Composite Rotor Research Articles

Combined tension and bending testing of tapered composite laminates

A simple beam element used at Bell Helicopter was incorporated in the Computational Mechanics Testbed (COMET) finite element code at the Langley Research Center (LaRC) to analyze the responce of tappered laminates typical of flexbeams in composite rotor hubs. This beam element incorporated the influence of membrane loads on the flexural response of the tapered laminate configurations modeled and tested in a combined axial tension and bending (ATB) hydraulic load frame designed and built at LaRC. The moments generated from the finite element model were used in a tapered laminated plate theory analysis to estimate axial stresses on the surface of the tapered laminates due to combined bending and tension loads. Surfaces strains were calculated and compared to surface strains measured using strain gages mounted along the laminate length. The strain distributions correlated reasonably well with the analysis. The analysis was then used to examine the surface strain distribution in a non-linear tapered laminate where a similarly good correlation was obtained. Results indicate that simple finite element beam models may be used to identify tapered laminate configurations best suited for simulating the response of a composite flexbeam in a full scale rotor hub.

Mass loaded composite rotor for vibro-acoustic application

Mass loaded composite rotor for vibro-acoustic application

A fully nonlinear theory of curved and twisted composite rotor blades accounting for warpings and three-dimensional stress effects

A new approach is used to develop a geometrically exact nonlinear beam model for naturally curved and twisted solid composite rotor blades undergoing large vibrations in three-dimensional space. A combination of the new concepts of local displacements and local engineering stresses and strains, a new interpretation and manipulation of the virtual local rotations, an exact coordinate transformation, and the extended Hamilton principle is used to derive six fully nonlinear equations of motion describing one extension, two bending, one torsion, and two shearing vibrations of composite beams. The formulation is based on an energy approach, but the derivation is fully correlated with the Newtonian approach and provides a straightforward explanation of all nonlinear structural terms without using complex tensor operations or asymptotic expansions. The theory accounts for in-plane and out-of-plane warpings due to bending, extensional, shearing and torsional loadings, elastic couplings among warpings, and three-dimensional stress effects by using the results of a two-dimensional, static, sectional, finite element analysis. Also, the theory fully accounts for extensionality, initial curvatures and geometric nonlinearities. The equations display linear elastic couplings due to structural anisotropy and initial curvatures and nonlinear geometric couplings. The theory contains most of the existing beam theories as special cases, and the final equations of motion are put in compact matrix form.

Aeroelastic response of composite rotor blades considering transverse shear and structural damping

The effects of transverse shear deformations and structural damping on the flutter phenomena of a composite rotor blade in hover have been investigated using the finite element method. First-order shear deformation theory with rotary inertia effects and a damped element model of composite laminates are employed for the structural formulation. A quasisteady aerodynamic theory with a dynamic inflow model is used. Torsion-related out-of-plane warping and noncirculatory aerodynamic components are also incorporated in the formulation. Using these structural and aerodynamic tools, several numerical studies are carried out, first to validate the current approach and second to show the effects of transverse shear deformations and structural damping on the aeroelastic stability of a composite rotor as a function of fiber orientation. The predictions derived by the frequency analysis of this model are found to be more accurate than those given by an alternative approach compared with experimental data. It is shown that the transverse shear flexibility tends to lower the frequency of the rotor, and generally has a destabilizing effect on the lag mode and a stabilizing effect on the flap mode. It is also presented that the magnitude of structural damping can be controlled by changing ply orientation angle. CT cd c, EA

Vibration and flutter of stiff-inplane elastically tailored composite rotor blades

Aeroelastic response, blade and hub loads, and shaft-fixed aeroelastic stability is investigated for a helicopter with elastically tailored stiff-inplane composite rotor blades. A free wake model for nonuniform rotor inflow is integrated with a recently developed finite-element-based aeroelastic analysis for helicopters with tailored composite blades. Pitch-flap and pitch-lag elastic couplings, introduced through the anisotropy of the plies in the blade spar, have a significant effect on the dynamic elastic torsion response. Positive and negative pitch-flap couplings reduce vertical hub shear forces approximately 20% in the high vibration transition flight regime, however, negative pitch-flap elastic coupling significantly increases inplane hub shear forces at all flight speeds. The influence of pitch-flap, pitch-lag, and extension-torsion elastic couplings on the rotating frame blade bending moments is small. Ply-induced composite couplings have a powerful effect on blade stability in both hover and forward flight. Positive pitch-flap, positive pitch-lag, and positive extension-torsion couplings each have a stabilizing effect on lag mode damping. Negative pitch-lag coupling has a strong destabilizing effect on blade lag stability, resulting in a mild instability at moderate flight speeds.

Optimization of composite rotor blades with advanced structural and aerodynamic modeling

This paper addresses the first significant step in the ongoing research towards the overall goal of developing an integrated dynamic, aerodynamic, and structural optimization methodology for helicopter rotor blades made of composite materials. It investigates the feasibility and the utility of the structural-dynamic phase of the optimization task with realistic blade cross-sections. As opposed to the often used, simplified cross-sections, the lift generating part of the rotor blade has a realistic airfoil shape in the present shape optimization task. A set of ‘primitive’ design variables form the representative design space in the present task of minimizing blade weight under several frequency, autorotational inertia, and airfoil mass-center related constraints. The investigation pertains to the hingeless rotors with frequency constraints for obtaining soft in-plane blades. The composite construction of the blade sections is analyzed via a two-dimensional finite element code, which establishes the beam constitutive law. The subsequent beam analysis is based on an hp-type displacement-finite-element strategy that incorporates all the geometric nonlinearities of the beam equations. The results indicate extreme sensitivity of the optimization process on the formulation of frequency constraints and starting designs. Autorotational inertia constraint also has a significant impact on the final design. The study shows the effectiveness of simpler and more numerous frequency constraints over the complicated and less numerous frequency constraints when trying to achieve an identical goal. The enhanced role of structural couplings (e.g., tension-bending, bending-torsion) was also observed under constraints on high modal frequencies. The work has highlighted the feasibility (and also the associated complexities) of including realistic blade details in a design optimization study. Research is progressing towards the overall objective of an integrated optimization study.

A new sensitivity analysis for structural optimization of composite rotor blades

This paper presents the mathematical details of the formulation for the sensitivity derivatives for the structural dynamic, aeroelastic stability, and response characteristics of a rotor blade in hover and forward flight. The formulation is denoted by the term semi-analytical approach, because certain derivatives are evaluated by a finite difference scheme. Using this formulation, sensitivity derivatives for structural dynamic and aeroelastic stability characteristics were evaluated for a composite rotor blade. Useful conclusions regarding the relative merits of the semi-analytical approach, when compared to a pure finite difference approach, are obtained. In addition, influence of ply orientation angles on the vibratory hub loads in forward flight is also considered.

A powder forged automotive connecting rod: L. Albano-Muller et al (Krebsoge Sinterholding GmbH, Radewormwald, Germany)

For ages flywheels have been used to achieve smooth operation of machines. The early models where purely mechanical consisting of only a stone wheel attached to an axle. Nowadays flywheels are complex constructions where energy is stored mechanically and transferred to and from the flywheel by an integrated motor/generator. The stone wheel has been replaced by a steel or composite rotor and magnetic bearings have been introduced. Today flywheels are used as supplementary UPS storage at several industries world over. Future applications span a wide range including electric vehicles, intermediate storage for renewable energy generation and direct grid applications from power quality issues to offering an alternative to strengthening transmission.One of the key issues for viable flywheel construction is a high overall efficiency, hence a reduction of the total losses. By increasing the voltage, current losses are decreased and otherwise necessary transformer steps become redundant. So far flywheels over 10 kV have not been constructed, mainly due to isolation problems associated with high voltage, but also because of limitations in the power electronics. Recent progress in semi-conductor technology enables faster switching and lower costs. The predominant part of prior studies have been directed towards optimising mechanical issues whereas the electro technical part now seem to show great potential for improvement. An overview of flywheel technology and previous projects are presented and moreover a 200 kW flywheel using high voltage technology is simulated.

Air and Ground Resonance of Helicopters with Elastically Tailored Composite Rotor Blades

The aeromechanical stability, including air resonance in hover, air resonance in forward flight, and ground resonance, of a helicopter with elastically tailored composite rotor blades is investigated. Five soft-inplane hingeless rotor configurations, featuring elastic pitch-lag, pitch-flap and extension-torsion couplings, are analyzed. Elastic couplings introduced through tailored composite blade spars can have a powerful effect on both air and ground resonance behavior. Elastic pitch-flap couplings (positive and negative) strongly affect body, rotor and dynamic inflow modes. Air resonance stability is diminished by elastic pitch-flap couplings in hover and forwrad flight. Negative pitch-lag elastic coupling has a stabilizing effect on the regressive lag mode in hover and forward flight. The negative pitch-lag coupling has a detrimental effect on ground resonance stability. Extension-torsion elastic coupling (blade pitch decreases due to tension) decreases regressive lag mode stability in both airborne and ground contact conditions. Increasing thrust levels has a beneficial influence on ground resonance stability for rotors with pitch-flap and extension-torsion coupling and is only marginally effective in improving stability of rotors with pitch-lag coupling.

Aeroelastic stability of composite hingeless rotor blades in hover—Part II: Results

In Part I of this two-part paper, an aeroelastic stability analysis was presented for isolated hingeless, composite rotor blades in the hovering flight condition, which was based on a mixed finite element method. Herein, the focus is to present numerical results obtained from this analysis. First, certain of these results are compared with those of existing aeroelastic stability analyses for validation. Next, the numerical accuracy and convergence characteristics of the current approach are quantified. Finally, parametric studies are performed to investigate the effects of composite elastic coupling and thrust condition on the blade's aeroelastic stability, especially that of the lightly damped lead-lag mode. The stability of some of the elastically coupled cases studied was sensitive to the nonclassical couplings; indeed, in one case a significant error appeared, accentuated at high thrust levels, when bending-shear coupling was neglected. Another significant effect stems from changes in the equilibrium solution for elastic twist due to extension-twist coupling. The necessity of including such effects in the blade model for general-purpose analysis is noted.

Aeroelastic stability of composite hingeless rotor blades in hover—Part I: Theory

A finite-element-based stability analysis is presented for isolated hingeless, composite rotor blades in the hovering flight condition. The formulation is comprised of separate, but compatible, cross-sectional (two-dimensional) and global or beam (one-dimensional) equations. The sectional analyses used account for all possible deformation in the three-dimensional representation of the blade. The global analysis is based on a mixed variational statement for the dynamics of moving beams; it can account for 6 × 6 cross-sectional stiffness and inertia matrices which, respectively, allow for the treatment of shear deformation and rotary inertia. There are no restrictions on the magnitudes of the displacements and rotations if the strain remains small compared to unity. The lift, drag, and pitching moment models are based on two-dimensional, quasi-steady strip theory, with induced inflow taken from momentum theory. The equilibrium operating configuration of the blade is obtained by an iterative solution of the complete nonlinear equations. The dynamic equations are linearized about this position, yielding an eigenproblem. In Part II, numerical results are presented for both extension-twist and bending-twist coupled rotor blades, which indicate that certain “nonclassical” couplings must be included in the analysis in general.

Nonlinear Large Amplitude Aeroelastic Behavior of Composite Rotor Blades

An investigation is made of the nonlinear, large amplitude aeroelastic behavior of hingeless composite rotor blades during hover. The aeroelastic model is capable of dealing with large vibration amplitudes as well as large static deflections including transverse shear and warping deformation and is based on an Euler angle formulation of the basic large deflection equations together with a harmonic balance, finite difference, and NewtonRaphson technique. Nonlinear stall aerodynamics is included by use of the ONERA airforces model. Analysis of large amplitude limit cycles that evolve from linear flutter solutions is performed on a [0/90]35 graphite/ epoxy composite blade model. Numerical results indicate that the nonlinear stall is dominant at moderate amplitudes, but that nonlinear static-dynamic couplings in the structure, which bring a softening effect into the general aeroelastic behavior, could be equally important at large amplitudes.

Aeroelastic Response, Loads, and Stability of a Composite Rotor in Forward Flight

The aeroelastic response, blade and hub loads, and shaft-fixed aeroelastic stability are investigated for a helicopter with elastically tailored composite rotor blades. A finite element analysis including nonclassical effects such as transverse shear, torsion related warping, and in-plane elasticity is integrated with the University of Margland Advanced Rotorcraft Code. The analysis is correlated against both experimental data and detailed finite element results. Correlation of rotating natural frequencies of coupled composite box-beams is generally within 5-10%

Leica Speeds Up Westland's Lynx Project

To keep pace with demand for its revolutionary Lynx composite main rotor blade, Westland Engineering's Composites Division installed a Wild TA10BL automatic cutting table which has enabled woven details to be cut out reliably and reproducibly in a greatly reduced timescale compared with manual cutting out.

Structural behavior of two-cell composite rotor blades with elastic couplings

This paper presents an analytical-cum-experimental study of the structural response of composite rotor blades with elastic couplings. Vlasov theory is expanded to analyze two-cell composite rotor blades made out of general composite laminates including the transverse shear deformation of the crosssection. Variation of shear stiffness along the contour of the section is included in the warping function . In order to validate this analysis, two-cell graphite-epoxy composite blades with extension-torsion coupling were fabricated using matched-die molding technique. These blades were tested under tip bending and torsional loads, and their structural response in terms of bending slope and twist was measured with a laser optical system. Good correlation between theory and experiment is achieved. Axial force induced twist rate of the order of 0.2 degree per inch length can be realized in extensiontorsion coupled blades with a hygrothermally stable [20/-70]2s layup for potential applications in the design of tilt rotors.

Approach for analysis and design of composite rotor blades

The developments in tilt rotor technology allow rotor blades to have different twist and rpm requirements in hover and in forward flight to optimize for operational conditions. The available twist of the rotor blade for different flight regimes bears on the performance of the flight. This article presents a detailed analysis of a typical rotor blade, the focus being the static response in the presence of extension-twist coupling induced due to asymmetric stacking sequence of laminas. The baseline rotor blade is modeled as a D-shaped spar using discrete Kirchoff theory-based finite elements. A method is proposed to derive six configurations of blade starting from a base rotor blade made of three laminated shells. The effects of varying rpm and lay-ups for the six configurations are presented as carpet plots in terms of the available tip twist angle. Such plots may be used in the rotor blade design trade-off studies.

Tiltrotor Research Aircraft composite blade repairs—Lessons learned

The XV-15, N703NA Tiltrotor Research Aircraft located at the NASA Ames Research Center, Moffett Field, California, currently uses a set of composite rotor blades of complex shape known as the advanced technology blades (ATBs). The main structural element of the blades is a D-spar constructed of unidirectional, angled fiberglass/graphite, with the aft-fairing portion of the blades constructed of a fiberglass cross-ply skin bonded to a Nomex honeycomb core. The blade tip is a removable laminate shell that fits over the outboard section of the spar structure, which contains a cavity to retain balance weights. Two types of tip shells are used for research. One is highly twisted (more than a conventional helicopter blade) and has a hollow core constructed of a thin Nomex-honeycomb-and-fiberglass-skin sandwich; the other is untwisted with a solid Nomex honeycomb core and a fiberglass cross-ply skin. During initial flight testing of the blades, a number of problems in the composite structure were encountered. These problems included debonding between the fiberglass skin and the honeycomb core, failure of the honeycomb core, failures in fiberglass splices, cracks in fiberglass blocks, misalignment of mated composite parts, and failures of retention of metal fasteners. Substantial time was spent in identifying and solving these problems. This paper discusses the types of problems encountered, the inspection procedures used to identify each problem, the repairs performed on the damaged or flawed areas, the level of criticality of the problems, and the subsequent monitoring of repaired areas. It is hoped that this discussion will help designers, analysts and experimenters in the future as the use of composites becomes more prevalent.

Experimental investigation of periodically excited rotating composite rotor blades

Experimental procedure for investigating the structural dynamics characteristics of thin-walled composite helicopter rotor blades undergoing periodical excitation during rotation is proposed and demonstrated. The testing setup, which is based on measurements in a vacuum chamber, provides the isolation of the structural dynamics behavior by eliminating the aerodynamic loads and introducing deliberate periodic concentrated loads at the blade's tip. The study presented in this paper includes the testing of thin-walled composite box beams in the nonrotating static case and in the case of periodic loading during rotation at various angular speeds along with comparisons with theoretical predictions. The investigation of the measured composite-related couplings and rotational effects showed some significant nonlinear characteristics.

The Sky is the Limit for Westland Helicopters

ROTOR blades on modern high performance helicopters are no longer entirely made of metal. New composit blades —made partly from carbon reinforced plastics — have enabled huge advances in rotor blade design. By freeing designers from the constraints of metal construction, performance has already been improved by 40 per cent. Composite rotor blades require low maintenance, are fatigue tolerant and offer a long service life.

Mechanical and Electrical Properties of Glass and Carbon Fiber-Reinforced Composites

The design of composite rotors for high-energy density pulsed power supplies demands accurate characterization of both the mechanical and electrical properties of fiber-reinforced epoxy. The mechanical properties of S-2 glass-epoxy, IM6 graphite-epoxy, and hybrid graphite-glass epoxy composites were measured in tension and torsion tests, providing strength and stiffness parameters for rotor dynamics modeling. Variable frequency electrical resistivity tests were conducted to allow estimation of eddy current losses arising in carbon-reinforced materials. Volume fraction measurements using electron microscopy and analysis by digestion allow for normalization of the test results with respect to composite fiber content. The experimental results were used to evaluate the micromechanical rule of mixtures and Halpin-Tsai correlations.