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At least 901 records · Page 50

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NASA NTRS1985

Hover and forward flight acoustics and performance of a small-scale helicopter rotor system

A 2.1-m diam., 1/6-scale model helicopter main rotor was tested in hover in the test section of the NASA Ames 40- by 80- Foot Wind Tunnel. Subsequently, it was tested in forward flight in the Ames 7- by 10-Foot Wind Tunnel. The primary objective of the tests was to obtain performance and noise data on a small-scale rotor at various thrust coefficients, tip Mach numbers, and, in the later case, various advance ratios, for comparisons with similar existing data on full-scale helicopter rotors. This comparison yielded a preliminary evaluation of the scaling of helicopter rotor performance and acoustic radiation in hover and in forward flight. Correlation between model-scale and full-scale performance and acoustics was quite good in hover. In forward flight, however, there were significant differences in both performance and acoustic characteristics. A secondary objective was to contribute to a data base that will permit the estimation of facility effects on acoustic testing.

Kitaplioglu, C.↗

NASA NTRS1986

Effect of a rotor wake on the local heat transfer on the forward half of a circular cylinder

Turbine rotor-stator wake dynamics was simulated by a spoked wheel rotating in annular flow, generating rotor wakes. Spanwise averaged circumferentially local heat transfer in the circular cylindrical leading edge region of a turbine airfoil was obtained. Reynolds numbers ranged from 35,000 to 175,000. Strouhal numbers ranged from 0.63 to 2.50. Wakes were generated by 2 sets of circular cylindrical bars, 1.59 and 3.18 mm in diameter. The rotor could be rotated either clockwise or counterclockwise. Grid turbulence was introduced upstream yielding freestream turbulence of 1.0 to 2.5% at the stator. Data represented an extensive body of local heat transfer coefficients, which can be used to model the leading edge region of a turbine airfoil. In the presence of rotor wakes, an asymmetry from the leeward to windward side was noted. Windward side levels were 30 to 40% higher than the corresponding leeward side.

Morehouse, K. A.↗

NASA NTRS1986

Effects of blade-to-blade dissimilarities on rotor-body lead-lag dynamics

Small blade-to-blade property differences are investigated to determine their effects on the behavior of a simple rotor-body system. An analytical approach is used which emphasizes the significance of these effects from the experimental point of view. It is found that the primary effect of blade-to-blade dissimilarities is the appearance of additional peaks in the frequency spectrum which are separated from the convention response modes by multiples of the rotor speed. These additional responses are potential experimental problems because when they occur near a mode of interest they act as contaminant frequencies which can make damping measurements difficult. The effects of increased rotor-body coupling and a rotor shaft degree of freedom act to improve the situation by altering the frequency separation of the modes.

Mcnulty, M. J.↗

NASA NTRS1985

Linear and nonlinear dynamic analysis of redundant load path bearingless rotor systems

The bearingless rotorcraft offers reduced weight, less complexity and superior flying qualities. Almost all the current industrial structural dynamic programs of conventional rotors which consist of single load path rotor blades employ the transfer matrix method to determine natural vibration characteristics because this method is ideally suited for one dimensional chain like structures. This method is extended to multiple load path rotor blades without resorting to an equivalent single load path approximation. Unlike the conventional blades, it isk necessary to introduce the axial-degree-of-freedom into the solution process to account for the differential axial displacements in the different load paths. With the present extension, the current rotor dynamic programs can be modified with relative ease to account for the multiple load paths without resorting to the equivalent single load path modeling. The results obtained by the transfer matrix method are validated by comparing with the finite element solutions. A differential stiffness matrix due to blade rotation is derived to facilitate the finite element solutions.

Murthy, V. R.↗

NASA NTRS1985

An experimental and analytical method for approximate determination of the tilt rotor research aircraft rotor/wing download

The XV-15 Tilt Rotor Research Aircraft download phenomenon was analyzed. This phenomenon is a direct result of the two rotor wakes impinging on the wing upper surface when the aircraft is in the hover configuration. For this study the analysis proceeded along tow lines. First was a method whereby results from actual hover tests of the XV-15 aircraft were combined with drag coefficient results from wind tunnel tests of a wing that was representative of the aircraft wing. Second, an analytical method was used that modeled that airflow caused gy the two rotors. Formulas were developed in such a way that acomputer program could be used to calculate the axial velocities were then used in conjunction with the aforementioned wind tunnel drag coefficinet results to produce download values. An attempt was made to validate the analytical results by modeling a model rotor system for which direct download values were determinrd..

Jordon, D. E.↗

NASA NTRS1985

Instability analysis procedure for 3-level multi-bearing rotor-foundation systems

A procedure for the instability analysis of a three-level multispan rotor systems is described. This procedure is based on a distributed mass elastic representation of the rotor system in several eight-coefficient bearings. Each bearing is supported from an elastic foundation on damped, elastic pedestals. The foundation is represented as a general distributed mass elastic structure on discrete supports, which may have different stiffness and damping properties in the horizontal and vertical directions. This system model is suited to studies of instability threshold conditions for multirotor turbomachines on either massive or flexible foundations. The instability conditions is found by obtaining the eigenvalues of the system determinant, which is obtained by the transfer matrix method from the three-level system model. The stability determinant is solved for the lowest rotational speed at which the system damping becomes zero in the complex eigenvalue, and for the whirl frequency corresponding to the natural frequency of the unstable mode. An efficient algorithm for achieving this is described. Application of this procedure to a rigid rotor in two damped-elastic bearings and flexible supports is described. A second example discusses a flexible rotor with four damped-elastic bearings. The third case compares the stability of a six-bearing 300 Mw turbine generator unit, using two different bearing types. These applications validate the computer program and various aspects of the analysis.

Zhou, S.↗

NASA NTRS1985

Backward whirl in a simple rotor supported on hydrodynamic bearings

The asymmetric nature of the fluid film stiffness and damping properties in rotors supported on fluid film bearings causes a forward or a backward whirl depending on the bearing parameters and the speed of the rotor. A rotor was designed to exhibit backward synchronous whirl. The rotor-bearing system exhibited split criticals, and a backward whirl was observed between the split criticals. The orbital diagrams show the whirl pattern.

Subbiah, R.↗

NASA NTRS1985

Partial rotor-to-stator rub demonstration

A rotor radial rub typically occurs in seals or at a blade tip or shroud when there is insufficient clearance, high vibration, or the shaft equilibrium position has been displaced to effectively limit the clearance (eccentricity). There are two extreme cases of radial rubs: full annular rub, when the rotor maintains continuous contact with the seal, etc.; and a partial rub, when the contact occurs during a fraction of the precession period. They both involve similar physical phenomena such as friction and modification of stiffness. In partial rubs with consecutive impacts, a significant average value of radial force is generated. This results in shaft average displacement in the direction opposite the rub location. The rotor rig demonstrates the characteristics of a partial lateral rub of varying severity and location. These characteristics include: (1) subharmonic components as a function of rotative speed/first balance resonance ratio and radial force; (2) higher harmonic content as a function of severity; (3) increased average rotor stiffness resulting in increased first balance resonance speed; and (4) change in overall orbital pattern as a sum of the unbalance response (1x) and subharmonic response (1nx).

Grissom, R.↗

NASA NTRS1984

Flight testing the fixed wing configuration of the Rotor Systems Research Aircraft (RSRA)

The Rotor Systems Research Aircraft (RSRA) is a unique research aircraft designed to flight test advanced helicopter rotor system. Its principal flight test configuration is as a compound helicopter. The fixed wing configuration of the RSRA was primarily considered an energy fly-home mode in the event it became necessary to sever an unstable rotor system in flight. While it had always been planned to flight test the fixed wing configuraion, the selection of the RSRA as the flight test bed for the x-wing rotor accelerated this schedule. This paper discusses the build-up to, and the test of, the RSRA fixed wing configuration. It is written primarily from the test pilot's perspective.

Hall, G. W.↗

NASA NTRS1986

Scaling of helicopter main rotor noise in hover

A 2.1-m-diam, one-sixth-scale model helicopter main rotor was tested in hover at the NASA Ames Outdoor Aerodynamic Research Facility. It had previously been tested in hover in the NASA Ames 40- by 80-Foot Wind Tunnel test section. The primary objective of the tests was to obtain acoustic and performance data on a medium-scale rotor at various thrust coefficients and tip Mach numbers to compare to similar existing data on a full-scale helicopter main rotor. Information is presented on the effects of thrust- and tip-speed variation, the effects of boundary-layer trip strips, and the effects of wind conditions. In addition, a preliminary evaluation of the scaling of helicopter main-rotor noise in hover is presented. A secondary objective was to contribute to a data base that will permit the estimation of facility effects on acoustic testing.

Kitaplioglu, C.↗

NASA NTRS1984

Applications of an analysis of axisymmetric body effects on rotor performance and loads

A computationally efficient body analysis is developed and coupled with a comprehensive helicopter analysis to calculate the body-induced aerodynamic effects on rotor performance and loads. A modified slender body theory is used as the body model. With the objective of demonstrating the accuracy, efficiency, and application of the method, the analysis at this stage is restricted to axisymmetric bodies at zero angle of attack. By comparing with results from an exact analysis for simple body shapes, it is found that the modified slender body theory provides an accurate potential flow solution for moderately thick bodies, with only a 10-20 percent increase in computational effort over that of an isolated rotor analysis. The computational ease of this method provides a means for routine assessment of body-induced effects on a rotor. Results are given for several configurations that typify those being used in the NASA Ames 40 x 80-ft Wind Tunnel and in the rotor-body interference tests being conducted at NASA Ames.

Johnson, W.↗

NASA NTRS1985

Forced response analysis of an aerodynamically detuned supersonic turbomachine rotor

The effect of aerodynamic detuning on the supersonic flow induced forced response behavior of a turbomachine blade row is analyzed using an aeroelastic model. The rotor is modeled as a flat plate airfoil cascade representing an unwrapped rotor annulus; the aerodynamic detuning is achieved by alternating the circumferential spacing of adjacent rotor blades. The total unsteady aerodynamic loading on the blading, due to the convection of the transverse gust past the airfoil cascade as well as that resulting from the motion of the cascade, is developed in terms of influence coefficients. The model developed here is then used to analyze the effect of aerodynamic detuning on the flow induced forced response behavior of a twelve-bladed rotor with Verdon's Cascade B flow geometry.

Hoyniak, D.↗

NASA NTRS1986

Wind tunnel test of a model rotor with a free-tip

A model helicopter rotor with a free tip was tested in a wind tunnel to show the effect of the free tip on rotor performance and oscillatory loads. The free-tip rotor has blade tips that are free to pitch independently of the remainder of the blade. A nearly constant pitching moment is applied to the tip so the tip will produce a nearly steady lift around the azimuth. The aerodynamic development of the free tip is reviewed. The development process concentrated on configuring the tip to have the capability of fast response to angle-of-attack perturbations. Wind tunnel test results show the free-tip rotor configuration requires less power than a fixed-tip configuration in forward flight. In addition, the free tip causes substantial reduction in blade-flapwise vibratory loads. However, blade-chordwise loads were increased. Pitch-link vibratory loads were also reduced by the free-tip configuration. Discussion is presented on the reasons that the use of the free tip caused reduced power and smaller vibratory loads.

Stroub, R. H.↗

NASA NTRS1986

Euler solutions for the flow around a hovering helicopter rotor

A method for computing the flow around a hovering helicopter rotor is presented. The solution procedure is divided into two parts. The first part is a finite volume solution of the Euler equations for the near field flow around a rotor blade. The second part is a free wake approach for determining the wake geometry and induced velocities. These two parts are solved in a coupled fashion. The rolled-up vortex wake of the rotor is included in the Euler computational domain through a nonlinear perturbation technique. This eliminates the numerical diffusion of the wake vorticity due to truncation error and artificial viscosity without the need for excessive grid resolution near the vortex core. The method is used to compute both hovering rotor and wing/vortex interaction flows, and the results are compared to experiment.

Roberts, T. W.↗

NASA NTRS1986

Computation of transonic flow about helicopter rotor blades

An inviscid, nonconservative, three-dimensional full-potential flow code, ROT22, has been developed for computing the quasi-steady flow about a lifting rotor blade. The code is valid throughout the subsonic and transonic regime. Calculations from the code are compared with detailed laser velocimeter measurements made in the tip region of a nonlifting rotor at a tip Mach number of 0.95 and zero advance ratio. In addition, comparisons are made with chordwise surface pressure measurements obtained in a wind tunnel for a nonlifting rotor blade at transonic tip speeds at advance ratios from 0.40 to 0.50. The overall agreement between theoretical calculations and experiment is very good. A typical run on a CRAY X-MP computer requires about 30 CPU seconds for one rotor position at transonic tip speed.

Arieli, R.↗

NASA NTRS1986

Rotor tip vortex geometry measurements using the wide-field shadowgraph technique

Rotor tip vortex geometry data have been acquired using the wide-field shadowgraph technique. Shadowgraphs were taken of the wakes of two model main-rotor systems in hover. These shadowgraphs provided detailed tip vortex trajectories from which axial and radial vortex coordinates were measured. Using these coordinates, prescribed-wake parameters were determined and input into a hover-performance prediction code. The resultant predictions were compared with experimental performance data to help assess the capabilities of this code. Another result of the experimental work was the development of a method to predict the visibility of rotor tip vortices on shadowgraphs. Using this method, the ability of the shadowgraph technique to visualize tip vortices in the rotor wake was quantified.

Norman, T. R.↗

NASA NTRS1986

Eigenvalue assignment strategies in rotor systems

The work done to establish the control and direction of effective eigenvalue excursions of lightly damped, speed dependent rotor systems using passive control is discussed. Both second order and sixth order bi-axis, quasi-linear, speed dependent generic models were investigated. In every case a single, bi-directional control bearing was used in a passive feedback stabilization loop to resist modal destabilization above the rotor critical speed. Assuming incomplete state measurement, sub-optimal control strategies were used to define the preferred location of the control bearing, the most effective measurement locations, and the best set of control gains to extend the speed range of stable operation. Speed dependent control gains were found by Powell's method to maximize the minimum modal damping ratio for the speed dependent linear model. An increase of 300 percent in stable speed operation was obtained for the sixth order linear system using passive control. Simulations were run to examine the effectiveness of the linear control law on nonlinear rotor models with bearing deadband. The maximum level of control effort (force) required by the control bearing to stabilize the rotor at speeds above the critical was determined for the models with bearing deadband.

Youngblood, J. N.↗

NASA NTRS1987

Comparison of composite rotor blade models: A coupled-beam analysis and an MSC/NASTRAN finite-element model

A methodology was developed for the structural analysis of composite rotor blades. This coupled-beam analysis is relatively simple to use compared with alternative analysis techniques. The beam analysis was developed for thin-wall single-cell rotor structures and includes the effects of elastic coupling. This paper demonstrates the effectiveness of the new composite-beam analysis method through comparison of its results with those of an established baseline analysis technique. The baseline analysis is an MSC/NASTRAN finite-element model built up from anisotropic shell elements. Deformations are compared for three linear static load cases of centrifugal force at design rotor speed, applied torque, and lift for an ideal rotor in hover. A D-spar designed to twist under axial loading is the subject of the analysis. Results indicate the coupled-beam analysis is well within engineering accuracy.

Hodges, Robert V.↗