Showing papers on "Flutter published in 1971"

Airfoil and Bridge Deck Flutter Derivatives

Abstract: The writers compare the flutter phenomena of the suspension bridge and the airfoil and employ a free-oscillation experimental method to measure model bridge flutter coefficients analogous to airfoil flutter coefficients. They employ the airfoil as a check on the experimental method, both as a theoretical backdrop and to test out the nature of aerodynamic oscillatory forces under exponentially modified motion. A short catalogue of bridge deck flutter coefficients is then experimentally obtained and presented covering a range of bridge deck forms. Detailed results are described to account for a number of phenomena observed in the wind tunnel and in the field.

An Approximate True Damping Solution of the Flutter Equation by Determinant Iteration

Abstract: The difference between a true damping, or rate-of-decay, solution of the flutter equation and the structural-damping-type of solution is highlighted. True damping solutions are possible if unsteady aerodynamics can be expressed in terms of the complex variable p. If the aerodynamics are given at discrete values of the reduced frequency fc, an approximate determination of the true damping is possible by assuming that the aerodynamic forces for harmonic motion are a good approximation for the cases of slowly increasing or decreasing amplitude. A determinant iteration method for obtaining the solution is presented. Results obtained by different methods of solving the flutter equation are compared.

Air foil and bridge deck flutter derivatives

Abstract: THE FLUTTER PHENOMENA OF THE SUSPENSION BRIDGE IS COMPARED TO THE AIRFOIL. A FREE-OSCILLATION EXPERIMENTAL METHOD IS USED TO MEASURE MODEL BRIDGE FLUTTER COEFFICIENTS ANALOGOUS TO AIRFOIL FLUTTER COEFFICIENTS. THE AIRFOIL IS EMPLOYED AS A CHECK ON THE EXPERIMENTAL METHOD, BOTH AS A THEORETICAL BAKCDROP AND TO TEST OUT THE NATURE OF AERODYNAMIC OSCILLATORY FORCES UNDER EXPONENTIALLY MODIFIED MOTION. A SHORT CATALOGUE OF BRIDGE DECK FLUTTER COEFFICIENTS WAS EXPERIMENTALLY OBTAINED AND COVERS A RANGE OF BRIDGE DECK FORMS. DETAILED RESULTS ARE DESCRIBED TO ACCOUNT FOR A NUMBER OF PHENOMENA OBSERVED IN THE WIND TUNNEL AND IN THE FIELD. /AUTHOR/

Optimization of complex structures to satisfy flutter requirements

Abstract: Aircraft structural parameters optimization satisfying flutter velocity constraint and minimum mass, applying to box beam design

Subsonic Unsteady Aerodynamics for General Configurations. Part 1. Volume I. Direct Application of the Nonplanar Doublet-Lattice Method

Abstract: : Two methods of accounting for body-lifting surface interference in unsteady flow are considered. The first method is a direct application of nonplanar lifting surface elements to both the lifting surfaces and the body surfaces. This type of idealization must be used with an axial doublet introduced to account for body incidence effects. The undesirable effects of the annular wing representation are then reduced. The second approach uses an image system and an axial singularity system to account for the effects of the bodies. The methods described are intended to be used by airplane designers to calculate with improved accuracy, the unsteady aerodynamic pressures that act on a lifting surface being propelled at subsonic speeds. The new feature of these calculations is that the effects on the pressure field induced by interference between the fuselage, for example, and the wing or the wing, pyon and nacelle, are taken into account. These calculations are an essential ingredient of flutter analyses and will improve the confidence level of such calculations in preventing wing-store flutter and flutter of advanced vehicles where fuselages are relatively large, provide some lifting capability and cause noticeable interference effects.

Analysis of nonlinear panel flutter and response under random excitation or nonlinear aerodynamic loading

Abstract: Nonlinear panel flutter analysis and response under random excitation or nonlinear aerodynamic loading, using Rayleigh-Ritz approximation to Hamilton variational principle

On the Flutter of a Smooth Circular Cylinder in a Wake

Abstract: The stability of a smooth circular cylinder, free to translate horizontally and vertically against linear springs in the wake from an identical neighbouring cylinder, is studied using quasi-static aerodynamic derivatives and simple flutter theory. It is found that at spacing values between ten and twenty cylinder diameters (typical of the spacings employed on “bundled” overhead transmission lines) classical flutter of the leeward cylinder can occur in a certain critical range of wind speeds at certain orientations of this cylinder in the wake. However, the occurrence of flutter appears to be conditional on a positive difference of natural frequency between vertical and horizontal motions of the leeward cylinder in still air. Classical static instability (divergence) of the leeward cylinder is also shown to be possible over the entire “incidence” range in the wake, but this occurs in a much higher wind speed range than that associated with flutter.

Suppression of flutter

Abstract: An active aerodynamic control system to control flutter over a large range of oscillatory frequencies unaffected by mass, stiffness, elastic axis, or center-of-gravity location of the system, mode of vibration or subsonic Mach number consisting of one or more pairs of leading edge and trailing edge hinged or deformable control surfaces, each pair operated in concert by a stability augmentation system. Torsion and bending motions or deflections of the fluttering member are sensed and converted by the stability augmentation system into leading and trailing edge control surface deflections which produce lift forces and pitching moments to suppress flutter.

Wake Induced Flutter of Circular Cylinders: Mechanical Aspects

Abstract: In a recent paper, a theory explaining the wake-induced flutter of smooth circular cylinders was developed and vindicated. It was shown that, for moderate spacing of the cylinders (where the aerodynamic coupling between the constituent motions is small), flutter could occur only when the natural frequency, in still air, of vertical oscillations exceeded that of the horizontal motions. The theory, however, took no account of the possibility of mechanical coupling between the constituent motions and, while the results from this theory were corroborated by wind-tunnel tests in which a statically uncoupled mechanical support system was used, no indications were given of the changes in aeroelastic behaviour which might accompany the introduction of simple coupling terms. In this paper, the class of cases wherein the mechanical support system for the leeward cylinder exhibits static coupling is studied using “undamped flutter theory”. It is demonstrated that the appearance of static coupling terms can lead to quite dramatic changes in the flutter characteristics, and that considerable care must be exercised in the design and operation of wind-tunnel dynamic models if meaningful results are to be obtained. An Appendix deals with the general problem of mechanical coupling, using the normal coordinates approach, and aspects of the problem which bear on the subconductor oscillation phenomenon experienced on “bundled” overhead power transmission lines are highlighted.

Failure detection of a space shuttle wing flutter model by random decrement

Abstract: On-line monitoring of characteristic structural response of space shuttle wing flutter model for failure detection and repair

Flutter analysis of rotating cylindrical shells immersed in a circular helical flowfield of air

Abstract: The aeroelastic stability of a long, thin cylindrical shell with the outer surface exposed to an inviscid, helical flow of ah* is investigated. The cylinder behavior is described by classical shell equations, whereas the aerodynamic forces are described by the linearized potential theory. The approach that is used herein examines the nature of stability of the system when the system is "slightly" perturbed from its initial equilibrium state. In this paper, numerical results are presented only for the special case of swirl flow around a nonrotating shell, i.e., the axial flow velocity is set to zero. These results indicate that traveling wave type of flutter can be caused by coalescence of backward and forward traveling waves. Two approximate theories are presented and the results are compared.

Nonconservative Stability by Finite Element

Abstract: Nonconservative stability of continuous systems has received considerable theoretical attention in recent years. This class of stability problems is examined herein by application of the finite element—Ritz method to the extended Hamilton's principle. The technique is illustrated by the detailed analysis of two examples. The first is the classical problem concerning the stability of a cantilever under follower force excitation. The principal problem is to determine the follower force at which the column will oscillate in an unstable manner (flutter). The second problem is a cantilevered tube containing an inviscid fluid in slug flow. In this example, primary interest is in the fluid velocity at which dynamic instability occurs. Results of both problems, which are presented in graphical or tabular form, or both, clearly demonstrate the power of the methods.

Flutter of Clamped Skew Panels in Supersonic Flow

Abstract: The supersonic panel flutter problem of clamped skew panels with in-plane forces is formulated on the basis of the classical, small deflection, thin plate theory using oblique coordinates. The two-dimensional, static approximation is made use of for the aerodynamic loading. Galerkin method with the flutter mode represented in terms of a double series of beam characteristic functions is employed. Results of numerical calculations made for unstressed panels for different combinations of side ratio, angle of skew, and angle of yaw are presented here. The majority of the calculations were made using 16 terms in the series. Convergence is examined in a few typical cases. The dynamic pressure parameter for flutter is found to increase monotonically with the angle of skew for side ratio 1 and to decrease initially before beginning to increase for side ratio 0.5. The results are also compared with those obtained earlier for simply supported panels.

Flutter and Divergence Characteristics of Four Low Mass Ratio Hydrofoils

Abstract: : Four low mass ratio hydrofoil models of aspect ratio 5 were flutter tested. The flutter speed of the mass ratio 0.963 model was 24.7 knots. The other three models, of mass ratios 0.455, 0.395, and 0.202, were subject to static failure in bending at about 36 knots and did not flutter below this speed. The results were compared with the predictions of three flutter theories. All theories gave conservative flutter speed predictions at mass ratio 0.963. Two of the theories were also conservative at mass ratio 0.455.

A Two-Dimensional Theory for Rotor Blade Flutter in Forward Flight

Abstract: Presented is a theoretical method for determining rotor blade flutter in forward flight. The theory accounts for the unsteady aerodynamic contribution of the wake below the rotor. This is made possible due to certain simplifying assumptions of the authors regarding the rotor's wake at the onset of flutter. In particular, it is assumed at the onset of flutter that oscillations begin to build up. prior to the blade reaching a critical azimuth position, then decay as the blade moves beyond this point. Based upon this a wake model is postulated and the theory developed. The resulting lift deficiency function is compared with that of Loewy and Theodorsen. It is shown in limiting cases that the work presented is consistent with earlier flutter theory. The theory is applied to bending-torsion flutter for the tip segment of a rotor blade. Here, beyond a certain value of advance ratio, the influence of advance ratio on flutter speed is found to be essentially constant.

Wind-tunnel systems and techniques for aircraft/stores compatibilitystudies.

Abstract: A comprehensive presentation of advanced wind-tunnel techniques and facilities used in aircraft store carriage and delivery studies is presented. Extensive static stability, control, and metric store tests aid in predictions of aircraft performance and structural requirements. Investigations with scaled dynamic models are used to determine the flutter boundaries and aeroelastic effects caused by large store aerodynamic and inertia forces. Methods used to obtain mutual aerodynamic interference of wing-pylon-st ore combinations and external store aerodynamic interference on control surface effectiveness are described. State-of-theart scaled dynamic separation and captive trajectory systems, their current and potential capabilities and limitations, are discussed. The quality of wind-tunnel simulation, in the general sense, is discussed and present limitations and potential improvements are pointed out.

Vibration and flutter of unstiffened and orthogonally stiffened circular cylindrical shells

Abstract: The problem of vibration and flutter analysis of simply-supported unstiffened and orthogonally stiffened circular cylindrical shells which are typical of missile bodies has been developed and programmed for digital computer solution. An extensive review of the existing literature covering various aspects of the shell flutter problem is given with a critical appraisal of the assumptions made, results obtained, etc. A comprehensive chronological bibliography is also included. The analysis and the programme which have been developed are capable of handling shells of arbitrary geometrical, modal and flow parameters. In the case of stiffened shells, the stiffener effects may be treated as 'averaged' ('smeared') or 'discrete' and in each case the influence of eccentricity, in-plane and rotary inertias may be studied. The aerodynamic generalised forces may be calculated using the linear piston theory, the linear piston theory with a correction for curvature, and the exact potential flow solution. By combining the invacuo-natural vibration analysis and the aerodynamic generalised forces the cylindrical shell flutter problem may be solved and the flutter boundaries may be obtained in each of the above cases. The procedures have been illustrated with typical examples in each of the above cases and the results discussed. A few shells have been tested using an experimental vibration rig designed and built for the purpose and compared with the theoretically predicted invacuo-natural frequencies and mode shapes.

A Comparison of Methods for the Analysis of Wing-Tail Interaction Flutter

Abstract: Results of an analytical investigation of the aeroelastic stability of variable sweep aircraft, specifically with respect to wing-tail interaction flutter, are presented. Three aerodynamic representations are employed: 1) modified two-dimensional strip theory containing no wingtail aerodynamic interaction, 2) vortex lattice theory containing aerodynamic interaction of wing on tail only, and 3) subsonic kernel function theory containing a complete evaluation of wing-tail aerodynamic interaction. The capability of the interaction methods is established by an application to an experimental flutter model where wing-tail aerodynamic interaction is known to be of importance. The aerodynamic methods are then applied to a particular high performance variable sweep aircraft. The basic flutter mechanisms for this hypothetical aircraft are generated by wing-fuselage mechanical interaction and are predicted by wing aerodynamics alone. Component aerodynamics on wing and tail without interaction do not predict the mechanism. Aerodynamic interaction causes the reappearance of flutter at a velocity 30% lower than generated by the wing alone.

Thermoelastic damping and its effect on flutter of stressed panels situated in a supersonic airflow

Abstract: The effects of material damping on flutter of stressed rectangular panels are studied within the context of linear thermoelasticity theory. The closed-form expression for the thermoelastic (material) damping coefficient is obtained as a function of frequency, panel temperature and dimensions, and material properties. The solution of the stability boundary value problem is obtained by use of a generalized Galerkin method in the cross stream direction which reduces the governing partial differential equations to a system of ordinary differential equations in the streamwise direction. These equations are then solved exactly. Numerical results are given for the thermoelastic damping coefficients and for the flutter speeds of partially and fully clamped panels subjected to midplane stress.

Unsteady supersonic aerodynamic forces on an oscillating circular cylindrical shell.

Abstract: Unsteady supersonic aerodynamic forces on an oscillating circular cylindrical shell are derived from the linearized equation of a potential flow on the basis of the assumption of circumferential wave number n ^> 1. The generalized aerodynamic forces calculated by the present aerodynamic theory are compared with those of the "exact" theory, the slenderbody theory, and the two-dimensional quasi-steady theory. The numerical results of the present theory are in good agreement with those of the "exact" theory except for small n. It is concluded from the qualitative argument that the slender-body theory may lead to an erroneous conclusion in flutter analysis, even though quantitative agreement between the generalized aerodynamic forces of the present and the slender-body theories is obtained. As for the two-dimensional quasi-steady theory, some doubts are clearly cast upon its applicability to the flutter analysis of a cylindrical shell except when the length-to-radius ratio and the reduced frequency are small enough compared with 1.

Aeroelastic Stability of Plates and Shells: An Innocent’s Guide to the Literature

Abstract: In the present paper we attempt to provide a guide to the basic literature on the aeroelastic stability of plates and shells or “panel flutter” for the non-expert. In doing so it is hoped that the accomplishments obtained in the field will become available to a wider spectrum of readers and that this will have the twin effects of (i) attracting new workers to the remaining unsolved problems and (ii) indicating how techniques developed for panel flutter may be more widely applied to other physical problems involving the stability or instability of nonlinear, non-conservative systems.