Document
June 1975 B75-10132
NASA TECH BRIEF
S.
Ames Research Center
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage commercial application. Tech Briefs are available on a subscription basis from the National Technical Information Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546.
New Design of Hingeless Helicopter Rotor Improves Stability The blades of conventional articulated rotor heli- structurally by special design of the blade material copters are attached to the rotor hub with hinges to elastic properties or by blade cross-section geometries permit flapping and lead-lag motion of the blades that displace the blade center of mass vertically with perpendicular and parallel to the plane of rotation.
respect to the shear center.
In hingeless rotor helicopters, cantilever blades are Because of hysteresis in the blade material, typical attached directly to the rotor hub, thereby substan- hingeless rotor blades exhibit roughly one-half to tially reducing the cost and complexity and increasing one-percent structural damping, but about four to the reliability of the helicopter rotor. However, some five percent damping (depending on the lead-lag form of aerolastic instability may be exhibited partly natural frequency of the rotor blade) is required to because the damping of lead-lag bending motion of prevent ground resonance instability. The combina- the cantilever blades is very small. For many hinge- tion of structural flap-lag coupling and pitch-lag less rotors, the minimum lead-lag damping occurs coupling described above provides damping of six to when the rotor is operating at low thrust and the ten percent, depending on the magnitude of the
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aerodynamic forces acting on the blades are small; coupling parameters.
accordingly, during ground contact, ground resonance Note: instability may be encountered if the structural Requests for further information may be directed damping of the rotor blades is not sufficiently high to: or if auxiliary lead-lag dampers are not installed.
Technology Utilization Officer Attempts to prevent hingeless rotor instabilities by Ames Research Center design modifications have not always been successful; Moffett Field, California 94035 auxiliary lead-lag dampers may be installed, but in- Reference: TSP 75-10132 creases in weight, cost, and complexity compromise the inherent advantage of the hingeless rotor.
Patent status: Increased damping of the blade lead-lag motion at This invention is owned by NASA, and a patent zero thrust is provided aerodynamically by a new application has been filed. Inquiries concerning non- design that combines structural coupling of flap exclusive or exclusive license for its commercial de- bending and lead-lag bending with the coupling be- velopment should be addressed to: tween pitching and lead-lag bending of the blade.
NASA Patent Counsel The structural flap-lag coupling is provided by Mail Code 200-1 1A orienting the principal flexural axes of the blade Ames Research Center cross section at an angle of up to 30° to with Moffett Field, California 94035 respect to the plane of rotation rather than roughly to of conventional helicopter rotor blades. Source: Robert A. Ormiston, William G. Bousman, Coupling of the blade pitch angle with lead-lag Dewey H. Hodges, and David A. Peters bending of the blade can be realized kinematically by Ames Research Center and proper arrangement of the geometry of the blade U.S. Army Air Mobility R & D Laboratory pitch controls. Pitch-lag coupling can also be realized (ARC-10807) .
Category 06 Government assumes any liability resulting from the use of the This document was prepared under the sponsorship of the National Aeronautics and Space Administration. Neither the United States information contained in this document, or warrants that such use Government nor any person acting on behalf of the United States will be free from privately owned rights.