Showing papers on "Helicopter rotor published in 2001"

Location of rotating sources by phased array measurements

Abstract: A method is described for the location of moving sources by a microphone array. This method can be applied to out-of-flow measurements in an open jet wind tunnel. For that purpose, an expression is derived for the pressure field of a moving monopole in a uniform flow. It is argued that the open jet shear layer does not form a serious obstacle. A technique is described for reconstruction of power spectra with high signal/noise ratio. The method was implemented for rotating sources, resulting in the computer program ROSI ("Rotating Source Identifier"). Applications of ROSI are given for rotating whistles, blades of a helicopter in hover and wind turbine blades. The test with the rotating whistles demonstrated convincingly the capability to reconstruct the emitted sound. On the helicopter blades, rotating broadband noise sources were made clearly visible. On the wind turbine blades, noise emitted from the leading and trailing edge could be distinguished well. Paper presented at the 7th AIAA/CEAS Aeroacoustics Conference, Maastricht, The Netherlands, 28-30 May 2001.

Wind tunnel testing of a Mach-scaled rotor model with trailing-edge flaps

Abstract: This paper presents the wind tunnel testing of a four-bladed Mach-scaled rotor model with piezoelectric bender actuated trailing-edge flaps. Correctly phased, this flap motion can be used for the active suppression of vibratory hub loads. First, the University of Maryland Advanced Rotorcraft Code was used to conduct a parametric study to determine the optimal flap sizing and location as well as the required deflection amplitudes. Next, a simplified rotor analysis which explicitly models the actuator dynamics was used to design a multi-layer actuator configuration that is capable of achieving the required flap deflections. Based on the above design studies, a matched set of four Mach-scaled rotor blades with piezo-bender actuated trailing-edge flaps were fabricated in-house. This rotor model was operated using a one-seventh scale Bell-412 Mach-scaled rotor hub. Finally, the rotor system was tested in forward flight in the Glenn L Martin wind tunnel. These tests consisted of open-loop single-frequency tests at different rotor speeds and collective settings. Some preliminary closed-loop tests using a neural network control algorithm were also conducted. Presented at the American Helicopter Society 56th Annual Forum, Virginia Beach, VA. Copyright 2000 by the American Helicopter Society, Inc. All rights reserved.

Development of a piezoelectric actuator for trailing edge flap control of full scale rotor blades

Abstract: The present paper covers the development of a piezoelectric actuator for trailing edge flap control on a 34 ft diameter helicopter main rotor. The design of an actuator using biaxial stack columns, and its bench, shake, and spin testing are described. Actuator bench testing proves the basic actuator concept, but also points to required performance improvements. Actuator robustness is demonstrated in shake and spin tests simulating the full range of dynamic conditions inside the rotor blade. A series of actuator improvements are implemented, resulting in almost doubled performance. Projections using the latest stack technology show that the improved actuator will meet the performance requirements. The next steps in this program are development of the actuator and full scale rotor system for whirl tower testing and flight testing on the MD Explorer. Presented at SPIE's Symposium on Smart Structures and Materials, Paper Number 3668-104, Newport Beach, 1999.

In-flight tracking of helicopter rotor blades using shape memory alloy actuators

Abstract: This paper describes the concept, design, fabrication and control of a shape memory alloy (SMA) wire actuator for tracking helicopter rotor blades while in-flight. A NACA 0012 wing section that has a 12 in chord and span was constructed with a trailing-edge tab with a 4 in span and a 2.4 in chord. A shape memory alloy wire actuator was embedded into the wing section. The actuator consists of a wire clamp, a hinge tube and several pre-strained, 0.015 in diameter SMA (Nitinol) wires. It was shown that with SMA wires that have 3.158% initial pre-strain, a tab deflection of 29° could be obtained.

Dynamic response of active twist rotor blades

Abstract: Dynamic characteristics of active twist rotor (ATR) blades are investigated analytically and experimentally in this paper. The ATR system is intended for vibration and potentially for noise reductions in helicopters through individual blade control. An aeroelastic model is developed to identify frequency response characteristics of the ATR blade with integral, generally anisotropic, strain actuators embedded in its composite construction. An ATR prototype blade was designed and manufactured to experimentally study the vibration reduction capabilities of such systems. Several bench and hover tests were conducted and those results are presented and discussed here. Selected results on sensitivity of the ATR system to collective setting (i.e. blade loading), blade rpm (i.e. centrifugal force and blade station velocity), and media density (i.e. altitude) are presented. They indicated that the twist actuation authority of the ATR blade is independent of the collective setting up to approximately 10P, and dependent on rotational speed and altitude near the torsional resonance frequency due to its dependency on the aerodynamic damping. The proposed model captures very well the physics and sensitivities to selected test parameters of the ATR system. The numerical result of the blade torsional loads show an average error of 20% in magnitude and virtually no difference in phase for the blade frequency response. Overall, the active blade model is in very good agreement with the experiments and can be used to analyze and design future active helicopter blade systems.

Helicopter in-flight rotor tracking system, method, and smart actuator therefor

Abstract: A system for quasi-statically correcting tracking of rotor blades of a helicopter rotor during operation includes a trim tab mounted on each rotor blade and deflectable relative to the rotor blade so as to change a tracking path of the rotor blade, and a rotor track and balance analyzer that includes sensors for acquiring rotor tracking and balance information and is operable to process the rotor tracking and balance information so as to predetermine a new tab position to which the trim tab should be deflected for bringing the tracking path of a mistracking rotor blade into alignment with a reference path The trim tab is deflected by an actuator mounted to the rotor blade, the actuator having an actuating element coupled with the trim tab and formed of a smart material such that the actuating element deforms proportional to a stimulus applied to the actuating element An electrical locking device is included which is operable when de-energized to lock the trim tab against deflection, and operable when energized to unlock to permit the trim tab to be deflected A trim actuation control system in communication with the rotor track and balance analyzer and the locking device is arranged to energize the locking device and to supply electrical power to the actuator so as to deflect the trim tab to the new tab position predetermined in the rotor track and balance analyzer, and to then de-energize the locking device and cut off the electrical power to the actuator Thus, electrical power is supplied to the actuator only while the trim tab is being deflected, and is then discontinued once the tab has reached the desired position and the locking device has been de-energized to lock the tab against further movement

A Fuzzy Logic System for Ground Based Structural Health Monitoring of a Helicopter Rotor Using Modal Data

Abstract: A fuzzy logic system (FLS) is developed for ground based health monitoring of a helicopter rotor blade. Structural damage is modeled as a loss of stiffness at the damaged location that can result from delamination. Composite materials, which are widely used for fabricating rotor blades, are susceptible to such delaminations from barely visible impact damage. The rotor blade is modeled as an elastic beam undergoing transverse (flap) and inplane (lag) bending, axial and torsion deformations. A finite element model of the rotor blade is used to calculate the change in blade frequencies (both rotating and nonrotating) because of structural damage. The measurements used for health monitoring are the first four flap (transverse bending) frequencies of the rotor blade. The measurement deviations due to damage are then fuzzified and mapped to a set of faults using a fuzzy logic system. The output faults of the fuzzy logic system are four levels of damage (undamaged, slight, moderate and severe) at five locations along the blade (root, inboard, center, outboard, tip). Numerical results with noisy data show that the FLS detects damage with an accuracy of 100% for noise levels below 15% when nonrotating frequencies are used. The FLS also correctly classifies the ‘‘undamaged’’ condition up to noise levels of 30% thereby reducing the possibility of false alarms, a key problem for diagnostics systems. The fuzzy logic approach is thus able to extract maximum information from very limited and uncertain data. Using rotating frequencies lowers the success rate for small damage because the centrifugal stiffening caused by rotation counters the stiffness reduction caused by structural damage. The fuzzy logic system in this study is proposed as an information-processing tool to help the maintenance engineer by locating the damage area roughly but accurately for further nondestructive inspections.

Model Based Fault Identification in Rotor Systems by Least Squares Fitting.

Abstract: In the present paper a model based method for the on-line identification of malfunctions in rotor systems is proposed. The fault-induced change of the rotor system is taken into account by equivalent loads which are virtual forces and moments acting on the linear undamaged system model to generate a dynamic behaviour identical to the measured one of the damaged system.

Analysis of a bending-torsion coupled actuator for a smart rotor with active blade tips

Abstract: Active rotorblade tips offer an alternative approach to the challenge of main rotor active vibration control. The tips are pitched with respect to the main blade via a piezo-driven bending-torsion coupled actuator beam that runs down the length of the blade. A Vlasov based, specialized one-dimensional finite beam element is developed to model the rotating actuator beam and is validated with the free-vibration and static forced response of 4:1 and 2:1 aspect ratio, bending-torsion coupled, active and passive plates. A one-eighth scale, reduced tip-speed rotor model (tip Mach 0.26), incorporating the bending-torsion actuator beam, has been previously hover tested (open loop). In these tests, blade tip deflections of the order of 2° (half peak-to-peak) were achieved at 2, 3, 4, 5/rev with corresponding dynamic vertical blade root shear variations of the order of 10-20% of the nominal blade lift at 8° collective (CT/σ = 0.07). The test results are used to validate a coupled actuator and elastic rotorblade model. The correlation of the predicted active blade-tip pitch deflections and the experimental data is within 20%. The predicted values for the active vertical root shears are within the same margin for 4° and 6° collective.

On the twist performance of a multiple-cell active helicopter blade

Abstract: This paper discusses how torsional stiffness is related with twist actuation in an integrally twist actuated rotor blade. It shows that increase in torsional stiffness does not necessarily reduce the twist actuation. Improvements in twist actuation for a realistic airfoil-shaped beam can be achieved while controlling the blade torsional stiffness within aeroelastic requirements. These design trends can be studied once an appropriate anisotropic beam analytical formulation with integral anisotropic piezoelectric actuators is developed. The new analytical model revises and extends the closed form solution for thin-walled multi-cell beams based on the variational-asymptotical method and introduces the distributed anisotropic piezoelectric actuator effects along the cross section of the blade. The model is applied to specifically investigate the relationship between twist actuation performance and torsional stiffness of a realistic active blade. In this study, both analytical expressions and numerical examples are examined. The results show that the torsional stiffness can be increased by up to 20% with an increase in twist actuation of approximately 5% from their corresponding baseline values. Moreover, the results also indicate that a single-cell model is insufficient to address qualitatively and quantitatively the complex interaction that exists among the wall members of a two-cell airfoil cross section.

Design of a variable twist tilt-rotor blade using shape memory alloy (SMA) actuators

Abstract: This paper presents research aimed at actively altering the twist distribution of a tiltrotor blade between hover and forward flight. Three different concepts-extension-twist coupled composites, bimoment actuation and discrete SMA torque tube actuation - are considered, and the torque tube appears the most feasible. Parametric design of the torque tube and attachment technique is presented with actuation torque, heat transfer and bandwidth issues being considered to arrive at the configuration of the tube. The effect of heat treatment of the SMA in tuning the actuation characteristics is discussed. A dramatic improvement in the actuation cooling time is demonstrated through the use of active cooling using thermodelectric modules. An extension of the one-dimensional formulation of Brinson's model to the torsional case is presented. The model is shown to have good correlation with room temperature characteristics. The criterion for impedance matching between the actuator and the host structure is derived. The torsional actuator is tested both under no load and acting against a restoring spring and shows repeatable actuation characteristics.

Effect of Compressibility on Suppression of Dynamic Stall Using a Slotted Airfoil

Abstract: A multielement airfoil designed for helicopter application has been tested for compressible dynamic stall behavior and has been proven to be a robust dynamic stall-free concept. This slotted airfoil has operated into poststall areas without the dynamic stall vortex that is normally present whenever airfoils are tested beyond their static stall boundary. Point diffraction interferogram images of the dynamic flow over the airfoil are presented, showing details of the flow development during the oscillation cycle, and instantaneous pressure distributions on the airfoil and slat during dynamic airfoil motion are included

Aeroelastic Considerations For Rotorcraft Primary Control with On-Blade Elevons

Abstract: Replacing the helicopter rotor swashplate and blade pitch control system with on-blade elevon control surfaces for primary flight control may significantly reduce weight and drag to improve mission performance. Simplified analyses are used to examine the basic aeroelastic characteristics of such rotor blades, including pitch and flap dynamic response, elevon reversal, and elevon control effectiveness. The profile power penalty associated with deflections of elevon control surfaces buried within the blade planform is also evaluated. Results suggest that with aeroelastic design for pitch frequencies in the neighborhood of 2/rev, reasonable elevon control effectiveness may be achieved and that, together with collective pitch indexing, the aerodynamic profile power penalty of on-blade control surface deflections may be minimized.

Vibration analysis of a misaligned rotor—coupling—bearing system passing through the critical speed:

Abstract: The transient response of a misaligned rotor—coupling—bearing system passing through the critical speed has been analysed by using the finite element method (FEM) for flexural vibrations. The coupling has been modelled in two ways: a frictionless joint and a joint with stiffness and damping. From the vibration analysis, the subcritical speeds at one-half, one-third and one-fourth the critical speed have been found when the misaligned rotor—coupling—bearing system passes through its critical speed. The continuous wavelet transform (CWT) has been used as a tool to extract the silent features from the time response of the rotor system. A parametric study has been carried out to investigate the transient response of this rotor system for different angular accelerations in different types of misalignment.

Magnetorheological fluid damper feedback linearization control for helicopter rotor application

Abstract: This study explores the possible use of a magnetorheological (MR) damper with feedback linearization control law for helicopter lag damping applications. Since this control law is based on an assumed damper model, the helicopter may be susceptible to limit cycle instabilities in the presence of model errors due to uncertainties (even if the prescribed damping using feedback linearization control is greater than the system negative damping). For specified uncertainty bounds, a robust control law can be synthesized which will eliminate limit cycle instability by using a much larger value of prescribed damping ratio. The resulting periodic loads in forward flight, however, would be excessively high. To reduce these periodic loads a band-rejecting filter is introduced that eliminates the 1/rev component of velocity from the feedback control signal. By doing so, stability of perturbation motions can be ensured and periodic loads can be drastically reduced, even in the presence of MR damper model errors.

Identification of the harmonic transfer functions of a helicopter rotor

Abstract: Identification of the time periodic dynamics of a helicopter rotor system is necessary for effective controller design and analysis. The transfer properties of a linear time periodic (LTP) system can be described by harmonic transfer functions (HTF) that give the input-output relationship between the Fourier coefficients of the input signal and those of the output signal. A method for identifying harmonic transfer functions of linear time periodic systems is developed in this thesis. The system identification scheme employs a least square estimation technique to obtain HTF estimates. This least square estimation problem is underdetermined; therefore, an additional assumption, that the transfer function is smooth, is made in the ID scheme to achieve a well-posed problem. The estimates are calculated by applying a quadratic penalty to the curvature of the transfer functions. The identification scheme has been implemented in MATLAB, and partly coded in C programming language for maximum computational efficiency. This system ID method has been validated with analytical results for a few well-known LTP systems. The validation results show excellent agreement between the identified and analytical transfer functions. Thesis Supervisor: Steven R. Hall Title: Professor of Aeronautics and Astronautics

Multigrid acceleration of an upwind Euler method for hovering rotor flows

Abstract: The effect of multigrid acceleration implemented within an upwind-biased Euler method for hovering rotor flows is presented. The requirement to capture the vortical wake development over several turns means a long numerical integration time is required for hovering rotors, and the solution (wake) away from the blade is significant. Furthermore, the flow in the region near the blade root is effectively incompressible. Hence, the solution evolution and convergence is different to a fixed wing case where convergence depends primarily on propagating errors away from the surface as quickly as possible, and multigrid acceleration is shown to be less effective for hovering rotor flows. It is found that a simple V-cycle is the most effective, smoothing in the decreasing mesh density direction only, with a relaxed trilinear prolongation operator. Results are presented for multigrid computations with 2, 3, 4, and 5 mesh levels, and a CPU reduction of approximately 80% is demonstrated for five mesh levels.

Blade-vortex interaction noise reduction with active twist smart rotor technology

Abstract: The results of this analytical feasibility study suggest that active blade twist technology is a viable means to reduce blade-vortex interaction (BVI) noise in rotorcraft systems. A linearized unsteady aerodynamics analysis was formulated and successfully validated with computation fluid dynamics (CFD) analysis. A simple control scheme with three control points was found to be effective for active BVI noise reduction. Based on current-day actuation technology where one to two degrees of twist per blade activation span is expected, measurable noise reductions of 2-4 dB were predicted for the relatively strong, close vortex interactions. For weaker vortex interactions, reductions of 7-10 dB were predicted. The required twist actuation per blade span for complete unsteady loading cancellation, however, may be infeasible because of the large stroke and high-frequency activation requirements.

The Analytical Imbalance Response of Jeffcott Rotor During Acceleration

Abstract: The characteristics of the transition vibration of a rotor system when it passes its critical speeds during acceleration are of great interest for active vibration control, active real-time balancing @1#, and rotor design. In the past, a few analyses @2‐8# have dealt with speed varying transient rotor dynamics. These researchers used numerical integration techniques to calculate numerical solutions to the transient dynamic model. Although these models can be used to predict the transient vibration for a complicated rotor system, it remains hard to obtain the quantitative characteristics of the transient vibration. Lewis @9# and Dimentberg @10# presented an analytical solution of the problem of running a rotor system through its critical speeds at a uniform acceleration. The basic characteristic of the ‘‘envelope’’ ~amplitude! of the transient vibration was studied by an approximation method. In this paper, their work is extended. An analytical expression of the motion of the geometric center of a simple Jeffcott rotor is derived. The exact ‘‘envelope’’ and ‘‘phase’’ of the transient vibration are presented. As stated in Dimentberg @10#, it is found that the transient vibration through critical speeds consists of free vibration and synchronous vibration. Explicit expressions of these two components are presented in this paper.

Tiltrotor Descent Aerodynamics: A Small-Scale Experimental Investigation of Vortex Ring State

Abstract: An experimental investigation was completed in the NASA Ames 80- by 120-Foot Wind Tunnel with the objective of studying the aerodynamic characteristics of a tiltrotor at high descent angles in helicopter mode. A specific objective was to determine whether tiltrotors behave differently from helicopter rotors when operating in the Vortex Ring State (VRS). A single, 4-foot diameter, 3-bladed rotor, with twist and solidity similar to that of rotors on current tiltrotor aircraft, was tested with an image plane to simulate the mean effect of a second rotor. Rotor performance data were obtained over a wide range of simulated descent conditions, both with and without a semispan wing. Results of this study are presented, including changes in both mean and oscillatory rotor thrust, which are reasonably consistent with helicopter rotor VRS characteristics. Large reductions in mean rotor thrust were found in VRS. These were associated with high levels of low-frequency rotor thrust fluctuations. The image plane had a significant effect on the results, while the wing had a relatively small effect. Implications of these results for the safe operation of tiltrotor aircraft are discussed.

Damage detection testing on a helicopter flexbeam

Abstract: The main and tail rotor systems of a helicopter must be maintained in a damage-free condition to ensure the safety of the helicopter. The components of the rotor system require regular maintenance and inspection, and some of the components are difficult to inspect. Accordingly, an integrated sensor system that continuously monitors the structural integrity of the rotor system is needed to assure the safety of flight and reduce the cost of maintenance of the rotor system. This smart monitoring system could also provide immediate information on the dynamic loading in the rotor system. This information could define the performance limit of the helicopter based on the condition of the rotor system. The operator will then be able to adjust the flight condition and prevent damage from occurring or propagating. The challenges in designing this monitoring system are related to the operating conditions of the helicopter. These include significant aerodynamic and external damping in bending and torsion modes, moder...

A study of rotor tip vortex structure alteration techniques

Abstract: Numerical studies of the tip-vortex structure from a hovering rotor with blowing are presented, and compared with the tip-vortex structure from a clean rotor, and from a rotorusing a passive trailing-edgetip device. A hybrid, Navier‐Stokes potential e ow method is used to model the e owe eld. A scheme that is e fth-order accurate in space is used to accurately capture the tip vortices. Velocity and vorticity data in the core of the vortex are studied at various planes behind the blade trailing edge. These data for the clean rotor are e rst compared with experimental results obtained for the same rotor. Previously published results for the tip-vortex structure from the same rotor employing a passive tip device are discussed next. Finally, results for a rotor blade with upper and lower surface blowing are presented. It is concluded that the tip-vortex strength may be modie ed through blowing. Blowing is found to be just as effective as a spoiler in altering the tip-vortex strength, but does not have the high drag and torque penalty associated with spoilers.

Effect of Wake Adaptation on Rotor Hover Simulations Using Unstructured Meshes

Abstract: A three-dimensional inviscid flow solver is developed for simulating the flowfield of hovering helicopter rotor using unstructured meshes. The flow solver utilizes a cell-centered finite volume scheme that is based on the Roe's flux-difference splitting with an implicit Jacobi/Gauss-Seidel time integration. Calculations are performed at two operating conditions of subsonic and transonic tip Mach numbers. A solution-adaptive mesh refinement technique is adopted to improve the resolution of flow features on the blade surface. It is demonstrated that the trajectory of the tip vortex can be captured through a series of adaptive mesh refinements starting from a very coarse initial grid