Showing papers on "Helicopter rotor published in 2009"

Bifurcation Behavior of Airfoil Undergoing Stall Flutter Oscillations in Low-Speed Wind Tunnel

Abstract: Stall flutter is a nonlinear aeroelastic phenomenon that can affect several types of aeroelastic systems such as helicopter rotor blades, wind turbine blades, and highly flexible wings. Although the related aerodynamic phenomenon of dynamic stall has been the subject of many experimental studies, stall flutter itself has rarely been investigated. This paper presents a set of experiments conducted on a NACA0012 airfoil undergoing stall flutter oscillations in a low-speed wind tunnel. The aeroelastic responses are analyzed with the objective of characterizing the local bifurcation behavior of the system. It is shown that symmetric stall flutter oscillations are encountered as a result of a subcritical Hopf bifurcation, followed by a fold bifurcation. The cause of these bifurcations is the occurrence of dynamic stall, which allows the transfer of energy from the freestream to the wing. A second bifurcation occurs at the system's static divergence airspeed. As a consequence, the wing starts to undergo asymmetric stall flutter bifurcations at only positive (or only negative) pitch angles. The dynamic stall mechanism itself does not change but the flow only separates on one side of the wing.

Multiple-Surrogate Approach to Helicopter Rotor Blade Vibration Reduction

Abstract: The advantages of using multiple surrogates for approximation andreduction of helicopter vibration are studied. Multiple approximation methods, including a weighted-average approach, are considered so that pitfalls associated with only using a single best surrogate for the rotor blade vibration-reduction problem are avoided. A vibration objective function corresponding to a flight condition in which blade–vortex interaction causes high levels of vibration is considered. The design variables consist of cross-sectional dimensions of the structural member of the blade and nonstructural masses. The optimized designs are compared with a baseline design resembling a Messerschmitt–Bolkow–Blohm BO-105 blade. The results indicate that at relatively little additional cost compared with optimizing a single surrogate, multiple surrogates can be used to locate various reduced-vibration designs that wouldbeoverlookedifonly asingleapproximationmethodwasemployed,andthemostaccurate surrogatemaynot lead to the best design.

Real-Time Stabilization of an Eight-Rotor UAV Using Optical Flow

Abstract: An original configuration of a small aerial vehicle having eight rotors is presented. Four rotors are devoted to the stabilization of the orientation of the helicopter, and the other four are used to drive the lateral displacements. A precompensation on the roll and pitch angles has been introduced so that the attitude dynamics is practically independent of the translational dynamics. This compensation is directly related to the velocity of the lateral motors. The dynamical model is obtained using the Euler-Lagrange approach. The proposed configuration is particularly useful for image processing since the the camera orientation is held constant. The eight-rotor rotorcraft is simpler to pilot than other rotorcrafts. A control strategy is proposed that uses the optical flow measurements to achieve a hover flight that is robust with respect to perturbations like wind. The new aerial configuration and control strategy have been tested in real-time experiments.

Computational Study of Helicopter Rotor-Fuselage Aerodynamic Interactions

Abstract: Aerodynamic interactions between the main rotor, fuselage, and tail rotor must be considered during the design phase of a helicopter, and their effect on performance must be quantified. However, interactional helicopter aerodynamics has so far been considered by very few researchers. In this work, the Helicopter Multi-Block flow solver is used to investigate the flow around two generic rotor–fuselage cases before moving on to a more realistic fullhelicopter geometry under investigation in the European Commission Framework 6 Generation of Advanced Helicopter Experimental Aerodynamic Database project. A comparison of the computational fluid dynamics results obtained with experimental data shows that the method is capable of resolving the main interactional flow features for the generic cases. A similar comparison with experimental data for the Generation of Advanced Helicopter Experimental Aerodynamic Database test case has not yet been conducted, but the obtained results show that even for a test case of high complexity, state-of-the-art rotorcraft computational fluid dynamics methods are capable of providing realistic predictions. However, comparisons with isolated rotor cases clearly show the increased loading as the blade passes over the nose of the helicopter as the result of a fuselage-induced upwash. Similarly, the fuselage induces a reduction of the blade loading for inboard stations when the blades passes through the rear part of the rotor disk. The present results highlight and quantify the radial and azimuthal extent of the rotor–fuselage interactional effect on the rotor loading.

Steady-state dynamics of a non-ideal rotor with internal damping and gyroscopic effects

Abstract: A rotor driven by an ideal source, i.e., a source capable of delivering unlimited amount of power, becomes unstable beyond a certain threshold spin speed due to non-conservative circulatory forces. The circulatory forces considered in this paper arise out of rotating internal damping. If the drive is non-ideal then the rotor spin speed cannot exceed the stability threshold. This phenomenon is a type of the Sommerfeld effect. In this work, a DC motor driving four-degrees-of-freedom rotor with internal damping and gyroscopic effects is considered and the corresponding steady-state spin frequency and the whirl orbit amplitude are analytically derived as functions of the parameters of the drive and the rotor system.

Euler and Navier-Stokes Simulations of Helicopter Rotor Blade in Forward Flight Using an Overlapped Grid Solver

Abstract: Three-dimensional Euler and Navier-Stokes equations are solved using an overlapped grid system to compute the unsteady flow field around helicopter rotor blade in forward flight. To verify the automated overlapped grid solver, the unsteady flow field and airloads on Caradonna-Tung NACA0012 and AH-1G rotor blade models in forward flight were computed. Two equation turbulence model k-ω Wilcox Durbin(WD+) is used. For spatial discretization Roe FDS(Flux Difference Splitting) scheme and Weighted ENO(Essentially Non-Oscillatory) scheme are applied. Good agreement between computed and measured surface pressure distributions was seen for the Caradonna and Tung rotor’s forward flight simulation. The effects of background wake grid spacing, spatial accuracy and viscosity were examined in the AH-1G rotor simulation. Surface pressure distributions, sectional thrust and oscillatory pitching moment coefficients were compared with flight test data. A better variation for sectional thrust and oscillatory pitching moment were found by using the Navier-Stokes analysis.

Potential Panel and Time-Marching Free-Wake-Coupling Analysis for Helicopter Rotor

Abstract: A potential-based panel method coupled with advanced time-marching free-wake techniques is developed to achieve fast and accurate prediction for unsteady aerodynamics and wake dynamics of helicopter rotating blades. This coupling analysis is enabled by using the equivalence of the doublet-wake panels and the vortex filaments. The coupled panel method allows the inclusion of the self-induced velocity of curved vortex filaments and high-order time integration for the computation of wake convection. A parallel computation is applied to the wake convection for fast numerical calculation. The computation of the induced velocity from each vortex filament is parallelized and computed separately. The velocity-field integration technique is used to avoid numerical singularity during the interaction between the wake and blades. It is found that blade-pressure predictions and the wake roll-up agree well with the measured data for helicopter rotors, both in hover and forward flight. Tip-vortex pairing phenomena are also predicted and compared with the measured data.

Wind Tunnel Test of the SMART Active Flap Rotor

Abstract: Boeing and a team from Air Force, NASA, Army, DARPA, MIT, UCLA, and U. of Maryland have successfully completed a wind-tunnel test of the smart material actuated rotor technology (SMART) rotor in the 40- by 80-foot wind-tunnel of the National Full-Scale Aerodynamic Complex at NASA Ames Research Center. The Boeing SMART rotor is a full-scale, five-bladed bearingless MD 900 helicopter rotor modified with a piezoelectric-actuated trailing edge flap on each blade. The eleven-week test program evaluated the forward flight characteristics of the active-flap rotor at speeds up to 155 knots, gathered data to validate state-of-the-art codes for rotor aero-acoustic analysis, and quantified the effects of open and closed loop active flap control on rotor loads, noise, and performance. The test demonstrated on-blade smart material control of flaps on a full-scale rotor for the first time in a wind tunnel. The effectiveness of the active flap control on noise and vibration was conclusively demonstrated. Results showed significant reductions up to 6dB in blade-vortex-interaction and in-plane noise, as well as reductions in vibratory hub loads up to 80%. Trailing-edge flap deflections were controlled within 0.1 degrees of the commanded value. The impact of the active flap on control power, rotor smoothing, and performance was also demonstrated. Finally, the reliability of the flap actuation system was successfully proven in more than 60 hours of wind-tunnel testing.

System Identification of a Miniature Helicopter

Abstract: Miniature helicopters provide a unique design choice for performing missions autonomously, vital to which is knowledge of a dynamic model to enable the development of control and state estimation algorithms. Toward these goals, a linear model for a miniature electric helicopter in hovering flight was identified from data obtained from a visual positioning system. Model structure determination was performed using first principles, previous work, and stepwise regression. Model parameters were computed using a two-step equation-error/output-error process in both the time and frequency domains, resulting in two complete sets of parameter estimates. Modal characteristics and predictive capability of the identified models were compared with those of a model previously identified for this aircraft using the frequency-response method.

Monitoring of wind turbines

Abstract: Method and apparatus for determining the deflection or curvature of a rotating blade, such as a wind turbine blade or a helicopter blade. Also, methods and apparatus for establishing an inertial reference system on a rotating blade.

SMART Rotor Development and Wind Tunnel Test

Abstract: Boeing and a team from Air Force, NASA, Army, Massachusetts Institute of Technology, University of California at Los Angeles, and University of Maryland have successfully completed a wind-tunnel test of the smart material actuated rotor technology (SMART) rotor in the 40- by 80-foot wind-tunnel of the National Full-Scale Aerodynamic Complex at NASA Ames Research Center, figure 1. The SMART rotor is a full-scale, five-bladed bearingless MD 900 helicopter rotor modified with a piezoelectric-actuated trailing-edge flap on each blade. The development effort included design, fabrication, and component testing of the rotor blades, the trailing-edge flaps, the piezoelectric actuators, the switching power amplifiers, the actuator control system, and the data/power system. Development of the smart rotor culminated in a whirl-tower hover test which demonstrated the functionality, robustness, and required authority of the active flap system. The eleven-week wind tunnel test program evaluated the forward flight characteristics of the active-flap rotor, gathered data to validate state-of-the-art codes for rotor noise analysis, and quantified the effects of open- and closed-loop active-flap control on rotor loads, noise, and performance. The test demonstrated on-blade smart material control of flaps on a full-scale rotor for the first time in a wind tunnel. The effectiveness and the reliability of the flap actuation system were successfully demonstrated in more than 60 hours of wind-tunnel testing. The data acquired and lessons learned will be instrumental in maturing this technology and transitioning it into production. The development effort, test hardware, wind-tunnel test program, and test results will be presented in the full paper.

Dynamic response of a rub-impact rotor system under axial thrust

Abstract: A model of a rigid rotor system under axial thrust with rotor-to-stator is developed based on the classic impact theory and is analyzed by the Lagrangian dynamics. The rubbing condition is modeled using the elastic impact-contact idealization, which consists of normal and tangential forces at the rotor-to-stator contact point. Mass eccentricity and rotating speed are used as control parameters to simulate the response of rotor system. The motions of periodic, quasi-periodic and chaotic are found in the rotor system response. Mass eccentricity plays an important role in creating chaotic phenomena.

The AVINOR Aeroelastic Simulation Code and its Application to Reduced Vibration Composite Rotor Blade Design

Abstract: The Active Vibration and Noise Reduction (AVINOR) aeroelastic simulation code for helicopter rotor blades is described. AVINOR is a research code that has been developed at UCLA and the University of Michigan for the purpose of conducting computationally ecient aeroelastic response analyses while maintaining a level of fidelity so as to be suitable for preliminary design of helicopter rotor blades. The current capabilities of AVINOR are illustrated by considering aeroelastic tailoring of a composite rotor blade for minimum vibration over the entire flight envelope. The AVINOR composite blade model is based on geometrically nonlinear kinematics suitable for moderate deflection analysis and the University of Michigan/Variational Asymptotic Beam Sectional (UM/VABS) analysis is used for cross-sectional modeling. A surrogate-based optimization (SBO) approach is utilized to locate a blade design which exhibits the best trade-o between reducing vibration due to blade-vortex interaction (BVI) at low advance ratios, and alleviating vibration due to dynamic stall which occurs at high advance ratios.

Tilt-rotor plane operated and propelled by thrust scull and slipstream rudder

Abstract: A tilt rotor aircraft adopting a thrust tail rotor and a slipstream rudder for operation and propelling and a design of two parallel rotary wings and common pneumatic distribution comprises an airframe, wings, an empennage, a system of thrust tail rotor and slipstream rudder, an undercarriage, a power-fuel system, a transmission system, a rotary wing system, a rotary-wing nacelle and a tilt rotor system of the rotary-wing nacelle The aircraft adopts the system of the thrust tail rotor and slipstream rudder to operate the VTOL and the pitching and drifting of the forward flight; the system comprising the thrust tail rotor, an elevator and a rudder is arranged on the empennage; a tilting wingtip appears as a small-area wing which is arranged on the outside of the rotary-wing nacelle and rotates together with the rotary-wing nacelle; the plane shape of the tilting wingtip appears as a trapezoid of leading edge sweepback and trailing edge sweepforward and the aspect ratio is 15 and the area of the tilting wingtip occupies about 15 to 20 percent of the whole wing area and the tilting wingtip is just integrated with the appearance of the rotary-wing nacelle and fixedly connected with a nacelle bevel gear box The tilt rotor aircraft is a novel aircraft type with the development potential and prosperous prospect

Prediction of UH-60A Structural Loads Using Multibody Analysis and Swashplate Dynamics

Abstract: The first part of this paper compares three rotor blade structural dynamic formulations: a finite element formulation with modal reduction, a full finite element formulation without modal reduction, and a multibody-based full finite element formulation for arbitrary large deformations. The second part of this paper studies the effect of swashplate dynamics on blade loads and servo-actuator loads. In all cases, measured airloads, damper force, and control pitch angles from the UH-60A flight tests are used to predict and analyze the structural loads. In the first part, the emphasis is on the validation of a multibody formulation, which is first verified with analytical solutions for beams undergoing hypothetical large deformations (elastica), then validated with the Princeton beam large deformation tests, and then finally used to predict the UH-60A structural loads. Two flight conditions are considered: a high-speed, high-vibration flight and a highly loaded dynamic stall flight. Predictions from the multibody analysis are compared with the full finite element and finite element based modal methods. It is observed that the predicted blade loads do not show any significant difference between the three formulations. In the second part, the four-bladed multibody rotor model is coupled to a swashplate-servo model to predict servo loads and to study the effect of swashplate dynamics on blade loads. It is observed that the higher frequencies of servo loads, 8/rev and 12/rev for this rotor, require modeling the swashplate dynamics. The low-frequency component, which is a dominant 4/rev load for this rotor, is less affected by swashplate dynamics and is determined primarily by the accuracy of the 3, 4, and 5/rev pitch-link loads. The 3-5/rev pitch-link loads, and in general the structural loads on the rotor blade, are not affected by swashplate dynamics.

On a Bistable Flap for an Airfoil

Abstract: This paper presents the design and wind tunnel testing results of a full scale helicopter rotor blade section with an electromechanically actuated bistable trailing edge flap. The bistability of the flap derives from the use of bistable composite material. In one state the flap follows the profile of the standard rotor blade section. In its second stable state the flap deflects the trailing edge downwards by ten degrees. The flap system is designed to change between these two positions when the helicopter moves between hover and forward flight conditions. The bistability of the flap system means that the electromechanical actuator is only required to do work to transit between these two stable states, not to maintain the states. Tests were conducted in the University of Bristol’s large low speed wind tunnel. These open loop tests consisted of measurements of lift, drag, and pitching moment when the flap is in both stable states. Some preliminary transitory analysis was also conducted.

Computational Investigation of Gurney Flap Effects on Rotors in Forward Flight

Abstract: A hybrid Navier-Stokes/free-wake solver has been employed to investigate the performance of a rotor equipped with a Gurney flap in steady level flight and in descent. A scaled model of the BO-105 rotor was studied. The calculations were coupled to a comprehensive analysis to properly account for the effects of the elastic deformations on the aerodynamic loads and to account for trim. Fixed and dynamically deployed Gurney flaps were considered. In forward flight, it was found that a properly chosen azimuthal deployment schedule of the Gurney flaps may reduce the peak-to-peak variations in hub loads, potentially reducing vibrations. In descent flight, it was found that the deployment of a fixed Gurney flap decreased the descent rate needed to maintain autorotation.

Synchronous Response to Rotor Imbalance Using a Damped Gas Bearing

Abstract: One type of test performed for evaluating bearings for application into turbomachinery is the synchronous bearing response to rotor imbalance. This paper presents rotordynamic tests on a rotor system using a 70 mm diameter damped gas bearing reaching ultra-high speeds of 50,000 rpm. The main objective of the study was to experimentally evaluate the ability of the damped gas bearing to withstand large rotor excursions and provide adequate damping through critical speed transitions. Two critical speeds were excited through varying amounts and configurations of rotor imbalance while measuring the synchronous rotordynamic response at two different axial locations. The results indicated a well-damped rotor system and demonstrated the ability of the gas bearing to safely withstand rotor vibration levels while subjected to severe imbalance loading. Also, a waterfall plot was used to verify ultra-high-speed stability of the rotor system throughout the speed range of the test vehicle. In addition to the experimental tests, a rotordynamic computer model was developed for the rotor-bearing system. Using the amplitude/ frequency dependent stiffness and damping coefficients for the ball bearing support and the damped gas-bearing support, a pseudononlinear rotordynamic response to imbalance was performed and compared with the experiments.

Active Flap Control of the SMART Rotor for Vibration Reduction

Abstract: Active control methodologies were applied to a full-scale active flap rotor obtained during a joint Boeing/ DARPA/NASA/Army test in the Air Force National Full-Scale Aerodynamic Complex 40- by 80-foot anechoic wind tunnel. The active flap rotor is a full-scale MD 900 helicopter main rotor with each of its five blades modified to include an on-blade piezoelectric actuator-driven flap with a span of 18% of radius, 25% of chord, and located at 83% radius. Vibration control demonstrated the potential of active flaps for effective control of vibratory loads, especially normal force loads. Active control of normal force vibratory loads using active flaps and a continuous-time higher harmonic control algorithm was very effective, reducing harmonic (1-5P) normal force vibratory loads by 95% in both cruise and approach conditions. Control of vibratory roll and pitch moments was also demonstrated, although moment control was less effective than normal force control. Finally, active control was used to precisely control blade flap position for correlation with pretest predictions of rotor aeroacoustics. Flap displacements were commanded to follow specific harmonic profiles of 2 deg or more in amplitude, and the flap deflection errors obtained were less than 0.2 deg r.m.s.

Optimal Placement of Piezoelectric Actuated Trailing-Edge Flaps for Helicopter Vibration Control

Abstract: This study aims to determine the optimal locations for dual trailing-edge flaps on a helicopter blade in the presence of actuator hysteresis. An aeroelastic analysis based on a finite element approach in space and time is used in conjunction with an optimal control algorithm to determine the actuator control input for vibration minimization. The reduced hub-vibration level and the flap power are the two optimization indices considered in this study. The location of the flaps along the blade are the design variables. The hysteresis in the piezo-actuators is modeled using a dynamic hysteresis model based on an extension to the classical Preisach model. It is found that second-order polynomial response surfaces based on the central composite design of the theory of design of experiments describe both objectives adequately. Numerical studies for a four-bladed hingeless rotor show that both objectives are more sensitive to outboard flap location compared with inboard flap location. Optimization studies indicate that the dual-flap configuration for the least hub-vibration level is different from the configuration for the least flap power. The Pareto front between the two objectives is found to be discontinuous. However, a reasonable tradeoff configuration is obtained by careful inspection of the Pareto front. This configuration yields about a 70% reduction in hub-vibration levels from the baseline conditions at an advance ratio of 0.30, while requiring about 21% more flap power from the initial configuration of the optimization study.

Robust decoupling control design for twin rotor system using Hadamard weights

Abstract: This paper considers cross-coupling affects in a twin rotor system that leads to degraded performance during precise helicopter maneuvering. This cross-coupling can be suppressed implicitly either by declaring it as disturbance or explicitly by introducing decoupling techniques. The standard H ∞ controller synthesized by loop-shaping design procedure (LSDP) offers robustness at the cost of performance to overcome cross-coupling. However, Hadamard weights are used to decouple the system dynamics to give desired performance as well. This idea has been successfully proved by simulations and verified through implementing it on a twin rotor system.

Theory of Adaptive Fiber Composites: From Piezoelectric Material Behavior to Dynamics of Rotating Structures

Abstract: Adaptive structural systems in conjunction with multifunctional materials facilitate technical solutions with a wide spectrum of applications and a high degree of integration. By virtue of combining the actuation and sensing capabilities of piezoelectric materials with the advantages of fiber composites, the anisotropic constitutive properties may be tailored according to requirements and the failure behavior can be improved. Such adaptive fiber composites are very well-suited for the task of noise and vibration reduction. In this respect the helicopter rotor system represents a very interesting and widely perceptible field of application. The occurring oscillations can be reduced with aid of aerodynamic couplings via fast manipulation of the angle of attack, being induced by twist actuation of the rotor blade. On the one hand the sensing properties may be used to determine the current state of deformation, while on the other hand the actuation properties may be used to attain the required state of deformation. The implementation of such concepts requires comprehensive knowledge of the theoretical context, which shall be illuminated in the work at hand from the examination of the material behavior to the simulation of the rotating structure.