Alfred G. Striz

Flutter analysis of a NACA 64A006 airfoil in small disturbance transonic flow

Flutter analyses are performed for a NACA 64A006 airfoil pitching and plunging in small-disturbance, unsteady transonic flow. Aerodynamic coefficients are obtained for M^ = 0.7, 0.8, 0.85, 0.8625, and 0.87, and for various values of low reduced frequencies. Two computer codes are used: 1) STRANS2 and UTRANS2 based upon the relaxation method and 2) LTRAN2 based upon the time-integration (indicial) method. Flutter results are presented as plots of flutter speed and corresponding reduced frequency vs one of the four parameters: airfoil/air mass density ratio, position of mass center, position of elastic axis, and freestream Mach number. In each figure, several sets of curves for different values of plunge/pitch frequency ratios are shown. The two sets of results based upon the two separate computer codes are, in general, in good agreement. For a special flutter analysis of a flat plate at M^ =0.7, the present methods agree well with the linear flat plate theory. The effect of each parameter on the trend of each curve of flutter speed is discussed in detail. The examples demonstrate the dip phenomenon of the curves for flutter speed in the transonic regime. Flutter results of transonic codes are compared with those obtained by linear flat plate theory.

Application of Transonic Codes to Flutter Analysis of Conventional and Supercritical Airfoils

Transonic flutter analyses are performed for two conventional airfoils, NACA 64A006 and NACA 64A010, and three supercritical airfoils, MBB A-3, CAST 7, and the NASA TF-8A wing section at 65.3% semispan. Two degrees of freedom, plunging and pitching about the quarter-chord axis, are considered. The aerodynamic data are obtained by using the two transonic aerodynamics codes LTRAN2 (indicial and time integration methods) and STRANS2/UTRANS2 (harmonic analysis method). The unsteady aerodynamic data are computed within the low reduced frequency range. For all airfoils, the effect of Mach number on flutter speed is studied for several values of four different aeroelastic parameters. For the MBB A-3 supercritical airfoil, the effect of angle of attack on flutter speed is studied. For the cases of a flat plate and a NACA 64A006 airfoil, time response results are obtained by LTRAN2. Applicability and limitations of the two transonic codes are evaluated. Results for the transonic flutter characteristics of these airfoils are discussed and some comparisons are made. ah b,c

Influence of Model Complexity and Aeroelastic Constraints on Multidisciplinary Optimization of Wings

This investigation focuses on the performance of the Automated Structural Optimization System multidisciplinary optimization code in the structural weight optimized design of two e nite element models of a low-aspect ratio e ghter-type wing. Optimal designs for the wing with the structure represented by a coarse and a more complex e nite element model are obtained with constraints imposed on strength, aileron reversal, and e utter using subsonic and supersonic aerodynamic theories. The results demonstrate the ability of ASTROS to effectively e nd local minima with weight as the objective function. The trends in the e nal weights illustrate the impact of changing design constraints on optimal design.

Optimization of wing tip store modeling

I N present three-dimensional flutter investigations of aircraft with external stores, the stores are often approximated by flat plates to reduce the cost and complexity of the analyses. In this Synoptic, the doublet lattice and kernel function methods are utilized to investigate the validity of this flatplate approximation in comparison to store models of other geometries for an F-5 wing with tip-mounted launcher/store combination. Various cross sections are found that show improved results with only a moderate increase in model complexity and thus computer cost.

Optimal design of aircraft structures with damage tolerance requirements

Damage tolerance analysis (DTA) was considered in the global design optimization of an aircraft wing structure. Residual strength and fatigue life requirements, based on the damage tolerance philosophy, were investigated as new design constraints. The global/local finite element approach allowed local fatigue requirements to be considered in the global design optimization. AFGROW fatigue crack growth analysis provided a new strength criterion for satisfying damage tolerance requirements within a global optimization environment. Initial research with the ASTROS program used this damage tolerance constraint to optimize cracked skin panels on the lower wing of a fighter/attack aircraft. For an aerodynamic and structural model of this type of aircraft, ASTROS simulated symmetric and asymmetric maneuvers during the optimization. Symmetric maneuvers, without underwing stores, produced the highest stresses and drove the optimization of the inboard lower wing skin. Asymmetric maneuvers, with underwing stores, affected the optimum thickness of the outboard hard points. Subsequent design optimizations included DTA and von Mises stress constraints simultaneously. In the configuration with no stores, the optimization was driven by the DTA constraint and, therefore, DTA requirements can have an active role to play in preliminary aircraft design.

Influence of structural and aerodynamic modeling on flutter analysis

Abstract : The influences of structural and aerodynamic modeling on the flutter analysis and multidisciplinary optimization of fully built-up finite element wing models in an aeroelastic environment are not yet well understood. Therefore, the dynamic aeroelastic and optimization capabilities in the Automated Structural Optimization System (ASTROS) were used to evaluate the flutter behavior and the behavior of structural optimization with flutter constraints of various representative fully built-up finite element wing models in subsonic and supersonic flow. ASTROS was here used as a tool to calculate flutter speeds and frequencies and to minimize the weight of these wing models in subsonic and supersonic flow under given flutter and frequency constraints to determine the effect that these modeling factors have. Structures, ASTROS, Aeroelasticity, Modelling, Optimization.

ISSUES OF MULTIOBJECTIVE FUNCTION OPTIMIZATION IN ENGINEERING DESIGN

As mathematical optimization methodologies become more and more accepted in the various areas of engineering, the complex problems at hand often require multicriteria or multiobjective function optimizations since most real-life design or decision problems involve multiple and conflicting objectives and constraints. In order to decrease the complexity of such optimizations, it seems of interest to investigate how the various objectives and constraints of a given problem influence and complement each other. Such knowledge could reduce the number of objectives and constraints by eliminating from the optimization loosely coupled parameters. In the present approach, various scenarios were developed, and studies in mathematical, structural, aircraft performance, and aircraft multi-disciplinary design optimization were suggested to address these issues. Investigations in structural and aircraft multidisciplinary design optimization were initiated.

MULTIDISCIPLINARY DESIGN OPTIMIZATION WITH DAMAGE TOLERANCE CONSTRAINTS AND A PROBABILISTIC LOAD ENVIRONMENT

Damage tolerance analysis (DTA) was considered in the multidisciplinary design optimization of an aircraft wing structure. Residual strength and fatigue life requirements, based on the damage tolerance philosophy, were investigated as new design constraints. A probabilistic approach was used to describe the uncertain load environment. Probabilistic load spectra models were developed from flight recorder data. AFGROW fatigue crack growth analysis provided a new strength criterion for satisfying damage tolerance requirements within a global optimization environment. Initial research with the ASTROS program used the probabilistic load model and this damage tolerance constraint to optimize cracked skin panels on the lower wing of a fighter/attack aircraft. For an aerodynamic and structural model of a fighter, ASTROS simulated symmetric and asymmetric maneuvers during the optimization. Symmetric maneuvers, without underwing stores, produced the highest stresses and drove the optimization of the inboard lower wing skin. Asymmetric maneuvers, with underwing stores, affected the optimum thickness of the outboard hard points. Subsequent multidisciplinary design optimizations included von Mises stress, aileron effectiveness, and lift effectiveness constraints simultaneously. This optimization was driven by the DTA and von Mises stress constraints and, therefore, DTA requirements can have an active role to play in preliminary aircraft design.