Rotor Whirl Research Articles

Philosophy, design, and evaluation of soft-mounted engine rotor systems

Gas turbine engine rotors are conventionally supported by bearings mounted on relatively stiff supports. The resulting vibratory loads and deflections can be reduced significantly by judiciously soft-mounting the bearings through squirrel cages and/or squeeze films. In addition to minimizing loads and stresses in an engine, it is important that clearances during conditions of maneuvers, thermal bow, and rotor whirl due to unbalance (even under extraordinary conditions such as loss of blades) be controlled. For high-speed rotors, it becomes necessary to support the rotors on resiliently mounted bearings to achieve vibration-free, long-life, close-clearance engines. In this paper, the design philosophy, criteria, and methods of evaluation for soft-mounted turbine engine rotor systems used in General Electric aircraft engine design are described. A major constituent of this method is a computer program for system vibration and static analysis [VAST]. This program is capable of finding natural frequencies, normalized modes, and responses due to any distribution of exciting forces considering gyroscopic and shear-deflection effects. Aircraft mounting and excitations from the helicopter rotor are also included in the computer analysis. General Electric's T700 turboshaft engine, under development for the U.S. Army, serves to illustrate the squeeze film, softmounting concept of design. Results from tests of the T700 engine, Advanced Technology Axial Centrifugal Compressor (ATACC), T64 turboshaft, TF34 turbofan, and other engines are summarized verifying the advantages of soft-mounted rotor systems.

Friction Induced Rotor Whirl—A Study in Stability

Of deep concern to designers of high speed, close clearance rotating machinery is that type of self-excited rotor whirl induced by rubbing friction between rotor and stator. Since the probability of transient or continuous rubbing is high, it is important to determine whether the consequent whirl will be sustained, whether it will decay, or whether it will increase destructively. This paper develops, for a simple flexible overhung rotor, an analysis showing that regimes of stability, dependent upon damping and stiffness characteristics, do indeed exist. A stability chart is computed for the selected model and this may be used to determine whether a specific disturbance deflection will decay, grow, or remain constant during the disturbance or after its removal.

On self-excited whirl of rotors

The paper presents a study of the non-synchronous, self-excited whirl of continuous rotors caused by internal damping forces and by hydrodynamic bearing-film forces. Taking into account gyroscopic effects, the effect of shear on bending, and external damping, the equations of motion are derived by the variational principle. By means of two mode expansions simple expressions are obtained for the limit of stability and the corresponding whirling frequency that are valid for small values of the velocity dependent forces. Numerical methods are used to obtain better approximations to the limits of stability and the whirling frequencies. The results show that for large values of internal and external damping forces and gyroscopic moments, the widely used two-mode approximation may be greatly in error. It is found that if the limit of stability is raised by adding external damping to the rotor system, then even a small amount of internal damping may considerably reduce the limit of stability. In accordance with experimental work done by other authors, it is found that for certain damping conditions a second stability region exists.

Oil whirl of rotors: A numerical method of prediction

A numerical precedure is suggested for the forecast of the onset speed for oil whirl in shafts rotating on lubricated plain bearings. The procedure uses matrix manipulations of the type recently proposed in a generalization of the Myklestadt-Holzer method for the prediction of amplitudes of vibration in unbalanced shafts.

Oil whirl of rotors. Part II: Horizontal shafts

Pursuing the research began in [2], we examine here the stability of the steady rotations of a heavy horizontal shaft supported on plain short lubricated bearings, under conditions where cavitation occurs in the lubricant. A complete linearized analysis is first carried out; its results are confirmed by numerical analysis of the full non linear equations of motion.

Oil whirl of rotors. Part I: Vertical shafts

Self excited vibrations are studied which occur in a balanced vertical shaft running on plain lubricated bearings. The effect of weight is excluded; also conditions of full lubrication at the bearings are assumed.

Whirl Dynamics of a Rotor Partially Filled With Liquid

A spinning rotor, mounted on an elastic shaft and containing a symmetrically located cylindrical chamber partially filled with liquid, is assumed to undergo a small whirling motion. In the analysis, rotor unbalance, gravity, and gyroscopic terms are considered negligible, and the liquid motion is taken as being axially uniform. The added liquid produces a lowering of the usual synchronous critical speed. In addition, waves in the liquid layer couple with rotor whirl motion to produce a range of self-excited asynchronous whirl speeds above the critical, similar to those observed for nonsymmetrical solid rotors. Experiments completely corroborate the analytical results.

A Study of High Speed Machines With Rubber Stabilized Air Bearings

This paper discusses the influence of rubber properties on the ability of rubber “O” rings to suppress the self-excited whirl of rotors supported in resiliently mounted air bearings.

The Influence of Trapped Fluids on High Speed Rotor Vibration

Fluids are often introduced accidentally and trapped inside of high speed rotors. This has given concern as to the possible deleterious effects of such trapped fluids on the system’s vibration characteristics. Trapped fluids will induce asynchronous vibratory motion of high speed rotors at supercritical rotational speeds. This tendency is examined in analytic detail. Assuming that the circumferential velocity of the trapped fluid varies linearly with radius, the generalized shape of the fluid film is derived. Integrating the fluid pressure on the cavity walls gives the net fluid forces and permits computation of whirl frequency and whirl amplitude as functions of rotative speed. Generalized plots are given of the film geometry, of the rotative speed at which asynchronous whirl starts, and of the induced whirl speed. General response curves are also given, showing whirl amplitude as a function of rotative speed. The detailed results indicate that whirl occurs at a rotative speed approximately double the induced whirl frequency (as happens with many rotor whirl mechanisms). Higher values of the system parameter g result in somewhat lower whirl onset speeds. The whirl velocity is approximately equal to the rotor critical speed, or slightly lower for large masses of trapped fluid. Rotor whirl amplitude increases sharply with rotative speed above onset speed until a limiting condition where the fluid film (and analytic solution) break down, at a rotative speed about 6 percent above onset speed. Trapped fluids will also influence the normal synchronous vibrations induced by rotor unbalance. Analyses of a simple model of synchronous, solid body rotation are made which give exact solutions for the condition of a fully welted cavity periphery, and give approximate solutions for a partially welted periphery. It is concluded that the trapped fluid generally reduces the critical frequency, reduces critical amplitude, and reduces high speed (supercritical) amplitude. The effect is quite small with a partially welted circumference, but is surprisingly large in the case where the periphery of the cavity stays fully welted at maximum vibration amplitude. In this case, the rotor acts as if the cavity were entirely full of fluid, even though actual trapped fluid may fill only a small fraction of the cavity volume! Significant reductions in critical frequencies and amplitude are thereby observed. In the case of systems that are partially welted in going through their resonance peak, a “rightward leaning” resonance peak is observed which results in jump phenomena and hysteresis in amplitude on accelerating and decelerating through the peak.

Investigation and prediction of helicopter rotor noise part I. Wessex whirl tower results

This report includes noise measurements taken from a three-bladed Wessex rotor of 56 ft diameter, mounted on a rotor whirl tower. The blades used for the measurements were of constant chord and made to a N.A.C.A. 0012 profile section. The range of noise measurements taken was concentrated in the speed range from 180 to 250 rev/min and at collective pitch angles (measured at the cuff) from 7° to 17°. These measurements have been compared with theoretical predictions of broad-band noise. The magnitude of the higher harmonics of the ordered noise found from the tests appears to be in good agreement with the narrow pressure pulse width acting on the blade surface. This is discussed in the report and illustrated in Figure 10. The broad-band or “vortex” noise has been found to obey a reasonable relationship with the sixth power of the blade tip velocity and also to be dependent upon C L 1·66, where C L is a tip-referred lift coefficient. The limitations on the test results imposed by the use of the whirl tower are discussed in section 6 and suggestions for further work on other rotor configurations are indicated.

Helicopter blade slap

Blade slap is the sharp increase in helicopter rotor noise, at the blade passing frequency, that is characteristic of certain model helicopters during some régimes of flight. An investigation has been conducted to determine the nature of the phenomenon. This has included flight tests with the Wessex and Belvedere helicopters and a comprehensive operational survey, military and commercial, in the United States and United Kingdom. Blade slap appears to be caused by a blade passing through the tip vortex shed by another blade in its proximity. This has been simulated on a rotor whirl stand under controlled conditions and the effect of various parameters investigated. A theory has been developed to predict the noise generated during a blade slap condition. There is good correlation between the flight tests, model tests and theory. The paper discusses all aspects of the investigation.

Protecting Turbomachinery From Self-Excited Rotor Whirl

Aerodynamic exciting forces have caused severe rotor whirl of axial compressors and turbines. One disturbing force investigated is due to circumferential variation of static pressure acting on the cylindrical surface of rotor, particularly within labyrinth seals. Another aerodynamic disturbing force is due to eccentricity of rotor causing circumferential variation of blade-tip clearance, and a corresponding variation of local efficiency and unbalanced torque. Seal deflection criteria and torque deflection criteria are presented as design guides for stable rotor systems. These criteria, the form of which comes from analysis of rotor dynamics, correlate design parameters of four examples of unstable rotor systems which exhibited whirl.

Discussion: “Protecting Turbomachinery From Self-Excited Rotor Whirl” (Alford, J. S., 1965, ASME J. Eng. Power, 87, pp. 333–343)

Discussion: “Protecting Turbomachinery From Self-Excited Rotor Whirl” (Alford, J. S., 1965, ASME J. Eng. Power, 87, pp. 333–343)

Discussion: “Protecting Turbomachinery From Self-Excited Rotor Whirl” (Alford, J. S., 1965, ASME J. Eng. Power, 87, pp. 333–343)

Discussion: “Protecting Turbomachinery From Self-Excited Rotor Whirl” (Alford, J. S., 1965, ASME J. Eng. Power, 87, pp. 333–343)

Discussion: “Protecting Turbomachinery From Self-Excited Rotor Whirl” (Alford, J. S., 1965, ASME J. Eng. Power, 87, pp. 333–343)

Discussion: “Protecting Turbomachinery From Self-Excited Rotor Whirl” (Alford, J. S., 1965, ASME J. Eng. Power, 87, pp. 333–343)

Closure to “Discussions of ‘Protecting Turbomachinery From Self-Excited Rotor Whirl’” (1965, ASME J. Eng. Power, 87, pp. 343–344)

Closure to “Discussions of ‘Protecting Turbomachinery From Self-Excited Rotor Whirl’” (1965, ASME J. Eng. Power, 87, pp. 343–344)

Oil Whirl of Flexible Rotors

The equations for a flexible rotor in journal bearings are set up and solved, extending the method previously used by Hummel and Cameron. It is shown that if w is the natural elastic frequency of the rotor, A the resonant oil frequency for an infinitely stiff but otherwise identical rotor, then the combined frequency s is given by l/ s2 = 1/Λ2 + 1/ w2. This resonant frequency will usually occur when it equals 1/2 the shaft rotational frequency The frequency calculated by the method given here is applied to a number of cases of large turbines and alternators and found to give quite satisfactory results

Oil whirl of flexible rotors : Z. Parszewski and A. Cameron. Inst. Mech. Engrs., Preprint P 22/62, (1962) 3–11; 7 figs., 1 table, 15 refs.

Oil whirl of flexible rotors : Z. Parszewski and A. Cameron. Inst. Mech. Engrs., Preprint P 22/62, (1962) 3–11; 7 figs., 1 table, 15 refs.