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  • 1
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    Testing of a stall flutter suppression system for helicopter rotors using individual-blade-control (1984)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: The development and testing of a feedback system designed to alleviate rotor blade first torsion mode oscillations associated with stall flutter are described. The system employs blade-mounted sensors to detect torsional oscillations and provide feedback in order to increase the damping of the first torsional mode. A model of the blade and control system dynamics is developed and is used to give qualitative and quantitative guidance in the design process, as well as to aid in the analysis of experimental results. System performance in wind tunnel tests in forward flight is described, and experimental results show that the system can provide substantial additional damping to stall-induced oscillations.
    Keywords: AIRCRAFT STABILITY AND CONTROL
    Type: American Helicopter Society, Journal (ISSN 0002-8711); 29; 38-44
    Format: text
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  • 2
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    The influence of the rotor wake on rotorcraft stability and control (1989)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The effect of the time-averaged rotor wake flowfield on the aerodynamic behavior of the tail rotor and fixed tail surfaces is discussed. The flowfield at the location of these surfaces is predicted by two wake models, a simplified flat wake model and an accurate free wake model. Both models are shown to give similar predictions of the flowfield in the vicinity of the empennage that are generally in agreement with experiment. The contributions of these aerodynamic interactions to the helicopter stability derivatives are described and control responses using different wake models are compared with flight test.
    Keywords: AIRCRAFT STABILITY AND CONTROL
    Format: text
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  • 3
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    A new approach to the free wake problem for hovering rotors (1985)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: In the present approach to the hovering rotor free wake problem, an influence coefficient solution method is used to find that rotor wake solution which is steady in a reference frame that rotates with the blades; this scheme solves directly for the conditions of free wake equilibrium by a procedure that does not involve time-stepping and the associated use of numerical damping or special convergence methods. The solution method has been implemented in a hover wake computer program having a three-part wake model for the tip vortex. All three wake regions are represented by the new Basic Curved Vortex Elements. Sample hover calculations are presented for single blade and multiblade rotors.
    Keywords: AERODYNAMICS
    Format: text
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  • 4
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    Feasibility of predicting performance degradation of airfoils in heavy rain (1989)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The heavy rain aerodynamic performance penalty program is detailed. This effort supported the design of a fullscale test program as well as examined the feasibility of estimating the degradation of performance of airfoils from first principles. The analytic efforts were supplemented by a droplet splashback test program in an attempt to observe the physics of impact and generation of ejecta. These tests demonstrated that the interaction of rain with an airfoil is a highly complex phenomenon and this interaction is not likely to be analyzed analytically with existing tools.
    Keywords: AERODYNAMICS
    Type: NASA-CR-181842 , NAS 1.26:181842 , CDI-89-04
    Format: application/pdf
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  • 5
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    Testing and evaluation of a stall-flutter-suppression system for helicopter rotors using individual-blade-control (1981)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The development and testing of a feedback system designed to alleviate the violent blade first torsion mode oscillations associated with stall flutter are described. The system, based on previously developed M.I.T. Individual-Blade-Control hardware, employs blade-mounted accelerometers to sense torsional oscillations and feeds back rate informaton to increase the damping of the first torsion mode. A linear model of the blade and control system dynamics is developed and is used to give qualitative and quantitative guidance in the design process as well as to aid in analysis of experimental results. System performance in wind tunnel tests, both in hover and forward flight, is described, and evidence is given of the system's ability to provide substantial additional damping to stall-induced blade oscillations.
    Keywords: AIRCRAFT STABILITY AND CONTROL
    Type: NASA-CR-166233 , ASRL-TR-196-3
    Format: application/pdf
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  • 6
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    A simple system for helicopter Individual-Blade-Control and its application to stall flutter suppression (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-07-27
    Description: A new, advanced type of active control for helicopters and its application to a system for stall flutter suppression is described. The system, based on previously developed M.I.T. Individual-Blade-Control hardware, employs blade-mounted accelerometers to sense torsional oscillations and feeds back rate information to increase the damping of the first torsion mode. A linear model of the blade and control system dynamics is used to give qualitative and quantitative guidance in the design process as well as to aid in analysis of experimental results. System performance in wind tunnel tests is described, and evidence is given of the system's ability to provide substantial additional damping to stall-induced blade oscillations.
    Keywords: AIRCRAFT STABILITY AND CONTROL
    Type: European Rotorcraft and Powered Lift Aircraft Forum; Sept. 8-11, 1981; Garmish-Partenkirchen
    Format: text
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  • 7
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    Computation of high resolution unsteady airloads using a constant vorticity contour free wake model (1992)
    In:  Other Sources
    Details
    Publication Date: 2019-07-27
    Description: Recent work in the study of helicopter aerodynamic loading for acoustics applications has involved research on the development of an exceptionally efficient simulation of the velocity field induced by the rotor's vortex wake. This paper summarizes the work to date on the development of this analysis, which builds on the refined constant vorticity contour (CVC) free wake model recently developed for application to the study of vibratory loading. The particular focus of this paper is on demonstrations of a reconstruction approach that efficiently computes both the flow fields and airloads induced by CVC wakes on lifting rotor blades. Results of recent calculations on both main rotor and tail rotors are presented. These calculations show that by employing flow field 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 the direct calculations used in traditional methods.
    Keywords: AERODYNAMICS
    Type: AAAF, European Rotorcraft Forum; Sept. 15-18, 1992; Avignon; France|; 13 p.
    Format: text
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  • 8
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    Performance optimization for rotors in hover and axial flight (1989)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Performance optimization for rotors in hover and axial flight is a topic of continuing importance to rotorcraft designers. The aim of this Phase 1 effort has been to demonstrate that a linear optimization algorithm could be coupled to an existing influence coefficient hover performance code. This code, dubbed EHPIC (Evaluation of Hover Performance using Influence Coefficients), uses a quasi-linear wake relaxation to solve for the rotor performance. The coupling was accomplished by expanding of the matrix of linearized influence coefficients in EHPIC to accommodate design variables and deriving new coefficients for linearized equations governing perturbations in power and thrust. These coefficients formed the input to a linear optimization analysis, which used the flow tangency conditions on the blade and in the wake to impose equality constraints on the expanded system of equations; user-specified inequality contraints were also employed to bound the changes in the design. It was found that this locally linearized analysis could be invoked to predict a design change that would produce a reduction in the power required by the rotor at constant thrust. Thus, an efficient search for improved versions of the baseline design can be carried out while retaining the accuracy inherent in a free wake/lifting surface performance analysis.
    Keywords: AERODYNAMICS
    Type: NASA-CR-177524 , NAS 1.26:177524
    Format: application/pdf
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  • 9
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    Optimization of rotor performance in hover using a free wake analysis (1991)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Keywords: AIRCRAFT DESIGN, TESTING AND PERFORMANCE
    Type: Journal of Aircraft (ISSN 0021-8669); 28; 200-207
    Format: text
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  • 10
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    Analyzing Wakes From Hovering-Helicopter Rotor Blades (1987)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: New method for analyzing free wake of hovering-helicopter rotor produces more reliable results and requires less computer time. Copes with wake instabilities, both physical and numerical, that afflict usual time-stepping analysis methods. Tip-vortex position, in coordinates moving with rotor blade, updated in relaxation procedure. Wake-airflow solution reached when updates converge to steady wake shape.
    Keywords: MECHANICS
    Type: ARC-11675 , NASA Tech Briefs (ISSN 0145-319X); 11; 9; P. 80
    Format: text
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