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At least 235 records · Page 13

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NASA NTRS1981

Testing and evaluation of a stall-flutter-suppression system for helicopter rotors using individual-blade-control

The development and testing of a feedback system designed to alleviate the violent blade first torsion mode oscillations associated with stall flutter are described. The system, based on previously developed M.I.T. Individual-Blade-Control hardware, employs blade-mounted accelerometers to sense torsional oscillations and feeds back rate informaton to increase the damping of the first torsion mode. A linear model of the blade and control system dynamics is developed and is used to give qualitative and quantitative guidance in the design process as well as to aid in analysis of experimental results. System performance in wind tunnel tests, both in hover and forward flight, is described, and evidence is given of the system's ability to provide substantial additional damping to stall-induced blade oscillations.

Quackenbush, T. R.↗

NASA NTRS1978

Two-dimensional dynamic stall as simulated in a varying freestream

A low speed wind tunnel equipped with a axial gust generator to simulate the aerodynamic environment of a helicopter rotor was used to study the dynamic stall of a pitching blade in an effort to ascertain to what extent harmonic velocity perturbations in the freestream affect dynamic stall. The aerodynamic moment on a two dimensional, pitching blade model in both constant and pulsating airstream was measured. An operational analog computer was used to perform on-line data reduction and plots of moment versus angle of attack and work done by the moment were obtained. The data taken in the varying freestream were then compared to constant freestream data and to the results of two analytical methods. These comparisons show that the velocity perturbations have a significant effect on the pitching moment which can not be consistently predicted by the analytical methods, but had no drastic effect on the blade stability.

Pierce, G. A.↗

NASA NTRS1981

Supersonic stall flutter of high-speed fans

An analytical model is proposed for predicting the onset of supersonic stall bending flutter in high-speed rotors. The analysis is based on a modified two-dimensional, compressible, unsteady actuator disk theory. The stability boundary predicted by the analysis is shown to be in good agreement with the measured boundary of a high speed fan. The prediction that the flutter mode would be a forward traveling wave sensitive to wheel speed and aerodynamic loading is confirmed by experimental measurements. In addition, the analysis shows that reduced frequency and dynamic head also play a significant role in establishing the supersonic stall bending flutter boundary of an unshrouded fan.

Adamczyk, J. J.↗

NASA NTRS1982

Analytical investigation of nonrecoverable stall

A lumped parameter model of the TF34 engine is formulated to study nonrecoverable stall. Features of the model include forward and reverse flow, radial flow in the fan, and variable corrected speed. The purpose of the study is to point out those parameters to which recoverability is highly sensitive but are not well known. Experimental research may then be directed toward identification of the parameters in that category. Compressor performance in the positive flow region and radial flow in the fan are shown to be important but unknown parameters determining recoverability. Other parameters such as compressor performance during reverse flow and in-stall efficiency have relatively small impact on recoverability.

Wenzel, L. M.↗

NASA NTRS1981

A simple system for helicopter Individual-Blade-Control and its application to stall flutter suppression

A new, advanced type of active control for helicopters and its application to a system for stall flutter suppression is described. The system, based on previously developed M.I.T. Individual-Blade-Control hardware, employs blade-mounted accelerometers to sense torsional oscillations and feeds back rate information to increase the damping of the first torsion mode. A linear model of the blade and control system dynamics is used to give qualitative and quantitative guidance in the design process as well as to aid in analysis of experimental results. System performance in wind tunnel tests is described, and evidence is given of the system's ability to provide substantial additional damping to stall-induced blade oscillations.

Ham, N. D.↗

NASA NTRS1981

A simple system for helicopter individual-blade-control and its application to stall-induced vibration alleviation

A new, advanced type of active control for helicopters and its application to a system for stall flutter suppression is described. The system, based on previously developed M.I.T. Individual-Blade-Control hardware, employs blade-mounted accelerometers to sense torsional oscillations and feeds back rate information to increase the damping of the first torsion mode. A linear model of the blade and control system dynamics is used to give qualitative and quantitative guidance in the design process as well as to aid in analysis of experimental results. System performance in wind tunnel tests is described and evidence is given of the system's ability to provide substantial additional damping to stall-induced blade oscillations.

Ham, N. D.↗

NASA NTRS1982

Stall induced instability of a teetered rotor

Recent tests on the 38m Mod-0 horizontal experimental wind turbine yielded quantitative information on stall induced instability of a teetered rotor. Tests were conducted on rotor blades with NACA 230 series and NACA 643-618 airfoils at low rotor speeds to produce high angles of attack at relatively low wind speeds and power levels. The behavior of the rotor shows good agreement with predicted rotor response based on blade angle of attack calculations and airfoil section properties. The untwisted blades with the 64 series airfoil sections had a slower rate of onset of rotor instability when compared with the twisted 230 series blades, but high teeter angles and teeter stop impacts were experienced with both rotors as wind speeds increased to produce high angles of attack on the outboard portion of the blade. The relative importance of blade twist and airfoil section stall characteristics on the rate of onset of rotor unstability with increasing wind speed was not established however. Blade pitch was shown to be effective in eliminating rotor instability at the expense of some loss in rotor performance near rated wind speed.

Glasgow, J. C.↗

NASA NTRS1983

Laser holographic interferometry for an unsteady airfoil in dynamic stall

Laser holographic interferometry was used to study a two-dimensional NACA 0012 airfoil undergoing dynamic stall. The airfoil, fabricated from graphite fiber and epoxy, was tested at Mach numbers of 0.3 to 0.6, at Reynolds numbers of 500,000-2,000,000, at reduced frequencies of 0.015 to 0.15, and at mean angles of attack of 0-10 deg with amplitudes of 10 deg. Density and pressure fields were obtained from dual-plate interferograms. Double-pulse interferograms, which seemed to show the wake boundaries better, were also taken. Comparisons of pressures with orifice pressures were good for the attached flow cases. For the separated flow cases, which had a vortex enbedded in the flow, the comparisons were poor. Vortices, wake structures, and the dynamic stall process can be seen by holographic interferometry.

Lee, G.↗

NASA NTRS1983

The effect of a leading-edge slat on the dynamic stall of an oscillating airfoil

The dynamic stall characteristics of a slatted airfoil were investigated experimentally on a 2-ft-chord airfoil oscillating in pitch at M = 0.2 for a range of reduced frequency and mean angle of oscillation. The slat produced a flow that remained attached to the airfoil for angles well above those normally attained by the retreating blade of a helicopter during high speed flight. The dynamic stall vortex usually associated with these flight conditions was completely eliminated for all angles under 30 deg. Instantaneous surface pressure, lift, and pitching moment data are presented as a function of incidence throughout the oscillation cycle; a detailed analysis of instantaneous boundary-layer flow behavior for the various test conditions is included.

Carr, L. W.↗

NASA NTRS1984

Determination of compressor in-stall characteristics from engine surge transients

A technique for extracting the in-stall pumping characteristics for an axial flow compressor operating in an engine system environment is developed. The technique utilizes a Hybrid computer simulation of the compressor momentum equation into which actual transient data are used to provide all terms but the desired compressor characteristic. The compressor force characteristic as a function of corrected flow and speed result from the computation. The critical problem of data filtering is addressed. Results for a compressor operating in a turbofan engine are presented and comparison is made with the conventional compressor map. The relationship of the compressor surge characteristic with its rotating stall characteristic is explored. Initial interpretation of the measured results is presented.

Lorenzo, C. F.↗

NASA NTRS1984

Development of dynamic simulation of TF34-GE-100 turbofan engine with post-stall capability

This paper describes the development of a hybrid computer simulation of a TF34-GE-100 turbofan engine with post-stall capability. The simulation operates in real-time and will be used to test and evaluate stall recovery control modes for this engine. The simulation calculations are performed by an analog computer with a peripheral multivariable function generation unit used for computing bivariate functions. Tabular listings of a simulation variables are obtained by interfacing to a digital computer and using a custom software package for data collection and display.

Krosel, S. M.↗

NASA NTRS1984

Laser velocimeter measurements of dynamic stall

Laser velocimeter measurements were made during the study of a two-dimensional NACA 0012 airfoil undergoing conditions of dynamic stall. The measurements, which were obtained in the Ames 2 foot wind tunnel at reduced frequencies of 0.12 and 1.2, show significant flow field hysteresis around the static stall angle. Comparisons were also made with dual-plate interferograms and good agreement was found for the attached flow cases. For separated flow, characteristic vortex shedding caused poor agreement and significantly increased the measured Reynolds shear stresses.

Owen, F. K.↗

NASA NTRS1984

Determination of compressor in-stall characteristics from engine surge transients

A technique for extracting the in-stall pumping characteristics for an axial flow compressor operating in an engine system environment is developed. The technique utilzes a Hybrid computer simulation of the compressor momentum equation in which actual transient data are used to provide all terms but the desired compressor characteristic. The compressor force characteristic as a function of corrected flow and speed results from the computation. The critical problem of data filtering is addressed. Results for a compressor operating in a turbofan engine are presented and comparison is made with the conventional compressor map. The relationship of the compressor surge charcteristic with its rotating stall characteristic is explored. Initial interpretation of the measured results is presented. Previously announced in STAR as N84-22566

Lorenzo, C. F.↗

NASA NTRS1984

Testing of a stall flutter suppression system for helicopter rotors using individual-blade-control

The development and testing of a feedback system designed to alleviate rotor blade first torsion mode oscillations associated with stall flutter are described. The system employs blade-mounted sensors to detect torsional oscillations and provide feedback in order to increase the damping of the first torsional mode. A model of the blade and control system dynamics is developed and is used to give qualitative and quantitative guidance in the design process, as well as to aid in the analysis of experimental results. System performance in wind tunnel tests in forward flight is described, and experimental results show that the system can provide substantial additional damping to stall-induced oscillations.

Quackenbush, T. R.↗

NASA NTRS1984

Development of dynamic simulation of TF34-GE-100 turbofan engine with post-stall capability

This paper describes the development of a hybrid computer simulation of a TF34-GE-100 turbofan engine with post-stall capability. The simulation operates in real-time and will be used to test and evaluate stall recovery control modes for this engine. The simulation calculations are performed by an analog computer with a peripheral multivariable function generation unit used for computing bivariate functions. Tabular listings of simulation variables are obtained by interfacing to a digital computer and using a custom software package for data collection and display.

Krosel, S. M.↗

NASA NTRS1984

Application of the ONERA model of dynamic stall

A semiempirical model to predict the unsteady loads on an airfoil that is experiencing dynamic stall, is investigated. The mathematical model is described from an engineering point of view, demonstrates the procedure for obtaining various empirical parameters, and compares the loads predicted by the model with those obtained in the experiment. It is found that the procedure is straightforward, and the final calculations are in qualitative agreement with the experimental results. Comparisons between calculations and measurements also indicate that a decrease in accuracy results when the values of both the reduced frequency and the amplitude of oscillation are large. Potential quantitative improvements in the accuracy of the calculations are discussed for accounting of both the hysteresis in the static data and the effects of stall delay in the governing equations.

Mcalister, K. W.↗

NASA NTRS1983

Synthesized airfoil data method for prediction of dynamic stall and unsteady airloads

A detailed analysis of dynamic stall experiments has led to a set of relatively compact analytical expressions, called synthesized unsteady airfoil data, which accurately describe in the time-domain the unsteady aerodynamic characteristics of stalled airfoils. An analytical research program was conducted to expand and improve this synthesized unsteady airfoil data method using additional available sets of unsteady airfoil data. The primary objectives were to reduce these data to synthesized form for use in rotor airload prediction analyses and to generalize the results. Unsteady drag data were synthesized which provided the basis for successful expansion of the formulation to include computation of the unsteady pressure drag of airfoils and rotor blades. Also, an improved prediction model for airfoil flow reattachment was incorporated in the method. Application of this improved unsteady aerodynamics model has resulted in an improved correlation between analytic predictions and measured full scale helicopter blade loads and stress data.

Gangwani, S. T.↗

NASA NTRS1985

Flight investigation of stall, spin and recovery characteristics of a low-wing, single-engine, T-tail light airplane

Flight tests were performed to investigate the stall, spin, and recovery characteristics of a four-place, low-wing, single-engine, T-tail, general aviation research airplane at an aft center-of-gravity position. Most stalls resulted in roll-offs. Spins were oscillatory in roll and pitch at 43 deg angle of attack; the magnitude of the oscillations was determined by aileron position. Power, flap deflection, and landing gear position did not affect the angle of attack to the spin. Antispin rudder followed by forward wheel with ailerons neutral produced the fastest and most consistent recoveries but the initial application of recovery controls did not always stop a spin.

Stough, H. P., III↗