Showing papers on "Helicopter rotor published in 1989"

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.

Rotor system for winged aircraft

Abstract: A rotor system for a rotor convertible fixed winged aircraft comprising a rotatable rotor shaft mounted on one of the wings of the aircraft, the shaft tiltable between a substantially vertical position and a substantially horizontal position, a rotor mounted to the shaft and including at least one blade, the blade having an adjustable blade angle relative to the shaft, apparatus for tilting the shaft between the vertical and horizontal positions, controls responsive to the tilting of the shaft for automatically adjusting the blade angle of a blade to an autogyro blade angle when the shaft is in the vertical position and to a substantially feathered blade angle when the shaft is in the horizontal position.

Piezoelectric helicopter blade flap actuator

Abstract: Electrically deformable material such as piezoelectric material is used to deform a deflectable flap on an airfoil such as a helicopter blade. The electrically deformable material is controlled to deflect the flap in a manner to control vibrations transmitted from a helicopter blade to the helicopter air frame. In a preferred embodiment, the electrically deformable material and mechanical linkages are segmented along the length of the rotor blade. In this way, differential actuation of the deformable elements results in variable flap deflections along the length of the rotor.

Flying multi-purpose aircraft carrier and method of V/STOL assisted flight

Abstract: The Flying Multi-Purpose Aircraft Carrier (FMPAC) comprises a runway body which is provided with V/STOL capability by at least two rotary wings, located outside the two lateral ends of the runway body in a side-by-side configuration. The rotary wings are, preferably, of a large diameter helicopter rotor blade type, and are powered by respective shaft turbine engines. A substantially flat runway platform is provided along the middle portion of the runway body, between the left and right helicopter rotary wings. The runway platform is located as low as possible, in order to secure flight stability of the composite aircraft. The top surface of the runway platform has a substantially flat and rectangular-like shape defining a conventional runway, and may allow a fixed wing CA to take-off from or to land on it, during flight of the FMPAC, allowing the pilot of the CA to operate his aircraft with a considerable margin of error. During composite flight, the brakes of the CA's landing gear should be actuated and in addition, its landing gear should be locked down to the surface of the FMPAC. Any CA may be assisted for V/STOL by means of a FMPAC, provided that its weight is not larger than the payload capability of the FMPAC. The V/STOL assisted flight of CA may be performed from any relatively flat ground surface, or alternatively, it may be performed from a water surface, if the FMPAC is equipped with a flying boat undercarriage.

Actuator design for rotor control

Abstract: Active control of dynamical behavior of rotor systems involves the following modeling of the rotor system including active elements as a control process, positioning of actuators and sensors, implementation of problem-adapted control concepts. The key to attaining the desired influence on the dynamic behavior of rotor systems lies in the presence of suitable actuators. Some types of actuators (magnetic, electrohydraulic and piezoelectric) will be discussed and their respective drawbacks indicated. Their design will be outlined and functional correlations will be presented. Theoretical and experimental results demonstrate that an improvement of the system dynamics by the use of control forces is always possible.

Aeroelastic Optimization of a Helicopter Rotor

Abstract: Structural optimization of a hingeless rotor is investigated to reduce oscillatory hub loads while maintaining aeroelastic stability in forward flight. Design variables include spanwise distribution of nonstructural mass, chordwise location of blade center of gravity and blade bending stiffnesses (flap, lag and torsion). A comprehensive aeroelastic analysis of rotors, based on a finite element method in space and time, is linked with optimization algorithms to perform optimization of rotor blades. Sensitivity derivatives of blade response, hub loads, and eigenvalues with respect to the design variables are derived using a direct analytical approach, and constitute an integral part of the basic blade response and stability analyses. This approach reduces the computation time substantially; an 80 percent reduction of CPU time to achieve an optimum solution, as compared to the widely adopted finite difference approach. Through stiffness and nonstructural mass distributions, a 60-90 percent reduction in all six 4/rev hub loads is achieved for a four-bladed soft-inplane rotor.

Calculation of blade-vortex interaction airloads on helicopter rotors

Abstract: Two alternative approaches area developed to calculate blade-vortex interaction airloads on helicopter rotors, second-order lifting-line theory and a lifting-surface theory correction. The common approach of using a larger vortex core radius to account for lifting-surface effects is quantified. The second-order lifting-line theory also improves the modeling of low aspect-ratio blades yawed flow, and swept tips. Calculated results are compared with wind-tunnel measurements of lateral flapping, and with flight test measurements of blade section lift on SA349/2 and H-34 helicopter rotors. The tip vortex core radius required for good correlation with the flight test data is about a 20-percent chord, which is within the range of measured viscous core sizes for helicopter rotors.

Main Rotor Broadband Noise Study in the DNW

Abstract: An acoustics test of a 2/5 scale model BO-105 helicopter main rotor was conducted in the Duits-Nederlandse Windtunnel (DNW). A range of operating conditions was tested from hover to moderately high flight speeds for various climb and descent rates at different thrust settings. Diagnostic tests including rotor speed and blade geometry changes were made to better isolate and study particular broadband self noise sources. Acoustic data in the form of acoustic pressure time histories and power spectra are used to demonstrate the regions of importance of the different broadband noise sources and their sensitivity to operating conditions. To help interpret the data, comparisons are made to predictions of rotor broadband noise. The predictions are based on self noise data previously obtained from isolated airfoil sections and the use of the NASA ROTONET program to define rotor performance and to sum contributions of noise from individual blade segments. An important result herein is the identification and articulation of a previously unheralded rotor broadband noise source. This source is blade-turbulent wake interaction (BWI) noise which dominates the spectra in the mid-frequencies for off-peak blade-vortex interaction (BVI) noise flight conditions.

Integrated multidisciplinary design optimization of rotorcraft

Abstract: The NASA/Army research plan for developing the logic elements for helicopter rotor design optimization by integrating appropriate disciplines and accounting for important interactions among the disciplines is discussed. The optimization formulation is described in terms of the objective function, design variables, and constraints. The analysis aspects are discussed, and an initial effort at defining the interdisciplinary coupling is summarized. Results are presented on the achievements made in the rotor dynamic optimization for vibration reduction, rotor structural optimization for minimum weight, and integrated aerodynamic load/dynamics optimization for minimum vibration and weight.

Active control of transient rotordynamic vibration by optimal control methods

Abstract: Although considerable effort has been put into the study of steady state vibration control, there are few methods applicable to transient vibration control of rotorbearing systems. In this paper optimal control theory has been adopted to minimize rotor vibration due to sudden imbalance, e.g., blade loss. The system gain matrix is obtained by choosing the weighting matrices and solving the Riccati equation. Control forces are applied to the system via a feedback loop. A seven mass rotor system is simulated for illustration. A relationship between the number of sensors and the number of modes used in the optimal control model is investigated. Comparisons of responses are made for various configurations of modes, sensors, and actuators. Furthermore, spillover effect is examined by comparing results from collocated and noncollocated sensor configurations. Results show that shaft vibration is significantly attenuated in the closed loop system.

Numerical and experimental investigation of isolated propeller wakesin axial flight

Abstract: Complementary numerical and experimental studies were used to investigate the isolated propeller aerodynamic field over a large range of operating parameters of the axial flight regime. The numerical approach is based on a free-wake-analysis calculation that uses a new complete equilibrium procedure for the wake divided into near and far regions. Measurements of instantaneous velocities, as well as overall thrust and torque, were performed to determine the wake geometry and the three-dimensional vortex flow streaming back from the propeller. A large check of the code efficiency to predict both overall and local aerodynamic quantities is realized through extensive comparisons with experimental data obtained on a model scale of a four-bladed propeller. From the present results, the code predictions are shown to be in good agreement with experiments for different axial flight conditions of the isolated propeller.

Noise produced by turbulent flow into a rotor: Theory manual for noise calculation

Abstract: A users manual for a computer program for the calculation of noise produced by turbulent flow into a helicopter rotor is presented These inputs to the program are obtained from the atmospheric turbulence model and mean flow distortion calculation, described in another volume of this set of reports Descriptions of the various program modules and subroutines, their function, programming structure, and the required input and output variables are included This routine is incorporated as one module of NASA's ROTONET helicopter noise prediction program

Fully integrated aerodynamic/dynamic optimization of helicopter rotor blades

Abstract: A fully integrated aerodynamic/dynamic optimization procedure is described for helicopter rotor blades. The procedure combines performance and dynamic analyses with a general purpose optimizer. The procedure minimizes a linear combination of power required (in hover, forward flight, and maneuver) and vibratory hub shear. The design variables include pretwist, taper initiation, taper ratio, root chord, blade stiffnesses, tuning masses, and tuning mass locations. Aerodynamic constraints consist of limits on power required in hover, forward flight and maneuvers; airfoil section stall; drag divergence Mach number; minimum tip chord; and trim. Dynamic constraints are on frequencies, minimum autorotational inertia, and maximum blade weight. The procedure is demonstrated for two cases. In the first case, the objective function involves power required (in hover, forward flight and maneuver) and dynamics. The second case involves only hover power and dynamics. The designs from the integrated procedure are compared with designs from a sequential optimization approach in which the blade is first optimized for performance and then for dynamics. In both cases, the integrated approach is superior.

Individual blade control system for helicopters

Abstract: A control system for providing individual blade control inputs to a four-bladed helicopter rotor. Limited authority series actuators located in a non-rotating portion of the helicopter are driven by a computer which receives inputs from individual blade sensors and aircraft stabilization sensors, summed with cockpit control inputs, and the resultant motions transmitted to the rotor blades through a conventional swashplate which drives four blades of the rotor and a translatable differential sleeve and summing linkage which drives only two blades. The differential sleeve is adapted for axially raising or lowering the swashplate. The combined motion of swashplate and differential sleeve is shown mathematically to result in individual blade control in pitch. Computer individual blade commands are converted from the rotating reference frame to cyclic, collective and differential blade pitch commands in the fixed reference frame through a coordinate transformation matrix.

Rub Interactions of Flexible Casing Rotor Systems

Abstract: Rub interactions between a rotor assembly and its corresponding casing structure has long been one of the major causes for machine failure. This paper presents a comprehensive analysis of a complex rotor-bearing-blade-casing system during component rub interactions. The modal method is used in this study. The dynamic characteristics of the system are examined in both the time domain and the frequency domain using a numerical FFT procedure. A multibearing flexible casing rotor system will be used as an example

Application of higher harmonic control (hhc) to hingeless rotor systems

Abstract: A comprehensive analytical formulation has been developed to predict the vibratory hub loads of a helicopter rotor system in forward flight. The analysis is used to calculate the optimal higher harmonic control inputs and associated actuator power required to minimize these hub loads. The formulation is based on a finite element method in space and time. A nonlinear time domain unsteady aerodynamic model is used to obtain the airloads, and the rotor induced inflow is calculated using a free wake model. Predicted vibratory hub loads are correlated with experimental data obtained from a scaled model rotor. Results of a parametric study on a hingeless rotor show that blade flap, lag and torsion vibration characteristics, offset of blade center of mass from elastic axis, offset of elastic axis from quarter-chord, and rotor thrust all have large effects on the HHC actuator power requirement.

Drive shaft and rotor hub for helicopter flexible rotor system

Abstract: A rotor system according to this invention includes a structurally flexible coupling for transmitting rotor torque and other rotor loads to a rotor hub. The rotor hub is adapted to have rotor blades mounted thereon and is mounted by an elastomeric spherical bearing whose center is located at the rotor center. A flexible coupling made from fiber reinforced resin matrix material, connected to the bearing at a connection located below the bearing center, extends vertically from that location through the bearing to a position located above the center where it is connected to a connecting member fixed to the rotor hub. The flexible coupling is structurally stiff with respect to the mode in which it transmits rotor torque compared to the rotor torque stiffness of the other components. However, the bending stiffness and axial stiffness of the flexible coupling is substantially less compared to the mode in which rotor moments and forces are transmitted from the other components to the rotor shaft.

Static and dynamic behavior of composite helicopter rotor blades under large deflections

Abstract: Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1989.

Correlation of Puma airfoils - Evaluation of CFD prediction methods

Abstract: A cooperative program was undertaken by research organizations in England, France, Australia and the U.S. to study the capabilities of computational fluid dynamics codes (CFD) to predict the aerodynamic loading on helicopter rotor blades. The program goal is to compare predictions with experimental data for flight tests of a research Puma helicopter with rectangular and swept tip blades. Two topics are studied. First, computed results from three CFD codes are compared for flight test cases where all three codes use the same partial inflow-angle boundary conditions. Second, one of the CFD codes (FPR) is iteratively coupled with the CAMRAD/JA helicopter performance code. These results are compared with experimental data and with an uncoupled CAMRAD/JA solution. The influence of flow field unsteadiness is found to play an important role in the blade aerodynamics. Alternate boundary conditions are suggested in order to properly model this unsteadiness in the CFD codes.

Icing research tunnel test of a model helicopter rotor

Abstract: An experimental program has been conducted in the NASA Lewis Research Center Icing Research Tunnel (IRT) in which an OH-58 tail rotor assembly was operated in a horizontal plane to simulate the action of a typical main rotor. Ice was accreted on the blades in a variety of rotor and tunnel operating conditions and documentation of the resulting shapes was performed. Rotor torque and vibration are presented as functions of time for several representative test runs, and the effects of various parametric variations on the blade ice shapes are shown. This OH-58 test was the first of its kind in the United States and will encourage additional model rotor icing tunnel testing. Although not a scaled representative of any actual full-scale main rotor system, this rig has produced torque and vibration data which will be useful in assessing the quality of existing rotor icing analyses.

A study of fundamental issues in higher harmonic control using aeroelastic simulation

Abstract: This paper describes a higher harmonic control (HHC) study of a four-bladed helicopter rotor using a coupled flap-lag-torsional aeroelastic stability and response analysis which incorporates finite-state, time-domain aerodyamics. The rotor trim condition is determined using a simplified flap-lag-torsional aeroelastic analysis. Deterministic and cautious controllers based on local and global HHC models are implemented to reduce 4/rev hub loads on roughly equivalent articulate and hingeless rotors. Results obtained for the case when the objective function requires reduction of hub shears alone are compared with results obtained when hub shears and moments are reduced simultaneously. The effects of HHC on blade aeroelastic stability are also considered. Finally, the power requirements for implementing HHC on a hingeless rotor are compared with those obtained for an articulated rotor. It is concluded that hingeless rotors require considerably more power and higher control angles than comparable articulated rotors.

Helicopter autorotation detection and recovery

Abstract: The existence of autorotation of a load (e.g., a helicopter rotor system) coupled to the free turbine of a gas turbine engine is determined and a signal indicative thereof enables an integrator which, upon autorotation, integrates rate-based recovery anticipation provided thereto, the integrator output providing a time-composite bias signal to the main control portion of the engine fuel control to increase the speed of the gas generator of the engine in anticipation of rotor reengagement with the free turbine. The amount of recovery error is determined from the rotor speed error and is multiplied with the integrator output, the product being fed, upon completion of autorotation recovery, to the integrator input whose output now provides a controlled removal of the bias signal from the main control portion so as to minimize transients induced therein.

Computing induced velocity perturbations due to a helicopter fuselage in a free stream

Abstract: The velocity field of a representative helicopter fuselage in a free stream is computed. Perturbation velocities due to the fuselage are computed in a plan above the location of the helicopter rotor (rotor removed). The velocity perturbations computed by a source-panel model of the fuselage are compared with experimental measurements taken with a laser velocimeter. Three paneled fuselage models are studied: fuselage shape, fuselage shape with hub shape, and a body of revolution. The velocity perturbations computed for both fuselage shape models agree well with the measured velocity field except in the close vicinity of the rotor hub. In the hub region, without knowing the extent of separation, modeling of the effective source shape is difficult. The effects of the fuselage perturbations are not well-predicted with a simplified ellipsoid fuselage. The velocity perturbations due to the fuselage at the plane of the measurements have magnitudes of less than 8 percent of free-stream velocity. The velocity perturbations computed by the panel method are tabulated for the same locations at which previously reported rotor-inflow velocity measurements were made.

Helicopter rotor blades

Abstract: A rotor blade arrangement for an aircraft comprises a disc member (2) and a plurality of blades (3) which are extendable outwardly from the periphery of the disc member (2). Both the disc member (2) and blades (3) are shaped aerodynamically to provide aerodynamic lift of the aircraft during travel. The outward extension of the rotor blades (3) is variable.

A numerical analysis of the British Experimental Rotor Program blade

Abstract: Two Computational Fluid Dynamic codes which solve the compressible full-potential and the Reynolds-Averaged Thin-Layer Navier-Stokes equations were used to analyze the nonrotating aerodynamic characteristics of the British Experimental Rotor Program (BERP) helicopter blade at three flow regimes: low angle of attack, high angle of attack and transonic. Excellent agreement was found between the numerical results and experiment. In the low angle of attack regime, the BERP had less induced drag than a comparable aspect ratio rectangular planform wing. At high angle of attack, the blade attained high-lift by maintaining attached flow at the outermost spanwise locations. In the transonic regime, the BERP design reduces the shock strength at the outer spanwise locations which affects wave drag and shock-induced separation. Overall, the BERP blade exhibited many favorable aerodynamic characteristics in comparison to conventional helicopter rotor blades.

Numerical solutions of forward-flight rotor flow using an upwind method

Abstract: A finite-volume upwind algorithm for solving the 3-D Euler equations with a moving grid has been developed for computing helicopter forward-flight rotor flows. The computed pressure distributions and shock positions of high-speed rotor flow are compared with various experimental data as well as with other numerical results, and the agreement is encouraging. A comparison of quasi-steady solutions with unsteady solutions reveals that when a shock occurs in the flowfield, the assumption of quasi-steady flow may fail due to the time-lag of the shock motion. Similarly, three-dimensional effects cannot be neglected. Sufficient subiterations for each time step are required to avoid numerical lag effects in using the present method. The redistribution of the residual due to the coordinate transformation is discussed. For high-order MUSCL-type schemes, a coordinate-independent solution can be obtained by interpolating primitive variables.