Showing papers on "Helicopter rotor published in 2002"

Free-Vortex Filament Methods for the Analysis of Helicopter Rotor Wakes

Abstract: The theoretical basis and the numerical implementation of free-vortex filament methods are reviewed for application to the prediction and analysis of helicopter rotor wakes. The governing equations for the problem are described, with a discussion of finite difference approximations to these equations and various numerical solution techniques. Both relaxation and time-marching wake solution techniques are reviewed. It is emphasized how the careful consideration of stability and convergence (grid-independent behavior) are important to ensure a physically correct wake solution. The implementation of viscous diffusion and filament straining effects are also discussed. The need for boundary condition corrections to compensate for the inevitable wake truncation are described. Algorithms to accelerate the wake solution using velocity field interpolation are shown to reduce computational costs without a loss of accuracy. Several challenging examples of the application of free-vortex filament methods to helicopter rotor problems are shown, including multirotor configurations, flight near the ground, maneuvering flight conditions, and descending flight through the vortex ring state

Computational Modeling of Hovering Rotor and Wake Aerodynamics

Abstract: Steady-state Reynolds-averaged Navier ‐Stokes computations are presented for a range of UH-60A model-rotor testcasesinhover. Thecomputationsaredesignedto assessgrid-related effectson thenumericalresultsandemploy 1)structuredoversetgridswithhighresolution ontherotorblades,2 )asystematicvariationofgridresolutioninthe rotor wake, and 3 ) a systematic variation of outer-boundary locations. Computed rotor performance values agree very well with experimental measurements and show little sensitivity to either grid resolution or outer-boundary locations. However, the computations uniformly overpredict the blade sectional thrust near the rotor tip. This overprediction of blade tip thrust is explained by an analysis of the circulation distribution in the computed rotor wake system.

Navier-Stokes Analysis of Helicopter Rotor Aerodynamics in Hover and Forward Flight

Abstract: Reynolds-averaged compressible Navier ‐Stokes computations are presented for the hovering 7A model rotor and a low aspect ratio NACA0012 rotor in nonlifting forward e ight. For enhanced hover performance prediction, aeroelastic effects are taken into account by tightly coupling the e ow solver with a e nite element model of the blade based on Timoshenko beam theory. Good agreement between computed and measured rotor performance is achieved for three collective pitch settings on structured periodic grids featuring noncongruent cell faces along the periodicity planes. Results of a hybrid Navier ‐Stokes/Euler chimera hover computation are presented, in addition to the periodic grid analysis demonstrating the superiority of the overset grid approach with respect to tip vortex conservation. The unsteady Navier ‐Stokes computations for the nonlifting forward e ight test case show more sensitivity relative to time step size than comparable Euler analyses. Overall correlation of computed and experimental near-tip pressure distributions over blade azimuth is considered to be fair, and the strong transonic effects on the advancing blade are adequately captured by the numerical analysis.

Regular and chaotic dynamics of a discontinuously nonlinear rotor system

Abstract: The nonlinear vibrations are considered in a two-degree of freedom rotordynamic system subjected to a bearing clearance effect The excitation is provided by an out-of-balance within the system, and the nonlinearity, in the form of a discontinuous stiffness, is effected by means of a radial clearance between the elastically supported rotor and the elastically supported outer ring Different nonlinear dynamics analysis techniques are employed to unveil the global dynamics of the rotor system In particular the system has been investigated with the help of time trajectories, phase portraits, bifurcation diagrams, Poincare maps, power spectrum analysis and the construction of basins of attraction A numerical study is presented which encompasses the effects of different system parameters in order to demonstrate the severity of the vibrations It is also shown that the response of the system can be extremely sensitive to changes in these parameters, and that chaos can exist over large regions of parameter space

Calculation and analysis of electromagnetic scattering by helicopter rotating blades

Abstract: This paper describes an application of physical optics and the method of equivalent currents to the calculation of radar cross section (RCS) of a helicopter rotor. The problem is treated using a quasi-stationary approach. The calculation can be parameterized as a function of the locations of the radar transmitter and receiver in relation to the rotor center. Therefore, this offers the possibility of monostatic and bistatic simulations in the far field and near field. Blade geometry is taken into account using a triangular meshing generated by the I-DEAS meshing software. Digital applications are presented and the effects on the RCS spectrum of incidence, frequency, blade number, and the near field are analyzed.

Apache Helicopter Stabilization Using Neural Dynamic Programming

Abstract: A new form of neural control is introduced, neural dynamic programming (NDP), a model-free online learning control scheme. NDP is shown to perform exceedingly well as a learning controller for practical systems of higher dimension, such as helicopters. The discussion is focused on providing a viable alternative helicopter control system design approach rather than providing extensive comparisons among various available controllers. A comprehensive treatise of NDP and extensive simulation studies of NDP designs for controlling an Apache helicopter under different flight conditions is presented. Design robustness is addressed by performing simulations under various disturbance conditions. All of the designs are based on FLYRT, a sophisticated industry-scale nonlinear validated model of the Apache helicopter.

Optimum speed tilt rotor

Abstract: A variable speed tilt rotor system and method for operating such a system are provided which allow the rotor to be operated at an optimal angular velocity in revolutions per minute (RPM) minimizing the power required to turn the rotor thereby resulting in performance efficiency improvements, reduction in noise, and improvements in rotor, transmission and engine life. The system and method provide for an increase in endurance and range. The system and method also provide a substantial improvement in performance during take-off, hover and maneuver. All such improvements are valid in helicopter mode when the helicopter is supported by rotor vertical lift, in airplane mode when the helicopter is supported by wing lift and the rotor is tilted forward to provide propulsive thrust and in conversion from helicopter mode to airplane mode.

Helicopter flight around a ship's' superstructure

Abstract: A computationaluid dynamics model of a hovering helicopter main rotor is developed to examine airow in the presence of ship structures and side winds. An illustratio n of the problem is given. The rotor is modelled by modifying the governing N avier±Stokes equations in the region of the disc. The extra terms added to the governing equations apply a downward force to theuid; these forces are independent of theow around the rotor and are equal to the helicopter weight. The helicopter rotor model and the ship model are combined to yield oneow solution, which, due to the severe non-linearities of the problem, cannot be achieved by superpositio n. The resultantow yields valuable data about the induced velocities at the rotor, which ultima tely determine the control pitch and power required to maintain the hover in a given location. Indeed, the interactions between the rotor downwash and ship airow are known to produce unexpected and adverseight dynamic behaviour of the aircraft.

Coupled aircraft rotor system

Abstract: An aircraft with a tilt rotor assembly comprising a craft body, a plurality of rotor blades which are subject to three modes of flight operation, a tilting mast coupling the plurality of rotor blades to the craft body and for selectively moving the plurality of rotor blades between the three modes of flight operation, a hub coupling the plurality of rotor blades to the tilting mast in a manner which transfers torque and thrust while allowing tilting of a rotor thrust vector, a main swashplate for tilting in response to operator input to control the direction of the rotor thrust vector, a plurality of pitch horns, each mechanically coupled to a particular one of the plurality of rotor blades and to the main swashplate, for communicating swashplate input to each of the plurality of rotor blades, a plurality of links coupling the main swashplate to the plurality of pitch horns, wherein each of the plurality of pitch horns is mechanically coupled to a particular one of the plurality of rotor blades by one of the plurality of links in a particular position which yields a delta-3 value which is not optimum, and a feedback swashplate and cooperating feedback links for receiving disk tilting input from the plurality of rotor blades during flight, and for supplying a mechanical input to the main swashplate to compensate for the less than optimum delta-3 coupling between the plurality of pitch horns and the plurality of links. The three modes of flight include: (1) an airplane mode of flight with the plurality of rotor blades in a rotor disk position which is substantially transverse to the craft body; (2) a helicopter mode of flight with the plurality of rotor blades in a rotor disk position substantially parallel to the craft body with direction of flight being controlled by a rotor thrust vector; and (3) a transition mode of flight with the plurality of rotor blades moving between the rotor disk positions associated with the airplane mode of flight and the helicopter mode of flight.

Recent Improvements to a Hybrid Method for Rotors in Forward Flight

Abstract: Ahybrid Navier ‐Stokes/full potential solver has been developed for the efe cient prediction of three-dimensional unsteady viscous e ow phenomena that occur over helicopter rotors in forward e ight. The method combines a Navier‐Stokes analysis near the blade, modeling the viscous e ow and near wake with a potential e ow analysis in the far e eld, modeling inviscid isentropic e ow. A grid motion module has been developed to account for the blade motion and elastic deformations. Free and prescribed wake models have been developed to account for the tip vortex effects once the vortex leaves the viscous e ow region and enters the potential e ow region. Sample results are presented for a two-bladed AH-1G rotor in descent and for a UH-60A rotor in high-speed forward e ight. Comparisons with experiments, e ight test data, and other numerical simulations are given.

Unstructured Mesh Navier-Stokes Calculations of the Flow Field of a Helicopter Rotor in Hover

Abstract: Three-dimensional viscous flow field around a lifting helicopter rotor in hover is calculated by using an unstructured mesh methodology. The flow solver utilizes a cell-centered finite-volume scheme that is based on the Roe’s flux-difference splitting and an implicit Jacobi/Gauss-Seidel time integration. The effect of turbulence is estimated by the Spalart-Allmaras one-equation model coupled with a wall function boundary condition. A solution-adaptive mesh refinement technique is used for efficient capturing of the tip vortex. Calculations are performed at two operating conditions with varying tip Mach number and collective pitch setting. Formation of the tip vortex is well captured through a series of adaptive mesh refinement procedure starting from a coarse initial mesh. Good agreements are obtained between the numerical result and the experiment for both the blade loading and the tip vortex behavior. It is demonstrated that the rotor performance and the flow field are significantly affected by viscosity. The process of tip vortex formation around the blade tip is also qualitatively investigated.

Health monitoring of a helicopter rotor in forward flight using fuzzy logic

Abstract: A fuzzy logic system is developed for helicopter rotor system fault isolation. Inputs to the fuzzy logic system are measurement deviations of blade bending and torsion response and vibration from a "good" undamaged helicopter rotor. The rotor system measurements used are flap and lag bending tip deflections, elastic twist deflection at the tip, and three forces and three moments at the rotor hub. The fuzzy logic system uses rules developed from an aeroelastic model of the helicopter rotor with implanted faults to isolate the fault while accounting for uncertainty in the measurements. The faults modeled include moisture absorption, loss of trim mass, damaged lag damper, damaged pitch control system, misadjusted pitch link, and damaged flap. Tests with simulated data show that the fuzzy system isolates rotor system faults with an accuracy of about 90-100%. Furthermore, the fuzzy system is robust and gives excellent results, even when some measurements are not available. A rule-based expert system based on similar rules from the aeroelastic model performs much more poorly than the fuzzy system in the presence of high levels of uncertainty.

Rotor system for helicopters

Abstract: A tail rotor system for a helicopter includes a gear box housing retainer ( 66 ) coupled to the tail boom ( 16 ) of the helicopter. The tail rotor system also includes a power-limiting device ( 50 ) configured to disconnect the tail rotor from a power plant for driving the tail rotor about a rotor axis of rotation.

Further examination of the vibratory loads reduction results from the nasa/army/mit active twist rotor test

Abstract: The vibration reduction capabilities of a model rotor system utilizing controlled, strain-induced blade twisting are examined The model rotor blades, which utilize piezoelectric active fiber composite actuators, were tested in the NASA Langley Transonic Dynamics Tunnel using open-loop control to determine the effect of active-twist on rotor vibratory loads The results of this testing have been encouraging, and have demonstrated that active-twist rotor designs offer the potential for significant load reductions in future helicopter rotor systems Active twist control was found to use less than 1% of the power necessary to operate the rotor system and had a pronounced effect on both rotating- and fixed-system loads, offering reductions in individual harmonic loads of up to 100% A review of the vibration reduction results obtained is presented, which includes a limited set of comparisons with results generated using the second-generation version of the Comprehensive Analytical Model of Rotorcraft Aerodynamics and Dynamics (CAMRAD II) rotorcraft comprehensive analysis

Cooling system for a hybrid aircraft

Abstract: A cooling system for a hybrid aircraft includes an inlet which extends through the body to communicate airflow to a powerplant subsystem and out through an exhaust within a rotor duct. In a hover mode, there is a significant low-pressure area created inside the rotor duct by the rotor system. The low-pressure area within the rotor duct assists in drawing air through the inlet and over the engine via the exhaust. A cooling fan is located adjacent the inlet to augment cooling-air flow. The cooling fan is smaller than conventional practice because it does not have to provide the entire pressure difference to force air-cooling flow over the engine. In a transition mode, the low-pressure area created inside the rotor duct decreases but ram air pressure through the inlet increases. In a forward flight mode, the pressure inside the rotor duct is approximately atmospheric but significant ram air is provided from the inlet due to forward flight speed.

Structural monitoring system for helicopter rotor components

Abstract: A system for monitoring the structural condition of a helicopter rotor assembly component includes a piezoelectric sensor. Acoustic emission signal techniques allow for recognizing the relatively high frequency stress waves associated with the propagation of cracks or defects in a rotor assembly component. A signal conditioner processes the sensor signal and provides an output signal that includes at least one characteristic that is indicative of the content of the sensor signal and, therefore, the structural condition of the item of interest. A signal analyzer utilizes the output signal from the signal processor to determine the structural condition of the component.

Rotor system vibration absorber

Abstract: A rotor system vibration absorber for use with a helicopter of other rotocraft is disclosed in which spring forces are provided by a plurality of elongated rods (73) arranged in a selected pattern. The rods are coupled at one end to a fixed base (79) that is coupled to a rotor hub (55), and at the other end to a tuning weight (81).

Boundary Conforming Discontinuous Galerkin Finite Element Approach for Rotorcraft Simulations

Abstract: A numerical method has been developed for predicting the complex vortex wake for a helicopter rotor in hover and in forward flight. The method is based on the solution of the three-dimensional, compressible Euler equations expressed in an arbitrary Lagrangian Eulerian reference frame. A second-order accurate discontinuous Galerkin finite element method is used to discretize the governing equations on a hexahedral mesh. Unstructured, local mesh refinement is performed to enable prediction of the structure of the vortex wake. The capabilities of this computational fluid dynamics method are demonstrated by simulations of the flow around the Caradonna-Tung helicopter rotor in hover and simulations of the flow around the Operational Loads Survey helicopter rotor in forward flight

Low-Speed Wind Tunnel Investigation of a Full-Scale UH-60 Rotor System

Abstract: An experimental program to test a full-scale UH-60 rotor system in the NASA Ames 80- by 120-Foot Wind Tunnel was completed. The rotor system was installed and tested using a new test stand/facility, the Large Rotor Test Apparatus (LRTA). The experimental program had both operational and research objectives, including 1) demonstration of LRTA capabilities, 2) evaluation of an Individual Blade Control system to reduce vibration and noise, 3) acquisition of low-speed performance and load data for comparison with flight test results and analyses, and 4) validation of a new flow measurement technique. In this paper, the specific objectives and approach for the wind tunnel test are presented along with examples of the research results. Particular attention is placed on describing the experimental program, including the new testing capabilities available with the LRTA. NOTATION

Vertical take-off and landing, aerodynamically self-sustained horizontal flight hybrid aircraft

Abstract: An vertical take-off and landing (VTOL) and horizontal flight (HF) aircraft has a fuselage with wings providing lift during horizontal flight, a rotor on the nose of the fuselage that is stowable during horizontal flight, a first pair of ducted fan propellers on the wings, a second pair of tiltable ducted fan propellers adjacent to the tail, a hydraulic system powered by the main engine and generating propulsive hydraulic power for the rotor system and the first and second pairs of propellers. A control system for the hydraulic system provides propulsive hydraulic power to the rotor system and to the second pair of propellers during vertical take-off and landing and not to the first pair of propellers during vertical take-off and landing and provides propulsive power to the first and second pairs of propellers during horizontal flight and not to the rotor system during horizontal flight.

Analysis of rotorcraft flight dynamics in autorotation

Abstract: Autorotation is a flight state whereby no powerplant torque is applied to the main rotor of a rotorcraft. For helicopters, autorotation is an abnormal mode of operation utilized in descending flight following total power failure. However it is the normal mode of operation for autogyros. It is common in helicopter mathematical modeling to assume constant rotorspeed, but this is invalid in autorotation. Introduction of the rotorspeed degree of freedom in modeling gives rise to an additional mode and coupling with the existing rigid-body modes. This paper therefore aims to explore the development of generic analytical expressions, exposing the role of the rotorspeed degree of freedom in modifying the classical rigid-body modes of motion. The analytical expressions are verified by quantifying their validity using autogyro derivatives obtained from a full nonlinear individual blade model. Comparison with the helicopter case then extends the generality and applicability of the analysis. It is concluded that there is an intimate relationship between the rotorspeed and low frequency rigid-body modes of rotorcraft in autorotation. The analytical expressions highlight the essence of coupled rotorspeed and rigid-body dynamics in autorotation, clearly identifying the conditions under which this relationship is stabilizing or otherwise.

Flight-Path Management/Control Methodology to Reduce Helicopter Blade-Vortex Interaction Noise

Abstract: A quasi-static acoustic mapping method has been developed to predict rotorcraft ex ernal noise This takes advantage of an expansion, to first order, about a solution to the helicopter no dimensional-state trim equations to include the effects of acceleration parallel to the flight path and X-fo c anges on e radiated noise. Application of the new method to predict helicopter blade-vortex interaction (BVI) noise has shown that choice of flight-path angle, X-force, and vehicle acceleration all have an important influence on ground noise exposure during approach and landing. The effect of constant flight-path-angle approaches on BVI ground noise, with and without X-force control, is compared with decelerating approaches at the same flight-path angle. Two different quiet flight trajectories are suggested that use a combination of these controls to minimize BVI noise exposure on the ground during a helicopter approach to a landing.

Optimal Short Takeoff of Tiltrotor Aircraft in One Engine Failure

Abstract: Optimal tiltrotor flight paths in the event of a single engine failure during short takeoff operations are investigated. A vertical plane rigid-body model is used that has as state variables aircraft position, body components of aircraft velocity, pitch angle and rate, and rotor angular speed. The control variables are the thrust coefficient of one rotor, the pilot's longitudinal stick displacement, and the nacelle angle. The model uses both parameters and aerodynamic data of the XV-15 research aircraft. The tabular aerodynamic data are interpolated with smooth functions. Tiltrotor flights after engine failure are formulated as nonlinear optimal control problems. Both continued takeoff and rejected takeoff flight after an engine failure are studied. Performance indices are selected to minimize runway length, subject to various constraints from safety considerations and tiltrotor performance limitations. These optimal control problems are parameterized via collocation into parameter optimizations for numerical solutions. Extensive numerical solutions are obtained, and sensitivity analyses are conducted to examine effects of model uncertainties

Predicting aerodynamic rotor-fuselage interactions by using unstructured meshes

Abstract: A numerical method has been developed for the analysis of interactional aerodynamics between helicopter rotor and fuselage in forward flight A 3-D steady compressible Euler solver is used to compute time-averaged interactional effects between the rotor and the fuselage The rotor is modeled as an actuator disk with zero thickness carrying pressure jump across it Collective and cyclic pitch angles are calculated to satisfy the rotor trim condition Unstructured meshes are used to model complex rotor-fuselage configurations Calculations are performed for two generic helicopter fuselage geometries The results are compared with available experimental data for validation