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At least 145 records · Page 8

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NASA NTRS1985

Test results from a dynamic model dynaflex rotor

A one-fifth scale dynamic model of the Sikorsky Dynaflex rotor was tested in hover and in forward flight conditions in a wind tunnel. The Dynaflex rotor features an advanced composite structure which flexes to provide a constant speed universal joint action. Testing concentrated on confirming that the stability and dynamic response of the rotor were satisfactory. Lift conditions of up to .11 Ct/sigma and advance ratios as high as .46 were reached. Vibratory loads were compared to those of articulated rotors. The Dynaflex rotor concept appears to be a practical concept from the standpoint of dynamic response and stability.

Niebanck, C. F.↗

NASA NTRS1984

Helicopter model rotor-blade vortex interaction impulsive noise: Scalability and parametric variations

Acoustic data taken in the anechoic Deutsch-Niederlaendischer Windkanal (DNW) have documented the blade vortex interaction (BVI) impulsive noise radiated from a 1/7-scale model main rotor of the AH-1 series helicopter. Averaged model scale data were compared with averaged full scale, inflight acoustic data under similar nondimensional test conditions. At low advance ratios (mu = 0.164 to 0.194), the data scale remarkable well in level and waveform shape, and also duplicate the directivity pattern of BVI impulsive noise. At moderate advance ratios (mu = 0.224 to 0.270), the scaling deteriorates, suggesting that the model scale rotor is not adequately simulating the full scale BVI noise; presently, no proved explanation of this discrepancy exists. Carefully performed parametric variations over a complete matrix of testing conditions have shown that all of the four governing nondimensional parameters - tip Mach number at hover, advance ratio, local inflow ratio, and thrust coefficient - are highly sensitive to BVI noise radiation.

Splettstoesser, W. R.↗

NASA NTRS1985

Helicopter model rotor-blade vortex interaction impulsive noise - Scalability and parametric variations

Acoustic data taken in the anechoic Deutsch-Niederlaendischer Windkanal (DNW) have documented the blade vortex interaction (BVI) impulsive noise radiated from a 1/7-scale model main rotor of the AH-1 series helicopter. Averaged model scale data were compared with averaged full scale, inflight acoustic data under similar nondimensional test conditions. At low advance ratios (mu = 0.164 to 0.194), the data scale remarkable well in level and waveform shape, and also duplicate the directivity pattern of BVI impulsive noise. At moderate advance ratios (mu = 0.224 to 0.270), the scalig deteriorates, suggesting that the model scale rotor is not adequately simulating the full scale BVI noise; presently, no proved explanation of this discrepancy exists. Carefully performed parametric variations over a complete matrix of testing conditions have shown that all of the four governing nondimensional parameters - tip Mach number at hover, advance ratio, local inflow ratio, and thrust coefficient - are highly sensitive to BVI noise radiation.

Boxwell, D. A.↗

NASA NTRS1983

Linear and Nonlinear Aspects of Rotordynamics

Excessive vibrations of the liquid oxygen pump in the Space Shuttle's Main Engine have been recorded during hot firing ground testing. In order to determine mathematical explanations of this possibility, destructive phenomenon differential equations have been examined which describe the rotordynamics of the pump. Modeling the rotor as a random eigenvalue problem was considered. Analytical expressions were derived for the solution in the case of symmetric damping and stiffness. This enables one to determine accuracy estimates when testing numerical techniques to solve both asymmetric and nonlinear problems. Finally, the rotor model has had nonlinear elements incorporated to improve its simulation of the pump and to expand the corresponding mathematical theory.

Day, W. B.↗

NASA NTRS2014

Tone and Broadband Noise Separation from Acoustic Data of a Scale-Model Counter-Rotating Open Rotor

Renewed interest in contra-rotating open rotor technology for aircraft propulsion application has prompted the development of advanced diagnostic tools for better design and improved acoustical performance. In particular, the determination of tonal and broadband components of open rotor acoustic spectra is essential for properly assessing the noise control parameters and also for validating the open rotor noise simulation codes. The technique of phase averaging has been employed to separate the tone and broadband components from a single rotor, but this method does not work for the two-shaft contra-rotating open rotor. A new signal processing technique was recently developed to process the contra-rotating open rotor acoustic data. The technique was first tested using acoustic data taken of a hobby aircraft open rotor propeller, and reported previously. The intent of the present work is to verify and validate the applicability of the new technique to a realistic one-fifth scale open rotor model which has 12 forward and 10 aft contra-rotating blades operating at realistic forward flight Mach numbers and tip speeds. The results and discussions of that study are presented in this paper.

Acoustic Data↗

NASA NTRS2014

Tone and Broadband Noise Separation from Acoustic Data of a Scale-Model Contra-Rotating Open Rotor

Noise Separation↗

NASA NTRS1987

Helicopter Model Rotor-Blade Vortex Interaction Impulsive Noise: Scalability and Parametric Variations

Acoustic data taken in the anechoic Deutsch-Niederlaendischer Windkanal (DNW) have documented the blade-vortex interaction (BVI) impulsive noise radiated from a 1/7-scale model main rotor of the AH-1 series helicopter. Averaged model-scale data were compared with averaged full-scale, in-flight acoustic data under similar non-dimensional test conditions using an improved data analysis technique. At low advance ratios (mu = 0.164 - 0.194), the BVI impulsive noise data scale remarkably well in level, waveform, and directivity patterns. At moderate advance ratios (mu = 0.224 - 0.270), the scaling deteriorates, suggesting that the model-scale rotor is not adequately simulating the full-scale BVI noise. Presently, no proved explanation of this discrepancy exists. Measured BVI noise radiation is highly sensitive to all of the four governing nondimensional parameters--hover tip Mach number, advance ratio, local inflow ratio, and thrust coefficient.

Boxwell, D. A.↗

NASA NTRS1983

A comparison of model helicopter rotor Primary and Secondary blade/vortex interaction blade slap

A study of the relative importance of blade/vortex interactions which occur on the retreating side of a model helicopter rotor disk is described. Some of the salient characteristics of this phenomenon are presented and discussed. It is shown that the resulting Secondary blade slap may be of equal or greater intensity than the advancing side (Primary) blade slap. Instrumented model helicopter rotor data is presented which reveals the nature of the retreating blade/vortex interaction. The importance of Secondary blade slap as it applies to predictive techniques or approaches is discussed. When Secondary blade slap occurs it acts to enlarge the window of operating conditions for which blade slap exists.

Hubbard, J. E., Jr.↗

NASA NTRS1983

Labyrinth seal forces on a whirling rotor

An experimental investigation of air labyrinth seal forces on a subsynchronously whirling model rotor is described and test results are given for diverging, converging, and straight two-strip seals. The effects of pressure drop, provide basic experimental data needed in the development of design methods for predicting and preventing self-excited whirl of turbine rotors and other machines having labyrinth seals. The total dynamic seal forces on the whirling model rotor are measured accurately by means of an active damping and stiffness system that is adjusted to obtain neutral whirl stability of the model rotor system. In addition, the whirling pressure pattern in the seal annulus is measured for a few test conditions and the corresponding pressure forces on the rotor are compared with the total measured forces. This comparison shows that either radial and axial pressure gradients in the seal annulus or drag forces on the rotor are significant. Comparisons made between the measured seal forces and theoretical results show that present theory is inadequate.

Wright, D. V.↗

NASA NTRS1983

Labyrinth seal forces on a whirling rotor

An experimental investigation of air labyrinth seal forces on a subsynchronously whirling model rotor is described and test results are given for diverging, converging, and straight two-strip seals. The effects of pressure drop, back pressure, whirl direction, and whirl frequency are shown. These results provide basic experimental data needed in the development of design methods for predicting and preventing self-excited whirl of turbine rotors and other machines having labyrinth seals. The total dynamic seal forces on the whirling model rotor are measured accurately by means of a novel active damping and stiffness system that is adjusted to obtain neutral whirl stability of the model rotor system. In addition, the whirling pressure pattern in the seal annulus is measured for a few test conditions and the corresponding pressure forces on the rotor are compared with the total measured forces. This comparison shows that either radial and axial pressure gradients in the seal annulus or drag forces on the rotor are significant.

Wright, D. V.↗

NASA NTRS1996

Macroscopic balance model for wave rotors

A mathematical model for multi-port wave rotors is described. The wave processes that effect energy exchange within the rotor passage are modeled using one-dimensional gas dynamics. Macroscopic mass and energy balances relate volume-averaged thermodynamic properties in the rotor passage control volume to the mass, momentum, and energy fluxes at the ports. Loss models account for entropy production in boundary layers and in separating flows caused by blade-blockage, incidence, and gradual opening and closing of rotor passages. The mathematical model provides a basis for predicting design-point wave rotor performance, port timing, and machine size. Model predictions are evaluated through comparisons with CFD calculations and three-port wave rotor experimental data. A four-port wave rotor design example is provided to demonstrate model applicability. The modeling approach is amenable to wave rotor optimization studies and rapid assessment of the trade-offs associated with integrating wave rotors into gas turbine engine systems.

Welch, Gerard E.↗

NASA NTRSDate not supplied

A Comparison of Rotor Disk Modeling and Blade-Resolved CFD Simulations for NASA's Tiltwing Air Taxi

A multi-fidelity computational fluid dynamics analysis is carried out for NASA’s tiltwing air taxi concept operating in airplane and helicopter mode. High-fidelity simulations are computationally expensive due to individual rotor blade modeling in a time-dependent computational domain with rotating grids. The mid-fidelity rotor disk option, in its source term implementation, is explored as a more affordable alternative. Computations are performed with NASA’s OVERFLOW flow solver loosely-coupled with the comprehensive code CAMRAD II for appropriate rotor trim. Detailed comparisons are shown for the trim solution, airloads, wake geometry, and rotor performance. While the rotor disk model is able to capture the flow field with satisfactory agreement in airplane mode, it faces difficulties in helicopter mode due to the three-dimensional effects of the wake. Although this study is limited to a specific vehicle geometry, it is expected that the results are somewhat generalizable to the analysis of multi-rotor configurations.

ARMD↗

NASA NTRS1983

On high frequency broadband noise from model helicopter rotors

An experimental investigation was carried out to study high frequency broadband noise (HFBN) from model helicopter rotors at a range of tip Mach numbers up to 0.40. The type of HFBN in the present case was due to boundary layer self noise (BLSN). The effects of rotor advance ratio, tip speed, blade tip geometry, and free stream turbulence on the intensity and spectrum of BLSN have been investigated. Saturation or fall-off of the peak sound pressure level was observed as the tip Mach number was varied from 0.20 to 0.40. This result is different from measurements on square tip blades reported earlier by Aravamudan, Lee and Harris (1979). An explanation of this difference along with a discussion of the effects of blade tip geometry on BLSN are provided.

Humbad, N. G.↗

NASA NTRS2001

Rotor Design Options for Improving V-22 Whirl-Mode Stability

With current tilt-rotor technology, very stiff, thick wings of limited aspect ratio are essential to meet aeroelastic stability requirements, which severely limits cruise efficiency and maximum speed. Larger and more efficient tiltrotors; will need longer and lighter wings, for which whirl-mode flutter is a serious design issue. Numerous approaches to improving the whirl-mode airspeed boundary have been investigated, typically focusing on wing design, active stability augmentation, and variable geometry rotors. The research to be reported applies the much simpler approaches of sweeping the outboard blade sections and extending balance weights at the tip. Improving proprotor whirl-mode stability margins is an ongoing research activity at NASA Ames Research Center. Previous efforts focussed on the XV-15 rotor. This is the first application of these design principles to a full V-22 rotor model in CAMRAD II (a previous publication presented results for a preliminary analytical model). The analysis to be discussed in the proposed paper includes a dual load-path blade model, a compressible-flow aerodynamic model, a representation of swashplate/rotor modal coupling, and other new features. The paper will include discussions of the mechanisms by which blade sweep and tip-mass offsets improve whirl-flutter margins. The V-22 was modeled with CAMRAD II; the rotor model is shown. It is also shows an example swept blade derived from the V-22 rotor system. For this rotor, the primary significance of sweep is the improved whirl-flutter boundary, not the reduced Mach-number effects. An offset tip mass is also shown.

Acree, C. W., Jr.↗

NASA NTRS1980

Surface pressure measurements at two tips of a model helicopter rotor in hover

Surface pressures were measured near the tip of a hovering single-bladed model helicopter rotor with two tip shapes. The rotor had a constant-chord, untwisted blade with a square, flat tip which could be modified to a body-of-revolution tip. Pressure measurements were made on the blade surface along the chordwise direction at six radial stations outboard of the 94 percent blade radius. Data for each blade tip configuration were taken at blade collective pitch angles of 0, 6.18 and 11.4 degrees at a Reynolds number of 736,000 and a Mach number of 0.25 both based on tip speed. Chordwise pressure distributions and constant surface pressure contours are presented and discussed.

Gray, R. B.↗

NASA NTRS1983

Analytical model of rotor wake aerodynamics in ground effect

The model and the computer program developed provides the velocity, location, and circulation of the tip vortices of a two-blade helicopter in and out of the ground effect. Comparison of the theoretical results with some experimental measurements for the location of the wake indicate that there is excellent accuracy in the vicinity of the rotor and fair amount of accuracy far from it. Having the location of the wake at all times enables us to compute the history of the velocity and the location of any point in the flow. The main goal of out study, induced velocity at the rotor, can also be calculated in addition to stream lines and streak lines. Since the wake location close to the rotor is known more accurately than at other places, the calculated induced velocity over the disc should be a good estimate of the real induced velocity, with the exception of the blade location, because each blade was replaced only by a vortex line. Because no experimental measurements of the wake close to the ground were available to us, quantitative evaluation of the theoretical wake was not possible. But qualitatively we have been able to show excellent agreement. Comparison of flow visualization with out results has indicated the location of the ground vortex is estimated excellently. Also the flow field in hover is well represented.

Saberi, H. A.↗

NASA NTRS2012

Application of a High-Fidelity Icing Analysis Method to a Model-Scale Rotor in Forward Flight

An icing analysis process involving the loose coupling of OVERFLOW-RCAS for rotor performance prediction and with LEWICE3D for thermal analysis and ice accretion is applied to a model-scale rotor for validation. The process offers high-fidelity rotor analysis for the noniced and iced rotor performance evaluation that accounts for the interaction of nonlinear aerodynamics with blade elastic deformations. Ice accumulation prediction also involves loosely coupled data exchanges between OVERFLOW and LEWICE3D to produce accurate ice shapes. Validation of the process uses data collected in the 1993 icing test involving Sikorsky's Powered Force Model. Non-iced and iced rotor performance predictions are compared to experimental measurements as are predicted ice shapes.

Narducci, Robert↗

NASA NTRS1975

A Study of Gust and Control Response of Model Rotor-Propellers in a Wind Tunnel Airstream

One-ninth-scale model hingeless and gimballed rotor-propellers were tested at an advance ratio of 0.7 in the Wright Brothers Wind Tunnel at MIT, in the presence of sinusoidal longitudinal and vertical gusts produced by a gust generator of novel design. The gimballed rotor was also subjected to sinusoidal collective and cyclic control inputs. Model test data in terms of blade inplane and out-of-plane bending, longitudinal and lateral gimbal motion, wing vertical and chordwise bending, and blade and wing torsion are presented and compared with theory.

Ham, N. D.↗