Experimental Flutter Research Articles

Some considerations on the effects of the P-derivatives on bridge deck flutter

Abstract Using two degrees of freedom (dof) experimental flutter derivatives to perform three-dimensional flutter analysis for a cable-supported bridge is a widely practiced method. It is important to consider the P-derivatives effect to have more accurate analysis for a long-span bridge. Through a case example, this paper studied some of the issues relating to the P-derivatives effects on flutter. The operational condition in two-dof experiments was discussed. It was suggested that due to the strong aeroelastic coupling effect of the sectional model studied in this research, there was an inherent weakness of two-dof experiments. The effect of the P-derivatives was studied for an example bridge by comparing the flutter analysis results using two-dof and three-dof experimental flutter derivatives.

Impaired Longitudinal Conduction in Crista Terminalis is Necessary for Sustenance of Experimental Atrial Flutter

Sustained atrial flutter (AFL) can be induced by creating a lesion between the vena cava in dogs. In previous studies on this model, the crista terminalis (CT) was often injured, and thus, role of CT in sustained reentry was not well understood. We hypothesized that impaired longitudinal conduction in CT is necessary for sustained AFL. In 16 anesthetized, open-chest dogs, linear radiofrequency ablation of the intercaval region was performed without interrupting CT. Intra-atrial conduction times (IAT) along CT were measured using a plaque electrode (25x35 mm) containing 30 bipolar electrodes before and after additional ablation of CT (group A, n=10) or the pectinate muscle (PM) region (group B, n=6). In group A, IAT along CT was 27 +/- 5 ms at baseline and was increased to 43 +/- 3 ms after ablation of CT (P<0.001). In group B, IAT along CT was 28 +/- 4 ms at baseline and 27 +/- 3 ms after ablation of PM (P=NS). Sustained AFL lasting >20 minutes was induced in 10/10 dogs in group A only after additional ablation of CT, and in 0/6 dogs in group B (P<0.001). The cycle lengths of AFL after ablation of the intercaval region and additional ablation of CT were 119 +/- 14 and 140 +/- 14 ms, respectively (P<0.01). There was a significant positive correlation between the cycle length of AFL and IAT along CT (r2=0.63, P<0.001). These results indicate that longitudinal conduction property in CT and not in PM strongly affects sustenance of AFL in this model.

Nonlinear effects in transonic flutter with emphasis on manifestations of limit cycle oscillations

This paper presents flutter and forced oscillation experiments in a transonic wind tunnel. For an aeroelastic supercritical 2-D airfoil configuration we studied typical transonic phenomena in as pure a form as possible. Various manifestations of small-amplitude limit cycle oscillations were observed for different flow conditions as well as coexisting limit cycles. We demonstrated how very small control forces were sufficient to excite or suppress flutter oscillations. Limit cycle oscillations occurred under free and forced turbulent boundary layer transition in a perforated wall test-section. Flutter calculations based on experimental aerodynamic forces yield stability limits which show good agreement with directly measured experimental flutter values. The results indicate that flow separation at the trailing edge, and the interactions between the shock and the marginal region of separated flow beneath it, may be responsible for limiting the amplitude of the observed limit cycle oscillations.

Flutter Limits and Behaviors of Flexible Webs Having a Simplified Basic Configuration in High-Speed Flow

Fluttering conditions were analyzed for webs with a simplified basic configuration with both leading and trailing edges fixed in a uniform flow. The predicted flutter limits are expressed in terms of a ratio of fluid force to tension (σ*), a ratio of tension to bending stiffness (τ*), and a reduced frequency fR. Three characteristic zones of the behavior are seen to appear depending on the magnitude of τ*. For medium τ* of 1×103 to 1×106, flutter-limit values of σ* and fR remain nearly constant, respectively. For low τ*&lt;1×103 effect of bending stiffness becomes significant and buckling-like instabilities tend to occur preceding the flutter. For high τ*&gt;1×106 ripple-like modes tend to occur and σ* falls drastically and fR scatters much. Experimental flutter limits obtained in the wind tunnel were seen on the average to agree with the expected ones for the tested range of 9×102&lt;τ*&lt;4×104.

Flutter and limit cycle oscillations of two-dimensional panels in three-dimensional axial flow

Experimental flutter and limit cycle oscillations (LCO) of two-dimensional elastic plates in three-dimensional axial flow were observed. The plate is clamped at its leading edge and free at its trailing edge, i.e., it is a “flag” albeit one dominated by its bending stiffness. In the companion theoretical model the structural nonlinearity arises in both the bending stiffness and the mass inertia. Aerodynamic nonlinearities are neglected, however, and linear three-dimensional incompressible vortex lattice aerodynamic theory and a corresponding reduced order aerodynamic model were used to calculate the linear flutter boundary and also the LCO (that occur beyond the linear flutter boundary). The results from the theory and experiment are in good agreement for the onset of flutter, including the critical flow velocity at which the aeroelastic system becomes unstable, as well as the aeroelastic mode of oscillation and frequency. However, there are significant differences between the present theory and experiment for large-amplitude LCO. It is hypothesized that aerodynamic nonlinearities (not modelled in the present theory) are the primary cause of these differences.

평판 날개의 아음속 플러터 실험 및 해석

Experimental flutter test for a flat plate wing is performed and the flutter analysis methods are verified by comparing with the experimental results. Wing model and experimental equipment are established in the subsonic wind-tunnel. From the response of the wing, the flutter speed is estimated by using the system identification technique. MSC/NASTRAN, V-g method and root-locus method are used for the flutter analysis of the wing. The computed flutter speed is compared with the estimated one from the experiment, and they show good agreement. Wing model in the present study can be used as a benchmark model for the flutter analysis.

ACTIVE WING FLUTTER SUPPRESSION USING A TRAILING EDGE FLAP

The aeroservoelastic behaviour of a thin rectangular wing with a controllable trailing edge flap is investigated. A rather high aspect ratio motivates a numerical model based on linear beam theory for the structural dynamics and strip theory for the unsteady aerodynamic loads. Experimental flutter testing shows good agreement with the numerical stability analysis, and the impact of the trailing edge flap on the dynamics is verified by open-loop testing. The problem of stabilizing the wing utilizing the trailing edge flap is posed, and the design of a fixed-structure feedback controller is performed using numerical optimization. The problem of maximizing closed-loop modal damping with constraints on actuator performance is solved for a sequence of flow speeds and the obtained controller is synthesized using gain scheduling. The fairly large predicted increase in critical speed is experimentally verified with satisfactory accuracy.

Flutter Limits of Flexible Thin Webs with a Basic Configuration in High-Speed Flow. 1st Report. Analytical Prediction and Experimental Confirmation of the Flutter Limits.

Fluttering conditions are analyzed for webs with a simplified basic configuration with both leading and trailing edges fixed in a uniform flow by the method proposed in a previous study by one of the authors. The predicted flutter limits are expressed in terms of a ratio of fluid force to tension (σ*) and a ratio of tension to bending stiffness (τ*). Three regions for τ* are seen to form characteristic zones of behavior. For τ* greater than 1×(10)3 and less than 1×(10)6, σ* and reduced frequency are of nearly constant values, respectively. For τ* less than 1×(10)3, effect of bending stiffness appears and divergence-like instabilities having lower frequencies occur before the flutter. For τ* greater than 1×(10)6, σ* tends to become drastically lower and the reduced frequency becomes higher, with oscillation modes changing into higher order ones. Experimental flutter limits obtained in the wind tunnel, though scattered much, are seen on the average to conform to the expected ones in the limited range of τ* of 1×(10)3-4×(10)4.

Computational Investigation of Wing Flutter

Numerical aeroelastic simulation of a high-aspect-ratio transport type wing model in transonic region is presented. The aeroelastic responses of the wing are extracted by integrating compressible thin-layer Navier-Stokes equations coupled with the equations of motion of the wing structure, in a time dependent manner. The Yee-Harten implicit TVD scheme and the Wilson's θ method are employed to integrate these equations, respectively. Flutter boundaries were found for Mach number range, 0.7 to 0.85 and the results were compared with experimental flutter boundaries. Futhermore, Limit Cycle Oscillations were found and the characteristics of the flutter and limit cycle oscillations are investigated and discussed.

Effect of concentrated mass on stability of cantilevers under rocket thrust

The paper describes the effect of an intermediate concentrated mass on the dynamic stability of cantilevered columns subjected to a rocket thrust. It is assumed that the rocket thrust is produced by the installation of a solid rocket motor at the tip end of the cantilevered columns having the intermediate mass. The rocket motor is assumed to be a rigid body having finite sizes but not a mass point. The importance of the magnitude and size of the intermediate mass is demonstrated by theory and experiment. The experimental results are compared with theoretical predictions made by taking into account the mass and size of the rocket motor as well as intermediate mass effect. The internal damping was neglected in the theoretical predictions. It is shown that theoretical stability predictions and experimental flutter limits agreed well.

Flutter of Cantilevered Column under Rocket Thrust

This paper describes an experimental investigation on the flutter of viscoelastic cantilevers subjected to a tangential follower force. The force was produced by the direct installation of a real solid rocket motor to the tip end of the cantilevered columns. The columns lost their stability by flutter. The results were compared with theoretical flutter predictions made by accounting for an internal damping of the test columns, as well as the mass and size of the installed rocket motor. The introduction of the concept of instability in a finite time interval is of vital importance in predicting the experimental flutter force.

Application of transonic small disturbance theory to the active flexible wing model

The CAP-TSD code, developed at the NASA Langley Research Center, is applied to the active flexible wing windtunnel model for prediction of transonic aeroelastic behavior. A semispan computational model is used for evaluation of symmetric motions, and a full-span model is used for evaluation of antisymmetric motions. Static aeroelastic solutions using the computational aeroelasticity program-transonic small disturbance, are computed. Dynamic (flutter) analyses are then performed as perturbations about the static aeroelastic deformations and presented as flutter boundaries in terms of Mach number and dynamic pressure. Flutter boundaries that take into account modal refinements, vorticity and entropy corrections, antisymmetric motions, and sensitivity to the modeling of the wingtip ballast stores are also presented and compared with experimental flutter results.

Experimental investigation of counter-rotating propfan flutter at cruise conditions

The paper presents wind tunnel experimental flutter results, at transonic relative flows, for a 0.62-m diameter composite propfan model. A blade row that fluttered was tested alone, and with a stable aft counter-rotating blade row. The major objectives of the experiment were to study the effect of the second blade row on the row in flutter, and to investigate the flutter. Results show that the second row had a stabilizing effect. Two distinct flutter modes were found. For both flutter modes: flutter boundary, frequency, nodal diameter, and blade displacement data are given. The blade displacement data, obtained with an optical method, gives an indication of the flutter mode shape at a span near the blade tip.

Three-dimensional time-marching aeroelastic analyses using an unstructured-grid Euler method

Modifications to a three-dimensional, implicit, upwind, unstructured-grid Euler code for aeroelastic analysis of complete aircraft configurations are described. The modifications involve the addition of the structural equations of motion for their simultaneous time integration with the governing flow equations. The paper presents a detailed description of the time-marching aeroelastic procedures and presents comparisons with experimental data to provide an assessment of the capability. Flutter results are shown for an isolated 45-deg swept-back wing and a supersonic transport configuration with a fuselage, clipped delta wing, and two identical rearward-mounted engine nacelles. Comparisons are made between computed and experimental flutter characteristics to assess the accuracy of the aeroelastic results.

Aeroelastic effects of spoiler surfaces on a low-aspect-ratio rectangular wing

An experimental research study to determine the effectiveness of spoiler surfaces in suppressing flutter onset for a low-aspect-ratio, rectangular wing has been conducted in the Langley Transonic Dynamics Tunnel (TDT). The wing model used in this flutter test consisted of a rigid wing mounted to the wind-tunnel wall by a flexible, rectangular beam. The flexible beam was connected to the wing root and cantilever mounted to the wind-tunnel wall. The wing had a 1.5 aspect ratio based on wing semispan and a NACA 64AGIO airfoil shape. The spoiler surfaces consisted of thin, rectangular aluminum plates that were vertically mounted to the wing surface. The spoiler surface geometry and location on the wing surface were varied to determine the effects of these parameters on the classical flutter of the wing model. Subsonically, the experiment showed that spoiler surfaces increased the flutter dynamic pressure with each successive increase in spoiler height or width. This subsonic increase in flutter dynamic pressure was approximately 15% for the maximum height spoiler configuration and for the maximum width spoiler configuration. At transonic Mach numbers, the flutter dynamic pressure conditions were increased even more substantially than at subsonic Mach numbers for some of the smaller spoiler surfaces. But for larger spoiler sizes (in terms of either height or width) the spoilers forced a torsional instability in the transonic regime that was highly Mach number dependent. This detrimental torsional instability was found at dynamic pressures well below the expected flutter conditions. Variations in the span wise location of the spoiler surfaces on the wing showed little effect on flutter. Flutter analysis was conducted for the basic configuration (clean wing with all spoiler surface mass properties included). The analysis correlated well with the clean wing experimental flutter results.

Aeroelastic analysis of wings using the Euler equations with a deforming mesh

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.

Antiarrhythmic effects of MS-551, a new class III antiarrhythmic agent, on experimental canine atrial flutter

Antiarrhythmic effects of MS-551, a new class III antiarrhythmic agent, on experimental canine atrial flutter

Wing-flutter calculations with the CAP-TSD unsteady transonic small-disturbance program

The application and assessment is described of CAP-TSD (Computational Aeroelasticity Program - Transonic Small Disturbance) code for flutter prediction. The CAP-TSD program was developed for aeroelastic analysis of complete aircraft configurations and was previously applied to the calculation of steady and unsteady pressures. Flutter calculations are presented for two thin, swept-and-tapered wing planforms with well defined modal properties. The calculations are for Mach numbers from low subsonic to low supersonic values, including the transonic range, and are compared with subsonic linear theory and experimental flutter data. The CAP-TSD flutter results are generally in good agreement with the experimental values and are in good agreement with subsonic linear theory when wing thickness is neglected.

Modern wing flutter analysis by computational fluid dynamics methods

The application and assessment of the recently developed CAP-TSD transonic small-disturbance code for flutter prediction is described. The CAP-TSD code has been developed for aeroelastic analysis of complete aircraft configurations and was previously applied to the calculation of steady and unsteady pressures with favorable results. Generalized aerodynamic forces and flutter characteristics are calculated and compared with linear theory results and with experimental data for a 45 deg sweptback wing. These results are in good agreement with the experimental flutter data which is the first step toward validating CAP-TSD for general transonic aeroelastic applications. The paper presents these results and comparisons along with general remarks regarding modern wing flutter analysis by computational fluid dynamics methods.

Flutter prediction involving trailing-edge control surfaces

AFLUTTER incident occurred during flight testing of the T-46A that was not predicted by analyses. The flutter mode consisted largely of wing bending and aileron rotation. Further calculations showed that the predicted effectiveness of the trailing-edge control surfaces had to be decreased in order to accurately predict flutter. Contents An unexpected flutter incident occurred during flight flutter testing of the T-46A jet trainer. The static unbalance of the ailerons had been changed in order to avoid a flutter mode predicted by analyses, and the intention was to fly up to 200 knots and excite the plane via stick raps. At approximately 205 knots, the pilot displaced the stick to excite the ailerons; this action was enough to induce flutter. Onboard measuring equipment showed oscillations of approximately 8 Hz. The flutter mode apparently involved the first bending mode of the wing coupling with an aileron rotation mode. The pilot was able to slow the plane quickly, and flutter stopped at approximately 150 knots. For this effort, the experimental flutter speed was assumed to be 150 knots. The T-46A is interesting to the dynamicist and the flutter analyst. The ailerons are not directly connected to the stick and are not connected to each other. Instead, they are supported by soft springs and driven by a small tab at the trailing edge of each aileron. The tabs are, in turn, connected to the stick via a cable arrangement. The result is that ailerons are quite free to move independently, and also that the first vibrational mode of the aircraft is the symmetrical aileron rotation mode at about 1.8 Hz. The next two modes both involve large amounts of wing bending and occur at about 8.2 and 8.9 Hz, respectively. The proximity of an aileron rotation mode and wing bending modes is unusual in an age of aircraft whose control surfaces are driven by very stiff electrical or hydraulic actuators. Military requirements state that aircraft must be free of flutter up to 115% of the design limit speed. The limit speed of 400 knots for the T-46 meant that the plane had to be free of flutter up to 460 knots. Analyses performed prior to the flight test in question indicated that a 33 Hz mode was present below 460 knots for aileron mass balance values above about 58%; 58% mass balance in this case means that weight was added ahead of the hinge line to equal 58% of the static unbalance of the aileron. These analyses also predicted an 8