Showing papers on "Flutter published in 1987"

AGARD standard aeroelastic configurations for dynamic response. Candidate configuration I.-wing 445.6

Abstract: To promote the evaluation of existing and emerging unsteady aerodynamic codes and methods for applying them to aeroelastic problems, especially for the transonic range, a limited number of aerodynamic configurations and experimental dynamic response data sets are to be designated by the AGARD Structures and Materials Panel as standards for comparison. This set is a sequel to that established several years ago for comparisons of calculated and measured aerodynamic pressures and forces. This report presents the information needed to perform flutter calculations for the first candidate standard configuration for dynamic response along with the related experimental flutter data.

Aeroelastic tailoring - Creative uses of unusual materials

Abstract: This paper discusses the fundamental aspects of the use of aeroelastic tailoring to enhance the performance of flexible fixed wing aircraft. Tailoring with advanced composites is seen as part of a natural evolutionary process in the everchanging field of design. Among topics discussed are: static divergence; lift effectiveness; drag reduction; control effectiveness; and, vibration and flutter. In addition, terminology is reviewed, together with descriptive formulas that characterize stiffness coupled structures. Finally, a summary of design features favorable to different facets of aeroelastic tailoring is given to illustrate conflicts and compromises that must be considered.

AGARD Manual on Aeroelasticity in Axial-Flow Turbomachines. Volume 1. Unsteady Turbomachinery Aerodynamics

Abstract: : Partial contents: Linearized Unsteady Aerodynamic Theory; Classical Two-dimensional Methods; Three dimensional flows; Numerical Methods Unsteady Transonic Flow; Stall Flutter; Unsteady Aerodynamic Measurements Flutter Research and in Forced Vibration Research; Understanding Fan Blade Flutter through Linear Cascade Aeroelastic Testing and Unsteady Aerodynamic Measurements Rotors.

Vortex excitation of rectangular cylinders with a long side normal to the flow

Abstract: The vortex excitation of rectangular cylinders having side ratios of 0.2, 0.4 and 0.6, with a long side normal to the flow, in a mode of lateral translation, is investigated experimentally in a wind tunnel using free- and forced-oscillation methods. The range of reduced wind speeds investigated, 3-12, includes the vortex-resonance regime. The forced-oscillation experiment includes measurements of the fluctuating lift force at amplitudes up to 10% of the length of a long side. The experiments were also performed on cylinders with a long fixed splitter plate. The results of the measurements show that vortex excitation of a rectangular cylinder is strongly dependent on the side ratio. It is suggested that the critical change of the mean base pressure of an oscillating rectangular cylinder with increasing side ratio is closely correlated with the vortex-excitation characteristics. The concept of vortex excitation as aeroelastic flutter occurring in a fluid-body coupled system is proposed on the basis of the experimental results. The most essential feature of vortex excitation as a coupled flutter is that the fluid subsystem can be set into resonance by the body motion. A rapid phase-angle change in the lift force through vortex resonance produces a large out-of-phase force component which excites the motion of the body subsystem.

Aeroelastic stability characteristics of a composite swept wing withtip weights for an unrestrained vehicle

Abstract: An analytical investigation to determine aeroelastic flutter and divergence behavior of a composite, forwardswept rectangular wing was conducted. It is assumed that the wing is carrying a fuselage at its semispan, a pylon at the wing tip, and that the aircraft is in a free-flight condition (unrestrained vehicle). The influence, due to the variation in the bending-torsion stiffness coupling of the tailored wing on the flutter and divergence critical dynamic pressure, is analyzed. The paper discusses the influence of the warping effect on the system's flutter and divergence velocities. The aeroelastic stability behavior of the system is obtained by applying an optimization procedure that solves exactly the coupled bending-torsion equations of motion for the unrestrained swept-wing aircraft. The results of the present study indicate that the warping effect significantly influences the system's aeroelastic characteristics.

Aeroelastic tailoring of aft-swept high-aspect-ratio composite wings

Abstract: The integrating matrix method is used to study the aeroelastic performance of aft-swept high-aspect-ratio wings. Aeroelastic stability boundaries are shown as a function of fiber angle and dimensionless modulus parameters for straight wings of aspect ratio 14. Certain laminates show divergence for wings with low sweep angles, but for most laminates and wing sweeps, flutter was the instability. The bending-torsion coupling that is beneficial for forward-swept wings is shown to be of no advantage for aft-swept wings, for which torsional stiffness is much more significant. Both symmetric and nonsymmetric ply orientations were studied, and it is demonstrated that there need not be a severe penalty for using general laminates.

Flutter analysis of a two-dimensional airfoil containing structural nonlinearities

Abstract: Nonlinear flutter of a two-dimensional airfoil undergoing plunging and pitching motions is studied using a time marching finite difference scheme. The structural nonlinearity considered is of the type due to a spring with preload and freeplay. Flutter is determined from solutions of the structural dynamic equations of motion when either divergent or limited amplitude oscillations are encountered. Case studies using various airfoil parameters and values of preload and freeplay are carried out. The effect of initial condition, which is important in nonlinear problems, is investigated by varying the displacement from equilibrium of the pitch angle at the beginning of the airfoil motion. For nonzero values of the preload, three types of oscillatory motion are possible, namely: damped, limited amplitude and divergent. The divergent flutter boundary is practically identical to that for the linear flutter case. The location of the limit-cycle flutter boundary varies for different airfoil and spring parameters. For zero preload, damped oscillations are not encountered even for air speeds down to 15 percent of the linear flutter speed which is the lowest used in this study. The limited amplitudes of the pitch and plunge motions are found to be independent of initial angular displacement. The characteristics of the oscillations and the development of higher harmonies in the various regions defined by the flutter boundary curves are investigated.

Active Control of Aerofoil Flutter

Abstract: On decrit une methode d'elimination du flottement par une technique de commande active. La commande est assuree par l'addition d'une force commandee d'amplitude et de phase appropriees pour annuler l'effet destabilisant naturel de la charge aerodynamique

Experimental aeroelastic behavior of forward-swept graphite/epoxy wings with rigid-body freedom

Abstract: An analytical and experimental investigation was made of the aeroelastic flutter and divergence behavior of graphite/epoxy forward-swept wings with rigid-body pitch and plunge freedoms present. A complete, two-sided, 30 deg forward-swept wing aircraft model was constructed and mounted with low-friction bearings in a lowspeed wind tunnel. Four different ply layup wings could be interchanged on the model, namely, [02/90]s, [152/0]s, [302/0]s, and [ -152/0]s. Wind-tunnel tests on the "free*'-flying models revealed body freedom flutter, bending-torsion flutter, and a support dynamic instability that could be eliminated by proper adjustment of the support stiffness. Good agreement with linear theory was found for all the observed instabilities. Additional tests on the models with rigid-body pitch only gave lower critical speeds, while tests on the cantilever wings gave higher speeds. The [152/0] s wing gave the best tailored aeroelastic behavior.

Computerized symbolic solution for a nonconservative system in which instability occurs by flutter in one range of a parameter and by divergence in another

Abstract: The instability of a uniform column which is simply supported at one end, resting on a support at some intermediate location q, and has the other end subjected to a follower force is studied. The problem is solved by the Galerkin method in conjunction with computerized symbolic algebra. It is shown that there exists a location for the intermediate support at q = q∗ so that the structure loses its stability by flutter for q q∗. At q = q∗ the critical load undergoes a jump, implying the transition of the instability mode from flutter to divergence. Moreover, the location q = q∗+ is an optimal location in the sense that the critical load assumes a maximum.

Control of flutter in bridges

Abstract: In recent years there has been increasing interest in the application of modern control theory to the problem of vibration suppression in civil structures. The loads on such structures can depend on the environment. For example, the motion of a flexible suspension bridge, such as the original Tacoma Narrows Bridge [Ref. 1], can include unsteady aerodynamic forces. The motion can be described by two partial differential equations for bending and torsional vibration [Ref. 2], with the airstream velocity V as a parameter. The stability of motion is governed by the real part of the system eigenvalues, where the eigenvalues depend on V. For V = 0, the system is self-adjoint [Ref. 3] and the eigenvlaues consist of pairs of mere imaginary complex conjugates. For small V, the eigenvalues have negative real parts, so that the motion is asymptotically stable. As V increases, some real parts can turn positive, rendering the system unstable. The air speed corresponding to zero real part is known as the critical speed V er. If the critical speed corresponds to a value for which the imaginary part of an eigenvalue is different from zero, the structure is said to be in flutter condition [Ref. 3].

Aeroelastic Behavior of Low-Aspect-Ratio Metal and Composite Blades

Abstract: The aeroelastic stability of titanium and composite blades of low aspect ratio is examined over a range of design parameters, using a Rayleigh-Ritz formulation. The blade modes include a plate-type mode to account for chordwise bending. Chord-wise flexibility is found to have a significant effect on the unstalled supersonic flutter of low-aspect-ratio blades, and also on the stability of tip sections of shrouded fan blades. For blades with a thickness of less than approximately 4 percent of chord, the chordwise, second bending, and first torsion branches are all unstable at moderately high supersonic Mach numbers. For composite blades, the important structural coupling between bending and torsion cannot be modeled properly unless chordwise bending is accounted for. Typically, aft fiber sweep produces beneficial bending-torsion coupling that is stabilizing, whereas forward fiber sweep has the opposite effect. By using crossed-ply laminate configurations, critical aeroelastic modes can be stabilized.

Transonic aeroelasticity of wings with active control surfaces

Abstract: Transonic aeroelasticity of wings with active control surfaces is studied by using the unsteady-small disturbance transonic aerodynamic equations coupled with modal structural equations of motion. The aerodynamic and structural equations of motion are simultaneously integrated by a time-accurate numerical scheme. A procedure of synthesizing active controls with unsteady transonics is presented. Flutter suppression in the transonic regime using active controls is demonstrated for a rectangular wing. Characteristics of a selected control law in the transonic regime are studied. The results from this study are useful in the design of active control systems in the transonic regime.

The effects of aeroelastic deformation on the unaugmented stopped-rotor dynamics of an X-Wing aircraft

Abstract: A new design concept in the development of vertical takeoff and landing aircraft with high forward flight speed capability is that of the X-Wing. The X-Wing is a stiff, bearingless helicopter rotor system which can be stopped in flight and the blades used as two forward-swept wings and two aft-swept wings. Because of the unusual configuration in the fixed-wing mode, there is a high potential for aeroelastic divergence or flutter and coupling of blade vibration modes with rigid-body modes. An aeroelastic stability analysis of an X-Wing configuration aircraft was undertaken to determine if these problems could exist. This paper reports on the results of dynamic stability analyses in the lateral and longitudinal directions including the vehicle rigid-body and flexible modes. A static aeroelastic analysis using the normal vibration mode equations of motion was performed to determine the cause of a loss of longitudinal static margin with increasing airspeed. This loss of static margin was found to be due to aeroelastic 'washin' of the forward-swept blades and 'washout' of the aft-swept blades moving the aircraft aerodynamic center forward of the center of gravity. This phenomenon is likely to be generic to X-Wing aircraft.

Analytical and experimental investigation of mistuning in propfan flutter

Abstract: An analytical and experimental investigation of the effects of mistuning on propfan subsonic flutter was performed. The analytical model is based on the normal modes of a rotating composite blade and a three-dimensinal subsonic unsteady lifting surface aerodynamic theory. Theoretical and experimental results are compared for selected cases at different blade pitch angles, rotational speeds, and free-stream Mach numbers. The comparison shows a reasonably good agreement between theory and experiment. Both theory and experiment showed that combined mode shape, frequency, and aerodynamic mistuning can have a beneficial or adverse effect on blade damping depending on Mach number. Additional parametric results showed that alternative blade frequency mistuning does not have enough potential for it to be used as a passive flutter control in propfans similar to the one studied. It can be inferred from the results that a laminated composite propfan blade can be tailored to optimize its flutter speed by selecting the proper ply angles.

Accelerometer placement in active flutter suppression systems

Abstract: An elementary study of the placement of accelerometers when used as sensors in active flutter suppression systems is presented. The model analyzed is a two-dimensional typical section with various combinations of leading- and trailing-edge controllers. The relationship between the control surface placement and accelerometer placement is examined. The results for the numerical examples presented show that accelerometers must be on the same side of the center of gravity as the control surface or right half-plane transmission zeros will arise. The detrimental effects of the right half-plane zeros are discussed in the context of linear-quadratic-regulator plus loop-transfer-recovered observer designs.

Stabilizing Winglet Pair for Slender Bridge Decks

Abstract: This paper describes the effectiveness, as an aerodynamic damper, of a pair of winglets mounted parallel to, and clear from, bridge decks that might otherwise be aeroelastically unstable. The winglets typically are mounted above or below, and slightly outboard of, the bridge deck, and typically would have a chord dimension of ten percent or less of the bridge deck chord. They can be rigidly attached to the bridge deck or can be clipped to hangers if the bridge is a suspension bridge. If the winglet pair is sufficiently removed from the bridge deck, the aerodynamic flutter coefficients of the winglet pair can be determined theoretically, and then superimposed upon the aerodynamic flutter coefficients for the bridge deck section. Comparisons between theory and experiment are made.

Analytical flutter investigation of a composite propfan model

Abstract: A theoretical model and an associated computer program for predicting subsonic bending-torsion flutter in propfans are presented. The model is based on two-dimensional unsteady cascade strip theory and three-dimensional steady and unsteady lifting surface aerodynamic theory in conjunction with a finite element structural model for the blade. The analytical results compare well with published experimental data. Additional parametric studies are also presented illustrating the effects on flutter speed of steady aeroelastic deformations, blade setting angle, rotational speed, number of blades, structural damping, and number of modes.

Structural tailoring of advanced turboprops

Abstract: The Structural Tailoring of Advanced Turboprops (STAT) computer program was developed to perform numerical optimization on highly swept propfan blades. The optimization procedure seeks to minimize an objective function defined as either: (1) direct operating cost of full scale blade or, (2) aeroelastic differences between a blade and its scaled model, by tuning internal and external geometry variables that must satisfy realistic blade design constraints. The STAT analysis system includes an aerodynamic efficiency evaluation, a finite element stress and vibration analysis, an acoustic analysis, a flutter analysis, and a once-per-revolution forced response life prediction capability. STAT includes all relevant propfan design constraints.

Suspension bridge structure with flutter damping means

Abstract: The invention concerns a suspension bridge comprising an essentially flat main structure, the upper surface of which forms the roadway for the transport means crossing the bridge, and a suspension structure formed of a plurality of catenary wires connected to end piers of the bridge and of a plurality of vertical stays for suspending the main flat bridge structure to the catenary wires. According to the invention, to the bridge structure there are associated wing control surfaces having an aerodynamic lifting and/or negative lifting action, the flutter speed proper to said wing control surfaces being considerably higher than the flutter speed proper to the bridge structure, and furthermore, the bridge structure and the wing control surfaces are stiffly interconnected and interact dynamically in order to shift the flutter speed of the whole at least above the top speed of the wind expected in the bridge area.

Flutter study of an advanced composite wing with external stores

Abstract: A flutter test using a scaled model of an advanced composite wing for a Navy attack aircraft has been conducted in the NASA Langley Research Center Transonic Dynamics Tunnel. The model was a wall-mounted half-span wing with a semi-span of 6.63 ft. The wing had an aspect ratio of 5.31, taper ratio of 0.312, and quarter-chord sweep of 25 degrees. The model was supported in a manner that simulated the load path in the carry-through structure of the aircraft and the symmetric boundary condition at the fuselage centerline. The model was capable of carrying external stores from three pylon locations on the wing. Flutter tests were conducted for the wing with and without external stores. No flutter was encountered for the clean wing at test conditions which simulated the scaled airplane operating envelope. Flutter boundaries were obtained for several external store configurations. The flutter boundaries for the fuel tanks were nearly Mach number independent (occurring at constant dynamic pressure). To study the aerodynamic effect of the fuel tank stores, pencil stores (slender cylindrical rods) which had the same mass and pitch and yaw inertia as the fuel tanks were tested on the model. These pencil store configurations exhibited a transonic dip in the flutter dynamic pressure, indicating that the aerodynamic effect of the actual fuel tanks on flutter was significant. Several flutter analyses methods were used in an attempt to predict the flutter phenomenon exhibited during the wind-tunnel test. The analysis gave satisfactory predictions of flutter for the pencil store configurations, but unsatisfactory correlation for the actual fuel tank configurations.

Optimization of wing tip store modeling

Abstract: I N present three-dimensional flutter investigations of aircraft with external stores, the stores are often approximated by flat plates to reduce the cost and complexity of the analyses. In this Synoptic, the doublet lattice and kernel function methods are utilized to investigate the validity of this flatplate approximation in comparison to store models of other geometries for an F-5 wing with tip-mounted launcher/store combination. Various cross sections are found that show improved results with only a moderate increase in model complexity and thus computer cost.