Helicopter Rotor Blades Research Articles

Параметры высоковольтных разрядов на лопастях винтов вертолета и создаваемых ими электромагнитных помех

Abstract Parameters of pulsed electric discharges arising between insulated metal parts of helicopter rotor blades, which generate electromagnetic interferences for onboard radio stations operating in the very short wave (VSW) and ultrashort wave (USW) bands have been studied. These discharges appear as a result of the in-flight engine-driven differential charging of helicopters. In-flight and laboratory experiments were used to determine the range of discharge voltages and currents and the amplitude and spectral characteristic of electromagnetic interferences produced by these discharges for onboard antennas.

Improving the Mechanical Behavior of a Helicopter Tail Rotor Blade Through the Use of Polyurethane Foams

The tail rotor blade represents a very important component in the structure of any helicopter, being subjected to many aerodynamic loads. The integrity of this component is essential for maintaining flight safety and improving its characteristics can increase the overall performance of the helicopter. Thus, in some cases, some improvements can be made to the structure, in order to reduce the weight of the component and to improve its mechanical properties and its reliability in flight. The current paper presents a comparative result analysis in the study of the tail rotor blade of the IAR 330 helicopter, where the honeycomb hexagonal core made of aluminum alloy is replaced with a full structural core made of polyurethane foam. The result of the analysis is of practical importance, because it shows the benefits of using polymers in the construction of helicopter rotor blades and similar structures.

Improving the Mechanical Behavior of a Helicopter Tail Rotor Blade Through the Use of Polyurethane Foams

The tail rotor blade represents a very important component in the structure of any helicopter, being subjected to many aerodynamic loads. The integrity of this component is essential for maintaining flight safety and improving its characteristics can increase the overall performance of the helicopter. Thus, in some cases, some improvements can be made to the structure, in order to reduce the weight of the component and to improve its mechanical properties and its reliability in flight. The current paper presents a comparative result analysis in the study of the tail rotor blade of the IAR 330 helicopter, where the honeycomb hexagonal core made of aluminum alloy is replaced with a full structural core made of polyurethane foam. The result of the analysis is of practical importance, because it shows the benefits of using polymers in the construction of helicopter rotor blades and similar structures. Keywords: helicopter tail rotor blade, honeycomb structure, polymers, polyurethane foam, finite element method

Simultaneous pressure and deformation field measurement on helicopter rotor blades using a grid-pattern pressure-sensitive paint system

A grid-pattern pressure-sensitive paint (grid PSP) system is developed and then integrated with a stereo vision system to achieve simultaneous pressure and deformation measurements on rotating surfaces. The grid PSP features a dot pattern, which is prepared by spraying PSP dots on top of a polymer-ceramic binder. A single-shot lifetime method is adopted to obtain the transient pressure field on a rotor blade coated with PSP. Meanwhile, the PSP dots provide feature points for blade surface reconstruction through stereo cameras. A data processing algorithm is developed for calculating the blade motion angles based on the registration targets. This system was applied on a helicopter model test stand to capture a sequence of images on the rotating blade surface. The results from grid PSP were compared with regular PSP to evaluate the performance of pressure measurement. The accuracy of blade motion measurement was also evaluated for pitch angle from 15° to +15°, flap angle from −10° to +10° and lag angle from −10° to +10°, respectively.

A New Approach to Modelling and Testing the Fatigue Strength of Helicopter Rotor Blades during Repair Process

Abstract The fatigue test was carried out on an element of a rotor blade removed from the Mi-2 helicopter. The purpose of the test was to check the fatigue strength of the repaired rotor blade. Metal composite rotor blades have a metal spar in the form of a box and the trailing sections in the form of metallic honeycomb sandwich panels. The trailing sections are bonded to the spar. The repair had been carried out at the point where the trailing section became debonded from the spar at the Air Force Institute of Technology in Warsaw using a methodology developed for carrying out repairs of rotor blades’ damage. All types of the Mi family helicopters are equipped with metal composite rotors blades. Depending on MTOW (Maximum Take-Off Weight) and destination of helicopters, blades differ in dimensions, but their design solutions are practically the same. For this reason, the developed repair methodology can be used for all characteristic rotor blades structures for Mi helicopters. The fatigue test was performed at the Łukasiewicz - Institute of Aviation in Warsaw, using a hydraulically driven fatigue machine. The fatigue test was carried out by performing over 1.1 million load cycles. In repair places, upon completion of fatigue testing, no damage was found.

Comparison of micro‐Doppler signatures registered using RBM of helicopters and WSM of vehicles

Jet engine modulation (JEM) are micro-Doppler features obtained from the radar data of aircraft, including jet aircraft and helicopters, and generally viewed as unique phenomenon suitable for identifying aerial targets. The authors demonstrate however, that JEM are not unique, and that the rotating wheel spokes on the wheels of ground vehicles produce a modulation similar to the rotating rotor blades of jet engines and helicopter rotor blades. They also develop a mathematical model to analyse the radar signal scattering mechanism of wheel spokes, and also employ a Ku-band Doppler radar system to collect radar echoes from a wheeled vehicle and a helicopter. Both the theoretical and experimental results verify the existence of rotor blade modulation (RBM) phenomena from helicopters and wheel spoke modulation (WSM) phenomena from vehicles. Both RBM and WSM are similar to JEM features. This fact indicates that JEM is a necessary but insufficient condition for the identification of aerial targets, and additional recognition features inherent in surface surveillance radar are needed to distinguish the JEM spectra arising from aerial targets and ground vehicles.

Performance Analysis of Composite Helicopter Blade Using Synergistic Damage Mechanics Approach

The damage behavior of a helicopter rotor blade made of carbon fiber polymer composite has been numerically investigated using a synergistic damage mechanics (SDM) model. Laminates of the quasi-isotropic stacking sequence have been considered in this study. The simulations were performed, corresponding to a hovering condition and for a range of the angles of attack and rotational speeds of the blade. The aerodynamic and centrifugal loads due to different angles of attack and rotational speeds have been computed by using fluid dynamics simulations. A structural analysis was then performed using the aerodynamic loads. Using the SDM model, the matrix crack density and crack multiplication were evaluated to predict the damage initiation site and maximum crack density occurring in the plies of the laminate under the operational conditions. It was observed that maximum crack appeared in the blade at extreme operational conditions; among the different plies, a ply was cracked by the maximum amount. A crack density of around was predicted in the ply at an operational condition with a 500 rpm blade rotation and an 18 deg angle of attack of the blade. The present study demonstrates application of the progressive damage modeling in designing a composite helicopter blade by considering its performance parameters.

Aerodynamic/aeroacoustic variable-fidelity optimization of helicopter rotor based on hierarchical Kriging model

Rotor noise is one of the most important reasons for restricting helicopter development; hence, the optimization design of rotor blade considering aeroacoustic and aerodynamic performance at the same time has always been the focus of research attention. For complex rotor design problems with a large number of design variables, the efficiency of the traditional Kriging model needs to be improved. Thus, Hierarchical Kriging (HK) model is employed in this study for rotor optimization design. By using the validated RANS solver and acoustic method based on the FW–Hpds equation, an efficient aerodynamic/aeroacoustic optimization method for high-dimensional problem of rotors in hover based on HK model is developed. By using present HK model and new infill-sampling criteria, the number of design variables is increased from less than 20–53. Results of two analytical function test cases show that the HK model is efficient and accurate in calculation. Subsequently, the helicopter rotor blade is optimally designed for aerodynamic/aeroacoustic performance in hover based on the HK model with high dimensional design variables. The objective function is adopted to improve the rotational noise characteristics by reducing the absolute peak of the acoustic pressure. In addition, the constraints of thrust, hover efficiency, solidity, and airfoils thickness are strictly satisfied. Optimization results show that the Kriging model finds the objective of reducing the noise by 2.87 dB after 248 iterations while the HK model does it only after 164 iterations. The optimization efficiency of the HK model is significantly higher than that of the traditional Kriging model. In the case analyzed, the HK model saves 35% of the time used by the Kriging model.

Speculation of local aerodynamic loads on helicopter rotor blade in forward flight

Design of rotor blade is mainly the most important consideration in helicopter industry. A sufficient input data is needed for various flight conditions that should be accounted for in rotor design process. In this paper, blade element theory is applied to helicopter rotor blade in forward flight. The main objective is to assure the fidelity of using blade element theory in helicopter rotor blade design. Blade element theory is used for calculating the average lift, thrust, power for rotary wings and the local aerodynamic loads. The local non-dimensional normal force coefficients obtained from blade element theory were compared with experimental results of the 7A rotor at five non dimensional radial positions for the high speed test point 312. The results showed the same pattern of variation along the azimuth with a considered convergence along the blade radius.

Vibration analysis of rotating fully-bonded and delaminated sandwich beam with CNTRC face sheets and AL-foam flexible core in thermal and moisture environments

In the present study, the high-order free vibration analysis of rotating fully-bonded and delaminated sandwich beams; with and without vertical contact; containing AL-foam flexible core and carbon nanotubes reinforced composite (CNTRC) face sheets subjected to thermal and moisture field are investigated by using generalized differential quadrature method (GDQM). The compressible core and face sheets of sandwich beam, respectively, are composed of Aluminum alloy foam with variable mechanical properties in the thickness direction and CNTRC with temperature dependent material properties. In this study, the high-order sandwich panel theory (HSAPT) for AL-foam flexible core and Euler-Bernoulli beam theory for CNTRC face sheets are considered. By employing Hamilton’s principle, the governing partial differential equations of motion and associated boundary and continuity conditions for various types of regions (fully-bonded, delaminated with contact, delaminated without contact) are derived and then discretized by using GDQM. The final formulations lead to 14 partial differential equations for the entire structure including five equations for fully-bonded two-headed parts of AL-foam cored sandwich beam (AL-FCSB) and four equations for delaminated middle part of AL-FCSB beam which are combined in axial and transverse deformations. A parametric study is performed to investigate the influence of some important parameters such as existence of delaminated region, type of delaminated region (with or without contact), longitudinal position of delaminated region, slenderness ratio, face sheet thickness ratio, CNT volume fraction, temperature rise, moisture concentration, rotating speed, and hub radius. The obtained results reveal that the 1st frequency of delaminated AL-FCSB beam, whether with or without vertical contact, is less remarkably than ones of fully-bonded AL-FCSB beam which its value for the case of delaminated ‘with contact’ is larger than that of ‘without contact’. Moreover, the 1st frequency variation of the delaminated AL-FCSB beam is symmetrical with regard to the longitudinal position of the debonded region such that the 1st natural frequency declines with moving the debonded region toward the center of the beam. The study of vibration behavior of rotating sandwich beams is very important in design of rotating structural systems, specially damaged ones, such as airplanes, helicopter rotor blades, and robot arms. One of the most important types of damage encountered in mentioned cases is the decomposition of two layers or delamination. Working these rotating structures in the media, are always along with variations of temperature and humidity and hence their mechanical properties may be changed due to the environment conditions.Communicated by S. Velinsky

Research of Precise Time Integration Method and its Derived Formats on Helicopter Rotor Dynamics

The aeroelastic coupling dynamic equation of helicopter rotor is essentially a set of nonlinear and inhomogeneous equations with large rigidity, in which the inhomogeneous term is a function of blade motion and aerodynamic load. In this paper, the precise time integration method and its derived formats are introduced to solve the rotor blade dynamic equation, and the Duhamel integral item can be calculated by various numerical methods. In terms of computational accuracy and numerical stability, the precise Kutta method and high precision direct integration method (HPD method) are carefully selected to compare with classical Runge–Kutta method numerically. HPD method is used to solve the rotor blade dynamic equation, and the transient response of the rotor blade is examined by Newmark and implicit trapezoidal methods. Results indicate that HPD method dominates the classical Runge–Kutta method in step size independence, and gets close to implicit methods in numerical stability and accuracy for dynamic equation of helicopter rotor blade.

Joint Occlusion Detection and Phase Estimation Algorithm for Helicopter Satellite Communication

In helicopter satellite communication systems, the received signal is blocked by the helicopter rotor blades during propagation, incurring the unstableness of traditional methods of channel estimation due to unknown channel state. To this end, a joint occlusion detection and phase estimation algorithm is proposed by factor graph in this paper. For occlusion detection to acquire channel state, the occlusion channel model is established relying on a Markov chain with a priori blocking ratio, in which the probability distribution of the channel state is calculated by the proposed message-passing algorithm. To obtain the occlusion position and update the blocking ratio, a cross-correlation method is designed. Furthermore, a joint block detection and phase estimation algorithm is developed based on the proposed occlusion detection algorithm to realize phase estimation by iterative receiver, where occlusion detection, phase estimation, and decoding are performed in turn for one turbo iteration and the result of decoding is then sent back to the occlusion detection step for the next iteration. Simulation results show that the proposed algorithm is adaptive to the dynamical change in the blocking ratio and can achieve bit error rate performance that is close to the performance bounds based on perfect channel state.

Systematic evaluation of the helicopter rotor blades: design variables and interactions

PurposeThe effects of rotor blade design variables and their mutual interactions on aerodynamic efficiency of helicopters are investigated. The aerodynamic efficiency is defined based on figure of merit (FM) and lift-to-drag responses developed for hover and forward flight, respectively.Design/methodology/approachThe approach is to couple a general flight dynamic simulation code, previously validated in the time domain, with design of experiment (DOE) required for the response surface development. DOE includes I-optimality criteria to preselect the data and improve data acquisition process. Desirability approach is also implemented for a better understanding of the optimum rotor blade planform in both hover and forward flight.FindingsThe resulting system provides a systematic manner to examine the rotor blade design variables and their interactions, thus reducing the time and cost of designing rotor blades. The obtained results show that the blade taper ratio of 0.3, the point of taper initiation of about 0.64 R within a SC1095R8 airfoil satisfy the maximum FM of 0.73 and the maximum lift-to-drag ratio of about 5.5 in hover and forward flight.Practical implicationsThe work shows the practical possibility to implement the proposed optimization process that can be used for the advanced rotor blade design.Originality/valueThe work presents the rapid and reliable optimization process efficiently used for designing advanced rotor blades in hover and forward flight.

Determination of fatigue resistance characteristics of helicopter rotor blade

This paper presents an investigation of the fatigue resistance testing of structural elements of the helicopter by the example of testing of helicopter rotor blades samples. Endurance tests of components and structural elements of aviation equipment are the laboratory reproduction of external influences corresponding to the typical operating conditions, cyclic loading and functioning. However, these tests do not include research related to the gradual accumulation of damage leading to the emergence and propagation of cracks and construction failure. In this regard, a special interest is learning the process of cracks growth during the specimens’ full-scale tests. A brief description of the blades endurance tests process with simultaneous video shooting has been given. Stretching static load is applied to the samples, after those samples are additionally loaded by alternating bending moment. Videos of the cracks growth are processed and the data about the time of subcritical crack growth are obtained. This information is displayed as a graph. The control and measurement system is considered which is used in the process of testing for recording of values of stress occurring in tested specimens because of alternating bending load applied to them. The characteristics of fatigue strength are determined, fatigue graph is plotted, durability arithmetic mean and standard deviation are determined, assemblage of durability distribution curves, assemblage of fatigue curves and endurance distribution curve are plotted, endurance limit arithmetic mean and standard deviation are determined. The achieved results have importance for the prediction of the safe operation term of the helicopter rotor blades.

Vortical lift on retreating rotor blades at high advance ratios

A retreating helicopter rotor blade encounters reverse flow over large portions of the rotor disk in high advance ratio flights. Measurements of flow velocity and loads are described for a two-bladed teetering rotor for advance ratios ranging from 0.6 to 1.2 in a low speed wind tunnel. The tip vortex trajectory is separately mapped. A pressure extraction technique is used to map the static pressure field on the lower surface of the rotor blade near sharp edge. The reverse flow region shows a proof of an attached, helical Sharp Edge Vortex (SEV). The proximity of the SEV to the rotor blade creates sharp excursions in the static pressures at the sharp edge. The negative lift generated by the vortex is comparable to the sectional bound circulation. The presence of the SEV explains the shift in the center of pressure from 75 to 70% of chord. Stereo particle image velocimetry, a pressure extraction technique, and load measurements provide closure in estimating aerodynamic loads. As the blade approaches 270 $$^\circ$$ azimuth vortex strength stops growing and its core detaches from the blade surface but the vortex stays close to the blade. Flow on blunt edge of the blade in reverse flow is found to be largely attached, unlike expectations of vortex shedding.

Multifidelity Aerodynamic Optimization of a Helicopter Rotor Blade

A multifidelity optimization technique is applied to the design of a helicopter rotor blade to improve its performance in forward flight. This optimization technique is based on a surrogate model that replaces the high-fidelity computational fluid dynamics (CFD)/computational structural dynamics (CSD) simulations necessary to capture the three-dimensional unsteady effects generated in the flowfield of a complex blade geometry. The single low-fidelity model based on kriging methodology and generated by lifting-line simulations leads to a power benefit of 2.5%, which is not reproducible by an a posteriori high-fidelity CSD/CFD computation. The optimization procedure using cokriging surrogate models based on two levels of fidelity (lifting-line and CSD/CFD simulations) leads to a realistic blade planform, for which the power benefit is estimated at 2.2%. This optimized solution, obtained after a factor-of-six reduction in CPU time, shows the advantages of using a cokriging surrogate model (rather than a single-fidelity kriging model) for aerodynamic optimizations.

A Comprehensive Optimization Design Method of Aerodynamic, Acoustic, and Stealth of Helicopter Rotor Blades Based on Genetic Algorithm

In this paper, a comprehensive optimization approach is presented to analyze the aerodynamic, acoustic, and stealth characteristics of helicopter rotor blades in hover flight based on the genetic algorithm (GA). The aerodynamic characteristics are simulated by the blade element momentum theory. And the acoustics are computed by the Farassat theory. The stealth performances are calculated through the combination of physical optics (PO) and equivalent currents (MEC). Furthermore, an advanced geometry representation algorithm which applies the class function/shape function transformation (CST) is introduced to generate the airfoil coordinates. This method is utilized to discuss the airfoil shape in terms of server design variables. The aerodynamic, acoustic, and stealth integrated design aims to achieve the minimum radar cross section (RCS) under the constraint of aerodynamic and acoustic requirement through the adjustment of airfoil shape design variables. Two types of rotor are used to illustrate the optimization method. The results obtained in this work show that the proposed technique is effective and acceptable.

Material and Mechanical Characterization of Multi-Functional Carbon Nanotube Reinforced Hybrid Composite Materials

Fiber reinforced polymer (FRP) composite structures are widely being used in aircraft wings, wind turbine blades, helicopter rotor blades and tail rotors. These structures are often exposed to external dynamic loads which shorten their lifetime. Carbon nanotubes (CNT) reinforced epoxy resin have distinctive characteristic in providing a significant increase in mechanical properties and stiffness of the FRP composite. The present study investigates the micro, macro and structural analysis of composites with and without reinforcement of multi walled carbon nanotubes (MWCNT). The carboxylic acid (COOH) functionalized MWCNT with more than 95% chemical purity having average dimensions of 17 nm outer diameter and 10 μm length were used to characterize their chemical properties and evaluate the mechanical and free and forced vibration response of composites with and without MWCNT reinforcement. Initially, the powder form of the MWCNT was taken for the identification of true density (ρ) using the gas displacement technique. The COOH-MWCNT were then randomly dispersed in low viscosity epoxy resin (LY556) through an organic solvent using the ultrasonic liquid processor. Test samples were fabricated by adding the hardener (HY951) at 10:1 ratio in the sonicated solution to obtain the Young’s Modulus (E) of MWCNT using Nano Indentation. Following this, Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), Fourier Transform Infrared spectroscopy (FT-IR), Thermo gravimetric analysis (TGA) were also used to quantify the dispersion, distribution, structural integrity, aspect ratio, functional group and purity level of nanotubes. Further, the impact hammer test based on ASTM E1876, tensile test based on ASTM D3039 and free and forced vibration analysis of the hybrid composite beams were carried out to identify the elastic properties, fundamental natural frequencies, damping ratio and transverse deflection of the hybrid structure. It was shown that the addition of 1 wt% of COOH-MWCNT in fiber reinforced composite beam increases the stiffness of the structure and consequently increases the natural frequencies and damping at each resonant response dominant peaks. The strong adhesion of bonding and proper dispersion of CNTs in the wide surface area of composite strengthen the polymer composites substantially than those of the Glass/epoxy composite structures without reinforcement of MWCNT.

Resonance avoidance for variable speed rotor blades using an applied compressive load

Varying the rotational speed of the main rotor is one method being considered to improve the performance of future rotorcraft. However, changes in rotor speeds often lead to resonant interactions between rotor blade modes and the rotor's excitation frequencies which increase the vibratory loads in the rotor. This research investigates the use of a compressive load to reduce a blade's natural frequencies and its potential to be used as a resonance avoidance technique by improving separation between the natural and excitation frequencies of a blade. The research presented herein describes and validates a model of a pretwisted rotating beam with non-coincident mass and elastic axes with an applied compressive load. The compressive load is applied at the elastic axis at the tip of the beam and is orientated towards the root of the beam. The beam model is then used in a case study to represent the rotor blade of a typical mid-sized civilian helicopter. The case study is performed to calculate the natural frequencies of a compressed blade for a reduction in rotor speed of up to 40% and evaluate the performance of the compressive load resonance avoidance technique. The results of the case study show that the compressive load improves the separation between natural and excitation frequencies over the full range of rotor speeds evaluated. The improved separation allows the rotor to operate safely with a reduction in rotor speed of up to 19%.

Cracking of Erosion Strip of Main Rotor Blade of a Military Helicopter

Erosion strip fitted on main rotor blade (MRB) of a military helicopter (flying in coastal/marine environment) had cracked in service. The erosion strip was made of austenitic stainless steel material of specification MIL-S-5059D Type 301 and was in quarter hard condition. The chemical composition, the hardness and the microstructure (condition) of the erosion strip met all the specified requirements. Two cracks were observed in the erosion strip. Scanning electron microscopy of the fracture surfaces showed the presence of fatigue striations amidst thick corrosion products indicating toward failure by corrosion fatigue. Energy-dispersive X-ray spectroscopy (EDS) showed that the corrosion products on the fracture surfaces were predominantly oxides of iron consisting of corrosive species like chlorine, which is typical of corrosion in marine/coastal environment (i.e., the service environment of the helicopter). Fractographic studies showed that fatigue failure had originated from the inner surface of the erosion strip that is bonded to the MRB. Intergranular and transgranular cracks and corrosion were observed in the erosion strip material from its inner surface at the failure origin. These appeared to be stress corrosion cracks as a result of crevice corrosion that had occurred on the inner surface of the erosion strip at the failure origin. Chlorides present in the environment (viz. marine/coastal) had apparently concentrated inside the crevice (at the bonding between the erosion strip and the MRB), attacked the protective oxide surface film on stainless steel and also prevented the re-forming of the protective oxide layer. Thus, the material in the crevice was attacked rapidly and the corrosion continued. Stress corrosion cracks can develop from the base of crevice where the stress concentration is greatest. Thus, the cracking of the erosion strip is a typical example of environment-induced mechanical failure of a fairly passive metallic material.