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Results for “HOVERING STABILITY”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

V/STOL hover stability impact on hover control task

Longitudinal static stability requirements for V/STOL aircraft are discussed in terms of vertical landing on a small ship at night, response of the aircraft to air turbulence, and hovering flight. The increased pilot workload encountered under these flight conditions is considered. Results of a flight simulation study performed to evaluate a specific hover control task, two control systems - attitude rate command and translational velocity command, and the pilots' performance with and without turbulence are reported. It is concluded that the translational velocity command system provides the control requirements for good spot hovering capability and reduced pilot workload.

Whitaker, A. B.↗
NASA NTRS1988

Effects of static equilibrium and higher-order nonlinearities on rotor blade stability in hover

The equilibrium and stability of the coupled elastic lead/lag, flap, and torsion motion of a cantilever rotor blade in hover are addressed, and the influence of several higher-order terms in the equations of motion of the blade is determined for a range of values of collective pitch. The blade is assumed to be untwisted and to have uniform properties along its span. In addition, chordwise offsets between its elastic, tension, mass, and aerodynamic centers are assumed to be negligible for simplicity. The aerodynamic forces acting on the blade are modeled using a quasi-steady, strip-theory approximation.

Crespodasilva, Marcelo R. M.↗
NASA NTRS1985

Application of the finite-state arbitrary-motion aerodynamics to rotor blade aeroelastic response and stability in hover and forward flight

The primary objective of this paper is to illustrate the influence of finite-state arbitrary-motion time-domain aerodynamics on rotor blade aeroelastic stability in hover and forward flight. The essential ingredients of the generalized Greenberg type time-domain unsteady aerodynamics are presented and incorporated in a coupled nonlinear flap-lag analysis. Aeroelastic stability boundaries for both hover and forward flight are obtained using both arbitrary-motion time-domain aerodynamics and quasi-steady aerodynamics. The sensitivity of flap-lag aeroelastic stability boundaries to time-domain arbitrary-motion unsteady aerodynamics is evaluated by comparing the two sets of results for various blade configurations and flight conditions.

Dinyavari, M. A. H.↗
NASA NTRS1975

Sensitivity of Hingeless Rotor Blade Flap-lag Stability in Hover to Analytical Modelling Assumptions

Prediction of flap-lag stability using a single bending mode for each degree-of-freedom is examined in the case in which the bending modes are assumed to be the same in the flap and lag directions and are independent of pitch angle and stiffness distribution. It is shown that this model gives results analogous to those obtained by Ormiston employing a rigid blade model with the blade and hub stiffness represented by springs in the limiting cases of the elastic coupling parameter R = 0 and 1. For intermediate values of R the results are shown to be quite different. The mode shape assumptions are shown to result in what is referred to as the parallel spring model in contrast to Ormiston's model which is referred to as a series spring model. The similarities and differences between these two models are developed in some details. The differences between these two models are examined for various typical rotor blade characteristics. Other aspects of the sensitivity of this problem are also considered.

Curtiss, H. C., Jr.↗
NASA NTRS1978

An experimental investigation of hingeless helicopter rotor-body stability in hover

Model tests of a 1.62 m diameter rotor were performed to investigate the aeromechanical stability of coupled rotor-body systems in hover. Experimental measurements were made of modal frequencies and damping over a wide range of rotor speeds. Good data were obtained for the frequencies of the rotor lead-lag regressing mode. The quality of the damping measurements of the body modes was poor due to nonlinear damping in the gimbal ball bearings. Simulated vacuum testing was performed using substitute blades of tantalum that reduced the effective lock number to 0.2% of the model scale value while keeping the blade inertia constant. The experimental data were compared with theoretical predictions, and the correlation was in general very good.

Bousman, W. G.↗
NASA NTRS1990

The effects of structural flap-lag and pitch-lag coupling on soft inplane hingeless rotor stability in hover

A 1.62-m-diameter rotor model was tested in hover to examine the effects of structural flap-lag and pitch-lag coupling on isolated rotor blade lead-lag stability. Flap-lag coupling was introduced by inclining the principal axes of the blade structure up to 60 degrees. Pitch-lag coupling was obtained either alone or in combination with flap-lag coupling through the use of skewed flexural hinges. The principal results confirm the predictions of theory, and show that both structural flap-lag and pitch-lag coupling when used separately are beneficial to blade stability. Moreover, when the couplings are combined, the lead-lag damping is significantly greater than it would be if the individual contributions were superimposed. Pitch-flap coupling is shown to have only a minor effect on blade lead-lag damping. Differences between theory and experiment observed at zero blade pitch and flexure angles during the initial testing were determined in a second test to be caused by stand flexibility. Other differences between theory and experiment warrant further investigation.

Bousman, William G.↗
NASA NTRS1985

Investigation of outside visual cues required for low speed and hover

Knowledge of the visual cues required in the performance of stabilized hover in VTOL aircraft is a prerequisite for the development of both cockpit displays and ground-based simulation systems. Attention is presently given to the viability of experimental test flight techniques as the bases for the identification of essential external cues in aggressive and precise low speed and hovering tasks. The analysis and flight test program conducted employed a helicopter and a pilot wearing lenses that could be electronically fogged, where the primary variables were field-of-view, large object 'macrotexture', and fine detail 'microtexture', in six different fields-of-view. Fundamental metrics are proposed for the quantification of the visual field, to allow comparisons between tests, simulations, and aircraft displays.

Hoh, R. H.↗
NASA NTRSDate not supplied

Simplified Vehicle Control Concept for a Lift Plus Cruise eVTOL Vehicle

Electric Vertical Takeoff and Landing (eVTOL) vehicles have the potential to enable cost effective Urban Air Mobility (UAM) applications. These concepts may also pose several challenging handling and control problems, which must be addressed prior to safe and efficient urban operations. This paper investigates a simplified vehicle control concept that is designed to address some of these challenges for a conceptual Lift Plus Cruise vehicle. This concept also includes several variations of a transition-to-hover function, designed to assist pilots in capturing and maintaining a stabilized hover prior to landing. The different simplified control architectures for this concept are presented along with preliminary findings. Preliminary results show that the simplified control concept can be effective in assisting pilots to achieve safe and reliable hover landings, and that transition-to-hover functions have the potential of reducing workload while increasing accuracy. A follow on evaluation will be performed in the Vertical Motion Simulator (VMS) at NASA Ames Research Center. The planned tests will focus on more operationally representative scenarios, while building upon these operations with additional test cases exploring variations in glidepath angles and wind conditions.

simplified vehicle control↗
NASA NTRS1983

Measured inplane stability characteristics in hover for an advanced bearingless rotor

A study was made of the inplane stability characteristics of a model of an advanced bearingless main rotor. The four-bladed rotor system was tested for aeroelastic stability in hover for both isolated and body-free conditions. Variations in several hub design parameters were tested, including blade coning and sweep angles, blade inplane structural damping, pitch-link location (delta-free effect), and fuselage structural damping. An analysis of the experimental results shows that parametric stability trends observed from isolated-rotor studies may yield incorrect conclusions regarding coupled rotor-body characteristics. For body-free conditions, the baseline rotor configuration resulted in the best stability margins at 1-g thrust as well as at the nominal design rotor speed. Blade built-in coning had little effect on the body-free damping levels at the inplane/body resonance points. Introducing blade sweep destabilized the rotor at the inplane/body pitch resonance.

Weller, W. H.↗
NASA NTRS1992

Visual cues in flight simulation - An evaluation of stereo effectiveness

The purpose of the present study was to quantify the effects of visual scene information on precision hovering tasks under biocular and stereo viewing conditions in a rotorcraft helmet-mounted display simulator. Four NASA test pilots performed a series of precision hover maneuvers in the context of three different scene contents, two ground textures, and three levels of control configuration difficulty. The results revealed that vertical position variability was greatest when vertical cues were absent, and that patterned ground texture aided hover stability only when other position-reference cues were absent from the scene. The stereo viewing condition showed no uniform advantages over the biocular condition. The results are discussed in terms of their implications for simulator design and qualification.

Johnson, Walter W.↗
NASA NTRS1984

Aeromechanical stability of a full-scale hingeless rotor in hover

The aeroelastic stability in hover of a 9.8-m-diam, hingeless helicopter rotor system is evaluated. Rotor inplane damping data were obtained for rotor operation between 350 and 425 rpm for thrust coefficients between 0.0 and 0.12. At constant rotor rotational speed, a minimum inplane damping level was obtained at a thrust coefficient of approximately 0.02. At constant rotor thrust, a minimum in rotor inplane damping was measured at 400 rpm. Correlation with a comprehensive aeroelastic stability program is found to be good. The influence of different aerodynamic inflow models on predicted damping levels is shown. Best correlation with experimental data is obtained using a dynamic inflow model rather than static or quasi-static inflow models. Comparison with other full-scale, hingeless rotor stability data in hover is presented. Also, direct comparisons are made with data from a full-scale bearingless main rotor test performed on the same general-purpose test apparatus. Although the bearingless rotor is more highly damped at design tip speed and 1-g thrust operation, greater sensitivity to operating conditions is shown.

Peterson, R. L.↗
NASA NTRS1985

Stability and control of VTOL capable airships in hovering flight

The stability and control characteristics of an airship equipped with lifting rotors to provide a modest VTOL capability are discussed. The rotors are used for control and maneuvering in near-hovering flight. Configurations with two, three, and four lifting rotors are examined and compared with respect to control capabilities and dynamic response characteristics. Linearized models of the dynamics are employed for this study. A new approach to the prediction of rotor derivatives for operation near zero thrust in hover is presented. It is found that all three configurations have similar control and response characteristics. The responses are characterized by long time constants and low levels of angular damping.

Curtiss, H. C., Jr.↗
NASA NTRS1988

Rotorcraft aeroelastic stability

Theoretical and experimental developments in the aeroelastic and aeromechanical stability of helicopters and tilt-rotor aircraft are addressed. Included are the underlying nonlinear structural mechanics of slender rotating beams, necessary for accurate modeling of elastic cantilever rotor blades, and the development of dynamic inflow, an unsteady aerodynamic theory for low-frequency aeroelastic stability applications. Analytical treatment of isolated rotor stability in hover and forward flight, coupled rotor-fuselage stability in hover and forward flight, and analysis of tilt-rotor dynamic stability are considered. Results of parametric investigations of system behavior are presented, and correlation between theoretical results and experimental data from small and large scale wind tunnel and flight testing are discussed.

Ormiston, Robert A.↗
NASA NTRS1985

Dynamic stability of a bearingless circulation control rotor blade in hover

The aeroelastic stability of flap bending, lead-lag bending and torsion of a bearingless circulation control rotor blade in hover is investigated using a finite element formulation based on Hamilton's principle. The flexbeam, the torque tube and the outboard blade are discretized into beam elements, each with fifteen nodal degrees of freedom. Quasisteady strip theory is used to evaluate the aerodynamic forces and the airfoil characteristics are represented either in the form of simple analytical expressions or in the form of data tables. A correlation study of analytical results with the experimental data is attempted for selected bearingless blade configurations with conventional airfoil characteristics.

Chopra, I.↗
NASA NTRS1975

An approximate closed-form solution for lead lag damping of rotor blades in hover

Simple stability methods are used to derive an approximate, closed-form expression for the lead-lag damping of rotor blades in hover. Destabilizing terms are shown to be a result of two dynamic mechanisms. First, the destabilizing aerodynamic forces that can occur when blade lift is higher than a critical value are maximized when the blade motion is in a straight line equidistant from the blade chord and the average direction of the air flow velocity. This condition occurs when the Coriolis terms vanish and when the elastic coupling terms align the blade motion with this least stable direction. Second, the nonconservative stiffness terms that result from pitch-flap or pitch-lag coupling can add or subtract energy from the system depending upon whether the motion of the blade tip is clockwise or counterclockwise.

Peters, D. A.↗
NASA NTRS1989

Theoretical and experimental investigation of the aeroelastic stability of an advanced bearingless rotor in hover and forward flight

The aeroelastic stability of a shaft-fixed, 1/8th Froude scaled bearingless rotor was investigated in a series of wind tunnel experiments simulating a wide range of operating conditions. A finite element formulation was used to perform a parallel theoretical analysis, with the goal of determining whether a bearingless rotor system could be made aeroelastically stable without the incorporation of auxilliary dampers. A quick estimate of lag mode damping was provided by a refined moving-block analysis implemented in real time which predicted similar damping values. Model rotor and blade properties were also determined, and these properties were used as inputs for a newly refined bearingless rotor analysis. Predicted results were compared with experimental results in hover and forward flight. Results indicated that soft pitch link stiffness increases pitch-lag coupling and stabilizes lag mode stability in hover and at low advance ratios, but destabilizes at higher advance ratios.

Wang, James M.↗