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  • Other Sources  (27)
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  • 1
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    Direct periodic solutions of rotor free wake calculations (1993)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Keywords: AERODYNAMICS
    Type: American Helicopter Society, Journal (ISSN 0002-8711); 38; 2; p. 53-60.
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  • 2
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    Efficient free wake calculations using analytical/numerical matching (1993)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Keywords: AERODYNAMICS
    Type: American Helicopter Society, Journal (ISSN 0002-8711); 38; 2; p. 43-52.
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  • 3
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    Comparisons of predicted and measured rotor performance in hover using a new free wake analysis (1988)
    In:  Other Sources
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    Publication Date: 2018-12-01
    Description: A new free-wake analysis has been developed to predict the performance of rotors in hover and axial flight. The analysis finds the rotor wake geometry using a novel influence coefficient/relaxation scheme. In this paper, the features of the analysis are reviewed, and the accuracy of the analysis is assessed for a wide range of rotor configurations in hover and vertical climb. The effects of variations in the analysis parameters are addressed, and recommendations for future development are presented. Generally good results were obtained for the predicted performance of a wide range of rotor geometries and operating conditions without any special 'tuning' of the analysis. The analysis also provides predictions of rotor performance in vertical climb that are substantially more accurate than momentum theory.
    Keywords: AIRCRAFT DESIGN, TESTING AND PERFORMANCE
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  • 4
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    Computational analysis of high resolution unsteady airloads for rotor aeroacoustics (1994)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The study of helicopter aerodynamic loading for acoustics applications requires the application of efficient yet accurate simulations of the velocity field induced by the rotor's vortex wake. This report summarizes work to date on the development of such an analysis, which builds on the Constant Vorticity Contour (CVC) free wake model, previously implemented for the study of vibratory loading in the RotorCRAFT computer code. The present effort has focused on implementation of an airload reconstruction approach that computes high resolution airload solutions of rotor/rotor-wake interactions required for acoustics computations. Supplementary efforts on the development of improved vortex core modeling, unsteady aerodynamic effects, higher spatial resolution of rotor loading, and fast vortex wake implementations have substantially enhanced the capabilities of the resulting software, denoted RotorCRAFT/AA (AeroAcoustics). Results of validation calculations using recently acquired model rotor data show that by employing airload reconstruction it is possible to apply the CVC wake analysis with temporal and spatial resolution suitable for acoustics applications while reducing the computation time required by one to two orders of magnitude relative to that required by direct calculations. Promising correlation with this body of airload and noise data has been obtained for a variety of rotor configurations and operating conditions.
    Keywords: ACOUSTICS
    Type: NASA-CR-194894 , NAS 1.26:194894 , CDI-93-12
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  • 5
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    Free wake analysis of hover performance using a new influence coefficient method (1990)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: 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.
    Keywords: AERODYNAMICS
    Type: NASA-CR-4309 , A-90091 , NAS 1.26:4309
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  • 6
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    A new methodology for free wake analysis using curved vortex elements (1987)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: A method using curved vortex elements was developed for helicopter rotor free wake calculations. The Basic Curve Vortex Element (BCVE) is derived from the approximate Biot-Savart integration for a parabolic arc filament. When used in conjunction with a scheme to fit the elements along a vortex filament contour, this method has a significant advantage in overall accuracy and efficiency when compared to the traditional straight-line element approach. A theoretical and numerical analysis shows that free wake flows involving close interactions between filaments should utilize curved vortex elements in order to guarantee a consistent level of accuracy. The curved element method was implemented into a forward flight free wake analysis, featuring an adaptive far wake model that utilizes free wake information to extend the vortex filaments beyond the free wake regions. The curved vortex element free wake, coupled with this far wake model, exhibited rapid convergence, even in regions where the free wake and far wake turns are interlaced. Sample calculations are presented for tip vortex motion at various advance ratios for single and multiple blade rotors. Cross-flow plots reveal that the overall downstream wake flow resembles a trailing vortex pair. A preliminary assessment shows that the rotor downwash field is insensitive to element size, even for relatively large curved elements.
    Keywords: AERODYNAMICS
    Type: NASA-CR-3958 , NAS 1.26:3958 , CDI-84-6
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  • 7
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    Analysis of rotor vibratory loads using higher harmonic pitch control (1992)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Experimental studies of isolated rotors in forward flight have indicated that higher harmonic pitch control can reduce rotor noise. These tests also show that such pitch inputs can generate substantial vibratory loads. The modification is summarized of the RotorCRAFT (Computation of Rotor Aerodynamics in Forward flighT) analysis of isolated rotors to study the vibratory loading generated by high frequency pitch inputs. The original RotorCRAFT code was developed for use in the computation of such loading, and uses a highly refined rotor wake model to facilitate this task. The extended version of RotorCRAFT incorporates a variety of new features including: arbitrary periodic root pitch control; computation of blade stresses and hub loads; improved modeling of near wake unsteady effects; and preliminary implementation of a coupled prediction of rotor airloads and noise. Correlation studies are carried out with existing blade stress and vibratory hub load data to assess the performance of the extended code.
    Keywords: ACOUSTICS
    Type: NASA-CR-189591 , NAS 1.26:189591 , CDI-91-03
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  • 8
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    Interior noise reduction by alternate resonance tuning (1987)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Existing interior noise reduction techniques for aircraft fuselages perform reasonably well at higher frequencies, but are inadequate at low frequencies, particularly with respect to the low blade passage harmonics with high forcing levels found in propeller aircraft. A method is studied which considers aircraft fuselages lined with panels alternately tuned to frequencies above and below the frequency that must be attenuated. Adjacent panel would oscillate at equal amplitude, to give equal acoustic source strength, but with opposite phase. Provided these adjacent panels are acoustically compact, the resulting cancellation causes the interior acoustic modes to be cut off, and therefore be nonpropagating and evanescent. This interior noise reduction method, called Alternate Resonance Tuning (ART), is being investigated theoretically and experimentally. Progress to date is discussed.
    Keywords: ACOUSTICS
    Type: NASA-CR-181189 , NAS 1.26:181189
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  • 9
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    Computation of rotor aerodynamic loads in forward flight using a full-span free wake analysis (1990)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The development of an advanced computational analysis of unsteady aerodynamic loads on isolated helicopter rotors in forward flight is described. The primary technical focus of the development was the implementation of a freely distorting filamentary wake model composed of curved vortex elements laid out along contours of constant vortex sheet strength in the wake. This model captures the wake generated by the full span of each rotor blade and makes possible a unified treatment of the shed and trailed vorticity in the wake. This wake model was coupled to a modal analysis of the rotor blade dynamics and a vortex lattice treatment of the aerodynamic loads to produce a comprehensive model for rotor performance and air loads in forward flight dubbed RotorCRAFT (Computation of Rotor Aerodynamics in Forward Flight). The technical background on the major components of this analysis are discussed and the correlation of predictions of performance, trim, and unsteady air loads with experimental data from several representative rotor configurations is examined. The primary conclusions of this study are that the RotorCRAFT analysis correlates well with measured loads on a variety of configurations and that application of the full span free wake model is required to capture several important features of the vibratory loading on rotor blades in forward flight.
    Keywords: AERODYNAMICS
    Type: NASA-CR-177611 , A-91136 , NAS 1.26:177611
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  • 10
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    General flow field analysis methods for helicopter rotor aeroacoustics (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Previous work in the analysis of rotor flow fields for aeroacoustic applications involved the preliminary development of an efficient and accurate Lagrangian simulation of the unsteady vorticity field in the vicinity of helicopter main rotor that could analyze a limited class of rotor/wake interactions. The capabilities of this analysis have subsequently been considerably enhanced to allow it to serve as the foundation for a general analysis of the rotor/wake interaction noise. This paper presents the details of these enhancements, which focus on the expansion of the reconstruction approach developed previously to handle arbitrary vortex wake interactions within three-dimensional regions located near or within the rotor disk. Also, the development of nearfield velocity corrections appropriate for the analysis of such interactions is described, as is a preliminary study of methods for using the new high-resolution flow field analysis for noise predictions. The results show that by employing this novel flow field reconstruction technique it is possible to employ full-span free wake analyses with temporal and spatial resolution suitable for acoustic applications while reducing the computation time required by one to two orders of magnitude relative to traditional methods.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 91-0591
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