Showing papers on "Helicopter rotor published in 1977"

Noise Produced by Turbulent Flow into a Propeller or Helicopter Rotor

Abstract: than can be applied to calculate the instantaneous sound spectrum produced by the rotor at each azimuthal rotor position, and this instantaneous spectrum can be averaged over the azimuthal rotor position to find an averaged far-field sound spectrum. In taking this average, account must be taken of the different amount of retarded time that the rotor spends at each azimuthal rotor position. Further discussion of this point is given in another paper.4 A further factor taken into account in the analysis is the existence of blade-to-blad e correlation. If a given turbulent eddy is chopped by more than one rotor blade, the blade-toblade correlation leads to narrow-band noise peaked around the rotor harmonics. The far-field sound for an airfoil moving in rectilinear motion through a turbulent flow can be expressed in terms of a single wavevector component of the turbulence. The presence of blade-to-blade correlations requires that the single wavevector component be replaced by a summation over several wavevector components. This summation generally is carried out numerically for the calculations presented herein, but, if the frequency of interest is high enough, the summation can be replaced by an integral that can be evaluated in closed form; i.e., the blade-to-blade correlation becomes unimportant, and the result reduces to that for a single blade in rectilinear motion. The preceding description of the procedure for calculating the far-field sound applies to the sound produced by a spanwise segment of the rotor. This segment must have a spanwise dimension small enough so that the velocity does not vary significantly over the segment but large enough so that the loading correlation from segment to segment is not significant. This latter assumption is consistent with the highfrequency assumption mentioned previously, since high frequency corresponds to small correlation length. Thus, to find the noise contributed by the entire rotor, an integral over span must be performed.

Elastomeric bearing for helicopter rotor

Abstract: An elastomeric bearing for a helicopter rotor in which the laminates of the bearing are shaped and positioned so that the center of hydrostatic pressure generated in each elastomer laminate and the cross-sectional center of gravity of each metal laminate are in alignment so that the integrated load vectors reacted through the metal and elastomer laminates of the bearing in imparting blade centrifugal loads to the hub therethrough are substantially in alignment to thereby prevent the creating of eccentric internal moments and hence avoid life reducing bending and hoop stresses in the metal laminates and unnecessarily high hydrostatic pressures in the elastomer laminates.

A nonlinear dynamical theory for heterogeneous, anisotropic, elasticrods

Abstract: A large deformation, small-strain theory is presented for heterogeneous, transverse isotropic, elastic rods with pre-twist. The theory is applicable to practical problems related to the dynamics of cable systems, helicopter blades, space antennae, and similar structures. Two elementary examples are included: reduction of the general theory to particular differential equations governing the planar, steady-state towing of cables, and the steadystate motion of helicopter rotor blades.

A new capability for predicting helicopter rotor and propeller noise including the effect of forward motion

Abstract: The governing equation and computing technique for the prediction of helicopter rotor and propeller noise are described. The method which gives both the acoustic pressure time history and spectrum of the noise includes the thickness and the loading noise. It was adapted to computers resulting in a new capability in noise prediction by removing many of the restrictions and limitations of previous theories. The capability results from the fact that the theory is developed entirely in the time domain. The formulation and the technique used are not limited to compact sources, steady level flight or to the far-field. In addition, the inputs to the computer program are normally available or are amenable to experimental measurements. This program can be used to study rotor and propeller noise with the aim of minimizing the radiated noise to reduce annoyance to the public. Several examples demonstrating the features and capability of the computer program are presented.

Effect of Modified Aerodynamic Strip Theories on Rotor Blade Aeroelastic Stability

Abstract: Various existing unsteady aerodynamic strip theories which have been developed in the past for both fixed and rotary wing aeroelastic analyses are modified in the paper so as to make them applicable to the coupled flap-lag-torsional aeroelastic problem of a rotor blade in hover. These corrections are primarily due to constant angle of attack, constant inflow and variable free stream velocity due to lead-lag motion. Next, the modified strip theories are incorporated in a coupled flap-lag-torsional aeroelastic analysis of the rotor blade in hover and the sensitivity of the aeroelastic stability boundaries to the aerodynamic assumptions is examined.

Influence of Modeling and Blade Parameters on the Aeroelastic Stability of a Cantilevered Rotor

Abstract: A set of equations describing the coupled flap-lag-torsional dynamics of a cantilevered rotor blade in hover is presented. This set of equations is used to evaluate the influence of structural damping, preconing, and offsets on the linearized aeroelastic stability of some representative blade configurations. The sensitivity of the stability boundaries to the assumptions of approximate linear vs exact nonlinear static blade equilibrium position is considered. Finally, results with the distributed torsional representation of blade properties are compared with those obtained when the root torsional model is used.

High frequency broadband rotor noise

Abstract: A method has been developed to find the absolute spectral level of high frequency far field sound of a rotor in terms of random load fluctuations on the rotor blades. The analysis deals with frequencies where the radiated sound spectrum is smooth, i.e., above 300 to 400 Hz for a typical helicopter. This is in constrast to the low frequency regions where the spectrum is continuous but peaked near bladed passing harmonics. We first show that the smooth, broadband part of the spectrum corresponds to load fluctuations which are uncorrelated between blade passages. Then the spectral intensities from the individual blades are additive. A point load approximation with spanwise loading corrections is used and the blade loading spectrum is specifically derived for upwash fluctuations due to inflow turbulence. Analytic approximations are made to simplify the evaluation of certain integrals and series. The method is compared to the more general method of Homicz and George, where practical, and to published experimental data. The agreement between the two theories is excellent. The comparison to the experiments is good although it is not clear how to estimate the increase in intensity of atmospheric turbulence as it is distorted while being drawn into the rotor. The results indicate that atmospheric turbulence is perhaps the major contribution to broadband noise in hover. The approach is also applicable to other load fluctuation mechanisms.

Stability of Hingeless Rotor Blades in Hover with Pitch-Link Flexibility

Abstract: A stability analysis of a single cantilevered helicopter rotor blade in hover is presented. The blade is represented by an elastic uniform beam, cantilevered in bending and having a torsional root spring to simulate pitch-link flexibility. Nonlinear equations are adapted for a linearized stability analysis about the blade equilibrium operating condition. Numerical results are obtained for hingeless rotor configurations having pitchlink flexibility, precone, droop, twist, and flap-lag structural coupling. The results indicate that hingeless rotor stability characteristics are sensitive to changes in most configuration parameters. For a given torsion frequency, the effect of pitch-link flexibility is generally found to be similar to the effect of blade torisional flexibility. Droop and precone, although physically similar, exhibit different effects on stability when pitch-link flexibility is present. Twist is shown to influence the stability by altering the flap-lag structural coupling.

An Assessment of the Hover Performance of The XH-59A Advancing Blade Concept Demonstration Helicopter

Abstract: : This report documents a study of the hovering characteristics of the XH-59A helicopter, which was built to demonstrate the feasibility of the Advancing Blade Concept (ABC). The study examined in- and out-of-ground hover characteristics, aircraft and rotor figures of merit, and hover performance at 10- and 20-foot wheel heights. The XH-59A's performance is also compared to the performances of other Army helicopters. This indicates that the XH-59A performs better than other helicopters, largely because it lacks a tail rotor.

Two-dimensional wind tunnel test of an oscillating rotor airfoil, volume 1

Abstract: A two dimensional wind tunnel test was conducted to obtain the quasisteady and unsteady characteristics of an advanced airfoil designed for helicopter rotor applications. Differential pressures were measured at 17 locations along the chord of the airfoil model. The airfoil motions were sinusoidal forced-pitch oscillations about the quarter chord at amplitudes varying from 2.5 to 10.0 degrees and at frequencies from 23 Hz to 90 Hz. The quasisteady tests were conducted at Mach numbers from 0.2 to 0.9, and the oscillatory tests between M = 0.2 and M = 0.7. At quasisteady conditions a limited number of drag measurements were made with a wake-traversing probe.

Helicopter lifting and propelling apparatus

Abstract: A helicopter rotor is formed with spars and ribs. The spars form parts of the surfaces, and long slots are constructed in the spars to provide suction and blower slots. Air is withdrawn in slots nearest the leading edge, and engine exhaust is conducted to a suction/blowing device which in turn blows air through slots near the trailing edge. Helicopter engine exhaust is mixed with air and fuel and is recombusted. Air is drawn into a recombustion chamber of a suction/blowing device from suction surfaces on the helicopter rotor blades. This suctioned air is then re-routed to the slots near the trailing edge and blown over the upper and lower surfaces of the blade.

Damper means for helicopter rotors

Abstract: A helicopter rotor includes a rotor head having, for each of a plurality of rotor blades, an elastomeric bearing and a damper that is arranged to provide controlled restraint of rotor blade movement in both the flap and lead/lag planes. The damper can be arranged to exhibit either substantially equal stiffness in both operational planes or any desired stiffness ratio. In a preferred embodiment of the latter arrangement, the desired stiffness in the respective planes are provided by separate components incorporated in the single damper. The damper may be situated either inboard or outboard of the elastomeric bearing.

Helicopter rotor having lead-lag damper operable with pitch and flap decoupling

Abstract: A helicopter rotor having at least one blade projecting from the central hub and mounted for pitch change, flapping and lead-lag motion and including a lead-lag damper mounted to be pivotally movable in a selected plane and connected to the blade through a pivotal link connection to avoid the adverse effects of pitch and flap coupling.

Semi-articulated flexstrap

Abstract: A semi-articulated flexstrap for use in connecting a rotor blade to the hub of a helicopter rotor is disclosed. The flexstrap includes a series of straps which alternatingly crisscross at a point between the blade and hub. The straps are substantially parallel to the plane of rotation of the rotor at the crossover point and unrestrained so as to permit relative movement between the straps. The relative movement of the straps of articulation of the flexstrap in the plane of rotation of the rotor permits lead lag motion of the rotor blade to occur.

Theoretical study of the effect of ground proximity on the induced efficiency of helicopter rotors

Abstract: A study of rotors in forward flight within ground effect showed that the ground-induced interference is an upwash and a decrease in forward velocity. The interference velocities are large, oppose the normal flow through the rotor, and have large effects on the induced efficiency. Hovering with small ground clearances may result in significant blade stall. As speed is increased from hover in ground effect, power initially increases rather than decreases. At very low heights above the ground, the power requirements become nonlinear with speed as a result of the streamwise interference. The streamwise interference becomes greater as the wake approaches the ground and eventually distorts the wake to form the ground vortex which contributes to certain observed directional stability problems.

Helicopter rotor head

Abstract: The present rotor head for a helicopter is free of flapping hinges and free of drag or lagging hinges. The rotor blades or wings are connected to the rotor head by a plurality of connecting plates which are flexible to bending moments. These plates are inserted into the blade roots of the helicopter wings and connected to the rotor head proper. The planes defined by these connecting plates intersect in the axis of the rotor blades or wings.

Helicopter rotor structure

Abstract: The present rotor structure for rotary wing aircraft, such as helicopters,s constructed to minimize the mass unbalance which normally occurs due to the lead-lag of the rotor blades or wings. For this purpose all the bearings supporting the rotors on the rotor heads are constructed as bearings permitting an axial movement of the tension bars which interconnect the wings of a pair of wings. In addition, the wing interconnecting bars are bending-resistant and connected with each other at their crossing point by a centering device which is freely movable in the rotor plane and relative to the rotor head. The connection of the bars by the centering device is rigid in the longitudinal, axial direction of the tension bars.

Measurements of helicopter rotor tip vortices

Abstract: This paper presents results from a recent wind-tunnel investigation of model helicopter rotor tip vortices. Measurements were made of the vortex positions, core sizes, and velocity distributions. A laser velocimeter was used to make the measurements, and a minicomputer-based data system was used to process the data and to aid in controlling the experiment. The velocimeter, the data system, and the software developed for the minicomputer are briefly described. The rotors investigated were two-bladed, teetering rotors with diameters of 2.1 m. Two sets of blades were used, one set with zero twist and one set with -11 deg of linear twist. The vortex positions were obtained by making flow field traverses while strobing the data system at a fixed azimuth. Aging of a vortex element was also studied by following the convected element while strobing the data system at different azimuths. By this method, the effects on the vortex of a close interaction with a blade and another vortex were studied.

Wind Tunnel Investigation of the Controllable Twist Rotor Performance and Dynamic Behavior

Abstract: : The Controllable Twist Rotor (CTR) principle is based on a torsionally flexible helicopter blade, utilizing inboard and outboard collective and cyclic controls to optimize the blade pitch distribution along the radius and around the azimuth. The benefits of this unique rotor system include improved performance, delayed retreating-blade stall, and reduced bending moments and vibration levels.

Gust response and its alleviation for a hingeless helicopter rotor in cruising flight.

Abstract: Thesis. 1977. Ph.D.--Massachusetts Institute of Technology. Dept. of Aeronautics and Astronautics.

High speed rotor system

Abstract: A magnetically suspended rotor system is provided capable of operating at rotational speeds of 100,000 rpm or greater. The system comprises a rotor assembly and electric motor means for producing rotation of the assembly about an axis thereof. Electromagnetic means disposed near one end of the rotor is energized to exert an attractive force on the rotor to support it and preferably cooperates with permanent magnet means affixed to the rotor assembly near one (the upper) end thereof. An additional permanent magnet affixed to the rotor near the opposite (lower) end thereof cooperates with another permanent magnet disposed in juxtaposition thereto to exert an attractive force on the rotor tending to oppose the force exerted by electromagnetic means and produces a radial constraining force on the rotor tending to center the lower end of the rotor assembly. A conductive element is interposed between the lower end of the rotor assembly and the latter permanent magnet which cooperates with the magnetic field produced by the permanent magnet in the lower end of the rotor assembly to inhibit nutation of the lower end of the rotor assembly by virtue of eddy currents induced therein. Means are provided responsive to displacement of the rotor in the direction of its axis of rotation for controlling the currents supplied to the electromagnetic means so as to tend to restore the rotor to its position prior to such displacement. The rotor may be driven by a motor utilizing regenerative commutation of the sort disclosed in co-pending application Ser. No. 695,507, filed June 11, 1976. The system disclosed makes possible the achievement of unusually high rotational speeds by minimizing or eliminating undesired resonances in the rotor assembly.

Helicopter rotor blade control system - has HF generator coupled to rotor shaft to drive trigger-plate with impulse pick=ups delivering signals to band-pass filters

Abstract: The system for cyclically varying rotor blade incidence angles in a helicopter, is based on a conventional swash-plate arrangement. A high frequency generator positively coupled to the rotor shaft drives a trigger plate with two fixed impulse pickups 90 deg. apart and aligned to produce respectively pure Cosine-and Sine-voltages relative to the rotor position. Bandpass filters separate these rotor speed harmonics into various Sine- and Cosine-components, usable for either automatic or programmed suppression of rotor vibrations.

Helicopter blade and rotor

Abstract: A torsionally compliant helicopter blade having its aerodynamic center axis positioned selectively forward of its center of gravity and elastic axes throughout a selected portion of the blade span so that, during operation, the blade will experience airloading which will induce two per rev torsional deflection of the blade to thereby improve rotor performance by reducing the total power required to drive the rotor.

Some results of the testing of a full-scale Ogee tip helicopter rotor; acoustics, loads, and performance

Abstract: Full-scale tests were utilized to investigate the effect of the Ogee tip on helicopter rotor acoustics, performance, and loads. Two facilities were used for this study: the Langley whirl tower and a UH-1H helicopter. The test matrix for hover on the whirl tower involved thrust values from 0 to 44,480 N (10,000 lbs) at several tip Mach numbers for both standard and Ogee rotors. The full-scale testing on the UH-1H encompassed the major portion of the flight envelope for that aircraft. Both near-field acoustic measurements as well as far-field flyover data were obtained for both the Ogee and standard rotors. Data analysis of the whirl-tower test shows that the Ogee tip does significantly diffuse the tip vortex while providing some improvement in hover performance. Flight testing of both rotors indicates that the strong impulsive noise signature of the standard rotor can be reduced with the Ogee rotor. Forward flight performance was significantly improved with the Ogee configuration for a large number of flight conditions. Further, rotor control loads and vibrations were reduced through use of this advanced tip rotor.

Circulation Control Applied to a High Speed Helicopter Rotor

Abstract: : An advanced circulation control rotor concept identified as the Reverse Blowing-Circulation Control Rotor (RB-CCR) is discussed from the standpoint of general requirements for high speed flight. This discussion centers on a rotor solidity ratio compromise between hover, transition and cruise requirements. It is shown that the critical solidity requirement occurs in transition where high lift capability is needed while maintaining rotor moment trim. An analytical and experimental investigation of the aerodynamic environment in the transition flight regime (advance ratios of 0.5 to 1.4) indicates that large local yawed flow angles do not severely affect the lift augmentation and maximum lift coefficient of circulation control airfoils. A RB- CCR model was designed and tested at the David W. Taylor Naval Ship Research and Development Center (DTNSRDC). This rotor is unique in its employment of a special circulation control airfoil which has a slot in the leading and trailing edge. The results of several test programs verified the capability of the rotor to perform efficiently in hover and at advance ratios up to 4.0. The model data also demonstrated that the rotor is capable of developing sufficient lift to fly through the critical advance ratio of 0.7.

A Study of Helicopter Rotor Rotational Noise

Abstract: The rotational noise of model helicopter rotors in forward flight was studied in an anechoic wind tunnel. The parameters under study were the rotor thrust (blade loading), blade number, and advance ratio. The separate effects of each parameter were identified with the other parameters being held constant. The directivity of the noise was also measured. Twelve sets of data for rotational noise as a function of frequency were compared with the theory of Lowson and Ollerhead. In general, the agreement is reasonably good, except for the cases of 1) low and high disk loadings, 2) the four-bladed rotor, and 3) low advance ratios. The theory always underestimates the rotational noise at high harmonics.

Helicopter blade ice protection systems

Abstract: An external system installed on a helicopter to spray a freezing point depressant fluid onto the main rotor blade of a single rotor helicopter to prevent the collection of ice when flying through icing weather. Actuation of this ice protection system will cause a pump to force a glycerine-alcohol mixture through a series of fixed nozzles to create a spray of freezing point depressant. The main rotor blade will move through this spray and be coated, thus preventing the collection of ice on the blade. Such protection is of vital importance to all-weather helicopter operations as experience has shown that the collection of ice on the rotor blades of helicopters is a safety of flight problem.