Showing papers on "Helicopter rotor published in 2004"

Stabilization of a mini-rotorcraft having four rotors

Abstract: In this paper we present a controller design and implementation on a mini-rotorcraft having four rotors A Lagrangian model of the helicopter was used for the controller synthesis The proposed controller is based on Lyapunov analysis Experimental results show that the controller is able to perform autonomously the tasks of taking-off, hovering and landing

Smart Material-Actuated Rotor Technology – SMART

Abstract: Vibration, noise, and aerodynamic design compromises are primary barriers to further improvements in effectiveness of the helicopter. The MD900 light utility helicopter main rotor system is modified to include in-blade smart material actuation for active control. A piezoelectric (PE)-driven trailing edge flap is used for vibration, noise, and aerodynamic performance improvements. A shape memory alloy (SMA)-driven trailing edge trim tab is used for in-flight blade tracking.Sizing and conceptual design of an active flap and trim tab system for the MD900 helicopter were completed. Several two-dimensional airfoil and flap/tab models were wind tunnel tested and an aerodynamic database was established. Structural samples of the flap actuator mounts, flap section, tab, and full-span flap were fabricated and successfully tested. Several prototype actuators were developed and extensively tested to establish their performance and robustness in the dynamic operating environment. The flap actuator uses two biaxial PE...

Development and whirl tower test of the SMART active flap rotor

Abstract: A full scale Smart Material Actuated Rotor Technology (SMART) system with piezoelectric actuated blade flaps was developed and whirl tower tested. The development effort included design, fabrication, and component testing of rotor blades, trailing edge flaps, piezoelectric actuators, switching power amplifiers, and the data/power system. Simulations and model scale wind tunnel tests have shown that this system can provide 80% vibration reduction, 10dB noise reduction for a helicopter passing overhead, and substantial aerodynamic performance gains. Whirl tower testing of the 34-foot diameter rotor demonstrated the functionality, robustness, and required authority of the active flap system. The program involved extensive development work and risk reduction tests which resulted in a robust, high performance actuator and a tightly integrated actuator, flap, and blade system. The actuator demonstrated excellent performance during bench testing and has accumulated over 60 million cycles under a spectrum of loading conditions. The flight worthy active flap rotor blades were based on a modified design of the FAA certified MD900 Explorer production rotor blade. Whirl tower testing was conducted with full rotor instrumentation and a 5-component balance. The rotor was tested for 13 hours under a range of conditions, including 7 hours of flap operation. Flap inputs included open loop static and dynamic commands. The flaps showed excellent authority with oscillatory thrust greater than 10% of the steady baseline thrust. Various flap actuation frequency sweeps were run to investigate the dynamics of the rotor and the flap system. Limited closed loop tests used hub accelerations and hub loads for feedback. Proving the integration, robust operation, and authority of the flap system were the key objectives met by the whirl tower test. This success depended on tailoring the piezoelectric materials and actuator to the application and meeting actuator/blade integration requirements. Test results demonstrate the feasibility and practicality of applying smart materials for limited authority, active control on a helicopter rotor. Follow-on forward flight demonstrations are needed to quantify the expected significant improvements in vibrations, noise, and aerodynamic performance. Extensions of this technology are a prime candidate for on-blade flight control, i.e. elimination of the swashplate. This program was performed as part of DARPA's Smart Materials and Structures Demonstrations. Funding was provided by DARPA, The Boeing Company, NASA, and the U.S. Army. Additional cost share funds were provided by the University of Maryland, MIT, and UCLA.

Compact co-axial rotor system for a rotary wing aircraft and a control system therefor

Abstract: A dual, counter rotating, coaxial rotor system provides an upper and lower rotor system, with a reduced axial rotor separation distance along a common axis by way of rotor tip position sensing and rotor position controls to avoid tip contact.

Scaling effects and dynamic characteristics of miniature rotorcraft

Abstract: The dynamic characteristics of miniature rotorcraft, starting from a parameterized linear model developed for the identification of a Yamaha R-50 helicopter (3.04-m rotor diameter), and later applied to a smaller, more agile X-Cell .60 helicopter (1.52-m rotor diameter), are described. From this model, key flying qualities metrics are extracted and related to physical parameters. Based on these metrics, the identified data, and fundamental Froude and Mach scaling hypotheses, the effects of rotorcraft size on flying qualities and performance characteristics are analyzed and scaling trends inferred. These results are used to highlight the mechanical features and flight characteristics that are typical of small-scale rotorcraft, as well as to provide basic design guidelines for this class of vehicles.

Study of the Geometric Stiffening Effect: Comparison of Different Formulations

Abstract: This paper reviews different formulations to account for the stressstiffening or geometric stiffening effect arising from deflections largeenough to cause significant changes in the configuration of the systemThe importance of such effect on many engineering applications, such asthe dynamic behaviour of helicopter blades, flexible rotor arms, turbineblades, etc., is well known. The analysis is carried out only for one-dimensional elements in 2D.

Development of the Smart Spring for Active Vibration Control of Helicopter Blades

Abstract: Significant structural vibration is an undesirable characteristic in helicopter flight that leads to structural fatigue, poor ride quality for passengers and high acoustic signature for the vehicle...

Aeroelastic Analysis of Helicopter Rotor Blades on Deformable Chimera Grids

Abstract: Aeroelastic Reynolds-averaged Navier‐Stokes and Euler computations are presented for an articulated model rotor in hover and forward flight. Comparative rigid-blade simulations are carried out to assess the effects of blade dynamics and elasticity on the numerical results. The INROT flow solver operates on deformable structured overset grids and can be tightly coupled with a finite element model of the rotor blade structure (DYNROT) based on Timoshenko beam theory. The order of time accuracy of fluid and structure modules is maintained in the overall analysis by an appropriate staggered coupling scheme. At the investigated thrust setting, global hover performance values computed by the coupled fully turbulent Navier‐Stokes analysis agree fairly well with available experimental data. In forward flight, the aeroelastic results are in much better agreement with the measurements than those obtained from rigid-blade simulations with prescribed articulation. Apart from superior rotor power predicition, the local pitching moment coefficients computed by the viscous analysis are found to correlate better with wind-tunnel data than the corresponding Euler output.

Interdependence of Diffusion and Straining of Helicopter Blade Tip Vortices

Abstract: An experiment was performed to help quantify the interdependence of viscous/turbulent diffusion and straining effects on the development of helicopter rotor tip vortices. The properties of the blade tip vortices were measured in the wake of a small-scale hovering rotor and compared to the results for the case when the wake approached a solid boundary. The presence of the boundary created velocity gradients that forced the tip vortex filaments to strain, allowing the effects of this process on the vortices to be measured relative to the baseline case without the boundary. It is shown that vortex stretching begins to decrease the viscous core size, and when the strain rates become large, this can balance the normal growth in the vortex core resulting from diffusion. The present results were used to help develop a more general tip vortex model suitable for use in a variety of helicopter rotor aeroacoustic applications. The proposed engineering model combines the effects of turbulent diffusion and strain on the vortex core growth. The empirical coefficients of this model have been derived based on the best available results from rotating-wing tip vortex measurements.

Nonlinear responses of a rub-impact overhung rotor

Abstract: For a rotor system with bearings and step-diameter shaft in the oxygen pump of an engine, the contact between the rotor and the case is considered, and the chaotic response and bifurcation are investigated. The system is divided into elements of elastic support, shaft and disk, and based on the transfer matrix method, the motion equation of the system is derived, and solved by Newmark integration method. It is found that hardening the support can delay the occurrence of chaos. When rubbing begins, the grazing bifurcation will cause periodic motion to become quasi-period. With variation of system parameters, such as rotating speed, imbalance and external damping, chaotic response can be observed, along with other complex dynamics such as period- doubling bifurcation and torus bifurcation in the response.

Identification of linear parameter-varying state-space models with application to helicopter rotor dynamics

Abstract: A considerable amount of work has been dedicated in the past to the problem of the system identification of helicopter flight dynamics, while much less activity has been oriented to the goal of developing suitable identification procedures for rotor dynamics, mainly because of the difficulties associated with the task. This paper shows that subspace and optimization based identification techniques can be used to determine discrete-time linear parameter-varying models that have the potential to provide accurate descriptions for the (intrinsically time-varying) dynamics of a rotor blade. The identification techniques are presented and applied to simulated data generated by a physical model that describes the out-of-plane bending dynamics of a helicopter rotor blade.

Alleviation of airfoil dynamic stall moments via trailing-edge-flap flow control

Abstract: Trailing-edge-flap flow control for the mitigation of large negative pitching moments and negative aerodynamic damping caused by helicopter rotor blade dynamic stall was studied by means of computational fluid dynamics. A discrete vortex method was used for the simulations. The model geometry was a NACA 0012 airfoil oscillating in an α(t) = 15 deg + 10 deg sin(ωt) motion at the reduced frequency of k = 0.173. The freestream flow conditions were of M = 0.117 and Re = 1.463296 x 10 6 . The flap actuation was a brief pulse signal of a sinusoidal shape

Investigation of helicopter rotor-blade-tip-vortex alleviation using a slotted tip

Abstract: A slotted tip was developed to modify the characteristics of the strong vortex trailed from the tip of a small-scale helicopter blade. Forward-facing slots directed a slight amount of the incident flow in the spanwise direction, which was vented at the side edge of the blade tip. This caused the tip vortex to detach from the blade tip face, and also introduced turbulent vortlets into the laminar core of the developing vortex. The resulting wake flowfield was investigated using flow visualization and laser Doppler velocimetry. Measurements were conducted to quantify the vortex swirl velocity components, the inner core development, and the overall vortical flow inside the vortex trails. The results were then compared to a baseline blade with a standard unmodified rectangular tip

Recording and evaluation methods of PIV investigations on a helicopter rotor model

Abstract: Three-component particle image velocimetry measurements at moderate speeds and observation distances can now be accomplished on a routine basis. This article discusses the experiment performed on a 4 m-diameter model rotor in the 6-m×8-m open test section of the Large Low Speed Facility of the German–Dutch Wind Tunnels. More than half a terabyte of raw data were recorded at various positions on the advancing and retreating sides of the rotor in order to obtain detailed measurements of the trailing vortex in the frame of an international project. This paper addresses measuring techniques and possible sources of errors and presents a limited number of cases for the purpose of illustrating the solutions to numerous technical challenges relating to the acquisition and analysis of vortical flows.

Rotor-Bearing Analysis for Turbomachinery Single- and Dual-Rotor Systems

Abstract: Finite element analysis models were developed to investigate the dynamic characteristics of single- and dual-rotor-bearing turbomachinery systems. When an inertial coordinate system was used, the dynamic models of the rotor-bearing systems included gyroscopic moments, rotary inertias, and bending and shear deformations. The models were analyzed to predict the natural frequencies, to produce critical speed maps, and to estimate the bearing stiffnesses. These rotor-bearing system analyses were then applied to both single-rotor and dual-rotor system applications. In the single-rotor system application, a small turbojet engine and its rotor components were used as a basis for the model. Both theoretical and experimental analyses were used to study this engine rotor-bearing system. Modal testing and a dynamic engine test were used to verify the analytical results, including the predicted critical speed map and the bearing stiffnesses. Very good agreement was found between the analyses and the test data. In the dual-rotor application, the effects of the speed ratio of the high-speed to low-speed shafts of the dual-rotor system on the critical speeds was studied. It was demonstrated that this speed ratio could be used as one of the dual-rotor system design parameters. Finally, it was noted that the interrotor bearing stiffness between the high-speed and the low-speed shafts of the dual-rotor system affected the mode shapes of the shafts within the system, in addition to the rotor system critical speeds.

Swashplateless Helicopter Rotor with Trailing-Edge Flaps

Abstract: A helicopter primary control system with trailing-edge flaps was investigated numerically for its potential to replace a conventional swashplate system. Eliminating the swashplate and associated control system can lead to significant reductions in weight, drag, and cost and an improvement of rotor performance. A comprehensive rotorcraft analysis was developed for analyzing the swashplateless rotor configuration and was implemented to examine the actuation requirements for rotor primary control with trailing-edge flaps. A multicyclic controller was implemented with the swashplateless rotor analysis, and the feasibility of trailing-edge flap performing both primary control and active vibration control was examined. Flap control inputs of a swashplateless rotor are presented at several advance ratios. With optimal selection of blade collective pitch index angle, the flap was shown to be able to trim the rotor with moderate flap inputs. Simulations of flaps performing both primary control and active vibration control were carried out, with the conclusion that trailing-edge flaps are capable of trimming the rotor and minimizing vibratory rotor hub loads simultaneously

Helicopter vibration reduction in forward flight with induced-shear based piezoceramic actuation

Abstract: Governing equations are obtained for helicopter rotor blades with surface bonded piezoceramic actuators using Hamilton's principle. The equations are then solved for dynamic response using finite element discretization in the spatial and time domains. A time domain unsteady aerodynamic model is used to obtain the airloads. The nonlinear relationship between the piezoelectric shear coefficient and applied ac field is represented as a polynomial curve fit. The nonlinear effects are investigated by applying a sinusoidal voltage to the helicopter rotor blade. The rotor blade is modeled as a two-cell box section with piezoelectric layers surface bonded to the top and bottom of the box beam. Comparison of results with linear and nonlinear shear coefficients is presented. Use of a nonlinear relationship (compared to linear) to achieve targeted reductions in strains or displacements results in a reduction in the requirement of applied amplitude of the sinusoidal field. A rate feedback control law is implemented which feeds back the higher harmonics of the time rate of change of strain in the azimuthal direction. The sensed voltage is then applied to the rotor blade, resulting in a vibration reduction of approximately 43% for a four-bladed, soft-in-plane hingeless rotor in forward flight.

A near wake model for trailing vorticity compared with the blade element momentum theory

Abstract: A near wake model for trailing vorticity originally proposed by Beddoes for high-resolution helicopter blade vortex interaction computations has been implemented and compared with the usual blade element momentum models used for wind turbine calculations. The model is in principle a lifting line model for the rotating blade, where only a quarter revolution of the wake system behind the blade is taken into account. This simplification of the wake enables a fast computation of the downwash from the trailed vortex system along the blade using the indicial function method and thus makes it realistic to use the model in aeroelastic time simulations. The downwash from the shed vorticity is also computed with a fast indicial function algorithm. In particular the model is investigated for use in calculations of aerodynamic damping for the different mode shapes of an operating wind turbine. Numerical results for the downwash of a wing in straight flow with elliptical circulation are compared with analytical results. Further, the downwash distribution of a 40 m long rotating blade is computed. Aerodynamic damping of the blade in axial harmonic translation and in the first flapwise mode is computed with the near wake model and compared with the results of a standard momentum model including a model for dynamic inflow. Copyright © 2004 John Wiley & Sons, Ltd.

An unsteady multiblock multigrid scheme for lifting forward flight rotor simulation

Abstract: Numerical simulation of multi-bladed lifting rotors in forward flight is considered. The flow-solver presented is multiblock and unsteady, which is essential for forward flight, and also includes multigrid acceleration to reduce run-times. A structured multiblock grid generator specifically for rotor blades has also been developed and is presented here. Previous work has shown that hovering lifting rotor flows are particularly expensive to simulate, since the capture of the vortical wake below the disc requires a long numerical integration time; more than 20 revolutions for an unsteady simulation, or more than 40000 time-steps for a single grid steady simulation. It is demonstrated here that only two or three revolutions are required to obtain a converged solution for forward flight, since the wake is swept downstream. This requires less than 1.5 × the run-time of a steady hovering simulation, for the same grid density around each blade, even though an unsteady simulation is required and the complete disk must be solved rather than one blade as in hover. It is demonstrated that very fine meshes are required to capture the unsteady tip vortex motion, and the effects on blade loading of blade-vortex interaction and rotor shaft inclination are also considered. Copyright © 2004 John Wiley & Sons, Ltd.

Development of an SMA Actuator for In-flight Rotor Blade Tracking:

Abstract: A new and improved solution to the helicopter blade-tracking problem has been investigated. A discrete, blade-installed actuator utilizing shape memory alloy (SMA) torsion tubes has been developed to enable in-flight rotor tracking. SMA torsion tubes from three materials were tested and one was selected for its cyclic stability. Improved tube fabrication, heating, end attachment methods, and locking schemes were developed. The final prototype actuator employed two biaxial SMA tubes for actuation/bias, an SMA-activated lock for power off operation, and integrated microprocessor control electronics. Tests under static and dynamic loading showed that actuation requirements, except for bandwidth, were met. Increased bandwidth may be obtained by improved control algorithms or cooling. The developed SMA actuator enables an in-flight, real-time adjustment of a blade-tracking tab. Projected payoffs are reduced maintenance cost as well as reduced helicopter vibrations and improved performance.

Control of a laboratory helicopter using switched 2-step feedback linearization

Abstract: A control structure based on feedback (input-output) linearization has been applied to the longitudinal subsystem of a laboratory double-rotor helicopter. This article focuses on the longitudinal subsystem which is underactuated in sense that the number of control variables is less than the number of degrees of freedom. A switching control law between exact and approximate input-output linearization is proposed. The feedback linearization has been applied in two steps, first to the nonlinear actuator, and then to the entire system. This law has been tested by simulated and experimental results.

Evaluation of CFD to Determine Two-Dimensional Airfoil Characteristics for Rotorcraft Applications

Abstract: The efficient prediction of helicopter rotor performance, vibratory loads, and aeroelastic properties still relies heavily on the use of comprehensive analysis codes by the rotorcraft industry. These comprehensive codes utilize look-up tables to provide two-dimensional aerodynamic characteristics. Typically these tables are comprised of a combination of wind tunnel data, empirical data and numerical analyses. The potential to rely more heavily on numerical computations based on Computational Fluid Dynamics (CFD) simulations has become more of a reality with the advent of faster computers and more sophisticated physical models. The ability of five different CFD codes applied independently to predict the lift, drag and pitching moments of rotor airfoils is examined for the SC1095 airfoil, which is utilized in the UH-60A main rotor. Extensive comparisons with the results of ten wind tunnel tests are performed. These CFD computations are found to be as good as experimental data in predicting many of the aerodynamic performance characteristics. Four turbulence models were examined (Baldwin-Lomax, Spalart-Allmaras, Menter SST, and k-ω ).

Rotor Performance of a UH-60 Rotor System in the NASA Ames 80- by 120-Foot Wind Tunnel

Abstract: : A full-scale four-bladed UH-60 rotor system was tested in the NASA Ames 80- by 120-Foot Wind Tunnel. A quality data set at low forward speed, 0 to 80 knots, has been obtained to support future rotor developments and analysis improvements. To evaluate the NASA Ames 80- by 120- Foot Wind Tunnel as a hover testing facility, rotor performance data were compared with predictions, UH-60 aircraft flight test data, and UH-60 model-scale data from other test facilities. Results indicate that valid hover data for this size rotor can be obtained from this facility at low to medium thrust conditions. Comparisons with flight test and model-scale data demonstrate the variability between existing data sets. Predictions show good agreement with full-scale data. To evaluate the analytical modeling in the 0 to 80 knot speed range, forward flight rotor performance data were acquired and compared with predictions. Comparisons were also made with existing model-scale and flight test data. Power calculations show fair to good agreement with full-scale wind tunnel data at advance ratios between 0.10 to 0.19 and poor agreement at advance ratios below 0.10. Comparisons with flight test and model-scale data show good agreement at all advance ratios tested. Propulsive force calculations show good correlation with full-scale wind tunnel data at advance ratios of 0.10 to 0.19.

Modeling active vibration control of a geared rotor system

Abstract: This paper analyzes the practicality of active internal shaft transverse vibration control using a piezoelectric stack actuator for reducing external gearbox housing structure response due to transmission error excitation from a gear pair. The proposed adaptive controller that is designed specifically for tackling mesh frequency vibrations is based on an enhanced filtered-x least mean square algorithm with frequency estimation to synthesize the required reference signal. The vital system secondary path characteristic that relates the actuation force to the housing response of interest is identified in offline or online mode using the additive random noise technique. Numerical studies employing practical design parameters in order to provide a realistic assessment of the proposed approach are performed for a geared rotor system (dynamic plant) that is represented as finite and lumped elements. The simulations reveal very promising vibration control results, which will be utilized to guide future experimental studies.