Analysis of Stall Flutter of a Helicopter Rotor Blade

Analysis of Stall Flutter of a Helicopter Rotor Blade PDF

Author: Peter Crimi

Publisher:

Published: 1973

Total Pages: 138

ISBN-13:

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A study of rotor blade aeroelastic stability was carried out, using an analytic model of a two-dimensional airfoil undergoing dynamic stall and an elastomechanical representation including flapping, flapwise bending and torsional degrees of freedom. Results for a hovering rotor demonstrated that the models used are capable of reproducing both classical and stall flutter. The minimum rotor speed for the occurrence of stall flutter in hover, was found to be determined from coupling between torsion and flapping. Instabilities analogous to both classical and stall flutter were found to occur in forward flight. However, the large stall-related torsional oscillations which commonly limit aircraft forward speed appear to be the response to rapid changes in aerodynamic moment which accompany stall and unstall, rather than the result of an aeroelastic instability. The severity of stall-related instabilities and response was found to depend to some extent on linear stability. Increasing linear stability lessens the susceptibility to stall flutter and reduced the magnitude of the torsional response to stall and unstall.

Investigation of Helicopter Control Loads Induced by Stall Flutter

Investigation of Helicopter Control Loads Induced by Stall Flutter PDF

Author: P. J. Arcidiacono

Publisher:

Published: 1970

Total Pages: 188

ISBN-13:

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An analytical study was conducted to determine if available unsteady normal force and moment aerodynamic test data could be used in conjunction with existing helicopter rotor aeroelastic and variable inflow analyses to provide a method for predicting the stall flutter response of a helicopter rotor blade. For this purpose, incompressible unsteady aerodynamic data for an NACA 0012 airfoil executing pure sinusoidal pitching motions were employed. To apply such data under rotor blade operating conditions where multiharmonic motions and velocity variations exist, the data were expressed as functions of instantaneous section angle of attack, angular velocity, and angular acceleration. In addition, scaling procedures were developed in an attempt to account for the effects of compressibility. Limited application of the resulting analysis to define the aeroelastic characteristics of several blade designs showed that significant self-excited torsional oscillations of the stall flutter type could, in fact, be predicted for certain combinations of flight conditions and blade designs. Correlation studies, to evaluate the ability of the analysis to predict control loads, were performed with CH-53A maneuvering flight test data and with level flight test data from the NH-3A (S-61F). (Author).

Investigation of Helicopter Control Loads Induced by Stall Flutter

Investigation of Helicopter Control Loads Induced by Stall Flutter PDF

Author: Peter J. Arcidiacono

Publisher:

Published: 1970

Total Pages: 179

ISBN-13:

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An analytical study was conducted to determine if available unsteady normal force and moment aerodynamic test data could be used in conjunction with existing helicopter rotor aeroelastic and variable inflow analyses to provide a method for predicting the stall flutter response of a helicopter rotor blade. For this purpose, incompressible unsteady aerodynamic data for an NACA 0012 airfoil executing pure sinusoidal pitching motions were employed. To apply such data under rotor blade operating conditions where multiharmonic motions and velocity variations exist, the data were expressed as functions of instantaneous section angle of attack, angular velocity, and angular acceleration. In addition, scaling procedures were developed in an attempt to account for the effects of compressibility. Limited application of the resulting analysis to define the aeroelastic characteristics of several blade designs showed that significant self-excited torsional oscillations of the stall flutter type could, in fact, be predicted for certain combinations of flight conditions and blade designs. Correlation studies, to evaluate the ability of the analysis to predict control loads, were performed with CH-53A maneuvering flight test data and with level flight test data from the NH-3A (S-61F). (Author).

Prediction of Rotor Instability at High Forward Speeds. Volume 3. Stall Flutter

Prediction of Rotor Instability at High Forward Speeds. Volume 3. Stall Flutter PDF

Author: Franklin O. Carta

Publisher:

Published: 1969

Total Pages: 105

ISBN-13:

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The analytical study in this volume was carried out to determine the susceptibility of helicopter rotor blades to a stall flutter instability. This analysis was based on the use of unsteady aerodynamic data previously obtained for an NACA 0012 airfoil oscillating in pitch about its quarter-chord over a wide range of values of incidence angle, oscillatory frequency, amplitude of motion, and free-stream velocity. These data were originally available in the form of moment coefficient-incidence angle loops, and a twofold task was performed in carrying out this study. First, it was necessary to convert the moment coefficient data to an aerodynamic damping parameter form. This was accomplished by integrating the moment over one cycle of motion to yield the aerodynamic work per cycle, and this in turn was multiplied by appropriate conversion factors to produce the desired two-dimensional aerodynamic damping. Second, it was necessary to apply these two-dimensional results to a helicopter rotor to evaluate the weighted three-dimensional damping at each azimuth station, and to interpret the implications of any predicted region of instability. The stall flutter analysis was used in conjunction with the blade motion solution of Volume I to provide flight condition boundaries for stall flutter intensity.

A Stall Flutter of Helicopter Rotor Blades: a Special Case of the Dynamic Stall Phenomenon

A Stall Flutter of Helicopter Rotor Blades: a Special Case of the Dynamic Stall Phenomenon PDF

Author: Norman D. Ham

Publisher:

Published: 1967

Total Pages: 10

ISBN-13:

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Several conclusions were drawn with respect to stall flutter and airload prediction of high speed and/or highly loaded helicopter rotor blades. The stall of an airfoil section during rapid transient high angle of attack changes is delayed well above the static stall angle and results in a large transient negative pressure disturbance leading to large transient lift and nose down pitching moment. The magnitude of the pitching moment is such as to generate substantial nose down pitching displacements of the blade. These pitching displacements can substantially alter the angle of attack distribution of the rotor blade. The dynamic stall phenomenon of a helicopter rotor blade can be separated into three major phases: (1) A delay in the loss of blade leading edge suction to an angle of attack far above the static stall angle, with associated airloads of the type described by classical unsteady airfoil theory. (2) A subsequent loss of leading edge suction accompanied by the formation of large negative pressure disturbance (due to the shedding of vorticity from the vicinity of the blade leading edge) which moves aft over the upper surface of the blade. Associated with this phase are high transient lift, drag, and nose-down pitching moment associated with the greatly altered pressure distribution on the airfoil. (3) Complete upper surface separation of the classic static type, characterized by low lift, high drag, and moderate nose-down pitching moment. (Author).

Dynamic Stall

Dynamic Stall PDF

Author: Peter Crimi

Publisher:

Published: 1973

Total Pages: 58

ISBN-13:

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Problems associated with unsteady stall are summarized and past experimental and theoretical studies, relating primarily to dynamic stall of helicopter rotor blades, are reviewed. The problems attendant to analytic treatment of dynamic stall, including identification of relevant flow elements and definition of unsteady separation, are then discussed, and the basis for a theory which accounts for viscous effects and viscous-inviscid interactions analytically is presented. Results of computations are compared with measured loading on an airfoil undergoing sinusoidal pitching motion. The amounts of lift overshoot and their variation with frequency are in good agreement. Analyses of wake-induced stall and stall flutter of a helicopter rotor blade are then presented. The results indicate that the large stall-related torsional oscillations which commonly limit helicopter forward speed are the response to rapid changes in aerodynamic moment which accompany stall and unstall, rather than the consequence of an aeroelastic instability. (Author).

Principles of Helicopter Aerodynamics with CD Extra

Principles of Helicopter Aerodynamics with CD Extra PDF

Author: Gordon J. Leishman

Publisher: Cambridge University Press

Published: 2006-04-24

Total Pages: 860

ISBN-13: 9780521858601

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Written by an internationally recognized teacher and researcher, this book provides a thorough, modern treatment of the aerodynamic principles of helicopters and other rotating-wing vertical lift aircraft such as tilt rotors and autogiros. The text begins with a unique technical history of helicopter flight, and then covers basic methods of rotor aerodynamic analysis, and related issues associated with the performance of the helicopter and its aerodynamic design. It goes on to cover more advanced topics in helicopter aerodynamics, including airfoil flows, unsteady aerodynamics, dynamic stall, and rotor wakes, and rotor-airframe aerodynamic interactions, with final chapters on autogiros and advanced methods of helicopter aerodynamic analysis. Extensively illustrated throughout, each chapter includes a set of homework problems. Advanced undergraduate and graduate students, practising engineers, and researchers will welcome this thoroughly revised and updated text on rotating-wing aerodynamics.

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