Showing papers on "Helicopter rotor published in 1986"

Prediction of helicopter rotor discrete frequency noise: A computer program incorporating realistic blade motions and advanced acoustic formulation

Abstract: A computer program has been developed at the Langley Research Center to predict the discrete frequency noise of conventional and advanced helicopter rotors. The program, called WOPWOP, uses the most advanced subsonic formulation of Farassat that is less sensitive to errors and is valid for nearly all helicopter rotor geometries and flight conditions. A brief derivation of the acoustic formulation is presented along with a discussion of the numerical implementation of the formulation. The computer program uses realistic helicopter blade motion and aerodynamic loadings, input by the user, for noise calculation in the time domain. A detailed definition of all the input variables, default values, and output data is included. A comparison with experimental data shows good agreement between prediction and experiment; however, accurate aerodynamic loading is needed.

Helicopter rotor dynamics by finite element time approximation

Abstract: Starting from a variational formulation, based on Hamilton's principle, the paper exploits the finite element technique in the time domain in order to generate, by an automated procedure, a numerical approximation of the nonlinear periodic equations peculiar to the dynamics of helicopter rotors. With this method, a unified approach is developed, which can be used to solve both the response and stability problem. Two simple examples are presented that show how the method can be used to study the trimmed response of a fully articulated rigid blade and the mechanical instability analysis of a four-bladed rotor.

The Dynamics of a Rotor System with a Cracked Shaft

Abstract: A theoretical analysis of the dynamics of a rotor-bearing system with a transversely cracked rotor is presented. The rotating assembly is modeled using finite rotating shaft elements and the presence of a crack is taken into account by a rotating stiffness variation. This stiffness variation is a function of the rotor’s bending curvature at the crack location and is represented by a Fourier series expansion. The resulting parametrically excited system is nonlinear and is analyzed using a perturbation method coupled with an iteration procedure. The system equations are written in terms of complex variables and an associated computer code has been developed for simulation studies. Results obtained by this analysis procedure are compared with previous analytical and experimental work presented by Grabowski.

Helicopter individual-blade-control research at MIT 1977-1985

Abstract: A new advanced system for active control of helicopters and its application to the solution of rotor aerodynamic and aeroelastic problems is described. Each blade is individually controlled in the rotating frame over a wide range of frequencies. Application of the system to gust alleviation, attitude stabilization, vibration alleviation, blade lag damping augmentation, stall flutter suppression, blade flapping stabilization, stall alleviation, and performance enhancement is outlined. The effectiveness of the system in achieving most of these applications is demonstrated by experimental results from wind tunnel tests of a model helicopter rotor with individual blade control. The feasibility of achieving many or all of the applications of individual blade control using the conventional helicopter swash plate is demonstrated, and the necessary control laws are presented.

Euler calculations for flowfield of a helicopter rotor in hover

Abstract: Aerodynamic loads on a multibladed helicopter rotor in hovering flight are calculated by solving the threedimensional Euler equations in a rotating coordinate system on body-conforming curvilinear grids around the blades Euler equations are recast in the absolute flow variables so that the absolute flow in the far field is uniform but the relative flow is nonuniform Equations are solved for the absolute flow variables employing Jameson's finite-volume explicit Runge-Kutta time-stepping scheme Rotor-wake effects are modeled in the form of a correction applied to the geometric angle of attack along the blades This correction is obtained by computing the local induced downwash with a free-wake analysis program The calculations are performed on a CRAY X/MP-48 for a model helicopter rotor in hover at various collective pitch angles The results compared with experimental data

Advanced composite rotor blade

Abstract: An advanced composite blade construction for a helicopter rotor blade using for the most part fiber reinforced plastic materials in which the major assembly of blade elements is a single step co-cure operation and in which an integral spar/skin carries centrifugal, flapwise and chordwise loads and also incorporates redundant means to carry torsional loads and a honeycomb core carries shear and aerodynamic pressures loads.

Design of helicopter rotor blades for optimum dynamic characteristics

Abstract: The optimal design of helicopter rotor blades is addressed. The forced response of an initial (i.e., non-optimized) blade to those of a final (optimized) blade are compared. Response of starting design and optimal designs for varying forcing frequencies, blade response to harmonics of rotor speed, and derivation of mass and stiffness matrices or functions of natural frequencies are discussed.

Numerical methods for determining the stability and response of periodic systems with applications to helicopter rotor dynamics and aeroelasticity

Abstract: This paper reviews and extends some recently developed numerical techniques for analyzing the stability, linear response and nonlinear response of periodic systems, which are governed by ordinary differential equations with periodic coefficients. The main emphasis is on applications aimed at rotor blade aeroelastic stability and response calculations in forward flight. First Floquer theory is br4iefly reviewed and numerically efficient methods for evaluating the transition matrix at the end of one period are described. Next the numerical treatment of the linear response problem is discussed. Finally the numerical solution of the nonlinear response problem is treated using quasilinearization and periodic shooting. Applications illustrating numerical properties of the methods are described.

An experimental investigation of the flap-lag-torsion aeroelastic stability of a small-scale hingeless helicopter rotor in hover

Abstract: A small scale, 1.92 m diam, torsionally soft, hingeless helicopter rotor was investigated in hover to determine isolated rotor stability characteristics. The two-bladed, untwisted rotor was tested on a rigid test stand at tip speeds up to 101 m/sec. The rotor mode of interest is the lightly damped lead-lag mode. The dimensionless lead-lag frequency of the mode is approximately 1.5 at the highest tip speed. The hub was designed to allow variation in precone, blade droop, pitch control stiffness, and blade pitch angle. Measurements of modal frequency and damping were obtained for several combinations of these hub parameters at several values of rotor speed. Steady blade bending moments were also measured. The lead-lag damping measurements were found to agree well with theoretical predictions for low values of blade pitch angle. The test data confirmed the predicted effects of precone, droop, and pitch control stiffness parameters on lead-lag damping. The correlation between theory and experiment was found to be poor for the mid-to-high range of pitch angles where the theory substantially overpredicted the experimental lead-lag damping. The poor correlation in the mid-to-high blade pitch angle range is attributed to low Reynolds number nonlinear aerodynamics effects not included in the theory. The experimental results also revealed an asymmetry in lead-lag damping between positive and negative thrust conditions.

Nonlinear flexure and torsion of rotating beams, with application to helicopter rotor blades. I - Formulation. II - Response and stability results

Abstract: The dynamic response and aeroelastic stability of rotating beams such as helicopter blades is investigated analytically. The Hamilton principle is used to formulate the equations of motion for extensional and inextensional beams with precone angles and variable pitch angles, taking higher-order nonlinearities into account. The derivation of the equations and their approximate solution by a Galerkin procedure are explained in detail, and numerical results of equilibrium solutions and stability analyses are presented graphically.

Nonlinear flexure and torsion of rotating beams, with application to helicopter rotor blades- i. formulation

Abstract: The differential equations of motion and their boundary conditions for a rotating beam such as a helicopter rotor blade are formulated via Hamilton's principle. The equations are va lid for both extensional and inextensional beams that have a pre-cone angle and a variable pitch angle. The equations are developed with the objective of retaining contributions due to higher order non-linearities which are generally disregarded in the literature due to their complexity. The influence of these higher order non­ linearities on the motion of a helicopter rotor blade is investigated by the authors in Part II of this paper that also appears in this issue of Vertica.

An experimental investigation of blade-vortex interaction at normal incidence

Abstract: An experimental investigation has been conducted for the blade-vortex interaction (BVI) of a rotor at normal incidence, where the vortex is generally parallel to the rotor axis. Tip Mach number, radial BVI station, and free stream velocity were varied during measurements of fluctuating blade pressures, far field sound pressure levels and directivity, incident vortex velocity field, and blade-vortex interaction angles. The experimental setup is representative of the chopping of helicopter main rotor tip vortices by the tail rotor. This interaction is found to generate impulsive noise which radiates primarily ahead of the blade.

Multi-Variable Control of the GE T700 Engine using the LQG/LTR Design Methodology

Abstract: In this paper we examine the design of scalar and multi-variable feedback control systems for the GE T700 turboshaft engine coupled to a helicopter rotor system. A series of linearized models are presented and analyzed. Robustness and performance specifications are posed in the frequency domain. The LQG/LTR methodology is used to obtain a sequence of three feedback designs. Even in the single-input single-output case, comparison of the current control system with that derived from the LQG/LTR approach shows significant performance improvement. The multi-variable designs, evaluated using linear and nonlinear simulations, show even more potential for performance improvement.

Aerodynamic Loads Induced by a Rotor on a Body of Revolution

Abstract: A wind-tunnel investigation was conducted in which aerodynamic loads were measured on a small-scale helicopter rotor and a body of revolution located close to it as an idealized model of a fuselage. The objective was to study the aerodynamic interactions as a function of forward speed, rotor thrust, and rotor/body position. Results show that body loads, normalized by rotor thrust, are functions of the ratio between free-stream velocity and the hover-induced velocity predicted by momentum theory.

Helicopter low-g monitor, recorder and warning system

Abstract: When helicopters are accomplishing certain special maneuvers such as powered descent, or the like involving low "g" conditions, the helicopter blades may teeter excessively with respect to the mast, and under severe conditions, the mast may shear. To sense such low "g" conditions, which may be dangerous, an accelerometer is mounted on the helicopter frame close to the center of gravity of the helicopter and is coupled to data processing circuitry which classifies the low-g conditions in bands or levels of severity, recording the duration of exceedances in which the low-g conditions are below predetermined levels, and issues warning signals to the helicopter pilot through the helicopter intercom system and a suitable warning light.

Assessment of aerodynamic and dynamic models in a comprehensive analysis for rotorcraft

Abstract: The history, status and lessons of a comprehensive analysis for rotorcraft are reviewed. The development, features, and capabilities of the analysis are summarized, including the aerodynamic and dynamic models used. Examples of correlation of the computational results with experimental data are given, extensions of the analysis for research in several topics of helicopter technology are discussed, and the experiences of outside users are summarized. Finally, the required capabilities and approach for the next comprehensive analysis are described.

Investigation of the Steady-State Response of a Dual-Rotor System With Intershaft Squeeze Film Damper

Abstract: This paper presents an analysis of the steady-state unbalance response of a dual-rotor gas turbine engine with a flexible intershaft squeeze film damper using a simplified transfer matrix method. The simplified transfer matrix method is convenient for the evaluation of the critical speed and response of the rotor system with various supports, shaft coupling, intershaft bearing, etc. The steady-state unbalance response of the rotor system is calculated for different shaft rotation speeds. The damping effects of an intershaft squeeze film damper with different radial clearances under various levels of rotor unbalance are investigated.

Influence of nonlinear blade damping on helicopter ground resonance instability

Abstract: The lagging motion of each helicopter blade is assumed to be of equal amplitude and equally apportioned phase, thus allowing a simplified analytical method to be used to calculate the ground resonance instability of a helicopter model with nonlinear dampers in both the landing gear and blades. The geometrical nonlinearities of the blade lag motion and the influence of initial disturbances on ground resonance instability are also discussed. Finally an experiment is carried out using a helicopter scale model. The experimental data agree well with analysis.

Parameter Sensitivity in the Dynamics of Rotor-Bearing Systems

Abstract: When designing a rotor system it is frequently desirable to have at hand a set of design sensitivity coefficients which quantitatively predict a change in specific system characteristics to changes in design parameters. This paper presents eigenvalue sensitivity coefficients for the damped natural frequencies of whirl of general linear rotor system modelled by finite element discretization. In addition, a simple and direct method for calculation of the damped critical speeds is presented, which utilizes the eigenvalue sensitivity with respect to the spin speed. It is shown that the combination of design parameter and spin speed whirl frequency sensitivity coefficients may be used to also evaluate the damped critical speed sensitivity coefficients.

Helicopter blade and the like stand-off and folding device

Abstract: The instant invention provide a helicopter blade and the like stand-off and folding device that permits a helicopter blades to be folded up to positions close to the tail of the aircraft during inclement weather, storage, transport, or maintenance, etcetera. During a fold or unfold a one-to-one correspondence between the blades and the main rotor head is maintained and critical adjustments of the rotor blade assembly are not disturbed.

Helicopter rotor blade control

Abstract: The present invention relates to a circulation flow control rotor system for an aircraft which includes rotor blades provided with slit shaped blower nozzles further comprising control devices for obtaining a cyclic or multicyclic or collective rotor blade angle control under utilization of individual pressure gas flow from a pressure chamber to the nozzle slits; further including deformable control members which are going to be deformed in accordance with certain rules underlying the requisite control.

Rotor control system

Abstract: A helicopter rotor control system (13) including a stop azimuth controller (32) for establishing the value of a deceleration command (15') to a deceleration controller (23), a transition azimuth predictor (41) and a position reference generator (55), which are effective during the last revolution of said rotor (14) to establish a correction indication (38) to adjust the deceleration command (15') to ensure that one of the rotor blades (27) stops at a predetermined angular position.

Helicopter rotor noise due to ingestion of atmospheric turbulence

Abstract: A theoretical study was conducted to develop an analytical prediction method for helicopter main rotor noise due to the ingestion of atmospheric turbulence. This study incorporates an atmospheric turbulence model, a rotor mean flow contraction model and a rapid distortion turbulence model which together determine the statistics of the non-isotropic turbulence at the rotor plane. Inputs to the combined mean inflow and turbulence models are controlled by atmospheric wind characteristics and helicopter operating conditions. A generalized acoustic source model was used to predict the far field noise generated by the non-isotropic flow incident on the rotor. Absolute levels for acoustic spectra and directivity patterns were calculated for full scale helicopters, without the use of empirical or adjustable constants. Comparisons between isotropic and non-isotropic turbulence at the rotor face demonstrated pronounced differences in acoustic spectra. Turning and contraction of the flow for hover and low speed vertical ascent cases result in a 3 dB increase in the acoustic spectrum energy and a 10 dB increase in tone levels. Compared to trailing edge noise, turbulence ingestion noise is the dominant noise mechanism below approximately 30 rotor harmonics, while above 100 harmonics, trailing edge noise levels exceed turbulence ingestion noise by 25 dB.

Free vibration characteristics of multiple load path blades by the transfer matrix method

Abstract: The determination of free vibrational characteristics is basic to any dynamic design, and these characteristics can form the basis for aeroelastic stability analyses. Conventional helicopter blades are typically idealized as single-load-path blades, and the transfer matrix method is well suited to analyze such blades. Several current helicopter dynamic programs employ transfer matrices to analyze the rotor blades. In this paper, however, the transfer matrix method is extended to treat multiple-load-path blades, without resorting to an equivalent single-load-path approximation. With such an extension, these current rotor dynamic programs which employ the transfer matrix method can be modified with relative ease to account for the multiple load paths. Unlike the conventional blades, the multiple-load-path blades require the introduction of the axial degree-of-freedom into the solution process to account for the differential axial displacements of the different load paths. The transfer matrix formulation is validated through comparison with the finite-element solutions.

Helicopter blade cyclic pitch control system

Abstract: A control and actuating system for adjusting the cyclic pitch of the blades of a helicopter automatically and independently in response to the blade gyrating angular position and to the helicopter forward velocity. This action is also independent of the control and adjustment of the collective pitch. The actuation of the blade pitch is performed by means of cams and followers. A hydraulic force-amplifying system is used between the follower/cam assemblies and the blade pitch actuator so as to isolate the follower/cam assemblies from the load carrying components of the system. The displacement signals generated by two cams are combined into a single linear displacement signal which causes a corresponding rotational movement of the blade. The axial position of one cam is determined by the pilot's action and the axial position of the other cam is defined by the helicopter forward speed. The two cams have surfaces that are not of revolution and which remain fixed for a given set combination of pilot's action and forward speed. The blade cyclic pitch variation is caused by the gyration of a housing to which the blades are attached and which contains and encloses the blade pitch control and actuating system.

Performance and loads data from a hover test of a full-scale advanced technology XV-15 rotor

Abstract: A hover test of a full-scale, composite, advanced technology XV-15 rotor was conducted at the Outdoor Aerodynamic Research Facility at Ames Research Center. The primary objective of the test was to obtain accurate measurements of the hover performance of this rotor system. Data were acquired for rotor tip Mach numbers ranging from 0.35 to 0.73. The rotor was tested with several alternate blade root and blade-tip configurations. Data are presented on rotor performance, rotor-wake downwash velocities, and rotor system loads.

Special Opportunities in Helicopter Aerodynamics

Abstract: Aerodynamic research relating to modern helicopters includes the study of three-dimensional, unsteady, nonlinear flow fields. A selective review is made of some of the phenomena that hamper the development of satisfactory engineering prediction techniques, but which provide a rich source of research opportunities: flow separation, compressibility effects, complex vortical wakes, and aerodynamic interference between components. Several examples of work in progress are given, including dynamic stall alleviation, the development of computational methods for transonic flow, rotor-wake predictions, and blade-vortex interactions.

Computation of transonic flow about helicopter rotor blades

Abstract: An inviscid, nonconservative, three-dimensional full-potential flow code, ROT22, has been developed for computing the quasi-steady flow about a lifting rotor blade. The code is valid throughout the subsonic and transonic regime. Calculations from the code are compared with detailed laser velocimeter measurements made in the tip region of a nonlifting rotor at a tip Mach number of 0.95 and zero advance ratio. In addition, comparisons are made with chordwise surface pressure measurements obtained in a wind tunnel for a nonlifting rotor blade at transonic tip speeds at advance ratios from 0.40 to 0.50. The overall agreement between theoretical calculations and experiment is very good. A typical run on a CRAY X-MP computer requires about 30 CPU seconds for one rotor position at transonic tip speed.

The Critical Role of Computational Fluid Dynamics in Rotary-Wing Aerodynamics

Abstract: The field of helicopter rotor aerodynamics is basically the study of unsteady aerodynamic flows in a rotating and translating coordinate system. Current trends in this field are briefly reviewed by examining recent advances in lifting-surface theory, wake modeling, panel methods, and finite-difference models. Examples are used to illustrate selected current methods and some indications of promising future directions are highlighted.