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  • 1
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    Experimentally determined flutter from two- and three-bladed model bearingless rotors in hover (1985)
    In:  CASI
    Details
    Publication Date: 2016-06-07
    Description: A series of experiments was performed on a 1.8-m-diam model rotor in hover for the principal purpose of investigating the lead-lag stability of isolated bearingless rotors. Incidental to those tests, at least three types of pitch-flap flutter were encountered. Type 1 flutter occurred approximately at the second flap-mode frequency on both two-and three-bladed rotors for both small and large pitch angles and appeared to be a classic pitch-flap flutter. Type 2 flutter showed mostly torsional motion and was seen on both two- and three-bladed rotors. Type 3 flutter was a regressing flap flutter that occurred for only the three-bladed rotor configurations and appears to be a wake excited flutter. Although flutter occurred on a number of different configurations, no rotor parameters were identified that were clearly stabilizing or destabilizing.
    Keywords: AIRCRAFT STABILITY AND CONTROL
    Type: NASA. Ames Research Center Rotorcraft Dynamics 1984; p 69-88
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 2
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    A Theoretical and Experimental Investigation of Flap-Lag Stability of Hingeless Helicopter Rotor Blades (1972)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: The stability of hingeless rotor blade oscillations in hover is examined theoretically using a simplified centrally-hinged, spring-restrained, rigid blade to approximate the deflections of actual elastic blades. The aerodynamic and inertial coupling between the flap and lead-lag degrees of freedom is primarily responsible for instability, however elastic coupling and kinematic pitch-lag coupling both exert a powerful influence on hingeless rotor blade stability. Experimental results obtained from a two-bladed 1.81m diameter model rotor designed for minimum elastic coupling have confirmed the results of linear theory. For this model configuration rotor blade stall at high pitch angles was found to counteract the destabilizing flap-lag coupling and increase the damping of lead-lag oscillations. It was possible to account for this effect with the theory by using drag data for stalled airfoils.
    Keywords: AIRCRAFT
    Type: NASA-TM-X-62179
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 3
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    An analog technique for the measurement of damping from transient decay signals (1976)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: A tracking filter that includes an output that is logarithmically proportional to the amplitude of the filtered signal is used to measure the damping exponent of a transient decay. This method is analogous to the digital technique referred to as the moving-block or peak-plot method. The method is simple to use and quite accurate, avoids the numerical computations associated with similar digital techniques, but is constrained by the poor time-domain response characteristics of commercial tracking filters presently available.
    Keywords: INSTRUMENTATION AND PHOTOGRAPHY
    Type: NASA-TM-X-73121 , A-6535
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 4
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    Hingeless helicopter rotor with improved stability (1976)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Improved stability was provided in a hingeless helicopter rotor by inclining the principal elastic flexural axes and coupling pitching of the rotor blade with the lead-lag bending of the blade. The primary elastic flex axes were inclined by constructing the blade of materials that display non-uniform stiffness, and the specification described various cross section distributions and the resulting inclined flex axes. Arrangements for varying the pitch of the rotor blade in a predetermined relationship with lead-lag bending of the blade, i.e., bending of the blade in a plane parallel to its plane of rotation were constructed.
    Keywords: AIRCRAFT DESIGN, TESTING AND PERFORMANCE
    Format: application/pdf
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  • 5
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    An experimental investigation of hingeless helicopter rotor-body stability in hover (1978)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Model tests of a 1.62 m diameter rotor were performed to investigate the aeromechanical stability of coupled rotor-body systems in hover. Experimental measurements were made of modal frequencies and damping over a wide range of rotor speeds. Good data were obtained for the frequencies of the rotor lead-lag regressing mode. The quality of the damping measurements of the body modes was poor due to nonlinear damping in the gimbal ball bearings. Simulated vacuum testing was performed using substitute blades of tantalum that reduced the effective lock number to 0.2% of the model scale value while keeping the blade inertia constant. The experimental data were compared with theoretical predictions, and the correlation was in general very good.
    Keywords: AERONAUTICS (GENERAL)
    Type: NASA-TM-78489 , AVRADCOM-TR-78-17(AM) , A-7430
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 6
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    New design of hingeless helicopter rotor improves stability (1975)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Cantilever blades are attached directly to rotor hub, thereby substantially reducing cost and complexity and increasing reliability of helicopter rotor. Combination of structural flap-lag coupling and pitch-lag coupling provides damping of 6 to 10%, depending on magnitude of coupling parameters.
    Keywords: MECHANICS
    Type: ARC-10807
    Format: application/pdf
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    NASA TECHNICAL REPORTS
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