Showing papers on "Helicopter rotor published in 1991"

Numerical solutions of forward-flight rotor flow using an upwind method

Abstract: A finite-volume upwind algorithm for solving the three-dimensional Euler equations with a moving grid has been developed for computing helicopter forward-flight rotor flows. The computed pressure distributions and shock positions of high-speed rotor flow are compared with various experimental data as well as with other numerical results, and the agreement is encouraging

Helicopter rotor blades

Abstract: Flow separation behind the notch region of a swept tip of a helicopter rotor blade is reduced by features of the geometry and aerodynamic characteristics of the notch region itself. In particular it has been found beneficial for a forwardly swept leading edge portion to extend at an angle between 30 degrees and 55 degrees from a reference line parallel to a blade pitch change axis and the leading edge of the aerofoil in the notch region incorporates blade droop.

Navier-Stokes computation of wing/rotor interaction for a tilt rotor in hover

Abstract: A method has been developed to analyze the wing/rotor interaction of tilt rotor aircraft in hover. The unsteady, thin-layer compressible Navier-Stokes equations are solved using an implicit, finite difference scheme that employs LU-ADI factorization. The rotor is modeled as an actuator disk which imparts a radial and azimuthal distribution of pressure rise and swirl to the flowfield. The 'chimera' approach of grid point blanking is used to update the rotor boundary conditions. Results are presented for both a rotor alone and for wing/rotor interaction where the thrust coefficient is 0.0164 and wing flap deflection is 67 degrees. Many of the complex flow features are captured including the fountain effect, leading and trailing edge separation, and the unsteady wake beneath the wing. Wing surface pressures compare fairly well with experimental data although the time-averaged download is about twenty percent higher than the measured value. This discrepancy is due to a combination of factors that are discussed.

Helicopter low-speed yaw control

Abstract: A system for improving yaw control at low speeds consists of one strake placed on the upper portion of the fuselage facing the retreating rotor blade and another strake placed on the lower portion of the fuselage facing the advancing rotor blade. These strakes spoil the airflow on the helicopter tail boom during hover, low speed flight and right or left sidewards flight so that less side thrust is required from the tail rotor.

Flexbeam helicopter rotor with improved snubber-vibration damper between the torque tube and the flexible spar member

Abstract: of the Disclosure Flexbeam Helicopter Rotor With Improved Snubber-Vibration Damper Between the Torque Tube and the Flexible Spar Member A flexbeam helicopter rotor having an improved snubber-vibration damper for positioning the flexbeam spar relative to the torque tube, wherein the snubber-damper is an elastomeric bearing having a spherical and a flat portion, and whose laminates are continuous, selectively preloaded and of selected material to provide increased fatigue life, size reduction, reduced laminate stress concentration, and reduced critical design stress in the snubber-vibration damper.

Aeroelastic optimization of a helicopter rotor using an efficient sensitivity analysis

Abstract: A structural optimization analysis of a hingeless helicopter rotor is developed and applied with the objective of reducing oscillatory hub loads in forward flight. The aeroelastic analysis of the rotor is based on a finite element method in space and time and is linked with automated optimization algorithms. Two types of structural blade representations are used: a generic stiffness-distribution beam and a single-cell, thin-walled beam. For the generic beam representation the design variables are nonstructural mass and its placement, chordwise center of gravity offset from the elastic axis, and structural stiffness (flap, lag, and torsion). For the second type of structural representation, spar width, height, and thickness are used as design variables instead of blade stiffness. Constraints on frequency placement, autorotational inertia, and aeroelastic stability of the blade are included. Sensitivity derivatives are efficiently calculated using a direct analytical approach, with a resulting 80% reduction in total CPU time required to obtain an optimum solution compared with a commonly used finite-difference approach. Optimum solutions resulted in reductions of 25-77% for the generic blade, and 30-50% for the box-beam blade relative to baseline values of the objective function that was comprised of all six components of hub load.

Main helicopter rotor

Abstract: A main helicopter rotor has a number of blades, and a drift shaft supporting at its top end, a hub consisting of a rigid annular body integral and coaxial with the drive shaft. An elastically deformable torodial body surrounds and is connected to the annular body for defining, with the same, a closed-section torodial assembly. Each blade is connected to the hub by a connecting element extending at least partially through the annular body and the torodial body, and connected respectively to the same by a first and second spherical elastomeric bearing.

Helicopter with retractable rotor for transport

Abstract: A helicopter with a retractable rotor for transport wherein the rotor and its blades, the swashplate, the pitch change rods and support mast are lowered or raised in unison by the hydraulic flight control servos as commanded by the helicopter computerized flight control when being commanded by special rotor retraction/extension software.

Helicopter anti-torque device direct pitch control

Abstract: The blade angle controlling pitch beam servo (26) of a helicopter tail rotor (22) is responsive to a main rotor torque signal (92) indicative of torque coupled to a helicopter main rotor (10) for providing torque compensation so that the helicopter airframe will not counter-rotate under the main rotor. The torque coupled to the main rotor is that amount of engine torque (76, 114) in excess of torque coupled to the tail rotor (81, 115) and helicopter auxiliaries (84, 116). The amount of torque compensation provided by the tail rotor is reduced by an amount indicative of aerodynamic forces (130) on the helicopter airframe.

Prediction of blade wake interaction noise based on a turbulent vortex model

Abstract: Blade wake interaction is defined as the broadband noise generated by the interaction of helicopter rotor blades with their own wake. Experimental observations have shown that this is a strong function of advance ratio and tip path plane angle. This paper describes how this noise source can be associated with the blade vortex interactions in the forward sector of the rotor. Measured levels of turbulence in the vortex core are used to predict the broadband noise levels with some success. However, more detailed information on the turbulence spectrum and the trajectory of the shed vortices is required before more accurate noise predictions can be made.

Response and bifurcation analysis of a MDOF rotor system with a strong nonlinearity

Abstract: A new HB (Harmonic Balance)/AFT (Alternating Frequency Time) method is further developed to obtain synchronous and subsynchronous whirling response of nonlinear MDOF rotor systems. Using the HBM, the nonlinear differential equations of a rotor system can be transformed to algebraic equations with unknown harmonic coefficients. A technique is applied to reduce the algebraic equations to only those of the nonlinear coordinates. Stability analysis of the periodic solutions is performed via perturbation of the solutions. To further reduce the computational time for the stability analysis, the reduced system parameters (mass, damping, and stiffness) are calculated in terms of the already known harmonic coefficients. For illustration, a simple MDOF rotor system with a piecewise-linear bearing clearance is used to demonstrate the accuracy of the calculated steady-state solutions and their bifurcation boundaries. Employing ideas from modern dynamics theory, the example MDOF nonlinear rotor system is shown to exhibit subsynchronous, quasi-periodic and chaotic whirling motions.

Nonlinear control of a twin-lift helicopter configuration

Abstract: Significant pilot work load involved in flying a twin-lift helicopter configuration necessitates the development of highly augmented flight control systems for satisfactory performance. Two nonlinear control philosophies based on feedback linearization are advanced for this configuration. The controller performance, together with the sensitivity to a few parameter variations, is studied in a nonlinear simulation. Controller implementation aspects are discussed.

Results of a sub-scale model rotor icing test

Abstract: A heavily instrumented sub-scale model of a helicopter main rotor was tested in the NASA Lewis Research Center Icing Research Tunnel (IRT) in September and November 1989. The four-bladed main rotor had a diameter of 1.83 m (6.00 ft) and the 0.124 m (4.9 in) chord rotor blades were specially fabricated for this experiment. The instrumented rotor was mounted on a Sikorsky Aircraft Powered Force Model, which enclosed a rotor balance and other measurement systems. The model rotor was exposed to a range of icing conditions that included variations in temperature, liquid water content, and median droplet diameter, and was operated over ranges of advance ratio, shaft angle, tip Mach number (rotor speed) and weight coefficient to determine the effect of these parameters on ice accretion. In addition to strain gage and balance data, the test was documented with still, video, and high speed photography, ice profile tracings, and ice molds. The sensitivity of the model rotor to the test parameters, is given, and the result to theoretical predictions are compared. Test data quality was excellent, and ice accretion prediction methods and rotor performance prediction methods (using published icing lift and drag relationships) reproduced the performance trends observed in the test. Adjustments to the correlation coefficients to improve the level of correlation are suggested.

Incremental force and moment coefficients for a parallel blade-vortex interaction

Abstract: Blade-vortex interactions occur in helicopter rotors when a rotor blade passes close to or through a tip vortex trailing from the same or another blade. An unsteady, parallel blade-vortex interaction model was developed using a discrete free-vortex method and classical potential flow theory. The tip vortex was modeled by a simple vortex and, for a close encounter, by a cluster of vortices.

Helicopter rotor dynamics optimization with experimental verification

Abstract: An automated design optimization analysis was developed to efficiently predict helicopter blade structural properties leading to improved dynamic behavior. Modal-based optimization criteria were defined for calculation by a coupled-mode eigenvalue analysis. The optimization analysis was applied to various rotor dynamics problems to predict potential design benefits and evaluate techniques for improving optimizer performance. Design parameter scaling, algorithm selection, and objective function formulation were shown to have a significant influence on optimizer performance. Two experimental programs were also conducted to verify the analysis. In each program, formal optimization techniques were applied to modify the structural properties of dynamically scaled models to improve certain operating characteristics. In the first, the edgewise structural bending-stiffness distribution of a bearingless rotor model flexure was optimized to maximize the edgewise structural damping ratio, while maintaining acceptable blade frequency placement. In the second, the blade spanwise mass distribution and structural bending stiffness of an articulated rotor model were varied to minimize rotor vibratory loads in forward flight. Direct comparison of experimental results from baseline and optimized rotors for each test program verified the reliability of the selected optimization criteria and showed that the modal-based analysis can be used to achieve significant improvements in aeromechanical stability and rotor vibratory response.

A study of fundamental issues in higher harmonic control using aeroelastic simulation

Abstract: This paper describes a higher harmonic control (HHC) study of a four-bladed helicopter rotor using a coupled flap-lag-torsional aeroelastic stability and response analysis which incorporates finite-state, time-domain aerodyamics. The rotor trim condition is determined using a simplified flap-lag-torsional aeroelastic analysis. Deterministic and cautious controllers based on local and global HHC models are implemented to reduce 4/rev hub loads on roughly equivalent articulate and hingeless rotors. Results obtained for the case when the objective function requires reduction of hub shears alone are compared with results obtained when hub shears and moments are reduced simultaneously. The effects of HHC on blade aeroelastic stability are also considered. Finally, the power requirements for implementing HHC on a hingeless rotor are compared with those obtained for an articulated rotor. It is concluded that hingeless rotors require considerably more power and higher control angles than comparable articulated rotors.

Rotor design optimization using a multidisciplinary approach

Abstract: A multidisciplinary optimization tool for helicopter rotor blade design has been developed. It uses a comprehensive analysis program, CAMRAD/JA, capable of performing analyses in all involved disciplines in a consistent and efficient manner, together with CONMIN's method of feasible directions. Design variables, constraints, and objective functions have been chosen to address actual design requirements in a realistic manner. The optimization procedure setup provides the flexibility to take full advantage of the comprehensive nature of the analysis code, allowing optimization driven by aerodynamic, aeroelastic, and flight mechanics design requirements. The optimization tool is applied to the McDonnell Douglas Helicopter Company AH-64A, a modern, high performance helicopter. Results are presented for combined hover/forward flight performance optimization, fuselage vibration reduction, and combined performance/vibration optimization. Blade aerodynamic and structural properties are used as design variables. The optimized designs show significant improvements and demonstrate that a practical and efficient optimization tool has been developed.

Fundamental studies of rotor wakes in low speed forward flight using wide-field shadowgraphy

Abstract: Experiments were conducted using the wide-field shadowgraph method to visualize the wake geometry from a helicopter rotor in low speed forward flight. The experiments were performed with an isolated rotor and with a body representing a helicopter fuselage. Particular attention was paid to documenting the isolated rotor wake geometry, blade vortex interaction phenomena near the rotor plane, the distortion made to the wake due to the presence of the body, and detailing the interaction of the tip vortices with the body surface. Quantitative measurements were made of the wake trajectories as functions of wake age, as well as estimates of the tip vortex core radius. The results have provided many details of the rotor wake structure that are useful for validating rotor wake and rotor/body interactional models in forward flight. The use of the wide-field shadowgraph method offers an important tool for visualizing rotor wake vortices, and helping to understand the complex three-dimensional nature of rotor wakes in forward flight.

Performance optimization of helicopter rotor blades

Abstract: As part of a center-wide activity at NASA Langley Research Center to develop multidisciplinary design procedures by accounting for discipline interactions, a performance design optimization procedure is developed. The procedure optimizes the aerodynamic performance of rotor blades by selecting the point of taper initiation, root chord, taper ratio, and maximum twist which minimize hover horsepower while not degrading forward flight performance. The procedure uses HOVT (a strip theory momentum analysis) to compute the horse power required for hover and the comprehensive helicopter analysis program CAMRAD to compute the horsepower required for forward flight and maneuver. The optimization algorithm consists of the general purpose optimization program CONMIN and approximate analyses. Sensitivity analyses consisting of derivatives of the objective function and constraints are carried out by forward finite differences. The procedure is applied to a test problem which is an analytical model of a wind tunnel model of a utility rotor blade.

Computational and experimental evaluation of helicopter rotor tips for high-speed flight

Abstract: A computational and experimental method for the evaluation of helicopter rotor tips in high-speed forward flight is presented which uses an unsteady full-potential solver on the advancing side of the rotor disk. Forces and moments measured during angle-of-attack sweeps reveal the soft-stall phenomenon of the double-swept planforms as well as the delayed stall of the single-swept hyperbolic tip. Double-swept planforms are shown to exhibit the most favorable performance, creating counter-rotating vortices that augment the lifting capabilities of the blade at high angles of attack and delay the onset of stall.

Nonlinear large amplitude vibration of composite helicopter blade atlarge static deflection

Abstract: The nonlinear, large amplitude nonrotating free vibration of composite helicopter blades under large static deflection is investigated analytically and experimentally. A new model capable of handling large amplitudes as well as large deflections was developed based on the use of Euler angles and a harmonic balance, finite difference solution of the basic large deflection equations. The behavior of the first and second bending, the first fore-and-aft, and the first torsional modes of [0/90]3s and [45/0]$ graphite/epoxy flat beams were explored analytically as tip static deflection and tip amplitudes vary* Free vibration tests of several different lay-ups of composite blades show good agreement between theory and experiment. It is found that both large static deflection and large amplitudes can affect the fore-and-aft and torsional modes significantly, but bending modes are not influenced much by the geometrical nonlinearities.

Articulated helicopter rotor with an improved blade-to-hub connection

Abstract: An articulated helicopter rotor wherein the blade-to-hub connection consists of a spherical elastomeric bearing connected to the hub and a composite yoke connected thereto in series and consisting of a plurality of unidirectional high strength fibers bonded together to form a continuous strap which passes through a central opening in the elastomeric bearing and is pin-wrap connected to the hub at a single station and to the blade at two spaced stations produces a construction which is light in weight, easy to manufacture, ballistrically tolerant, redundant, and readily visually inspectable for structural integrity.

Recent rotorcraft aeroelastic testing in the Langley Transonic Dynamics Tunnel

Abstract: Wind-tunnel testing of a properly scaled aeroelastic model helicopter rotor is considered a necessary phase in the design and development of new rotor systems. For this reason, extensive testing of aeroelastically scaled model rotors is done in the Transonic Dynamics Tunnel (TDT) located at the Langley Research Center. A unique capability of this facility, which enables proper dynamic scaling, is the use of diflourodichloromethane, or Refrigerant-12 (R-12) as a test medium. The paper presents a description of the TDT and a discussion of the benefits of using R-12 as a test medium. A description of the system used to conduct model tests is provided and examples of recent rotor tests are cited to illustrate the types of aeroelastic model rotor tests conducted in the TDT.

HHC Study in the DNW to Reduce BVI Noise-An Analysis

Abstract: The noise of an aeroelastically scaled helicopter rotor has been studied in the German-Dutch wind tunnel in order to assess the utility of higher-harmonic control (HHC) in reducing blade-vortex interaction (BVI) noise Acoustic data are presented for 3/rev, 4/rev, and 5/rev HHC, as applied to a typical landing approach rotor operating condition; noise reduction of up to 6 dB were found for advancing-blade BVI noise radiating upstream of the rotor, as well as for retreating blade BVI noise radiating below and downstream of the rotor

Introduction of the M-85 high-speed rotorcraft concept

Abstract: As a result of studying possible requirements for high-speed rotorcraft and studying many high-speed concepts, a new high-speed rotorcraft concept, designated as M-85, was derived. The M-85 is a helicopter that is reconfigured to a fixed-wing aircraft for high-speed cruise. The concept was derived as an approach to enable smooth, stable conversion between fixed-wing and rotary-wing while retaining hover and low-speed flight characteristics of a low disk loading helicopter. The name, M-85, reflects the high-speed goals of 0.85 Mach number at high altitude. For a high-speed rotorcraft, it is expected that a viable concept must be a cruise-efficient, fixed-wing aircraft so it may be attractive for a multiplicity of missions. It is also expected that a viable high-speed rotorcraft concept must be cruise efficient first and secondly, efficient in hover. What makes the M-85 unique is the large circular hub fairing that is large enough to support the aircraft during conversion between rotary-wind and fixed-wing modes. With the aircraft supported by this hub fairing, the rotor blades can be unloaded during the 100 percent change in rotor rpm. With the blades unloaded, the potential for vibratory loads would be lessened. In cruise, the large circular hub fairing would be part of the lifting system with additional lifting panels deployed for better cruise efficiency. In hover, the circular hub fairing would slightly reduce lift potential and/or decrease hover efficiency of the rotor system. The M-85 concept is described and estimated forward flight performance characteristics are presented in terms of thrust requirements and L/D with airspeed. The forward flight performance characteristics reflect recent completed wind tunnel tests of the wing concept. Also presented is a control system technique that is critical to achieving low oscillatory loads in rotary-wing mode. Hover characteristics, C(sub p) versus C(sub T) from test data, is discussed. Other techniques pertinent to the M-85 concept such as passively controlling inplane vibration during starting and stopping of the rotor system, aircraft control system, and rotor drive technologies are discussed.

Helicopter blade crack detection system

Abstract: The invention is a helicopter blade crack detection system for blades that are hollow and pressurized or evacuated to a low pressure A rotating assembly having a rotating indicator assembly photo-optically communicates to an airframe detection assembly providing high reliability, ease of maintenance, and EMI secured.