Showing papers on "Helicopter rotor published in 2010"

Method and apparatus for in-flight blade folding

Abstract: A foldable rotor system for a rotorcraft, the foldable rotor system comprising a rotor assembly operably associated with a driveshaft, the driveshaft being operable associated with an engine, the rotor assembly comprising a rotor blade connected to a grip pin. A swashplate is operable associated with the grip pin in order selectively change a pitch of the rotor blade. A blade fold actuator is operably associated with the grip pin such that the blade fold actuator is configured to fold and unfold the rotor blade about a blade fold axis. During an airplane mode, the rotorcraft can stop and fold the rotor blades so that the rotorcraft relies upon thrust from the engine for propulsion. The rotor blades are folded in a spiral fold path so that the rotor blades remain substantially edgewise, or feathered, during the folding process. The spiral fold path minimizes the aerodynamic drag experienced by the rotor blades while being folded during flight of the rotorcraft.

Blade Deflection Measurements of a Full-Scale UH-60A Rotor System

Abstract: Blade deflection (BD) measurements using stereo photogrammetry have been made during the individual blade control (IBC) testing of a UH-60A 4-bladed rotor system in the 40 by 80-foot test section of the National Full-Scale Aerodynamic Complex (NFAC). Measurements were made in quadrants one and two, encompassing advance ratios from 0.15 to 0.40, thrust coefficient/solidities from 0.05 to 0.12 and rotor-system drive shaft angles from 0.0 to -9.6 deg. The experiment represents a significant step toward providing benchmark databases to be utilized by theoreticians in the development and validation of rotorcraft prediction techniques. In addition to describing the stereo measurement technique and reporting on preliminary measurements made to date, the intent of this paper is to encourage feedback from the rotorcraft community concerning continued analysis of acquired data and to solicit suggestions for improved test technique and areas of emphasis for measurements in the upcoming UH-60A Airloads test at the NFAC.

Tilt rotor aircraft adopting parallel coaxial dual rotors

Abstract: The invention relates to a tilt rotor aircraft adopting parallel coaxial dual rotors, which comprises a fuselage, wings, an empennage, a pitch control scull system, a landing gear, a power and fuel system, a transmission system, a rotor system, a rotor nacelle and a tilt system, wherein the wings are arranged at the center section of the fuselage; the empennage and the pitch control scull system are arranged at the tail of the fuselage; the landing gear is positioned at the belly of the fuselage; the power and fuel system is arranged inside the center section of the fuselage and is connected with the rotor system and the pitch control scull system through the wings and the transmission system in the fuselage; the rotor system is arranged on the rotor nacelle at the tip of the wings; partial wing which is fixedly connected with the rotor nacelle and simultaneously can tilt is arranged at the inner side of the rotor nacelle; and the tilt system is arranged in the wings and is connected with the rotor nacelle and the partial wing which can tilt. The tilt rotor aircraft is mainly characterized by adopting the pitch control scull system, the parallel coaxial dual rotors and the partialwing which can tilt to realize flight status transformation and conventional taxiing and landing, thereby improving the forward speed and the propulsive efficiency.

Helicopter rotor shape optimization for the improvement of aeroacoustic performance in hover

Abstract: A helicopter rotor is optimally designed for aeroacoustic performance improvement. As shown in previous reports, the blade shapes can be designed to minimize high-speed impulsive noise but tend to have excessively high tapers and be swept back. Since an overly short chord length around the blade-tip region may cause structural problems and safety issues in autorotation, an autorotation index has been introduced to keep the tip region from having excessive taper ratios. In addition, the changes in thickness and camber of the airfoils can also be taken into account to better represent realistic rotor shapes. Aeroacoustic analysis is performed using Kirchhoff's method coupled with computational fluid dynamics analysis, and the optimization is performed using the kriging-model-based genetic algorithm method. Optimization results are presented that show that the designed blades have improved aerodynamic performance and reduced high-speed impulsive noise characteristics. It is found that a more practical blade shape can be obtained by using airfoil transitions and an autorotation index. The results of the analysis of variance and self-organization map indicate that the taper ratios, the swept back, the tip chord length, the protrusion shape, the camber, and the thickness of the root airfoil are the prominent features affecting the aeroacoustic performance of the rotor.

Instability and Chaos of a Flexible Rotor Ball Bearing System: An Investigation on the Influence of Rotating Imbalance and Bearing Clearance

Abstract: In this paper, a horizontal flexible rotor supported on two deep groove ball bearings is theoretically investigated for instability and chaos. The system is bi-periodically excited. The two sources of excitation are: rotating imbalance and self excitation due to varying compliance effect of ball bearing. A generalized Timoshenko beam FE formulation, which can be used for both flexible and rigid rotor systems with equal effectiveness, is developed. The novel scheme proposed in the literature to analyze quasi-periodic response is coupled with the existing non-autonomous shooting method and thus modified; shooting method is used to obtain steady state quasi-periodic solution. The eigenvalues of monodromy matrix provide information about stability and nature of bifurcation of quasi-periodic solution. The maximum value of Lyapunov exponent is used for quantitative measure of chaos in the dynamic response. The effect of three parameters, viz.: rotating unbalance, bearing clearance and rotor flexibility, on unstable and chaotic behaviour of horizontal flexible rotor is studied. Interactive effects between the three parameters are examined in detail in respect of rotor system instability and chaos, and finally the range of parameters is established for the same.Copyright © 2010 by ASME

Experimental study of structural damping of composite helicopter blades with different cores

Abstract: In this paper, the experimental results of structural vibratory testing for the light multipurpose helicopter main rotor blade of composite laminated materials are presented. The aim of the main rotor blade vibratory testing was to define the basic aeroelastic properties of the blade. This testing included the determination of the natural oscillation modes and natural frequency of the structure in oscillations, as well as defining the blade structural damping. The measurement was performed for three different full scale models with three different types of cores of the main rotor blade: Rohacell 71 polyurethane foam, Nomex aramid honeycomb and Nomex phenolic honeycomb. The results are presented in this paper.

Development and Conversion Flight Test of a Small Tiltrotor Unmanned Aerial Vehicle

Abstract: T HE tiltrotor configuration was selected as the unmanned aerial vehicle (UAV) platform for the Smart UAV (SUAV) development program, which is one of the 21st Frontier research and development programs supported by the Korean government [1]. Some of the subsystems of the SUAV, such as the rotor system, drive system, andflight control system, were especially challenging for the Korea Aerospace Research Institute to develop, due to lack of previous development experience. To reduce the risk of failure during the development, the ironbird test of the rotor and drive system and flight simulation was conducted. Although the flight simulation mitigated the risk in the flight control system development, a downscaled model was constructed in an effort to ensure the safety in the flight test of the full-scale vehicle. The small-scale platform is shown in Fig. 1. It is expected from the flight test of the small scale model to enhance the understanding of the features of the actual tiltrotor vehicle. The size of the downscaledmodelwas determined to be 1=2:5 (2m overall length) of the full-scale SUAV (5 m overall length) so as to have large enough space for the simple flight control computer, the navigation system, and the other components such as the engine and actuators. The aerodynamic performance of the 1=2:5-scale tiltrotor was analyzed using the in-house performance code. The calculated performance data were used for scheduling the control devices, such as the collective pitch of the rotors and the flaperon deflection, in the flight control logic. In this Note, sizing and performance analysis of the downscaled tiltrotor are presented in which simple codes based on blade element and momentum theory were used. The conversion corridor of the tiltrotor was predicted and the nacelle tilt angle and air speed were compared with flight-test results. After a series of progressive flight tests, conversion flight from helicopter to fixed-wing mode was accomplished. This verified that the stability and control augmentation algorithmwork properly in the flight control software. This small tiltrotor flight test is expected to reduce risks in flight test of the 5-m-span full-scale tiltrotor UAV called Smart UAV. II. Sizing of the Tiltrotor

Bond Graph Modeling of an Internally Damped Nonideal Flexible Spinning Shaft

Abstract: The rotating internal damping or nonconservative circulatory force in a rotor shaft system causes instability beyond a certain threshold rotor spinning speed. However, if the source loading of the drive is considered, then the rotor spin is entrained at the stability threshold and a stable whirl orbit is observed about the unstable equilibrium. As we move toward the use of more and more lightweight rotor dynamic components such as the shaft and the motor, overlooking this frequency entrainment phenomenon while sizing the actuator in the design stage may lead to undesirable performance. This applies to many emerging areas of strategic importance such as in vivo medical robots where flexible probes are used and space robotics applications involving rotating tools. We analyze this spin entrainment phenomenon in a distributed parameter model of a spinning shaft, which is driven by a nonideal dc motor. A drive whose dynamics is influenced by the dynamics of the driven system is called a nonideal source and the whole system is referred to as a nonideal system. In particular, we show the advantages of representing such nonideal drive-system interactions in a modular manner through bond graph modeling as compared to standard equation models where the energetic couplings between dynamic variables are not explicitly shown. The developed modular bond graph model can be extended to include rotor disks and bearings placed at different locations on the shaft. Moreover, the power conserving property of the junction structure of the bond graph model is exploited to derive the source loading expressions, which are then used to analytically derive the steady-state spinning frequency and whirl orbit amplitude as functions of the drive and the rotor system parameters. We show that the higher transverse modes may become unstable before the lower ones under certain parametric conditions. The shaft spinning speed is entrained at the lowest stability threshold among all transverse modes. The bond graph model is used for numerical simulation of the system to validate the steady-state results obtained from the theoretical study.

Computational-Fluid-Dynamics-Based Twist Optimization of Hovering Rotors

Abstract: Twist optimization of a helicopter rotor in hover is presented using compressible computational fluid dynamics as the aerodynamic model. A domain-element shape parameterization method has been developed, which solves both the geometry control and the volumemesh deformation problems simultaneously, using radial basis function global interpolation. This provides direct transfer of domain-element movements into deformations of the design surface and the computational fluid dynamics volume mesh, which is deformed in a high-quality fashion. The method is independent ofmesh type (structured or unstructured), and it has been linked to an advancedparallel gradient-based algorithm, for which independence from the flow solver is achieved by obtaining sensitivity information by finite differences. This has resulted in aflexible andversatilemodularmethod ofwraparound optimization. Previousfixedwing results have shown that a large proportion of the design space is accessible with the parameterization method, and heavily constrained drag optimization demonstrated significant performance improvements. In the present work, themethod is extended to a rotor blade, and this is optimized forminimum torque in hovering flight with strict constraints. Twist optimization results are presented for three tip Mach numbers, and the effects of different parameterization levels are analyzedusing various combinations of two levels: global and local. Torque reductions of over 12% are shown for a fully subsonic case, and for over 24% for a transonic case, using only three global and 15 local twist parameters.

Rotor system health monitoring using shaft load measurements and virtual monitoring of loads

Abstract: A method of real-time rotor fault detection includes measuring a set of loads to obtain measured signals and virtually monitoring the set of loads to obtain estimated signals. The estimated signals are subtracted from the measured signals to obtain residuals and the residuals are compared to a categorical model. A categorical output representative of a rotor fault is identified within the categorical model.

Wind Tunnel Testing of a Helicopter Rotor Trailing Edge Flap Actuated via Pneumatic Artificial Muscles

Abstract: A novel active trailing edge flap actuation system is under development. This system differs significantly from previous trailing edge flap systems in that it is driven by a pneumatic actuator technology. Pneumatic Artificial Muscles (PAMs) were chosen because of several attractive properties, including high specific work and power output, an expendable operating fluid, and robustness. The actuation system is sized for a full scale active rotor system for a Bell 407 scale helicopter. This system is designed to produce large flap deflections (±20°) at the main rotor rotation frequency (1/rev) to create large amplitude thrust variation for primary control of the helicopter. Additionally, it is designed to produce smaller magnitude deflections at higher frequencies, up to 5/rev (N+1/rev), to provide vibration mitigation capability. The basic configuration has a pair of Pneumatic Artificial Muscles mounted antagonistically in the root of each blade. A bellcrank and linkage system transfers the force and motion of these actuators to a trailing edge flap on the outboard portion of the rotor. A reduced span wind tunnel test model of this system has been built and tested in the Glenn L. Martin Wind Tunnel at the University of Maryland at wind speeds up to M = 0.3. The test article consisted of a 5-ft long tip section of a Bell 407 rotor blade cantilevered from the base of the tunnel with a 34 in, 15% chord plain flap that was driven by the PAM actuation system. Testing over a wide range of aerodynamic conditions and actuation parameters established the considerable control authority and bandwidth of the system at the aerodynamic load levels available in the tunnel. Comparison of quasi-static experimental results shows good agreement with predictions made using a simple system model.Copyright © 2010 by ASME

Experimental and Numerical Studies on Nonlinear Dynamic Behavior of Rotor System Supported by Ball Bearings

Abstract: Ball bearings are important mechanical components in high-speed turbomachinery that is liable for severe vibration and noise due to the inherent nonlinearity of ball bearings. Using experiments and the numerical approach, the nonlinear dynamic behavior of a flexible rotor supported by ball bearings is investigated in this paper. An experimental ball bearing-rotor test rig is presented in order to investigate the nonlinear dynamic performance of the rotor systems, as the speed is beyond the first synchroresonance frequency. The finite element method and two-degree-of-freedom dynamic model of a ball bearing are employed for modeling the flexible rotor system. The discrete model of a shaft is built with the aid of the finite element technique, and the ball bearing model includes the nonlinear effects of the Hertzian contact force, bearing internal clearance, and so on. The nonlinear unbalance response is observed by experimental and numerical analysis. All of the predicted results are in good agreement with experimental data, thus validating the proposed model. Numerical and experimental results show that the resonance frequency is provoked when the speed is about twice the synchroresonance frequency, while the subharmonic resonance occurs due to the nonlinearity of ball bearings and causes severe vibration and strong noise. The results show that the effect of a ball bearing on the dynamic behavior is noticeable in optimum design and failure diagnosis of high-speed turbomachinery.

Investigation of Rotor Performance and Loads of a UH-60A Individual Blade Control System

Abstract: A full-scale wind tunnel test was recently conducted (March 2009) in the National Full-Scale Aerodynamics Complex (NFAC) 40- by 80-FootWind Tunnel to evaluate the potential of an individual blade control (IBC) system to improve rotor performance and reduce vibrations, loads, and noise for a UH-60A rotor system [1]. This test was the culmination of a long-termcollaborative effort between NASA, U.S. Army, Sikorsky Aircraft Corporation, and ZF Luftfahrttechnik GmbH (ZFL) to demonstrate the benefits of IBC for a UH-60Arotor. Figure 1 shows the UH-60Arotor and IBC system mounted on the NFAC Large Rotor Test Apparatus (LRTA). The IBC concept used in the current study utilizes actuators placed in the rotating frame, one per blade. In particular, the pitch link of the rotor blade was replacedwith an actuator, so that the blade root pitch can be changed independently. This concept, designed for a full-scale UH-60A rotor, was previously tested in the NFAC 80- by 120-FootWind Tunnel in September 2001 at speeds up to 85 knots [2]. For the current test, the same UH-60A rotor and IBC system were tested in the 40- by 80-FootWind Tunnel at speeds up to 170 knots. Figure 2 shows the servo-hydraulic IBC actuator installed between the swashplate and the blade pitch horn. Although previous wind tunnel experiments [3, 4] and analytical studies on IBC [5, 6] have shown the promise to improve the rotor s performance, in-depth correlation studies have not been performed. Thus, the current test provides a unique resource that can be used to assess the accuracy and reliability of prediction methods and refine theoretical models, with the ultimate goal of providing the technology for timely and cost-effective design and development of new rotors. In this paper, rotor performance and loads calculations are carried out using the analyses CAMRAD II and coupled OVERFLOW-2/CAMRAD II and the results are compared with these UH-60A/IBC wind tunnel test data.

Reviewing and Investigating the Use of Co-Axial Rotor Systems in Small UAVs

Abstract: Co-axial propeller systems have been used successfully in fixed-wing aircraft for many years due to their inherently good performance. Likewise, several pioneering rotarywinged aircraft projects used co-axial rotors, such as the Westland WG-25 Mote, Gyrodyne DASH, and a range of rotorcraft from Kamov in Russia. There has been much debate over the years about whether the co-axial rotor arrangement is more efficient than traditional layouts. Our findings point to the fact that although the co-axial arrangement has a reduced power output of up to 15% when compared to an equivalent single rotor system, this can be offset by the elimination of the need for a tail rotor, which could save up to 20% of the required power. It is only recently that this type of technology has reached the small UAV market in the form of a series of semi-autonomous and autonomous rotorcraft which are starting to make their mark in the military, homeland security and civilian fields. This paper investigates the use of such systems and discusses their advantages, disadvantages and performance metrics.

Airfoil for a helicopter rotor blade

Abstract: An airfoil family for a helicopter rotor blade, designated SC362XX SC362XX essentially removes the large lower surface suction peak associated with ‘drag creep’ at moderate lift coefficients while reducing the peak Mach number and shock strength at high lift/Mach number conditions Another optional airfoil family for use at inboard regions of the helicopter rotor, which is designated SC3252XX airfoil family, is a relatively thicker airfoil section that includes a significant increase in thickness forward of the 30% x/c location to provide a relatively thick and rigid inboard section The lift coefficient at which the drag divergence Mach number was optimized is the same in both families thereby readily providing application to a single rotor blade

Applications of Low-Speed Dynamic-Stall Model to the NREL Airfoils

Abstract: National Renewable Energy Laboratory, USA (NREL) airfoils have been specially developed for wind turbine applications, and projected to yield more annual energy without increasing the maximum power level. These airfoils are designed to have a limited maximum lift and relatively low sensitivity to leading-edge roughness. As a result, these airfoils have quite different leading-edge profiles from airfoils applied to helicopter blades, and thus, quite different dynamic-stall characteristics. Unfortunately for wind turbine aerodynamics, the dynamic-stall models in use are still those specially developed and refined for helicopter applications. A good example is the Leishman―Beddoes dynamic-stall model, which is one of the most popular models in wind turbine applications. The consequence is that the application of such dynamic-stall model to low-speed cases can be problematic. Recently, some specific dynamic-stall models have been proposed or tuned for the cases of low Mach numbers, but their universality needs further validation. This paper considers the application of the modified dynamic low-speed stall model of Sheng et al. ( "A Modified Dynamic Stall Model for Low Mach Numbers, " 2008, ASME J. Sol. Energy Eng., 130(3), pp. 031013) to the NREL airfoils. The predictions are compared with the data of the NREL airfoils tested at the Ohio State University. The current research has two objectives: to justify the suitability of the low-speed dynamic-stall model, and to provide the relevant parameters for the NREL airfoils.

Design of Extendable Chord Sections for Morphing Helicopter Rotor Blades

Abstract: Chord extension in helicopter rotors allows for expansion of the flight envelope, with the helicopter capable of flying at higher gross weights, altitudes and maximum speeds. A fixed large chord, however, results in a penalty when the helicopter is well within the envelope (for example, at low to moderate gross weight, sea level, and at moderate speed cruise). Chord morphing allows the helicopter to perform optimally in these diverse conditions. In this paper, the authors present a morphing mechanism to extend the chord of a section of the helicopter rotor blade. The region aft of the leading-edge spar contains a morphing cellular structure. In the “compact” state the edge of the cellular structure aligns with the trailing-edge of the rest of the blade. When the morphing cellular structure is in the “extended” state the chord of that section of the blade is increased by close to 30% (with the trailing-edge extending beyond that of the rest of the blade). In transitioning from compact to extended states, the cellular structure slides along ribs which define the boundaries of the morphing section in the span-wise direction. The cellular section has mini-spars running along the span-wise direction to attach the flexible skin and provide stiffness against camber-like deformations due to aerodynamic loads. The paper presents a finite element analysis and a design study of the morphing cellular structure, ensuring that the local strains in the cellular structure do not exceed maximum allowables even as the section undergoes large global strain. On the other hand, the morphing cellular structure is required to be stiff enough so that the pre-stretched skin that is attached to the surfaces does not result in deformation. Another question that is considered in detail is various methods of attachment of the flexible skin to the morphing substructure, the levels of pre-strain required, and their ramifications. A model of a blade section is fabricated and shown to undergo chord morphing, as designed.Copyright © 2010 by ASME

Helicopter rotor control system

Abstract: A rotor control system operatively linked to a plurality of rotor blades includes a swashplate assembly having a stationary member and a rotating member. A blade attachment member is operatively connected to the plurality of rotor blades and a control horn is operatively connected to the blade attachment member and one of the plurality of rotor blades. At least one hydraulic actuator member is operatively coupled to the control horn and at least one hydraulic actuator element is operatively coupled to the swashplate assembly and the at least one hydraulic actuator member. The at least one hydraulic actuator member transmits control signals from the swashplate assembly to the one of the plurality of rotor blades through the at least one hydraulic actuator member.

High-resolution simulations of parallel blade-vortex interactions

Abstract: The physics of a parallel blade―vortex interaction is studied numerically and the predicted pressure and acoustic results are compared with experimental measurements. A high-resolution solution of the compressible Euler equations is performed on structured overset meshes. Initially, a two-dimensional airfoil-vortex interaction is studied for both a case where the vortex misses the blade and a case of direct impact. The vortex is initiated in the flow as a perturbation to the freestream conditions and is free to evolve, thus allowing for the deformation of the vortex as it interacts with the blade to be studied. The simulation is seen to accurately reproduce the experimental results and the emission of the acoustic waves from the airfoil surface is observed in detail. Acoustic energy generated by the interaction is seen to primarily radiate from the leading-edge section of the airfoil with a weaker contribution coming from the trailing edge. The simulations are then extended to three-dimensional moving overset meshes where the vortex generation and convection is also resolved. The numerical methodology is seen to accurately preserve the vortex strength and accurately reproduce the experimentally measured blade surface pressures and acoustics. The computations presented here face similar challenges to that encountered in the simulation of realistic helicopter blade—vortex interaction, but the computational costs are such that the solutions can be well resolved, and comprehensively validated using moderate resources.

Self-powered smart blade: helicopter blade energy harvesting

Abstract: A novel energy harvesting device powered by aeroelastic flutter vibrations is proposed to generate power for embedded wireless sensors on a helicopter rotor blade Such wireless sensing and on-board power generation system would eliminate the need for maintenance intensive slip ring systems that are required for hardwired sensors A model of the system has been developed to predict the response and output of the device as a function of the incident wind speed A system of coupled equations that describe the structural, aerodynamic, and electromechanical aspects of the system are presented The model uses semi-empirical, unsteady, nonlinear aerodynamics modeling to predict the aerodynamic forces and moments acting on the structure and to account for the effects of vortex shedding and dynamic stall These nonlinear effects are included to predict the limit cycle behavior of the system over a range of wind speeds The model results are compared to preliminary wind tunnel tests of a low speed aeroelastic energy harvesting experiment

Multi-objective aerodynamic shape optimization for unsteady viscous flows

Abstract: This paper presents an adjoint method for the multi-objective aerodynamic shape optimization of unsteady viscous flows. The goal is to introduce a Mach number variation into the Non-Linear Frequency Domain (NLFD) method and implement a novel approach to present a time-varying cost function through a multi-objective adjoint boundary condition. The paper presents the complete formulation of the time dependent optimal design problem. The approach is firstly demonstrated for the redesign of a helicopter rotor blade in two-dimensional flow and in three-dimensional viscous flow, the technique is employed to validate and redesign the NASA Rectangular Supercritical Wing (RSW).

Aircraft with helicopter rotor, thrust generator and assymetric wing configuration

Abstract: An aircraft features a source of forward thrust on a fuselage having a helicopter rotor assembly and an asymmetric primary wing configuration providing more wing-generated lift on one side of the fuselage than the other. The primary wing configuration counteracts the rotor's dissymmetry of lift during forward cruising, and reliance on the separate thrust source for such cruising reduces demand on the main rotor, keeping the angle of attack on the rotor blades low to avoid the stalling and violent vibration experienced by conventional helicopters at relatively high speeds. In some embodiments, an oppositely asymmetric tail wing or horizontal stabilizer acts alone, or together with an offset vertical stabilizer laterally outward from the tail, to counteract yaw-inducing drag of the primary wing.

Dynamic behaviours of the rotor non-linear system with fixed—tilting-pad journal bearings support

Abstract: The variational constraint approach is used to calculate the non-linear oil force of fixed—tilting-pad journal bearings. The non-linear oil force of a single pad is calculated by the proposed method in its coordinate system and its Jacobi matrix is calculated simultaneously for compatibility. The oil force, damping, and stiffness coefficients of the bearing are obtained by the assembly method. The unbalanced responses of the symmetrical rigid rotor dynamic system supported by fixed—tilting-pad journal bearings are analysed by the Runge—Kutta method and a Poincare map. The effects of the pivot ratio and the preload on the stability of the rotor are analysed. The numerical results reveal complex non-linear behaviours of the system, such as periodic, period-doubling, period-13, and quasi-periodic motion.

A rational approach to comparing the performance of coaxial and conventional rotors

Abstract: The merit, in terms of its efficiency and performance, of the twin, contrarotating coaxial rotor configuration over the more conventional single rotor system has long been a point of contention. Previously published comparisons yield seemingly inconsistent and conflicting conclusions. In this paper, the basis for a fair, like-for-like comparison of the performance of coaxial and single rotor systems is discussed. A comparison between experimentally measured data and numerical predictions of rotor performance obtained using the vorticity transport model shows that a computational approach can be used reliably to decompose the power consumption into induced and profile constituents. These comparisons show that a somewhat stronger similarity in geometry needs to be enforced between the two types of rotor system than previously suggested in order that the systems be directly comparable. If the equivalent single rotor system is constructed to have the same disk area, blade geometry, and total number of blades as that of the coaxial rotor, then the geometric differences between the two systems are confined to the defining characteristics of the two types of rotor system, in other words to the vertical separation between the rotor blades and their relative direction of rotation. The differences in aerodynamic performance between a coaxial rotor and an equivalent single rotor defined in this way then arise solely as a result of the differences in the detailed interaction between the blades and their wakes that arise within the two types of system. Using this form of comparison, the articulated coaxial system is shown to consume marginally less induced power than the equivalent single rotor system. The difference is small enough, however, to be obscured if the profile drag of the blades is overtly sensitive to operating condition, as for instance might be the case at low Reynolds number.

Computational-Fluid-Dynamics- and Computational-Structural- Dynamics-Based Time-Accurate Aeroelasticity of Helicopter Rotor Blades

Abstract: A modular capability to compute dynamic aeroelastic characteristics of rotor blades using the Euler/Navier-Stokes flow equations and finite element structural equations is presented. The approach is based on a time-accurate analysis procedure that is suitable for nonlinear fluid-structure interaction problems. Fluids and structural solvers are time-accurately coupled in the C++ environment. Unsteady aerodynamic and aeroelastic results are validated with experimental data for nonrotating and rotating isolated blades.

A surrogate based approach to reduced-order dynamic stall modeling

Abstract: The surrogate-based recurrence framework (SBRF) approach to reduced-order nonlinear unsteady aerodynamic modeling associated with pitching/plunging airfoils subject to xed or time-varying freestream Mach numbers is described. Using full-order solutions generated by the OVERFLOW CFD code, the SBRF reduced-order modeling approach is shown to eectively mimic full-order solutions of unsteady lift, moment, and drag under dynamic stall conditions, but at a fraction of the computational cost. In addition to accounting for realistic helicopter rotor blade dynamics, it is shown that the SBRF can model advancing rotor blade stall due to shock induced separation, as well as retreating blade stall associated with excessive angles of attack. Therefore, the SBRF reduced-order modeling approach is ideally suited for a variety of aeroelasticity and active/passive design optimization studies that require high delity aerodynamic response solutions with minimal computational expense.