Showing papers in "Journal of Aircraft in 2008"
TL;DR: In this article, the fundamental parametric geometry representation method is used to describe an essentially limitless design space composed entirely of analytically smooth geometries, which is then transformed into the physical space in which the actual geometry definition is obtained.
Abstract: andsimplemathematicalfunctionshavingeasilyobservedphysicalfeatures.Thefundamentalparametricgeometry representation method is shown to describe an essentially limitless design space composed entirely of analytically smooth geometries. The class function/shape function methodology is then extended to more general threedimensional applications such as wing, body, ducts, and nacelles. It is shown that a general three-dimensional geometry can be represented by a distribution of fundamental shapes, and that the class function/shape function methodology can be used to describe the fundamental shapes as well as the distributions of the fundamental shapes. Withthisveryrobust,versatile,andsimplemethod, athree-dimensional geometry isdefinedinadesignspacebythe distribution of class functions and the shape functions. This design space geometry is then transformed into the physical space in which the actual geometry definition is obtained. A number of applications of the class function/ shape function transformation method to nacelles, ducts, wings, and bodies are presented to illustrate the versatility of this new methodology. It is shown that relatively few numbers of variables are required to represent arbitrary three-dimensional geometries such as an aircraft wing, nacelle, or body.
567 citations
TL;DR: AIAA Drag Prediction Workshop (DPW-III) as discussed by the authors focused on the prediction of both absolute and differential drag levels for wing-body and wing-alone configurations that are representative of transonic transport aircraft.
Abstract: Results from the Third AIAA Drag Prediction Workshop (DPW-III) are summarized. The workshop focused on the prediction of both absolute and differential drag levels for wing-body and wing-alone configurations that are representative of transonic transport aircraft The baseline DLR-F6 wing-body geometry, previously used in DPW-II, is also augmented with a side-of-body fairing to help reduce the complexity of the flow physics in the wing-body juncture region. In addition, two new wing-alone geometries have been developed for DPW-III. Numerical calculations are performed using industry-relevant test cases that include lift-specific and fixed-alpha flight conditions, as well as full drag polars. Drag, lift, and pitching-moment predictions from numerous Reynolds-averaged Navier-Stokes computational fluid dynamics methods are presented, focused on fully turbulent flows. Solutions are performed on structured, unstructured, and hybrid grid systems. The structured grid sets include point-matched multiblock meshes and overset grid systems. The unstructured and hybrid grid sets are composed of tetrahedral, pyramid, and prismatic elements. Effort was made to provide a high-quality and parametrically consistent family of grids for each grid type about each configuration under study. The wing-body families are composed of a coarse, medium, and fine grid, whereas the wing-alone families also include an extra-fine mesh. These mesh sequences are used to help determine how the provided flow solutions fare with respect to asymptotic grid convergence, and are used to estimate an absolute drag for each configuration.
150 citations
TL;DR: In this article, the authors examined the voltage requirements for the plasma actuators to reattach the flow at the leading edge of airfoils at poststall angles of attack for a range of flow parameters in order to establish scaling between laboratory and full flight conditions.
Abstract: We present experimental results to yield insight into the scalability and control effectiveness of single-dielectricbarrier-discharge plasma actuators for leading-edge separation control on airfoils. The parameters investigated are chord Reynolds number, Mach number, leading-edge radius, actuator amplitude, and unsteady frequency. This includes chord Reynolds numbers up to 1:0 � 106 and a maximum freestream speed of 60 m=s corresponding to a Mach number of 0.176. The main objective of this work is to examine the voltage requirements for the plasma actuators to reattach the flow at the leading edge of airfoils at poststall angles of attack for a range of flow parameters in order to establish scaling between laboratory and full-flight conditions. For the full range of conditions, an optimum unsteady actuator frequency f is found to minimize the actuator voltage needed to reattach the flow, such that F� � fLsep=U1 � 1. At the optimum frequencies, the minimum voltage required to reattach the flow is weakly dependent on chord Reynolds number and strongly dependent on the poststall angle of attack and leading-edge radius. The results indicate that the voltage required to reattach the flow scales as the square of the leading-edge radius.
136 citations
TL;DR: In this article, an analysis and parametric study of the flight dynamics of highly flexible aircraft is presented, where the aircraft structure is represented as a collection of geometrically exact, intrinsic beam elements, with continuity conditions enforced where beams intersect.
Abstract: An analysis and parametric study of the flight dynamics of highly flexible aircraft are presented. The analysis extends previous work of the authors, used to predict the atypical flight dynamic characteristics of highly flexible flying wings, to conventional configurations with one or more fuselages, wings, and/or tails. The aircraft structure is represented as a collection of geometrically exact, intrinsic beam elements, with continuity conditions enforced where beams intersect. The structural model is coupled with an aerodynamic model consisting of two-dimensional, large-angle-of-attack, unsteady theory for the lifting surfaces, and a fuselage model based on application of slender-body theory to a cylindrical beam. Influences of various design parameters such as wing flexibility, horizontal/vertical tail aerodynamics, and offset are investigated for aeroelasticity and flight dynamics of highly flexible aircraft. Results for prototype configurations illustrate the relationships between its design parameters and flight dynamic behavior.
136 citations
TL;DR: In this article, the T-wing tail-sitter unmanned air vehicle has undergone an extensive program of flight tests, resulting in a total of more than 50 flights, many under autonomous control from takeoff to landing.
Abstract: Since October 2005, the T-wing tail-sitter unmanned air vehicle has undergone an extensive program of flight tests, resulting in a total of more than 50 flights, many under autonomous control from takeoff to landing. Starting in August 2006, free flights with conversion between vertical and horizontal flight modes have also been undertaken. Although the latter flights have required some guidance-level ground-pilot input, significant portions of them were performed in autonomous mode, including the transitions between horizontal and vertical flight. This paper considers the overall control architecture of the vehicle, including the different control modes that the vehicle was flown under during the recent series of tests. Although the individual controllers for each flight mode are unremarkable in themselves, it is notable that the aggregate system allows the vehicle to fly throughout its entire flight envelope, which is considerably broader than that of conventional fixed- or rotary-wing vehicles. The performance of the controllers for the different flight modes will also be considered, with a particular focus on hover dispersion results, in differing wind conditions. The majority of these flights were performed on a tether test rig during autonomous control development, to ensure vehicle safety with minimal impact on vehicle dynamics. The demonstration of autonomous flight under the constraints imposed by the tether system in winds up to 18 kt is a significant achievement Results from the more recent horizontal flight tests with conversions between vertical and horizontal flight are also presented. Most important, these results confirm the basic feasibility of tail-sitter vehicles that use control surfaces submerged in propeller wash for vertical flight control.
111 citations
TL;DR: A hierarchical multifidelity design approach where high-f fidelity models are only used where and when they are needed to correct the shortcomings of the low-fidelity models is proposed.
Abstract: The practical use of high-fidelity multidisciplinary optimization techniques in low-boom supersonic business-jet designs has been limited because of the high computational cost associated with computational fluid dynamics-based evaluations of both the performance and the loudness of the ground boom of the aircraft. This is particularly true of designs that involve the sonic boom loudness as either a cost function or a constraint because gradient-free optimization techniques may become necessary, leading to even larger numbers of function evaluations. If, in addition, the objective of the design method is to account for the performance of the aircraft throughout its full-flight mission while including important multidisciplinary tradeoffs between the relevant disciplines the situation only complicates. To overcome these limitations, we propose a hierarchical multifidelity design approach where high-fidelity models are only used where and when they are needed to correct the shortcomings of the low-fidelity models. Our design approach consists of two basic components: a multidisciplinary aircraft synthesis tool (PASS) that uses highly tuned low-fidelity models of all of the relevant disciplines and computes the complete mission profile of the aircraft, and a hierarchical, multifidelity environment for the creation of response surfaces for aerodynamic performance and sonic boom loudness (BOOM-UA) that attempts to achieve the accuracy of an Euler-based design strategy. This procedure is used to create three design alternatives for a Mach 1.6, 6-8 passenger supersonic business-jet configuration with a range of 4500 n mile and with a takeoff field length that is shorter than 6000 ft. Optimized results are obtained with much lower computational cost than the direct, high-fidelity design alternative. The validation of these design results using the high-fidelity model shows very good agreement for the aircraft performance and highlights the need for improved response surface fitting techniques for the boom loudness approximations.
105 citations
TL;DR: In this paper, a cohesive element for shell analysis is presented, which can be used to simulate the initiation and growth of delaminations between stacked, non-coincident layers of shell elements.
Abstract: A cohesive element for shell analysis is presented. The element can be used to simulate the initiation and growth of delaminations between stacked, non-coincident layers of shell elements. The procedure to construct the element accounts for the thickness offset by applying the kinematic relations of shell deformation to transform the stiffness and internal force of a zero-thickness cohesive element such that interfacial continuity between the layers is enforced. The procedure is demonstrated by simulating the response and failure of the Mixed Mode Bending test and a skin-stiffener debond specimen. In addition, it is shown that stacks of shell elements can be used to create effective models to predict the inplane and delamination failure modes of thick components. The results indicate that simple shell models can retain many of the necessary predictive attributes of much more complex 3D models while providing the computational efficiency that is necessary for design.
102 citations
TL;DR: In this paper, the structural and material considerations for fiber/metal composite technology for future primary and secondary aircraft structures were discussed, and it was concluded that a composite technology approach, in which both metals and fibers are combined to form a tailored structural material, can lead to significant weight reduction in future structural applications.
Abstract: This paper discusses the structural and material considerations for fiber/metal composite technology for future primary and secondary aircraft structures. Based on these considerations and the experience obtained so far with fiber/metal laminates in primary aircraft structures, the potential field of further development of fiber/metal composite technology will be explained. It is concluded that a composite technology approach, in which both metals and fibers are combined to form a tailored structural material, can lead to significant weight reduction in future structural applications.
96 citations
TL;DR: In this paper, a high-resolution particle image velocimetry system was used to quantify the transient behavior of vortex and turbulent flow structures around the flexible-membrane airfoils/wings.
Abstract: An experimental study was conducted to assess the benefits of using flexible-membrane airfoils/wings at low Reynolds numbers for micro air vehicle applications compared with using a conventional rigid airfoil/wing. In addition to measuring aerodynamic forces acting on flexible-membrane airfoils/wings, a high-resolution particle image velocimetry system was used to conduct flowfield measurements to quantify the transient behavior of vortex and turbulent flow structures around the flexible-membrane airfoils/wings to elucidate the associated underlying fundamental physics. The aerodynamic force measurements revealed that flexible-membrane airfoils could provide better aerodynamic performance compared with their rigid counterpart at low Reynolds numbers. The flexibility (or rigidity) of the membrane skins of the airfoils was found to greatly affect their aerodynamic performance. Particle image velocimetry measurements elucidated that flexible-membrane airfoils could change their camber (i.e., crosssectional shape) automatically to adapt incoming flows to balance the pressure differences on the upper and lower surfaces of the airfoils, therefore suppressing flow separation on the airfoil upper surfaces. Meanwhile, deformation of the flexible-membrane skins was found to cause significant airfoil trailing-edge deflection (i.e., lift the airfoil trailing edge up from its original designed position), which resulted in a reduction of the effective angles of attack of the flexible-membrane airfoils, thereby delaying airfoil stall at high angles of attack. The nonuniform spanwise deformation of the flexible-membrane skins of the flexible-membrane airfoils was found to significantly affect the characteristics of vortex and turbulent flow structures around the flexible-membrane airfoils.
95 citations
TL;DR: In this article, a pair of winglets with adjustable cant angle, independently actuated and mounted at the tips of a baseline flying wing, are used for the control of morphing aircraft.
Abstract: This paper investigates a novel method for the control of “morphing” aircraft. The concept consists of a pair of winglets with adjustable cant angle, independently actuated and mounted at the tips of a baseline flying wing. The generalphilosophybehindtheconceptwasthatforspecific flightconditionssuchasacoordinatedturn,theuseoftwo control devices would be sufficient for adequate control. Computations with a vortex lattice model and subsequent wind-tunnel tests demonstrate the viability of the concept, with individual and/or dual winglet deflection producing multi-axis coupled control moments. Comparisons between the experimental and computational results showed reasonable to good agreement, with the major discrepancies thought to be due to wind-tunnel model aeroelastic effects.
95 citations
TL;DR: Results showed the necessity of leading-edge slats for the proposed tail-sitter vertical takeoff and landing unmanned aerial vehicle and provided a satisfactory improvement during the transition and enabled a level inbound transition.
Abstract: A new design for a tail-sitter vertical takeoff and landing unmanned aerial vehicle was proposed. A nonlinear mathematical model of the vehicle dynamics was constructed by combining simple estimation methods. The flight characteristics were revealed through a trim analysis and an optimized transitional flight path analysis by using the mathematical model. The trim analysis revealed the existence of a flight path constraint to avoid stall; the vehicle could not descend in low-speed flight without high-lift devices such as flaps and slats. These devices improved the descentperformance. Inparticular, slatsprovidedasubstantialimprovement; theyenabled adescent rateof2 m=s. In the optimized transitional flight path analysis, a level outbound transition without high-lift devices was achieved althoughatrimmedlevel flightatlowspeed,aswasshowninthetrimanalysis,wasnotpossible;thiswasbecausethe outboundtransition wasanaccelerative flight.Onthe contrary,without high-lift devices, the vehicle could notavoid climbing to avoid stall during inbound transitions. The slats provided a satisfactory improvement during the transition and enabled a level inbound transition. These results showed the necessity of leading-edge slats for the proposed tail-sitter vertical takeoff and landing unmanned aerial vehicle.
TL;DR: In this paper, an analysis of data obtained in an automated aerial refueling test flight conducted with a KC135 as the tanker and a Learjet 25 as the surrogate receiver unmanned aerial vehicle is carried out.
Abstract: This paper presents an analysis of data obtained in an automated aerial refueling test flight conducted with a KC135 as the tanker and a Learjet 25 as the surrogate receiver unmanned aerial vehicle. The purpose is to identify the wind induced by the tanker wake and its effect on the receiver aircraft. From the available flight data, a direct computation of the winds experienced by the tanker and receiver is carried out. The mean variation of the receiver wind is compared with the tanker wind when the receiver is at the observation and contact positions. This results in the identification of the wake-induced wind. A spectrum analysis is conducted to characterize the turbulence and to identify the pilot effects. The paper also presents methods used to model 1) prevailing wind, 2) wake-vortex-induced wind, and 3) turbulence as the three sources of wind that the aircraft are exposed to and the approach used for incorporating the wind effect into the dynamic simulation of the aircraft. The test flight is simulated in various caseswithdifferentturbulencemodelsand flightcontrollers.Thesimulationresultsareanalyzedandcomparedwith the flight data in terms of the power spectral densities and mean variations to validate the wind and turbulence modeling techniques.
TL;DR: In this article, the aerodynamic design of a flapping-wing micro air vehicle requires a careful study of the wing design space to ascertainthe bestcombination of parameter metrics, and the effects of wing geometry on aerodynamic performance of such flapping wings are investigated by comparing the influence on a numberof synthetic plan-form shapes while varying only one parameter at a time.
Abstract: The aerodynamic design of a flapping-wing micro air vehicle requires a careful study of the wing design space to ascertainthebestcombinationofparameters.Anonlinearunsteadyaerodynamic modeldevelopedbytheauthorsis used to make such a study for hovering insectlike flapping wings. The work is characterized, in particular, by the insights it provides into flapping-wing flows and the use of these insights for aerodynamic design. The effects of wing geometry on the aerodynamic performance of such flapping wings are investigated by comparing the influence on a numberofsyntheticplanformshapeswhilevaryingonlyoneparameteratatime.Bestperformanceappearstobefor wingshapesthathavenearlystraight leadingedges andmoreareaoutboard,where flowvelocities arehigher.Other important trends are also identified and practical considerations are noted. When possible, comparisons are also drawn with quasi-steady expectations and discrepancies are explained.
TL;DR: In this article, the authors explored the use of magnetorheological dampers in a semi-active seat suspension system for helicopter crew seats to enhance occupant comfort, which reduced the dominant rotor-induced vertical vibration transmitted to a 50th percentile male aviator by 76%.
Abstract: This study explores the use of magnetorheological dampers in a semi-active seat suspension system for helicopter crew seats to enhance occupant comfort. Key concepts in designing a magnetorheological seat suspension system to isolate the occupant from rotorcraft vibration are identified. Using these design concepts, a magnetorheological damper is designed, fabricated, and retrofitted into a tactical SH-60 Seahawk crew seat. This magnetorheological damper is implemented in series with the existing fixed load energy absorbers such that the crashworthiness capability of the seat is not impaired. Semi-active control is implemented and performance is evaluated both analytically and experimentally. Experimental test results have shown that this system reduced the dominant rotor-induced vertical vibration (4 per rev) transmitted to a 50th percentile male aviator by 76%, which is a 61-70% improvement over the unmodified SH-60 crew seat depending upon whether a soft seat cushion is used. Furthermore, these experimental tests also show that this system significantly reduces vertically induced seat rocking that occurs as a result of an offset center of gravity in the crew seat design.
TL;DR: Practical aspects of identifying dynamic models for aircraft in real time were studied and Estimated parameter standard errors, prediction cases, and comparisons with results from postflight analysis using the output-error method in the time domain were used to demonstrate the accuracy of the identified real-time models.
Abstract: Practical aspects of identifying dynamic models for aircraft in real time were studied. Topics include formulation of an equation-error method in the frequency domain to estimate non-dimensional stability and control derivatives in real time, data information content for accurate modeling results, and data information management techniques such as data forgetting, incorporating prior information, and optimized excitation. Real-time dynamic modeling was applied to simulation data and flight test data from a modified F-15B fighter aircraft, and to operational flight data from a subscale jet transport aircraft. Estimated parameter standard errors, prediction cases, and comparisons with results from a batch output-error method in the time domain were used to demonstrate the accuracy of the identified real-time models.
TL;DR: In this paper, the aerodynamics of a Gurney-flap-equipped airfoil have been explored by means of low-speed wind-tunnel experiments performed at a chord Reynolds number of 1:0 10.
Abstract: The aerodynamics of a Gurney-flap-equipped airfoil has been explored by means of low-speed wind-tunnel experiments performed at a chord Reynolds number of 1:0 10. Various chordwise locations and sizes of Gurney flaps were tested. Surface-pressure distributions and the wake momentum deficit were measured and used to determine lift, pitching moment, and drag. Compared with the clean airfoil, the measured maximum lift coefficient can be increased by nearly 30%with these simple devices. The amount of lift increase has a nearly linear dependency on the chordwise location and size of the Gurney flap. Minimum drag is primarily affected by the flap size and, to a lesser extent, by the chordwise location. The Gurney flap increases in maximum lift are obtained by increasing the lower-surface pressures over the aft part of the airfoil. At the same time, themagnitude of pressure peak on the upper surface near the leading edge is reduced such that the upper-surface pressures over themiddle parts of the airfoil are reduced and the separation point is moved aft by the reduced pressure-recovery gradients. As expected, this increases the aft loading and results in an increased nose-down pitchingmoment. As the angle of attack is decreased, the influence of a Gurney flap extending from the lower surface likewise decreases as the flap is increasingly immersed in the thickening boundary layer. A Gurney flap mounted to the upper surface behaves in the opposite way: increasing the negative lift at low angles of attack and having less and less influence as the angle of attack is increased. AlthoughGurney flaps result in significantly higher drags for airfoils with extensive runs of laminar flow, this disadvantage disappears as the amount of turbulent boundary-layer flow increases, as is the case with fixed transition near the leading edge of the airfoil.
TL;DR: In this article, the problem of tailoring for the pressure-pillowing problem of a fuselage skin is addressed using steered fibers, where the problem is modeled as a two-dimensional plate using von Karman plate equations.
Abstract: Manufacturing of high-quality fiber-reinforced composite structures with spatially varying fiber orientation is possible using advanced tow-placement machines. Changing the fiber-orientation angle within a layer produces variable-stiffness properties. Contrary to traditional composites with straight fibers, this method allows the designer to fully benefit from the directional material properties of the composite to improve laminate performance by determining optimal fiber paths. In this paper, design tailoring for the pressure-pillowing problem of a fuselage skin is addressed using steered fibers. The problem is modeled as a two-dimensional plate using von Karman plate equations. The analysis is performed using the Rayleigh-Ritz method, and the nonlinear response is traced using a normal flow algorithm. The design objective is to determine the optimal fiber paths over the panel for maximum failure load. Different designs are obtained for different loading cases. The results indicate that by using steered fibers, the pressure-pillowing problem can be alleviated and the load-carrying capacity of the structure can be improved, compared with designs using straight fibers.
TL;DR: In this paper, a significant improvement to the development of computational-fluid-dynamics-based unsteady aerodynamic reduced-order models is presented, which involves the simultaneous excitation of the structural modes of the computational fluid dynamics-based aerodynamic system.
Abstract: A significant improvement to the development of computational-fluid-dynamics-based unsteady aerodynamic reduced-order models is presented. The improvement involves the simultaneous excitation of the structural modes of the computational-fluid-dynamics-based unsteady aerodynamic system. This improvement enables the computation of the unsteady aerodynamic state-space model using a single computational fluid dynamics execution, independent of the number of structural modes. Two new types of input functions are presented that can be used for the simultaneous excitation of the unsteady aerodynamic system via the excitation of the structural modes. Results are presented for a semispan configuration using the CFL3Dv6.4 code.
TL;DR: In this paper, a detailed derivation of an Eulerian model of the droplet-wall interaction process is presented, along with a comparison of numerical and experimental collection efficiency distributions demonstrating the model's current simulation capabilities.
Abstract: To accurately simulate the impingement behavior and associated icing potential of supercooled large droplets, droplet-wall interactions including splashing and bouncing phenomena must be accounted for in the governing Eulerian model. Because of current limitations in computational capacity, an industrially viable simulation is necessarily based on a semi-empirical description of the droplet-wall interaction process. Because empirical correlations are inherently Lagrangian in nature, the associated information must be transformed from a Lagrangian to an Eulerian frame of reference. A detailed derivation of an Eulerian model of the droplet-wall interaction process is presented, along with a comparison of numerical and experimental collection efficiency distributions demonstrating the model's current simulation capabilities.
TL;DR: In this paper, the effect of wing geometry and kinematics on the aerodynamic performance of a single-wing micro-air vehicle (FMAV) was investigated using a fruit fly.
Abstract: A GILE flight inside buildings, caves, and tunnels is of significant military and civilian value and is an attractive application for micro air vehicles (MAVs), defined here as flying machines of the order of 150 mm in size Indoor flight imposes particular design and performance requirements, including small size, low speed, hovering capability, high maneuverability at low speeds, and (for covert operations) small acoustic signature, among other things As discussed elsewhere [1–4], insectlike flapping is a solution thatmeets these requirements and is proven in nature Although a number of elements characterize the design of a flapping-wing MAV, the focus here is on its wing aerodynamic design This is crucial because for a flapping-wing MAV (FMAV) the wings are not only responsible for lift, but also for propulsion and maneuvers Although insect flapping wings offer a proven solution and are abundant in nature (there are over 170,000 species of flying insects), little is known about the optimality of their wing design Unlike forfixed or rotarywings, the parametric space associatedwith flapping wings is largely unexplored A study that addresses the effects of both wing kinematics and wing geometry on the aerodynamic performance of flapping wings is required, and the former forms the underlying theme of this paper The effect of wing geometry is considered elsewhere [5] This work also provides insights into flapping-wing flow physics and uses these insights for aerodynamic design Although Ellington’s [6] seminal work rejuvenated interest in insect flight, it is only recently that attention has been directed toward the design of vehicles that use insectlike flapping wings, particularly at the MAV scale [1–3] In a later study, Ellington [7] proposed design guidelines based on scaling fromnature, but this does not give physical insight or allow design optimization Dickinson et al [8] investigated the effect of advancing or delaying pitch rotation of the flapping wing with respect to its translational motion, using experiments on Dickinson’s Robofly: a scaled-up mechanical model of the fruit fly Drosophila Ramamurti and Sandberg [9] used a computational fluid dynamics (CFD) method to demonstrate this effect and presented some useful flow visualization Sun and Tang [10] also used a CFD code to investigate the effect of advancing and delaying pitch rotation on insectlike flapping flight and the effect of varying the duration of stroke reversals [11] In an earlier study [12], they investigated the effect of Reynolds number and the duration of wing stroke reversal They also studied the effect of advance ratio (the ratio of flight speed to wing mean tip speed) in forward flapping flight [13] Yu and Tong [14] used an aerodynamic modeling approach [15] to study forward flapping flight at various advance ratios by varying asymmetries between upand downstrokes However, none of the preceding studies aimed to produce an optimized wing aerodynamic design Milano and Gharib [16] made probably the only study thus far aimed at optimizing wing kinematics They used a genetic algorithm paired with digital particle-image velocimetry experiments on a flapping wing in a water-filled towing tank By using insectlike kinematics, they optimized for average lift over four flapping cycles and found a number of convergent solutions in the parameter space They noted that the optimally efficient solutions all tended to generate leading-edge vortices ofmaximum strength However, their Received 26 October 2007; revision received 11 June 2008; accepted for publication 13 June 2008 Copyright © 2008 by SalmanA Ansari Published by the American Institute of Aeronautics and Astronautics, Inc, with permission Copies of this paper may be made for personal or internal use, on condition that the copier pay the $1000 per-copy fee to the Copyright Clearance Center, Inc, 222 Rosewood Drive, Danvers, MA 01923; include the code 0021-8669/08 $1000 in correspondence with the CCC ResearchOfficer, Department ofAerospace, Power and SensorsMember AIAA Professor of Aeromechanical Systems, Department of Aerospace, Power and Sensors Associate Fellow AIAA Reader in Control Engineering, Department of Aerospace, Power and Sensors Member AIAA JOURNAL OF AIRCRAFT Vol 45, No 6, November–December 2008
TL;DR: In this article, the second derivative of the pressure distribution is calculated, using two interpolation schemes: piecewise cubic Hermite interpolating polynomial and Spline, from which it is determined that transition may be identified as the location of maximum curvature.
Abstract: airfoil types: NACA 4415 and WTEA-TE1, as well as for 17 modified WTEA-TE1 airfoil shapes, obtained by displacing the flexible wing upper surface using a single point control mechanism. The second derivative of the pressure distribution is calculated, using two interpolation schemes: piecewise cubic Hermite interpolating polynomial and Spline, from which it is determined that transition may be identified as the location of maximum curvature in the pressure distribution. The results of this method are validated using the well-known XFoil code, which is used to theoretically calculate the transition point position. Advantages of this new method in the real-time control of the location of the transition point are presented.
TL;DR: In this article, a lifting surface method is presented that uses elements having distributed vorticity to model lifting surfaces and their shed wakes, allowing the representation of a force-free continuous wake-vortex sheet that is free of numerical singularities and is thus robust in its numerical rollup behavior.
Abstract: A lifting-surface method is presented that uses elements having distributed vorticity to model lifting surfaces and their shed wakes. Using such distributed vorticity elements allows the representation of a force-free continuous wake-vortex sheet that is free of numerical singularities and is thus robust in its numerical rollup behavior. Unlike other potential-flow methods that use discrete vortex filaments having solid-core models at their centers to avoid problems with the singularities, the numerical robustness of the new method is achieved without the subsequent solution being dependent on the choice of a cutoff distance or core size. The computed loads compare well with results of classical theory and other potential-flow methods. Its numerical robustness, computational speed, and ability to predict loads accurately make the new method ideal for the investigation of applications in which the loadings on a lifting surface depend strongly on the influence of the wake and its shape, as is the case for the two application examples presented: formation flight and rotating-wing systems.
TL;DR: In this paper, a biaxial tensile test and a pressurized cylinder test were carried out to find the stress that initiates the tear propagation in an airship envelope material.
Abstract: In a design criterion of a nonrigid airship, such as FAA-P-8110-2, Airship Design Criteria, it is specified to measure the tear strength of the envelope material. However, the tear strength itself has no definite relationship with the actual tear propagation characteristics of an airship envelope material. Therefore, there are several investigations to establish the relationship. To study it, tests are conducted to measure the tear propagation stress to find the appropriate formula. To simulate the actual stress field, two kinds of tests are carried out: a biaxial tensile test and a pressurized cylinder test. Both tests simulate the biaxial stress field on an airship envelope. The material tested is the high-strength and lightweight envelope material, Z2929T-AB. This is one of the envelope materials with Zylon as its base fabric and is developed especially for the technology demonstrator of a stratospheric platform. This material has a density as low as 157 g/m 2 ; nevertheless, the tensile strength is as high as 997 N/cm. The measurement is made to find the stress that initiates the tear propagation. The data are fitted to Thiele's formula and the correlation is excellent. By using Thiele's empirical equation, the minimum slit size of the tear propagation under the limit stress is estimated. For the technology demonstrator of the stratospheric platform, which is planned to have an overall length of 150 m and the maximum diameter of 38 m, the slit size of less than 40 mm does not allow tear propagation under the limit load. The consideration of the stress field near the slit introduces a different approach for the empirical formula, and this leads to the equation which correlates very well the tear propagation stress with the tensile strength of the envelope material. These two results are described, as well as the test details.
TL;DR: In this paper, a computational fluid dynamics technique was used to investigate the dispersion characteristics of sneezed/coughing particles by both the Eulerian and Lagrangian methods, and the results showed that personalized ventilation, through the distribution of fresh air directly in the breathing zone, was able to shield up to 60% of air pollutants in a passenger's inhalation.
Abstract: Complaints about cabin air quality and persistent reports of spreading infections on commercial flights indicate that continued investigations of cabin air systems and effective measures to improve cabin air quality are required. This study used a computational fluid dynamics technique to investigate the dispersion characteristics of sneezed/ coughed particles by both the Eulerian and Lagrangian methods. These particles can be transported to a location more than three rows in front of the sneezing person, and less than 20% of the particles were exhausted, whereas the remainders are deposited owing to the high surface-to-volume ratio. Personalized ventilation, through the distribution of fresh air directly in the breathing zone, was able to shield up to 60% of air pollutants in a passenger's inhalation.
TL;DR: In this paper, the effect of uncertainty in composite material properties on the cross-sectional stiffness properties, natural frequencies, and aeroelastic responses of a composite helicopter rotor blade is investigated.
Abstract: This study investigates the effect of uncertainty in composite material properties on the cross-sectional stiffness properties, natural frequencies, and aeroelastic responses of a composite helicopter rotor blade. The elastic moduli and Poisson’s ratio of the composite material are considered as random variables with a coefficient of variation of around 4%, which was taken from published experimental work. An analytical box beam model is used for evaluating blade cross-sectional properties. Aeroelastic analysis based on finite elements in space and time is used to evaluate the helicopter rotor blade response in forward flight. The stochastic cross-sectional and aeroelastic analyses are carried out with Monte Carlo simulations. It is found that the blade cross-sectional stiffness matrix elements show a coefficient of variation of about 6%. The nonrotating rotor blade natural frequencies show a coefficient of variation of around 3%. The impact of material uncertainty on rotating natural frequencies varies from that on nonrotating blade frequencies because of centrifugal stiffening. The propagation of material uncertainty into aeroelastic response causes large deviations, particularly in the higher-harmonic components that are critical for the accurate prediction of helicopter blade loads and vibration. The numerical results clearly show the need to consider randomness of composite material properties in the helicopter aeroelastic analysis.
TL;DR: In this article, a multidisciplinary design exploration technique with a high-fidelity analysis applied to the winglet design for a commercial jet aircraft was described, where the minimization of the block fuel at a fixed aircraft operating range and a maximum takeoff weight were selected as design objectives.
Abstract: In this paper, we describe a multidisciplinary design exploration technique with a high-fidelity analysis applied to the winglet design for a commercial jet aircraft. The minimization of the block fuel at a fixed aircraft operating range and a maximum takeoff weight were selected as design objectives. Both objective functions were estimated from a computational fluid dynamics based aerodynamic drag and a finite element method based structural weight. Various computational fluid dynamics and optimization techniques, such as the midfield drag decomposition method, the automatic computational fluid dynamics mesh generation, the kriging surrogate model, and multi-objective genetic algorithms, were integrated and applied to the detail design exploration. Computational fluid dynamics with the midfield drag decomposition method showed the effect on wave, induced, and profile drag components due to different winglet defining parameters. Practical design decision was explored based on the Pareto front and some design criteria that were uncovered within the numerical optimization. Finally, the design process was validated through the validation of the kriging approximation and aerodynamic characteristics based on the wind-tunnel test.
TL;DR: In this article, a tilt-body configuration of the vertical takeoff and landing micro air vehicle is proposed based on a propulsion system consisting of two coaxial contrarotating motors and propellers.
Abstract: The research and development efforts outlined in this paper address the aerodynamic design of micro air vehicles with hovering and vertical takeoff and landing capabilities. The tilt-body configuration of the vertical takeoff and landing micro air vehicle is proposed based on a propulsion system consisting of two coaxial contrarotating motors and propellers. Values of thrust, torque, power, and efficiency of this propulsion system were measured in pusher and tractor arrangements of propellers and compared against single motor-propeller propulsion. With comparable efficiency, the developed propulsion system has very little propeller torque. Hot-wire measurements have been conducted to investigate the velocity profile in slipstream. The lower average velocity and significant decrease in velocity in the core of the slipstream found in the tractor arrangement are mostly due to the parasite drag caused by the motors. It causes the decrease of the thrust force observed for the tractor arrangement in comparison with the pusher arrangement. Wind-tunnel testing was conducted for a motor, a wing, and an arrangement of a wing with a motor. The drag force on the wing is produced by two mixing airflows: freestream and propeller-induced pulsating slipstream. The zero-lift drag coefficient increases by about 4 times with propeller-induced speed increased from 0 to 7.5 m/s. The results of this study were realized in the design of a vertical takeoff and landing micro air vehicle prototype that was successfully flight tested.
TL;DR: A mathematical footing from network theory is introduced for examining transport networks in the U.S. domestic air transportation system using data for the 2004 travel year, and the structure of the transport network is exposed in terms of degree distribution.
Abstract: Research reported in this paper is motivated by the need to better understand the structure of connectivity in air transportation networks.Theaim istoinvestigateanalysismodelsandtechniquesfrom modernnetworktheoryas a framework to provide both characterization of network structure and a useful systems analysis approach to derive implications from both local and global topology characteristics. Recent developments in network theory establish meanstoquantifytopological structureinamannerthatmayindicateexpectedperformanceandrobustness.Inthis paper, a mathematical footing from network theory is introduced for examining transport networks in the U.S. domestic air transportation system. Using data for the 2004 travel year, the structure of the transport network (service routes between airports) and several subnetworks is exposed in terms of degree distribution, and the importance of airportsis assessed through several networkmeasures. Useful implications are drawn from measures andfurther analysis that directly maps these measures to system performance is presented. The general approachis found to merit further investigation as part of a larger, more comprehensive, and design-oriented systems analysis framework for air transportation.
TL;DR: In this paper, both the angle of attack and the deflection are modeled as a sinusoidal wave and the outputs (the deflection and the transition point position) are well controlled and the results are very good.
Abstract: steps for the deflection but adds a sinusoidal component for the angle of attack; this simulation is closer to the cruise flight regime. During the third simulation, both the angle of attack and the deflection are modeled as a sinusoidal wave. The outputs (the deflection and the transition point position) are well controlled and the results are very good. Hence, it is concluded that this original method of control is suitable for the control of the transition point position from the laminar to turbulent region on a morphing wing airfoil.
TL;DR: In this paper, a comparison study of computations for the Third AIAA Computational Fluid Dynamics Drag Prediction Workshop is performed on the DLR-F6 wing-body configurations with and without the wing body fairing using UPACS and unstructured grid solver TAS code.
Abstract: Comparison study of computations for the Third AIAA Computational Fluid Dynamics Drag Prediction Workshop is performed on the DLR-F6 wing-body configurations with and without the wing-body fairing using the structured grid solver UPACS and unstructured grid solver TAS code. Grid convergence study at a fixed C L using a family of three difference density grids and the results by a sweep are discussed. The self-made multiblock structured grids and mixed-element unstructured grids are employed. Another participant's grid is also compared. Comparisons between the two codes are conducted using the same turbulence model. Moreover, the detailed comparisons are conducted on the grid topology at the comer of the wing-body junction, the turbulence models, and the thin-layer approximation in viscous terms using the multiblock structured grids. The reconstruction schemes to realize the second-order spatial accuracy are also compared on unstructured grids. By comparing the results, the sensitivity of drag prediction to these factors is discussed.