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  • 1
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    Some approximations to the flapping stability of helicopter rotors (1974)
    In:  CASI
    Details
    Publication Date: 2016-06-07
    Description: The flapping equation for a helicopter in forward flight are reported which have coefficients that are periodic in time, and this effect complicates the calculation of stability. A constant coefficient approximation which will allow the use of all the well known methods for analyzing constant coefficient equations are presented. The flapping equation is first transformed into the nonrotating coordinate frame, where some of the periodic coefficients are transformed into constant terms. The constant coefficient approximation is then made by using time averaged coefficients in the nonrotating frame. Stability calculations based on the approximation are compared to results from a theory which correctly includes all of the periodicity. The comparison indicates that the approximation is reasonably accurate at advance ratios up to 0.5.
    Keywords: AIRCRAFT
    Type: Its Rotorcraft Dyn.; p 45-53
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 2
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    Longitudinal and lateral-directional aerodynamic characteristics of a large-scale, V/STOL model with four tilting ducted fans arranged in a dual tandem configuration (1966)
    In:  CASI
    Details
    Publication Date: 2019-05-29
    Description: Longitudinal and lateral-directional aerodynamic characteristics of large scale, V/STOL model with four tilting ducted fans arranged in dual tandem configuration
    Keywords: AIRCRAFT
    Type: NASA-TN-D-3490
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 3
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    Wind-tunnel test of a full-scale, 1.1 pressure ratio, ducted lift-cruise fan (1964)
    In:  CASI
    Details
    Publication Date: 2019-05-23
    Description: Fixed blade angle, ducted, tip-turbine-driven, cruise fan as aircraft propulsive device at high subsonic speed
    Keywords: AERODYNAMICS
    Type: NASA-TN-D-2498
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 4
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    A note on multicyclic control by swashplate oscillation (1978)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: It was shown that for two, three, or four bladed rotors, simple oscillation of the nonrotating swashplate controls can produce prescribed blade pitch schedules of the sort which were suggested for vibration alleviation. Equations were given which relate the swashplate motions to the resulting blade pitch schedules.
    Keywords: AIRCRAFT STABILITY AND CONTROL
    Type: NASA-TM-78475 , A-7367
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 5
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    Application of a parameter identification technique to a hingeless helicopter rotor (1974)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: A mathematical model of a gyro-controlled, three-bladed hingeless helicopter rotor was developed and parameters of the model were estimated using a parameter identification technique. The flapping and feathering degrees of freedom of the blades were modeled. The equations of the model contain time-varying, periodic coefficients due to the forward speed of the rotor. A digital simulation of the analytical model was compared with wind-tunnel measurements to establish the validity of the model. Comparisons of steady-state and transient solutions of the analytical model with the tunnel measurements gave reasonably good matching of gyro angle but less satisfactory matching of hub moment measurements. Further improvements were obtained by use of a parameter identification technique to adjust as many as 10 parameters of the analytical model. The sensitivity of the blade response to small changes in the parameters was also calculated.
    Keywords: AERODYNAMICS
    Type: NASA-TN-D-7834 , A-5289
    Format: application/pdf
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  • 6
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    Shake test of rotor test apparatus in the 40- by 80-foot wind tunnel (1975)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: A shake test was conducted to determine the dynamic characteristics of a rotor test apparatus on strut systems. The rotor-off hub transfer function (acceleration per unit force as a function of frequency) was measured in the longitudinal and lateral directions, using a combination of broadband and discrete frequency excitation techniques. The dynamic data is summarized for the configurations tested, giving the following properties for each mode identified: the natural frequency, the hub response at resonance, and the fixed system damping. The complete transfer functions are presented, and the detailed test results are included as an appendix. Data analysis techniques developed to obtain on-line measurements of the system modal properties, including the damping coefficient and the damping ratio are discussed.
    Keywords: RESEARCH AND SUPPORT FACILITIES (AIR)
    Type: NASA-TM-X-62418 , A-5973
    Format: application/pdf
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  • 7
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    Investigating complex aerodynamic flows with a laser velocimeter (1976)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: The paper discusses the application of the laser velocimeter in the study of two highly complex aerodynamic flows. In the first experiment, the laser velocimeter has been used with frequency tracking electronics to survey the multiple vortex wake structure behind a model of a large jet transport. The second application is to the study of the induced instantaneous inflow velocities near the blades of a model helicopter rotor; counter type processing was used in these measurements. In each experiment, the data output channels of these processors were handled in an on-line fashion, including both velocity computations and the plotting of fully reduced data.
    Keywords: INSTRUMENTATION AND PHOTOGRAPHY
    Type: The accuracy of flow measurements by laser Doppler methods; Aug 25, 1975 - Aug 28, 1975; Copenhagen; Denmark
    Format: text
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    NASA TECHNICAL REPORTS
  • 8
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    Laser velocimeter measurements of the helicopter rotor-induced flow field (1974)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: The use of a two-color laser velocimeter to measure the flow velocities in the wake of a helicopter rotor is discussed, including methods for obtaining two components of both instantaneous and time-averaged velocities. Results are presented from an experiment using a 2.13 m diameter model helicopter rotor operating at a tip speed ratio of 0.18 in a wind tunnel. The location of the tip vortex from the preceding blade was determined on the advancing side, and the diameter of the vortex core was found to be 15 percent of the blade chord (1.5 percent of the radius). The effects of the airfoil's bound vorticity were observed in the velocity distributions vry near the blade. These effects suggest that the laser velocimeter may be used to determine the aerodynamic loading (circulation) at a spanwise station on the blade. Also, the structure and boundary of the time-averaged wake were investigated.
    Keywords: INSTRUMENTATION AND PHOTOGRAPHY
    Type: AHS PREPRINT 800 , Annual National V/STOL Forum; May 07, 1974 - May 09, 1974; Washington, DC
    Format: text
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    NASA TECHNICAL REPORTS
  • 9
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    Measurements of helicopter rotor tip vortices (1977)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: This paper presents results from a recent wind-tunnel investigation of model helicopter rotor tip vortices. Measurements were made of the vortex positions, core sizes, and velocity distributions. A laser velocimeter was used to make the measurements, and a minicomputer-based data system was used to process the data and to aid in controlling the experiment. The velocimeter, the data system, and the software developed for the minicomputer are briefly described. The rotors investigated were two-bladed, teetering rotors with diameters of 2.1 m. Two sets of blades were used, one set with zero twist and one set with -11 deg of linear twist. The vortex positions were obtained by making flow field traverses while strobing the data system at a fixed azimuth. Aging of a vortex element was also studied by following the convected element while strobing the data system at different azimuths. By this method, the effects on the vortex of a close interaction with a blade and another vortex were studied.
    Keywords: AERODYNAMICS
    Type: AHS 77-33-06 , Annual National Forum; May 09, 1977 - May 11, 1977; Washington, DC
    Format: text
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  • 10
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    An investigation of full-scale helicopter rotors at high advance ratios and advancing tip Mach numbers (1968)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Tabulated performance data of full-scale rotary wings at high advance ratios and advancing tip Mach numbers based on wind tunnel tests
    Keywords: AERODYNAMICS
    Type: NASA-TN-D-4632
    Format: application/pdf
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