Structural analysis for designing rotor blades with integral actuators

Additional investigation of the optimization of cross-sectional properties for rotor blades was performed with methodology based on the Genetic Algorithm (GA). In previous work by the authors, a multilevel optimization was conducted using gradient-based methods that are appropriate for continuous variables. In contrast to that work, here optimization is conducted with the main focus of assessing the impact of discrete variables that are inherent to the problem at both local and global levels. The GA was modified based on existing routines in MATLAB to make the code suitable for the methodology presented and to utilize interfaces with the multi-body dynamics program DYMORE, used for global analysis, and the cross-sectional analysis program VABS, used at the local level. At the first exploratory phase of the optimization, values of discrete variables are determined using the GA along with several promising regions for global minima that serve as starting points for the second phase optimization, which deals only with continuous variables. Furthermore, if more than one optimal design exists, further analysis is performed to pick the best design from among candidate configurations. ptimization of the cross-sectional layout of composite rotor blades is a challenging problem because of the large number of design parameters and the wide-ranging impact of those parameters on overall vehicle characteristics. The availability of high-fidelity structural beam analysis tools naturally allows a hierarchical decomposition of the problem that separates the local (cross-sectional) sub-problem from the global optimization. By encapsulating the intricate details of the cross-sectional design into a relatively small number of equivalent beam properties, one can significantly reduce the overall complexity of the optimization. An additional advantage of such a decoupled approach is that it follows a venerable tradition of analyzing rotorcraft systems by splitting three-dimensional (3D) problems into separate sub-problems over the crosssection (2D) and span (1D). Modern computer tools for analyzing composite rotor blades with arbitrary cross-sectional configurations such as VABS 1, 2 provide a fast and accurate means of assessing both static and dynamic structural properties of rotor blades in terms of an equivalent beam models. Calculation of the cross-sectional stiffness matrix is conducted using a finite element discretization. Results of this calculation also include a set of cross-sectional stress/strain recovery relations. Once the equivalent cross-sectional stiffnesses are obtained, a beam analysis can be conducted very rapidly, for various loading and boundary conditions as appropriate. Results from the 1D analysis can then be fed back into 2D recovering relations to obtain detailed stress and strain components over the cross-section. The overall analysis possesses accuracy comparable to that of 3D finite element methods with a reduction in computational cost by two to three orders of magnitude. 3 Optimization based on a parametric cross-sectional model was considered in Ref. 4. The layout was sufficiently realistic to allow optimization of a baseline cross-section in which the ply angles, thicknesses, and spar location were varied in order to place the shear center in a specified location while holding the crosssectional stiffnesses within a specified tolerance. This work was focused on the lower level of optimization and was continued with a still higher level of realism in Ref. 5.

This paper deals with modeling of the first damage mode, matrix micro-cracking, in helicopter rotor/wind turbine blades and how this effects the overall cross-sectional stiffness. The helicopter/wind turbine rotor system operates in a highly dynamic and unsteady environment leading to severe vibratory loads present in the system. Repeated exposure to this loading condition can induce damage in the composite rotor blades. These rotor/turbine blades are generally made of fiber-reinforced laminated composites and exhibit various competing modes of damage such as matrix micro-cracking, delamination, and fiber breakage. There is a need to study the behavior of the composite rotor system under various key damage modes in composite materials for developing Structural Health Monitoring (SHM) system. Each blade is modeled as a beam based on geometrically non-linear 3-D elasticity theory. Each blade thus splits into 2-D analyzes of cross-sections and non-linear 1-D analyzes along the beam reference curves. Two different tools are used here for complete 3-D analysis: VABS for 2-D cross-sectional analysis and GEBT for 1-D beam analysis. The physically-based failure models for matrix in compression and tension loading are used in the present work. Matrix cracking is detected using two failure criterion: Matrix Failure in Compression and Matrix Failure in Tension which are based on the recovered field. A strain variable is set which drives the damage variable for matrix cracking and this damage variable is used to estimate the reduced cross-sectional stiffness. The matrix micro-cracking is performed in two different approaches: (i) Element-wise, and (ii) Node-wise. The procedure presented in this paper is implemented in VABS as matrix micro-cracking modeling module. Three examples are presented to investigate the matrix failure model which illustrate the effect of matrix cracking on cross-sectional stiffness by varying the applied cyclic load.

A probabilistic stress analysis tool predicting reliability of composite wind turbine rotor blades was developed and validated by comparing with results from a three‐dimensional shell finite element model of a blade. Stress analysis was based on thin wall multicellular Euler–Bernoulli beam theory using as input section stress resultants directly from aeroelastic simulations; a finite strip method was implemented for elastic stability calculations. Reliability analysis was performed at the ply level of the multidirectional laminates implementing various methods such as the response surface method, β‐index and crude Monte Carlo simulation. Physical and statistical uncertainties of the basic variables was taken into account while several model uncertainties related to the material properties were further introduced and quantified in the light of appropriate test results. To prove the efficiency of the code as a design tool, the effect of various probabilistic assumptions concerning the material properties was directly investigated on the estimated reliability β‐index values for two rotor blade design cases typical of stall‐regulated and pitch‐regulated wind turbines. Copyright © 2014 John Wiley & Sons, Ltd.

The material and ply angles of a fiber-reinforced plastic (FRP) rotor blade of a helicopter are optimized using a genetic algorithm (GA) with local search. The variation of the structural stiffness with material and ply angle is investigated. The aim of the optimization is to find an optimal stacking sequence that minimizes twist deformation. An optimum rotor blade is obtained using the ply angles as variables; failure, natural frequency, and minimum moment of inertia are used to constrain the optimization. The GA is employed as an optimization tool. A simplified local search is implemented in the final stage of optimization to reduce the searching time.

Geometrically exact nonlinear analysis of pre-twisted composite rotor blades

Dynamic characteristics of active twist rotor (ATR) blades during forward flight is investigated analytically in this paper. An aeroelastic model for an active rotor system is developed to identify dynamic characteristics of ATR blades with integral strain actuators embedded in their composite construction. More specifically, a time domain integration scheme for the geometrically exact formulation of passive beams is extended with the active materials constitutive relations for the forward flight analysis of the ATR system. In parallel, forward flight wind-tunnel tests are conducted at NASA Langley to collect control sensitivity functions experimentally using dynamically-scaled four-active-bladed rotor system. Preliminary results from the present analytical model are presented and compare well with experimental observations. The theoretical model will be used as a design and evaluation tool for closed-loop controller for twist actuation of the ATR system.

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.

Safety factors of the Ansat helicopter main rotor blade are determined in the root section at various flight modes with taking into account the possible damages.

: The mission of the AH-1Z attack helicopter is to provide rotary wing close air support, anti-armor, armed escort, armed/visual reconnaissance and fire support coordination capabilities under day/night and adverse weather conditions for the United States Marine Corps. The mission of the UH-1Y utility helicopter is to provide command, control and assault support under day/night and adverse weather conditions. Both the AH-1Z and UH-1Y aircraft incorporate state-of-the-art designs, which serve to improve capability, lethality, and survivability. Major modifications include a new four-bladed rotor system with semi-automatic blade fold of the new composite rotor blades, new performance matched transmissions, a new four-bladed tail rotor and drive system, upgraded landing gear, and pylon structural modifications. The H-1 Upgrades aircraft have increased maneuverability, speed, and payload capability. Both aircraft have fully integrated common cockpits/avionics that reduce operator workload and improve situational awareness, thus increasing safety.

The twin beam composite rotor blade

Rotor blade–vortex interaction noise

The outstanding fatigue tolerance features of composite structures were the reason for introducing composite rotor blades in the helicopter design. One goal was to eliminate dynamically loaded metallic hinges. In 1967, the BO 105, a product of the former helicopter division of MBB, now Eurocopter Deutschland GmbH, flew for the first time. Its hingeless rotor was the first serial design with composite blades. Later in 1996, the EC 135 was certified according to the latest damage tolerance and fatigue certification rules. As a consequent design improvement this helicopter has a sophisticated modern rotor without any hinges and bearings. The EC 135 with the “Flexbeam-Rotor” including the bending and torsion element is shown in Fig. 1. More than 1000 helicopters have been produced until today. [Bansemir, Muller: The EC 135...].

<div class="htmlview paragraph">Design studies supported by laboratory test data have shown that graphite fiber composite fan blades in subsonic turbofan engines offer significant potential reductions in weight for both the fan rotor and containment structure as well as reduced sensitivity to fatigue failure from foreign object damage. However, substantial progress in the application of composite material technology including mechanical properties, design, development, processing and inspection methods must be accomplished in order to realize these potential improvements over conventional titanium fan blade materials. Turbofan aircraft engines are subjected to foreign object impact by birds, ice balls, ice slabs, rocks, sand, rain, and miscellaneous objects like blown tire treads in addition to operation with inlet distortion. Graphite composite blades must be designed to tolerate these types of damage and still produce power comparable to current engines before they can seriously be considered for flight applications.</div> <div class="htmlview paragraph">In 1969, the General Electric Company selected a high bypass engine as the test vehicle for initial application of graphite fiber composite fan blades. A two phase design, manufacturing and development program was initiated to produce a graphite composite first stage fan rotor suitable for flight application. The first phase of this program, completed in 1970, was directed toward identifying the problems and producing graphite composite fan blades for engine test evaluation with unshrouded titanium blades in the same rotor.</div> <div class="htmlview paragraph">This report presents a status summary of the composite fan blade design, manufacturing and development effort including the results of the crosswind engine testing of these blades at General Electric's engine test facility at Peebles, Ohio.</div>

In this paper a new method for an experimental determination of the mass distribution of rotor blades is presented. This mass distribution measurement method is developed for the testing of on active twist blades, but it is applicable to the mass property determination of any type of rotating blade. Usually X-ray computer tomography (CT) is used for non-destructive inspection, such as to find voids in composite components. In this research CT is used to get information regarding the spatial distribution of mass in a rotor blade. The CT-analysis of an active twist rotor blade and the implemented determination of the mass distribution are presented in this paper. Image reconstruction due to X-ray computer tomography makes possible to obtain a cross-section image of the rotor blade. Each pixel of this image has a CT-Number which is proportional to the attenuation coefficient of the material forming the blade. Due to the fact that the attenuation of X-ray is characteristic for each material, it is theoretically possible to correlate CT-Numbers with material properties. This method is “quasi”-experimental because the attenuation coefficient for each pixel is experimentally determined and then densities are assigned to these pixels. Once that correlation is done, calculating mass properties such as total mass, mass per length, local center of mass and local moment of inertia for each cross section is possible.

Open architecture dynamic data system at Langley's Transonic Dynamics Tunnel