Showing papers in "Journal of Aircraft in 2009"

Plasma Flaps and Slats: An Application of Weakly Ionized Plasma Actuators

Abstract: The experimental validation of an application of weakly-ionized plasma actuators for improved aerodynamic performance of multi-element wings and wings with movable control surfaces is presented. Two spanwise arrays of plasma actuators, configured to produce a wall-jet effect, were applied on the suction surface of a two-dimensional NACA 0015 wing model, one at the leading edge and the other near the trailing edge to mimic the effects of a wing leading-edge slat and a trailing-edge flap, respectively. Flow control tests were conducted at chord Reynolds numbers, corrected for blockage, of 0.217 x 10 6 and 0.307 x 10 6 in a low-speed wind tunnel at the University of Notre Dame. The leading-edge-separation control resulted in an increase in both the maximum lift coefficient and the stall angle of attack and a lift-to-drag improvement of as much as 340%. An optimum frequency was found to exist for unsteady excitation of the leading-edge separation. Under this condition, the power to the actuator was estimated to be only 2 W. The trailing-edge actuator was found to produce the same effect as a plain trailing-edge flap. This included a uniform shift at all angles of attack of the lift coefficient and a shift toward higher lift coefficients of the drag bucket. In addition, there was a slight decrease in the minimum drag coefficient. The obvious advantages of this approach are its simplicity, as there are no moving parts, and its lack of hinge gaps, which add drag. An example of their use as ailerons for roll control produces a comparable roll moment coefficient to a sample general aviation aircraft.

In Pursuit of Grid Convergence for Two-Dimensional Euler Solutions

Abstract: Grid-convergence trends of two-dimensional Euler solutions are investigated. The airfoil geometry under study is based on the NACA0012 equation. However, unlike the NACA0012 airfoil, which has a blunt base at the trailing edge, the study geometry is extended in chord so that its trailing edge is sharp. The flow solutions use extremely- high-quality grids that are developed with the aid of the Karman-Trefftz conformal transformation. The topology of each grid is that of a standard O-mesh. The grids naturally extend to a far-field boundary approximately 150 chord lengths away from the airfoil. Each quadrilateral cell of the resulting mesh has an aspect ratio of one. The intersecting lines of the grid are essentially orthogonal at each vertex within the mesh. A family of grids is recursively derived starting with the finest mesh. Here, each successively coarser grid in the sequence is constructed by eliminating every other node of the current grid, in both computational directions. In all, a total of eight grids comprise the family, with the coarsest-to-finest meshes having dimensions of 32 x 32-4096 x 4096 cells, respectively. Note that the finest grid in this family is composed of over 16 million cells, and is suitable for 13 levels of multigrid. The geometry and grids are all numerically defined such that they are exactly symmetrical about the horizontal axis to ensure that a nonlifting solution is possible at zero degrees angle-of-attack attitude. Characteristics of three well-known flow solvers (FLO82, OVERFLOW, and CFL3D) are studied using a matrix of four flow conditions: (subcritical and transonic) by (nonlifting and lifting). The matrix allows the ability to investigate grid-convergence trends of 1) drag with and without lifting effects, 2) drag with and without shocks, and 3) lift and moment at constant angles-of-attack. Results presented herein use 64-bit computations and are converged to machine-level-zero residuals. All three of the flow solvers have difficulty meeting this requirement on the finest meshes, especially at the transonic flow conditions. Some unexpected results are also discussed. Take for example the subcritical cases. FLO82 solutions do not reach asymptotic grid convergence of second-order accuracy until the mesh approaches one quarter of a million cells. OVERFLOW exhibits at best a first-order accuracy for a central-difference stencil. CFL3D shows second-order accuracy for drag, but only first-order trends for lift and pitching moment. For the transonic cases, the order of accuracy deteriorates for all of the methods. A comparison of the limiting values of the aerodynamic coefficients is provided. Drag for the subcritical cases nearly approach zero for all of the computational fluid dynamics methods reviewed. These and other results are discussed.

Full Configuration Drag Estimation

Abstract: Accurate drag estimation is critical in making computational design studies. Drag may be estimated thousands of times during a multidisciplinary design optimization, and computational fluid dynamics is not yet possible in these studies. The current model has been developed as part of an air-vehicle conceptual-design multidisciplinary design optimization framework. Its use for subsonic and transonic aircraft configurations is presented and validated. We present our parametric geometry definition, followed by the drag model description. The drag model includes induced, friction, wave, and interference drag. The model is compared with subsonic and transonic isolated wings, and a wing/body configuration used previously in drag prediction workshops. The agreement between the predictions of the drag model and test data is good, but lessens at high lift coefficients and high transonic Mach numbers. In some cases the accuracy of this drag estimation method exceeds much more elaborate analyses.

Optimizing Electric Propulsion Systems for Unmanned Aerial Vehicles

Abstract: The propeller model and a model of the electric system, together with various optimization schemes, are used to design optimal propulsion systems for a mini unmanned aerial vehicle for various goals and under various constraints. Important design trends are presented, discussed, and explained. Although the first part of the investigation is based on typical characteristics of the electric system, the second part includes a sensitivity study of the influence of variations of these characteristics on the optimal system design.

Discrete Sweeping Jets as Tools for Improving the Performance of the V-22

Abstract: Experiments aimed at delaying flow separation through discrete jets pointing in the direction of streaming and sweeping side to side along the span were conducted on a V-22 airfoil with and without deflected trailing-edge flaps. The results indicated substantial drag reduction and lift increase at moderately low inputs of mass and momentum. Additional experiments were carried out on a semispan V-22 wing/nacelle combination, and they too provided an increase in lift-to-drag ratio L/D of approximately 60% (although active flow control was applied to the wing only). The effectiveness of the sweeping jets on reducing the download force acting on a V-22 full-span powered model in hover was also examined. A 29% reduction in download was realized using the embedded sweeping jets, corresponding approximately to a 2000 1b increase in hover lift.

Field Evaluation of the Tailored Arrivals Concept for Datalink-Enabled Continuous Descent Approach

Abstract: Allowing aircraft to descend uninterrupted at low engine power, continuous descent operations promise to maximize fuel efficiency while minimizing environmental impact. Tailored arrivals is a concept for enabling continuous descents under constrained airspace conditions by integrating advanced air and ground automation through digital datalink. Operational trials were completed in January 2007 involving transpacific flights into San Francisco during early morning hours. Leveraging newly deployed Federal Aviation Administration automation in the oceanic environment, trajectory-based clearances were transmitted by datalink to Boeing 777 aircraft equipped with future air navigation system avionics. NASA's prototype ground-based automation for high-density arrival management tailored trajectory clearances to accommodate artificially imposed metering constraints. Upon sharing wind and descent-speed-intent data, ground-based and airborne automation were found to predict meter-fix arrival times to within a mean accuracy of 3 s over a 25 min prediction horizon. Corresponding mean altitude and along-track prediction errors of ground-based automation were -500 ft and -1.3 n mile, respectively, in comparison with surveillance truth. A benefits analysis suggests Boeing 777 fuel savings of between 200 and 3000 1b per flight (depending highly upon baseline traffic conditions) together with a corresponding reduction in CO 2 emissions of between 700 and 10,000 1b per flight.

Grid Quality and Resolution Issues from the Drag Prediction Workshop Series

Abstract: The drag prediction workshop series (DPW), held over the last six years, and sponsored by the AIAA Applied Aerodynamics Committee, has been extremely useful in providing an assessment of the state-of-the-art in computationally based aerodynamic drag prediction. An emerging consensus from the three workshop series has been the identification of spatial discretization errors as a dominant error source in absolute as well as incremental drag prediction. This paper provides an overview of the collective experience from the worksho series regarding the effect of grid-related issues on overall drag prediction accuracy. Examples based on workshop results are used to illustrate the effect of grid resolution and grid quality on drag prediction, and grid convergence behavior is examined in detail. For fully attached flows, various accurate and successful workshop results are demonstrated, while anomalous behavior is identified for a number of cases involving substantial regions of separated flow. Based on collective workshop experiences, recommendations for improvements in mesh generation technology which have the potential to impact the state-of-the-art of aerodynamic drag prediction are given.

Robust Aerodynamic Design Optimization Using Polynomial Chaos

Abstract: This paper investigates the potential of polynomial chaos methods, when used in conjunction with computational fluid dynamics, to quantify the effects of uncertainty in the computational aerodynamic design process. The technique is shown to be an efficient and accurate means of simulating the inherent uncertainty and variability in manufacturing and flow conditions and thus can provide the basis for computationally feasible robust optimization with computational fluid dynamics. This paper presents polynomial chaos theory and the nonintrusive spectral projection implementation, using this to demonstrate polynomial chaos as a basis for robust optimization, focusing on the problem of maximizing the lift-to-drag ratio of a two-dimensional airfoil while minimizing its sensitivity to uncertainty in the leading-edge thickness. The results demonstrate that the robustly optimized designs are significantly less sensitive to input variation, compared with nonrobustly optimized airfoils. The results also indicate that the inherent geometric uncertainty could degrade the on-design as well as the offdesign performance of the nonrobust airfoil. This leads to the further conclusion that the global optimum for some design problems is unreachable without accounting for uncertainty.

Modeling of Terminal-Area Airplane Fuel Consumption

Abstract: DOI: 10.2514/1.42025 Accurate modeling of airplane fuel consumption is necessary for air transportation policy-makers to properly adjudicate trades between competing environmental and economic demands. Existing public models used for computing terminal-area airplane fuel consumption have been shown to have substantial errors, potentially leading to erroneous policy and investment decisions. The method of modeling fuel consumption proposed in this paper was developed using data from a major airplane manufacturer. When compared with airline performance/operational data, this proposed method has been shown to accurately predict fuel consumption in the terminal area. The proposed method uses airplane performance data from publicly available environmental models supported by the Federal Aviation Administration and others. The proposed method has sufficient generality to protect the proprietary interests of the manufacturer, while still having adequate fidelity to analyze low-speed airplane operations in the terminal area. This improved methodology will enable more informed decisions by policy-makers seekingtoaccountfortheeffectsoffuelconsumptionandairplaneemissionsonplansforfutureairspaceandairport designs.

Two-Level Multifidelity Design Optimization Studies for Supersonic Jets

Abstract: The conceptual/preliminary design of supersonic jet configurations requires multidisciplinary analyses tools, which are able to provide a level of flexibility that permits the exploration of large areas of the design space. High-fidelity analysis for each discipline is desired for credible results; however, the corresponding computational cost can be prohibitively expensive, often limiting the ability to make drastic modifications to the aircraft configuration in question. Our work has progressed in this area, and we have introduced a truly hybrid, multifidelity approach in multidisciplinary analyses and demonstrated, in previous work, its application to the design optimization of a low-boom supersonic business jet. In this paper, we extend our multifidelity approach to the design procedure and present a two-level design of a supersonic business-jet configuration, in which we combine a conceptual low-fidelity optimization tool with a hierarchy of flow solvers of increasing fidelity and advanced adjoint-based sequential quadratic programming optimization approaches. In this work, we focus on the aerodynamic performance aspects alone: no attempt is made to reduce the acoustic signature. The results show that this particular combination of modeling and design techniques is quite effective for our design problem and the ones in general and that high-fidelity aerodynamic shape optimization techniques for complex configurations (such as the adjoint method) can be effectively used within the context of a truly multidisciplinary design environment. Detailed configuration results of our optimizations are also presented.

Cooperative Task Assignment/Path Planning of Multiple Unmanned Aerial Vehicles Using Genetic Algorithms

Abstract: This research was supported by the Korea Aerospace Research Institute for a program of development of the Communications, Navigation, Surveillance/Air Traffic Management system for the next generation.

Numerical Simulation of Maneuvering Aircraft by Aerodynamic, Flight Mechanics and Structural Mechanics Coupling

Abstract: Selected results of the DLR Project SikMa-"Simulation of Complex Maneuvers" are presented. The objective of the SikMa Project is to develop and validate a numerical tool to simulate the unsteady aerodynamics of a free flying aeroelastic combat aircraft, by use of coupled aerodynamic, flight mechanics and aeroelastic computations. To achieve this objective, the unstructured, time-accurate flow-solver TAU is coupled with a computational module solving the flight mechanics equations of motion and a structural mechanics code determining the structural deformations. The numerical results are validated by experimental data. For this purpose several specific wind tunnel experiments with different wind tunnel models are carried out.

Accelerating the Numerical Generation of Aerodynamic Models for Flight Simulation

Abstract: The generation of a tabular aerodynamic model for design related flight dynamics studies, based on simulation generated data, is considered. The framework described accommodates two design scenarios. The first emphasizes the representation of the aerodynamic nonlinearities, and is based on sampling. The second scenario assumes incremental change from an initial geometry, for which a hi-fidelity model from the first scenario is available. In this case data fusion is used to update the model. In both cases, Kriging is used to interpolate the samples computed using simulation. A commercial jet test case, using DATCOM as a source of data, is computed to illustrate the sampling and fusion. Future application using Computational Fluid Dynamics as the source of data is considered.

Eulerian Simulation of the Fluid Dynamics of Helicopter Brownout

Abstract: A computational model is presented that can be used to simulate the development of the dust cloud that can be entrained into the air when a helicopter is operated close to the ground in desert or dusty conditions. The physics of this problem, and the associated pathological condition known as ‘brownout’ where the pilot loses situational awareness as a result of his vision being occluded by dust suspended in the flow around the helicopter, is acknowledged to be very complex. The approach advocated here involves an approximation to the full dynamics of the coupled particulate-air system. Away from the ground, the model assumes that the suspended particles remain in near equilibrium under the action of aerodynamic forces. Close to the ground, this model is replaced by an algebraic sublayer model for the saltation and entrainment process. The origin of the model in the statistical mechanics of a distribution of particles governed by aerodynamic forces allows the validity of the method to be evaluated in context by comparing the physical properties of the suspended particulates to the local properties of the flow field surrounding the helicopter. The model applies in the Eulerian frame of reference of most conventional Computational Fluid Dynamics codes and has been coupled with Brown’s Vorticity Transport Model. Verification of the predictions of the coupled model against experimental data for particulate entrainment and transport in the flow around a model rotor are encouraging. An application of the coupled model to analyzing the differences in the geometry and extent of the dust clouds that are produced by single main rotor and tandem-rotor configurations as they decelerate to land has shown that the location of the ground vortex and the size of any regions of recirculatory flow, should they exist, play a primary role in governing the extent of the dust cloud that is created by the helicopter.

Automatic Transition Prediction in Hybrid Flow Solver, Part 1: Methodology and Sensitivities

Abstract: A hybrid Reynolds-averaged NavierA¢Â�Â�Stokes solver, a laminar boundary-layer code, and a fully automated local, linear stability code were coupled to predict the laminarA¢Â�Â�turbulent transition due to TollmienA¢Â�Â�Schlichting and crossflow instabilities using the eN method based on the two-N-factor approach. The coupled system was designed to be applied to three-dimensional aircraft configurations which are of industrial relevance. The transition prediction methodology provides two different approaches which are available to be used in different flow situations. Both approaches are described and tested in detail. The application of the complete coupled system to a two-dimensional two-element airfoil configuration and a three-dimensional generic full aircraft configuration is described and documented in this paper. The prediction of the laminarA¢Â�Â�turbulent transition lines was done in a fully automatic manner. It will be shown that complex aircraft configurations can be handled without a priori knowledge of the transition characteristics of the specific flow problem. The computational results are partially compared to experimental data. This article is the first of two companion papers: the first dealing with the transition prediction methodology and the second dealing with the practical application of the coupled system.

Aeroelastic Study of Bistable Composite Airfoils

Abstract: A bistable airfoil with the actuator system and the aerodynamic loads coupled to the structure is designed to understand the aeroelastic characteristics of the system. The bistable flap consists of a stack of six bistable prestressed buckled laminates with the plates having dimensions of 100 × 100 mm made from Hexcel 913 glass fiber. The bistable plates attached to a spar situated at 85% chord on the rotor blade. The foam tape placed between each of the plates at the trailing edge making the airfoil weather tight. The bistable plate spar is parallel to the direction of motion of the moving crosshead of the test machine. For the flap to be in equilibrium without any actuator input, the sum of the work done on the structure by the aerodynamic loads W f and the work done on the aerodynamic loads by the structure W b is zero. The modulus function shows that the work needs to be done on the structure to rotate the flap in either direction from an equilibrium point whereas the work done on the aerodynamic loads is dependant on direction.

Comparison of Measured and Block Structured Simulation Results for the F-16XL Aircraft

Abstract: This article presents a comparison of the predictions of three RANS codes for flight conditions of the F-16XL aircraft which feature vortical flow. The three codes, ENSOLV, PMB and PAB3D, solve on structured multi-block grids. Flight data for comparison was available in the form of surface pressures, skin friction, boundary layer data and photographs of tufts. The three codes provided predictions which were consistent with expectations based on the turbulence modelling used, which was k- , k- with vortex corrections and an Algebraic Stress Model. The agreement with flight data was good, with the exception of the outer wing primary vortex strength. The confidence in the application of the CFD codes to complex fighter configurations increased significantly through this study.

Knowledge-Based Engineering Approach to Support Aircraft Multidisciplinary Design and Optimization

Abstract: This paper introduces the concept of the design and engineering engine, which is a modular computational design system to support distributed multidisciplinary design and optimization of aircraft. In particular, this paper discusses the architecture and the functionalities of the multimodel generator module, which is a knowledge-based engineering application developed to model the geometry of both conventional and novel aircraft configurations and to automate the generation of dedicated models for low- and high-fidelity analysis tools. This paper demonstrates the capability of the knowledge-based engineering approach to record and automate complex engineering design processes, such as the generation of models for finite element analysis. The time reduction gained by process automation, together with the enabled use of high-fidelity analysis tools earlier in the design process, constitute significant achievements toward a broader exploitation of the multidisciplinary design and optimization methodology, as well as the development of novel aircraft configurations.

Development of an Onboard Doppler Lidar for Flight Safety

Abstract: Air turbulence has become a major cause of significant injuries and aircraft damages. Timely advanced warning of turbulence ahead of an aircraft may allow pilots to take appropriate action to minimize potential damage, such as reducing speed and securing passengers and unsecured objects, or to avoid the turbulence altogether. The aim of our research is to develop a practical, onboard, Lidar-based proactive sensor that will detect air turbulence in clear air at a range of 5 n miles (9.3 km) at cruising altitudes. In February 2007 we successfully measured wind speeds approximately 3 n miles (5.6 km) ahead of an aircraft in low-altitude flight experiments, and in a subsequent experiment in July of the same year, we succeeded in detecting air turbulence before encountering it. An upgraded 5-n-mile Lidar for low altitudes was developed in fiscal year 2007, and has successfully measured wind speeds at ranges up to 5 n miles in ground tests. This paper describes the master development plan of our Lidar turbulence sensor and the results of basic flight and ground experiments.

Design of Composite Wings Including Uncertainties : A Probabilistic Approach

Abstract: A probabilistic method is developed to optimize the design of an idealized composite wing through consideration of the uncertainties in the material properties, fiber-direction angle, and ply thickness. The polynomial chaos expansion method is used to predict the mean, variance, and probability density function of the flutter speed, making use of an efficient Latin hypercube sampling technique. One-dimensional, two-dimensional, and three-dimensional polynomial chaos expansions are introduced into the probabilistic flutter model for different combinations of material, fiber-direction-angle, and ply-thickness uncertainties. The results are compared with Monte Carlo simulation and it is found that the probability density functions obtained using second- and third-order polynomial chaos expansion models compare well but require much less computation. A reliability criterion is defined, indicating the probability of failure due to flutter, and is used to determine successfully the optimal robust design of the composite wing.

Optimization of Propeller Based Propulsion System

Abstract: Propeller design is a complex task that involves a variety of disciplines such as: aerodynamics, structural analysis, and acoustics. A new method of designing an optimal propeller which is based on a MDO (Multidisciplinary Design Optimization) approach is presented. By combining various analysis tools with an optimization tool, a powerful and flexible design method is obtained. During the design process three different optimization schemes are used, leading the design to its optimal goal. This new method is applied for the design of a propeller for an Ultralight aircraft. Several optional designs for different design goals are presented. The results of the new method are compared with results of the classical design method, based on Betz's condition, which considers only the aerodynamic performance of the propeller. The importance of addressing the characteristics of the entire air-vehicle, its aerodynamic characteristics and its propulsion system (engine, gear box, etc.), rather than only the isolated propeller, is emphasized.

Experimental Investigation of Bistable Winglets to Enhance Aircraft Wing Lift Takeoff Capability

Abstract: An experimental investigation into the use of a bistable winglet to enhance the lift characteristics of a wing transitioning from lower to higher subsonic flow speeds is presented in this paper. The concept centers around the use of a specifically designed composite winglet, manufactured with an unsymmetric layup, which, when increasingly loaded, snaps between two stable states. Initially, during low-speed operation, the winglet is fixed in one stable state that is specifically designed to be cambered, thus enhancing the lift capability of the wing. At higher dynamic pressures, the winglet snaps to a configuration more intuitive and conventional to current winglet design. Results presented in this paper show the concept to be viable at enhancing the lift produced by a swept wing as aerodynamic loading increases before snap-through. During snap-through, however, the absence of any method of controlling the snap-through process generated significant dynamic loading that was transmitted, unhindered, throughout the entire test rig.

Optimum Design of a Compound Helicopter

Abstract: A design and aeromechanics investigation was conducted for a 100,000-lb compound helicopter with a single main rotor, which is to cruise at 250 knots at 4000 ft/95 deg F condition. Performance, stability, and control analyses were conducted with the comprehensive rotorcraft analysis CAMRAD II. Wind tunnel test measurements of the performance of the H-34 and UH-1D rotors at high advance ratio were compared with calculations to assess the accuracy of the analysis for the design of a high speed helicopter. In general, good correlation was obtained when an increase of drag coefficients in the reverse flow region was implemented. An assessment of various design parameters (disk loading, blade loading, wing loading) on the performance of the compound helicopter was conducted. Lower wing loading (larger wing area) and higher blade loading (smaller blade chord) increased aircraft lift-to-drag ratio. However, disk loading has a small influence on aircraft lift-to-drag ratio. A rotor parametric study showed that most of the benefit of slowing the rotor occurred at the initial 20 to 30% reduction of the advancing blade tip Mach number. No stability issues were observed with the current design. Control derivatives did not change significantly with speed, but the did exhibit significant coupling.

Performance and Design Investigation of Heavy Lift Tilt-Rotor with Aerodynamic Interference Effects

Abstract: : The aerodynamic interference effects on tiltrotor performance in cruise are investigated using comprehensive calculations, to better understand the physics and to quantify the effects on the aircraft design. Performance calculations were conducted for 146,600-lb conventional and quad tiltrotors, which are to cruise at 300 knots at 4000 ft/95 deg F condition. A parametric study was conducted to understand the effects of design parameters on the performance of the aircraft. Aerodynamic interference improves the aircraft lift-to-drag ratio of the baseline conventional tiltrotor. However, interference degrades the aircraft performance of the baseline quad tiltrotor, due mostly to the unfavorable effects from the front wing to the rear wing. A reduction of rotor tip speed increased the aircraft lift-to-drag ratio the most among the design parameters investigated.

Numerical and Experimental Validation of Three-Dimensional Shock Control Bumps

Abstract: Numerical and experimental studies have been performed to show the potential for drag reductions of an array of discrete three-dimensional shock control bumps. The bump contour investigated was specifically designed by means of computational-fluid-dynamics-based numerical optimization for wind-tunnel testing on a modern transonic airfoil. The experimental investigations focused on turbulent flow at a Reynolds number of 5 million and were carried out at the Transonic Wind Tunnel Gottingen. Drag reductions of around 10% in the drag-rise region were found in the experiment even though the results were influenced by wind-tunnel interference effects. A detailed numerical study of the wind-tunnel environment reproduced the influence of the wind-tunnel walls on the bump performance and gave good agreement to the experimental results.

Effects of Vortex Generators on an Airfoil at Low Reynolds Numbers

Abstract: A low-speed wind-tunnel investigation is presented detailing the effects of vortex generators on an airfoil at low Reynolds numbers (80,000 and 160,000) Six different static vortex generator layouts were tested In addition, an oscillatory (or active) vortex generator was designed and tested Force balance measurements were recorded and interpreted with the aid of surface flow visualization The data suggest that the static vortex generators function similarly to those at higher Reynolds numbers; increasing the maximum lift coefficient and increasing the stall angle Different static vortex generator configurations appear preferable at the two tested Reynolds number ranges The oscillating vortex generator did not appear effective in its present configuration

Modeling Pilot Control Behavior with Sudden Changes in Vehicle Dynamics

Abstract: A pursuit tracking model of the human pilot adapting to sudden changes in vehicle dynamics is developed and exercised. The current model is based upon a simplified representation of the human pilot in multi-axis tasks previously reported in the literature. A key feature of the adaptive model is the simplicity afforded by only varying two gain parameters in each control loop to accommodate pilot adaptation. The model is first exercised in a single-loop task in which the controlled element dynamics are changed from rate command to acceleration command (with time delay), back to rate command, and finally, to position command. A second example applies the model to a simple two-axis task (two control inceptors) in which the controlled element dynamics in both control loops are changed simultaneously. The model employed for sudden changes in vehicle dynamics is also applied to a single-axis task in which the controlled element dynamics change gradually over a 10 s period. Finally, longitudinal control of a simplified model of a fighter aircraft undergoing sudden damage is considered.

Lessons Learned from Numerical Simulations of the F-16XL Aircraft at Flight Conditions

Abstract: Nine groups participating in the Cranked Arrow Wing Aerodynamics Project International (CAWAPI) project have contributed steady and unsteady viscous simulations of a full-scale, semi-span model of the F-16XL aircraft. Three different categories of flight Reynolds/Mach number combinations were computed and compared with flight-test measurements for the purpose of code validation and improved understanding of the flight physics. Steady-state simulations are done with several turbulence models of different complexity with no topology information required and which overcome Boussinesq-assumption problems in vortical flows. Detached-eddy simulation (DES) and its successor delayed detached-eddy simulation (DDES) have been used to compute the time accurate flow development. Common structured and unstructured grids as well as individually-adapted unstructured grids were used. Although discrepancies are observed in the comparisons, overall reasonable agreement is demonstrated for surface pressure distribution, local skin friction and boundary velocity profiles at subsonic speeds. The physical modeling, steady or unsteady, and the grid resolution both contribute to the discrepancies observed in the comparisons with flight data, but at this time it cannot be determined how much each part contributes to the whole. Overall it can be said that the technology readiness of CFD-simulation technology for the study of vehicle performance has matured since 2001 such that it can be used today with a reasonable level of confidence for complex configurations.

Vibration and Flutter Characteristics of a Folding Wing

Abstract: Studies are presented that characterize the dynamic aeroelastic aspects of a morphing aircraft design concept. The notion of interest is a folding wing design resulting in large-scale wing area changes. A finite element approach is used to investigate the sensitivity of natural frequencies and flutter instabilities to the wing position (e.g., fold angle), actuator stiffness, and vehicle weight. Sensitivities in these areas drive design requirements and raise flight envelope awareness issues. The study is presented in two parts as a comparison between two models of varying complexity. A simple folding wing model, based on the Goland wing, is analyzed and results are compared with a built-up structural model of the proposed full scale morphing vehicle.

Low-Speed Fan Noise Attenuation from a Foam-Metal Liner

Abstract: A foam-metal liner for attenuation of fan noise was developed for and tested on a low-speed fan. This type of liner represents a significant advance over traditional liners, due to the possibility of placement in close proximity to the rotor. An advantage of placing treatment in this region is that the acoustic near field is modified, thereby inhibiting the noise-generation mechanism. This can result in higher attenuation levels than could be achieved by liners located in the nacelle inlet. In addition, foam-metal liners could potentially replace the fan rub strip and containment components, ultimately reducing engine components and thus weight, which can result in a systematic increase in noise reduction and engine performance. Foam-metal liners have the potential to reduce fan noise by 4 dB based on this study.