Response, Loads and Stability of Rotors With Interconnected Blades a /-v I.K. Suresh Helicopter Design Bureau Hindustan Aeronautics Ltd. Bangalore 56001 7, Indra J.Nagabhushanarn Dept. oJAeros,>nce Ettgitreering Indiarl Irrstitute oJScierzce Banyalore 560 012, India An isolated rotor with hlades interconnected through viscoelastic elements is analyzed for response, loads and stability in moment trim under forward flight conditions. A conceptual model of a multibladed rotor with rigid flap and lag motions is considered. Although the interconnecting elements are placed in the in-plane direction, considerable cou- pling between the flap-lag motions of the hlades can occur in certain ranges of interhlade element stiffness. Interhlade coupling can yield significant changes in the response, loads and stability which are dependent on the interhlade ele- ment and rotor parameters. Notation Lift curve Slope, rad Sectional lift coefficient Sectional drag coefficient Damping coefficient of interhlade element Flap blade root damping coeficient Lag blade root damping coeff~ciie Blade hinge offset Olfset from lag hinge to interblade element attachment point Mass moment of inertia of blade at hinge Linear stiffness of interblade element Stiffness of Flap and Lag root springs at hinge Mass of interblade element (mJB per unit length) ~- Number of blades Running coordinate along blade Rotor radius Flap-lag structural coupling parameter Velocity components parallel and normal to the blade airfoil chord/RRb Resultant of UT and Up at airfoil section Interblade spacing, W N b Rotor coning angle Flap angle of i" blade Lock number Azimuth angle of ith blade Advance ratio Interblade element mass parameter irh blade pitch angle Interhlade element stiffness parameter Fundamental nondimensional lead-lag and flap natural fre- quencies in rotating coordinate system Multihlade flap collective, regressive Manuscript recicvcd June 1994: accepted June 1996. 283 Opp.Opd Progressive and differential mode frequencies oE.wSr Multiblade lag collective, regressive ;tp,~td progressive and differential mode frequencies Rotor speed, Nominal ti Lag angle of iIh blade ti-,. ti+, Lag angle of (i-1)" and (i+l)" blades 0 Nondimensional quantity, ( )lR ('1 Time derivative, d( )Idt or d ( ) / w Introduction Very few rotor designs with interconnected rotor hlades, which are essentially adopted to improve the ground resonance characteristics of the helicopter, are in existcnce currently. Recently, the French helicop- ter industry tested rotors with interconnected blades. Mscoelastic ele- ments were used as interconnecting elements.It was ohserved that these rotor designs can lead to helicopters with good ground resonance char- acteristics, decreased flight loads, extended flight envelope, reduced weight and cross-section and lower vibrational levels (Refs. l,2). Although there are some helicopters in existence with interconnected blades, hardly any Literahue is available on their response, loads and stabil- itv characteristicswhich is needed to fullv understand. evaluate and exoloit the potcnlial of their designs. Recently, the governing equations for a con- ceptual model of such arotor having rigid hlades with flap-lagmotions and interconnected with viscoelastic elements were developed (Ref. 3). In the present paper, using these equations, an isolated rotor is analyzed for the response, loads and stability characteristics in hover and forward flight un- der moment uim conditions.These characteristic~ are studied for the varia- tions of rotor blade and interconnecting viscoelastic element parameters. Analytical Model Theconceptual model of aNb-bladedrotor with interconnected blades is presented in figure I. The rotor blades are idealized as rigid with flap- lag motions executed about a coincident hinge located at a distance e from the hub centre. The flap and lead-lag motions arc rcstraincd at the