Deog-Kwan Kim

Structural Analysis of a Bearingless Rotor Using an Improved Flexible Multibody Model

This paper presents an improved structural analysis for a bearingless helicopter rotor. The bearingless rotor usually features a significantly large elastic twist in the flexbeam and additional unique structural characteristics. Thus, it will require sophisticated structural analysis and relevant numerical validation procedures due to its multiple load paths, as induced by the single or multiple flexbeams and the torque tube. In this paper, an extended finite element formulation was derived to consider the multiple components as individual beam elements. A geometrically exact beam formulation was adopted to describe the nonlinear behavior of these major components in the rotor precisely. To implement the interconnecting kinematic relationship with the major components, Lagrange multipliers were used. The present static analysis was validated through comparisons with the existing multi-body dynamics analysis DYMORE. Additional results were obtained for rotating conditions in both a vacuum and a set atmosph...

Helicopter Rotor Load Prediction Using a Geometrically Exact Beam with Multicomponent Model

In this paper, an accurate structural dynamic analysis was developed for a helicopter rotor system including rotor control components, which was coupled to various aerodynamic and wake models in order to predict an aeroelastic response and the loads acting on the rotor. Its blade analysis was based on an intrinsic formulation of moving beams implemented in the time domain. The rotor control system was modeled as a combination of rigid and elastic components. A multicomponent analysis was then developed by coupling the beam finite element model with the rotor control system model to obtain a complete rotor-blade/control-system aeroelastic analysis. The rotor blade analysis was in good agreement and validated by comparing with DYMORE. Numerical results were obtained for a four-bladed, small-scale, articulated rotor rotating in vacuum and in a wind tunnel to simulate forward-flight conditions and its aerodynamic effects. The complete rotor-blade/control-system model was loosely coupled with various inflow and wake models in order to simulate both hover and forward-flight conditions. The resulting rotor blade response and pitch link loads are in good agreement with those predicted by CAMRAD II. The present analysis features both model compactness and robustness in its solution procedure while capturing the sophisticated behavior of individual rotor components. The analysis is expected to be part of a framework useful in the preliminary design phase for helicopters.

Hub Parametric Investigation of Main Rotor Stability of Bearingless Helicopter

This paper describes a stability and dynamic characteristics of bearingless helicopter main rotor in hover. Baseline rotor configuration is defined and modal analysis for the configuration is taken to verify the dynamic characteristics. The kinematic pitch-lag couplings through ways of pitch link installation are analyzed to know effects on loads, frequencies and stability. The effects of pitch link attachments in spanwise direction and chordwise direction as well as pitch link inclination on thrust, power, flpa-lag-pitch mode frequencies and inplane damping are examined. Pitch link at trailing edge location in chordwise direction has influence on aeroelastic stability of the rotor. Also, the pitch link with negative inclination angle makes inplane damping increase.

Development of an Aerodynamic Performance Analysis Module for Rotorcraft Comprehensive Analysis Code

In this study, an aerodynamic performance analysis code has been developed as a part of rotorcraft comprehensive program. Airloads on rotor blades are calculated based on the blade element theory with look-up tables of aerodynamic coefficients of 2-D airfoils. In order to calculate rotor induced inflow, various inflow prediction methods such as linear inflow, dynamic inflow, prescribed wake and free wake model are integrated into the present module. The aerodynamic characteristics of each method are compared and validated against available experimental data such as Elliot`s inflow distribution and sectional normal force coefficients of AH-1G.

Numerical Investigation on Overlap Effects of Tandem Rotors in Forward Flight

A study on the interference effects of overlapping tandem rotors in forward flight is conducted using the time-marching free-wake panel method which adopts field velocity boundary integral formulation. The conventional boundary integral formulation is numerically unstable for the cases when the blade and the wake are in close proximity to each other. In order to avoid this problem, this study applies the field velocity method and modifies the boundary integration formulation. The improved method is used for the parametric study on the advance ratio and the distance between the rotors. These are the parameters that most affect the interference of the tandem rotor in forward flight. Comparison of the aerodynamic performance shows that the horizontal distance between the rotors negligibly influences the overlap-induced power factor for high advance ratio. In addition, it shows that the overlap-induced power factor is inversely proportional to the squared vertical distance between the rotors, and that the overlap-induced power factor increases to a certain extent and decrease back as the advance ratio increases.

Design and Vibratory Loads Reduction Analysis of Advanced Active Twist Rotor Blades Incorporating Single Crystal Piezoelectric Fiber Composites

This paper presents design optimization of a new Active Twist Rotor (ATR) blade and conducts its aeroelastic analysis in forward flight condition. In order to improve a twist actuation performance, the present ATR blade utilizes a single crystal piezoelectric fiber composite actuator and the blade cross-sectional layout is designed through an optimization procedure. The single crystal piezoelectric fiber composite actuator has excellent piezoelectric strain performance when compared with the previous piezoelectric fiber composites such as Active Fiber Composites (AFC) and Macro Fiber Composites (MFC). Further design optimization gives a cross-sectional layout that maximizes the static twist actuation while satisfying various blade design requirements. After the design optimization is completed successfully, an aeroelastic analysis of the present ATR blade in forward flight is conducted to confirm the efficiency in reducing the vibratory loads at both fixed- and rotating-systems. Numerical simulation shows that the present ATR blade utilizing single crystal piezoelectric fiber composites may reduce the vibratory loads significantly even with much lower input-voltage when compared with that used in the previous ATR blade. However, for an application of the present single crystal piezoelectric actuator to a full scaled rotor blade, several issues exist. Difficulty of manufacturing in a large size and severe brittleness in its material characteristics will need to be examined.

Dynamic Characteristics of Helicopter Bearingless Main Rotor

ABSTRACT The characteristics of bearingless main rotor of helicopter are investigated throughnon-rotatingtestsandrotatingtests.Thestiffnessandnaturalfrequenciesofrotorblades,flexbeam,andtorquetubewhicharecorecomponentsofbaearinglessrotoraremeasuredtoobtain inputmaterial propertiesfor rotor analysis. The functionaltest on ground forassemblyofonehubwithdamper,snubber,andnobladeiscarriedouttocheckinterfacesbetweencomponents,kinematicsofcomponents,andpitchmotionrangesunderappliedloadsincludingcentrifugalload.The4-bladedbearinglessrotorwith5.82mofrotorradiusistestedonthewhirltowerwithrotationplaneof9.65mheight. Thethrustandpoweraremeasuredtoobtainhoverperformanceandthefrequenciesanddampingsoftherotorareobtainedbyexcitationofcyclicpitchbyhydraulicactuators. 초 록 이 논문에서는 무베어링 로터의 동특성을 비회전 강성 및 진동수 측정 시험과 회전 훨타워 시험을 통해 살펴보았다.무베어링 로터의 주요 구성품인 블레이드,유연보,토크튜브에 대해 강성 및 고유진동수를 측정하여 해석을 위한 물성치를 확보하였고,훨타워 시험 전에 댐퍼와 스너버를 장착한 무베어링 로터를 조립하여 지상에서 하중이 부가되는 조건하에 기능시험을 수행하여 로터 구성품간의 간섭 여부,기구학적 거동 등을 확인하였다.4개의 블레이드를 가지고 회전반경이 5.82m인 무베어링 로터의 회전 동특성시험을 회전면 높이가 9.65m인 훨타워에서 수행하였다.정지비행성능을 확인하기 위해 추력과 사용동력을 측정하였으며,회전시 블레이드의 고유진동수 및 감쇠를 측정하기 위해 유압 가진기를 통하여 사이클릭 가진을 하였다.KeyWords:Helicopter(헬리콥터),Bearinglessrotor(무베어링 로터),Dynamiccharacteristics(동특성),Damping(감쇠),Whirltest(훨시험)

Predictions on the Internal Loads and Structural Deflection in a Full-scale Experimental Bearingless Rotor

In this paper, the unsteady aerodynamics and blade structural dynamics of an experimental bearingless rotor were analyzed. Due to the multiple load path and nonlinear behavior of a bearingless rotor, sophisticated structural modeling and structural-aerodynamic coupled analysis is required. To predict the internal load and deformation of an experimental bearingless rotor, trim analysis was implemented. The results showed good agreement when compared with those predicted by CAMRAD II the rotorcraft comprehensive analysis. It is possible to extend the present structural-aerodynamic combined analysis to further advanced configurations of the bearingless rotor in the future.

Aerodynamics-Structural Coupled Analysis of a Bearingless Rotor using a Flexible Multi-body model

A bearingless rotor has been used to take advantage of the reduced number of helicopter hub components and maintenance expense. It comprises of three primary components: a main rotor blade, torque tube, and flexbeams. It has another advantage that its structure is simple. But large nonlinear bending-torsion is induced because the flexbeams have low torsional stiffness. Furthemore, since it has multiple load path, its structural modeling and structure-aerodynamics coupled analysis will be complicated. In this paper, a geometrically exact beam formulation was used to describe nonlinear behavior of the rotating beam elements. To conduct the multi-body analysis, multiple beam elements were assembled. The present structural model was coupled with the finite-state dynamic inflow aerodynamics. Finally, trim analysis was carried out, and the blade deformation/internal load was predicted. Its results were compared with those obtained by the rotorcraft comprehensive analysis, CAMRAD II. The blade deformation discrepancies were less than 5% and internal loads average difference was smaller than 10%.

Fatigue Safe Life Analysis of Helicopter Bearingless Rotor Hub Composite Flexbeam

After we designed Bearingless rotor hub system for 7,000lb class helicopter, flexbeam fatigue analysis was conducted for validation of requirement life time 8,000 hours. sectional structural analysis method applying elastic beam model was used. Fatigue analysis for two sections of flexbeam which were expected to weak to fatigue damage from result of static analysis was conducted. Extension, bending and torsion stiffness of flexbeam section shape was calculated using VABS for structure analysis. S-N curve of two composite material which composed flexbeam was generated using wohler equation. Load analysis of bearingless rotor system was conducted using CAMRAD II and load analysis result was applied HELIX/FELIX standard load spectrum to generate bearingless rotor system load spectrum which was used flexbeam fatigue safe life analysis.