Toward a lightweight high-speed fin: structural and flutter analysis for thickness reduction

An aeroelastic analysis method for fighter aircraft operating at extreme flight conditions has been developed and tested. The method involves the use of state-of-the-art zonal grid generation methods, three-dimensional Reynoldsaveraged Navier-Stokes analysis, and linear structures to analyze the flow over complex, flexible aircraft. The main objective of this effort is to develop a method capable of analyzing aircraft operating at flight conditions where vortices, strong shock waves, separated flow, and even highly unsteady flow may be present. The present application focuses on the static aeroelastic analysis of fighter aircraft operating at high angle of attack and high transonic Mach number. The developed method has been compared against static aeroelastic wind-tunnel data on an aeroelastically tailored wing/fuselage configuration, and the results are very encouraging.

An aeroelastic analysis method for fighter aircraft operating at extreme flight conditions has been developed and tested. The method involves the use of state-of-the-art zonal grid generation methods, three-dimensional Reynoldsaveraged Navier-Stokes analysis, and linear structures to analyze the flow over complex, flexible aircraft. The main objective of this effort is to develop a method capable of analyzing aircraft operating at flight conditions where vortices, strong shock waves, separated flow, and even highly unsteady flow may be present. The present application focuses on the static aeroelastic analysis of fighter aircraft operating at high angle of attack and high transonic Mach number. The developed method has been compared against static aeroelastic wind-tunnel data on an aeroelastically tailored wing/fuselage configuration, and the results are very encouraging.

Modal analysis of structures with uncertain-but-bounded parameters via interval analysis

In the present paper the continuous model method is applied to the prototype of a wind turbine tower in order to perform its modal structural analysis. This mathematical analysis is used as an alternative approach to the modal analysis method that uses discrete models. It is well known that in discrete models with high-level discretization and a large number of finite elements, several open questions on the accuracy, the convergence and the stability of the solution arise during the modal or response history analysis. In this sense, the results of the analysis by means of discrete modeling are in several cases doubtful and, therefore, a modal analysis by applying a continuous model as an effective alternative is recommended. To this end, the present paper proposes a continuous model approach to calculate the eigen-frequencies, periods and mode shapes to a wind turbine tower prototype. Starting from the equilibrium of forces on a differential element of the structure, the equation of motion of the tower is formulated and using in turn the known boundary conditions at the two ends of the wind tower, the tower eigen-problem is numerically treated and solved. The action of the higher mode-shapes is very important and may become critical in the case that the tower is subjected to strong dynamic loading (cf. e.g. wind) and simultaneously is excited by a strong seismic motion.

Visual vibrometry is a computer vision methodology that allows a user to perform modal analysis on an object using typical video cameras, for example a standard smartphone. There is significant potential in using visual vibrometry for structural monitoring and building modal analysis; it significantly reduces cost and set-up time as compared to traditional contact and non-contact based modal analysis techniques, and allows for the measurement of non-conventional structures which might be difficult to otherwise instrument. This paper describes an initial benchmarking of the effectiveness of the visual vibrometry technique in structural modal analysis, with testing conducted on a simply supported beam and a complex semi-rigid frame structure using both visual vibrometry and classical contact-based methods. Results indicated that visual vibrometry was effectively able to detect natural frequencies under 20 Hz for both structures, with reasonable correlation seen between visual and contact-based measurements, as well as analytical and numerical solutions. The poor detection of higher frequency modes was attributed to the receptance frequency response, generated when the collected visual displacement data is transformed to the frequency domain. Doing so highlights low frequency responses but suppresses intermediate and high frequency responses. Use of high-speed cameras may improve high-frequency detection, but further work is needed to determine practical limitations.KeywordsVisual vibrometryStructural dynamicsStructural health monitoringModal analysis

Static and modal analyses of structures with different repeated patterns

Flight control design using non-linear inverse dynamics

Aircraft in extreme flight conditions such as stalls and spins experience nonlinear forces and moments generated from high angles of attack and high angular rates. Flight control systems based upon nonlinear inverse dynamics offer the potential for providing improved levels of safety and performance in these flight conditions over the competing designs developed using linearizing assumptions. Inverse dynamics are generated for specific command variable sets of a 12-state nonlinear aircraft model to develop a control system that provides satisfactory response over the entire flight envelope. Detailed descriptions of the inertial dynamic and aerodynamic models are given, and it is shown how the command variable sets are altered as functions of the system state to add stall prevention features to the system. Simulation results are presented for various mission objectives over a range of flight conditions to confirm the effectiveness of the design.

This article deals with aeroelastic certification analyses of the new-generation Czech twin turboprop utility aircraft. The article is focused on the first phase of analyses before the ground vibration test (GVT) of the prototype. It describes the analytical model as well as the used tools and methods. The description of specific blocks of analyses follows afterwards. It includes the preliminary flutter analyses, static aeroelastic analyses (control reversal and divergence), modal analyses, and large parametric flutter analyses (surface, control surfaces, and tab flutter). The flutter characteristics of the structure with respect to changes of the selected parameters are presented. Furthermore, the preparation of the GVT (i.e. the optimization of the exciter and accelerometer positions based on the analytical results, and the whirl flutter analyses by means of an optimization-based approach to find the critical stability boundaries are explained). Finally, the performed activities are summarized and the next phase of aeroelastic certification (GVT, final analyses, and flight flutter tests) and the aircraft development are outlined.

This paper presents a framework to develop data-driven parametric reduced order models (PROMs) for aeroelastic (AE) analysis of flexible vehicles within a broad flight envelop. It is based on the separate domain and mode shape perturbation method. The flight envelop is first partitioned by multiple grid points, on each of which an aerodynamic ROM (AeroROM) is constructed using system identification (SYSID) techniques to capture dependence of the generalized aerodynamic force on the generalized displacement using data generated by high-fidelity CFD simulation. Then AeroROMs not on the grid point are obtained by interpolating those at neighboring grid points. Two interpolation schemes, i.e., the output-based interpolation and coefficient interpolation are developed. The parametric AeroROM is then coupled with the mode-based structural ROMs to enable integrated AE analysis under various flight conditions. The state consistence enabled by different SYSID techniques and performance of both ROM interpolation methods are also investigated. For the first time, it is found that the autoregressive exogenous ROM allows state consistence and direct model coefficient interpolation. The ROMs exhibit excellent agreement with CFD simulations (< 3% relative error) and orders of magnitudes speedup. The effort opens up new opportunities for parametric AE analysis and flight control design.

Performance of an airplane defines its usefulness and is the primary reason for designing and manufacturing aircrafts. In this article we present simple methods for approximately evaluating the performance of an airplane in normal and accelerated flight conditions. Methods are presented for calculating maximum and minimum flight speed, maximum flight altitude, endurance range, etc. Some flight conditions involving acceleration like take-off, landing, pull-ups and turns are also considered.

The periodic azimuthal time dependence of a linearized simple floating wind turbine model with four degrees of freedom is dealt with by developing a particular procedure that enables a stability analysis calculation and reduces computational efforts to calculate time responses. We apply Hill’s method for the aeroelastic stability analysis by generating a constant state-space hyper-matrix that takes into account the periodic azimuthal dependence of the system. It is used to compute precisely the time domain response and to solve an overall eigenvalue problem where natural frequency and modal damping are determined through the eigenvalues. For a varying rotational speed with a fixed air density, eigenfrequencies from Hill’s method are displayed on a Campbell diagram that is based on a waterfall plot of the frequencies obtained by decay tests peaks using the time domain model. Through the modal analysis as well, an investigation of the impact of aerodynamic damping is done following the same procedures by rather fixing the rotational speed and varying air density. In the end, a forcing moment of the platform pitch motion is added to analyze the accuracy of fast time response calculations compared to the time domain model benchmark results.

Current aerodynamic interest has turned to the study of supermaneuverable fighters and weapon performance when launched in extreme flight conditions. The evaluation of design missile performance requires multiple runs of six degree-of-freedom (6-DQF) simulations, analyzing the missile behavior for a variety of launch and flight conditions. Before wind-tunnel tests, it is necessary to produce the aerodynamic loading of candidate missiles for 6-DOF analyses. Since semi-empirical formulas fail in regions of nonlinear aerodynamics, and solutions to the full Navier-Stokes equations are too costly and time consuming, an alternative method of discrete vortex analysis is re-examined. The present theory examines the three-dimensional nature of the shed vorticity and generalizes previous discrete vortex analyses. Consequently, the results demonstrate relative user independence in determining all slender-body loading at angles of attack from 0 to 70 deg. The rapid calculations of the discrete vortex method makes it a prime candidate for the determinations of high ahgle-of-attack aerodynamic databases.

The paper presents an application of genetic algorithms to the design of a longitudinal flight controller for a hypersonic accelerator vehicle which is to be used to launch small satellites. A feature of hypersonic air-breathing flight vehicles is the high level of engine integration with the airframe. As a result, maintenance of vehicle attitude is not simply an issue of stability, but also one of propulsive effectiveness, which itself varies with flight conditions and the vehicle attitude. There is therefore limited scope for departure from optimum operating conditions. This, together with the extreme flight conditions, performance uncertainty, and the inherent instability of the vehicle, contributes to a demanding control task. We examine the capacity of a genetic algorithm in designing a fuzzy logic controller for the task of closed loop flight control. With a fixed, preset control structure, the design task is to configure the control surface through selection of the rule consequents and input scaling. The genetic algorithm uses a collection of simulated flight response in its formulation of the objective function. This allows the generation of a controller design without linearization of the vehicle model and dynamics. Stability augmentation is shown through flight simulation at the low-speed end of the hypersonic trajectory and also at a higher flight speed.

Hypersonic reentry has a severe flight environment during spacecraft return. To analyze the influence of pneumatic loads on the flexible structure under the extreme load condition during the deceleration process of inflatable reentry and descent system, fluid–and thermal–solid couplings have been studied. In addition, the stress and structural deformation distributions and the temperature distribution of each functional layer of the flexible thermal protection system were obtained in this study. The influence of flight conditions and structural parameter changes on inflatable structure performance was studied, and the transient response mechanism of the pneumatic load to the inflatable structure was revealed. Ballistic analysis combined with engineering algorithm was used to predicted the flight envelope. The fluid-solid thermal coupling method in Workbench is utilized to realize the real-time transfer of aerodynamic and thermal loads to flexible structures. And the loose coupling method was applied to carried out the one-way transfer of aerodynamic load and thermal load to the surface of flexible structure. The results showed that the mechanical–thermal–structural coupling analysis method could predict the mechanical properties of flexible inflatable deceleration structures, which can provide references for the aerodynamic configuration and thermal protection design under extreme flight conditions.