Showing papers on "Vortex lattice method published in 1987"

Numerical analysis of three-dimensional elastic membrane wings

Abstract: This paper presents a numerical method for predicting the behavior of an elastic membrane wing under aerodynamic loading. A method for finding the pressure distribution generated by flow over a given threedimensional surface is combined with another for finding the shape of a given membrane under a given pressure distribution. The pressure is calculated using a vortex lattice simulation of potential flow, and the shape is determined using a finite element representation of the membrane. An iterative scheme is employed to solve the resulting nonlinear equations which relate the shape and loading to the displacements of the surface. A simple example is given, in which the lift and stress distribution are calculated for a membrane wing with the shape and boundary constraints of an idealized hang glider. The method is equally applicable to yacht sails.

The numerical simulation of subsonic flutter

Abstract: The present paper describes a numerical simulation of unsteady, subsonic aeroelastic responses. The technique accounts for aerodynamic nonlinearities associated with angles of attack, vortex-dominated flow, static deformations, and unsteady behavior. The fluid and the wing together are treated as a single dynamic system, and the equations of motion for the structure and flowfield are integrated simultaneously and interactively in the time domain. The method employs an iterative scheme based on a predictor-corrector technique. The aerodynamic loads are computed by the general unsteady vortex-lattice method and are determined simultaneously with the motion of the wing. Two models are used to demonstrate the technique: a rigid wing on an elastic support experiencing plunge and pitch about the elastic axis, and a continuous wing rigidly supported at the root chord experiencing spanwise bending and twisting. The time domain solution coupled with the unsteady vortex-lattice method provides the capability of graphically depicting wing and wake motion. Several graphs that illustrate the time domain behavior of the wing and wake are presented.

A new method to predict unsteady aeroelastic behavior

Abstract: A new method for predicting subsonic flutter and static deflections, including divergence, has been developed. The present method accounts for aspect ratio and, in the case of flutter, static deflections. The angle of attack is limited only by the occurrence of stall or vortex bursting near the wing. The innovation in the present method is to integrate simultaneously and interactively the equations of motion of the structure and the flowfield. The present approach employs an iterative scheme based on the predictor-corrector method. The general unsteady vortex-lattice method (UVLM) is used to predict the aerodynamic loads. Because the UVLM predicts the wakes as part of the solution, the history of the motion is taken into account; hysteresis is predicted. The deflection (for both bending and torsion) is expressed as an expansion in terms of the free-vibration modes. The time-dependent coefficients in these expansions serve as the generalized coordinates. Numerical examples illustrating the calculation of static deflections and transient dynamic responses above and below the flutter boundary are included.

Loss of lift due to thickness for low-aspect-ratio wings in incompressible flow

Abstract: The problem under consideration is a numerical study of the effects of thickness on lift for low-aspect-ratio wings in steady incompressible inviscid flow at moderate of attack. At these angles of attack the flow separates along the leading edge giving rise to a lift substantially higher than that computed by classical attached-flow potential theory. The problem is treated as a perturbation expansion in a small thickness parameter. The lifting elements of the flow are modeled using a nonlinear vortex-lattice method which replaces the leading and trailing-edge vortex sheets by segmented straight vortex filaments. The thickness elements of the flow are modeled with a mean-plane source distribution and a modification to the wing boundary conditions. Results are obtained for wings with biconvex and NACA 0012 sections which compare well with available experimental data. The important observation that the effect of thickness is to decrease the lift is made.

Pressure measurements on a thick cambered and twisted 58 deg delta wing at high subsonic speeds

Abstract: A pressure experiment at high subsonic speeds was conducted by a cambered and twisted thick delta wing at the design condition (Mach number 0.80), as well as at nearby Mach numbers (0.75 and 0.83) and over an angle-of-attack range. Effects of twin vertical tails on the wing pressure measurements were also assessed. Comparisons of detailed theoretical and experimental surface pressures and sectional characteristics for the wing alone are presented. The theoretical codes employed are FLO-57, FLO-28, PAN AIR, and the Vortex Lattice Method-Suction Analogy.

Structural Dynamics of Maneuvering Aircraft.

Abstract: : The technical objectives of the research are to establish the minimum level of modeling necessary for predicting the dynamic stresses in fighter aircraft during maneuvers and transitions between maneuvers, to identify the physical phenomena which are significant, to identify potential shortfalls in current aircraft design technology and to identify areas for further research. A symmetric wing, modeled as a beam with quasi-steady aerodynamic loads introduced through the vortex lattice method, is used for the dynamic evaluation. The dynamical equations of motion are solved in the time domain by the Runge-Kutta method specialized to second order equations. The maneuver dynamic analysis program, MANDYN, written in Fortran 77, uses the parameters of real variable-geometry airplane. An extensive investigation of dynamic loads resulting from a simple maneuver for various severity levels revealed that the lowest natural frequency and load application time are significant factors in determining the validity of the load factor approach. Recommendations based on the study results are made. Keywords: Symmetrical maneuvers.