Showing papers on "Helicopter rotor published in 1995"

Remote self-powered structure monitor

Abstract: An acoustic rotor monitor that is an autonomous self-powered measurement instrument which can detect embedded and hidden fatigue cracks in remotely inaccessible devices such as helicopter rotor system components. A predictive maintenance-related problem for rotor craft is the detection of fatigue cracks as a continuous real-time monitoring process under dynamic rotor system loading conditions. The rotor monitor focuses on the embedding an acoustic emission-based smart sensor directly into the rotor system to measure the high frequency stress waves indicating that a structural crack has propagated as a "self-powered" measurement without reducing structural integrity.

Rotor free-wake modeling using a pseudoimplicit relaxation algorithm

Abstract: A pseudoimplicit predictor-cor rector relaxation algorithm with five-point central differencing in space has been developed for the solution of the governing differential equations of the helicopter rotor free-wake problem. This new approach is compared and contrasted with more conventional explicit-type free-wake algorithms. A convergence analysis shows that the new algorithm provides for much more rapid convergence characteristics compared to explicit methods, with improvements in numerical efficiency and predictive accuracy. Nomenclature be = spatial boundary condition vector CT = rotor thrust coefficient, Tlp7rR2([lR)2 c = blade chord, m E — shift operator i,/, k = unit vectors in the x, y, and z directions, respectively L = spatial discretization operator / = length of discretized vortex element, m Nh = number of blades N.t. = number of vortex filaments P = iteration scheme operator R = rotor radius, m r(. = vortex core radius, m r, = spanwise location from which vortex filaments are trailed, m r = position vector of a point on a vortex filament, m S = source vector T = rotor thrust, N / = time, s V.,_ — freestream velocity, m/s V = time invariant flowfield velocity, m/s Vind = induced velocity, m/s Vloc = local velocity at a point in space, m/s VH = tangential velocity, m/s Cartesian coordinate system, origin at hub center rotor shaft angle (negative forward), deg )3() = blade coning angle, deg r = circulation, m2/s Ar" = nth iteration position-vector correction f = distance along trailed wake filament (wake age), rad A.- = uniform-induced inflow ratio

Dynamic Stall Control for Advanced Rotorcraft Application

Abstract: Advanced concepts designed to improve the lift, drag, and pitching moment characteristics of rotor blades have been investigated for the purpose of enhancing rotor maneuver capability. The advantages and disadvantages of these concepts have been evaluated using both computational and experimental means. The concepts that were considered in this study included a leading-edge slat, a deformable leading-edge, and upper-surface blowing. The results show the potential of these concepts for substantially improving the performance of a rotor. HE next generation of rotorcraft will be required to operate at much higher performance levels than in the past, particularly in the areas of nap-of-the-ea rth (NOE), deep-penetration operations, and air-to-air combat. These new requirements will require highly maneuverable, agile, and survivable rotorcraft, far exceeding the capabilities of those in the current inventory. The objectives of this project include an increase in the maneuverability/agility capability of the helicopter and a reduction in the acoustic detection range. The single most important element of the rotorcraft for meeting these requirements is the rotor itself, since it is the primary source of lift, control, and speed. At the same time, the rotor is also a major source of acoustically detectable radiation. Among the many factors affecting rotorcraft performance, the aerodynamic characteristics of the rotor system are the most important and are the main subject of this paper. The maneuvering capability of a rotorcraft can be improved by re- ducing or suppressing the vibratory loads on the rotor blades caused by aerodynamic separation and stall. This would have the effect of expanding the stall-limiting boundary of the rotor and thereby increase the available load factor in all flight regimes. The con- ventional way to obtain higher lift is to increase the blade area, however, this usually results in a heavier rotor that is also less ef- ficient. With regard to compressibili ty effects and acoustic radia- tion, improvements have been obtained by sweeping, tapering, and thinning the tip region of the rotor blade. As a result, numerous families of airfoils and planform shapes have evolved that offer bet- ter advancing-blade characteristics. However, improvements on the retreating-blade side have not been as impressive. One reason for this imbalance may be that design codes are available for treating blades at low angles of attack and high Mach number (characteris- tic of the advancing side), whereas the design strategy has had to depend heavily on costly empirical studies for blades at high angles of attack and having some amount of separation (characteristic of the retreating side). Increasing the tip speed of the rotor to achieve a maneuvering ad- vantage may produce a dangerous condition with regard to acoustic detection. Rapid advancements in passive acoustic sensor arrays and advanced signal processing technologies pose a serious threat to the mission effectiveness of Army helicopters. Since the rotor blade generates acoustic radiations that can be easily detected and identified, airfoil and planform shapes must be carefully optimized to reduce the detection range of the rotorcraft. The requirements for improved maneuverability and reduced sus- ceptibility will clearly demand a substantial growth in the technolo- gies for addressing rotor aerodynamics. New control techniques must be considered, both passive and active, and these must be ac- companied by a more thorough physical understanding of these flow phenomena along with substantially improved prediction capabili- ties. To meet these requirements, computational and experimental efforts have been initiated to evaluate the effectiveness of various concepts. At present these concepts include airfoils with slats and slots, airfoils that deform, and airfoils with flow energizers. Description of Experiment and Computational Fluid Dynamics Code

Aeroelasticity and structural optimization of composite helicopter rotor blades with swept tips

Abstract: This report describes the development of an aeroelastic analysis capability for composite helicopter rotor blades with straight and swept tips, and its application to the simulation of helicopter vibration reduction through structural optimization. A new aeroelastic model is developed in this study which is suitable for composite rotor blades with swept tips in hover and in forward flight. The hingeless blade is modeled by beam type finite elements. A single finite element is used to model the swept tip. Arbitrary cross-sectional shape, generally anisotropic material behavior, transverse shears and out-of-plane warping are included in the blade model. The nonlinear equations of motion, derived using Hamilton's principle, are based on a moderate deflection theory. Composite blade cross-sectbnal properties are calculated by a separate linear, two-dimensional cross section analysis. The aerodynamic loads are obtained from quasi-steady, incompressible aerodynamics, based on an implicit formulation. The trim and steady state blade aeroelastic response are solved in a fully coupled manner. In forward flight, where the blade equations of motion are periodic, the coupled trim-aeroelastic response solution is obtained from the harmonic balance method. Subsequently, the periodic system is linearized about the steady state response, and its stability is determined from Floquet theory.

Main rotor system for helicopters

Abstract: The invention pertains to the field of thrust rotors (1) for both model and full size helicopter (15). More particularly, the present invention relates to high lift rotors for all types of helicopters, and to simple and inexpensive rotors for use in model helicopter applications.

Aerodynamics and Acoustics of Rotor Blade-Vortex Interactions

Abstract: Helicopter rotor simulation codes of different organizations are validated against experimental data obtained in the DNW. The comparison addresses rotor blade loading, wake geometry, blade motion and noise radiation. Although specific differences exist the general prediciton of rotor noise is reasonably well.

A Study of the Response of a Discontinuously Nonlinear Rotor System

Abstract: In this paper the numerical and experimental response of a two degree of freedom, discontinuously nonlinear rotor system, which is subject to excitation by out-of-balance is considered. The nonlinearity in the form of a discontinuous stiffness is effected by a radial clearance between the elastically supported rotor and an elastically supported outer ring. The rotor is placed eccentrically within this ring so that it is just touching one side of the inner bearing housing.

Multilevel decomposition procedure for efficient design optimization of helicopter rotor blades

Abstract: This paper addresses a multilevel decomposition procedure, for efficient design optimization of helicopter blades, with the coupling of aerodynamics, blade dynamics, aeroelasticity, and structures. The multidisciplinary optimization problem is decomposed into three levels. The rotor is optimized for improved aerodynamic performance at the first level. At the second level, the objective is to improve the dynamic and aeroelastic characteristics of the rotor. A structural optimization is performed at the third level. Interdisciplinary coupling is established through the use of optimal sensitivity derivatives. The Kreisselmeier-Steinhauser function approach is used to formulate the optimization problem when multiple design objectives are involved. A nonlinear programming technique and an approximate analysis procedure are used for optimization. Results obtained show significant improvements in the rotor aerodynamic, dynamic, and structural characteristics, when compared with a reference or baseline rotor.

Engineering Feasibility of Induced Strain Actuators for Rotor Blade Active Vibration Control

Abstract: Rotor blade vibration reduction based on Higher Harmonic Control—Individual Blade Control (HHC-IBC) principles is presented as a possible area of application of Induced Strain Actuation (ISA). Recent theoretical and experimental work on achieving HHC-IBC through conventional and ISA means is reviewed. Though the force-displacement and power-energy estimates vary significantly, some common-base values are identified. Hence, a bench-mark specification for a tentative HHC-IBC device based on the aerodynamic servo-flap principle operated through ISA means is developed. Values for the invariant quantities of energy, power and force-displacement product are identified, along with actual displacement and force values of practical interest. The implementation feasibility of this specification into an actual ISA device is then discussed. It is shown that direct actuation is not feasible due to the large required length of the ISA device, resulting in excessive compressibility effects (displacement loss and parasit...

Summary of the interaction of a rotor wake with a circular cylinder

Abstract: To compute the aerodynamics of a rotorcraft in low-speed flight, the interaction of the strong vortices in the rotor wake with the airframe must be modeled. Using a hemisphere-cylinder airframe and a two-bladed rotor for reference, the various phenomena encountered during such interactions are summarized, combining previous results on various configurations with recent experimental results. Differences between the interaction at the front and aft portions of the wake are discussed. The precollision phase conforms to expectations from potential flow, and includes distortion of the vortex trajectory determined by the sense of rotation of the vortex. The collision phase involves complex boundary-layer interactions. The axial velocity in the vortex core causes substantial asymmetry and influences the surface pressure distribution on the airframe side under the advancing rotor blade, where the axial flow stagnates. The postinteraction vortex is much weaker, but still contains some swirl energy. Where flow separation occurs due to airframe shapes, the interaction is not modified significantly, because the vortex dominates the interaction with separated shear layers for parameter values of practical interest. Areas of remaining uncertainty are discussed.

Laser vibrometry measurements of rotating blade vibrations

Abstract: One of the most important design factors in modern turbomachinery is the vibration of turbomachinery blading. There is a need for developing an in-service, noncontacting, noninterfering method for the measurement and monitoring of gas turbine, jet engine, and steam turbine blade vibrations and stresses. Such a technique would also be useful for monitoring rotating helicopter blades. In the power generation industry, blade failures can result in millions of dollars of downtime. The measurement of blade vibrations and dynamic stresses is an important guide for preventive maintenance, which can be a major contributor to the availability of steam turbine, gas turbine, and helicopter operations. An experiment is designed to verify the feasibility of such a vibration monitoring system using the reference beam on-axis laser-Doppler technique. The experimental setup consists of two flat, cantilever blades mounted on a hub attached to the shaft of a dc motor. The motor rests on a linear bearing permitting motion only in the direction of the motor shaft. The motor and blade assembly is then excited via an electrodynamic shaker at the first natural frequency of the blades. The resulting blade vibration is then detected using a laser vibrometer. The vibration frequencies and amplitudes of the two rotating blades are successfully measured.

Retraction/extension mechanism for variable diameter rotors

Abstract: A retraction/extension mechanism (70) for a Variable Diameter Rotor system (10), the Variable Diameter Rotor system (10) having a plurality of rotor blade assemblies (16) mounted to and rotating with a rotor hub assembly (18) about an axis of rotation (20), each rotor blade assembly (16) having inboard and outboard blade sections (24, 22) wherein the outboard blade section (22) telescopically mounts to the inboard blade section (24). The retraction/extension mechanism (70) includes a reeling assembly (80) disposed in combination with the outboard blade section (22) of each rotor blade assembly (16) and an epicyclic gear train (100) disposed in combination 7ith the rotor hub assembly (18) and the reeling assembly (80). The epicylic gea2 train is, furthermore, operative for driving the reeling assembly (80) so as to effect telescopic transition of the outboard blade sections (22) with respect to the inboard blade sections (24).

Helicopter rotor tip shapes for reduced blade-vortex interaction - An experimental investigration. II

Abstract: Mach numbers of 0.75 and 0.78 with respective chord Reynolds numbers of 3.2 and 5.6 million. The latter Mach number-Reynolds number pair provide a full-scale operational match for the candidate rotor tip planforms. Total pressure surveys of the wake were conducted at three chordwise stations aft of each model at an equivalent rotor pitch angle of 8 deg. These tests revealed the initial roll-up region and provided a comparison of the effects of Mach number, Reynolds number, and tip shape on the vortex size, shape, location, and minimum stagnation pressure. The swept and swept-tapered planforms produced larger, more diffuse, tip vortices than the rectangular or tapered planforms. (Author)

A Solution Adaptive Structured/Unstructured Overset Grid Flow Solver with Applications to Helicopter Rotor Flows

Abstract: This paper summarizes a method that solves both the three dimensional thin-layer Navier-Stokes equations and the Euler equations using overset structured and solution adaptive unstructured grids with applications to helicopter rotor flowfields. The overset structured grids use an implicit finite-difference method to solve the thin-layer Navier-Stokes/Euler equations while the unstructured grid uses an explicit finite-volume method to solve the Euler equations. Solutions on a helicopter rotor in hover show the ability to accurately convect the rotor wake. However, isotropic subdivision of the tetrahedral mesh rapidly increases the overall problem size.

Computation of Helicopter Rotor Acoustics in Forward Flight

Abstract: This paper presents a new method for computing acoustic signals from helicopter rotors in forward flight. The aerodynamic and acoustic solutions in the near field are computed with a finite-difference solver for the Euler equations. A nonrotating cylindrical Kirchhoff surface is then placed around the entire rotor system. This Kirchhoff surface moves subsonically with the rotor in forward flight. The finite-difference solution is interpolated onto this cylindrical surface at each time step and a Kirchhoff integration is used to carry the acoustic signal to the far field. Computed values for high-speed impulsive noise show excellent agreement with model-rotor and flight-test experimental data. Results from the new method offer high accuracy with reasonable computer resource requirements.

Modeling and Control of HSFDs for Active Control of Rotor-Bearing Systems

Abstract: This paper summarizes the development of hybrid squeeze film dampers (HSFDs) for active control of rotor vibrations. Previously, it was shown both theoretically and experimentally that HSFDs can be used for controlling rotor vibrations (El-Shafei, 1993). This is done by controlling the flow in a squeeze film damper through movable end seals, thus achieving the ability to change the damper from a short damper to a long damper and vice versa. However, the control of the HSFD was manual. In this paper, an automatically controlled circuit is developed for the HSFD, incorporating a pressure control servovalve for controlling the pressure in the sealing chambers. A complete mathematical model of this open-loop system is developed and is implemented on a digital computer. The transient behavior of the system, including the sealing ring dynamics, illustrates that the open-loop system exhibits well-behaved, stable, and fast response. In addition it is shown that the HSFD can achieve any amount of damping between the short and long damper modes through the accurate positioning of the sealing rings. The simulation results illustrate that the automatically controlled HSFD can be a very useful device for the active control of rotors. A closed-loop control strategy with feedback on rotor speed is also investigated both from the points of view of steady state and transient behaviors. It is shown that this closed-loop strategy results in a much improved behavior of the rotor system.

Simulation of Rotor Blade Element Turbulence

Abstract: A piloted, motion-based simulation of Sikorsky's Black Hawk helicopter was used as a platform for the investigation of rotorcraft responses to vertical turbulence. By using an innovative temporal and geometrical distribution algorithm that preserved the statistical characteristics of the turbulence over the rotor disc, stochastic velocity components were applied at each of twenty blade-element stations. This model was implemented on NASA Ames' Vertical Motion Simulator (VMS), and ten test pilots were used to establish that the model created realistic cues. The objectives of this research included the establishment of a simulation-technology basis for future investigation into real-time turbulence modeling. This goal was achieved; our extensive additions to the rotor model added less than a 10 percent computational overhead. Using a VAX 9000 computer the entire simulation required a cycle time of less than 12 msec. Pilot opinion during this simulation was generally quite favorable. For low speed flight the consensus was that SORBET (acronym for title) was better than the conventional body-fixed model, which was used for comparison purposes, and was determined to be too violent (like a washboard). For high speed flight the pilots could not identify differences between these models. These opinions were something of a surprise because only the vertical turbulence component on the rotor system was implemented in SORBET. Because of the finite-element distribution of the inputs, induced outputs were observed in all translational and rotational axes. Extensive post-simulation spectral analyses of the SORBET model suggest that proper rotorcraft turbulence modeling requires that vertical atmospheric disturbances not be superimposed at the vehicle center of gravity but, rather, be input into the rotor system, where the rotor-to-body transfer function severely attenuates high frequency rotorcraft responses.

The verification of a theoretical helicopter rotor blade sailing method by means of windtunnel testing

Abstract: The performance of a helicopter rotor has always been dogged by the dissymmetry of air velocity over the blades caused by its translation in a direction substantially parallel to the plane of the rotor disc. Furthermore, in order to make the rotor even usable, this dissymmetry of lift has caused the introduction of major mechanical complications to the rotor hub. In forward flight the rotor is aerodynamically split into two halves either side of the flight direction. The advancing side, where the rotor rotation is in the same sense as the forward velocity, will see a greater dynamic head than the retreating side, where the rotor rotation and forward speed are in opposition. A rigid rotor would consequently suffer a major rolling moment and if a viable proposition is to be achieved the rolling moment must be avoided.

Continued development of a helicopter rotor model employing smart trailing-edge flaps for vibration suppression

Abstract: The continued development of a Froude-Scale helicopter rotor model featuring a trailing-edge flap driven by piezoceramic bimorph actuators for active vibration suppression is discussed. Block force and stroke of the current actuators are evaluated using two theories and compared with experimental results. Dynamic performance of the actuator as well as the actuator-flap assembly are examined. Earlier hover tests have shown severe degradation in flap deflections with increasing rotor speed, and flap deflections were too small to be effectively utilized for significant vibration control. To investigate the causes of the performance degradation, new blades are constructed and tested in-vacuo to isolate the effects of centrifugal loading on the actuator-flap system. A beam model of the piezo bimorph including propeller moment effects is formulated to better illustrate the physical mechanisms affecting the system in a rotating environment. The cause of the reduced deflections is traced to frictional forces created at the junction where the flap is supported during rotation of the blades. The use of a thrust bearing was found to alleviate this problem and subsequent tests on a hover stand showed a dramatic increase in flap detection at high excitation frequencies.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Experimental results of the European HELINOISE aeroacoustic rotor test

Abstract: In a cooperative research program between eight European partners, a 40% 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 The primary objectives of this experimental study were 1) to improve the physical understanding of the impulsive rotor noise sources by correlating blade pressure and acoustic characteristics and 2) to provide an extensive airload and acoustic database for code validation purposesConsequently, a comprehensive set of simultaneous acoustic and aerodynamic blade surface pressure data as well as blade dynamic and performance data were measured for the standard rotor with rectangular blade tips In addition, initial quantitative information of the blade- vortecx miss distance during blade-vortex interaction (BVI) was obtained

Effect of higher harmonic control on helicopter rotor blade-vortex interaction noise: Prediction and initial validation

Abstract: The paper presents a status of theoretical tools of AFDD, DLR, NASA and ONERA for prediction of the effect of HHC on helicopter main rotor BVI noise. Aeroacoustic predictions from the four research centers, concerning a wind tunnel simulation of a typical descent flight case without and with HHC are presented and compared. The results include blade deformation, geometry of interacting vortices, sectional loads and noise. Acoustic predictions are compared to experimental data. An analysis of the results provides a first insight of the mechanisms by which HHC may affect BVI noise.

Aeroelastic analysis of multibladed hingeless rotors in hover

Abstract: The coupled flap-lag-torsion aeroelastic response and stability of multibladed hingeless rotors in the hovering flight condition are investigated. The vortex lattice method, with a three-dimensional prescribed wake geometry, is used for the prediction of unsteady airloads of multibladed rotors undergoing disturbed dynamic motions. Interblade unsteady wake effects due to vortex-phasing phenomena beneath a rotor are numerically calculated by the phase control of wake vortices shed from each blade. The aeroelastic equations of motion of the rotor blade are formulated using a finite element beam model that has no artificial restrictions on the magnitudes of displacements and rotations due to the degree of nonlinearity. Numerical results of the steady equilibrium deflections and the lead-lag damping and frequency are presented for two-, three-, and four-bladed stiff-inplane rotors, and are compared with those obtained from a two-dimensional quasisteady strip theory with steady and uniform inflow.

Helicopter Track and Balance With Artificial Neural Nets

Abstract: Before a helicopter leaves the plant, it needs to be tuned so that its vibrations meet the required specifications. Helicopter track and balance is currently performed based on sensitivity coefficients which have been developed statistically after years of production experience. The fundamental problem with using these sensitivity coefficients, however, is that they do not account for the nonlinear coupling between modifications or their effect on high amplitude vibrations. In order to ensure the reliability of these sensitivity coefficients, only a limited number of modifications are simultaneously applied. As such, a number of flights are performed before the aircraft is tuned, resulting in increased production and maintenance cost. In this paper, the application of feedforward neural nets coupled with back-propagation training is demonstrated to learn the nonlinear effect ofmodifications, so that the appropriate set of modifications can be selected in fewer iterations (flights). The effectiveness of this system of neural nets for track and balance is currently being investigated at the Sikorsky production line.

Helicopter rotor blade of glass fibre reinforced synthetic resin

Abstract: The rotor blade (2) has a profile at the tip of the blade which is rotated about the rotor blade longitudinal axis via actuators. The torsion skin (6) of the rotor blade is anisotropic at the blade tip, for providing a tension-rotation coupling and associated with a control actuator (8), extending in the longitudinal direction of the rotor blade. Pref. the control actuator provides a controlled normal force at the end face of the rotor blade tip and is pref. provided by a piezoactuator, extended in length for rotation of the blade tip in one direction and reduced in length for its rotation in the opposite direction.