Showing papers on "Aeroelasticity published in 1980"

A Comprehensive Analytical Model of Rotorcraft Aerodynamics and Dynamics. Part 1. Analysis Development

Abstract: Structural, inertia, and aerodynamic models were combined to form a comprehensive model of rotor aerodynamics and dynamics that is applicable to a wide range of problems and a wide class of vehicles. A digital computer program is used to calculate rotor performance, loads, and noise; helicopter vibration and gust response; flight dynamics and handling qualities; and system aeroelastic stability. The analysis is intended for use in the design, testing, and evaluation of rotors and rotorcraft, and to be a basis for further development of rotary wing theories.

Divergence of Forward Swept Composite Wings

Abstract: Forward swept wing aircraft may have superior aerodynamic performance for certain missions. Algebraic expressions to predict the static aeroelastic divergence characteristics of forward swept wings constructed of composite materials have been developed using a laminated box beam model to describe the wing structure and aerodynamic strip theory to predict the loads due to wing bending and torsional deformation. The expressions presented show that, because of elastic coupling between wing bending and torsion, wing divergence may be precluded for reasonably large forward sweep angles if the composite structure is properly tailored. The structural parameters that maximize divergence speed are readily identified. Two illustrative examples are presented. ao b

Design for active and passive flutter suppression and gust alleviation

Abstract: Analytical design techniques for active and passive control of aeroelastic systems are based on a rational approximation of the unsteady aerodynamic loads in the entire Laplace domain, which yields matrix equations of motion with constant coefficients. Some existing schemes are reviewed, the matrix Pade approximant is modified, and a technique which yields a minimal number of augmented states for a desired accuracy is presented. The state-space aeroelastic model is used to design an active control system for simultaneous flutter suppression and gust alleviation. The design target is for a continuous controller which transfers some measurements taken on the vehicle to a control command applied to a control surface. Structural modifications are formulated in a way which enables the treatment of passive flutter suppression system with the same procedures by which active control systems are designed.

Application of the finite element method to rotary-wing aeroelasticity.

Abstract: A finite element method for the spatial discretization of the dynamic equations of equilibrium governing rotary-wing aeroelastic problems is presented. Formulation of the finite element equations is based on weighted Galerkin residuals. This Galerkin finite element method reduces algebraic manipulative labor significantly, when compared to the application of the global Galerkin method in similar problems. The coupled flap-lag aeroelastic stability boundaries of hingeless helicopter rotor blades in hover are calculated. The linearized dynamic equations are reduced to the standard eigenvalue problem from which the aeroelastic stability boundaries are obtained. The convergence properties of the Galerkin finite element method are studied numerically by refining the discretization process. Results indicate that four or five elements suffice to capture the dynamics of the blade with the same accuracy as the global Galerkin method.

Design for active flutter suppression and gust alleviation using state-space aeroelastic modeling

Abstract: An analytical design technique for an active flutter-suppression and gust-alleviation control system is presented. It is based on a rational approximation of the unsteady aerodynamic loads in the entire Laplace domain, which yields matrix equations of motion with constant coefficients. Some existing rational approximation schemes are reviewed, and a new technique which yields a minimal number of augmented states for a desired accuracy is presented. The state-space aeroelastic model is used to design a constant gain, partial-feedback control system, which simultaneously assures stability and optimizes any desired combination of gust response parameters throughout the entire flight envelope.

A Comprehensive Analytical Model of Rotorcraft Aerodynamics and Dynamics. Part 2. User's Manual

Abstract: The computer program for a comprehensive analytical model of rotorcraft aerodynamics and dynamics is described. This analysis is designed to calculate rotor performance, loads, and noise; the helicopter vibration and gust response; the flight dynamics and handling qualities; and the system aeroelastic stability. The analysis is a combination of structural, inertial, and aerodynamic models that is applicable to a wide range of problems and a wide class of vehicles. The analysis is intended for use in the design, testing, and evaluation of rotors and rotorcraft and to be a basis for further development of rotary wing theories.

Application of two design methods for active flutter suppression and wind-tunnel test results

Abstract: The synthesis, implementation, and wind tunnel test of two flutter suppression control laws for an aeroelastic model equipped with a trailing edge control surface are presented. One control law is based on the aerodynamic energy method, and the other is based on results of optimal control theory. Analytical methods used to design the control laws and evaluate their performance are described. At Mach 0.6, 0.8, and 0.9, increases in flutter dynamic pressure were obtained but the full 44 percent increase was not achieved. However at Mach 0.95, the 44 percent increase was achieved with both control laws. Experimental results indicate that the performance of the systems is not so effective as that predicted by analysis, and that wind tunnel turbulence plays an important role in both control law synthesis and demonstration of system performance.

Wind-tunnel experiments on divergence of forward-swept wings

Abstract: An experimental study to investigate the aeroelastic behavior of forward-swept wings was conducted in the Langley Transonic Dynamics Tunnel. Seven flat-plate models with varying aspect ratios and wing sweep angles were tested at low speeds in air. Three models having the same planform but different airfoil sections (i.e., flat-plate, conventional, and supercritical) were tested at transonic speeds in Freon 12. Linear analyses were performed to provide predictions to compare with the measured aeroelastic instabilities which include both static divergence and flutter. Six subcritical response testing techniques were formulated and evaluated at transonic speeds for accuracy in predicting static divergence. Two "divergence stoppers" were developed and evaluated for use in protecting the model from structural damage during tests.

Calculation of the transient motion of elastic airfoils forced by control surface motion and gusts

Abstract: The time-domain equations of motion of elastic airfoil sections forced by control surface motions and gusts were developed for the case of incompressible flow. Extensive use was made of special functions related to the inverse transform of Theodorsen's function. Approximations for the special cases of zero stream velocity, small time, large and time are given. A numerical solution technique for the solution of the general case is given. Examples of the exact transient response of an airfoil are presented.

Aeroelastic modelling of pneumatic and tensioned fabric structures

Abstract: The requirement for modelling the elastic properties of tensioned surfaces is examined and shown to be unnecessary in aeroelastic modelling of most tensioned structures. Simplifications arising from combining the scaling of internal pressurization and Froude number scaling for air-supported structures are illustrated and the importance of scaling the acoustic stiffness of volumes enclosed by such structures is emphasized.

Coupled Rotor/Tower Aeroelastic Analysis of Large Horizontal Axis Wind Turbines

Abstract: Formulation of the governing nonlinear equations of motion for the coupled rotor/tower dynamics of a large two-bladed horizontal axis wind turbine (HAWT) is presented. Each blade has elastic flap and lead-lag bending deflections and the supporting tower has bending-bending-torsion deflections. Rotor/tower coupling is accomplished by enforcing dynamic equilibrium between the rotor and the top of the tower. The nonlinear periodic coefficient equations of motion are used to study aeroelastic stability and response of the NASA/DOE 100 kW Mod-O wind turbine. The influence of the flexible tower and nonlinear terms on rotor stability is examined. Isolated rotor blade behavior is compared to the complete coupled rotor/tower system and the basic differences are identified. It is concluded that for high tower stiffness, the aeroelastic response is primarily dependent on isolated rotor forcing, yaw mechanism flexibility, and tower shadow effect. It is also shown that rotor stability can be improved by "tuning" the support system stiffness.

Status of NASA full-scale engine aeroelasticity research

Abstract: Data relevant to several types of aeroelastic instabilities were obtained using several types of turbojet and turbofan engines In particular, data relative to separated flow (stall) flutter, choke flutter, and system mode instabilities are presented The unique characteristics of these instabilities are discussed, and a number of correlations are presented that help identify the nature of the phenomena

VAWTDYN - A numerical package for the dynamic analysis of vertical axis wind turbines

Abstract: In order to design wind turbines which possess long life and at the same time use a minimum quantity of low-cost materials, it is essential that dynamic, in addition to static, structural analyses be performed to eliminate unnecessary margins of safety resulting from undetermined dynamic amplifications. The VAWTDYN package provides a tool for assessing these amplifications in two-bladed vertical axis wind turbines. The dynamic model on which the package is based contains turbine motions most commonly observed in existing research systems. Gyroscopic effects, structural damping, aerodynamic wind loading, and power generation through either an induction or synchronous generator are also included in the model. In addition to a comprehensive description of the model, this report documents the efforts that have gone into the verification, qualification, and demonstration of the package.

Rotor blade aeroelastic stability and response in forward flight

Abstract: The aeroelastic stability and response problem of the coupled flap-lag-torsional dynamics of a hingeless rotor blade in forward flight is treated in a comprehensive manner. The spatial dependence of the partial differential, nonlinear, equations of motion is discretized using a multimodal Galerkin method. The aeroelastic problem is coupled with the trim state of the helicopter obtained from improved, representative, trim procedures. The nonlinear time dependent equilibrium position, or response, about which the equations are linearized is obtained by solving a sequence of linear periodic response problems, using quasi-linearization. Numerous results illustrating blade behavior in forward flight are presented.

The Effects of Warhead-Induced Damage on the Aeroelastic Characteristics of Lifting Surfaces. Volume I. Aeroelastic Effects.

Abstract: : An investigation is being conducted to determine whether ballistic damage can seriously degrade the aeroelastic integrity of lifting surfaces on aircraft. A potential aeroelastic failure mode that was identified in the first year's study has been investigated here over a larger range of parameters. This failure mechanism results from the localized steady drag generated when a lifting surface encounters damage to its aerodynamic shape. Its modeling has been extended in this study to swept wing configurations and to possible multiple and distributed damage sites. In addition, a larger range of single damage site locations have also been considered to assess the possible tradeoffs between the influence of both structural and aerodynamic damage locations. A check on the validity of the strip theory aerodynamic modeling employed in this study has also been made by comparing these results with those obtained from a lifting surface theory modeling. Finally, an additional failure mechanism is identified that results from any unsteady but periodic fluctuating aerodynamic drag loads that are generated by the damage. A parametric and oscillatory instability can be induced by relatively low level drag loads in this case if they happened to be appropriately tuned to the structural frequencies of the wing. (Author)

Aerodynamic response of long h-sections

Abstract: Section models of several long H-section members of the Commodore Barry Bridge were tested in wind tunnels for aerodynamic coefficients and for vibration response when supported in a three degree-of-freedom mounting. Vibration testing was carried out in both smooth and turbulent flow. Comparisons were made with the response of an aeroelastic model tested at the Vertol Division of the Boeing Company. Tests results clearly show that for long H-section members, significant distress may occur due to static (mean) and fluctuating forces produced by wind effects. Vorttex excitation and self-excited motion are possible unless frequencies are relatively high and greater damping than the usual structural level is provided. Some remedies are considered; the actual solution adopted as worked out by others is noted.

Nonlinear Aeroelastic Equations of Motion of Twisted, Nonuniform, Flexible Horizontal-Axis Wind Turbine Blades

Abstract: The second-degree nonlinear equations of motion for a flexible, twisted, nonuniform, horizontal axis wind turbine blade were developed using Hamilton's principle. A mathematical ordering scheme which was consistent with the assumption of a slender beam was used to discard some higher-order elastic and inertial terms in the second-degree nonlinear equations. The blade aerodynamic loading which was employed accounted for both wind shear and tower shadow and was obtained from strip theory based on a quasi-steady approximation of two-dimensional, incompressible, unsteady, airfoil theory. The resulting equations had periodic coefficients and were suitable for determining the aeroelastic stability and response of large horizontal-axis wind turbine blades.

The prediction of pressure distributions on an arrow-wing configuration including the effect of camber, twist, and a wing fin

Abstract: Wind tunnel tests of an arrow wing body configuration consisting of flat, twisted, and cambered twisted wings were conducted at Mach numbers from 0.40 to 2.50 to provide an experimental data base for comparison with theoretical methods. A variety of leading and trailing edge control surface deflections were included in these tests, and in addition, the cambered twisted wing was tested with an outboard vertical fin to determine its effect on wing and control surface loads. Theory experiment comparisons show that current state of the art linear and nonlinear attached flow methods were adequate at small angles of attack typical of cruise conditions. The incremental effects of outboard fin, wing twist, and wing camber are most accurately predicted by the advanced panel method PANAIR. Results of the advanced panel separated flow method, obtained with an early version of the program, show promise that accurate detailed pressure predictions may soon be possible for an aeroelasticity deformed wing at high angles of attack.

Numerical optimization - An assessment of its role in transport aircraft aerodynamic design through a case study

Abstract: An efficient transonic wing design procedure based upon numerical optimization together with three-dimensional transonic methods has been developed and used to design an advanced transport wing. The method development included an examination of the use of both full potential and extended small disturbance analysis codes and demonstrated that the former formulation was more reliable. In either case, the design procedure is economical and easy to use. Design verification in a unique semi-span test arrangement demonstrated that the design method produced a wing which satisfied the study design requirements. However, aeroelastic deformation of the wing occurred during the wind tunnel test. The computational methods used in the design procedure were employed to assess the effect of the aeroelastic deformation. The paper concludes with an evaluation of the design procedure and recommendation for its improvement.

An assessment of the future roles of the National Transonic Facility and the Langley Transonic Dynamics Tunnel in aeroelastic and unsteady aerodynamic testing

Abstract: The characteristics and capabilities of the two tunnels, that relate to studies in the fields of aeroelasticity and unsteady aerodynamics are discussed. Scaling considerations for aeroelasticity and unsteady aerodynamics testing in the two facilities are reviewed, and some of the special features (or lack thereof) of the Langley Research Center Transonic Dynamics Tunnel (TDT) and the National Transonic Facility (NTF) that will weigh heavily in any decisions conducting a given study in the two tunnels are discussed. For illustrative purposes a fighter and a transport airplane are scaled for tests in the NTF and in the TDT, and the resulting model characteristics are compared. The NTF was designed specifically to meet the need for higher Reynolds number capability for flow simulation in aerodynamic performance testing of aircraft designs. However, the NTF can be a valuable tool for evaluating the severity of Reynolds number effects in the areas of dynamic aeroelasticity and unsteady aerodynamics. On the other hand, the TDT was constructed specifically for studies and tests in the field of aeroelasticity. Except for tests requiring the Reynolds number capability of NTF, the TDT will remain the primary facility for tests of dynamic aeroelasticity and unsteady aerodynamics.

Stochastic analysis of bridge motion in large-scale turbulent winds

Abstract: Using a linear formulation similar to one commonly used for the analysis of the motion of an airfoil, the presence of turbulence in the wind flow is shown to change the self-excited loads and it may cause instability of an otherwise stable motion of a suspension bridge. Furthermore, the self-excited loads and the buffeting loads are statistically correlated since both are related to the same turbulence fields. Therefore, even when the bridge motion remains stable, this correlation must be taken into account in the computation of stochastic moments of the structural response. Numerical examples are given to illustrate the application of this new theory.

A stability prediction method for application to nonlinear aeroelasticity

Abstract: An approximate p rocedure is developed which efficiently determines the amplitude-dependent stability of nonlinear systems. This p rocedure, which is referred to as the "method of imposed disturbances", is shown to be especially applicable to aeroelastic design variable sensitivity studies. The method, which is based on the principle of c onservation of energy in a limit cycle, is first demonstrated on several r epresentative nonlinear, nonconservative problems. Amplitude-dependent transonic airfoil flutter is then predicted for a pitching NACA 64A006 airfoil using the LTRAN2 code to predict the aerodynamic loads.

Wind tunnel investigation of active controls technology applied to a DC-10 derivative

Abstract: Application of active controls technology to reduce aeroelastic response offers a potential for significant payoffs in terms of aerodynamic efficiency and structural weight. As part of the NASA Energy Efficient Transport program, the impact upon flutter and gust load characteristics has been investigated by means of analysis and low-speed wind tunnel tests of a semispan model. The model represents a DC-10 derivative with increased wing span and an active aileron surface, responding to vertical acceleration at the wing tip. A control law satisfying both flutter and gust load constraints is presented and evaluated. In general, the beneficial effects predicted by analysis are in good agreement with experimental data.

Transonic small disturbances equation applied to the solution of two-dimensional nonsteady flows

Abstract: Transonic nonsteady flows are of large practical interest. Aeroelastic instability prediction, control figured vehicle techniques or rotary wings in forward flight are some examples justifying the effort undertaken to improve knowledge of these problems is described. The numerical solution of these problems under the potential flow hypothesis is described. The use of an alternating direction implicit scheme allows the efficient resolution of the two dimensional transonic small perturbations equation.

Aeroelastic and dynamic finite element analyses of a bladder shrouded disk

Abstract: The delivery and demonstration of a computer program for the analysis of aeroelastic and dynamic properties is reported. Approaches to flutter and forced vibration of mistuned discs, and transient aerothermoelasticity are described.