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At least 883 records · Page 49
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Effects of different rub models on simulated rotor dynamics

Using a direct integration, transient response rotor dynamics computer code, the response of turbine engine rotors to two different blade tip - seal interference rub models was studied. The first model, an abradable seal rub model, is based on an energy-loss-per-unit-volume theory (applicable to a ceramic turbine blade tip seal). The second, a smearin model, is based on viscous hydrodynamic theory (applicable to a metallic blade tip seal). The results from these two models were compared with those from a previously studied model based on dry friction theory. The abradable model was very senitive to small changes in the energy per unit volume, and once a threshold was exceeded, the rotor went into a backward whirl. The amplitude seemed to grow without limit. This was similar to the dry friction model when the coefficient of friction exceeded a particular threshold. The smearing model was not as sensitive to small changes in the viscosity, but a threshold viscosity was found. When it was exceeded, the rotor went into backward whirl, but the amplitude seemed to grow to a finite limit.

Kascak, A. F.↗
NASA NTRS1984

An assessment of the capability to calculate tilting prop-rotor aircraft performance, loads and stability

Calculated performance, loads, and stability of the XV-15 tilt rotor research aircraft are compared with wind tunnel and flight measurements, to define the level of the current analytical capability for tilting prop rotor aircraft, and to define the requirements for additional experimental data and further analysis development. The correlation between calculated and measured behavior is generally good, although there are some significant discrepancies. Based on this correlation, the analysis is assessed overall as being adequate for the design, evaluation, and testing of tilting prop rotor aircraft. A general assessment of the state of the art of tilt rotor predictive capability is given. Specific areas are identified where improvements in the capability to calculate performance, loads, and stability are desirable. Requirements for more accurate and detailed data which support the development of improved analytical models are identified as well.

Johnson, W.↗
NASA NTRS1984

Broadband rotor noise analyses

The various mechanisms which generate broadband noise on a range of rotors studied include load fluctuations due to inflow turbulence, due to turbulent boundary layers passing the blades' trailing edges, and due to tip vortex formation. Existing analyses are used and extensions to them are developed to make more accurate predictions of rotor noise spectra and to determine which mechanisms are important in which circumstances. Calculations based on the various prediction methods in existing experiments were compared. The present analyses are adequate to predict the spectra from a wide variety of experiments on fans, full scale and model scale helicopter rotors, wind turbines, and propellers to within about 5 to 10 dB. Better knowledge of the inflow turbulence improves the accuracy of the predictions. Results indicate that inflow turbulence noise depends strongly on ambient conditions and dominates at low frequencies. Trailing edge noise and tip vortex noise are important at higher frequencies if inflow turbulence is weak. Boundary layer trailing edge noise, important, for large sized rotors, increases slowly with angle of attack but not as rapidly as tip vortex noise.

George, A. R.↗
NASA NTRS1984

Results of the first complete static calibration of the RSRA rotor-load-measurement system

The compound Rotor System Research Aircraft (RSRA) is designed to make high-accuracy, simultaneous measurements of all rotor forces and moments in flight. Physical calibration of the rotor force- and moment-measurement system when installed in the aircraft is required to account for known errors and to ensure that measurement-system accuracy is traceable to the National Bureau of Standards. The first static calibration and associated analysis have been completed with good results. Hysteresis was a potential cause of static calibration errors, but was found to be negligible in flight compared to full-scale loads, and analytical methods have been devised to eliminate hysteresis effects on calibration data. Flight tests confirmed that the calibrated rotor-load-measurement system performs as expected in flight and that it can dependably make direct measurements of fuselage vertical drag in hover.

Acree, C. W., Jr.↗
NASA NTRS1984

A study of the aerodynamic interaction between a main rotor and a fuselage

The wake of a helicopter rotor can have a significant effect on a fuselage. Results from a recent wind-tunnel investigation show that certain fuselage characteristics, normalized by rotor thrust, scale proportionally to a rotor-wake-induced velocity parameter. Effects on the body of changes in velocity, thrust, tip-path-plane angle of attack, and rotor/body position are discussed. These results show that the rotor can have a favorable or unfavorable influence on the body, depending upon the operating condition.

Smith, C. A.↗
NASA NTRS1986

Tiltable-Wing, Tiltable-Rotor Aircraft

New airplane wing tilts 90 degrees, leading edge up, to feather into rotor downwash during hovering, takeoff, or landing. After hovering or takeoff, rotors tilted forward slightly to accelerate airplane into forward flight. As airplane accelerates, wings start to tilt forward. At speeds of 20 to 40 knots (10.3 to 20.6 m/s), wings are in horizontal orientation. In this speed range, rotor power requirements decreased sufficiently to allow wings to assume normal orientation. Tiltable wings movable independently of tiltable rotors. Tilted at whatever pitch schedule yields greatest efficiency.

Stroub, R. H.↗
NASA NTRS1984

Extended aeroelastic analysis for helicopter rotors with prescribed hub motion and blade appended penduluum vibration absorbers

The mathematical development for the expanded capabilities of the G400 rotor aeroelastic analysis was examined. The G400PA expanded analysis simulates the dynamics of all conventional rotors, blade pendulum vibration absorbers, and the higher harmonic excitations resulting from prescribed vibratory hub motions and higher harmonic blade pitch control. The methodology for modeling the unsteady stalled airloads of two dimensional airfoils is discussed. Formulations for calculating the rotor impedance matrix appropriate to the higher harmonic blade excitations are outlined. This impedance matrix, and the associated vibratory hub loads, are the rotor dynamic characteristic elements for use in the simplified coupled rotor/fuselage vibration analysis (SIMVIB). Updates to the development of the original G400 theory, program documentation, user instructions and information are presented.

Bielawa, R. L.↗
NASA NTRS1983

Clearance flow-generated transverse forces at the rotors of thermal turbomachines

Self-excited rotor whirl represents a serious hazard in the operation of turbomachines. The reported investigation has, therefore, the objective to measure the lateral forces acting on the rotor and to determine the characteristic pressure distribution in the rotor clearance area. A description is presented of an approach for calculating the leakage flow in the case of an eccentric rotor position on the basis of empirical loss coefficients. The results are reported of an experimental investigation with a turbine stage, taking into account a variation of the clearance characteristics. The pressure data measured are consistent with the theoretical considerations.

Urlichs, K.↗
NASA NTRS1985

Structural response of a rotating bladed disk to rotor whirl

A set of high speed rotating whirl experiments were performed in the vacuum of the MIT Blowdown Compressor Facility on the MIT Aeroelastic Rotor, which is structurally typical of a modern high bypass ratio turbofan stage. These tests identified the natural frequencies of whirl of the rotor system by forcing its response using an electromagnetic shaker whirl excitation system. The excitation was slowly swept in frequency at constant amplitude for several constant rotor speeds in both a forward and backward whirl direction. The natural frequencies of whirl determined by these experiments were compared to those predicted by an analytical 6 DOF model of a flexible blade-rigid disk-flexible shaft rotor. The model is also presented in terms of nondimensional parameters in order to assess the importance of the interation between the bladed disk dynamics and the shaft-disk dynamics. The correlation between the experimental and predicted natural frequencies is reasonable, given the uncertainty involved in determining the stiffness parameters of the system.

Crawley, E. F.↗
NASA NTRS1985

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 configuration, 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 NTRS1984

Comparison of experimental and analytical predictions of rotor blade-vortex interactions using model scale acoustic data

Helicopter blade-vortex interaction (BVI) noise is studied using a model scale rotor acoustic data base and an analytical rotor wake prediction method. The variation of BVI acoustic levels with vehicle flight conditions (forward speed and disk attitude) is presented. Calculations of probable BVI locations on the rotor disk are made for a range of operating conditions using the measured acoustic signals and an acoustic ray tracing technique. Analytical predictions of possible BVI locations on the rotor disk are made using a generalized distorted wake analysis program. Comparisons of the interaction locations are made with the results of both the analytic approach and the acoustic ray tracing technique.

Martin, R. M.↗
NASA NTRS1984

Calculation of unsteady fan rotor response caused by downstream flow distortions

The present model for fan rotor/support strut airfoil interaction uses a time-marching code for the rotor flow, coupled with a potential flow model for the stator-strut region. Study of the effect of strut design variables indicates that rotor flow disturbance is increased by the primary variables of larger strut thickness and circumferential spacing, while decreasing exponentially with increased rotor-strut separation. The time-marching code predicts local rotor pressure and flow perturbations in response to an unsteady downstream boundary condition.

Obrien, W. F.↗
NASA NTRS1985

Numerical solution of the full-potential equation for rotors and oblique wings using a new wake model

A three-dimensional, full-potential, quasi-steady code TFAR1 is proposed for calculating the transonic flow past a lifting helicopter rotor blade and oblique wing. The TFAR1 uses a two-dimensional nonlinear wake-model that allows a jump in velocity potential to propagate with the local fluid flow in the wake. Rotor calculations were made for a single blade at an advance ratio of 0.3, a rotational tip Mach number of 0.7, and at 0-degree incidence. A 1/7-scale model of the Cobra Operational Load Survey (OLS) rotor blade is calculated, and the pressure distributions are compared to the measurements for azimuth angles 0, 30, 60, 90, 120, and 150 degrees at the 95 percent spanwise station of the OLS blade. Furthermore, an oblique wing with Korn airfoil was calculated at the high transonic free-stream Mach number of 0.9791, zero incidence, and yaw angle of 40 degrees. The TFAR1, coupled with a helicopter performance code CAMRAD (Johnson, 1981), provides a full-potential code for calculating the entire flow field for a multiple-bladed rotor in transonic lifting forward flight.

Chang, I.-C.↗
NASA NTRS1984

Simulation validation of the XV-15 tilt-rotor research aircraft

The results of a simulation validation program of the XV-15 tilt-rotor research aircraft are detailed, covering such simulation aspects as the mathematical model, visual system, motion system, cab aural system, cab control loader system, pilot perceptual fidelity, and generic tilt rotor applications. Simulation validation was performed for the hover, low-speed, and sideward flight modes, with consideration of the in-ground rotor effect. Several deficiencies of the mathematical model and the simulation systems were identified in the course of the simulation validation project, and some were corrected. It is noted that NASA's Vertical Motion Simulator used in the program is an excellent tool for tilt-rotor and rotorcraft design, development, and pilot training.

Ferguson, S. W.↗
NASA NTRS1985

Wide-field shadowgraph flow visualization of tip vortices generated by a helicopter rotor

The vortex trajectory and vortex wake generated by helicopter rotors were visualized using a wide-field shadowgraph technique. Use of a retro-reflective Scotchlite screen made it possible to investigate the flow field generated by full-scale tail rotors. Tip vortex trajectories were visible in shadowgraphs for a range for tip Mach number of 0.38 - 0.60. The effect of the angle of attack was substantial. At an angle of attack greater than 8 degrees, the visibility of the vortex core was significant even at relatively low tip Mach numbers. The theoretical analysis of the visibility was carried out for a rotating blade. This analysis demonstrated that the visibility decreases with increasing dimensionless core radius (ro/c) and increases with increasing tip Mach number. Based on this investigation, it is concluded that the wide-field shadowgraph flow visualization technique should be feasible to study the flow field generated by a large main rotor in a wind tunnel and in an outdoor full-scale test stand. Of note is that the shadowgraph technique could easily be used with other on-going tests such as aerodynamic performance study, noise measurements, velocity measurements with LDV or hot-wire anemometer, local pressure measurement on the rotor surface, etc.

Parthasarathy, S. P.↗
NASA NTRS1985

Performance degradation of helicopter rotor in forward flight due to ice

This study addresses the analytical assessment of the degradation in the forward flight performance of the front rotor Boeing Vertol CH47D helicopter in a rime ice natural icing encounter. The front rotor disk was divided into 24 15-deg sections and the local Mach number and angle of attack were evaluated as a function of azimuthal and radial location for a specified flight condition. Profile drag increments were then calculated as a function of azimuthal and radial position for different times of exposure to icing, and the rotor performance was re-evaluated including these drag increments. The results of the analytical prediction method, such as horsepower required to maintain a specific flight condition, as a function of icing time have been generated. The method to illustrate the value of such an approach in assessing performance changes experienced by a helicopter rotor as a result of rime ice accretion is described.

Korkan, K. D.↗
NASA NTRS1986

Pitch Control for Helicopter Rotors

Pitch controller for helicopter rotors uses hub-mounted actuators located symmetrically between rotor blades. New controller designed for X-wing rotors requiring collective pitch control; pitch of all blades must be changed by same amount. Collective control allows rotor trimmed during variety of flight regimes, particularly during hovering and fixed-wing flight.

Jeffery, P.↗
NASA NTRS1985

A simulation of rotor-stator interaction using the Euler equations and patched grids

An unsteady Euler code to study rotor-stator interaction problem was developed. The code uses patched grids that move relative to each other to simulate the motion of the rotor airfoils with respect to the stator airfoils. The Osher integration scheme is used in conjunction with an implicit relaxation approach. The scheme is second order accurate in space and time, and is also TVD in each spatial direction. The numerical results were found to be periodic in time, thus demonstrating the capability of the integration and zonal schemes in simulating periodic time dependent flow. The pressure contours obtained are almost oscillation free because of the TVD nature of the scheme. A new procedure was developed to simulate flows about bodies that move relative to each other. This capability should prove to be very useful in the areas of rotor-stator interaction, propeller-nacelle interaction, and helicopter rotor-fuselage interaction.

Rai, M. M.↗