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At least 685 records · Page 38
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Laser anemometer measurements in a transonic axial-flow fan rotor

Laser anemometer surveys were made of the 3-D flow field in NASA rotor 67, a low aspect ratio transonic axial-flow fan rotor. The test rotor has a tip relative Mach number of 1.38. The flowfield was surveyed at design speed at near peak efficiency and near stall operating conditions. Data is presented in the form of relative Mach number and relative flow angle distributions on surfaces of revolution at nine spanwise locations evenly spaced from hub to tip. At each spanwise location, data was acquired upstream, within, and downstream of the rotor. Aerodynamic performance measurements and detailed rotor blade and annulus geometry are also presented so that the experimental results can be used as a test case for 3-D turbomachinery flow analysis codes.

Strazisar, Anthony J.↗
NASA NTRS1989

Prediction and measurement of low-frequency harmonic noise of a hovering model helicopter rotor

Far-field acoustic data for a model helicopter rotor have been gathered in a large open-jet, acoustically treated wind tunnel with the rotor operating in hover and out of ground-effect. The four-bladed Boeing 360 model rotor with advanced airfoils, planform, and tip shape was run over a range of conditions typical of today's modern helicopter main rotor. Near in-plane acoustic measurements were compared with two independent implementations of classical linear theory. Measured steady thrust and torque were used together with a free-wake analysis (to predict the thrust and drag distributions along the rotor radius) as input to this first-principles theoretical approach. Good agreement between theory and experiment was shown for both amplitude and phase for measurements made in those positions that minimized distortion of the radiated acoustic signature at low-frequencies.

Aggarawal, H. R.↗
NASA NTRS1990

Effects of higher harmonic control on rotor performance and control loads

An analytical study, based on an advanced Higher Harmonic Control (HHC) analysis for helicopter rotor systems, is carried out to investigate HHC application for rotor performance enhancement. The effects of HHC on stall characteristics of rotor and blade pitch-link loads when the system is configured to suppress vibration are also examined. For vibration control, simulated results indicate that HHC may promote early blade stall. Effects of blade torsion frequencies on HHC performance are moderate, and torsionally stiff blades require less actuator power than torsionally soft blades. For rotor performance improvement, a 3 to 5 percent reduction in rotor shaft power can be achieved with 2 deg of two-per-rev blade pitch control.

Nguyen, Khanh↗
NASA NTRS1989

The response of helicopter rotors to vibratory airload

The problems of oscillatory and vibratory loading are examined from the perspective of the rotor's structural response. Blade flap, chord, and torsion responses, from flight or wind tunnel tests of eight full-scale rotors are examined and compared for high-speed flight conditions in the absence of blade stall or maneuvers. Some important similarities and differences between rotors are considered, and it is pointed out that these observations are useful in determining the appropriate tests for development of theoretical prediction methods. Among other things, major similarities were found in the dominant flapping vibratory response which occurs at 3/rev, and the root oscillatory chord bending for the CH-34, the SA 349/2, and the UH-60A; differences noted include vibratory chord bending-moment behavior of the rotors, a difference in the damper-induced behavior of CH-53A root oscillatory chord bending, and unique torsion and pitch link loading behavior exhibited by the AH1-G articulated rotor.

Bousman, William G.↗
NASA NTRS1990

Calculation of rotor impedance for use in design analysis of helicopter airframe vibrations

Excessive vibration is one of the most prevalent technical obstacles encountered in the development of new rotorcraft. The inability to predict these vibrations is primarily due to deficiencies in analysis and simulation tools. The Langley Rotorcraft Structural Dynamics Program was instituted in 1984 to meet long term industry needs in the area of rotorcraft vibration prediction. As a part of the Langley program, this research endeavors to develop an efficient means of coupling the rotor to the airframe for preliminary design analysis of helicopter airframe vibrations. The main effort was to modify the existing computer program for modeling the dynamic and aerodynamic behavior of rotorcraft called DYSCO (DYnamic System COupler) to calculate the rotor impedance. DYSCO was recently developed for the U.S. Army and has proven to be adaptable for the inclusion of new solution methods. The solution procedure developed to use DYSCO for the calculation of rotor impedance is presented. Verification of the procedure by comparison with a known solution for a simple wind turbine model is about 75 percent completed, and initial results are encouraging. After the wind turbine impedance is confirmed, the verification effort will continue by comparison to solutions of a more sophisticated rotorcraft model. Future work includes determination of the sensitivity of the rotorcraft airframe vibrations to helicopter flight conditions and rotor modeling assumptions. When completed, this research will ascertain the feasibility and efficiency of the impedance matching method of rotor-airframe coupling for use in the analysis of airframe vibrations during the preliminary rotorcraft design process.

Nygren, Kip P.↗
NASA NTRS1990

Turbomachinery rotor support with damping

Damping seals, damping bearings, and a support sleeve are presented for the ball bearings of a high speed rotor. The ball bearings consist of a duplex set having the outer races packaged tightly within the sleeve while the sleeve provides a gap with a support member so that the bearings may float with the sleeve. The sleeve has a web extending radially between the pair of outer races and acts in conjunction with one or more springs to apply an axial preload to the outer races. The sleeves have a series of slits which provide the sleeve with a spring-like quality so that the spring acts to center the rotor upon which the bearings are mounted during start up and shut down. A damping seal or a damping bearing may be used in conjunction with the ball bearings and supporting sleeve, the damping seal and damping bearing having rotor portions including rigid outer surfaces mounted within the bore of a stator portion having triangular shaped pockets on the surface facing the rotor. Axial gates are provided between adjacent pockets in sections of the stator permitting fluid to flow with less resistance axially relative to the flow of fluids circumferentially between the rotor and the stator.

Vonpragenau, George L.↗
NASA NTRS1990

A flight-dynamic helicopter mathematical model with a single flap-lag-torsion main rotor

A mathematical model of a helicopter system with a single main rotor that includes rigid, hinge-restrained rotor blades with flap, lag, and torsion degrees of freedom is described. The model allows several hinge sequences and two offsets in the hinges. Quasi-steady Greenberg theory is used to calculate the blade-section aerodynamic forces, and inflow effects are accounted for by using three-state nonlinear dynamic inflow model. The motion of the rigid fuselage is defined by six degrees of freedom, and an optional rotor rpm degree of freedom is available. Empennage surfaces and the tail rotor are modeled, and the effect of main-rotor downwash on these elements is included. Model trim linearization, and time-integration operations are described and can be applied to a subset of the model in the rotating or nonrotating coordinate frame. A preliminary validation of the model is made by comparing its results with those of other analytical and experimental studies. This publication presents the results of research compiled in November 1989.

Takahashi, Marc D.↗
NASA NTRS1991

Dynamic modelling and analysis of a magnetically suspended flexible rotor

A 12-state lumped-element model is presented for a flexible rotor supported by two attractive force electromagnetic journal bearings. The rotor is modeled as a rigid disk with radial mass unbalance mounted on a flexible, massless shaft with internal damping (Jeffcott rotor). The disk is offset axially from the midspan of the shaft. Bearing dynamics in each radial direction are modeled as a parallel combination of a negative (unstable) spring and a linear current-to-force actuator. The model includes translation and rotation of the rigid mass and the first and second bending models of the flexible shaft, and it simultaneously includes internal shaft damping, gyroscopic effects, and the unstable nature of the attractive force magnetic bearings. The model is used to analyze the dependence of the system transmission zeros and open-loop poles on system parameters. The dominant open-loop poles occur in stable/unstable pairs with bandwidth dependent on the ratios of bearing (unstable) stiffnesses to rotor mass and damping dependent on the shaft spin rate. The zeros occur in complex conjugate pairs with bandwidth dependent on the ratios of shaft stiffness to rotor mass and damping dependent on the shaft spin rate. Some of the transmission zeros are non-minimum phase when the spin rate exceeds the shaft critical speed. The transmission zeros and open-loop poles impact the design of magnetic bearing control systems. The minimum loop cross-over frequency of the closed-loop system is the speed of the unstable open-loop poles. For the supercritical shaft spin rates, the presence of non-minimum phase zeros limits the distribution rejection achievable at frequencies near or above the shaft critical speed. Since non-minimum phase transmission zeros can only be changed by changing the system inputs and/or outputs, closed-loop performance is limited for supercritical spin rates unless additional force or torque actuators are added.

Mccallum, Duncan C.↗
NASA NTRS1991

Vibratory Loads Data from a Wind-Tunnel Test of Structurally Tailored Model Helicopter Rotors

An experimental study was conducted in the Langley Transonic Dynamics Tunnel to investigate the use of a Bell Helicopter Textron (BHT) rotor structural tailoring concept, known as rotor nodalization, in conjunction with advanced blade aerodynamics as well as to evaluate rotor blade aerodynamic design methodologies. A 1/5-size, four-bladed bearingless hub, three sets of Mach-scaled model rotor blades were tested in forward flight from transition up to an advance ratio of 0.35. The data presented pertain only to the evaluation of the structural tailoring concept and consist of fixed-system and rotating system vibratory loads. These data will be useful for evaluating the effects of tailoring blade structural properties on fixed-system vibratory loads, as well as validating analyses used in the design of advanced rotor systems.

Yeager, William T., Jr.↗
NASA NTRS1990

NASA/Army rotor system flight research leading to the UH-60 airloads program

A review is presented of some of the early rotor systems flight research leading to the present comprehensive NASA/Army rotor system airloads program with the UH-60 helicopter. The experimental and analytical plans and progress for this program are described, including the design and development of a rotor blade which incorporated 242 pressure transducers buried in the surface of the blade, and also the development of calibration hardware for regular calibration and testing of the transducers. Supporting analytical developments based on the comprehensive analytical model of rotorcraft aerodynamics and dynamics (CAMRAD) and various CFD codes are discussed. The highly instrumented UH-60 as well as companion programs of full-scale and model wind tunnel tests of the UH-60 rotor with identical instrumentation will provide the opportunity to explore a full range of rotor experiments and the data necessary to validate the advanced methodologies under development.

Snyder, W. J.↗
NASA NTRS1991

Three-dimensional flows in a transonic compressor rotor

This study involves an experimental and numerical investigation of the three-dimensional flows in a transonic compressor rotor. A variety of data which could be used, in a complementary fashion, to validate/calibrate the computational fluid dynamics turbomachinery code and improve understanding of the flow physics, were acquired. Detailed radial survey data which consisted of total pressure, total temperature, static pressure and flow angle were obtained at stations upstream and downstream of the rotor blade. Detailed velocity and turbulence profiles were obtained upstream of the rotor and used as the upstream boundary conditions for the numerical analysis. Calibrated flush-mounted hot film probes were used to measure wall shear stress on the hub and casing walls upstream of the rotor. The blade-to-blade shear-stress angle distributions were obtained at two axial locations on the rotor casing, using flush-mounted hot film probes. A numerical analysis conducted using a three-dimensional Navier-Stokes code was compared with the experimental results.

Reid, Lonnie↗
NASA NTRS1991

An experimental study of the sensitivity of helicopter rotor blade tracking to root pitch adjustment in hover

The sensitivity of blade tracking in hover to variations in root pitch was examined for two rotor configurations. Tests were conducted using a four bladed articulated rotor mounted on the NASA-Army aeroelastic rotor experimental system (ARES). Two rotor configurations were tested: one consisting of a blade set with flexible fiberglass spars and one with stiffer (by a factor of five in flapwise and torsional stiffnesses) aluminum spars. Both blade sets were identical in planform and airfoil distribution and were untwisted. The two configurations were ballasted to the same Lock number so that a direct comparison of the tracking sensitivity to a gross change in blade stiffness could be made. Experimental results show no large differences between the two sets of blades in the sensitivity of the blade tracking to root pitch adjustments. However, a measurable reduction in intrack coning of the fiberglass spar blades with respect to the aluminum blades is noted at higher rotor thrust conditions.

Wilkie, W. Keats↗
NASA NTRS1991

Identification of dynamic characteristics of flexible rotors as dynamic inverse problem

The problem of dynamic and balancing of flexible rotors were considered, which were set and solved as the problem of the identification of flexible rotor systems, which is the same as the inverse problem of the oscillation theory dealing with the task of the identifying the outside influences and system parameters on the basis of the known laws of motion. This approach to the problem allows the disclosure the picture of disbalances throughout the rotor-under-test (which traditional methods of flexible rotor balancing, based on natural oscillations, could not provide), and identify dynamic characteristics of the system, which correspond to a selected mathematical model. Eventually, various methods of balancing were developed depending on the special features of the machines as to their design, technology, and operation specifications. Also, theoretical and practical methods are given for the flexible rotor balancing at far from critical rotation frequencies, which does not necessarily require the knowledge forms of oscillation, dissipation, and elasticity and inertia characteristics, and to use testing masses.

Roisman, W. P.↗
NASA NTRS1991

Field Telemetry of Blade-rotor Coupled Torsional Vibration at Matuura Power Station Number 1 Unit

The quasi-modal reduction technique and finite element model (FEM) were used to construct an analytical model for the blade-rotor coupled torsional vibration of a steam turbine generator of the Matuura Power Station. A single rotor test was executed in order to evaluate umbrella vibration characteristics. Based on the single rotor test results and the quasi-modal procedure, the total rotor system was analyzed to predict coupled torsional frequencies. Finally, field measurement of the vibration of the last stage buckets was made, which confirmed that the double synchronous resonance was 124.2 Hz, meaning that the machine can be safely operated. The measured eigen values are very close to the predicted value. The single rotor test and this analytical procedure thus proved to be a valid technique to estimate coupled torsional vibration.

Isii, Kuniyoshi↗
NASA NTRS1992

A comparison of predicted and measured inlet distortion flows in a subsonic axial inlet flow compressor rotor

Detailed flow measurements were taken inside an isolated axial compressor rotor operating subsonically near peak efficiency. These Laser Anemometer measurements were made with two inlet velocity profiles. One profile consisted of an unmodified baseline flow, and the second profile was distorted by placing axisymmetric screens on the hub and shroud well upstream of the rotor. A detailed comparison in the rotor relative reference frame between a Navier-Stokes solver and the measured experimental results showed good agreement between the predicted and measured flows. A primary flow is defined in the rotor and deviations and the computed predictions is made to assess the development of a passage vortex due to the distortion of the inlet flow. Computer predictions indicate that a distorted inlet profile has a minimal effect on the development of the flow in the rotor passage and the resulting passage vortex.

Owen, Albert K.↗
NASA NTRS1991

Analysis of unsteady pressures induced on a body by a rotor

An analysis was conducted on the unsteady pressures measured on a body surface in proximity to a rotor. Unsteady pressures were measured at strategic points on the body in forward flight at various combination of advance ratio, rotor thrust, and tip-path-plane angle. It was found that the local values of unsteady pressure were significantly altered by small changes in rotor thrust and advance ratio; tip-path-plane angle variations had a much smaller effect. Four characteristic pressure signatures representative of (1) blade passage, (2) close wake interactions with the body, (3) wake impingement on the body, and (4) postwake impingement were distinguished. These classifications were supported by correlations with shadowgraphic flow visualization of the rotor wake/body interaction, as well as unsteady potential flow models. This general classification of the surface pressure signatures has permitted a greater physical understanding of the overall air loads and possible mechanisms responsible for the interactional effects between a helicopter rotor and its fuselage.

Bi, Nai-Pei↗
NASA NTRS1991

Results from a test of a 2/3-scale V-22 rotor and wing in the 40- by 80-Foot Wind Tunnel

A test of a 0.658-scale V-22 rotor and wing was conducted in the 40- by 80-Foot Wind Tunnel at Ames Research Center. The principal objectives of the test were to measure the wing download in hover for a variety of test configurations, and rotor performance in forward flight. Also, a limited amount of data on rotor performance in vertical climb were acquired. This paper presents the results from the test with predictions from appropriate analytical methods. A new method for presenting and interpreting wing surface pressure data in hover is described, and this method shows that the wing flap can produce substantial lift loads in hover. The rotor performance in vertical climb was underpredicted by CAMRAD/JA and by the free wake analysis EHPIC. A simple momentum theory is presented which provides good predictions of rotor performance in forward flight.

Felker, Fort F.↗
NASA NTRS1992

Analytical aeroelastic stability considerations and conversion loads for an XV-15 tilt-rotor in a wind tunnel simulation

A rotorcraft analysis is conducted to assess tilt-rotor stability and conversion loads for the XV-15 rotor with metal blades within its specified test envelope. A 38-DOF flutter analysis based on the code by Johnson (1988) is developed to simulate a wind-tunnel test in which the rotor torque is constant and thereby study stability. The same analytical model provides the simulated loads including hub loads, blade loads, and oscillatory pitch-link loads with attention given to the nonuniform inflow through the proprotor in the presence of the wing. Tilt-rotor stability during the cruise mode is found to be sensitive to coupling effects in the control system stiffness, and a stability problem is identified in the XV-15 Advanced Technology Blades. The present analysis demonstrates that the tilt-rotor is stable within the specified test envelope of the NASA 40 x 80-ft wind tunnel.

Kottapalli, Sesi↗