Showing papers in "Journal of Aircraft in 1991"

Aeroelastic Analysis of Wings Using the Euler Equations with a Deforming Mesh

Abstract: Modifications to the CFL3D three-dimensional unsteady Euler/Navier-Stokes code for the aeroelastic analysis of wings are described. The modifications involve including a deforming mesh capability that can move the mesh to continuously conform to the instantaneous shape of the aeroelastically deforming wing and also including the structural equations of motion for their simultaneous time integration with the governing flow equations. Calculations were performed using the Euler equations to verify the modifications to the code and as a first step toward aeroelastic analysis using the Navier-Stokes equations. Results are presented for the NACA 0012 airfoil and a 45-deg sweptback wing to demonstrate applications of CFL3D for generalized force computations and aeroelastic analysis. Comparisons are made with published Euler results for the NACA 0012 airfoil and with experimental flutter data for the 45-deg sweptback wing to access the accuracy of the present capability. These comparisons show good agreement and, thus, the CFL3D code may be used with confidence for aeroelastic analysis of wings. The paper describes the modifications that were made to the code and presents results and comparisons that assess the capability.

Multidisciplinary optimization in aircraft design using analytic technology models

Abstract: An approach to multidisciplinary optimization is presented which combines the Global Sensitivity Equation method, parametric optimization, and analytic technology models. The result is a powerful yet simple procedure for identifying key design issues. It can be used both to investigate technology integration issues very early in the design cycle, and to establish the information flow framework between disciplines for use in multidisciplinary optimization projects using much more computational intense representations of each technology. To illustrate the approach, an examination of the optimization of a short takeoff heavy transport aircraft is presented for numerous combinations of performance and technology constraints.

Helicopter vibration reduction using structural optimization with aeroelastic/multidisciplinary constraints - A survey

Abstract: This paper presents a survey of the state-of-the-art in the field of structural optimization when applied to vibration reduction of helicopters in forward flight with aeroelastic and multidisciplinary constraints. It emphasizes the application of the modern approach where the optimization is formulated as a mathematical programming problem, the objective function consists of the vibration levels at the hub, and behavior constraints are imposed on the blade frequencies and aeroelastic stability margins, as well as on a number of additional ingredients that can have a significant effect on the overall performance and flight mechanics of the helicopter. It is shown that the integrated multidisciplinary optimization of rotorcraft offers the potential for substantial improvements, which can be achieved by careful preliminary design and analysis without requiring additional hardware such as rotor vibration absorbers of isolation systems.

Aerodynamic and structural studies of joined-wing aircraft

Abstract: A method for rapidly evaluating the structural and aerodynamic characteristics of joined-wing aircraft was developed and used to study the fundamental advantages attributed to this concept. The technique involves a rapid turnaround aerodynamic analysis method for computing minimum trimmed drag combined with a simple structural optimization. A variety of joined-wing designs are compared on the basis of trimmed drag, structural weight, and, finally, trimmed drag with fixed structural weight. The range of joined-wing design parameters resulting in best cruise performance is identified. Structural weight savings and net drag reductions are predicted for certain joined-wing configurations compared with conventional cantilever-wing configurations.

Numerical solutions of forward-flight rotor flow using an upwind method

Abstract: A finite-volume upwind algorithm for solving the three-dimensional Euler equations with a moving grid has been developed for computing helicopter forward-flight rotor flows. The computed pressure distributions and shock positions of high-speed rotor flow are compared with various experimental data as well as with other numerical results, and the agreement is encouraging

Analytical Studies on Static Aeroelastic Behavior of Forward-Swept Composite Wing Structures

Abstract: An exact methodology allowing one to determine the aeroelastic lift distribution and the divergence instability of swept cantilevered composite wing structures is developed in this paper. The approach based on the Laplace transform technique enables one to solve, in a unified manner, both aeroelastic problems. The analysis encompasses the cases of free and constrained warping models for the wing twist. Numerical results are presented to demonstrate the effects played by the fiber orientation, ply lay-up, warping inhibition, and wing geometry on the subcritical static aeroelastic response and on the divergence instability of composite swept wings.

Airfoil design method using the Navier-Stokes equations

Abstract: An airfoil design procedure is described that was incorporated into an existing 2-D Navier-Stokes airfoil analysis method. The resulting design method, an iterative procedure based on a residual-correction algorithm, permits the automated design of airfoil sections with prescribed surface pressure distributions. The inverse design method and the technique used to specify target pressure distributions are described. It presents several example problems to demonstrate application of the design procedure. It shows that this inverse design method develops useful airfoil configurations with a reasonable expenditure of computer resources.

Moving surface boundary-layer control as applied to two-dimensional airfoils

Abstract: The concept of moving surface boundary-layer control, as applied to a Joukowsky airfoil, is investigated through an experimental program complemented by a flow visualisation study. The moving surface was provided by rotating cylinders located at the leading edge and upper surface of the airfoil. The results suggest that the leading-edge rotating cylinder effectively extends the lift curve without substantially affecting its slope, thus increasing the maximum lift and delaying stall. When used in conjunction with a second cylinder on the upper surface, further improvements in the maximum lift and stall angle are possible. The maximum coefficient of lift realised was around 2.73, approximately three times that of the base airfoil. The maximum delay in stall was around 48 deg. In general, the performance improves with an increase in the ratio of cylinder surface speed Uc to the freestream speed U.

Full-scale demonstration tests of cabin noise reduction using activevibration control

Abstract: We report the results of tests conducted to demonstrate the effectiveness of active vibration control techniques in reducing structure borne noise in aircraft cabins. A prototype active vibration control system was tested in the Douglas Aircraft Company Fuselage Acoustic Research Facility, using the aft section of a DC-9 aircraft as the test article

Transonic Low-Reynolds Number Airfoils

Abstract: Airfoils operating in the unexplored high-Mach—low-Reynolds number regime are computationally investigated. The motivations are 1) quantificatio n of achievable airfoil performance levels; 2) quantificatio n of parameter sensitivities which impact vehicle sizing; 3) identification of possible shortcomings in the computational methods employed; and 4) identification of test data required for adequate validation of the airfoil designs and performance prediction methods. The investigation centers on candidate airfoils developed for proposed ultrahigh altitude aircraft (UHAA) having both a high-ceiling and a long-range requirement. Computational studies indicate that 35-km ceiling performance at M — 0.60, Re — 200,000 hinges on the effective use of transonic flow to enhance transition and reduce separation-bubble losses. The separation bubbles become associated with large lambda shock structures at the highest tolerable Mach numbers. Airfoil performance predictions are parameterized by quantities dependent only on altitude and vehicle characteristics, and independent of flight trim conditions. For the airfoils designed, no flaps are necessary to achieve nearly optimal performance at both 35-km ceiling conditions as well as lower 15-25-km altitudes where long-range cruise would occur. Variation in airfoil thickness between 11-15% has surprisingly little impact on aerodynamic performance.

Separated flowfield on a slender wing undergoing transient pitching motions

Abstract: An experimental investigation on the flowfield surrounding a delta wing undergoing a transient pitching motion was conducted. The leading-edge vortices were marked with smoke, and the model and vortex motions were recorded using high-speed motion picture photography. These film records were then analyzed to yield information on the vortex location and trajectory as functions of both angle of attack and time. Hysteresis effects on the vortex dynamics were apparent for both the pitch-up and pitch-down motions

High-Intensity Acoustic Tests of a Thermally Stressed Plate

Abstract: An investigation was conducted in the Thermal Acoustic Fatigue Apparatus at the Langley Research Center to study the acoustically excited random motion of an aluminum plate which is buckled due to thermal stresses. The thermal buckling displacements were measured and compared with theory. The general trends of the changes in resonances frequencies and random responses of the plate agree with previous theoretical prediction and experimental results for a mechanically buckled plate.

Experiments on the unsteadiness associated with a ground vortex

Abstract: The ground vortex formed by a jet impinging on the ground in the presence of a crossflow has been studied experimentally. High speed motion pictures and spectral measurements were obtained to study the unsteady features of this flowfield. A very low-frequency pulsation or 'puffing' instability was observed. Since this unsteadiness could not be correlated with any other oscillations in the flowfield, the low-frequency oscillations must come from the gross features of the ground vortex itself. Namely, jet fluid accumulates in the ground vortex until the vortex is so large that the flowfield breaks up, the ground vortex is swept away, a new smaller vortex forms, and the process repeats itself. Measurements of the frequency of these oscillations are presented for the first time, and data on the vertical extent (height) of the ground vortex are also shown.

Forward sweep - A favorable concept for a laminar flow wing

Abstract: The application of laminar flow on swept wings is authoritatively limited at high Reynolds numbers by a sweep angle where crossflow instability and attachment line transition lead to fully turbulent boundary layers on the wing. For a laminar flow wing, the reduction in sweep in the case of a forward swept wing leads to a more stable laminar boundary layer concerning transition because of crossflow instability and attachment line transition. Thus, with this concept, a laminar forward swept wing can be realized more easily than a comparable swept back wing

Visualization Measurements of Vortex Flows

Abstract: Novel flow visualization experiments have been used on the separated vortex flow on a delta wing at low speed (3000 < Rec < 30,000) to give measurements of flow parameters for comparison with theory. Data on vortex core position, vortex sheet shape, vortex strength, and local velocity magnitude and direction in the core have been determined. The results show surprising consistency with several features of established theoretical models for high Re flows and further justify the use of low Re experiments for vortex flows. The results also indicate areas where care should be taken in extrapolating results at low Re to flight cases.

Aeroelastic optimization of a helicopter rotor using an efficient sensitivity analysis

Abstract: A structural optimization analysis of a hingeless helicopter rotor is developed and applied with the objective of reducing oscillatory hub loads in forward flight. The aeroelastic analysis of the rotor is based on a finite element method in space and time and is linked with automated optimization algorithms. Two types of structural blade representations are used: a generic stiffness-distribution beam and a single-cell, thin-walled beam. For the generic beam representation the design variables are nonstructural mass and its placement, chordwise center of gravity offset from the elastic axis, and structural stiffness (flap, lag, and torsion). For the second type of structural representation, spar width, height, and thickness are used as design variables instead of blade stiffness. Constraints on frequency placement, autorotational inertia, and aeroelastic stability of the blade are included. Sensitivity derivatives are efficiently calculated using a direct analytical approach, with a resulting 80% reduction in total CPU time required to obtain an optimum solution compared with a commonly used finite-difference approach. Optimum solutions resulted in reductions of 25-77% for the generic blade, and 30-50% for the box-beam blade relative to baseline values of the objective function that was comprised of all six components of hub load.

Fortified LEWICE with viscous effects

Abstract: A method for computing the prediction of ice shapes on airfoils and their effects on the airfoil lift and drag coefficients are described. The previously developed LEWICE code has been modified to avoid problems due to multiple stagnation points. The interactive boundary-layer method developed by Cebeci has been incorporated into LEWICE to improve the accuracy of predicting ice shapes as well as to compute the performance characteristics of iced airfoils. The paper also presents ice shapes calculated without and with viscous effects, the consequences for aerodynamic properties, particulary lift and drag, and evaluation of the time steps used in the ice accretion process.

Proposed modifications to ice accretion/icing scaling theory

Abstract: The difficulty of conducting full-scale icing tests has long been appreciated. Testing in an icing wind tunnel has been undertaken for decades. While aircraft size and speeds have increased, tunnel facilities have not, thus making subscale geometric tests a necessity. Scaling laws governing these tests are almost exclusively based on analysis performed over 30 years ago and have not been rigorously validated. The following work reviews past scaling analyses and suggests revision to these analyses based on recent experimental observation. It is also suggested, based on the analysis contained herein, that current ice accretion predictive technologies, such as LEWICE when utilized in the glaze ice accretion regime, may need upgrading to more accurately estimate the rate of ice build-up on aerodynamic surfaces.

Further studies of harmonic gradient method for supersonic aeroelastic applications

Abstract: Recent developments in the applications of the harmonic gradient method (HGM) to various lifting surfaces with/without control surfaces are described. Our objective is to validate the acceleration-potential version of HGM, also known as the ZONA51 code, with available measured data and existing methods, which include the constant pressure panel method, the pressure mode method, and the piston theory. Unsteady supersonic aerodynamics over a leading-edge flap of an F-18 wing and a trailing-edge flap of a British Aerospace Corporation vertical fin are studied. Measured pressure jumps along the flap hinge lines are captured by the ZONA51 code whereas other methods fail to do so. A supersonic flutter analysis is performed for four different wing planforms; these include a 45-deg swept wing, a NASA 70-deg delta wing, a National Aerospace Plane (NASP)-type wing body, and the active flexible wing (AFW). For all cases considered, it is found that the present method yields favorable flutter trends which follow closely with those measured. Conservative flutter boundaries are obtained in almost all cases, in contrast to the predicted results of other existing methods. Finally, the AFW with fuselage and wing-tip ballast store is conveniently modeled and computed by the ZONA51 code resulting in a reasonable flutter boundary. It is believed that the ZONA51 code with its robust structure along with the present validation effort will be upheld as an integral part of matured aeroelastic technology.

Euler/experiment correlations of sonic boom pressure signatures

Abstract: The ability of inviscid computational fluid dynamics (CFD) codes to compute sonic boom pressure signatures is examined using three different codes that solve the Euler equations of fluid flow on structured hexahedral and unstructured tetrahedral grids. The results of these Euler codes were evaluated by comparing the computed pressure signatures with near-field experimental data. The computational pressure signatures were determined at distances of one body length or less below the configuration in the plane of symmetry and extrapolated to experimental distances. The extrapolated CFD pressure signatures gave acceptable correlations with experimental data, provided that fine grids were used near the surface and downstream of the configuration.

Modified Exponential Series Approximation for the Theodorsen Function

Abstract: An improved method is developed for the approximation of unsteady aerodynamics in the time domain by a series of decaying exponentials. The new method is different from the previous procedures in that it consistently accounts for the case when the optimum values of the lag parameters in the exponential series are close to one another. This is achieved by introducing a time-weighted exponential series for the repeated pole case. The method uses a nongradient optimizing procedure. Approximations are presented for Theodorsen's lift deficiency function and results are compared with those of a gradient-based method that was published recently. HE representation of the general motion of an aeroelastic structure requires the availability pf the unsteady aerody- namic forces in the time domain. An important feature of these forces is the lag associated with the circulatory wake. Theodorsen1 employed a lift deficiency function in the re- duced frequency domain to represent this effect for the oscilla- tory flow over an airfoil. Jones2 used a two-term series of decaying exponentials in the time domain to approximate the effect of circulation for the transient aeroelastic motion and solved for the linear coefficients of the series by using the Fourier transform to convert the transfer function into the frequency domain where it must equal Theodorsen's circula- tion function. This idea was extended by Do well3 who used an exponential series in the time domain to represent the unsteady aerodynamic transfer function and then transformed it into a rational function in the Laplace domain. In contrast with this approach is the conventional least-squares method4 that be- gins with a suitable approximation in the Laplace domain and then transforms it into the time domain using the inverse Laplace transform. It is to be noted that, although certain effects, such as the aerodynamic damping and the aerody- namic inertia, can be included in the rational function approx- imation in the Laplace domain, they are essentially left out in the exponential series approximation in the time domain. However, this difference between the two methods is not present when only the circulatory effect is being approxi- mated, such as the lift deficiency function of Theodorsen. The accuracy of an exponential series approximation de- pends crucially on the values of the nonlinear lag parameters that occur in the exponentials. Recently, Peter son and Craw- ley5 have shown .that, when these parameters are optimized to give a minimum squared error between the exact and the approximate values of the transfer function, the accuracy of the approximation is greatly enhanced. However, the objec- tive-function minima obtained by Peterson and Crawley5 may not be unique because the gradient-based optimizer is unable to escape the local minima. Also, the frequently encountered cases when the optimum values of two or more lag parameters are nearly the same are mistaken to indicate that the same fit accuracy can be achieved by reducing the number of lag states. Moreover, their inclusion of the numerator as well as the denominator coefficients of the transfer function in the Laplace domain as the free parameters of optimization ap- pears to be a less efficient procedure than the one in which the numerator coefficients are determined by a least-squares fit. The latter option is utilized in the present method, which uses a simplex nongradient optimizer to locate the absolute minima of the objective function. The most significant improvement is achieved by using a consistent optimizing procedure, intro- duced earlier by the authors,6 which correctly accounts for the case of repeated lag parameters by employing a new approxi- mation that contains time-weighted exponentials. The time- weighting functions are polynomials in time of one degree less than the multiplicity of the corresponding lag parameters. By using this series, the repeated pole case not only becomes meaningful but also allows an improvement in the fit accuracy with the frequency domain data. Gradient-Based Method as Compared to the Present Method The present example compares the gradient-based opti- mization scheme of Peterson and Crawley,5 who used the exponential time-series approximation of Dowell3 as the basic transfer function, with the nongradient optimized scheme of the present method employing the same transfer function. For the purpose of comparison, both methods approximate the Theodorsen circulation function.

New device for controlling asymmetric flowfields on forebodies at large alpha

Abstract: An exploratory experimental investigation of a new device to control the asymmetric flowfield on forebodies at large angles of attack has been conducted. The device is a rotatable forebody tip, which varies in cross section from circular at its base to elliptic at its tip. The device itself extends over a small portion of the aircraft or missile forebody. The device provides two important improvements. First, it replaces the normally random behavior of the nose side force as a function of nose tip orientation with a predictable and generally sinusoidal distribution; second, the device shows promise for use as part of a vehicle control system to be deflected in a prescribed manner to provide additional directional control for the vehicle. The device was tested on a cone/cylinder model having a 10-deg semiapex angle and on a 3.0-caliber tangent ogive model. Data were taken with each model at a Reynolds number of 8.4 x 10 4 based on cylinder diameter and by a helium-bubble flow visualization technique at a Reynolds number of 2.4 x 10 4.

Numerical simulation of high-incidence flow over the F-18 fuselage forebody

Abstract: As part of the NASA High Alpha Technology Program, fine-grid Navier-Stokes solutions have been obtained for flow over the fuselage forebody and wing leading-edge extension of the F/A-18 High Alpha Research Vehicle at large incidence. The resulting flows are complex and exhibit cross-flow separation from the sides of the forebody and from the leading-edge extension. A well-defined vortex pattern is observed in the leeward-side flow. Results obtained for laminar flow show good agreement with flow visualizations obtained in ground-based experiments. Further, turbulent flows computed at high-Reynolds-number flight-test conditions show good agreement with surface and off-surface visualizations obtained in flight.

Impact ice stresses in rotating airfoils

Abstract: Finite element analysis is used to study the tensile and shear stresses at the interface between impact ice adhering to a rotating airfoil and the metal airfoil surface A simple rotating beam-ice structure is used to obtain basic understanding of stress distribution in the ice Calculations show that shear stresses increase linearly with ice thickness and tensile stresses tend to zero for a fully bonded surface When shear stresses exceed the ultimate strength, adhesive failure occurs and tensile stresses are developed in the unbonded ice, resulting in tensile failure of the impact ice A second model is used to study the OH-58 tail rotor with a measured ice profile Ice shedding predictions are compared to the resulting data using a statistical structural analysis

Thermostructural concepts for hypervelocity vehicles

Abstract: In the design of hypervelocity veh~cles (HVV). a key driving parameter is the structural mass fraction of the system. The mass fraction greatly influences the performance and payload capabilities of the vehicle. For example. a reduction of one pound of structural weight would effectively translate into an additional pound of payload, or alternatively, approximately ten fewer pounds of propellant. Through the optimization of thermostructural concepts. a significant increase in overall mission perfbrmance can be reallred. Such an optimiration requires not only good familiarity with the available material optlons and therrnostructural concepts. but also a thorough knowledge of the thermal. inertial, aerodynamic, and control loads on the vehicle as a consequence of hqpervelocity flight conditions. This optimiration process the establishment o l aeroheating and other critical loads, the consideration of material alternatives, and the formulation of thermostructural concep t wa the primary l'i~cus ol' t h ~ \ study. A suborbital vehicle (Figure I ) and the aswciated trajectory (Figure 2 ) a i described in Rckrence I were chosen as stud) baselines. These selections wcrc Idlowed b> a series of interrclated design and anal] tical tazks. during which a number of thcri~~ostructural trade \tudie\ \\el-c conducted. The primary results ol'the stud! include detailed w i g h t estlmates h r the candidate thernio~tructural concepts and the selecSUB-ORBITER SYNERGETIC PLANE CHANGE

Heat transfer measurements from a smooth NACA 0012 airfoil

Abstract: Local convective heat transfer coefficients were measured from a smooth NACA 0012 airfoil having a chord length of 0.533 m. Flight data were taken for the smooth airfoil at Reynolds numbers based on chord in the range 1.24 to 2.50 million and at various angles of attack up to 4 deg. During these flight tests, the freestream velocity turbulence intensity was found to be very low. Wind tunnel data were acquired in the Reynolds number range 1.20 to 4.52 million and at angles of attack from -4 to +8 deg. The turbulence intensity in the IRT was 0.5-0.7 percent with the cloud-generating sprays off. A direct comparison between the results obtained in flight and in the IRT showed that the higher level of turbulence intensity in the IRT had little effect on the heat transfer for the lower Reynolds numbers but caused a moderate increase in heat transfer at the higher Reynolds numbers. Turning on the cloud-generating spray nozzle atomizing air in the IRT did not alter the heat transfer. The present data were compared with leading-edge cylinder and flat plate heat transfer correlations that are often used to estimate airfoil heat transfer in computer codes.

Calculating time-correlated gust loads using matched filter and random process theories

Abstract: This paper describes and illustrates two ways of performing time-correlated gust-load calculations. The first is based on matched filter theory, the second on random process theory. Both approaches yield theoretically identical results, represent novel applications of the theories, are computationally fast, and may be applied to other dynamic-response problems. A theoretical development and example calculations using both matched filter theory and random process theory approaches are presented.