Showing papers on "Helicopter rotor published in 1971"

An Analytical and Experimental Investigation of Helicopter Rotor Hover Performance and Wake Geometry Characteristics

Abstract: : An analytical and experimental investigation was conducted to acquire systematic model rotor performance and wake geometry data and to evaluate the accuracy of various analytical methods in predicting the effects on performance of changes in helicopter rotor design and operating parameters. Both classical hover performance analyses and analytical methods recently developed at the United Aircraft Research Laboratories were evaluated. Of primary concern in the study was the assessment of assumptions in the analyses regarding the geometry of the rotor wake. Of particular interest was the prediction by the analysis of an instability of the tip vortex helix at moderate distances from the rotor which appeared to be substantiated by available experimental results.

Blade for high speed helicopter

Abstract: To alleviate a helicopter blade instability which manifests itself as a submiltiple oscillation of the rotor tip path plane during high speed flights, the blade tip is thinned between its inboard end and its outboard end so that the outboard end is approximately one half the thickness of the inboard end, while the tip chord remains substantially constant, and the blade tip is swept rearwardly to shift its aerodynamic center rearwardly and thereby establish a moment to counteract the instability and preferably, the blade tip is selectively cambered to delay retreating blade stall.

Detailed aerodynamic measurements on a model rotor in the blade stall regime

Abstract: Retreating blade stall experiments on model helicopter rotor, considering series of pressure distribution and boundary layer events

A method for predicting helicopter wake geometry, wake-induced flow and wake effects on blade airloads

Abstract: Wake model and computer program to compute geometries, flows and velocity influence coefficients for helicopter blade load calculations

An Investigation of the Quantitative Applicability of Model Helicopter Rotor Wake Patterns Obtained from a Water Tunnel

Abstract: : An analytical investigation was conducted to evaluate quantitative applications of available model rotor tip vortex patterns from a water tunnel. This evaluation consisted of reducing selected photographic wake data to coordinate form, and comparing the resulting wake geometries with both theoretical results and available experimental data taken in air. Additionally, the study included an examination of the sensitivity of water tunnel wake geometry to the water tunnel test parameters which did not duplicate full-scale rotor values and the applicability of a water tunnel wake geometry to determine the airloads of a full-scale rotor. Finally, the possibility of developing simplified wake documentation and generalization procedures and improved water tunnel test techniques was assessed.

System for determining the fatigue of a structure experiencing varying stresses

Abstract: A system for measuring fatigue damage in helicopter blades is based on a determination of the number of force cycles to which the blade is subjected and on the amplitude of the force experienced in each cycle.

Optimal Takeoff Trajectories of a Heavily Loaded Helicopter

Abstract: Optimal control theory has been applied to the STOL takeoff of a heavily loaded helicopter. The object of the analysis was to determine constrained and unconstrained extremal trajectories that maximize the terminal vertical distance for a given fixed horizontal distance. The resulting optimal trajectory consists of an initial acceleration segment followed by a decelerating segment in which most of the terminal altitude is gained. If the problem was constrained to nondecelerating flight, the resulting optimum consisted of a maximum acceleration segment followed by a constant velocity climbout segment. The slightly decreased performance of the constrained optimum was more than offset by its ease of implementation. Nomenclature A — area of rotor disc Ci-s = momentum theory expansion coefficients CPL = power loss eg = helicopter center of gravity / = wetted drag area of HLD helicopter /o = the derivative of the payoff function with respect to time g = acceleration of gravity HLD = heavily loaded hg = vertical distance measured from the ground plane J = payoff function m = mass of the helicopter Pd = power difference in steady level flight R = radius of rotor R/C = rate of climb ST = reference distance T = thrust t = time Vio = induced velocity V = velocity of the helicopter Vm = smallest velocity where equilibrium flight of the helicopter out of ground effect is possible Vr = reference velocity: induced velocity at hover out of ground effect Vsi = static best climb angle speed Vs2, — static best rate of climb speed W = weight xg = horizontal distance measured on the ground plane Z = vertical height of the rotor above the ground plane CLTP = angle of attack of the rotor tip path plane /3 = dy/dt = control variable rj = dV/dt = control variable 7 = flight path angle dvio = effective reduction of induced velocity of the rotor due to ground effect \H = inflow through the rotor plane nondimensionalized by SIR

Development and application of a method for predicting rotor free wake positions and resulting rotor blade air loads. Volume 1: Model and results

Abstract: Rotor wake geometries are predicted by a process similar to the startup of a rotor in a free stream. An array of discrete trailing and shed vortices is generated with vortex strengths corresponding to stepwise radial and azimuthal blade circulations. The array of shed and trailing vortices is limited to an arbitrary number of azimuthal steps behind each blade. The remainder of the wake model of each blade is an arbitrary number of trailing vortices. Vortex element end points were allowed to be transported by the resultant velocity of the free stream and vortex-induced velocities. Wake geometry, wake flow, and wake-induced velocity influence coefficients are generated by this program for use in the blade loads portion of the calculations. Blade loads computations include the effects of nonuniform inflow due to a free wake, nonlinear airfoil characteristics, and response of flexible blades to the applied loads. Computed wake flows and blade loads are compared with experimentally measured data. Predicted blade loads, response and shears and moments are obtained for a model rotor system having two independent rotors. The effects of advance ratio, vertical separation of rotors, different blade radius ratios, and different azimuthal spacing of the blades of one rotor with respect to the other are investigated.

Rotor system having viscoelastic lead-lag damper

Abstract: A rotor system for a helicopter or the like vertical lift aircraft in which lead-lag motions of a blade are effectively damped by a body of viscoelastic material that is stressed preferentially in shear by the lead-lag movement of the blade.

Helicopter rotor pitch control system

Abstract: A blade pitch control assembly in accordance with the invention comprises an inner disc rotating with the rotor shaft and an outer non-rotating connected connected to the first disc by a bearing and which can be tilted by pitch variation control means.

Critical speeds of a rotor in rigidly mounted, externally pressurized, air-lubricated bearings

Abstract: Calculation of rotor critical speeds for two bearing rotor system from rigid body theory using film stiffness as determined from zero speed, load eccentricity data

An Experimental Study of Helicopter Rotor Impulsive Noise

Abstract: : Results of a study of helicopter rotor impulsive noise (RIN) are presented. Rotor noise, together with rotor blade dynamic and pressure data, was measured during hover and cruise of a CH-53A helicopter for use in a correlation study of calculated and measured noise. In addition, the rotor rotational noise analysis described in U. S. Army Aviation Materiel Laboratories (USAAVLABS) technical Report 70-1B was modified to reduce computation time and to include blade flapping and coning motions. The inclusion of these motions, however, is shown to have little effect on the predicted noise. Correlation of calculated and measured noise harmonic amplitudes is generally within 5 dB through the third harmonic at distances less than 1000 feet in front of the helicopter. Waveform correlation of calculated and measured time histories of acoustic pressure is good. RIN is identified as being primarily a rotational noise phenomenon, ordered at the blade passage frequency and its harmonics, rather than amplitude modulated broadband noise. Hover RIN is shown to be due to vortex interference (blade/wake interaction RIN), while cruise RIN is shown to be due to the combination of acoustic effects of a high subsonic tip Mach number on wave propagation and blade drag, and is referred to as advancing blade RIN.

Analytical investigation of the effects of blade flexibility, unsteady aerodynamics, and variable inflow on helicopter rotor stall characteristics

Abstract: Analytical investigation of effects of blade flexibility, unsteady aerodynamics, and variable inflow on helicopter rotor stall characteristics

Rotary wing aircraft with folding rotor blades

Abstract: The invention relates to rotary wing aircraft having at least one four-bladed rotor system and discloses means for folding the four rotor blades into a stowed position in which the blades are arranged in two partly overlapping pairs located one pair on each side of the longitudinal centreline of the fuselage.

An Energy Method for Prediction of Helicopter Maneuverability

Abstract: : In response to Phase 3 of Contract F33615-70-C-1052, the necessary methodology has been developed to provide an independent analysis capability to Air Force Studies Analysis (AFSA) for accurately simulating the flight paths of single main rotor helicopters. The equations developed under Phase I of this contract have been generalized to represent other single main rotor helicopters that are currently operational and those designed for the near future. This report reviews the necessary equations for the determination of power required in accelerated flight and a method for determining the input coefficients based on the helicopter's physical parameters and power required in one g flight. The impact of the type of rotor system on the maximum thrust which can be produced is examined in light of the available flight test data. The concepts of energy maneuverability for a helicopter are examined using the AH-1G helicopter as an example. Several applications of these equations are considered including terrain following, decelerating turns, and low speed maneuvering.

Wake and boundary layer effects in helicopter rotor aerodynamics

Abstract: Helicopter wake and boundary layer effects on rotor aerodynamic performance in hovering, low and high speed forward flight

An Investigation of Noise Generation on a Hovering Rotor

Abstract: : This report presents the results of a program of helicopter rotor noise measurement. The program was carried out using a 60-foot diameter CH-47B 3-bladed rotor on the Boeing-Vertol engineering rotor whirl tower. The primary objectives were: to obtain acoustical data over a frequency range wide enough to define all elements of rotor noise under well-documented ambient conditions, to measure the tip vortex position with respect to a trailing blade using high- speed cameras and smoke to visualize the tip vortex and to relate blade-vortex separation distance to noise level, to determine the propagation characteristics of rotor noise, and to evaluate two current analytical procedures for predicting rotor noise against the measured data. Tipspeeds ranged from 600 to 900 fps and thrusts ranged from 6300 to 32,000 pounds (disk load 2.2 to 11.3 pounds per square foot).

A Full-Scale Experimental Feasibility Study of Helicopter Rotor Isolation Using the Dynamic Antiresonant Vibration Isolator

Abstract: : The report contains the results of a full-scale experimental feasibility study of rotor isolation employing the Dynamic Antiresonant Vibration Isolator (DAVI). The full-scale experiments were performed on a UH-2 helicopter fuselage isolated from a simulated rotor and transmission. Tests were conducted for three directions of vibratory input at the hub on the nonisolated vehicle and then compared to results obtained for the isolated vehicles. Tests on the isolated helicopter were conducted for two-bladed, three-bladed, and four-bladed rotor configurations. These rotor configurations were simulated by proper tuning of the DAVI isolation system to the predominant excitation frequency of the rotor systems. Results of this experimental study on a 6500-pound helicopter show that rotor isolation is feasible. Excellent reduction of vibration throughout the fuselage was obtained at the predominant excitation frequency (n-per-rev). This was accomplished with low static deflection, minimum weight penalty, and small relative vibratory deflections between the rotor and fuselage.

Full-Scale Wind Tunnel Investigation of the Advancing Blade Concept Rotor System

Abstract: : A 40-foot-diameter ABC (coaxial) rotor system was tested in a 40 ft x 80 ft wind tunnel The six rigid blades were instrumented to measure flatwise, edgewise, and torsional strain Advance ratios up to 091 and tip Mach numbers to 083 were tested Lateral displacement of individual rotor lift was varied The performance, control, stress, and vibration data recorded during these tests are presented and discussed

Autogyros, especially permitting the use of these devices as economical air-ground vehicles

Abstract: Improvements in the rotor system of autogyros permitting the easy folding back of the rotor blades and to ensure the coincidence of the blade axes in a single point.

Flight Evaluation of Elastomeric Bearings in an AH-1 Helicopter Main Rotor

Abstract: : Presented in this report are the results of a flight test program conducted to evaluate elastomeric bearings in the main rotor of an AH-1G helicopter. An experimental main rotor was fabricated and tested using only elastomeric bearings in both the flapping and pitch change axes. With this rotor, the flapping bearings carry the rotor drive and lift loads and allow the flapping motions. Two pitch change bearings were used in each grip to carry the blade bending and shear loads and transfer the blade centrifugal force to the rotor yoke. These bearings also accommodate the blade collective and cyclic pitch change motions. Torsional, radial, and axial load deflection curves are given for the flapping and pitch change bearings. In addition, the results from a limited endurance test program are included for the pitch change bearing. The purpose of the endurance tests was to assure that bearing metal parts would not fail prior to elastomer shredding. Thus, visual inspection of the elastomeric bearings was considered sufficient for safety-of-flight during the test program. The test results have shown the feasibility of elastomeric bearings for helicopter main rotor applications.

Test Section Size Influence on Model Helicopter Rotor Performance

Abstract: : The study indicates that the successful testing of model helicopter rotors in tunnels when in the hover and transitional flight regimes requires not only Reynolds number scaling, where the Reynolds number is recognized as controlling the rotor's maximum section lift coefficient, but also a scaling of a characteristic defined here as the wake energy dissipation pattern. The dissipation of the energy from the rotor into the surrounding fluid is a function of the viscosity of the test fluid; thus, operation of a reduced scale model in the same fluid as that in which the full-scale vehicle operates makes appropriate scaling of this wake characteristic difficult if Reynolds number scaling is to be maintained. These conflicting factors have led to the requirements of relatively large models and correspondingly large transitional flight modes are of interest. The use of water as the test fluid for scale model studies permits an easier maintaining of Reynolds number scaling while having the scale model produce a wake energy dissipation pattern similar to that of the full-scale vehicle because of the advantages water possesses over air in terms of kinematic and dynamic viscosities.

Trim, control, and stability of a gyro-stabilized hingeless rotor at high advance ratio and low rotor speed

Abstract: Development of methods for measuring and predicting behavior of rigid rotors with stiff blades at high advance ratios and low rotor speeds

An Experimental Investigation of Compound Helicopter Aerodynamics in Level and Descending Forward Flight and in Ground Proximity

Abstract: : A series of experiments was performed on the Princeton Dynamic Model Track on a compound helicopter model with an 8-foot-diameter hingeless rotor. Rotor, wing and fuselage forces and moments were measured as functions of advance ratio, rotor angle of attack, and collective pitch at various combinations of wing size and position, including wing off and wing and fuselage off. Test conditions included forward level flight, partial-power steep descent, and slow longitudinal and lateral flight in ground proximity. The results of these experiments are presented as functions of advance ratio in nondimensional coefficient form based on rotor tip speed and an appropriate characteristic area, such as rotor area or wing area.

Wind-Tunnel Investigation of a Quarter-Scale Two-Bladed High-Performance Rotor in a Freon Atmosphere

Abstract: : A joint USAAVLABS/NASA-Langley/Bell Helicopter Company experimental investigation of a quarter-scale two-bladed teetering rotor system was conducted in the NASA-Langley Transonic Dynamics Tunnel using Freon as a test medium. Rotors tested were a dynamically similar Bell 540 rotor having -10-degree twist, and a thin-tipped rotor having 0-degree twist. Aerodynamic and loads data were obtained at advance ratios from 0.30 to 0.72 in combination with advancing-tip Mach numbers varying from 0.70 to 0.96. These data were compared to full-scale data and theory. In general, the aerodynamic test data followed predicted trends and correlated well with full-scale tunnel data in the areas of low blade loading at low advance ratio and low advancing-tip Mach numbers. The blade loads data followed the expected trends, but their magnitudes could not be readily compared to existing flight test data due to the differences between the flight test conditions, model test conditions attained, and hub impedance differences. (Author)

Helicopter Rotor Periodic Differential Pressures and Structural Response Measured in Transient and Steady-State Maneuvers

Abstract: Helicopter rotor blade differential pressure and structural load characteristics in transient and steady state maneuvers