Showing papers on "Helicopter rotor published in 1997"

A Survey of Theoretical and Experimental Coaxial Rotor Aerodynamic Research

Abstract: The recent appearance of the Kamov Ka-50 helicopter and the application of coaxial rotors to unmanned aerial vehicles have renewed international interest in the coaxial rotor configuration. This report addresses the aerodynamic issues peculiar to coaxial rotors by surveying American, Russian, Japanese, British, and German research. (Herein, 'coaxial rotors' refers to helicopter, not propeller, rotors. The intermeshing rotor system was not investigated.) Issues addressed are separation distance, load sharing between rotors, wake structure, solidity effects, swirl recovery, and the effects of having no tail rotor. A general summary of the coaxial rotor configuration explores the configuration's advantages and applications.

Modern helicopter aerodynamics

Abstract: ▪ Abstract Modern helicopter aerodynamics is challenging because the flow field generated by a helicopter is extremely complicated and difficult to measure, model, and predict; moreover, experiments are expensive and difficult to conduct. In this article we discuss the basic principles of modern helicopter aerodynamics. Many sophisticated experimental and computational techniques have been employed in an effort to predict performance parameters. Of particular interest is the structure of the rotor wake, which is highly three-dimensional and unsteady, and the rotor-blade pressure distribution, which is significantly affected by the strength and position of the wake. We describe the various modern methods of computation and experiment which span the range from vortex techniques to full three-dimensional Navier-Stokes computations, and from classical probe methods to laser velocimetry techniques. Typical results for the structure of the wake and the blade pressure distribution in both hover and forward fligh...

Hover Testing of Smart Rotor with Induced-Strain Actuation of Blade Twist

Abstract: The experimental results are presented of hover test with a one-eighth dynamically scaled (Froude scale) helicopter rotor model with embedded piezoceramic elements as actuators to suppress vibrations. A 6-ft-dlam two-bladed bearingless rotor model was built with banks of piezoelectric torsional actuators capable of manipulating blade twist at harmonics of the rotational speed. The twist performances of several rotor blade configurations were investigated using accelerometers embedded in the blade tip. The change in oscillatory rotor lift due to plezoactuation was measured by a hub balance. Experimental results show that linear twist distributions of up to 0.6 deg, resulting in increases of up to 10% of the nominal rotor lift, are possible with existing piezoceramic technology. Although the twist amplitudes attained in this experiment were less than the target value (1-2 deg), it is expected that partial reduction of hub vibration can be achieved with the current smart rotors.

Unsteady Fluid Mechanics Applications of Neural Networks

Abstract: The capability to harness or alleviate unsteady aerodynamic forces and moments could dramatically enhance aircraft control during severe maneuvers as well as signie cantly extend the life span of both helicopter and wind turbine blade/rotor assemblies. Using recursive neural networks, time-dependent models that predict unsteady boundary-layer development, separation, dynamic stall, and dynamic reattachment have been developed. Further, these models of the e ow› wing interactions can be used as the foundation upon which to develop adaptive control systems. The present work describes these capabilities for three-dimensional unsteady surface pressures and two-dimensional unsteady shear-stress measurements obtained for harmonic and constant-rate pitch motions. In the near future, it is predicted that such techniques will provide a viable approach for the development of six degree-of-freedom motion simulators for severe vehicle maneuvers as well as a foundation for the active control of unsteady e uid mechanics in a variety of systems.

Analysis of recorded helicopter echo

Abstract: The problem of a hovering helicopter detection and classification with a radar system is presented. First, the possible components of a helicopter echo are analyzed. A theoretical model of helicopter rotor backscatter is described, showing the anticipated detection difficulties. An experiment, consisting of radar signal recording and off-line analysis is outlined. The analysis results show the possibility to identify a helicopter echo by time- or frequency-domain characteristics. The probability of detection during normal radar operation is, however, shown to be moderate.

Wind tunnel test of a smart rotor model with individual blade twist control

Abstract: The objective of this research is to develop a smart rotor with active control of blade twist using embedded piezoceramic elements as sensors and actuators to minimize rotor vibrations. A 1/8 Froude-scale (dynamically scaled) bearingless helicopter rotor model was built with banks of torsional actuators capable of manipulating blade twist at frequencies from 5 to 100 Hz. To assess the effectiveness of the torsional actuators and vibration suppression capabilities, systematic wind tunnel testing was conducted in the Glenn L. Martin Wind Tunnel. Using accelerometers embedded in the blade tip, the oscillatory blade twist response was measured. The changes in rotor vibratory loads due to piezo-induced twist were determined using a rotating hub balance located at the rotor hub. Experimental test results show that tip twist amplitudes on the order of 0.5 degrees are attainable by the current actuator configurations in forward flight. Although these amplitudes were less than the target value (I to 2 degrees for ...

Helicopter with a gyroscopic rotor and rotor propellers to provide vectored thrust

Abstract: A model helicopter creates lift using rotor propellers mounted to the rotor arms of a gyroscopic rotor assembly. A controller converts front-back, and left-right inputs into speed control signals used to vary the speeds of the rotor propellers at selected positions of the rotor assembly as it rotates. The varying speed of the rotor propellers at selected rotor positions produces thrust vectors at those positions. The resultant thrust vector determines the direction of the helicopter's flight and enables it to pitch and roll in response to the front-back and left-right inputs. The rotor assembly can have two or more rotor arms, each with a propeller. The helicopter provides left-right yaw control with a yaw propeller on the helicopter body. Electric motors, or motors using other conventional power and speed control methods, can be used to drive the rotor and yaw propellers. Power and speed control signals can be transferred to the rotor motors via commutators on the helicopter body and rotor assembly that make electrical contact as the rotor assembly rotates.

Oscillating air jets for helicopter rotor aerodynamic control and BVI noise reduction

Abstract: An active control system for reducing blade-vortex-interaction (BVI) noise generated by a rotor blade. The active control system includes a pressure sensor assembly, a device for changing a lift generated by the rotor blade, and a controller for activating the device upon a detected change in air pressure by the sensor assembly. The sensor assembly is disposed in close proximity to the rotor blade, and is adapted to detect a change in air pressure on a surface of the rotor blade near a leading edge of the rotor blade. The device is adapted to be activated by the controller, to thereby change a lift of the rotor blade. The controller activates the device to change a lift of the rotor blade in order to introduce a compensating pressure onto the surface of the rotor blade. This compensating pressure attenuates the magnitude of the change of air pressure. The device for changing a lift generated by the rotor blade can include at least one aperture on the rotor blade and a diaphragm in the interior of the rotor blade. The diaphragm can be activated and moved between a first position and a second position at a frequency. Movement of the diaphragm from the first position to the second position pushes air out of the at least one aperture, and movement of the diaphragm from the second position to the first position draws air into the at least one aperture.

Application of smart materials to helicopter rotor active control

Abstract: Helicopter design is limited by the compromise inherent in meeting hover and forward flight requirements, and the unsteady environment encountered in forward flight. Active control of helicopter rotors using smart material, in-blade actuation can overcome these barriers and provide substantial reductions in noise and vibrations and improved performance. The present study covers the blade/actuator integration and actuator development for a full scale system to demonstrate active control of noise and vibrations as well as inflight blade tracking on the MD Explorer helicopter. A piezoelectric multilayer stack actuator, driving a trailing edge flap, is used for active control. A shape memory alloy torsion actuator, driving a trailing edge trim tab, is used for inflight tracking. Overall, this DARPA sponsored program entails the design, development, and fabrication of the full scale active control rotor system. If successful, an entry in the NASA Ames 40 X 80 foot wind tunnel and flight tests are planned for a follow on program.

Detection of Helicopter Rotor System Simulated Faults Using Neural Networks

Abstract: Simulated fault data from a mathematical model of a damaged rotor system is used to develop a neural network-based approach for rotor system damage detection. The mathematical model of the damaged rotor is a comprehensive rotorcraft aeroelastic analysis based on a finite element approach in space and time. Selected helicopter rotor faults are simulated through changes in inertial, damping and stiffness properties of the damaged blade. A feed-forward neural network with back-propagation learning is trained using both 'ideal' and 'noisy' simulated data. Testing of the trained neural network shows that it can detect and identify damage in the rotor system from simulated blade response and vibratory hub loads data. (Author)

Navier-Stokes/Full Potential/Free-Wake Method for Rotor Flows

Abstract: A technique for modeling unsteady three-dimensional compressible viscous e ow over lifting rotors in hover and forward e ight is described. The e ow solver has three modules: 1 ) a compressible Navier› Stokes solver for modeling the viscous e ow over the rotor and the rotor near wake; 2 ) a compressible potential e ow solver for modeling the inviscid, isentropic potential e ow regions far away from the rotor, and 3) a Lagrangian model for capturing and convecting the rotor wake once it leaves the Navier› Stokes domain. Results are presented for viscous e ow over a two-bladed untwisted rotor and a UH-60A rotor in hover. Good agreement is found with measurements.

Influence of Elastomeric Damper Modeling on the Dynamic Response of Helicopter Rotors

Abstract: Several modeling approaches that describe the behavior of elastomeric materials used in helicopter rotor lag dampers are examined and evaluated, using laboratory test data. Two of the models are then used in a simulation of a helicopter rotor startup, and the simulation results are compared to one another and to flight test data. The models created using laboratory test results show that the simple, complex modulus model will yield good predictions of damper energy dissipation, but the simulation results indicate that this model is inadequate for predicting forced response. Although some of the models evaluated performed better than the others, none were free from limitations that would make them unsuitable for some applications. It is recommended that more effort be put into acquiring and analyzing damper test data to facilitate the development of more robust modeling approaches.

Comparison of computational aeroacoustic prediction methods for transonic rotor noise

Abstract: This paper compares two methods for predicting transonic rotor noise for helicopters in hover and forward flight. Both methods rely on a computational fluid dynamics (CFD) solution as input to predict the acoustic near and far fields. For this work, the same full-potential rotor code has been used to compute the CFD solution for both acoustic methods. The first method employs the acoustic analogy as embodied in the Ffowcs Williams-Hawkings equation, including the quadrupole term. The second method uses a rotating Kirchhoff formulation. Computed results from both methods are compared with one another and with experimental data for both hover and advancing rotor cases. The results are quite good for all cases tested. The Kirchhoff method was somewhat sensitive to the location of Kirchhoff surface, if the surface was positioned too close to the rotor blade. The acoustic analogy method was not as sensitive to the extent of volume included in the quadrupole calculation. The computational requirements of both methods are comparable; in both cases these requirements are much less than the requirements for the CFD solution.

Analysis and Testing of a Froude Scaled Helicopter Rotor with Piezoelectric Bender Actuated Trailing Edge Flaps

Abstract: This paper presents an analytical model and validation tests of a Froude scaled rotor featuring piezoelectric bender (bimorph) actuated trailing-edge flaps for active vibration suppression. The analytical model for the coupled bimorph actuator trailing-edge flap dynamic response in the rotating environment takes into account the aerodynamic, centrifugal, inertial and frictional loads acting on the actuator-flap system. The linkage arm length associated with the mechanical amplification mechanism is selected to maximize flap performance in the rotating environment. The bimorph clamping is improved to prevent actuator slippage under high centrifugal loads. The analytical model is validated by carrying out a series of bench tests, vacuum chamber tests and hover tests. In hover, flap deflections of ± 6 degrees at 4/rev flap excitation are achieved at 900 RPM, thus demonstrating the potential ofthe piezoceramic bender as a lightweight and compact actuation system for individual blade control purposes. This paper also includes a feasibility study for piezo-bimorph actuation of a trailing-edge flap for a Mach scaled rotor model.

Improvement of Stability of Rotor System by Introducing a Hydraulic Damper into an Active Journal Bearing

Abstract: Modelling and analysis of a rotor--bearing system with a new type of active oil bearing is presented. The active bearing is supplied with a flexible sleeve whose deformation can be changed during operation of the rotor. The flexible sleeve is also a part of a hydraulic damper whose parameters can be controlled during operation as well.Finite Element Method (FEM) and the Guyan condensation technique was utilised to create mathematical model of both, the rotor and the flexible sleeve. The hydrodynamic pressure distribution in the oil film, for the instantaneous position of the flexible sleeve and rotor, was approximated by Reynolds equation.The mathematical model of motion of a rotor system with the described active bearing developed in this paper allowed the influence of the introduced hydraulic damper on stability of the rotor-bearing system to be investigated. Results of the computer simulation shows that within a large region of configuration parameters of the rotor bearing system, the self exciting vibration can be eliminated or greatly reduced during operation by properly controlled deformation of the flexible sleeve and optimal choice of the hydraulic damper parameters.

Design of a Permanent~Electromagnetic Magnetic Bearing-controlled Rotor System

Abstract: This study proposes design procedures for the permanent-magnet-biased magnetic bearings (PEMBs) in rotor systems. Many aspects of designing magnetic bearings are discussed, e.g. the selection of a permanent magnet material, dimensions of electromagnets and permanent magnets, gap length, load capacity and maximum Ampere-turns. Linearization and DC current driver are the two constraints for determining feasible designs. According to an analytical model with a rigid body assumption for the rotor-bearing system, a decentralized output feedback control algorithm is employed to control this inherently unstable magnetic suspension rotor system. Experimental results indicate that the controlled rotor performs well at rotor speeds up to 12,000 rpm.

Helicopter rotor tip jet

Abstract: A helicopter includes a rotor tip jet engine which combines ram compression and centrifugal compression to increase the thrust. A first flow of fuel is dispersed in the ram air and is carried into a combustion area of the engine. A separate flow of fuel is injected into the centrifugally compressed air before the fuel air mixture enters the combustion area. For helicopters and other applications, the ram jet engine combines ram air and a separate mass of compressed air which is directed into the combustion area for increased thrust. A regulator or control mechanism is also provided for regulating the quantity of compressed air which is added. The regulator or control mechanism may also be used to cut off or stop the addition of the separate mass of compressed air.

A composite tip cap assembly for a helicopter main rotor blade

Abstract: A composite tip cap assembly (10) for a helicopter main rotor blade includes a composite main fairing (20), a unitary composite rib (60) detail internally integrated in combination with the composite main fairing (20), a composite closure fairing (50) bonded in combination with the composite main fairing (20), and an abrasion strip (70) bonded in combination with the composite main fairing (20) and the composite closure fairing (50).

Design and testing of a 1/12th-scale solid state adaptive rotor

Abstract: A new approach to helicopter flight control through active blade pitch manipulation is presented. By using piezoceramic directionally attached piezoelectric (DAP) torque plates mounted between the rotor shaft and the blade root, the pitch angle of the helicopter rotor blades may be adjusted as the blades sweep the azimuth. Analytical models based on classical laminated plate theory for steady torque-plate deflection are presented. To verify the models, a 1/12th-scale two-bladed experimental test article was constructed. The test article was Froude scaled with a 122 cm total diameter and a 5.5 cm chord. Static and dynamic testing showed that blade pitch deflections could be controlled from -4 through with good correlation between theory and experiment. Dynamic testing demonstrated that the first natural frequency in pitch was greater than 2.5 with a maximum power consumption of 194 mW under the most extreme conditions, thus making it feasible for control of collective, longitudinal and lateral cyclic. Whirl-stand testing at up to 600 RPM showed that the rotor could generate thrust coefficients ranging from -0.0046 through +0.014 with little sensitivity of blade deflection to increases in rotor speed.

Rotating Shake Test and Modal Analysis of a Model Helicopter Rotor Blade

Abstract: Rotating blade frequencies for a model generic helicopter rotor blade mounted on an articulated hub were experimentally determined. Testing was conducted using the Aeroelastic Rotor Experimental System (ARES) testbed in the Helicopter Hover Facility (HHF) at Langley Research Center. The measured data were compared to pretest analytical predictions of the rotating blade frequencies made using the MSC/NASTRAN finite-element computer code. The MSC/NASTRAN solution sequences used to analyze the model were modified to account for differential stiffening effects caused by the centrifugal force acting on the blade and rotating system dynamic effects. The correlation of the MSC/NASTRAN-derived frequencies with the experimental data is, in general, very good although discrepancies in the blade torsional frequency trends and magnitudes were observed. The procedures necessary to perform a rotating system modal analysis of a helicopter rotor blade with MSC/NASTRAN are outlined, and complete sample data deck listings are provided.

Rotor-Fuselage Interaction: Analysis and Validation with Experiment

Abstract: The problem of rotor-fuselage aerodynamic interaction has to be considered in industry applications from various aspects. First, in order to increase helicopter speed and reduce operational costs, rotorcraft tend to be more and more compact, with a main rotor closer to the fuselage surface. This creates significant perturbations both on the main rotor and on the fuselage, including steady and unsteady effects due to blade and wake passage and perturbed inflow at the rotor disk. Furthermore,the main rotor wake affects the tail boom, empennage and anti-torque system. This has important consequences for helicopter control and vibrations at low speeds and also on tail rotor acoustics (main rotor wake-tail rotor interactions). This report describes the US Army-France MOD cooperative work on this problem from both the theoretical and experimental aspects. Using experimental 3D velocity field and fuselage surface pressure measurements, three codes that model the interactions of a helicopter rotor with a fuselage are compared. These comparisons demonstrate some of the strengths and weaknesses of current models for the combined rotor-fuselage analysis.

Incrementally adjustable rotor-blade tracking tab using sma composites

Abstract: Advanced helicopter rotor systems are designed with non-metallic composite materials for fatigue strength and low radar signature for reduced observability. They incorporate non-metallic composite blades and non-metallic tracking tabs. Adjustment of the existing non-metallic tracking tabs is complex and inefficient since it requires heating and cooling a thermoplastic tab material for each track adjustment. Several track-tab adjustments are required until optimum tracking is achieved. In this paper, a method for achieving direct in- flight incremental adjustment of the tracking tab is presented. The method utilizes a special tab material that can be remotely activated by the pilot from the cabin while in forward flight. This material is a composite consisting of shape memory alloy (SMA) wires embedded into a polymeric matrix. The activation of the SMA composite tab is done through electric current controlled from the pilot cabin and sent through conductors along the blade to the tracking tab. The proposed method will achieve in-flight tracking tab adjustment, will minimize the adjustment cycles and will reduce by at least a factor of ten the total tracking time. The implementation of the proposed incrementally adjustable in-flight tracking tab will rapidly minimize the once-per-rev helicopter vibration forward flight. This will lead to reduced crew fatigue, increased component life, and reduced maintenance for the helicopter. Additionally, the proposed method will also be serve as emergency system to reduce unbalanced vibrations due to ballistic damage of the rotor blade .

Forward-Flight Analysis of Slatted Rotors Using Navier-Stokes Methods

Abstract: The objective of this study is to evaluate the aerodynamic effects of leading-edge slats on rotor blades under high-speed forward- ight conditions. A partial-span leading-edge slat is proposed for the inboard portion of the existing UH-60A rotor. Some two-dimensional slat studies are done to optimize the slat orientation at low subsonic Mach numbers. A multizone unsteady three-dimensional compressible Navier ­ Stokes solver is developed to compute such rotor/slat conŽ gurations. The rotor wake is captured from Ž rst principles. Some forward- ight results are presented for the UH-60A slatted rotor and compared with the baseline rotor case. The surface pressures and the force coefŽ cients indicate that the slat is beneŽ cial in reducing the retreating side dynamic stall. It was found that the slat reduces the torque and pitching moments on the retreating side compared to the baseline conŽ guration.