Helicopter Rotor Blades Research Articles

Shape optimization under uncertainty of morphing airfoils

This paper is devoted to the formulation of a novel optimization under uncertainty framework for the definition of optimal shapes for morphing airfoils, applied here to advancing/retreating 2D airfoils. In particular, the morphing strategy is conceived with the intent of changing the shape at a given frequency to enhance aerodynamic performance. The optimization of morphing airfoils presented here only takes into account the aerodynamic performance. The paper is then focused on an aerodynamic optimization to set the optimal shape with respect to performance, where technological aspects are inserted through geometrical constraints. In fact, this paper presents an exploratory work on morphing geometries which aims at understanding the relationship between shape degree of freedom and actual aerodynamic gain. Thus, exploring and demonstrating the gain of the aerodynamic shape may drive the development of new mechanism for the realization of morphing structures, which could be applied to helicopter rotor blades.

Design and Optimization of Helicopter Rotor Blade Using Different Materials

Design and Optimization of Helicopter Rotor Blade Using Different Materials

Further Improvements in the SNUF Blade Design by Numerical Design Optimization Framework

This paper focused on improving a small-scaled helicopter rotor blade prototype with a flap-driving mechanism termed the Seoul National University Flap (SNUF). The design of the SNUF included realizing vibratory load reduction. First, a multibody structural dynamics analysis was performed to determine the influence of the flap dimension and location within the rotor blade with respect to hub vibratory load reduction. This process was followed by selecting a specific blade configuration that maximized vibration reduction capability. Following this, a numerical optimization technique was applied to improve the cross sectional design of the SNUF blade. The design optimization procedure obtained improved blade sectional design with decreased first torsional frequency and reduced blade weight, and it satisfied sufficient structural integrity. Three-dimensional nonlinear static structural analysis also was performed for the optimized SNUF design. The von Mises stress distribution on the blade and components was predicted by considering external aerodynamic loads, centrifugal loads because of rotation, and contact among the internal components. Future studies will include fabricating the prototype blade on the basis of the optimized design and performing a whirl tower test.

Helicopter blades running elevation measurement using omnidirectional vision

Omnidirectional dynamic space parameters of high-speed rotating helicopter blades are precise 3D vector description of the blades. In particular, the elevation difference is directly related to the aerodynamic performance and maneuverability of the helicopter. The state of the art detection techniques based on optics and common vision have several drawbacks, such as high demands on devices but poor extensibility, limited measurement range and fixed measurement position. In this paper, a novel approach of helicopter blades running elevation measurement is proposed based on omnidirectional vision. With the advantages of panoramic visual imaging integration, 360° field of view and rotation in-variance, high-resolution images of all rotating blades positions are obtained at one time. By studying the non-linear calibration and calculation model of omnidirectional vision system, aiming at solving the problem of inaccurate visual space mapping model, the omnidirectional and full-scale measurement of the elevation difference are finalized. Experiments are carried out on our multifunctional simulation blades test system and the practical blades test tower, respectively. The experimental results demonstrate the effectiveness of the proposed method and show that the proposed method can considerably reduce the complexity of measurement.

Boundary-layer transition measurements on Mach-scaled helicopter rotor blades in climb

In this work, laminar-turbulent boundary-layer transition is investigated on the suction side of Mach-scaled helicopter rotor blades in climb. The phenomenon is assessed by means of temperature-sensitive paint (TSP). Results are compared to a data sample acquired by infrared (IR) thermography and accompanied by integral thrust- and local surface pressure measurements at two radial blade sections. Spatially, high-resolved data allow for precise detection of boundary-layer transition along the outer 60% of the blade span. Results obtained via TSP and IR show remarkable agreement with minor deviations due to different surface qualities of the respective blades tested. TSP data are obtained at various collective pitch angles and three different rotating speeds corresponding to chord Reynolds and Mach numbers based on blade tip speed of $$Re_{\rm tip} = 4.8 - 9.3\times 10^5$$ and $$M_{\rm tip} = 0.29 - 0.57$$ , respectively. The transition position is detected with an accuracy of better than 1% chord and the findings show overall coherence as blade loading and tip chord Reynolds number are varied. Experimental findings are shown to be consistent with two-dimensional simulations using the $$e^N$$ -envelope method for transition prediction. Based on quantitative agreement between measured and calculated surface pressures, a comparison of the corresponding transition results suggests a critical amplification factor of $$N_{\rm cr.} = 5.5$$ best suited for transition prediction in the rotating test facility of the DLR Gottingen.

Modeling of Laminar-Turbulent Transition in Boundary Layers and Rough Turbine Blades

A local, intermittency-function-based transition model was developed for the prediction of laminar-turbulent transitional flows with freestream turbulence intensity Tu at low (Tu < 1%), moderate (1% < Tu < 3%), and high Tu > 3% levels, and roughness effects in a broad range of industrial applications such as turbine and helicopter rotor blades, and in nature. There are many mechanisms (natural or bypass) that lead to transition. Surface roughness due to harsh working conditions could have great influence on transition. Accurately predicting both the onset location and length of transition has been persistently difficult. The current model is coupled with the k–ω Reynolds-averaged Navier–Stokes (RANS) model, that can be used for general computational fluid dynamics (CFD) purpose. It was validated on the ERCOFTAC experimental zero-pressure-gradient smooth flat plate boundary layer with both low and high leading-edge freestream turbulence intensities. Skin friction profiles agree well with the experimental data. The model was then tested on ERCOFTAC experimental flat plate boundary layer with favorable/adverse pressure gradients cases, periodic wakes, and flows over Stripf's turbine blades with roughness from hydraulically smooth to fully rough. The predicted skin friction and heat transfer properties by the current model agree well with the published experimental and numerical data.

Modeling and Design of a Full-Scale Rotor Blade with Embedded Piezocomposite Actuators

An optimization methodology for the design of a full-scale rotor blade with an active twist in order to enhance its ability to reduce vibrations and noise is presented. It is based on a 3D finite-element model, the planning of experiments, and the response surface technique to obtain high piezoelectric actuation forces and displacements with a minimum actuator weight and energy applied. To investigate an active twist of the helicopter rotor blade, a structural static analysis using a 3D finite-element model was carried out. Optimum results were obtained at two possible applications of macrofiber composite actuators. The torsion angle found from the finite-element simulation of helicopter rotor blades was successfully validated by its experimental values, which confirmed the modeling accuracy.

Study on the Performance of Infrared Thermal Imaging Light Source for Detection of Impact Defects in CFRP Composite Sandwich Panels

Recently, composite materials have been mainly used in the main wings, ailerons, and fuselages of aircraft and rotor blades of helicopters. Composite materials used in rapid moving structures are subject to impact by hail, lightning, and bird strike. Such an impact can destroy fiber tissues in the composite materials as well as deform the composite materials, resulting in various problems such as weakened rigidity of the composite structure and penetration of water into tiny cracks. In this study, experiments were conducted using a 2 kW halogen lamp which is most frequently used as a light source, a 2 kW near-infrared lamp, which is used for heating to a high temperature, and a 6 kW xenon flash lamp which emits a large amount of energy for a moment. CFRP composite sandwich panels using Nomex honeycomb core were used as the specimens. Experiments were carried out under impact damages of 1, 4 and 8 J. It was found that the detection of defects was fast when the xenon flash lamp was used. The detection of damaged regions was excellent when the halogen lamp was used. Furthermore, the near-infrared lamp is an effective technology for showing the surface of a test object.

Design and Development of Main Helicopter Rotor Blade

Design and Development of Main Helicopter Rotor Blade

Real-time estimation of helicopter rotor blade kinematics through measurement of rotation induced acceleration

This paper presents a novel approach to monitoring rotor blade flap, lead-lag and pitch using an embedded gyroscope and symmetrically mounted MEMS accelerometers. The central hypothesis is that differential accelerometer measurements are proportional only to blade motion; fuselage acceleration and blade bending are inherently compensated for. The inverse kinematic relationships (from blade position to acceleration and angular rate) are derived and simulated to validate this hypothesis. An algorithm to solve the forward kinematic relationships (from sensor measurement to blade position) is developed using these simulation results. This algorithm is experimentally validated using a prototype device. The experimental results justify continued development of this kinematic estimation approach.

Numerical investigation of the vortex roll-up from a helicopter blade tip using a novel fixed-wing adaptation method

This contribution relates to the simulation of the flow around the tip of a helicopter rotor blade in hovering flight conditions. We here propose a new methodology of framework adaptation, using a comprehensive rotor code and high-fidelity numerical simulations. We construct an equivalent fixed-wing configuration from a rotating blade, in which centrifugal and Coriolis forces are neglected. The effect of this approximation on the solution is analyzed. The method is validated by a detailed comparison with wind tunnel data from the literature, concerning aerodynamic properties and tip vortex roll-up. This validation also includes variations of the pitch angle and rotational speed, up to transonic tip velocities. Compared to previously published methods of framework adaptation, the new hybrid method is found to reproduce more accurately the flow around a rotating-blade tip.

Displacement measurement and shape acquisition of an RC helicopter blade using Digital Image Correlation

The present work addresses the development and implementation of an image sensing methodology for Structural Integrity Monitoring of rotating parts such as aircraft rotor blades. The availability of blade deformation data can enhance the quality of the numerical models and increase the confidence on the simulations. The proposed approach to the problem of deformation measurement on moving subjects uses simultaneously triggered high-speed cameras to bring the rotating subject to an apparent still position, and using 3D Digital Image Correlation (DIC) thereafter to acquire shape and measure total deformation and local strain and stress. The presented contribution involves the use of 3D DIC for displacement measurement and shape acquisition of a radio-controlled helicopter's rotor blade under dynamic loads resultant from rotation at its minimum and maximum speeds (approximately 600 and 1200 rpm respectively).

Optimisation Methodology of a Full-scale Active Twist Rotor Blade

This paper presents optimisation methodology for the design of the full scale rotor blade with an active twist to enhance its capability for vibration and noise reduction. This methodology is based on the 3Dfinite element model, planning of experiments and response surface technique to obtain high piezoelectric actuation forces and displacements with the minimal actuator weight and energy applied. To investigate an active twist of the helicopter rotor blade, the structural static analysis with thermal load using 3D finite element model was developed by finite element software ANSYS. Torsion angle obtained from the finite element simulation of helicopter rotor blades was successfully validated by experimental value to confirm the modelling accuracy.

Dynamic stall control on flapping wing airfoils

Flapping wing efficiency is limited by flow separation effects. The time dependent development of a leading edge vortex (LEV) during rapid pitch-up motion of a retreating helicopter rotor blade is known as dynamic stall vortex. Movement of this vortex along the airfoil upper surface first increases lift but later the vortex lifts off the airfoil surface causing strong drag rise, severe nose-down pitching moments, and possibly negative aerodynamic damping. Very similar effects can be observed on flapping airfoils and wings experiencing combined plunging (heaving) motion and pitching motion. With increasing plunge amplitude the flow on the flapping wing starts to separate and concentrated dynamic stall vortices may develop on both upper and lower wing surfaces. Under these conditions it is shown that wing propulsion efficiency is considerably reduced. Recent investigations of dynamic stall control have shown that a strong vortex may be avoided by appropriate airfoil deformation. It will be shown in the present paper that with dynamic airfoil deformation the propulsion efficiency can be improved considerably. The validity of the numerical calculations is first tested against existing data from literature.

Analysis of weak solution of Euler–Bernoulli beam with axial force

In this paper, we discuss about the existence and uniqueness of the weak form of the non-uniform cantilever Euler–Bernoulli beam equation with variable axial (tensile and compressive) force. We investigate the reason of the buckling from the coercivity analysis. The frequencies of the beam with tensile force are found by the Galerkin method in the Sobolev space H2 with proper norm. Using this method, a system of ordinary differential equations in time variable is formed and the corresponding mass and stiffness matrices are constructed. A very general form of these matrices, which is very simple and suitable for calculations, is derived here with a standard basis. Numerical results for rotating beams with polynomial stiffness and mass variation, typical of wind turbine and helicopter rotor blades, are obtained. These results match well with the published literature. A new polynomial generating set is found. Using two elements of this set, a formula to find the eigenfrequencies is derived. The proposed approach is easy to implement in symbolic computing software.

Modelling and analysis of coupled flap-lag-torsion vibration characteristics helicopter rotor blades

This paper presents the development of a mathematical approach targeting the modelling and analysis of coupled flap-lag-torsion vibration characteristics of non-uniform continuous rotor blades. The proposed method is based on the deployment of Lagrange’s equation of motion to the three-dimensional kinematics of rotor blades. Modal properties derived from classical-beam and torsion theories are utilized as assumed deformation functions. The formulation, which is valid for hingeless, freely hinged and spring-hinged articulated rotor blades, is reduced to a set of closed-form integral expressions. Numerical predictions for mode shapes and natural frequencies are compared with experimental measurements, non-linear finite element analyses and multi-body dynamics analyses for two small-scale hingeless rotor blades. Excellent agreement is observed. The effect of different geometrical parameters on the elastic and inertial coupling is assessed. Additionally, the effect of the inclusion of gyroscopic damping is evaluated. The proposed method, which is able to estimate the first seven coupled modes of vibration in a fraction of a second, exhibits excellent numerical stability. It constitutes a computationally efficient alternative to multi-body dynamics and finite element analysis for the integration of rotor blade flexible modelling into a wider comprehensive rotorcraft tool.

Full-Scale Testing of a Centrifugally Powered Pneumatic De-Icing System for Helicopter Rotor Blades

A centrifugally powered pneumatic de-icing system was developed and tested. Two prototype designs were fabricated and parametrically tested at representative icing conditions. The superior performing design was able to remove ice thicknesses as small as 0.078 in. at and 0.112 in. at . This configuration was reproduced and installed on the outboard 8 ft section of a full-scale 24 ft K-MAX blade. Full-scale icing tests were conducted at Kaman’s outdoor whirl tower using a custom portable icing cloud generator to produce a controlled icing cloud as well as a secondary cloud generator that produced large, uncontrolled water droplets. During full-scale icing tests, the de-icing system was able to remove ice thicknesses as small as 0.08 in. at 270 rpm. A method to calculate power drawn from the main rotor during de-icing system operation is presented. During depressurization, the system draws a peak power of 4.15 hp from the rotor and 0.02 hp during pressurization. The total weight of the de-icing system is comparable to that of existent erosion caps installed on in-service helicopters.

Assessment of comprehensive analysis predictions of helicopter rotor blade loads in forward flight

Comparisons of helicopter rotor blade loads, between rotorcraft comprehensive analysis predictions using a free vortex wake model and measured data, are provided for the UH-60A, SA 330 (research Puma), SA 349/2, and H-34 rotors in forward flight. The rotors are modeled as being isolated from the rest of the vehicle. The comparisons encompass a total of 24 test points featuring wide variations in advance ratio for a thorough assessment of the predictions. With the exception of chord bending moment for the case of UH-60A, the analysis correctly predicts trends in half peak-to-peak values of blade structural loads and pitch link force. Most of the predictions in half peak-to-peak blade structural loads and pitch link force deviate from the measured data by no more than 40% and most of the deviations are underpredictions. The predictions typically resemble the measured data in shapes of the waveforms for both flap bending moment and normal force, but this is less often the case for chord bending and torsion moments. While the analysis may capture the 1/rev harmonic contents in pitching moment waveforms, it consistently underpredicts any higher harmonics.

Operational Modal Analysis of a Helicopter Rotor Blade Using Digital Image Correlation

A novel procedure to perform operational modal analysis on a reduced-scale, 2 m diameter helicopter rotor blade is described. Images of the rotor blade rotating at 900 RPM are captured by a pair of high-speed digital cameras at a sampling rate of 1000 frames per second. From these images, the out-of-plane bending deformation of the rotor blade is measured using Digital Image Correlation, with a spatial resolution of 7.2 mm and an accuracy of 60 μm, or 0.006 % of the rotor radius. Modal parameters including natural frequencies and mode shapes are determined from the bending deformation through application of the Ibrahim Time Domain method. The first three out-of-plane bending modes were identified at each rotational speed and compared to an analytical finite element model of the rotor blade. The experimental and analytical natural frequencies agreed to within 0.2 % in the best case and 10.0 % in the worst case. The experimental mode shapes were also found to closely match the analytical predictions. The results of this study demonstrate the ability of this procedure to accurately determine the modal parameters of rotating helicopter rotor blades.

A fully automated framework for helicopter rotor blades design and analysis including aerodynamics, structure, and manufacturing

This study describes an integrated framework in which basic aerospace engineering aspects (performance, aerodynamics, and structure) and practical aspects (configuration visualization and manufacturing) are coupled and considered in one fully automated design optimization of rotor blades. A number of codes are developed to robustly perform estimation of helicopter configuration from sizing, performance analysis, trim analysis, to rotor blades configuration representation. These codes are then integrated with a two-dimensional airfoil analysis tool to fully design rotor blades configuration including rotor planform and airfoil shape for optimal aerodynamics in both hover and forward flights. A modular structure design methodology is developed for realistic composite rotor blades with a sophisticated cross-sectional geometry. A D-spar cross-sectional structure is chosen as a baseline. The framework is able to analyze all realistic inner configurations including thicknesses of D-spar, skin, web, number and ply angles of layers of each composite part, and materials. A number of codes and commercial software (ANSYS, Gridgen, VABS, PreVABS, etc.) are implemented to automate the structural analysis from aerodynamic data processing to sectional properties and stress analysis. An integrated model for manufacturing cost estimation of composite rotor blades developed at the Aerodynamic Analysis and Design Laboratory (AADL), Aerospace Information Engineering Department, Konkuk University is integrated into the framework to provide a rapid and dynamic feedback to configuration design. The integration of three modules has constructed a framework where the size of a helicopter, aerodynamic performance analysis, structure analysis, and manufacturing cost estimation could be quickly investigated. All aspects of a rotor blade including planform, airfoil shape, and inner structure are considered in a multidisciplinary design optimization without an exception of critical configuration.