Showing papers on "Helicopter rotor published in 1993"

TURNS - A free-wake Euler/Navier-Stokes numerical method for helicopter rotors

Abstract: Computational capabilities of a numerical procedure, called TURNS (transonic unsteady rotor Navier-Stokes), to calculate the aerodynamics and acoustics (high-speed impulsive noise) out to several rotor diameters are summarized. The procedure makes it possible to obtain the aerodynamics and acoustics information in one single calculation. The vortical wave and its influence, as well as the acoustics, are captured as part of the overall flowfield solution. The accuracy and suitability of the TURNS method is demonstrated through comparisons with experimental data.

A Finite-Volume Euler Solver for Computing Rotary-Wing Aerodynamics on Unstructured Meshes

Abstract: An unstructured-grid solver for the unsteady Euler equations has been developed for predicting the aerodynamics of helicopter rotor blades. This flow solver is a finite-volume scheme that computes flow quantities at the vertices of the mesh. Special treatments are used for the flux differencing and boundary conditions in order to compute rotary-wing flowfields, and these are detailed in the paper. The unstructured-grid solver permits adaptive grid refinement in order to improve the resolution of flow features such as shocks, rotor wakes and acoustic waves. These capabilities are demonstrated in the paper. Example calculations are presented for two hovering rotors. In both cases, adaptive-grid refinement is used to resolve high gradients near the rotor surface and also to capture the vortical regions in the rotor wake. The computed results show good agreement with experimental results for surface airloads and wake geometry.

Aeroelastic Response and Stability of Tiltrotors with Elastically-Coupled Composite Rotor Blades

Abstract: There is a potential for improving the performance and aeroelastic stability of tiltrotors through the use of elastically-coupled composite rotor blades. To study the characteristics of tiltrotors with these types of rotor blades it is necessary to formulate a new analysis which has the capabilities of modeling both a tiltrotor configuration and an anisotropic rotor blade. Background for these formulations is established in two preliminary investigations. In the first, the influence of several system design parameters on tiltrotor aeroelastic stability is examined for the high-speed axial flight mode using a newly-developed rigid-blade analysis with an elastic wing finite element model. The second preliminary investigation addresses the accuracy of using a one- dimensional beam analysis to predict frequencies of elastically-coupled highly-twisted rotor blades. Important aspects of the new aeroelastic formulations are the inclusion of a large steady pylon angle which controls tilt of the rotor system with respect to the airflow, the inclusion of elastic pitch-lag coupling terms related to rotor precone, the inclusion of hub-related degrees of freedom which enable modeling of a gimballed rotor system and engine drive-train dynamics, and additional elastic coupling terms which enable modeling of the anisotropic features for both the rotor blades and the tiltrotor wing. Accuracy of the new tiltrotor analysis is demonstrated by a comparison of the results produced for a baseline casewith analytical and experimental results reported in the open literature. Two investigations of elastically tailored blades on a baseline tiltrotor are then conducted. One investigation shows that elastic bending-twist coupling of the rotor blade is a very effective means for increasing the flutter velocity of a tiltrotor, and the magnitude of coupling required does not have an adverse effect on performance or blade loads. The second investigation shows that passive blade twist control via elastic extension-twist coupling of the rotor blade has the capability of significantly improving tiltrotor aerodynamic performance. This concept, however, is shown to have, in general, a negative impact on stability characteristics.

Feasibility study to build a smart rotor: trailing edge flap actuation

Abstract: This paper presents an experimental study on the development of a Froude scale helicopter rotor blade with trailing edge flap as a vibration reduction device actuated by piezoelectric crystals. A fixed wing model with NACA 0012 airfoil, 3.0 inch blade chord and 20% trailing edge flap is fabricated and tested in the open-jet tunnel to determine the dynamic flap response at various blade angles of attack and excitation frequencies. The results show that for a given velocity, flap response does not change appreciably with the excitation frequency and the blade angle of attack.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Aeroelastic response, loads, and stability of a composite rotor in forward flight

Abstract: The aeroelastic response, blade and hub loads, and shaft-fixed aeroelastic stability are investigated for a helicopter with elastically tailored composite rotor blades. A finite element analysis including nonclassical effects such as transverse shear, torsion related warping, and in-plane elasticity is integrated with the University of Margland Advanced Rotorcraft Code. The analysis is correlated against both experimental data and detailed finite element results. Correlation of rotating natural frequencies of coupled composite box-beams is generally within 5-10%

Experimental and Theoretical Study for Nonlinear Aeroelastic Behavior of a Flexible Rotor Blade

Abstract: The purpose of this paper is to study the flutter instability and forced response of a nonrotating flexible rotor blade model with a geometrical structural nonlinearity and a freeplay structural nonlinearity. The ONERA stall aerodynamic model is used. External excitations are provided by base harmonic excitations in the pitch angle and the chordwise direction, respectively. Two cases are considered in this paper. Case A is for a nonlinear blade structure with an unstalled unsteady aerodynamic model. Case B is for the nonlinear blade structure with a large effective mean angle of attack. The effects of the structural and aerodynamic nonlinearities and initial disturbance on instability and forced response behavior are discussed. A wind-tunnel test has also been carried out in the Duke University low-speed wind tunnel. The wind-tunnel tests show generally good agreement between theory and experiment for both static and dynamic behavior. Although the mathematical and experimental model of the nonrotating blade is different from the operational rotating hingeless rotor blade, the results from the experimental-theoretical correlation study provide fundamental understanding of the nonlinear aeroelastic behavior for a flexural-flexural-torsional hingeless rotor blade.

Tip vortex geometry of a hovering helicopter rotor in ground effect

Abstract: The wide-field shadowgraph method has been used to photograph the tip vortices of a hovering helicopter rotor in ground effect. The shadowgraphs were used to obtain quantitative measurements of the rotor tip vortex geometry both in and out of ground effect. Many important phenomena are visible in the rotor wake using this method. These include the variation in descent and contraction rates of the tip vortices in ground effect, and the interaction between tip vortices in the far wake. The tip vortex geometry from the shadowgraphs is compared with the tip vortex geometry predicted using a free wake hover performance analysis. The free wake analysis accurately predicts the tip vortex geometry both in and out of ground effect. Performance data from the test is compared with the performance predicted using several methods, including the free wake analysis. All methods provided reasonable predictions for the helicopter performance in ground effect.

Unbalance response of a dual rotor system : theory and experiment

Abstract: A dual rotor rig is developed and is briefly discussed. The rig is capable of simulating dynamically the two spool aeroengine, though it does not physically resemble the actual aeroengine configuration. Critical speeds, mode shape, and unbalance response are determined experimentally. An extended transfer matrix procedure in complex variables is developed for obtaining unbalance response of dual rotor system. Experimental results obtained are compared with theoretical results and are found to be in reasonable agreement.

Method of and apparatus for detection of ice accretion

Abstract: Method and apparatus for detecting icing, particularly on airfoils such as helicopter rotor blades, by detecting warming caused by latent heat released as water freezes. The sensors may be multiple or single, contact or remote.

Feasibility study to build a smart rotor: induced-strain actuation of airfoil twisting using piezoceramic crystals

Abstract: The objective of this research is to develop a dynamically-scaled helicopter rotor blade with embedded piezoceramic elements as sensors and actuators to control blade vibrations. A 6 ft diameter 2-bladed Froude-scale bearingless rotor model is built where each blade is embedded with banks of specially-shaped piezoelectric crystals at +/- 45 degree angles on the top and bottom surfaces. A twist distribution along the blade span is achieved through in-phase excitation of the top and bottom crystals at equal potentials. The non-rotating static torsional response of the piezoceramic blade is experimentally determined and then correlated with the prediction by theory.

Vertical take off and landing and horizontal flight aircraft

Abstract: The present invention pertains to an aircraft that is capable of converting between vertical flight or helicopter mode flight, and horizontal flight or airplane mode flight where a two-bladed rotor is employed as both helicopter rotor blades in vertical flight and as a fixed wing in horizontal flight. In vertical flight, a bearing connection between two fuselage sections enables a forward section supporting the rotor blades to rotate relative to an aft section of the aircraft fuselage about the longitudinal axis of the aircraft. The exhaust or thrust force created by the mode of power (either a propeller engine or a turbine jet engine) is partially routed over the exterior of the aircraft to provide both vertical and horizontal thrust force, and in one embodiment a portion of the exhaust is routed through the interiors of the rotor blades and out exhaust ports at the blades' distal ends to rotate the blades in vertical flight and to provide a thrust force for the blades in horizontal flight. A control system controls the pitch position of the rotor blades to convert from vertical flight to horizontal flight by rotating the blades about their common lateral axis through opposite angles of substantially 90°.

Helicopter with leading edge servo flaps for pitch positioning its rotor blades

Abstract: In a helicopter having a main rotor with blades controlled in pitch by servo flaps the servo flaps are arranged in advance of the leading edges of the blade to provide increased lift efficiency of the rotor system. The signals for controlling the pitches of the blade are generated by a computer implemented means responsive to pilot command signals and flight parameter signals and are transmitted from the stationary structure of the helicopter to the rotating structure through a non-mechanical stationary-to-rotary interface. An active stabilization system is associated with each rotor blade to overcome the instability previously associated with leading edge servo flaps and to reduce the complexity of the control signals needed to be produced by the control signal generating means.

Model for rotor tip vortex-airframe interaction. II: Comparison with experiment

Abstract: The interaction between a vortex and a solid boundary involves the development of a variety of length and time scales, some of which are due to very strong viscous effects. Prediction of these phenomena from first principles would remove a major obstacle in the computation of flows around rotorcraft. Quantitative comparisons are made with experimental results for a rotor tip vortex approaching a cylindrical airframe with a hemispherical leading edge.

System for controlling higher harmonic vibrations in helicopter rotor blades

Abstract: Higher harmonic control (hereinafter HHC) of helicopter rotor blade vibrations is provided by an actively controlled, rotatable, slotted cylinder which is mounted at an outboard section of each blade. Continuous rotation of each cylinder about its longitudinal axis produces a periodic aerodynamic force on the blade at a frequency of twice the rotational frequency of the cylinder. The amplitude of force is controlled by the size of a slot opening in the cylinder while the rotational speed of the cylinder is synchronized to run at a multiple of the speed of a rotor blade drive shaft. The amplitude and phase of the HHC force is regulated, either manually or by active feedback control, to minimize any vibratory load transmitted to the airframe through the rotor blade drive shaft. A significant advantage offered by this concept relative to other HHC methods, such as high-frequency blade pitch motions actuated either by the swash plate or by moveable tabs at the blade trailing edge, is its low power requirement.

Model for rotor tip vortex-airframe interaction. I - Theory

Abstract: The flowfield generated by a helicopter in flight is extemely complex, and it has been recognized that interactions among components can significantly affect helicopter performance. In the present work a simplified model for the interaction of a rotor tip vortex with the helicopter fuselage is developed. The tip vortex is idealized as a single three-dimensional vortex tube, and the fuselage is moleded as an infinite circular cylinder

Helicopter rotor damper

Abstract: A hollow rotor hub is provided at the top of the rotor shaft for a multi-bladed helicopter. The blades being mounted to the rotor hub on spherical elastomeric bearings which provide for coincident rotor blade hinges for blade pitch, flap and lead-lag motions. The invention discloses a lead-lag damper contained in a housing at the center of the hollow rotor hub. The damper consists of a series of damping plates, each plate being connected to one of the rotor blades. The damping plates are separated from each other and from the damper housing by elastomeric layers bonded to the plates and to the housing. Lead-lag damping is provided by the shear in the elastomeric layers. The damper may be fastened to the air frame in a conventional manner, it may be mounted on a spindle for rotation about the rotor axis independent of rotor shaft rotation or may be free-floating and attached only to the rotor blades and not to the rotor shaft. With suitable selection of elastomeric damping material properties, it can provide blade to hub damping, blade to blade damping or combinations thereof.

A multidisciplinary optimization approach for vibration reduction in helicopter rotor blades

Abstract: The paper addresses the integration of blade dynamics, aerodynamics, structures and aeroelasticity in the design of helicopter rotors using a formal optimization technique. The interaction of the disciplines is studied inside a closed-loop optimization process. The goal is to reduce vibratory shear forces at the blade root with constraints imposed on dynamic, structural and aeroelastic design requirements. Both structural and aerodynamic design variables are used. Multiobjective formulation. procedures are needed since more than one design objective is used. A nonlinear programming technique and an approximate analysis procedure are used for optimization. Substantial reductions are obtained in the vibratory root forces and moments while satisfying the remaining design criteria. The results of the optimization procedure using two multiobjective formulation procedures, are compared with a baseline or reference design.

A pitch actuation system restraint device for a helicopter blade folding system

Abstract: A pitch actuation system restraint (PASR) device (10) that provides protection for the pitch actuation system of a helicopter having a main rotor assembly configured for main rotor blade folding operations. The PASR device (10) includes permanent adapter brackets (12) that are permanently mounted in combination with the rotor hub arms (108) and temporary adapter bracket (14) and quick release pins (16) for each main rotor blade to be folded. Prior to implementing blade folding operations, a temporary adapter bracket (14) is secured in combination with the pitch control horn (126) of each main rotor blade and the permanent adapter bracket (12) of the adjacent rotor hub arm (108) by means of the quick release pins (16). Each temporary adapter bracket (14) and permanent adapter bracket (12) functions as a rigid structural interconnection that effectively locks the pitch control horn (126) in position during blade folding operations such that displacements induced in the main rotor blade during blade folding operations cannot be coupled into the pitch actuation system.

Euler Calculations of Unsteady Interaction of Advancing Rotor with a Line Vortex

Abstract: The unsteady, three-dimensional flowfield of a helicopter rotor blade in forward flight encountering a concentrated line vortex is calculated using an implicit, finite difference numerical procedure for the solution of Euler equations. A prescribed vortex method is adopted to preserve the structure of the interacting vortex. The test cases considered for computation correspond to the two-bladed model rotor experimental conditions of Caradonna et al. and consist of parallel and oblique interactions

The HELINOISE Aeroacoustic Rotor Test in the DNW

Abstract: In a major cooperative research program between 8 European partners, a geometrically and dynamically scaled and highly instrumented model of the ECD (formerly MBB) BO-105 helicopter main rotor was tested in the open-jet anechoic test section of the German-Dutch Wind Tunnel, DNW, in the Netherlands. A comprehensive set of simultaneous aerodynamic and acoustic pressure data as well as dynamic and performance data were measured for the standard rotor (NACA 23012 mod) with rectangular blade tips. The primary objective of this experimental study was to generate an extensive airload and acoustic data base for code validation, to examine the relation between the blade pressure characteristics and the acoustic radiation, and as a further objective to obtain initial detailed information on blade-tip vortex trajectories and blade positions during blade-vortex interactions. This report describes the instrumented model rotor, the Modular Wind-tunnel Model (MWM) test stand, the aerodynamic and acoustic data acquisition systems, and the scope of the test matrix and of test conduction. Moreover, the measurement techniques used to visualize the interacting vortex flow and to determine blade deflection and incidence at the blade tip are briefly outlined. Selected test results are presented in this report which is accompanied by a number of appendices and enclosures wherein the experimental results are documented in full. The data is expected to improve the understanding of rotor aeroacoustics and to further the validation of various aerodynamic and acoustic codes developed or improved by the partners of this joint European venture.

Vibration Response Reduction of a Rotor Shaft System Using Viscoelastic Polymeric Supports

Abstract: The main objective of the present work is to determine reduction in the unbalance response of a rotor shaft system by using a suitable polymeric or viscoelastic bearing support. For analysis, a simple rotor system has been taken with the rotor placed in the middle of a massless shaft with linear elastic bearings at the ends, having viscoelastic supports. A procedure is given for determining the frequency dependence of viscoelastic support characteristics so that the frequency of excitation never coincides with any of the undamped natural frequencies of the system, thus giving low unbalance response over a wide frequency range and the support material can be chosen accordingly.

Dynamic Stall Control by Airfoil Deformation

Abstract: With combined numerical and experimental investigations of the dynamic stall process on refreating helicopter rotor blades new insight into the complex unsteady flows involved have been achieved recently. With these new experiences in mind it is of increasing interest to suitable influence the flow. i.e. by dynamic airfoil deformation. The present paper describes first steps towards an improvement of the dynamic stall effects with respect to the time-dependent flows and overall forces by both numerical and experimental tools.

Blade retention pin system for a helicopter blade folding system

Abstract: A blade retention pin system is operative to secure the blade cuff of a main rotor blade of a helicopter in combination with a spindle assembly of the main rotor hub assembly for main rotor assembly operations and is readily reconfigurable to accommodate blade folding operations. The blade retention pin system includes first and second pin assemblies, each pin assembly including a retainer pin and a handle subassembly, which includes a retainer member and a spring member in superposed abutting combination, pivotably mounted in combination therewith. The first pin assembly includes, in addition, a safety pin subassembly secured in combination therewith. Each retainer pin is inserted into aligned apertures of the blade cuff and spindle assembly and locked in place by pivoting the handle subassembly from an unlocked to a locked position wherein the retainer member engages the retainer pin and the spring member is biased onto the retainer pin to abuttingly engage the retainer member.

Investigation of rotor tip vortex interactions with a body

Abstract: Experiments were conducted to examine rotor tip vortex interactions with a body in low-speed forward flight. Unsteady pressure measurements were made at points along the top and around the circumference of the body surface. Flow visualization of the rotor wake was performed using the wide-field shadowgraph method. Considerable insight into the tip vortex interaction processes was obtained by correlating the pressure loads with the vortex trajectories as they approached, distorted, and impinged on the body surface. Unsteady potential flow theory was explored as a means of predicting the unsteady pressure loads on the body surface, using prescribed tip vortex trajectories measured from flow visualization. The results have shown that the process of tip vortex interaction with a body can be divided into three regimes: 1) close tip vortex/body interactions, which is an inviscid flow regime; 2) vortex/surface impingement; and 3) postvortex/surface impingement; the latter involves viscous effects. The results have also shown that the pressures at points on the body exhibited a high sensitivity to tip vortex convection speed and location, which makes the general prediction of such interactional phenomena difficult with existing rotor/airframe interaction models.

LQR-Based Least-Squares Output Feedback Control of Rotor Vibrations Using the Complex Mode and Balanced Realization Methods

Abstract: The complex mode and balanced realization methods are used separately to obtain reduced-order models for general linear asymmetric rotor systems. The methods are outlined and then applied to a typical rotor system represented by a 52 degree-offreedom finite element model. The accuracy of the two methods is compared for this model and the complex model method is found to be more accurate than the balanced realization method for the desired frequency bandwidth and for models of the same reduced order. However, with some limitations, it is also shown that the balanced realization method can be applied to the reduced-order complex mode model to obtain further order reduction without loss of model accuracy. A “Linear-Quadratic-Regulator-based least-squares output feedback control” procedure is developed for the vibration control of rotor systems. This output feedback procedure eliminates the requirement of an observer for the use of an LQ regulator, and provides the advantage that the rotor vibration can be effectively controlled by monitoring only one single location along the rotor shaft while maintaining an acceptable performance. The procedures presented are quite general and may be applied to a large class of vibration problems including rotordynamics.

Unsteady panel method for flows with multiple bodies moving along various paths

Abstract: A potential flow based three-dimensional panel method was modified to treat time-dependent conditions in which several submerged bodies can move within the fluid along different trajectories. This modification was accomplished by formulating the momentary solution in an inertial frame of reference, attached to the undisturbed stationary fluid. Consequently, the numerical interpretation of the multiple-body, solid-surface boundary condition and the viscous wake rollup was considerably simplified. The usteady capability of this code was calibrated and validated by comparing computed results with closed-form analytical results available for an airfoil, which was impulsively set into a constant speed forward motion. To demonstrate the multicomponent capability, computations were made for two wings following closely intersecting paths (i.e., simulations aimed at avoiding mid-air collisions) and for a flowfield with relative rotation (i.e., the case of a helicopter rotor rotating relative to the fuselage). Computed results for the cases were compared to experimental data, when such data was available.