Showing papers on "Vortex lattice method published in 1986"

Modeling aerodynamic responses to aircraft maneuvers - A numerical validation

Abstract: The regime of validity of an aerodynamic mathematical model, applicable to describe the nonlinear aerodynamic reactions to a delta wing maneuvering at high angles of attack, is investigated. An unsteady vortexlattice method is used to compute the unsteady flowfields, and thus to evaluate the aerodynamic data required by the model, in terms of specified characteristic motions. Time-histories of the aerodynamic responses to complex motions are generated by means of the model and the evaluated aerodynamic data and are compared with baseline aerodynamic responses obtained from direct vortex-lattice computations. The validity of the mathematical modeling approach for the maneuvering delta wing is demonstrated by agreement of the force and moment responses obtained from the two approaches.

Axisymmetric vortex lattice method applied to parachute shapes

Abstract: The Naval Weapons Center is developing a dynamic, unsteady aerodynamic model to investigate the dynamics of inflating parachutes. In this report, analysis of axisymmetric, rigid shapes is presented. The model replaces the body with a series of vortex rings. The generated vorticity is shed as discrete vortices and allowed to freely convect. The predicted drag and pressure distributions are compared to available data for ribbon parachutes. Wake interactive, apparent mass calculations are also presented.

Application of the unsteady vortex-lattice method to the nonlinear two-degree-of-freedom aeroelastic equations

Abstract: A means of numerically simulating flutter is established by implementing a predictor-corrector algorithm to solve the equations of motion. Aerodynamic loads are provided by the unsteady vortex lattice method (UVLM). This method is illustrated via the obtainment of stable and unstable responses to initial disturbances in the case of two-degree-of-freedom motion. It was found that for some angles of attack and dynamic pressure, the initial disturbance decays, for others it grows (flutter). When flutter occurs, the solution yields the amplitude and period of the resulting limit cycle. The preliminaray results attest to the feasibility of this method for studying flutter in cases that would be difficult to treat using a classical approach.

Comment on 'Convergence characteristics of a vortex-lattice method for nonlinear configuration aerodynamics'

Abstract: It is proposed that the study of Rusak et al. (1985), which reports numerical modeling sensitivities on longitudinal force/moment properties for a vortex-lattice method incorporating free vortex filaments to represent the leading-edge vortex separation, employs a formula that is strongly affected by the particular points of analysis chosen. This results in a narrowly applicable curve fit, where numerical sensitivities of the theory are inappropriately traded off against physical effects that are not modeled in that theory. Attention is also given to questionable drag estimate computations.

Euler equations analysis of the initial roll-up of aircraft wakes

Abstract: A new approach has been developed to study the initial roll-up of aircraft wakes This approach is based on the solution of the three-dimensional Euler equations subject to inflow and initial conditions predicted by a vortex-lattice method This procedure allows for the solution of this complex problem without any empirical inputs However, valid solutions are limited to those flowfields generated by wings which are free of flow separation Results are presented for four configurations: a rectangular wing with part span flap, a rectangular wing, a typical transport-type configuration and a fighter configuration In addition, an assessment of an 'unsteady 2-D analogy' was undertaken using the 2-D time accurate Euler equation solutions in crossflow planes In general, predictions from the 2-D unsteady analogy coincided with the 3-D results which were found to be in good agreement with available experimental data

Nonplanar vortex lattice method for analysis of complex multiple lifting surfaces

Abstract: A method for determining the aerodynamic characteristics of complex multiple lifting surfaces in inviscid subcritical flows has been developed and programmed on UNIVAC 1100/60 computer. 13; Each lifting surface is represented by a network of non-planar horse shoe vortices distributed on the mean surface and trailing to infinity. The strengths of these vortices are determined by requiring the flow to be parallel to the surface at a number of control points. The force due13; to a vortex segment is calculated as the vector product of local velocity and the vortex strength multiplied by density.13; The programme can handle wings with breaks and span wise segmented flaps in leading and trailing edges, local dihedral, camber and twist. The code can be used to compute lift, induced drag and pitching moments for any lifting planar of non-planar surfaces and surfaces in combination13; like wing-canard or wing-horizontal tail. The programme has been validated for a number of configurations for which experimental data is available.

A study on the prediction of propeller slipstream and the numerical analysis of lifting surface

Abstract: In recent years, a highly advanced lifting surface method with high accuracy has been required in order to design the various types of propellers which demand high efficiency and to reduce ship vibration and noise.In this paper, an improved lifting surface procedure based on the vortex lattice method is presented. For the determination of a trailing vortex wake geometory, a numerical iterative procedure is described. The numerical results on propeller slipstream give a good agreement with the measured values by laser doppler velocimeter.The present method is applied to calculate the open water characteristics of a propeller. And a quasi-steady technique based on the present method is adopted to calculate the fluctuation of propeller forces and cavitation patterns behind the ship's wake field.The advantages of the present method are shown by comparing the numerical results with the experimental data of conventional and highly skewed propeller.

Summary of a high subsonic force/pressure experiment for 58 deg cambered/twisted thick delta wings

Abstract: This paper summarizes the results of a force, moment, and pressure experiment involving six thick, cambered and twisted, delta wings with 58 deg leading-edge sweep. This experiment was conducted in the NASA Langley 7- by 10-foot High-Speed Tunnel at Mach numbers of 0.75, 0.80, and 0.83. The design goal was a configuration which was self-trimming at a lift coefficient of 0.25 and Mach number of 0.80. Although the design goal was not met, the configuration which came closest and which had the best overall performance was selected for further study. Wing surface pressure data and limited surface oil flow data for this configuration are presented to show the extent of attached flow at the design point. For selected cases, inviscid solutions from vortex lattice method/suction analogy, PAN AIR, FLO-28, and FLO-57 are compared with the experimental force, moment, and pressure data.

Unsteady forces on counter-rotating propeller blades

Abstract: Unsteady forces experienced by counter-rotating propeller blades are examined in this paper. A fully coupled vortex lattice model of counter-rotation is used to obtain a quasi-steady solution to the propeller loadings, and an unsteady Sears (1941) analysis provides an estimate of the unsteady loads from the quasi-steady results. The vortex lattice method predicts the overall performance of counter-rotation well, based on comparisons of measured and predicted results. The effects of propeller spacing and blade number on the unsteady loadings are investigated. The peak-to-peak variation about the mean of the unsteady loads on the rear propeller varied from 9 percent for a 2 x 2 counter-rotation system to 2 percent for an 8 x 8 system.

Investigation of chord ratio, stagger, decalage angle, and flap angle for dual wing configurations

Abstract: Aerodynamical closely coupled dual wing configurations of unequal chords are investigated for medium speed general aviation applications. Vortex panel and momentum boundary layer analysis are utilized for the two-dimensional predictions. A multi-surface vortex lattice method is used for the three-dimensional predictions. In the process of searching for the highest lift to drag ratio upper airfoil to lower airfoil chord ratios, both greater than and less than one, are investigated in terms of stagger, decalage angle, and gap. With the optimum chord ratio, at the optimum stagger and gap, proper spanwise decalage distribution is shown to yield the lowest two dimensional drag results. Various wing taper ratios and wing twists are investigated to increase wing efficiency. Comparisons are made between optimized dual and optimized single wing configurations both with the same fuselage, stabilator surfaces, engines, payload, and fuel. The dual wing configuration is shown to have significantly less drag and hence longer range than the conventional single wing configuration.