Buffet Loads Research Articles

A Finite Element Approach to Modal Parameter Estimation of Vertical Tail Fin

The vertical stabilizers or fin of aircraft, missiles or bombs are typically found on the aft end of the fuselage or body, and are intended to reduce aerodynamic side slip and provide direction stability. Vertical tails designed for aircrafts suffer from reduced stability and control effectiveness at high dynamic pressures due to aeroelastic effects. Adequate tail performance requires a detailed study on dynamic performance before investigating aeroelastic effects. Experimental modal analysis is a very extensive, time consuming and expensive process while finite element analysis is quick, accurate, fast and economical. Hence, in the present work it is purposed to carry out a detailed finite element simulation of vertical tail fin for a UAV to evaluate modal parameters such as natural frequencies and its corresponding modal shapes. CAD modeling of the vertical tail fin was done in UG NX using spline and swept commands. The vertical fin is sandwiched with polyurethene foam of density 70kg/m 3 bonded with thin Aluminum sheet of 1mm thickness as face sheets. Similarly, biwoven e-glass fiber was considered as face sheets along with PU foam as core. The meshing of model was carried out using a 10 noded solid tetrahedral element. Boundary conditions were simulated and dynamic analysis was performed to evaluate natural frequencies and corresponding modal shapes. Keywords: Modal Analysis, Natural Frequencies, Mode Shapes, Vertical Tail, FEM I. Introduction For modern UAV's, the ability to fly and maneuver at high angles of attack and at high loading conditions gives tactical advantages. For UAV's the maneuverability at very high angles of attack is achieved through a combination of the wing root leading edge extensions (LEXs) and the placement of vertical tails. A pair of large vortices generated by highly swept wing root LEXs contribute to enhanced vortex lift by developing high suction areas over the wings. The UAVs are designed to utilize unique characteristics of these vortex structures which are prime contributors to the aerodynamics of an aircraft during high angle of attack flight. The vertical tails of the UAV's are placed in closer proximity to the LEX vortex flow field to take full advantage of the concentrated energy contained in the LEX vortices to provide the directional stability and control necessary for high angle of attack maneuverability. Large buffet loads and associated fatigue damage have been observed on the tails of such aircrafts during certain flight conditions, including rapid maneuvering at subsonic Mach numbers and dynamic pressures considerably less than the maximum allowable values (1, 2).The interactions of a flexible structure with the aerodynamic forces acting on it are severe enough to influence the structural and aerodynamic design. The dynamic and aeroelastic analysis of an aircraft with main reference to its lifting and control surfaces is an essential aspect for finalization of the design cycle and is also required for obtaining flight clearance and certification of the aircraft. Correlation of the dynamic characteristics of the aircraft obtained from analysis with the Ground Vibration Test results of the aircraft was studied (3). The airframes of high performance aircraft, have suffered from an aeroelastic tail buffet problem for many years. This problem is inherent to vertical flows used to generate lift at high angles of attack as they tend to break down causing severe empennage dynamic loading and premature fatigue failures. Better understanding of the empennage buffeting problem is required for development of reliable fatigue usage monitoring systems and for the fleet management of aircraft. The challenges associated with computational simulation of empennage buffet vary from prediction of the nonlinear separated vortical flows about complex configurations to the coupled interaction between the flow and the dynamic response of the tail structure (4).Vibration analysis of an aircraft component can characterize the structural dynamics, determine the fundamental frequencies and define the complete modal data of the component. From this data, engineers can objectively evaluate their concerns about the impact of vibrations on adjacent aircraft components. These concerns have very real consequences since excessive vibrations can lead to premature component fatigue and failure (5, 6). Vertical tail fin is a critical component that must meet very high quality standards. Vertical tails designed for aircrafts suffer from reduced stability and control effectiveness at high dynamic pressures due to aeroelastic effects. Therefore, adequate tail performance requires a detailed study of dynamic performance before investigating aeroelastic effects (7, 8). Also, modal analysis is not only useful in its own right, but it also provides the basis for a number of further dynamics analyses. To this end, modal test plays an important role in the certification process of any new or

Active vibration control based on piezoelectric smart composite

An aircraft’s vertical fin may experience dramatic buffet loads in high angle of attack flight conditions, and these buffet loads would cause huge vibration and dynamic stress on the vertical fin structure. To reduce the dynamic vibration of the vertical fin structure, macro fiber composite (MFC) actuators were used in this paper. The drive moment equations and sensing voltage equations of the MFC actuators were developed. Finite element analysis models based on three kinds of models of simplified vertical fin structures with surface-bonded MFC actuators were established in ABAQUS. The equivalent damping ratio of the structure was employed in finite element analysis, in order to measure the effectiveness of vibration control. Further, an open-loop test for the active vibration control system of the vertical fin with MFC actuators was designed and developed. The experimental results validated the effectiveness of the MFC actuators as well as the developed methodology.

An Approach to Buffet Clearance of a Combat Aircraft using Unsteady Pressure Measurements from a Wind Tunnel Test

The extension of the aircraft's performance envelope into the high angle of attack regime carries with it the penalty of undesirable flow instabilities. At these conditions the aircraft flow-field is complex, with extensive regions of highly separated unsteady flow which is more pronounced in the case of the vertical tail or fin. The excitation given to a structure by separated flows is termed as ‘Buffet’ and the response of the structure to this excitation is known as ‘buffeting’. In the present study an attempt is made to quantify the Buffet load spectrum from experimental data and determine the response of the vertical tail of a typical combat aircraft under this load and thereby establishing a broad procedure for buffet clearance.

Analysis of Ares Crew Launch Vehicle Transonic Alternating Flow Phenomenon

A transonic wind tunnel test of the Ares I-X Rigid Buffet Model (RBM) identified a Mach number regime where unusually large buffet loads are present. A subsequent investigation identified the cause of these loads to be an alternating flow phenomenon at the Crew Module-Service Module junction. The conical design of the Ares I-X Crew Module and the cylindrical design of the Service Module exposes the vehicle to unsteady pressure loads due to the sudden transition between a subsonic separated and a supersonic attached flow about the cone-cylinder junction as the local flow randomly fluctuates back and forth between the two flow states. These fluctuations produce a square-wave like pattern in the pressure time histories resulting in large amplitude, impulsive buffet loads. Subsequent testing of the Ares I RBM found much lower buffet loads since the evolved Ares I design includes an ogive fairing that covers the Crew Module-Service Module junction, thereby making the vehicle less susceptible to the onset of alternating flow. An analysis of the alternating flow separation and attachment phenomenon indicates that the phenomenon is most severe at low angles of attack and exacerbated by the presence of vehicle protuberances. A launch vehicle may experience either a single or, at most, a few impulsive loads since it is constantly accelerating during ascent rather than dwelling at constant flow conditions in a wind tunnel. A comparison of a windtunnel- test-data-derived impulsive load to flight-test-data-derived load indicates a significant over-prediction in the magnitude and duration of the buffet load. I. Introduction One

Comparison of Ares I-X Wind-Tunnel-Derived Buffet Environment with Flight Data

The Ares I-X Flight Test Vehicle (FTV), launched in October 2009, carried with it over 243 buffet verification pressure sensors and was one of the most heavily instrumented launch vehicle flight tests. This flight test represented a unique opportunity for NASA and its partners to compare the wind-tunnel derived buffet environment with that measured during the flight of Ares I-X. It is necessary to define the launch vehicle buffet loads to ensure that structural components and vehicle subsystems possess adequate strength, stress, and fatigue margins when the vehicle structural dynamic response to buffet forcing functions are considered. Ares I-X buffet forcing functions were obtained via wind-tunnel testing of a rigid buffet model (RBM) instrumented with hundreds of unsteady pressure transducers designed to measure the buffet environment across the desired frequency range. This paper discusses the comparison of RBM and FTV buffet environments, including fluctuating pressure coefficient and normalized sectional buffet forcing function root-mean-square magnitudes, frequency content of power-spectral density functions, and force magnitudes of an alternating flow phenomena. Comparison of wind-tunnel model and flight test vehicle buffet environments show very good agreement with root-mean-square magnitudes of buffet forcing functions at the majority of vehicle stations. Spectra proved a challenge to compare because of different wind-tunnel and flight test conditions and data acquisition rates. However, meaningful and promising comparisons of buffet spectra are presented. Lastly, the buffet loads resulting from the transition of subsonic separated flow to supersonic attached flow were significantly over-predicted by wind-tunnel results.

공력 Buffet 하중을 고려한 헬리콥터 탐색등의 구조진동해석

In this study, computational structural vibration analysis of helicopter search light exposing unsteady buffet load have been conducted using combined advanced numerical methods. Unsteady CFD method based on Navier-Stokes equations is used to predict viscous buffet load due to flow separation effects. Full three-dimensional finite element model is constructed in order to conduct static and structural dynamic analyses of the search light model for two different typical flight speeds. Also, the correct performance of the search light can be physically estimated to examine the actual lighting area considering the effects of structural deformations.

Active Control of Vertical Tail Buffeting by Piezoelectric Actuators

A simplified method for analyzing and designing a vertical tail buffeting alleviation system is developed. The vertical tail model in this study is equipped with surface-bonded piezoelectric actuators to suppress the buffeting responses induced by aerodynamic forces. The structural dynamics of the vertical tail and the electrodynamics of piezoelectric actuators are modeled using a finite element method and realized by commercial software packages ANSYS®. Afterward, the finite element model for both the host structure and the piezoelectric patches are imported into MATLAB®, in which the dynamic equations of the system in modal coordinates are obtained through modal truncation. The motion-induced aerodynamic forces are computed by using the doublet-lattice method. The buffet input excitations are simulated by the proper orthogonal decomposition-based method. The suboptimal controller is designed to compute suitable control voltages needed to drive piezoelectric actuators. Finally, the effectiveness of the piezoelectric actuators in reducing vibratory responses due to buffet loads on the vertical tail is investigated numerically. The results demonstrate that the proposed method is feasible and effective.

Effects of partially earth-anchored cable system on dynamic wind response of cable-stayed bridges

In this study, a partially earth-anchored cable system is studied in order to reduce the dynamic wind response of cable-stayed bridges. The employment of earth-anchored cables changes the dynamic characteristics of cable-stayed bridges under wind loads. In order to estimate the changes in the member forces, the spectral analysis for wind buffeting loads are performed and the peak responses are evaluated using 3-D finite element models of the three-span cable-stayed bridges with the partially earth-anchored cable system and with the self-anchored cable system, respectively. Comparing the results for the two different models, it is found that the earth-anchored cables affect longitudinal and vertical modes of the bridge. The changes of the natural frequencies for the longitudinal modes remarkably decrease the peak bending moment in the pylon and the movements at the expansion joints. The small changes of the natural frequencies for the vertical modes slightly increase bending moments and deflections in the girder. The original effects of the partially earth-anchored cable system are also shown under wind loads; the decrement of girder axial forces and bearing uplifting forces, and the increment of cable forces in the earth-anchored cables.

Damage tolerance demonstration testing for the Australian F/A-18

Over recent years Australia has been involved in a number of full-scale fatigue testing programs in support of the through-life structural integrity of the Royal Australian Air Force’s (RAAF) F/A-18 fleet. It was recognised early in the acquisition cycle that the certification testing conducted by the manufacturer failed to considered damage tolerance requirements and would be unlikely to cover the typically more severe and diverse RAAF operations. Given similar aircraft structural integrity management philosophies, major benefits were to be realised through collaboration with the Canadian Forces (CF). In particular, as fatigue testing under representative CF/RAAF loading was the basis for both countries’ structural integrity management, the International Follow-On Structural Test Project (IFOSTP) was successfully concluded. This paper emphasises the Australian components of IFOSTP, including the damage tolerance testing and demonstration for the aft fuselage that incorporated the simultaneous application of both manoeuvre and dynamic buffet loads. Many of the innovations and consequences of this work program are highlighted, and may be applicable to future fighter aircraft structural integrity programs.

Effect of small cycles and load spectrum truncation on the fatigue life scatter in 7050 Al alloy

This work examines the effect of dynamic buffet loads (small cycles) on the scatter in the fatigue life of aircraft aluminum. Current life cycle management of fighter airframes assumes, without engineering evidence, that buffet loads cause an increase in the scatter factor used in safe-life calculations. Hence, the role of small cycles in spectra representative of the CF-188 aileron inboard hinge was examined in this study. The base load spectrum with the dynamic content was filtered to remove specified amounts of dynamic damage as determined by the CI89 strain-based cumulative fatigue damage program. The effect of this filtering on the scatter in crack initiation life and total fatigue life of double edge notched fatigue coupons of 7075-T7451 aluminum alloy were examined. The results indicate that inclusion of the dynamic loading caused the distribution of the crack initiation life to become bimodal. Each mode could be described with a log-normal distribution, the standard deviation of which was lower than the standard deviation obtained for the filtered load spectra. There was no evidence that the standard deviation increased with small cycle content, once the true nature of the distribution was taken into account. In fact, the standard deviation associated with the truncated spectrum was considerably larger. The bimodal distribution is positted to be a result of included particles near the surface on the fatigue limit of individual specimens.

High Resolution Turbulence Treatment of F/A-18 Tail Buffet

Unsteady tail loads of the F/A-18 are computed using various turbulence models at an angle of attack consistent with buffet induced by leading-edge extension vortex breakdown. Comparison of these industry-standard Reynolds-averaged Navier-Stokes turbulence models with the detached-eddy simulation hybrid approach reveals the inadequacies of Reynolds-averaged Navier-Stokes methods and the ability of detached-eddy simulation to reproduce the observed unsteadiness at these conditions. Computed vortex breakdown position and frequencies of the detached-eddy simulation method are shown to be accurate by comparison to flight-test and experimental results. Finally, comparison of the detached-eddy simulation unsteady tail pressures with flight-test tail pressures reveal the ability of the method to accurately reproduce F/A-18 tail buffet loads and its readiness for inclusion in a fluid-structure interaction method.

The effect of dynamic loading on the fatigue scatter factor for Al 7050

This work examines the effect of dynamic buffet loads (small cycles) on the scatter in the fatigue life of aircraft aluminum. Current life cycle management of fighter airframes assumes, without engineering evidence, that buffet loads cause an increase in the scatter factor used in safe life calculations. Hence, the role of small cycles in spectra representative of the aft tail hanger position of the CF188 aircraft was examined in this study. The base load spectra with the dynamic content were filtered to remove specified amounts of dynamic damage as determined by the CI89 strain-based cumulative fatigue damage program. The effect of this filtering on the scatter in crack initiation life and total fatigue life of double edge notched fatigue coupons of 7075-T7451 aluminum alloy were examined. The results indicate that inclusion of the dynamic loading caused the distribution of the crack initiation life to become bimodal. Each mode could be described with a log-normal distribution, the standard deviation of which was lower than the standard deviation obtained for the filtered load spectra. There was no evidence that the standard deviation increased with small cycle content, once the true nature of the distribution was taken into account. For all of the spectra examined, the scatter in the crack initiation life was found to increase for specimens taken further from the centre of the block from which they were machined. There was also a slight increase in the mean fatigue life for specimens taken from outer sections of the block. In addition, the crack growth rate was related by a power law to the crack initiation life for all spectra independent of the small cycle content.

Unsteadiness of an axisymmetric separating-reattaching flow: Numerical investigation

The separated flow over a cylinder elongated by another cylinder of a smaller diameter is investigated numerically at the high subsonic regime using zonal detached eddy simulation (ZDES) and compared with the experimental data of Deprés, Reijasse, and Dussauge [AIAA J. 42, 2541 (2004)]. First, it is shown that this axisymmetric step flow has much in common with the two-dimensional facing step flows as regards the shear layer instability process. Second, the statistical and spectral properties of the pressure fluctuations are scrutinized. Close to the step, the surface pressure signature is characterized by low frequencies f.Lr∕U∞=O(0.08) (where Lr and U∞ denote, respectively, the mean reattachment length and free-stream velocity) and an upstream velocity of 0.26U∞ while in the second half-part of the recirculation higher frequencies fluctuations at f.Lr∕U∞≈0.6 and a downstream convection velocity 0.6U∞ are the dominant features. The current calculation shows that the separated bubble dynamics depends on very complex interactions of large eddies formed in the upstream free shear layer with the wall in the reattachment region. These structures are shed with a nondimensional frequency of about 0.2. Besides, it has been observed that the secondary corner vortex experiences a cycle of growth and decay. The correspondence between the frequencies of this secondary corner vortex dynamics and the flapping motion (f.Lr∕U∞≈0.08) suggests that there should be different aspects of the same motion. These results show that there is an ordered structure in this axisymmetric separating/reattaching flow which is dominated by large scale coherent motion. This is confirmed by a two-point correlation analysis of the pressure signals showing that the flow is dominated by highly coherent antisymmetric modes at the flapping and vortex shedding frequencies whose signatures are evidenced in the spectrum of the computed buffet loads. Possible onsets of a large-scale self-sustained motion of the separated area are finally discussed and the existence of an absolute instability of the axisymmetric recirculation bubble originating from a region located near the middle of the recirculating zone is conjectured.

Alleviation of buffet-induced vibration using piezoelectric actuators

The effectiveness of piezoelectric actuators in reducing vibratory response due to buffet loads on the F-16 ventral fin was investigated numerically. The study was initiated by tuning a finite element model of the ventral fin to existing ground vibration test data. Strain energy profiles were determined for the first five mode shapes. The modes most critical for active control were determined through a flutter analysis, in conjunction with existing flight test data, as well as a review of the structural detail of previous fin failures. Piezoelectric actuator patches were located within the regions of highest strain energy and aligned with the principal strain vectors in those regions.

Experimental Evaluation of an Advanced Buffet Suppression System on Full Scale F/A-18 Fin

Buffeting is an aeroelastic phenomenon that plagues high-performance aircraft, especially with those with twin vertical tails such as the F/A-18. At high angles of attack, unsteady vortices that emanate from wing leading-edge extensions interact with the vertical fin, resulting in premature fatigue failure. An advanced hybrid buffet suppression system was experimentally evaluated on a full-scale F/A-18 empennage as an international technical collaboration activity among Australia, Canada, and the United States under the auspices of The Technical Cooperative Program. The advanced hybrid buffet suppression system incorporated two distinct actuation systems to control the bending and torsion modes independently. The vertical fin bending mode was counteracted using the inertial loading from the rudder structure while the first torsion model was controlled using surface-mounted macrofiber composite conformable actuators optimally located on the fin. The buffet excitation input generated by the shakers matched the buffet modes and magnitude expected under aerodynamic loading, but it was difficult to match the aerodynamic damping. Therefore, closed-loop tests were conducted under two damping conditions that were lower and higher than the expected aerodynamic damping level. The lower damping condition was produced by imposing free vibration condition on the active fin, and the higher damping condition was tested using forced vibration. The real-time adaptive controllers were able to effectively suppress both the bending and torsion vibration modes under representative buffet load spectra. The performance of the hybrid actuation system was significantly greater at the low damping condition, which led to higher reduction in vibration. Results showed that the conventional rudder and macrofiber composite actuator systems were capable of simultaneously reducing both the bending and torsion modes sufficiently to double the fatigue life of the fin. The full-scale closed-loop tests demonstrated that the advanced hybrid buffet suppression system is a feasible solution to alleviate vertical tail fatigue due to buffeting in fighter aircraft.

Low Speed Wind Tunnel Buffet Testing on the F/A-22

Buffet results from the F/A-22 V-9 model test conducted in the Lockheed Martin Aeronautics Marietta Low Speed Wind Tunnel (LSWT) between 515-02 and 5-18-02 are presented. The purpose of the test was to determine the effect on stability and control parameters of CFD derived potential aerodynamic solutions for attenuating vertical tail buffet. Instrumentation was added to permit assessment of the effects of these potential configurations on buffet loads. The data from the test show buffet attenuation benefit from several of the configurations tested, with two fence configurations showing significant reductions in the buffet response up to approximately 32 degrees angle-of attack. CFD analysis predicted significant effects of Mach number, engine inlet mass flow, and scaling on buffet onset. Therefore, any consideration of incorporating such configurations as determined by wind tunnel test should be verified by model testing with these effects present and by full-scale evaluation. Significant buffet attenuation is also shown with side weapons bay doors open. (Note: The configuration showing most promise for buffet attenuation resulted in a loss in lateral stability at 32 degrees)

The inverse determination of aerodynamic loading from structural response data using neural networks

The prediction and monitoring of aircraft structural fatigue damage is vital for the safe operation of ageing aircraft. The ability to determine aerodynamic loading inversely using structural response data has the potential to significantly improve fatigue monitoring capabilities. This study examines how a Neural Network can be used to estimate and predict aerodynamic loading from structural response data. To simulate aerodynamic loading conditions F/A-18 Empennage fatigue test data which includes the application of both high frequency buffet and low frequency manoeuvre loading will be used. The neural network was trained using response data from several strain gauges as input and known applied loads as output. It was then tested with new data and compared to the known applied loading corresponding to the new data. The network was also tested in its ability to predict the aerodynamic loading across locations different to the locations of the training data.

Active Control of a hybrid actuation system for aircraft vertical fin buffet load alleviation

AbstractTwin-tail fighter aircraft may experience intense buffet loads when flying at high angles of attack. One such aircraft is the F/A-18 where the broadband buffet loads primarily excite the first bending and torsional modes of the vertical fin, resulting in significant vibration and dynamic stresses on the vertical tail structure. This buffet phenomenon reduces the fatigue life of the aircraft structure while decreasing mission availability.An international technical co-operation program was initiated to develop a novel hybrid actuation system to actively alleviate the buffet response of a full-scale F/A-18 vertical fin. A hydraulic rudder actuator was used to control the bending mode of the vertical fin using rudder inertia forces. Multiple macro fiber composite actuators were distributed optimally to provide maximum induced strain control authority for the torsional mode. In order to develop an effective control law, a system identification approach was conducted to obtain a state-space model of the vertical fin using open-loop test data. An LQG control law was selected to minimise the dynamic response of the vertical fin at critical locations. The effectiveness of the control law was verified through extensive simulation prior to closed-loop experiments. The LQG control law demonstrated high robustness in all excitation load conditions; both bending and torsional vibration modes of the vertical tail were suppressed effectively and simultaneously. The dynamic stress and acceleration response at critical locations were also reduced significantly. A closed-loop experiment was conducted on a full-scale F/A-18 empennage using the IFOSTP test rig, and the experimental results verified the effectiveness of the control law development methodology used for the full-scale hybrid buffet load system for the F/A-18 aircraft. In addition, the ground vibration test demonstrated that the hybrid actuation system is a feasible solution to alleviate the vertical tail buffet loads in high performance fighter aircraft.

Control of Flow Structure on Delta Wing with Steady Trailing-Edge Blow

The near-surface flow structure and topology are characterized on a delta wing of low sweep angle, which is subjected to trailing-edge blowing. A technique of high-image-density particle image velocimetry is employed to determine the topological critical points adjacent to the surface and in the near wake of the wing, in relation to the dimensionless magnitude of the blowing coefficient. These topological features are, in turn, interpreted in conjunction with patterns of surface-normal vorticity and near-surface velocity fluctuations, which provide an indication of buffet loading. Even though the jet blowing is at the trailing edge, it has a remarkable, global influence on the surface patterns located well upstream; at high angle of attack, it leads to eradication of large-scale, three-dimensional separation in the vicinity of the apex.

Aerodynamic Active Control For Fin-Buffet Load Alleviation

The efficiency of an active auxiliary rudder system in alleviating fin buffeting of modern fighter aircraft is investigated. Low-speed wind-tunnel tests are performed on a detailed 1/15-scale model of canard-delta-wing type. A digitally controlled auxiliary rudder is installed on a specific fin model providing harmonic oscillations at varying frequency and deflection angle. The vertical tail is instrumented to measure unsteady surface pressures, fin-tip accelerations, and auxiliary rudder moments. At open-loop tests the fin unsteady pressure field is fed with energy at the frequencies of the auxiliary rudder motions. The corresponding rms values increase with increasing frequency and deflection angle at all angles of attack tested. The motion-induced rms pressures reach values well above the levels of the nonoscillating case. Thus, a potential to reduce rms buffet loads by approximately 18% for closed-loop operations exists. There is no decrease in the rudder moment with increasing angle of attack substantiating the effectiveness of the auxiliary rudder concept also for high angles of attack. The active control system uses single-input single-output control laws to alleviate buffeting in the fin first bending and torsion mode, respectively. With active control the spectral density peaks of fin-tip accelerations at the frequencies of the considered eigenmodes can be reduced by as much as 60% at angles of attack up to 31 deg.