Helicopter Rotor Blades Research Articles

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.

Variable-Fidelity Methodology for the Aerodynamic Optimization of Helicopter Rotors

The design of helicopter rotor blades is a challenging task. On the one hand, there are the demanding simulations, which are a multidisciplinary endeavor. On the other hand, tools for parametric studies or optimizations require many simulations, making the design process even more costly. In the rotorcraft community, two routes for the numerical optimization task are observed. The first route is based upon local gradient search algorithms, which exploit low-fidelity tools or adjoint-based computational fluid dynamics (CFD) simulations. The second route is surrogate-based optimization in combination with high-fidelity CFD simulations. These surrogate-based optimizations can be further accelerated, when knowledge from low-fidelity models is used. This paper presents a framework that is developed for the multi-objective aerodynamic optimization of helicopter rotor blades including surrogate models based on different fidelities. The individual components necessary for performing a variable-fidelity, multi-objective optimization are reviewed before being applied. A novel technique to deal with unsuccessful simulations referred to as a failure map is additionally presented. The gain of the variable-fidelity optimizations in contrast to the single-fidelity optimizations is quantified, and a reduction in computational resources of up to 69% is observed. The failure map requires 78% less resources in contrast to the classical failure handling.

Optimization of helicopter rotor blade performance by spline-based taper distribution using neural networks based on CFD solutions

ABSTRACTThe taper distribution along the span of a helicopter blade is defined using a novel method applied for the first time and considered as the main contribution of this work. This method uses cubic splines to generate modified blade shapes. The thrust and the torque values, computed by a 3-D Reynolds Average Navier Stokes solver, are used to train a Neural Networks based model. After that a constrained optimization is conducted based on this model for two different rotor speeds under hover condition. The optimization variables are the chord lengths at three different span locations: root, mid-span and tip. The optimization constraints are the torque or thrust values of the original blade and the practical limits for the chord lengths. Two optimum cases are investigated: maximum Figure of Merit with greater thrust and maximum Figure of Merit with less torque than the baseline. The major challenge of this work is to use the taper distribution as the only design parameter to obtain comparable results to other studies in literature in which more than one parameter is used. The results show that the Figure of Merit can improve by around 5% and the torque can be reduced by around 20%.

High-Fidelity Multidisciplinary Sensitivity Analysis and Design Optimization for Rotorcraft Applications

A multidisciplinary sensitivity analysis of rotorcraft simulations involving tightly coupled high-fidelity computational fluid dynamics and comprehensive analysis solvers is presented and evaluated. A sensitivity-enabled fluid-dynamics solver and a nonlinear flexible multibody-dynamics solver are coupled to predict aerodynamic loads and structural responses of helicopter rotor blades. A discretely consistent adjoint-based sensitivity analysis available in the fluid-dynamics solver provides sensitivities arising from unsteady turbulent flows and unstructured dynamic overset meshes, whereas a complex-variable approach is used to compute structural sensitivities with respect to aerodynamic loads. The multidisciplinary sensitivity analysis is conducted through integrating the sensitivity components from each discipline of the coupled system. Sensitivities of lift computed by the multidisciplinary sensitivity analysis are verified by comparison with the sensitivities obtained by complex-variable simulations. The multidisciplinary sensitivity analysis is applied to a constrained gradient-based design optimization for a HART-II rotorcraft configuration.

On the Effect of Rotational Forces on Rotor Blade Boundary-Layer Transition

Laminar-turbulent boundary-layer transition is investigated on the suction side of Mach-scaled helicopter rotor blades in climb and analyzed in view of the effect of rotational forces. Transition positions are detected with temperature-sensitive paint and complemented by surface pressure measurements at two radial blade sections. The effect of rotational forces is investigated by systematic variation of the Rossby number from to at and . The findings do not show a significant effect on boundary-layer transition in the investigated parameter range and suggest predominantly two-dimensional flow behavior. The result is supported by subsequent validation with two-dimensional numerical tools. Based on quantitative agreement between measured and calculated surface pressures, measured transition positions are predicted to within chord if a critical amplification factor of is used in the coupled Euler/boundary equation solver MSES for transition prediction in the rotor test facility of the DLR, German Aerospace Center in Göttingen, Germany. The measured transition onset positions are also correlated with integral growth rates, obtained from separate two-dimensional compressible boundary-layer computations and subsequent local linear stability analysis to get transition factors of . Differences in factors are discussed in view of the different approaches used. Overall, transition -factor correlations are independent of relative chord Reynolds number and incompressible shape factor at the detected transition onset. The findings underline the capability of two-dimensional numerical techniques based on linear stability theory to model boundary-layer transition in the investigated parameter range.

Creation of an Integrated System for Monitoring the Technical Condition of High-Quality Helicopter Units based on Fiber-Optic Technology

This article discusses the problems of creating an integrated system for monitoring the technical condition of highly responsible helicopter units, and analyzed options for its creation. Fiber-optical technology is considered as a technology, on the basis of which it is possible to build an integrated system, since this technology makes it possible to measure various physical parameters such as vibration, deformation, temperature, acoustic emission and other parameters, and due to the miniature dimensions of the fiber-optic light guide. It can be built into the PCM design, which is a relevant factor due to the growth use of PCM in the design and manufacture of helicopters. The results of bench and flight tests of helicopter pilot and rotor blades with fasteners using fiber optic deformation sensors based on a Bragg fiber grille demonstrate the fundamental possibility of creating a system for monitoring the technical condition of helicopter rotors using fiber optic sensors .Also, there are examples of creating other elements of an integrated monitoring system based on the use of fiber-optic technology, such as a system for signaling the breakdown of air flow and flutter on the helicopter rotor blades, a system for measuring the weight and alignment of helicopter cargo and a system for monitoring the technical condition of the airframe.

Damage Identification Using Wavelet Packet Transform and Neural Network Ensembles

This study presents a damage identification method that combines wavelet packet transforms (WPTs) with neural network ensembles (NNEs). The WPT is used to extract damage features, which are taken as the input vectors in the NNEs used for damage identification. An experiment was performed on a helicopter rotor blades structure to verify the proposed method. First, the vibration responses collected by different sensors are decomposed using the WPT. Second, the relative band energy of each decomposed frequency band is calculated and fused as the damage feature vectors. Third, two types of the NNEs are designed. One is based on the backward propagation neural networks (BPNNs) for detecting the damage locations and severities and the other one is based on the probabilistic neural network (PNN) to detect the damage types. Finally, the trained NNEs are employed in damage identification. From the identification outcomes, it is concluded that damage information can be extracted effectively by the WPT and the identification accuracy of the NNEs is better than that of individual neural networks (INNs).

Optimized Comparative Analysis of an Active Twist for Helicopter Rotor Blades with C- and D-Spar Designs

The application of macrofiber composite (MFC) actuators to an active twist control of helicopter rotor blades significantly reduces the vibration and noise without the use of complex mechanisms in the rotating systems. The applicability of two main spar designs and two variants of utilization of MFC actuators for an active twist of advanced helicopter rotor blades is examined in the present study. For this reason, 3D finite-element models and the corresponding piezoelectric, static torsional, and modal analyses are developed using the ANSYS code. Due to the great dimension of the numerical problem to be solved, an optimization methodology is developed employing the design of experiments and the response surface technique. The numerical results found are validated successfully by quasi-static tests on a demonstration blade, which confirm the high accuracy of the 3D finite-element models developed and the corresponding analyses performed.

Experimental study of a pitching and plunging wing

PurposeThe complex flow behavior over an oscillating aerodynamic body, e.g. a helicopter rotor blade, a rotating wind turbine blade or the wing of a maneuvering airplane involves combinations of pitching and plunging motions. As the parameters of the problem (Re, St and phase difference between these two motions) vary, a quasi-steady analysis fails to provide realistic results for the aerodynamic response of the moving body, whereas this study aims to provide reliable experimental data.Design/methodology/approachIn the present study, a pitching and plunging mechanism was designed and built in a subsonic closed-circuit wind tunnel as well as a rectangular aluminum wing of a 2:1 aspect-ratio with a NACA64-418 airfoil, used in wind turbine blades. To measure the pressure distribution along the wing chord, a number of fast responding transducers were embedded into the mid span wing surface. Simultaneous pressure measurements were conducted along the wing chord for the Reynolds number of 0.85 × 106 for both steady and unsteady cases (pitching and plunging). A flow visualization technique was used to detect the flow separation line under steady conditions.FindingsElevated pressure fluctuations coincide with the flow separation line having been detected through surface flow visualization and flattened pressure distributions appear downstream of the flow separation line. Closed hysteresis loops of the lift coefficient versus angle of attack were measured for combined pitching and plunging motions.Practical implicationsThe experimental data can be used for improvement of unsteady fluid mechanics problem solvers.Originality/valueIn the present study, a new installation was built allowing the aerodynamic study of oscillating wings performing pitching and plunging motions with prescribed frequencies and phase lags between the two motions. The experimental data can be used for improvement of computational fluid dynamics codes in case that the examined aerodynamic body is oscillating.

Prediction of helicopter rotor noise in hover using FW-H analogy

The integral formulation of Ffowcs Williams and Hawkings (FW-H) analogy developed by Farassat is implemented to understand the sound generation and propagation of a rotating slender body like the helicopter rotor blade in hover. Using the linear acoustic theory, the thickness and loading noise terms are implemented in a flexible, new, in-house aero-acoustic tool. The pressure distribution on the blade surface is obtained from CFD simulation which sufficiently captures the correct flow characteristics of the rotor blade. Although, there is an improved estimation of the acoustic pressure signal in comparison to existing numerical noise signal results, there is still an under prediction of the peak amplitude compared to experimental values. The code is validated against the experiment for the UH-1H model helicopter rotor equipped with two rectangular blades having NACA0012 section.

Design optimization of helicopter rotor using kriging

Purpose The purpose of this study is to obtain optimum locations, peak deflection and chord of the twin trailing-edge flaps and optimum torsional stiffness of the helicopter rotor blade to minimize the vibration in the rotor hub with minimum requirement of flap control power. Design/methodology/approach Kriging metamodel with three-level five variable orthogonal array-based data points is used to decouple the optimization problem and actual aeroelastic analysis. Findings Some very good design solutions are obtained using this model. The best design point in minimizing vibration gives about 81 per cent reduction in the hub vibration with a penalization of increased flap power requirement, at normal cruise speed of rotor-craft flight. Practical implications One of the major challenges in the helicopters is the high vibration level in comparison with fixed wing aircraft. The reduction in vibration level in the helicopter improves passenger and crew comfort and reduces maintenance cost. Originality/value This paper presents design optimization of the helicopter rotor blade combining five design variables, such as the locations of twin trailing-edge flaps, peak deflection and flap chord and torsional stiffness of the rotor. Also, this study uses kriging metamodel to decouple the complex aeroelastic analysis and optimization problem.

비행 조건 별 헬리콥터 로터 블레이드 공력 소음 예측

헬리콥터 소음예측은 저소음 헬리콥터 기술 개발에 필수적인 과정이다. 본 논문에서는 헬리콥터 통합성능해석프로그램 CAMRAD-II와 자체개발한 소음해석코드를 이용하여 소음예측 기법을 구축하였다. 또한 헬리콥터 소음 중 가장 큰 원인인 블레이드-와류 간섭 소음을 분석하여 해석기법을 검증하였다. 실제적인 소음예측을 위해 국제민간항공기구(ICAO)에서 기준으로 하고 있는 헬리콥터 소음 측정 비행조건인 Flyover, Approach 조건에 대해서 소음해석을 수행하였으며, 최종적으로 비행시험결과 와의 비교·분석을 통해 해석방법의 적합성을 확인하였다.

Rotor blade shape reconstruction from strain measurements

Traditional helicopter blades are subject to significant deformations, which influence control forces and moments, as well as the helicopter aeroelastic and aeroacoustic behavior. Thus, the knowledge of rotor elastic states could help improving flight control efficiency, and reducing vibration level and acoustic emissions of next-generation helicopters. This paper presents an original and computationally efficient modal approach aimed at dynamic shape sensing of helicopter rotor blades. It is based on strain measurements in a limited number of points over the blade surface. Although the algorithm is based on the cascaded solution of linear algebraic equations, much like other modal-based algorithms, it is able to reconstruct nonlinear, moderate lag, flap and torsional deflections, which are typical in helicopter structural dynamics. The algorithm is tested on non-rotating and rotating hingeless blades through numerical simulations based upon a multibody dynamics solver for general nonlinear comprehensive aeroelastic analysis. Its capabilities are assessed against those of classical modal approaches. Numerical investigations show that the proposed algorithm is reliable, accurate and robust to measurement noise.

Design fabrication and performance analysis of length morphing rotor blade

The present work deals with helicopter theory involving the study, design and fabrication of the helicopter rotor blades with the length-morphing mechanism. The research of the rotor blades has enabled in a proper understanding of the aerodynamics and design of the same. The thrust produced by a blade is proportional to its area, and for every motor RPM, maximum thrust efficiency is achieved for a discrete length of the rotor blade. Facing this complexity, designers compute an optimal length for the average motor RPM while designing the heli-copter blades. Acknowledging the challenges, Length-Morphing rotor blades targeting maximum thrust efficiency for each motor RPM was developed with the aid of knowledge in Blade Element Theory. The rotor blade was designed and fabricated to be driven by the centrifugal force from the motor. The rotor blade was divided into fixed inboard section and sliding outboard part in a span-wise direction. The analy-sis was carried out to study and comprehend the operating conditions of the length-variable rotor during revolutions and to derive the design variables of extension-spring and rotor weight. Variation of thrust concerning the length of the rotor blade was studied, and the setup was fabricated. The project aims to enable maximum rotor blade thrust efficiency for each RPM of the motor by varying the length of the rotor blade and computing the performance characteristics of the same.

Investigating the Aeroelastic Behaviors of Rotor Blades with Nonlinear Energy Sinks

The aim of this paper is to investigate the effectiveness of the nonlinear energy sink on the aeroelastic behaviors of rotor blades. For this purpose, a single hingeless helicopter rotor blade in a hover flight aerodynamic condition is considered. The performance of two linear vibration absorbers is investigated in this aeroelastic system, as well for comparison. The linear stability and nonlinear responses of the systems are analyzed and compared with the primary rotor blade system. The results show that both the linear and nonlinear vibration absorbers can delay the flutter point. The obtained results from the nonlinear analysis give helpful details of different systems.

Unsteady RANS/DES analysis of flow around helicopter rotor blades at forword flight conditions

In this paper, the complex flows around forward-flying helicopter blades are numerically investigated. Both the Reynolds-averaged Navier–Stokes (RANS) and the Detached Eddy Simulation (DES) methods are used for the analysis of characteristics like local dynamic flow separation, effects of radial sweeping and reversed flow. The flow was solved by a highly efficient finite volume solver with multi-block structured grids. Focusing upon the complexity of the advance ratio effects, above properties are fully recognized. The current results showed significant agreements between both RANS and DES methods at phases with attached flow phases. Detailed information of separating flow near the withdrawal phases are given by DES results. The flow analysis of these blades under reversed flow reveals a significant interaction between the reversed flow and the span-wise sweeping.

Passive flow control application for rotorcraft in transonic conditions

PurposeHelicopter rotor blades are usually aerodynamically limited by the severe conditions present in every revolution: strong shock wave boundary layer interaction on the advancing side and dynamic stall on the retreating side. Therefore, different flow control strategies might be applied to improve the aerodynamic performance.Design/methodology/approachThe present research is focussed on the application of passive rod vortex generators (RVGs) to control the flow separation induced by strong shock waves on helicopter rotor blades. The formation and development in time of the streamwise vortices are also investigated for a channel flow.FindingsThe proposed RVGs are able to generate streamwise vortices as strong as the well-known air-jet vortex generators. It has been demonstrated a faster vortex formation for the rod type. Therefore, this flow control device is preferred for applications in which a quick vortex formation is required. Besides, RVGs were implemented on helicopters rotor blades improving their aerodynamic performance (ratio thrust/power consumption).Originality/valueA new type of vortex generator (rod) has been investigated in several configurations (channel flow and rotor blades).

Digital image correlation displacement measurement of a rotating RC helicopter blade

Abstract An image sensing methodology is developed to enable structural integrity monitoring of rotating parts. The proposed approach is based on simultaneously triggering two high-speed cameras in order to acquire images of the rotating subject in an apparent fixed position and employing 3D Digital Image Correlation (DIC) thereafter to acquire shape and measure total deformation. A controller was developed to generate the trigger signals at appropriate times and is capable of supporting multiple use cases. The present contribution includes the application of this method to a radio-controlled helicopter's rotor blade under dynamic loading from rotation at 680 rpm and a comparison of this method with previous results.

Multi-objective optimal design of composite box beam using hybrid adaptive harmony search with dynamically reconfigurable harmony memory

In the design of helicopter rotor blades, the composite materials are being popularly used, as they are lighter in weight with high specific strength and modulus apart from superior fatigue characteristics. As the performance of the composite materials can be tailored with the optimized layup sequence, the robust optimization algorithms, which cater to this, become necessary. We propose an approach for multi-objective optimization of composite box beam by considering the cross-sectional dimensions and the discrete ply angles as design variables. We present a new improved version of the basic harmony search optimization algorithm by building the features like adaptivity, hybrid search with a customized local search, and multiple sub harmonies with dynamic interaction among them. These features greatly improve both intensification and diversification capabilities of the proposed algorithm. Numerical studies carried out on a box beam clearly reflects the superiority of the proposed approach in providing wider as well as improved design configuration while handling multiple objectives of box-beam design. Finally, we have used a set of performance metrics to demonstrate the effectiveness of the proposed algorithm by comparing with other very recent stochastic algorithms.