Showing papers on "Helicopter rotor published in 1973"

Vortex Noise of Isolated Airfoils

Abstract: Hohenemser, K. H. and Yin, Shen-Kuang, "Some Applications of the Method of Multi-blade Coordinates," Journals of the American Helicopter Society, Vol. 17, No. 3, July 1972, pp. 3-12. Coleman, R. P., "Theory of Self-Excited Mechanical Oscillations of Hinged Rotor Blades," NACA Advanced Restricted Rept. 3G29, Republished as Rept. 1351,1943, NACA. Miller, R. H., "Helicopter Control and Stability in Hovering Flight," Journal of the Aeronautical Sciences, Vol. 15, No. 8, Aug. 1948, pp. 453-472. Azuma, A., "Dynamic Analysis of the Rigid Rotor System," Journal of Aircraft, Vol. 4, No. 3, May-June 1967, pp. 203-209. Curtiss, H. C., Jr. and Shupe, N. K., "A Stability and Control Theory for Hingeless Rotors," American Helicopter 27th Annual National V/STOL Forum, Washington, D. C., May 1971. Ward, J. F., "Exploratory Flight Investigation and Analysis of Structural Loads Encountered By A Helicopter Hingeless Rotor System," D-3676, Nov. 1966, NASA. Harris, F. D., "Articulated Rotor Blade Flapping Motion at Low Advance Ration," Journal of the American Helicopter Society, Vol. 17, No. 1, Jan. 1972, pp. 41-48. Ormiston, R. A. and Peters, D. A., "Hingeless Helicopter Rotor Response with Nonuniform Inflow and Elastic Blade Bending," Journal of Aircraft, Vol. 9, No. 10, Oct. 1972, pp. 730736.

An Experimental Investigation of Vortex Rings and Helicopter Rotor Wakes Using a Laser Doppler Velocimeter

Abstract: : The report describes a laser Doppler velocimeter (LDV) capable of performing measurements in vortex cores, presents detailed measurements of structure and stability of vortex rings and also presents measurements in the wake of a model helicopter rotor. The use of conventional instrumentation such as hot wire anemometer for measuring velocity in vortex filament is hampered by probe interference and the difficulty of calibrating the probe in a flow field where the velocity is changing rapidly in magnitude and direction. For these reasons, a laser Doppler velocimeter (LDV) is used throughout these experiments. The LDV measures the velocity of smoke particles by measuring the Doppler shift of laser light scattered from the particles. (Modified author abstract)

Dynamics of Slung Bodies Utilizing a Rotating Wheel for Stability

Abstract: filler, R. H., "Rotor Blade Harmonic Airloadings," AIAA Journal, Vol. 2, No. 7, July 1964, p. 1260. Curtiss, H. C. Jr., "The Use of Complex Coordinates in the study of Rotor Dynamics," Journal of Aircraft, Vol. 10, No. 5, May 1973, pp. 285-295. Ormiston, R; A. and Peters, D. A., "Hingeless Rotor Response with Non-Uniform Inflow and Elastic Blade Bending," Journal of Aircraft, Vol. 9, No. 10, Oct. 1972, pp. 730-736. Kerr, A. W., Potthast, A. J., and Anderson, W. D., "An Interdisciplinary Approach to Integrated Rotor/Body Mathematical Modeling," Mideast Region Symposium, American Helicopter Society, 1972. Hohenemser, K. H. and Crews, S. T., "Model Tests on Unsteady Rotor Wake Effects," Journal of Aircraft, Vol. 10, No. 1, Jan. 1973, pp. 58-60. Carpenter, P. J. and Fridovich, B., "Effect of a Rapid Blade Pitch Increase on the Thrust and Induced Velocity Response of a Full Scale Helicopter Rotor," TN 3044, Nov. 1953, NACA. Hohenemser, K. H. and Yin, S. K., "Some Applications of the Method of Miiltiblade Coordinates," Journal American Helicopter Society, Vol. 17, No. 4, July 1972, pp. 3-12. Dynamics of Slung Bodies Utilizing a Rotating Wheel for Stability

Helicopter Blade Flutter

Abstract: : Methods of analysis of helicopter blade flutter for both hinged and hingeless blades are presented The major types considered are bending-torsion flutter, flap-lag flutter, and stall flutter Both hover and forward flight are considered Means of avoiding flutter are described

Nonlinear Equations for Bending of Rotating Beams with Application to Linear Flap-Lag Stability of Hingeless Rotors

Abstract: The nonlinear partial differential equations for the flapping and lead-lag degrees of freedom of a torisonally rigid, rotating cantilevered beam are derived. These equations are linearized about an equilibrium condition to study the flap-lag stability characteristics of hingeless helicopter rotor blades with zero twist and uniform mass and stiffness in the hovering flight condition. The results indicate that these configurations are stable because the effect of elastic coupling more than compensates for the destabilizing flap-lag Coriolis and aerodynamic coupling. The effect of higher bending modes on the lead-lag damping was found to be small and the common, centrally hinged, spring restrained, rigid blade approximation for elastic rotor blades was shown to be resonably satisfactory for determining flap-lag stability. The effect of pre-cone was generally stabilizing and the effects of rotary inertia were negligible.

Aeroelastic Instabilities of Hingeless Helicopter Blades

Abstract: In this study the stability boundaries of a hingeless helicopter blade are studied using a system of coupled flap-lag-pitch equations of motion. Divergence and flutter boundaries for the linearized system of equations are presented. Flap-lag, flap-pitch and coupled flap-lag-pitcn instabilities in hovering flight are studied. The effect of the torsional degree of freedom on the flap-lag type of instability is investigated. Similarly the effect of lag on the flap-pitch type of instability is also considered. Results illustrating these effects, together with the effect of various important blade parameters on blade stability are presented. Nomenclature a _ - two-dimensional lift curve slope A,B = tip loss coefficients b = half-chord, nondimensionalized with respect _ to/2 Bl = generalized masses defined in Appendix A Cdo = profile-drag coefficient C/ = element of matrix {C| Ci = quantity defined in Appendix A ei = quantity defined in Fig. 1 ~e = e/l offset used in representing the blade by Young's model Fl = flapping coefficients defined in Appendix A F'x*2),Fx(2) ..,etc. = flutter derivatives associated with the flap equation, defined in Appendix C gi = generalized coordinate, first normal flapping mode !JDi,gD2,gD3 — equivalent damping terms, defined in Appendix C

Helicopter rotor with redundant load carrying capability

Abstract: A helicopter rotor in which the helicopter blades are mounted for rotation from a rotor hub which is shaped to provide redundant load carrying paths for blade centrifugal loading, torque loading, thrust loading and rotor head moment loading.

Helicopter vibration isolation

Abstract: A helicopter in which vibration is minimized by use of a lift unit including a multiblade helicopter main rotor system mounted to drive a nodalized module comprising structure which vibrates in response to blade developed vertical forces. A load unit is then connected to the vibrating structure only at vibration nodal points thereon.

Rotor dynamic considerations for large wind power generator systems

Abstract: Successful large, reliable, low maintenance wind turbines must be designed with full consideration for minimizing dynamic response to aerodynamic, inertial, and gravitational forces. Much of existing helicopter rotor technology is applicable to this problem. Compared with helicopter rotors, large wind turbines are likely to be relatively less flexible with higher dimensionless natural frequencies. For very large wind turbines, low power output per unit weight and stresses due to gravitational forces are limiting factors. The need to reduce rotor complexity to a minimum favors the use of cantilevered (hingeless) rotor configurations where stresses are relieved by elastic deformations.

Apparatus for folding blades without changing pitch due to folding

Abstract: A helicopter rotor has a blade mounted for folding movement. The blade is also mounted for feathering movement about its longitudinal axis and has a blade horn connected to a swashplate by linkage mechanism which provides a zero alpha pitch coupling during blade folding.

Complex Coordinates in Near Hovering Rotor Dynamics

Abstract: The application of complex coordinates to the study of the dynamic characteristics of the tip-pathplane of a helicopter rotor is considered. The coordinates describing rotor blade motions can be transformed from individual blade flapping angles to linear combinations of flapping angles. Two of the transformed coordinates can be interpreted as describing the tilting motion of the tip-path-plane. In a stationary reference frame, these two new coordinates are coupled. However, by defining a set of complex coordinates, for near hovering flight these two coupled differential equations can be combined into a single second-order differential equation describing the tilting motion of the tip-path-plane. This formulation provides a convenient and natural framework for investigation of the response characteristics of fully articulated and hingeless rotors. Considerable insight into the influence of various physical parameters on the behavior of the tip-pathrplane can be gained. The approach is illustrated by consideration of the transient and frequency response characteristics of the tip-pathplane and the influence of flapping feedback. An extension of the root locus method is described which makes the investigation of flapping feedback convenient. Nomenclature! a - rotor blade lift curve slope b = number of blades A(s) = transfer function c = rotor blade chord Ci = rotor hub rolling moment coefficient, positive for right roll, Ci = L/pKR2(VR)2R Cm — rotor hub pitching moment coefficient, positive nose up, Cm = M/pirR2(QR)2R Cmz = complex rotor hub moment coefficient, Cmz— Cm + iCi /i = rotor blade flapping moment of inertia j = constant of proportionali ty between complex inflow and complex rotor aerodynamic hub moment coefficient KH = gain parameter associated with integral flapping feedback Kp,e = parameters associated with proportional flapping feedback mz — dimensionless aerodynamic moment acting on rotor blade due to forward speed. Effect of cyclic pitch, angular rates and flapping not included p = rotor blade natural frequency in flapping divided by rotor rpm, also roll rate nondimensionalized by rotor rpm, positive roll right q = pitch rate nondimensionalized by rotor rpm, positive nose up

Model Wind Tunnel Tests of a Reverse Velocity Rotor System

Abstract: : A one-seventh scale model reverse velocity rotor system was manufactured and tested with the goal of substantiating the results that had been predicted for this system in previous analytical studies. The 8 ft diameter 4 bladed hydraulically powered model rotor was provided with remote operation of the controls and shaft angle. Tests were conducted in the 12 ft pressure wind tunnel at NASA Ames during June and July 1972. The tests did not cover the whole range of conditions desired, but results were obtained at advance ratios from 0. 3 to 2.46 and at tunnel speeds up to 350 knots. Significant results of the tests were the freedom of the rotor from instability, and the ability to trim the rotor laterally and longitudinally under all conditions. Reasonable agreement was found between the measured performance of the model rotor and that predicted using the results of two-dimensional wind tunnel tests made on three reversible airfoil sections of the model rotor blade. It is recommended that further tests be performed with this model to expand the envelope of test conditions, particularly to include testing with two-per-rev control angle input.

Unsteady lifting surface theory applied to a propeller and helicopter rotor

Abstract: This thesis is concerned with the development of a theory for determining the aerodynamic forces for unsteady, compressible subsonic flow on a propeller and helicopter rotor. The acceleration potential method was used in developing the basic equations and the method has been programmed for the propeller on a computer and some results are given. The integral equations was solved by the doublet-lattice method, which consists of placing "load" lines at certain locations on the chord and satisfying the down-vash condition at other selected positions. The examples presented include the spanvise and chordwise loading on a rotating propeller for incompressible flow, an example of compressible flow calculations and finally, a calculation illustrating the loss of aerodynamic damping of a propeller blade due to the passage of the blade over its own wake.

Helicopter rotor plenum chamber

Abstract: A floating helicopter rotor hub is disclosed for transporting a flow of air from within the rotor hub to hollow sections within each of a pair of rotor blades. A hollow rotor shaft, connected to an air compressor within the helicopter, conveys a flow of air to the rotor hub. Each of a pair of flexible bellows conveys portions of the flow of air from the rotor hub to the hollow section within one of the rotor blades. Each of the rotor blades includes bearing members which permit a predetermined amount of freedom in the rotor blade pitch axis, angular movement of the blade with respect to the rotor hub, and angular movement of the rotor hub and rotor blade assembly with respect to the rotor shaft. The flexible nature of the bellows, and appropriate seals, permits the bellows to compensate for the simple or compound angular movements of the rotor blades without disrupting the air flow through the rotor hub. The flow of air through each rotor blade is discharged through a nozzle disposed at the trialing edge on the tip of each rotor blade. The reaction force exerted by the discharging flow of air rotates the rotor blades about the rotor shaft.

Some conclusions regarding the aeroelastic stability of hingeless helicopter blades in hover and in forward flight

Abstract: In this paper results and conclusions obtained from the study of the aeroelastic instability of hingeless helicopter blades are presented. First, the large amplitude coupled flap-lag equations of motion of a hingeless elastic helicopter blade are solved using an asymptotic expansion procedure in multiple time scales. Both hover and forward flight cases are considered. Stability boundaries and amplitudes of nonlinear response are obtained. From these, the importance of the nonlinear coupling and the effect of the periodic coefficients is determined. Next, using a system of linearized coupled flap-lag-pitch equations in hover, various divergence mechanisms for hingeless blades are shown. Finally, the flutter boundaries for coupled flap-lag-pitch are obtained. The effect of the torsional degree of freedom on the flap-lag type of instability is investigated. Similarly the effect of lag on the flap-pitch type of instability is considered. In addition, the effect of various blade parameters on the stability boundaries is shown.

Vortex Modification by Mass Injection and by Tip Geometry Variation

Abstract: : The report describes an experimental research program in which the outer section of a UH-1D helicopter blade was modified to incorporate a system for injecting the trailing tip vortex produced by the blade with a mass of linearly-directed air, and also an Ogee-tip section to study its effect as a passive system on vortex dissipation. The effects of mass injection were investigated at low mass flow rates, at near-sonic injection velocities, and with a two-section nozzle. The results are presented in terms of quantitative measurements of circulation strength as a function of mass flow rate and thrust, and are correlated with the results from previous research done at RASA. Also presented are flow-visualization studies which were conducted using illuminated helium bubbles, smoke, and tuft grids. The results of this research program present additional confirming evidence that mass injection of the concentrated tip vortex is a practical approach to the elimination of the strong induced effects on a lifting surface of the circulatory flow associated with a concentrated vortex generated at the tip of a helicopter rotor blade.

Dynamic Model Wind Tunnel Tests of a Variable-Diameter, Telescoping- Blade Rotor System (TRAC Rotor)

Abstract: : An analytical and experimental program was conducted to establish feasibility and determine characteristics of the Sikorsky TRAC rotor system, which is a unique variable-diameter, telescoping-blade concept for advanced rotary-wing aircraft The program included a preliminary design study of a full-scale blade, wind tunnel tests of a dynamically scaled rotor model in various flight modes, and correlation of experimental results with theory Feasibility was established for the TRAC rotor operating both as a high-speed compound helicopter and as a stopped/stowed rotor configuration The basic blade structural design and the retraction system were verified with the model, which was scaled for operation at full tip speeds and forward speeds

V/STOL tilt rotor aircraft study: Wind tunnel tests of a full scale hingeless prop/rotor designed for the Boeing Model 222 tilt rotor aircraft

Abstract: The rotor system designed for the Boeing Model 222 tilt rotor aircraft is a soft-in-plane hingeless rotor design, 26 feet in diameter. This rotor has completed two test programs in the NASA Ames 40' X 80' wind tunnel. The first test was a windmilling rotor test on two dynamic wing test stands. The rotor was tested up to an advance ratio equivalence of 400 knots. The second test used the NASA powered propeller test rig and data were obtained in hover, transition and low speed cruise flight. Test data were obtained in the areas of wing-rotor dynamics, rotor loads, stability and control, feedback controls, and performance to meet the test objectives. These data are presented.

Analysis of helicopter maneuver-loads and rotor-loads flight test data

Abstract: A study was conducted in which available airload and blade response data for the NH-3A and CH-53A rotors were analyzed in an attempt to provide greater insight into the sources of rotor vibratory loads in both level and maneuvering flight. Primary emphasis in the study was placed on examining and understanding causes of high-frequency rotor control loads. Secondary objectives were: (1) to examine the effect of number of rotor blades on hub vibratory shear forces and (2) to assess which of the many terms appearing in the hub vibratory shear force expression were of most significance.

The reduction of VTOL operational noise through flight trajectory management

Abstract: Prop rotor and lift fan VTOL aircraft ground noise level reduction, using flight trajectory management

Balanced airflow control valve for helicopter blade

Abstract: An airflow control valve for a circulation control rotor helicopter blade wherein the airflow path is equally distributed about the closure element so as to prevent the generation of unbalanced pressure forces.

A Perturbation Solution of Helicopter Rotor Flapping Stability

Abstract: A SOLUTION for the flapping stability of a helicopter blade, including the influence of a periodic coefficients due to the rotor forward velocity, is obtained using the techniques of perturbation theory. Contents The dynamics of a helicopter rotor in forward flight are described by a set of linear, ordinary differential equations with periodic coefficients. Sophisticated and wellknown techniques are available for the analysis of constant coefficient equations, including stability determination and control system design. Even the determination of the stability of periodic coefficient equations requires, however, considerable numerical calculation (specifically, for one frequently used technique, numerical integration of the equations of motion is required), and other topics are considerably more difficult than the corresponding constant coefficient system analysis. An alternate mathematical approach to periodic coefficient equations is the use of small perturbation techniques. The base paper considers the application of these techniques to the calculation of the flapping stability of a single blade of a helicopter rotor. The dynamics of the flapping motion of a single blade of a helicopter rotor are governed by the following equation.

Specialists Meeting on Helicopter Rotor Loads Prediction Methods

Abstract: : ;Contents: Rotary wing design technology; Current loads technology for helicopter rotors; Helicopter rotor loads prediction; Rotor system design and evaluation using a general purpose helicopter flight simulation program; The prediction of loading actions on high speed semi-rigid rotor helicopters; Load prediction methods for hingeless rotor helicopters; Integrated rotor/body loads prediction

Nonlinear Helicopter Rotor Lifting Surface Theory. Part I

Abstract: : A numerical method is developed based on potential flow nonlinear lifting surface theory for predicting the surface velocities and pressures on a rotor blade of an arbitrary helicopter rotor system which is executing a constant rotational and constant axial translational motion including, specifically, the hover flight mode. The formulation of the problem is exact in the sense that the normal surface boundary condition is satisfied on the surface of the rotor blade. The problem is governed by a Fredholm integral equation of the first kind which relates a singular velocity doublet potential surface distribution applied on the rotor blades and wakes to the normal relative velocity on the rotor blade surface. The wake model is assumed to be of a prescribed shape.

Analysis of stall flutter of a helicopter rotor blade.

Abstract: A study of rotor blade aeroelastic stability was carried out, using an analytic model of a two-dimensional airfoil undergoing dynamic stall and an elastomechanical representation including flapping, flapwise bending and torsional degrees of freedom. Results for a hovering rotor demonstrated that the models used are capable of reproducing both classical and stall flutter. The minimum rotor speed for the occurrence of stall flutter in hover, was found to be determined from coupling between torsion and flapping. Instabilities analogous to both classical and stall flutter were found to occur in forward flight. However, the large stall-related torsional oscillations which commonly limit aircraft forward speed appear to be the response to rapid changes in aerodynamic moment which accompany stall and unstall, rather than the result of an aeroelastic instability. The severity of stall-related instabilities and response was found to depend to some extent on linear stability. Increasing linear stability lessens the susceptibility to stall flutter and reduced the magnitude of the torsional response to stall and unstall.