). The flow around LAR wings is characterized by complex three-dimensional flow phenomena. These phenomena include wing-tip vortices, flow separation and reattachment, laminar to turbulent transition, and a mutual interaction among these phenomena. The flow is studied using a commercial computational fluid dynamics (CFD) program and a strip method. The CFD code is used to investigate the three-dimensional flow aerodynamics of rectangular LAR wings with an aspect ratio between 0.5 and 2 at a Reynolds number of 1 x 10 5 . Simulations on a flat plate and a reflex-type low-Reynolds-number profile (S5010), which is representative for a flying-wing MAV, are performed and compared. Experimental data is used for comparison and validation. The effects of flow separation and low Reynolds numbers are further investigated using a strip method. Two accurate formalized methods to predict lift and drag are derived. The first method is applicable to profiled wings with moderate low-Reynolds-number effects. The second method, which is based on the strip method, is more general and is also applicable to flat plates and wings exhibiting large regions of flow separation.

Identification of new lift generation mechanism in flapping flight

In this paper the combined effect of two mechanisms for lift enhancement at low Reynolds numbers are considered, wing oscillations and wing flexibility. The force, deformation and flow fields of rigid and flexible low aspect ratio (AR=3) and high aspect ratio (AR=6) wings oscillating at a fixed post-stall angle of attack of 15° and amplitude of 15% of chord are measured. The force measurements show that flexibility can increase the time-averaged lift coefficient significantly. For low aspect ratio wings the maximum lift coefficient across all Strouhal numbers was Cl=1.38 for the rigid wing as opposed to Cl=2.77 for the flexible wing. Very similar trends were observed for the high aspect ratio wings. This increase is associated with significant deformation of the wing. The root is sinusoidally plunged with small amplitude but this motion is amplified along the span resulting in a larger tip motion but with a phase lag. The amount it is amplified strongly depends on Strouhal number. A Strouhal number of Src=1.5 was selected for detailed flow field measurements due to it being central to the high-lift region of the flexible wings, producing approximately double the lift of the rigid wing. For this Strouhal number the rigid wings exhibit a Leading Edge Vortex (LEV) ring. This is where the clockwise upper-surface LEV pairs with the counter-clockwise lower-surface LEV to form a vortex ring that self-advects upstream and away from the wing's upper surface. Conversely the deformation of the flexible wings inhibits the formation of the LEV ring. Instead a strong upper-surface LEV forms during the downward motion and convects close to the airfoil upper surface thus explaining the significantly higher lift. These measurements demonstrate the significant gains that can be achieved through the combination of unsteady aerodynamics with flexible structures.

Discovery of an ornithurine bird and its implication for Early Cretaceous avian radiation.

An ornithurine bird, Hongshanornis longicresta gen. et sp. nov., represented by a nearly complete and articulated skeleton in full plumage, has been recovered from the lacustrine deposits of the Lower Cretaceous Jehol Group in Inner Mongolia, northeast China. The bird had completely reduced teeth and possessed a beak in both the upper and lower jaws, representing the earliest known beaked ornithurine. The preservation of a predentary bone confirms that this structure is not unique to ornithischian dinosaurs but was common in early ornithurine birds. This small bird had a strong flying capability with a low aspect ratio wing. It was probably a wader, feeding in shallow water or marshes. This find confirms that the aquatic environment had played a key role in the origin and early radiation of ornithurines, one branch of which eventually gave rise to extant birds near the Cretaceous/Tertiary boundary. This discovery provides important information not only for studying the origin and early evolution of ornithurines but also for understanding the differentiation in morphology, body size, and diet of the Early Cretaceous birds.

Membrane Wing-Based Micro Air Vehicles

Micro air vehicles (MAVs) with a wingspan of 15cm or shorter, and flight speed around 10m∕s have attracted substantial interest in recent years. There are several prominent features of MAV flight: (i) low Reynolds number (104-105), resulting in degraded aerodynamic performance, (ii) small physical dimensions, resulting in certain favorable scaling characteristics including structural strength, reduced stall speed, and impact tolerance, and (iii) low flight speed, resulting in order one effect of the flight environment and intrinsically unsteady flight characteristics. Flexible wings utilizing membrane materials are employed by natural flyers such as bats and insects. Compared to a rigid wing, a membrane wing can better adapt to the stall and has the potential for morphing to achieve enhanced agility and storage consideration. We will discuss the aerodynamics of both rigid and membrane wings under the MAV flight condition. To understand membrane wing performance, the fluid and structure interaction is of critical importance. Flow structures associated with the low Reynolds number and low aspect ratio wing, such as pressure distribution, separation bubble, and tip vortex, as well as structural dynamics in response to the surrounding flow field are discussed. Based on the computational capabilities for treating moving boundary problems, an automated wing shape optimization technique is also developed. Salient features of the flexible-wing-based MAV, including the vehicle concept, flexible wing design, novel fabrication methods, aerodynamic assessment, and flight data analysis are highlighted.

Low Aspect Ratio Aerodynamics at Low Reynolds Numbers

The recent interest in the development of small unmanned aerial vehicles (UAVs) and micro air vehicles has revealed a need for a more thorough understanding of the aerodynamics of small airplanes flying at low speeds. In response to this need, a study of the lift, drag, and pitching moment characteristics of wings of low aspect ratio operating at low Reynolds numbers are presented. Wind-tunnel tests of wings with aspect ratios between 0.5 and 2.0, four distinct planforms, thickness-to-chord ratios of ≈ 2%, and 5-to-1 elliptical leading edges have been conducted as part of this research. The Reynolds numbers considered were in the range of 7 × × 10 4 to 2 × × 10 5 . Analysis of the data includes comparison of lift-curve slope, nonlinear equation approximations, maximum lift coefficient, and center of lift.

Biomechanic consequences of differences in wing morphology betweenTadarida brasiliensis andMyotis chiloensis

The wing morphology of bats is very diverse, and may correlate with energetic, behavioural, and ecological demands. If these demands conflict, wing shape may reflect compromise solutions. In this study, we compared the wing morphology of two bats,Tadarida brasiliensis (Geoffroy, 1824) andMyotis chiloensis (Waterhouse, 1828), that differ in body size, habitat, and foraging behaviour. We analyzed features of biomechanical and energetic relevance, and sought evidence of compromise solutions to energetic, ecological, and behavioural demands. We found that wing span of both species conformed to expectations based on allometric relationships, but that although the wing area ofM. chiloensis did not differ from predictions, the wing area ofT. brasiliensis was lower.M. chiloensis possessed an unusually low second moment of area of the humerus. Wing form ofM. chiloensis is consistent with highly maneuverable flight needed to live between shrubs and wooded habitats, and its low aspect ratio and low wing loading indicate a high energetic cost and a low flight speed, respectively. The low humeral second moment of area may be related to a reduction of wing mass and may result in decreased inertial power. In contrast,T. brasiliensis showed high aspect ratio and wing loading, characteristic of high speed, energetically economic flight.

Flow visualization on a low aspect ratio wing by liquid crystal thermography

Flow visualization on a low aspect ratio wing by liquid crystal thermography

Morphology, Velocity, and Intermittent Flight in Birds1

Body size, pectoralis composition, aspect ratio of the wing, and forward speed affect the use of intermittent flight in birds. During intermittent non-flapping phases, birds extend their wings and glide or flex their wings and bound. The pectoralis muscle is active during glides but not during bounds; activity in other primary flight muscles is variable. Mechanical power, altitude, and velocity vary among wingbeats in flapping phases; associated with this variation are changes in neuromuscular recruitment, wingbeat frequency, amplitude, and gait. Species of intermediate body mass (35–158 g) tend to flap-glide at slower speeds and flap-bound at faster speeds, regardless of the aspect ratio of their wings. Such behavior may reduce mechanical power output relative to continuous flapping. Smaller species (<20 g) with wings of low aspect ratio may flap-bound at all speeds, yet existing models do not predict an aerodynamic advantage for the flight style at slow speeds. The behavior of these species appears to be due to wing shape rather than pectoralis physiology. As body size increases among species, percent time spent flapping increases, and birds much larger than 300 g do not flap-bound. This pattern may be explained by adverse scaling of mass-specific power or lift per unit power output available from flight muscles. The size limit for the ability to bound intermittently may be offset somewhat by the scaling of pectoralis composition. The percentage of time spent flapping during intermittent flight also varies according to flight speed.

Effects of Angle of Attack on Nonlinear Flutter of a Delta Wing

Limit cycle oscillations (LCOs) have been observed in flight for certain modern high-performance aircraft. The nonlinear physical mechanism responsible for the LCOs is still in doubt, even to the point of it not yet being determined whether the nonlinearity is principally in the flexible elastic structure of the aircraft or due to the fluid behavior in the surrounding aerodynamic flowfield. One observation from flight tests is that by changing the angle of attack of aircraft, the flight velocity at which LCOs begin may be raised or lowered and that the amplitude of the LCOs may be reduced. It has been suggested that this sensitivity to angle of attack indicates the nonlinearity is in the fluid rather than in the structure. We show that such effects of an angle of attack change can be the result of a structural nonlinearity. Specifically, an investigation to determine the effects of a steady angle of attack on nonlinear flutter and LCO of a delta wing-plate model in low subsonic flow has been made. A three-dimensional time domain vortex lattice aerodynamic model and a reduced order aerodynamic technique were used, and the structure is modeled using von Karman plate theory that allows for geometric strain-displacement nonlinearities in the delta wing structure. The results provide new insights into nonlinear aeroelastic phenomena not previously widely appreciated, i.e., LCOs for low aspect ratio wings that have a platelike nonlinear structural behavior. The effects of a steady angle of attack on both the flutter boundary and the LCOs are found to be significant. For a small steady angle of attack, α 0 ≤ 0.1 deg, the flutter onset velocity increases, whereas for larger α 0 , it decreases. Moreover, as α 0 increases, the maximum LCO amplitude decreases substantially. Such effects have been observed by Bunton and Denegri in flight flutter experiments. It is noted that the present theoretical results do not prove that the LCOs phenomena observed in flight are due to structural nonlinearities; however, the results of the present analysis are consistent with those observed in flight and do show that a structural nonlinearity can give rise to the observed effects of angle of attack on the LCOs.

Biomechanics and physiology of gait selection in flying birds.

Two wing-beat gaits, distinguished by the presence or absence of lift production during the upstroke, are currently used to describe avian flight. Vortex-visualization studies indicate that lift is produced only during the downstroke in the vortex-ring gait and that lift is produced continuously in the continuous-vortex gait. Tip-reversal and feathered upstrokes represent different forms of vortex-ring gait distinguished by wing kinematics. Useful aerodynamic forces may be produced during tip-reversal upstroke in slow flight and during a feathered upstroke in fast flight, but it is probable that downstroke forces are much greater in magnitude. Uncertainty about the function of these types of upstroke may be resolved when more data are available on wake structure in different flight speeds and modes. Inferring from wing kinematics and available data on wake structure, birds with long wings or wings of high aspect ratio use a vortex-ring gait with tip-reversal upstroke at slow speeds, a vortex-ring gait with a feathered upstroke at intermediate speeds, and a continuous-vortex gait at fast speeds. Birds with short wings or wings of low aspect ratio use a vortex-ring gait with a feathered upstroke at all speeds. Regardless of wing shape, species tend to use a vortex-ring gait for acceleration and a continuous-vortex gait for deceleration. Some correlations may exist between gait selection and the function of the muscular and respiratory system. However, overall variation in wing kinematics, muscle activity, and respiratory activity is continuous rather than categorical. To further our understanding of gait selection in flying birds, it is important to test whether upstroke function varies in a similar manner. Transitions between lifting and nonlifting upstrokes may be more subtle and gradual than implied by a binomial scheme of classification.

Incompressible flow past oscillatory wings of low aspect ratio by the integral equations method

In the framework of the linearized perturbation theory, the pressure jump over the oscillating wing is the solution of a two-dimensional integral equation. Performing an asymptotic expansion with respect to the aspect ratio and keeping the leading terms, we reduce the integral equation to a one-dimensional one, obtaining a simplified method of solving the lifting surface integral equation for a class of thin wings of low aspect ratio with a straight trailing edge. The one-dimensional integral equation is solved for the delta flat plate and the pressure coefficient field and the lift and moment coefficients are calculated. The range of validity of the new method is discussed in the final part of the paper. Copyright © 1999 John Wiley & Sons, Ltd.