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

Bousman, W. G.

Hodges, D. H.

Ormiston, R. A.

Peters, D. A.

English

NASA Technical Reports Server

June 1975	 B75-10132 
S. NASA TECH BRIEF 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 
E 
.
The blades of conventional articulated rotor heli-
copters are attached to the rotor hub with hinges to 
permit flapping and lead-lag motion of the blades 
perpendicular and parallel to the plane of rotation. 
In hingeless rotor helicopters, cantilever blades are 
attached directly to the rotor hub, thereby substan-
tially reducing the cost and complexity and increasing 
the reliability of the helicopter rotor. However, some 
form of aerolastic instability may be exhibited partly 
because the damping of lead-lag bending motion of 
the cantilever blades is very small. For many hinge-
less rotors, the minimum lead-lag damping occurs 
when the rotor is operating at low thrust and the 
aerodynamic forces acting on the blades are small; 
accordingly, during ground contact, ground resonance 
instability may be encountered if the structural 
damping of the rotor blades is not sufficiently high 
or if auxiliary lead-lag dampers are not installed. 
Attempts to prevent hingeless rotor instabilities by 
design modifications have not always been successful; 
auxiliary lead-lag dampers may be installed, but in-
creases in weight, cost, and complexity compromise 
the inherent advantage of the hingeless rotor. 
Increased damping of the blade lead-lag motion at 
zero thrust is provided aerodynamically by a new 
design that combines structural coupling of flap 
bending and lead-lag bending with the coupling be-
tween pitching and lead-lag bending of the blade. 
The structural flap-lag coupling is provided by 
orienting the principal flexural axes of the blade 
cross section at an angle of up to 30° to 450 with 
respect to the plane of rotation rather than roughly 
00 to 50 of conventional helicopter rotor blades. 
Coupling of the blade pitch angle with lead-lag 
bending of the blade can be realized kinematically by 
proper arrangement of the geometry of the blade 
pitch controls. Pitch-lag coupling can also be realized
structurally by special design of the blade material 
elastic properties or by blade cross-section geometries 
that displace the blade center of mass vertically with 
respect to the shear center. 
Because of hysteresis in the blade material, typical 
hingeless rotor blades exhibit roughly one-half to 
one-percent structural damping, but about four to 
five percent damping (depending on the lead-lag 
natural frequency of the rotor blade) is required to 
prevent ground resonance instability. The combina-
tion of structural flap-lag coupling and pitch-lag 
coupling described above provides damping of six to 
ten percent, depending on the magnitude of the 
coupling parameters. 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: TSP 75-10132 
Patent status: 
This invention is owned by NASA, and a patent 
application has been filed. Inquiries concerning non-
exclusive or exclusive license for its commercial de-
velopment should be addressed to: 
NASA Patent Counsel 
Mail Code 200-1 1A 
Ames Research Center 
Moffett Field, California 94035 
Source: Robert A. Ormiston, William G. Bousman,

Dewey H. Hodges, and David A. Peters

Ames Research Center and

U.S. Army Air Mobility R & D Laboratory

(ARC-10807) 
Category 06 
This document was prepared under the sponsorship of the National
	
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information contained in this document, or warrants that such use 
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will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19750000132 2020-03-19T20:23:24+00:00Z


New design of hingeless helicopter rotor improves stability

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