Showing papers on "Flutter published in 1975"

Active Flutter Suppression—A Flight Test Demonstration

Abstract: The first flight test demonstration of active flutter suppression has been successfully completed. The Control Configured Vehicles (CCV) B-52 test airplane was twice flown 10 knots faster than its flutter speed relying solely on an automatic control system for adequate damping. The design, safety considerations, mechanization, ground testing, and flight testing of the flutter mode control system are reported. Comparisons between flight test and theoretical results are presented. The system was tested at heavy and light airplane weights and tested for compatibility with simultaneous ride control, maneuver load control, fatigue reduction, and augmented stability.

Advances in fan and compressor blade flutter analysis and predictions

Abstract: A unified approach to flutter prediction has been developed at Pratt & Whitney Aircraft (P&WA). The aeromechanical stability of the blade-disk system is expressed in terms of a stability parameter which measures the amount of unsteady work done by the air on the system, when the system is vibrating in one of its natural modes. In neutrally stable systems, the unsteady work done by the air on the blades will balance the work dissipated by friction and by material damping. An accurate prediction of the vibrational deflections and of the unsteady aerodynamic forces is required at every spanwise location on each blade, so that the work done by the unsteady aerodynamic forces may be calculated. Recent progress is described in the prediction of unsteady aerodynamic forces and the determination of mode shapes. The stability model is applied to the prediction of supersonic flutter, chord wise bending flutter, and stall flutter. Recommendations are made for additional work necessary to improve the prediction model.

Development and demonstration of a flutter-suppression system using active controls

Abstract: The application of active control technology to suppress flutter was demonstrated successfully in the transonic dynamics tunnel with a delta-wing model. The model was a simplified version of a proposed supersonic transport wing design. An active flutter suppression method based on an aerodynamic energy criterion was verified by using three different control laws. The first two control laws utilized both leading-edge and trailing-edge active control surfaces, whereas the third control law required only a single trailing-edge active control surface. At a Mach number of 0.9 the experimental results demonstrated increases in the flutter dynamic pressure from 12.5 percent to 30 percent with active controls. Analytical methods were developed to predict both open-loop and closed-loop stability, and the results agreed reasonably well with the experimental results.

Torsional flutter of rectangular prisms

Abstract: Prismatic bar models tested are of rectangular cross section and are provided with different side ratios (depth/height) of 1, 2, 4, and 5. The position of axis of rotation of the model, which is spring-mounted so as to oscillate in torsion, is varied along one of the center lines of the cross section to see the axis position effects on torsional flutter. The validity of the C M α criterion for torsional flutter is examined by finding a possible correlation between the onset of flutter and the sign of C M α (steady aerodynamic torsional moment derivative) at each axis position. Theoretical analyses of unsteady aerodynamic torsional moment acting on a bluff structure oscillating in torsion reveal that it has two constituents, one arising from the fluid memory effect, and other from the quasi-steady flow effect. It is indicated that the C M α criterion for torsional flutter is applicable to cases where the fluid memory effect is predominant. Further analysis comprising the measurements of the wake velocity variations and the flow visualization results emphasizes the significant role played by the fluid memory effect in the torsional flutter of structures.

Flutter of Conservative Systems of Pipes Conveying Incompressible Fluid

Abstract: An examination of the dynamics of beam-like motions of pipes conveying fluid, with both ends clamped, is presented by means of beam theory and, in the case of thin-walled pipes, by thin-shell theor...

Flutter of Flat Finite Element Panels in a Supersonic Potential Flow

Abstract: A finite element formulation and a solution procedure are developed for the flutter analysis of twodimensional flat panels with one surface exposed to a supersonic potential flow and supported at the leading and trailing edges. The aerodynamic forces due to supersonic flow are obtained from the exact theory for linearized two-dimensional unsteady flow where no limitations on the order of the frequency are imposed. Thus the Mach number need not be limited to beyond approximately 1.6. When using this theory, the aerodynamic matrix becomes location-dependent and must be formulated for each element. This difficulty is overcome by first assembling the element shape functions for the entire structure and then finding the aerodynamic forces and the element aerodynamic matrix by numerical integration. The effects of structural damping and initial inplane tension are included. Examples for finding the panel thickness required to prevent flutter at various Mach numbers are demonstrated and the results are compared with an alternative Galerkin's modal solution as well as experiments.

Parametric Constraints with Application to Optimization for Flutter Using a Continuous Flutter Constraint

Abstract: Many optimum structural-design problems involve constraints which have to be satisfied for an entire range of certain parameters. For example, in a structure under dynamic loads the stress constraint has to be satisfied for a given range of time. The properties of parametric constraints are investigated in the present paper. It is shown that a parametric constraint may be replaced by an equivalent "minimum value" constraint which is more efficient computationally for optimization problems. The efficiency of the minimum value constraint is due to the minimal amount of computation needed to update it as the structure is changed in the optimization process. The results obtained for the general case of parametric constraints are applied to the problem of optimal structural design under flutter constraints. It is shown that a recently developed continuous flutter constraint is a parametric constraint and may be replaced by the equivalent minimum value constraint. An example problem is solved using the continuous flutter constraint. It is shown that the design obtained for this problem could not be obtained by using a more conventional flutter constraint.

F100 fan stall flutter problem review and solution

Abstract: An experimental investigation was conducted to examine an airfoil durability problem in the first fan rotor of the F100 engine. This study incorporated laboratory and simulated engine flight tests, an empirical correlation of aeroelastic stability parameters from engine test data, and substantiation testing of the redesign. The results of this investigation's initial testing showed that rotor failure at high-flight Mach numbers and low altitudes was caused by torsional stall flutter instability. The results of the empirical correlation indicated that a design free of flutter required a decrease in both normalized incidence and reduced velocity. Further, the correlation indicated that the flutter was affected by inlet pressure, a heretofore undocumented phenomenon. The results of the substantiation testing confirmed that the redesign made the rotor flutter-free throughout the entire aircraft flight envelope. It was concluded that an improved stall flutter analysis was required to ensure stable fan and compressor rotor designs. It was further concluded that the effect of changes in inlet pressure level on rotor stability was, in part, the result of the accompanying changes in air density and steady-state aerodynamic loading.

Panel flutter optimization by gradient projection

Abstract: A gradient projection optimal control algorithm incorporating conjugate gradient directions of search is described and applied to several minimum weight panel design problems subject to a flutter speed constraint. New numerical solutions are obtained for both simply-supported and clamped homogeneous panels of infinite span for various levels of inplane loading and minimum thickness. The minimum thickness inequality constraint is enforced by a simple transformation of variables.

Development of Active Flutter Suppression Wind Tunnel Testing Technology

Abstract: : A research study was conducted to develop active flutter suppression wind tunnel testing technology. A one-thirtieth scale B-52 aeroelastic model was modified to represent the Control Configured Vehicles (CCV) B-52 flight test airplane with an active flutter mode control system (FMCS). The system was mechanized on the model using electromechanical actuation systems for the scaled CCV airplane outboard aileron and flaperon outboard segment control surfaces. The model was tested in the NASA-Langley Transonic Dynamics Tunnel to evaluate the unaugmented model flutter characteristics and performance of the flutter mode control system. Test results were compared with model analytical results and CCV program flight test results for equivalent weight and altitude conditions. The model flutter speed, in airplane scale, is 8.1 percent higher than the airplane flutter speed, less than 1.0 percent higher than the predicted difference. Flutter mode damping with the FMCS engaged is higher on the model than on the airplane, but the damping trends with increasing airspeed are similar. The good agreement attained between model and airplane test results demonstrates that dynamically scaled models can be used to verify analytical methods used to design active flutter mode control systems.

Aspects of dynamic stability of structures

Abstract: The dynamic nature of the transient of a structure from stability to instability is emphasized. The need to consider flutter and the paradoxical effect of mass distribution and damping on flutter loads are pointed out. A criterion is formulated for deciding upon a structure to be of the divergence type. For structures of this type, a static approach to stability is permissible. Fortunately, the majority of structures in civil engineering are of the divergence type. However, this does not exempt the civil engineer from being prepared to investigate stability by means of dynamics whenever necessary. The importance of Liapunov's direct method for stability investigations has been stressed.

Study of Dynamic Characteristics of Aeroelastic Systems Utilizing Randomdec Signatures

Abstract: The feasibility of utilizing the random decrement method in conjunction with a signature analysis procedure to determine the dynamic characteristics of an aeroelastic system for the purpose of on-line prediction of potential on-set of flutter was examined. Digital computer programs were developed to simulate sampled response signals of a two-mode aeroelastic system. Simulated response data were used to test the random decrement method. A special curve-fit approach was developed for analyzing the resulting signatures. A number of numerical 'experiments' were conducted on the combined processes. The method is capable of determining frequency and damping values accurately from randomdec signatures of carefully selected lengths.

Comparison of Two Types of Structural Optimization Procedures for Flutter Requirements

Abstract: A comparison is made of results obtained from mathematical programing and optimality criteria procedures for the minimum-mass design of typical aircraft wing structures to satisfy prescribed flutter requirements The mathematical programing method is based on an interior penalty function approach A Langrangian optimality criterion and an intuitive optimality criterion based on uniform strain energy density are considered An intuitive resizing procedure is used for both optimality criterion solutions All results are calculated using the same computer program, changing only the optimization procedure Both high- and low-aspect-ratio wings are examined Finite elements are used for structural modeling, and the generalized coordinates for the flutter solution are based on the natural vibration modes of the structure Second-order piston theory aerodynamics is used for supersonic conditions and kernel function aerodynamics for subsonic conditions Convergence of the optimality criteria procedures with respect to the number of natural modes is considered [A ] [B] [Bti] Nomenclature

Transonic Study of Active Flutter Suppression Based on an Energy Concept

Abstract: The application of active control technology to the suppression of flutter has been successfully demonstrated in the Langley transonic dynamics tunnel. This study involved the implementation of an aerodynamic-energy criterion to suppress flutter of a simplified delta-wing model. Use of this technique with both leadingand trailing-edge active controls has resulted in an increase in dynamic pressure of 22% above the basic wing flutter point and with only a trailing-edge active control has resulted in an increase in dynamic pressure of 30% above the basic wing flutter point at a Mach number of 0.9. Analytical methods used to predict the openand closedloop behavior of the model are also discussed.

Flutter of a panel supported on an elastic foundation

Abstract: The flutter behavior of a thin panel resting on a linear elastic foundation is studied, using an analogy with the flutter of a panel of the same geometry without spring support. With quasi-steady aerodynamic loading, it is observed that there is a change not only in the flutter frequency, but also in the total damping of the system, due to the elastic foundation. Furthermore, it is found that for small values of aerodynamic damping and spring stiffness, the quasi-static approach is reasonable, whereas for high values of aerodynamic damping or of spring stiffness, quasi-static theory is inadequate. For high values of spring stiffness, the flutter mode of the panel exhibits traveling wave motion.

Integration time required to extract accurate static and dynamic data from transonic wind-tunnel tests

Abstract: Because the forces and pressures on wind-tunnel models tested at transonic speeds are not steady, even for 'static' aerodynamic tests, integration time is required to obtain data of acceptable accuracy. The integration time required for both static and dynamic tests is evaluated analytically and confirmed by experimental measurements. It is shown that, for static and dynamic tests, the accuracy obtained is a function of integration time, frequency content of the signal, and the ratio of the dynamic amplitude to the full signal of interest. In addition, for the dynamic case, the frequency band width used in analysis is important. Results of this study indicate that, for typical data accuracy desired from models in a large transonic wind tunnel (11- by 11-ft), up to the following integration times are required: static force and moment tests, 0.5 sec; static pressure tests, 0.1 sec; flutter tests, 30 to 60 sec; and random-dynamic tests, 10 sec.

Vibration and flutter analysis of reusable surface insulation panels

Abstract: A study has been conducted of the aeroelastic behavior of heat shield panels for the thermal protection system of the space shuttle. Questions of structural and aerodynamic modeling are considered along with the analysis of a single isotropic tile and an isotropic main substructure. Representative flutter results are presented, taking into account a rigid and a flexible metallic substructure.

Application of an advanced computerized structural design system to an arrow-wing supersonic cruise aircraft

Abstract: A structural design study of an arrow-wing supersonic cruise aircraft has been made using the integrated design system, ATLAS, and a relatively large analytical finite-element model containing 8500 degrees of freedom. This paper focuses on structural design methods developed and used in support of the study with emphasis on aeroelasticity. The use of ATLAS permitted (1) automatic resizing of the wing structure for multiple load conditions, (2) rapid evaluation of aeroelastic effects, and (3) an iterative approach to the correction of flutter deficiencies. The significant results of the study are discussed along with the advantages derived from the use of an advanced structural design system in preliminary design studies.

FCAP - A new tool for the evaluation of active control technology

Abstract: A computer program has been developed for the evaluation of flight control systems designed for flexible aircraft. This Flight Control Analysis Program (FCAP) is designed in a modular fashion to incorporate sensor, actuator, and control logic element dynamics as well as aircraft dynamics and aerodynamics for complex configurations. Formulation of the total aircraft dynamic system is accomplished in matrix form by casting the equations in state vector format. The system stability and performance are determined in either the frequency or time domain using classical analysis techniques. The aerodynamic method used also permits evaluation of the flutter characteristics of the aircraft.

The Stability of a Damped Elastic System with a Follower Force

Abstract: A theoretical and experimental analysis is made of the behaviour of a double pendulum with viscoelastic hinges subjected to a follower force. The effects of variations in the masses, stiffnesses, geometry and the internal and external velocity-dependent forces on the stability of the system are examined in detail. The picture that emerges shows all these factors to be important. In all cases, instability occurs in the first mode of flutter motion and the results accurately confirm theoretical predictions.