Helicopter Rotor Research Articles

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.

Rotation-Frequency-Driven Extension–Torsion Coupled Self-Twisting Rotor Blades

This paper describes the effects of a composite coupled blade spar on the performance of a slowed-rotation-frequency helicopter rotor in high-speed edgewise flight. Antisymmetric composite coupling in the spar of a UH-60A-like rotor can provide a significant efficiency increase when rotation frequency is reduced. Comprehensive analysis was performed using a full three-dimensional finite element analysis-based aeroelastic computational structural dynamics solver with the inclusion of a free-wake aerodynamics model. This was validated using existing UH-60A test data for high advance ratios. The current study shows that a composite blade can achieve a maximum increase in the lift-to-drag ratio of 1.3 (20% improvement) at 85% of the nominal rotation frequency (NR) of . This efficiency gain is achieved through a combination of delayed stall drag along the retreating side of the rotor and reduced negative lift along the advancing side. A further reduction to 65NR showed a maximum efficiency improvement of 15% and was attributed only to the alleviation of negative lift. A hygrothermally stable layup was shown to perform as well as a nominal coupled layup at 85NR and marginally better at 65NR. Close study of the strains in the rotor showed that these rotors would be within the realm of practical manufacturability as axial strains fell within IM7/8552’s allowable tensile strain of 6000 microstrain.

Predicting Formation of Manufacturing Defects in Contoured Composites

Composite helicopter rotor components are typically thick and often have areas with a tight radius of curvature, which make them especially prone to process-induced defects, including wrinkles and voids at ply interfaces. Such flaws cause high rejection rates in production of flight-critical components and structures. This work seeks to fill the gaps in understanding generation of the noted defects in contoured polymer–matrix composite laminates. In particular, understanding and modeling defect formation at the early stages of the manufacturing process might be the missing link to enable the development of practical engineering solutions allowing for better control of the manufacturing process of contoured composite parts. In this work, an approach based on a continuum description of the uncured prepreg material, including the initial bulk or void content, and finite element modeling (FEM) is used to simulate the consolidation process at the early stages of manufacturing of contoured laminates. The simulation predicts instabilities leading to formation of both wrinkles and voids at ply interfaces during laminate debulking or vacuum consolidation. Applicability of the method to consolidation in both closed-cavity and open-face tooling is also demonstrated. FEM results show good correlation with X-ray computed tomography data. This work also introduces a new simulation concept based on finite element and discrete modeling of voids at ply interfaces to improve the accuracy of predicting their evolution during the debulking operations.

A handling qualities analysis tool for rotorcraft conceptual designs

ABSTRACTOver the past decade, NASA, under a succession of rotary-wing programs, has been moving towards coupling multiple discipline analyses to evaluate rotorcraft conceptual designs. Handling qualities is one of the component analyses to be included in such a future Multidisciplinary Analysis and Optimization framework for conceptual design of Vertical Take-Off and Landing (VTOL) aircraft. Similarly, the future vision for the capability of the Concept Design and Assessment Technology Area of the U.S Army Aviation Development Directorate also includes a handling qualities component. SIMPLI-FLYD is a tool jointly developed by NASA and the U.S. Army to perform modelling and analysis for the assessment of the handling qualities of rotorcraft conceptual designs. Illustrative scenarios of a tiltrotor in forward flight and a single-main rotor helicopter at hover are analysed using a combined process of SIMPLI-FLYD integrated with the conceptual design sizing tool NDARC. The effects of variations of input parameters such as horizontal tail and tail rotor geometry were evaluated in the form of margins to fixed- and rotary-wing handling qualities metrics and the computed vehicle empty weight. The handling qualities Design Margins are shown to vary across the flight envelope due to both changing flight dynamics and control characteristics and changing handling qualities specification requirements. The current SIMPLI-FLYD capability, lessons learned from its use and future developments are discussed.

Surface roughness effect on rotor broadband noise

The change of helicopter rotor broadband noise due to different surface roughness during ice accretion is investigated. Comprehensive rotor broadband noise measurements are carried out on rotor blades with different roughness sizes and rotation speeds in two facilities: the Adverse Environment Rotor Test Stand facility at The Pennsylvania State University, and the University of Maryland Acoustic Chamber. In both facilities, the measured high-frequency broadband noise increases significantly with increasing surface roughness height. Rotor broadband noise source identification is conducted and the broadband noise related to ice accretion is thought to be turbulent boundary layer-trailing edge noise. Theory suggests turbulent boundary layer-trailing edge noise scales with Mach number to the fifth power, which is also observed in the experimental data confirming that the dominant broadband noise mechanism during ice accretion is trailing edge noise. A correlation between the ice-induced surface roughness and the broadband noise level is developed. The correlation is strong, which can be used as an ice accretion early detection tool for helicopters, as well as to quantify the ice-induced roughness at the early stage of rotor ice accretion. The trailing edge noise theories developed by Ffowcs Williams and Hall, and Howe both identify two important parameters: boundary layer thickness and turbulence intensity. Numerical studies of two-dimensional airfoils with different ice-induced surface roughness heights are conducted to investigate the extent that surface roughness impacts the boundary layer thickness and turbulence intensity (and ultimately the turbulent boundary layer-trailing edge noise). The results show that boundary layer thickness and turbulence intensity at the trailing edge increase with the increased roughness height. Using Howe’s trailing edge noise model, the increased sound pressure level of the trailing edge noise due to the increased displacement thickness and normalized integrated turbulence intensity are 6.2 and 1.6 dB for large and small accreted ice roughness heights, respectively. The estimated increased sound pressure level values agree reasonably well with the experimental results, which are 5.8 and 2.6 dB for large and small roughness height, respectively.

Emission surfaces and noise prediction from rotating sources

An accurate and efficient prediction method for noise emanating from rotating sources such as helicopter rotors and aircraft propellers, is presented. Combining bifurcation analysis of the retarded time equation with different emission surface topologies, a new Emission Porous Surface (EPS) algorithm has been developed. The algorithm can be used in the framework of hybrid methods that combine Computational Fluid Dynamics, for noise generation, and integral formulations for noise propagation. A panel-by-panel reconstruction of the supersonic emission surface is also proposed, which allows to manage an unstructured mesh. The noise prediction method is applied to the analysis of the UH1H rotor in strong shock delocalization conditions, a well known and extensively studied case of High Speed Impulsive (HSI) noise. The present results are compared with experimental and numerical data available in literature.

Helicopter rotor noise prediction using a convected FW-H equation in the frequency domain

In this paper, high-speed impulsive noise radiated from helicopter rotors in hover and forward flight is computed by using a convected FW-H equation in the frequency domain with a rotating permeable data surface. Compared to the time-domain formulations, the frequency-domain formula avoids retarded time computation and flow data interpolation with respect to the retarded time. In addition, the complicated emission surface construction is obviated, while a supersonic rotating data surface is chosen to account for quadrupole sources outside the sonic cylinder. The selection of a rotating data surface close to the rotor blades improves noise source resolution and thus ensures noise prediction accuracy at a low computational cost, compared to a stationary data surface approach. Besides, the use of a rotating data surface for hovering helicopter rotors enables noise source identification by a steady flow simulation. Application of the methodology to hovering and forward-flight helicopter rotors has shown good agreement with experimental data, thus indicating that the approach is an accurate and efficient computational tool for noise prediction of helicopter rotors operating at high tip Mach number. Since the formula is derived in a moving medium frame, the effect of forward-flight speed on the radiated noise can be explicitly taken into account.

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.

State-space coaxial rotors inflow modelling derived from high-fidelity aerodynamic simulations

Wake inflow modelling is a crucial issue in the development of efficient and reliable computational tools for flight dynamics and aeroelastic analyses of rotorcraft. The aim of this work is the development of state-space, wake inflow modelling techniques for coaxial rotors perturbed from steady flight conditions, based on simulations provided by high-fidelity aerodynamic solvers. These provide relationships of the coefficients of an approximated linear distribution of wake inflow over upper and lower rotor discs either with rotor controls and helicopter kinematic variables or with thrust and in-plane moments generated by the rotors. A three-step identification procedure is proposed. It consists in: (i) evaluation of wake inflow due to harmonic perturbations of rotor kinematics; (ii) determination of the corresponding inflow coefficients (and rotors loads) transfer functions, followed by (iii) their rational approximation and transformation into time domain to obtain the final differential form. Aerodynamic responses are predicted through a boundary element method tool for potential flows, capable of capturing effects due to wake distortion, multi-body interference (like that in coaxial rotors) and severe blade–vortex interactions. The derived state-space models are validated by correlation with inflow distributions directly calculated by the aerodynamic solver, for a coaxial rotor subject to arbitrary perturbations.

Rotor Airload and Acoustics Prediction Based on CFD/CSD Coupling Method

An advanced airload and noise prediction method based on computational fluid dynamics/computational structural dynamics (CFD/CSD) coupling for helicopter rotor has been developed in this paper. In the present method, Navier-Stokes equation is applied as the governing equation, and a moving overset grid system is generated in order to account for the blade motions in rotation, flapping and pitching. The blade structural analysis is based on 14-DOF Euler beam model, and the finite element discretization is conducted on Hamilton's variational principle and moderate deflection theory. Aerodynamic noise is calculated by Farassat 1A formula derived from FW-H equation. Using the developed method, numerical example of UH-60A is performed for aeroelastic loads calculation in a low-speed forward flight, and the calculated results are compared with both those from isolated CFD method and available experimental data. Then, rotor noise is emphatically calculated by CFD/CSD coupling method and compared with the isolated CFD method. The results show that the aerodynamic loads calculated from CFD/CSD method are more satisfactory than those from isolated CFD method, and the exclusion of blade structural deformation in rotor noise calculation may cause inaccurate results in low-speed forward flight state.

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).

Flowfield analysis of helicopter rotor in hover and forward flight based on CFD

The helicopter rotor field is simulated in hover and forward flight based on Computational Fluid Dynamics(CFD). In hover case only one rotor is simulated with the periodic boundary condition in the rotational coordinate system and the grid is fixed. In the non-lift forward flight case, the total rotor is simulated in inertia coordinate system and the whole grid moves rigidly. The dual-time implicit scheme is applied to simulate the unsteady flowfield on the movement grids. The k – ω turbulence model is employed in order to capture the effects of turbulence. To verify the solver, the flowfield around the Caradonna-Tung rotor is computed. The comparison shows a good agreement between the numerical results and the experimental data.

Uncertainty quantification for the aeroacoustics of rotating blades in the time domain

Aeroacoustics has received great attention in the past decade, owing to the ever stricter noise regulations. Despite the stochastic nature of most aeroacoustic systems, non-deterministic investigations in regards to computational aeroacoustics (CAA) are limited. In this paper, uncertainty quantification has been achieved for the noise propagation stage of hybrid CAA, and also on the noise prediction of a non-lifting helicopter rotor in hover. Analytical and computational fluid dynamics test cases have been analyzed, with uncertainties propagated through these systems using non-intrusive polynomial chaos methods. It is shown here that the source of the uncertainty in the noise is dominated by the major characteristic properties of the simulations, such as the mean flow Mach number and blade tip Mach number. Only at a low tip Mach number uncertainties in the blade thickness may contribute significantly to the noise uncertainty. Apart from this, it is seen to be unlikely that small uncertainties in the geometry, ambient conditions and observer properties will contribute significantly to the noise uncertainty. A peak pressure uncertainty of up to 20% is seen in the hovering helicopter test case, from small, realistic uncertainties. This highlights the importance of considering uncertainties in CAA investigations.

Design, control, and visual navigation of the DelftaCopter VTOL tail‐sitter UAV

AbstractTo participate in the Outback Medical Express UAV Challenge 2016, a vehicle was designed and tested that can autonomously hover precisely, takeoff and land vertically, fly fast forward efficiently, and use computer vision to locate a person and a suitable landing location. The vehicle is a novel hybrid tail‐sitter combining a delta‐shaped biplane fixed‐wing and a conventional helicopter rotor. The rotor and wing are mounted perpendicularly to each other,and the entire vehicle pitches down to transition from hover to fast forward flight where the rotor serves as propulsion. To deliver sufficient thrust in hover while still being efficient in fast forward flight, a custom rotor system was designed. The theoretical design was validated with energy measurements, wind tunnel tests, and application in real‐world missions. A rotor‐head and corresponding control algorithm were developed to allow transitioning flight with the nonconventional rotor dynamics that are caused by the fuselage rotor interaction. Dedicated electronics were designed that meet vehicle needs and comply with regulations to allow safe flight beyond visual line of sight. Vision‐based search and guidance algorithms running on a stereo‐vision fish‐eye camera were developed and tested to locate a person in cluttered terrain never seen before. Flight tests and a competition participation illustrate the applicability of the DelftaCopter concept.

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.

Numerical simulation for three-dimensional rotor icing in forward flight

Icing will occur when helicopters pass through clouds containing supercooled droplets. The ice accreted on rotor blade affects the safety of helicopter. A numerical simulation method of helicopter rotor icing in forward flight is presented, and the icing characteristics of rotor are studied. The flow field is calculated based on the overlapping grids, and the Eulerian model is applied in the droplet trajectory calculation. The unsteady method is used to calculate rotor flow field and droplet trajectory. The icing model considering the effect of centrifugal force is adopted to simulate the ice shapes of rotor in forward flight. The average ice growth rate is compared with the ice growth rate at each azimuth angle, and the effects of blade pitch and forward flight speed on the droplet impingement and icing characteristics are summarized. Finally, the influence of time step on the calculation results is analyzed. The calculation results show that the droplet impingement characteristics and icing characterist...

Pressure feedback-based blade–vortex interaction noise controller for helicopter rotors

With the aim of alleviating the noise annoyance emitted by blade–vortex interactions occurring on helicopter main rotors, the present work presents a methodology suitable for the identification of a multi-cyclic harmonic controller based on the actuation of rotor blades equipped with Miniature Trailing Edge Effectors. The objective of the control methodology is the direct suppression of the aerodynamic noise sources by generation of localized high-harmonic blade–vortex interaction counter-actions. The set-up of control devices is selected on the basis of the blade–vortex interaction scenario, taking into account a trade-off between effectiveness and power requirement. The control law is efficiently identified by means of an optimal controller synthesized through suitable two-dimensional multi-vortex, parallel blade–vortex interaction problems. The proposed methodology is validated by the application to realistic helicopter main rotors during low-speed descent flights, numerically simulated through high-fidelity aerodynamic and aeroacoustic solvers based, respectively, upon a three-dimensional free-wake boundary element method to solve the potential flow around rotors in blade–vortex interaction conditions and the Farassat 1A formulation. Results concerning the capability of the proposed controller to alleviate the blade–vortex interaction noise emitted by a realistic helicopter main rotor are presented and discussed.

Analysis of Dynamic Stall Mechanisms in Helicopter Rotor Environment

Analysis of Dynamic Stall Mechanisms in Helicopter Rotor Environment

Identification of an effective nondestructive technique for bond defect determination in laminate composites—A technical review

Laminate composites are commonly used for the production of critical mechanical structures and components such as wind turbine blades, helicopter rotors, unmanned aerial vehicle wings and honeycomb structures for aircraft wings. During the manufacturing process of these composite structures, zones or areas with weak bond strength are always issues, which may affect the strength and performance of components. The identification and quantification of these zones are always challenging and necessary for the mass production. Non-destructive testing methods available, including ultrasonic A, B, and C-Scan, laser shearography, X-ray tomography, and thermography can be useful for the mentioned purposes. A comparison of these techniques concerning their capacity of identification and quantification of bond defects; however, still needs a comprehensive review. In this paper, a detailed comparison of several non-destructive testing techniques is provided. Emphasis is placed to institute a guideline to select the most suitable technique for the identification of zones with bond defects in laminated composites. Experimental tests on different composite based machined components are also discussed in detail. The discussion provides practical evidence about the effectiveness of different non-destructive testing techniques.