Showing papers on "Helicopter rotor published in 1992"

Flowfield of a Lifting Rotor in Hover: A Navier-Stokes Simulation

Abstract: The viscous, three-dimensional flowfield of a lifting helicopter rotor in hover is calculated by using an upwind, implicit, finite-difference numerical method for solving the thin layer Navier-Stokes equations. The induced effects of the wake, including the interaction of tip vortices with successive blades, are calculated as part off the overall flowfield solution without using any ad hoc wake models. Comparison of the numerical results for the subsonic and transonic conditions show good agreement with the experimental data and with the previously published Navier-Stokes calculations using a simple wake model. Some comparisons with Euler calculations are also presented, along with some discussions of the grid refinement studies.

Dynamic analysis of geared rotors by finite elements

Abstract: A finite element model of a geared rotor system on flexible bearings has been developed. The model includes the rotary inertia of on shaft elements, the axial loading on shafts, flexibility and damping of bearings, material damping of shafts and the stiffness and the damping of gear mesh. The coupling between the torsional and transverse vibrations of gears were considered in the model. A constant mesh stiffness was assumed. The analysis procedure can be used for forced vibration analysis geared rotors by calculating the critical speeds and determining the response of any point on the shafts to mass unbalances, geometric eccentricities of gears, and displacement transmission error excitation at the mesh point. The dynamic mesh forces due to these excitations can also be calculated. The model has been applied to several systems for the demonstration of its accuracy and for studying the effect of bearing compliances on system dynamics.

A finite-volume Euler solver for computing rotary-wing aerodynamics on unstructured meshes

Abstract: An unstructured-grid solver for the unsteady Euler equations has been developed for predicting the aerodynamics of helicopter rotor blades. This flow solver is a finite-volume scheme that computes flow quantities at the vertices of the mesh. Special treatments are used for the flux differencing and boundary conditions in order to compute rotary-wing flowfields, and these are detailed in the paper. The unstructured-grid solver permits adaptive grid refinement in order to improve the resolution of flow features such as shocks, rotor wakes and acoustic waves. These capabilities are demonstrated in the paper. Example calculations are presented for two hovering rotors. In both cases, adaptive-grid refinement is used to resolve high gradients near the rotor surface and also to capture the vortical regions in the rotor wake. The computed results show good agreement with experimental results for surface airloads and wake geometry.

Flutter and Stall Response of a Helicopter Blade with Structural Nonlinearity

Abstract: The purpose of the present paper is to study the flutter instability and forced response of a nonrotating helicopter blade model with a NACA-0012 airfoil and a pitch freeplay structural nonlinearity. In this paper, three typical combinations of linear and nonlinear structure with a linear and nonlinear (ONERA) aerodynamic model are considered. Characteristic results are used to display the limit cycle oscillation and chaotic behavior of both the flutter instability and forced response for all three cases. The effects of various initial disturbance amplitudes on the forced response behavior are discussed. Comparisons of the results for the three cases are helpful in understanding physically the nonlinear aeroelasticity phenomena and chaotic oscillations.

Vibration reduction in helicopter rotors using an active control surface located on the blade

Abstract: A feasibility study of vibration reduction in a four-bladed helicopter rotor using individual blade control (IBC), which is implemented by an individually controlled aerodynamic surface located on each blade, is presented. For this exploratory study, a simple offset-hinged spring restrained model of the blade is used with fully coupled flap-lag-torsional dynamics for each blade. Deterministic controllers based on local and global system models are implemented to reduce 4/rev hub loads using both an actively controlled aerodynamic surface on each blade as well as conventional IBC, where the complete blade undergoes cyclic pitch change. The effectiveness of the two approaches for simultaneous reduction of the 4/rev hub shears and hub moments is compared. Conventional IBC requires considerably more power to achieve approximately the same level of vibration reduction as that obtained by implementing IBC using an active control surface located on the outboard segment of the blade. The effect of blade torsional flexibility on the vibration reduction effectiveness of the actively controlled surface was also considered and it was found that this parameter has a very substantial influence.

Design and development of a compressible dynamic stall facility

Abstract: A dynamic stall facility offering a unique new capability for studies of compressibility effects on dynamic stall is described. This facility features complete visual access by mounting the test airfoil between optical-quality glass windows which are rotated in unison to produce the oscillating airfoil motion associated with helicopter rotor dynamic stall. By using the density gradients associated with the rapidly changing dynamic stall flow field, this facility permits simultaneous detailed investigation of the flow on the surface as well as in the flow field surrounding airfoils experiencing dynamic stall.

Robust flight control system design with multiple model approach

Abstract: References 1 Lukes, D. L., Differential Equations: Classical to Controlled, Academic, New York, 1982, Chap. 8. Kretz, M., Aubrun, J. N., and Larch, M., "Wind Tunnel Tests of the Dorand DH 2011 Jet Flap Rotor," Vol. 1, NASA-CR-114693, June 1973. Molusis, J. A., Hammond, C. E., and Cline, J. H., "A Unified Approach to the Optimal Design of Adaptive and Gain Scheduled Controllers to Achieve Minimum Helicopter Rotor Vibration," 37th Annual Forum of the American Helicopter Society, New Orleans, LA, May 1981. Johnson, W., "Self Tuning Regulators for Multicyclic Control of Helicopter Vibration," NASA TP 1996, March 1982. Bryson, A. E., and Ho, Y. C., Applied Optimal Control: Optimization, Estimation, and Control, Hemisphere, New York, 1975, Chap. 12. Wittenmark, B., "Stochastic Adaptive Control Methods: A Survey," International Journal of Control, Vol. 21, No. 5, 1975, pp. 705-730. Haviland, J. K., and Knospe, C., "A Wide-Band, Time-Domain Adaptive Control for Helicopter Vibrations," Second Technical Workshop on Dynamics and Aeroelastic Stability Modeling of Rotorcraft Systems, Boca Raton, FL, Nov. 1987.

Helicopter servoflap actuator having mechanical stop and oil pump

Abstract: In a servoflap system for helicopters, angular positions of the helicopter's servoflaps are controlled by actuators located near the tips of the helicopter blades Each actuator contains a pump for circulating a lubricant onto the actuator's gears when blade pitch is being changed The pump is built into the actuator's mechanical stops

Experimental study of rotor wake/body interactions in hover

Abstract: Experiments were conducted to document the tip vortex geometries and interactional effects betwen a hovering rotor and a body representing a simplified helicopter fuselage. The wide-field shadowgraph technique was used to visualize the rotor tip vortices and to obtain quantitative information on the trajectories, with and without the presence of the body. It was found that the effects of the body caused significant changes to both the radial contraction and axial displacements of the tip vortices compared to the isolated case. Direct impingement of the tip vortices on the body surface was also observed, and found to cause large local wake deformations. The rotor performance was significantly affected by the body, producing a higher figure of merit relative to the isolated case.

Effects of higher harmonic control on rotor performance and control loads

Abstract: An analytical study, based on an advanced Higher Harmonic Control (HHC) analysis for helicopter rotor systems, is carried out to investigate HHC application for rotor performance enhancement. The effects of HHC on stall characteristics of rotor and blade pitch-link loads when the system is configured to suppress vibration are also examined. For vibration control, simulated results indicate that HHC may promote early blade stall. Effects of blade torsion frequencies on HHC performance are moderate, and torsionally stiff blades require less actuator power than torsionally soft blades. For rotor performance improvement, a 3 to 5 percent reduction in rotor shaft power can be achieved with 2 deg of two-per-rev blade pitch control.

Method for folding helicopter main rotor blades

Abstract: A method of folding the main rotor blades (21, 23, 25, 27) of a helicopter (11) for storage in which blade supports (31, 33) are removeably attached to the nose (13) and tail (15) of the fuselage of the helicopter (11).

Measurements of the Dynamic Stall Vortex Convection Speed

Abstract: This paper considers the dynamic stall vortex of importance in helicopter rotor aerodynamics and discusses previous measurements of its convection speed. It emerges that an anomaly exists between the available data sets, i.e. that some workers find that the convection speed is dependent upon the aerofoil motion, while others find that this is not the case. Measurements of the convection speed from data gathered at Glasgow University for a variety of aerofoil shapes and motion types are then presented, which support the conclusion that the dynamic stall vortex convection speed is independent of aerofoil type and motion type to a first order.

Extension and validation of an unsteady wake model for rotors

Abstract: A new three-dimensional, finite-state induced-flow model is extended to treat nonlinearities associated with the mass flow induced through the rotor plane. This new theory is then applied to the correlation of a recent set of unsteady, hover laser Doppler velocimetry inflow measurements conducted in the Aeroelastic Rotor Test Chamber at Georgia Institute of Technology. Although the model is intended primarily as a representation of unsteady aerodynamics for aeroelasticity applications, the results show that it has an excellent capability in predicting the inflow distribution in hover except near the root and tip. In addition, the computed unsteady spanwise lift distribution of a rotor is compared with that from an unsteady vortex lattice method for pitch oscillations at various frequencies. The new model is shown to be capable of prediction of unsteady loads typical of aeroelastic response.

Optimization of rotor blades for combined structural, dynamic, and aerodynamic properties

Abstract: A helicopter is intrinsically interdisciplinary due to the close coupling among aerodynamics, dynamics, and the blade structural details. Therefore a design optimization with proper interactions among appropriate disciplines (such as structure, dynamics, and aerodynamics) can offer significant benefit to improve rotor performance. This paper studies the integration of structure, dynamics, and aerodynamics in design optimization of helicopter rotor blades. The optimization is performed to minimize the rotor power required and to satisfy design requirements from structure (minimum blade weight and safe stress margin and fatigue life) and dynamics (proper placement of blade natural frequencies and free of flutter). An effort is made to formulate an effective strategy for combining these various requirements in the optimization process. The paper also presents a way for an intelligent phasing of this interdisciplinary optimization to overcome the hurdles due to conflicting demands on the design variables which arise from different disciplines.

A demonstration of passive blade twist control using extension-twist coupling

Abstract: The results from a study aimed at improving the dynamic and aerodynamic characteristics of composite rotor blades through the use of extension-twist coupling are presented. A set of low twist model-scale helicopter rotor blades was manufactured with a view towards demonstrating the passive blade twist control concept. Hover testing of the blades was conducted to measure the change in blade twist as a function of rotor speed. The blades were spun through the 0-800 rpm range, with a corresponding sweep of collective pitch to determine the effect on the blade elastic twist. Hover data were obtained for both a ballasted and unballasted blade configuration in atmospheric conditions, where maximum twist changes of 2.54 and 5.24 degrees were respectively observed. These results compared well with those from a finite element analysis of the blade, which yielded maximum twists of 3.01 and 5.61 degrees for the unballasted and ballasted blade configurations, respectively. The aerodynamic-induced effects on the blade elastic twist, determined by testing a ballasted blade configuration in a near-vacuum condition, were found to be minimal with a maximum twist difference of 0.17 degrees observed between the two test environments. The effect of collective pitch sweep on the elastic twist was minimal.

Adequacy of modeling turbulence and related effects on helicopter response

Abstract: 'Blade-fixed' atmospheric turbulence encountered by a helicopter substantially differs from body-fixed (i.e., fuselage) turbulence, because the rotational velocity moves the blade station fore and aft through the turbulence. The present closed-form solution of a frequency-time spectrum for the dominant vertical turbulence velocity at arbitrary blade station defines in what way and in what degree the rotational velocity affects rotor-disk turbulence frequency and temporal characteristics. Blade-flapping response to blade- and body-fixed turbulence is also presented over a range of turbulence-scale length and advance ratio.

A Study on the Feasibility of Using Adaptive Structures in the Attenuation of Vibration Characteristics of Rotary Wings.

Abstract: This paper investigates the feasibility of employing adaptive material to build both sensors and actuators to attenuate the higher harmonic loads developed at the helicopter rotor blades using the elastic flatwise bending (second for hingeless rotors) and the first elastic torsion modes of a single blade deserve special attention in the vibration control. Theoretical investigations, supported by wind tunnel and flight tests, confirmed that these modes are responsible for the larger amplitude loads at 3/rev in four-blade hingeless rotors. This is a situation for which IBC, based on a collocated actuator-sensor arrangement along the blade, and tailored to act specifically on the bending and the torsion modes, is expected to bring further improvements to the reduction of the overall dynamic response of helicopters. The results indicate that there are already real situations for which the adaptive material has enough power to accomplish the task without saturation of the applied electrical field.

Anti-air weapon system for helicopter neutralisation - has ground system launching projectile which opens out revealing wires which impale on rotor blades

Abstract: The projectile launcher and detector (2) detects a helicopter and sets up a firing trajectory (21) to intercept the target. The projectile is launched towards the target, and the target is detected at the point of intercept. The projectile then enters its deployment phase, releasing a central module. The module consists of a cable connecting the front and rear sections (38,35) with sub-modules (34) deploying a number of radial wires (40). The wires are designed to slice through the helicopter rotor blades. The central cable is kept tensioned by jets on the front and rear projectile section pointing in opposite directions. ADVANTAGE - Type of helicopter does not need to be known for successful engagement to occur.

Unsteady transition measurements on a pitching three-dimensional wing

Abstract: Boundary layer transition measurements were made during an experimental study of the aerodynamics of a rectangular wing undergoing unsteady pitching motions. The wing was tested at chordwise Mach numbers between 0.2 and 0.6, at sweep angles of 0, 15, and 30 deg, and for steady state, sinusoidal, and constant pitch rate motions. The model was scaled to represent a full size helicopter rotor blade, with chord Reynolds numbers between 2 and 6 x 10(exp 6). Sixteen surface hot-film gages were located along three spanwise stations: 0.08, 0.27, and 0.70 chords from the wing tip. Qualitative heat transfer information was obtained to identify the unsteady motion of the point of transition to turbulence. In combination with simultaneous measurements of the unsteady surface pressure distributions, the results illustrate the effects of compressibility, sweep, pitch rate, and proximity to the wing tip on the transition and relaminarization locations.

Helicopter rotor blade aeroelasticity in forward flight with an implicit structural formulation

Abstract: This paper describes an aeroelastic stability and response analysis in which the structural operator is formulated numerically, without expanding analytically the various algebraic expressions that make up the beam model.

Analytic Formulation of Unsteady Profile Aerodynamics and its Application to Simulation of Rotors

Abstract: The steady aerodynamics characteristics of airfoils are represented by analytical formulas. Unsteady aerodynamics are modeled using the approximations to the Wagner and Kussner functions. The dynamic stall angle of attack is modeled in a similar way. The results are validated using steady and dynamic experimental data in attached and separated flow. Finally the application to helicopter rotor simulation is shown.

Rotorcraft blade-vortex interaction controller

Abstract: Blade-vortex interaction noises, sometimes referred to as "blade slap", are avoided by increasing the absolute value of inflow to the rotor system of a rotorcraft. This is accomplished by creating a drag force which causes the angle of the tip-path plane of the rotor system to become more negative or more positive.

Recent advances in integrated multidisciplinary optimization of rotorcraft

Abstract: A joint activity involving NASA and Army researchers at NASA LaRC to develop optimization procedures to improve the rotor blade design process by integrating appropriate disciplines and accounting for all of the important interactions among the disciplines is described. The disciplines involved include rotor aerodynamics, rotor dynamics, rotor structures, airframe dynamics, and acoustics. The work is focused on combining these five key disciplines in an optimization procedure capable of designing a rotor system to satisfy multidisciplinary design requirements. Fundamental to the plan is a three-phased approach. In phase 1, the disciplines of blade dynamics, blade aerodynamics, and blade structure are closely coupled while acoustics and airframe dynamics are decoupled and are accounted for as effective constraints on the design for the first three disciplines. In phase 2, acoustics is integrated with the first three disciplines. Finally, in phase 3, airframe dynamics is integrated with the other four disciplines. Representative results from work performed to date are described. These include optimal placement of tuning masses for reduction of blade vibratory shear forces, integrated aerodynamic/dynamic optimization, and integrated aerodynamic/dynamic/structural optimization. Examples of validating procedures are described.