Showing papers on "Vortex lattice method published in 1995"

Aerodynamics and Acoustics of Rotor Blade-Vortex Interactions

Abstract: Helicopter rotor simulation codes of different organizations are validated against experimental data obtained in the DNW. The comparison addresses rotor blade loading, wake geometry, blade motion and noise radiation. Although specific differences exist the general prediciton of rotor noise is reasonably well.

Lateral and Directional Stability and Control in Air-to-Air Refuelling:

Abstract: Application of a wake roll-up method coupled with the vortex lattice method and approximate expressions for the receiver fuselage effect have been used to determine the induced loads on a Hercules receiver aircraft behind a KC10 tanker. The induced loads depend strongly on the vertical position of the receiver wing and fin relative to the tanker wing wake. In the case of steady sideslip there is a large decrease in the directional stability of the receiver as quantified by the gradient of the rudder angle versus sideslip. This is due mainly to the combined effects of the yawing moments due to bank, yaw and side displacements. Minimum directional stability corresponds to the tip of the receiver fin intersecting the tanker wing wake. The associated aileron angle is two to three times the value in free air in agreement with flight test data. Solution of the linearized equations of motion gives three lateral characteristic oscillations for the air-to-air refuelling case. These include the usual Dutch roll osc...

Aeroelastic analysis of multibladed hingeless rotors in hover

Abstract: The coupled flap-lag-torsion aeroelastic response and stability of multibladed hingeless rotors in the hovering flight condition are investigated. The vortex lattice method, with a three-dimensional prescribed wake geometry, is used for the prediction of unsteady airloads of multibladed rotors undergoing disturbed dynamic motions. Interblade unsteady wake effects due to vortex-phasing phenomena beneath a rotor are numerically calculated by the phase control of wake vortices shed from each blade. The aeroelastic equations of motion of the rotor blade are formulated using a finite element beam model that has no artificial restrictions on the magnitudes of displacements and rotations due to the degree of nonlinearity. Numerical results of the steady equilibrium deflections and the lead-lag damping and frequency are presented for two-, three-, and four-bladed stiff-inplane rotors, and are compared with those obtained from a two-dimensional quasisteady strip theory with steady and uniform inflow.

Trailing Vortex Effects on Large Receiver Aircraft

Abstract: The aerodynamic forces and moments on the Hercules receiver aircraft, due to its horizontal and vertical position and bank, yaw, and pitch attitudes in the wake of the KC10 tanker aircraft, are assessed relative to the receiver's aerodynamic characteristics in free air. Large changes in lift, drag, and pitching moment are predicted near the tanker wake centerline. As the receiver is displaced sideways towards the tanker wingtip vortices it experiences large side force and yawing moment and particularly high rolling moment. The most significant term due to the receiver attitude is the rolling moment due to bank. b C»t CL C. ^'V. cm '" H ,

Kill Probability in Antiaircraft Firing Theory

Abstract: The control parameter Kf in Eq. (9) is the only variable needed to fine-tune the vortex-burst predictions. It was adjusted for the 60-deg delta wing to achieve onset of vortex burst at the wing trailing edge at a = 13 deg. A value of 2.6 for Kf gave this result. When this same value of Kf was used for VBM/VLM computations over a range of angles of attack for each of the wing shapes tested, the vortex burst locations shown in Fig. 1 resulted. Wind-tunnel values for burst locations on similar models from Refs. 8 and 9 are plotted in Fig. 2 for comparison. Note the good agreement between the model and wind-tunnel data for all shapes tested.

Study of propeller-rudder interaction based on a linear method

Abstract: A linear method which calculates the steady forces of the propeller-rudder system working in a uniform flow is presented. A propeller with an infinite number of blades is used and modelled by a bound vortex sheet, a hub vortex filament and a series of free vortex sheets with an assumption that the free vortex sheets are undisturbed by the rudder. A lifting-line analysis method is incorporated to estimate the propeller performance characteristics. The rudder is represented by vortex and source/sink filaments on its mean plane. A vortex lattice method is used to estimate the rudder performance characteristics. Comparisons of the steady forces with experiments show a reasonable agreement. A rudder located in the accelerating flow just behind a propeller experiences a pressure drag. It is shown that this drag is an internal force which is counterbalanced by an increased propeller thrust. Due to the propeller induced tangential velocities in the slipstream a rudder thrust is created and the work done by the rudder thrust is a measure of the energy recovered by the rudder. The rudder in the test cases can recover up to 40% of the rotational energy and can increase the open water efficiency by up to 1.7%.

A simple surface panel method to predict steady marine propeller performance

Abstract: We present a simple surface panel method to predict steady marine propeller performance. This method uses source distributions (Hess and Smith type) on the wing surface and discrete vortex distributions arranged on the camber surface according to Lan's quasi-continuous vortex lattice method (QCM). Then these singularities are determined at a time by solving simultaneously both the wing surface and the camber surface conditions that the normal velocity should be zero. Since these singularities satisfy automatically the Kutta condition, we need not use any iterative procedure. We named this method SQCM (Source and QCM).We show numerical results for four kinds propellers (DTRC 4119, DTRC 4842, Seiunmaru conventional and highly skewed propellers) in this paper. Pressure distributions obtained by the present method are in good agreement with the experimental data and other numerical results. Though the viscous effects are not taken into consideration in the present calculations, the calculated thrust and torque coefficients agree qualitatively with experimental results.

Design of high performance supercavitating propellers based on a vortex lattice method

Abstract: This paper describes a rigorous design method for supercavitating propellers (SCP). An optimum circulation distribution was calculated by Lerbs' lifting line theory. Lifting surface correction based on a vortex lattice method was directly made by using the most favourable load distribution based on a non-linear supercavitating flow theory. Three SCPs were designed and they all generated the required thrust at the design point except the highly loaded one. The efficiencies were reasonably high for all propellers. It is concluded that the method is one of the most promising and reliable tools for designing high performance SCPs.

A review of the influence of high angle of attack aerodynamics on aircraft dynamic stability

Abstract: This paper presents a review of the most important flow phenomena associated with flying at high angles of attack, as well as behavior of aerodynamic stability derivatives under such flight conditions A short note is given of problems encountered by todays fighter aircraft, when flying at a high angle of attack Discussion of reliable aerodynamic and flight mechanics models is also included The paper emphasises the fact that, for a correct prediction of aircraft motion at high angle of attack, significant changes in aerodynamic stability derivatives should be taken into consideration New concepts in combat aircraft design require more attention to be given to the prediction of aircraft dynamic behavior at extreme maneuvers At the early stages of the developing program, where wind-tunnel results are still not fully available, simple and reliable computational and prediction techniques of non-linear aerodynamic characteristics are required In some flow regimes, especially for thin wings having sharp leading edges and/or tips the nonlinear Vortex Lattice Method can be recommended as giving load distribution and stability derivatives in good agreement with the experimental results

Investigation into hub effect of marine propeller by surface vortex lattice method

Abstract: In this paper the hub effects of some model propellers are investigated by the surface vortex lattice method for the hub and by the ordinary vortex lattice method on the lifting surface for the blade. The surface, where the surface vortex lattices are distributed, is taken inside of the real surface of the hub in order to avoid the strong interference effect between the root vortex lattices of the blade and the surface vortex lattices of the hub. It is found that the hub effect is noticeable on the pressure distribution on the blade surface near the root but smaller on the outer blade surface toward the tip, but that the hydrodynamic forces of the propeller such as thrust and torque are scarcely affected by the existence of the hub of the normal size. The calculated pressure distributions on the blade including the hub effect show good or poor agreement with the measured results. The pressure distribution on the hub surface is calculated and assessed. Calculations also show that the hub effect is more significant for the larger hub diameter such as controllable pitch propellers, as expected,

Quasi-Steady Aerodynamic Analysis of Propeller-wing Interaction

Abstract: The Quasi-steady aerodynamic coupling between a propeller and a wing is analyzed using the 3-D vortex lattice method based on the unsteady lifting line theory for the propeller and the 3-D panel method based on the unsteady lifting surface theory for the wing. The periodic loads are decomposed into harmonics and the harmonic amplitudes are found iteratively. Each stage of the iteration involves the solution of an isolated propeller or wing problem, the interaction being done through the Fourier Transform of the induced velocity field. This method was validated by comparing the predicted velocity field about an isolated propeller with detailed laser doppler velocimeter measurements and by comparison with mean loads measured in a wing-propeller experiments. Comparisons have also been made among the fluctuating loads predicted by the present method an unsteady panel scheme and a quasi-steady vortex lattice scheme.