Showing papers on "Helicopter rotor published in 1990"

An unsteady helicopter rotor-fuselage aerodynamic interaction analysis

Abstract: A computational method has been developed to treat the unsteady aerodynamic interaction between a helicopter rotor, wake, and fuselage. Two existing codes, a lifting line-prescribed wake rotor analysis and a source panel fuselage analysis, were modified and coupled to allow prediction of unsteady fuselage pressures and airloads. A prescribed displacement technique was developed to position the rotor wake about the fuselage. Also coupled into the method were optional blade dynamics or rigid blade performance analyses to set the rotor operating conditions. Sensitivity studies were performed to determine the influence of the wake and fuselage geometry on the computational results. Solutions were computed for an ellipsoidal fuselage and a four bladed rotor at several advance ratios, using both the classical helix and the generalized distorted wake model. Results are presented that describe the induced velocities, pressures, and airloads on the fuselage and the induced velocities and bound circulation at the rotor. The ability to treat arbitrary geometries was demonstrated using a simulated helicopter fuselage. Initial computations were made to simulate the geometry of an experimental rotor-fuselage interaction study performed at the Georgia Institute of Technology.

Helicopter rotor dynamics and aeroelasticity - Some key ideas and insights

Abstract: Four important current topics in helicopter rotor dynamics and aeroelasticity are discussed: (1) the role of geometric nonlinearities in rotary-wing aeroelasticity; (2) structural modeling, free vibration, and aeroelastic analysis of composite rotor blades; (3) modeling of coupled rotor/fuselage areomechanical problems and their active control; and (4) use of higher-harmonic control for vibration reduction in helicopter rotors in forward flight. The discussion attempts to provide an improved fundamental understanding of the current state of the art. In this way, future research can be focused on problems which remain to be solved instead of producing marginal improvements on problems which are already understood.

Helicopter trim with flap-lag-torsion and stall by an optimized controller

Abstract: An autopilot is applied to helicopter rotor flap-lag-torsion equations to obtain the control settings for a trimmed flight condition. The rotor aerodynamic description includes a stage-space dynamic stall model for lift and for pitching moments. Thus, the rotor is trimmed for flight conditions in which significant stall and torsional deformations are present. The autopilot is extended to Q-bladed rotors by a series of time-delay terms. As a result, the optimum gains and time constants depend upon the number of blades as well as upon the torsional stiffness.

Aerodynamic characteristics of two rotorcraft airfoils designed for application to the inboard region of a main rotor blade

Abstract: A wind tunnel investigation was conducted to determine the 2-D aerodynamic characteristics of two new rotorcraft airfoils designed especially for application to the inboard region of a helicopter main rotor blade. The two new airfoils, the RC(4)-10 and RC(5)-10, and a baseline airfoil, the VR-7, were all studied in the Langley Transonic Tunnel at Mach nos. from about 0.34 to 0.84 and at Reynolds nos. from about 4.7 to 9.3 x 10 (exp 6). The VR-7 airfoil had a trailing edge tab which is deflected upwards 4.6 degs. In addition, the RC(4)-10 airfoil was studied in the Langley Low Turbulence Pressure Tunnel at Mach nos. from 0.10 to 0.44 and at Reynolds nos. from 1.4 to 5.4 x 10 (exp 6) respectively. Some comparisons were made of the experimental data for the new airfoils and the predictions of two different theories. The results of this study indicates that both of the new airfoils offer advantages over the baseline airfoil. These advantages are discussed.

Development of UMARC (University of Maryland Advanced Rotorcraft Code)

Abstract: The University of Maryland Advanced Rotorcraft Code (UMARC) is a user-friendly, FEM-based comprehensive helicopter rotor simulation code of high numerical robustness and computational efficiency. UMARC formulates the rotor-fuselage equations using Hamilton's principle, and are discretized using finite elements in space and time. The FEM formulation allows the code to analyze a wide variety of rotor designs. Dynamic inflow modeling is used for unsteady wake inflow computations. Predicted stability, response, and blade-load data are validated with experimental data for several configurations, including representative articulated, hingeless, and bearingless rotors.

Vibration Transmission through Rolling Element Bearings in Geared Rotor Systems

Abstract: A new mathematical model is proposed to examine the vibration transmission through rolling element bearings in geared rotor systems. Current bearing models, based on either ideal boundary conditions for the shaft or purely translational stiffness element description, cannot explain how the vibratory motion may be transmitted from the rotating shaft to the casing. Experimental results have shown that the casing plate motion is primarily flexural. Here, this issue is clarified qualitatively and quantitatively by developing a comprehensive bearing stiffness matrix of dimension 6 to model precision rolling element bearings using basic principles. The proposed bearing stiffness matrix is partially verified using available analytical and experimental data, and is completely characterized. The study extends the proposed bearing formulation to analyze the overall geared rotor system dynamics including casing and mounts. The bearing stiffness matrix is included in discrete system models using lumped parameter and/or dynamic finite element techniques.

Free wake analysis of hover performance using a new influence coefficient method

Abstract: A new approach to the prediction of helicopter rotor performance using a free wake analysis was developed. This new method uses a relaxation process that does not suffer from the convergence problems associated with previous time marching simulations. This wake relaxation procedure was coupled to a vortex-lattice, lifting surface loads analysis to produce a novel, self contained performance prediction code: EHPIC (Evaluation of Helicopter Performance using Influence Coefficients). The major technical features of the EHPIC code are described and a substantial amount of background information on the capabilities and proper operation of the code is supplied. Sample problems were undertaken to demonstrate the robustness and flexibility of the basic approach. Also, a performance correlation study was carried out to establish the breadth of applicability of the code, with very favorable results.

Dynamic Analysis of a Rotor-Journal Bearing System for Twin Rotary Compressors

Abstract: A rotor-journal bearing system for a twin rotary compressor has been nlliDerically analyzed to estimate the reliability and to optimize the bearing design For rotary compressors, large dynamic loads act on the rotor They are unbalanced forces due to eccentric rotation parts and gas forces induced by the difference in pressure between compression and suction gases In such a case, the rotor-journal bearing system becomes nonlinear, because the stiffness and damping coefficients of the lubricating oil film in the bearings vary Such a rotor system is solved as a coupled problem of momentum equations and Reynolds equations for all the bearings The analytical procedure and the results are described in this paper

Minimum weight design of rotorcraft blades with multiple frequency and stress constraints

Abstract: Minimum weight designs of helicopter rotor blades with constraints on multiple coupled flap-lag natural frequencies are studied. Constraints are imposed on the minimum value of the blade autorotational inertia to ensure sufficient rotary inertia to autorotate in case of engine failure and on stresses to guard against structural failure due to blade centrifugal forces. Design variables include blade taper ratio, dimensions of the box beam located inside the airfoil and magnitudes of nonstructural weights. The program CAMRAD is used for the blade modal analysis; the program CONMIN is used for the optimization. A linear approximation involving Taylor series expansion is used to reduce the analysis effort. The procedure contains a sensitivity analysis consisting of analytical derivatives for objective function and constraints on autorotational inertia and stresses. Central finite difference derivatives are used for frequency constraints. Optimal designs are obtained for both rectangular and tapered blades. Using this method, it is possible to design a rotor blade with reduced weight, when compared to a baseline blade, while satisfying all the imposed design requirements.

Optimal placement of tuning masses for vibration reduction in helicopter rotor blades

Abstract: Described are methods for reducing vibration in helicopter rotor blades by determining optimum sizes and locations of tuning masses through formal mathematical optimization techniques. An optimization procedure is developed which employs the tuning masses and corresponding locations as design variables which are systematically changed to achieve low values of shear without a large mass penalty. The finite-element structural analysis of the blade and the optimization formulation require development of discretized expressions for two performance parameters: modal shaping parameter and modal shear amplitude. Matrix expressions for both quantities and their sensitivity derivatives are developed. Three optimization strategies are developed and tested. The first is based on minimizing the modal shaping parameter which indirectly reduces the modal shear amplitudes corresponding to each harmonic of airload. The second strategy reduces these amplitudes directly, and the third strategy reduces the shear as a function of time during a revolution of the blade. The first strategy works well for reducing the shear for one mode responding to a single harmonic of the airload, but has been found in some cases to be ineffective for more than one mode. The second and third strategies give similar results and show excellent reduction of the shear with a low mass penalty.

Response and hub loads sensitivity analysis of a helicopter rotor

Abstract: A sensitivity study of blade response and oscillatory hub loads in forward flight for a hingeless rotor with respect to structural design variables is examined. Structural design variables include nonstructural mass distribution (spanwise and chord wise), chordwise offset of center of gravity, and blade bending stiffnesses (flap, lag, and torsion). The blade is discretized into a number of beam elements, and response equations are transformed into the normal mode space. The nonlinear, periodic, steady response of the blade is calculated using a finite element in time approach. Then, the vehicle trim and blade steady response are calculated iteratively as one coupled solution using a modified Newton method. The formulation for the derivatives of blade response and hub loads is implemented, using a direct analytical approach (chain rule differentiation), and forms an integral part of the basic response analysis. For calculation of the sensitivity derivatives, a 96% reduction of CPU time is achieved by using the direct analytical approach, compared with the finite-difference approach.

Stability sensitivity analysis of a helicopter rotor

Abstract: A sensitivity study of blade stability in forward flight for a hingeless rotor with respect to design variables is carried out using a direct analytical method. Design variables include nonstructural mass distribution (spanwise and chord wise), chordwise offset of center of gravity, and blade bending stiffnesses (flap, lag, and torsion). The formulation for blade steady response is based on a finite-element method in space and time. The vehicle trim and blade steady response are calculated iteratively as one coupled solution using a modified Newton method (coupled trim analysis). Eigenvalues corresponding to different blade modes are calculated using Floquet transition matrix theory. The formulation for the derivatives of the eigenvalues with respect to the design variables is implemented using a direct analytical approach and constitutes an integral part of the regular stability analysis. For the calculation of the stability derivatives with respect to a total of 30 design variables, there is an 85% reduction in CPU time using the direct analytical approach compared to the frequently adopted finite-difference approach. A parametric study showed that nonstructural mass and chordwise blade e.g. offset of outboard elements, and lag bending stiffness of inboard elements, have powerful influence on blade stability.

Development of active magnetic bearings for high speed rotors

Abstract: A magnetically fully levitated rotor system has been designed and built. It consists of stator and rotor bearings for the rotating shaft, an electronic control system, an optical sensing system, and power amplifiers. For an application in which the rotor spins at high speeds in a vacuum, its position is controlled by infrared light sensors. The system has been shown to spin at frequencies of up to 140 Hz, and an extension of the rotational frequency to 1000 Hz is envisaged. The rotor vibrations have been shown to occur at theoretically predicted frequencies. The amplitudes of radial vibrations were kept below +or-22 mu m and mainly occurred at around +or-10 mu m. >

Rotor noise due to atmospheric turbulence ingestion. II - Aeroacoustic results

Abstract: A computer program for the prediction of noise due to the turbulence of inflow to a propeller or helicopter rotor is extended to the case of nonisotropic turbulence, on the basis of a combined mean flow contraction model and rapid distortion theory. The mean flow distortion is noted to stretch the turbulence, decreasing the velocities along the principal axis of the stretching. In the case of a principal stretching axis lying close to the rotor axis, the distortion acts to decrease the upwash velocities of the rotor: thereby decreasing the noise from levels associated with isotropic turbulence. Acoustic energies are calculated at observer location for several cases, and compared to the turbulence energy as affected by the contraction.

Application of the Wide-Field Shadowgraph Technique to Helicopters in Forward Flight

Abstract: A test was conducted to examine the feasibility of using the wide-field shadowgraph method for visualizing the wake of a small-scale helicopter rotor in forward flight. A wide range of test conditions were examined, including thrust and forward speed variations. Shadowgraphs from the side and top of the rotor were obtained. The visibility of the tip vortices using the shadowgraph method was documented for advance ratios up to 0.175. It was demonstrated that shadowgraphs of the rotor wake in forward flight can be used to obtain qualitative and quantitative information about wake geometry, wake/body interactions, and blade/vortex interactions. Video cameras were used for the first time to record the shadowgraphs. These provided several advantages compared to still cameras and greatly enhanced the capabilities of the shadowgraph method.

Flowfield of a lifting hovering rotor: A Navier-Stokes simulation

Abstract: The viscous, three-dimensional flowfield of a lifting helicopter rotor in hover is calculated by using an upwind, implicit, finite-difference numerical method for solving the thin layer Navier-Stokes equations. The induced effects of the wake, including the interaction of tip vortices with successive blades, are calculated as a part of the overall flowfield solution without using any ad hoc wake models. Comparison of the numerical results for the subsonic and transonic conditions show good agreement with the experimental data and with the previously published Navier-Stokes calculations using a simple wake model. Some comparisons with Euler calculations are also presented, along with some discussions of the grid refinement studies.

Helicopter control with multiple schedule rotor speed decay anticipator

Abstract: The speed 54,56 of the free turbine 40 of a helicopter engine 22 is compared 134,188 with the speed 142,140 of the helicopter rotor 10 to indicate 106,108 a specific magnitude of autorotatian and the deceleration 150 of the rotor above either one of two threshold magnitudes 220,222 (dependent on the magnitude of autorotation) is utilized 81,68,69 to increase fuel flow 72 to the engine according to a specific schedule 160,162, determined by the type of autorotation, in anticipation of rotor speed droop which would otherwise occur during recovery from the autorotation maneuver.

Superposition Method for Stress Stiffening in Flexible Multibody Dynamics.

Abstract: For dynamics of flexible multibody systems the deformation of the bodies is described by global assumed modes. The development of geometric stiffness terms due to steady state loads is presented. In particular, the stress stiffening matrix with respect to nominal stresses is derived by superposition of unit stress stiffening matrices and scaling them by actual nominal loads. The method is applied to a flexible pendulum and a helicopter rotor blade. The results are compared with those obtained by a finite elements analysis.

Blades of helicopter rotor - are stabilised by stabilising rods connected to swash plate

Abstract: The rotor of a helicopter, especially a model helicopter, is stabilised by means of cyclical adjustable blades (7) which are actuated by means of a swash plate (4). The rotor has three main blades (1) and a corresponding number of stabilising rods (6) which are attached to a swinging cardan ring (5) which is cyclically controlled by the swash plate (4). The inclination of the cardan ring, corresponding to the cyclical actuation of the stabilising rods (6), is transmitted with a phase difference of 90 deg. to the main rotor blades (1). USE - Helicopter rotors.

The Acoustic Results of a United Technologies Scale Model Helicopter Rotor Tested at DNW

Abstract: In a major cooperative program between U.S. Government agencies (represented by the U.S. Army Aeroflightdynamics Directorate and NASA Ames and Langley Research Centers) and United Technologies Corp., a 1/6 geometrically and aeroelastically scaled UTC model helicopter rotor was tested in the open-jet anechoic test section of the Duits-Nederlandse Windtunnel in the Netherlands. As the fourth entry under the Aerodynamic and Acoustic Testing of Model Rotors Program, several comprehensive acoustic and aerodynamic databases were obtained relating the important aerodynamic phenomena to both the near- and far-field acoustic radiation. In particular, high speed impulsive noise and blade-vortex interaction are of primary interest. This paper provides an initial summary of the acoustic measurements acquired for some of the different configurations tested. A review of the baseline swept tip rotor acoustic characteristics in the regimes of high speed forward flight, where high speed impulsive noise dominates, and low speed descent, where severe blade vortex interaction noise occurs, is presented. The trends of these primary noise sources are studied as the first step in validating the data for release and application.

Wind-Tunnel Survey of an Oscillating Flow Field for Application to Model Helicopter Rotor Testing

Abstract: A survey was conducted of the flow field produced by the Airstream Oscillator System (AOS) in the Langley Transonic Dynamics Tunnel (TDT). The magnitude of a simulated gust field was measured at 15 locations in the plane of a typical model helicopter rotor when tested in the TDT using the Aeroelastic Rotor Experimental System (ARES) model. These measurements were made over a range of tunnel dynamic pressures typical of those used for an ARES test. The data indicate that the gust field produced by the AOS is non-uniform across the tunnel test section, but should be sufficient to excite a model rotor.

Tabulation of data from the tip aerodynamics and acoustics test

Abstract: In a continuing effort to understand helicopter rotor tip aerodynamics and acoustics, researchers at Ames Research Center conducted a flight test. The test was performed using the NASA White Cobra and a set of highly instrumented blades. Tabular and graphic summaries of two data subsets from the Tip Aerodynamics and Acoustics Test are given. The data presented are for airloads, blade structural loads, blade vibrations, with summary tables of the aircraft states for each test point. The tabular data consist of the first 15 harmonics only, whereas the plots contain the entire measured frequency content.

Can shock waves on helicopter rotors generate noise? - A study of the quadrupole source

Abstract: An analysis has previously established that local shock surfaces attached to helicopter rotor blades moving at high subsonic speeds are potent noise generators; in pursuit of this insight, a novel formulation is presented for the prediction of the noise of a deformable shock, whose area changes as a function of the azimuthal position of the blade. The derivation of this formulation has its basis in a mapping of the moving shock to a time-independent region. In virtue of this mapping, the implementation of the main result on a computer becomes straightforward enough for incorporation into the available rotor-noise prediction code. A problem illustrating the importance of rotor shocks in the generation of high-intensity noise is presented.