Assessment of Composite Rotor Blade Modeling Techniques Sung Natn Jung V T. Nagaraj lndcrjit Chopli~ Viriring A:sirro,~r Professor Reseatdt Scierrrisr A(frrd Cesso~v Pr,feisor nr~d llircc~o, Clronbuk Nrrrior~nl Ut~ivemir,, Cltofljrt, Korea Alfird Gessow Ruro~rqli Cenre~ Deljorl~rtei~r oJAero.rl,oce E,rgi,~er,btg Uf~ivorily of Matyln,~rl.College PC,&, MD 20742. USA In the present paper, an assessment is made of the state-of-the-art in modeling composite rotors with a view of bringing out the influence of special characteristics of compnsite materials. The review encompasses the modeling of thin- and thick-walled composite rotors, the influences of non-classical effects such as out-of-plane warping, warping restraints, and transverse shear, non-uniformities in blade construction. laree deformations. aeroelastic stabilitv in hover and i n forward flieht. aeromechani- , - cal stability and optimization. The paper includes recommendations for future directions in these topics. Introduction Background During the past two decades, advanced composites havc round in- creasing applications in the construction o f helicopter rotor blades. At the present time, there is a good understandingof the design and manufactur- ing practices of composite rotor blades with balanced laminates. The fa- vorable propelties of fiber reinforced composites, such as higher specilic strength and stiffness, superior damage tolerance and thc possibility of manufacturing complex gcomelrical shapes, make them more attractive than metals for rotorcraft applications. Thcorelical studies have shown that compositcs offer the potential o f certain structural couplings that can have a favorable influence on the aeroelastic behavior of rotor blades to im- prove performance, increase aeromechanical stability and minimize vibra- tion and blade loads. Such couplings have not been exploited in current rotor designs. Composite rotor blades are normally of closed single- or multi-celled cross-sections and are thin-walled except near the root where they become thick-walled. The flex-beams of bearingless rotors are often of open cross- section such as solid rectangular, l-section, or cruciform-section beams. Apalt from the possibility of using different materials such as glass, graphite and kevlar, various lay-up schemes can be used for the construc- tion of the rotor blades. It is challenging to optimally dcsign a rotor blade that is inherently efficient and cost effective to build. The acromechanical analysis of helicopter rotors involves the interaction of structural, inertial and aerodynamic forces. The modeling of all these forces is essential tu Presented at AHS International Meeting on Adv.mced Rolorcmlt Technology and Disaster Relief. Gifu. Japan, April 21-23, 1998. Ma~uscript received June 1998; ac- ccpbd Much 1999. predict aeromechanical responses, including the resulting structural loads and stability characteristics. During the past decade, there has been a phenomenal growth o f rc- search activities to develop methodology lo analyze composite rotor blades. These range from simple analytical models to detailed finite cle- ment methods. These methods havc been validated using other analyses and experimental data from simple specimens and, sometimes, scalc mod- els. These nudies have led to a better understanding of the structural be- ha\,ior of composite rotor bladcs and to the importance of nonclassical eC fects such as out-of-planc warping, warping restraint and transverse shear on the behavior of campasite blades. The bulk of the research has been de- voted to uniform blades and towards prediction of dynamic responses; there have been "cry few studies dealing with practical aspects like non- uniformities arising from taper, ply-drop-oft, edge delaminations etc., es- pecially with rotor blades with unbalanced laminates. In this paper, the state-of-the-artmethods for modclingco~nposite rotor blades tbr static and dynamics analyses are presented. Special characteris- tics of anisotropic composite materials that include claslic coupling. trans- verse shear, section walping and warping restraint are discussed. The paper messes the present status in the modeling of thin- and thick-walled com- posite rotors, non-classicalstructural effects, non-uniformitiesi n blade con- struction, largc dehrmations, aeroelastic stability io hover arid forward flight, aeromechanical stability, and structural optimization.The paper con- cludes with recommendations for future directions in these topics. Special Characteristics of Composite Rotor Blades Composite rotor blades are designed [rum low-stress and long-cycle fatigue considerations that imply low strain conditions. Moderate-to-large bending slopes and twist of a rotor blade introduce gcometric nonlineari- ties that give rise to kinematic couplings belwcen piah, tlap, lag and axial deflections. Key issues involved in the modeling of anisotropic composite