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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 73 records · Page 4
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NASA NTRS1987

Aeroelastic stability of bearingless rotors in forward flight

The finite element method was used to determine the dynamic stability of bearingless rotor blades (BR) in forward flight. The analysis was applied to four different BR configurations and the results were correlated with experimental data. The analysis was correlated with hover lag mode stability data for a simple three-blade BR rotor tested in various pitch link configurations. In addition, a more advanced BR which includes precone, blade twist, blade sweep, and a lag shear restraint is analyzed and compared to experimental data in both hover and forward flight.

Dull, Andrew L.↗
NASA NTRS1987

Aeroelastic effects in multirotor vehicles. Part 2: Methods of solution and results illustrating coupled rotor/body aeromechanical stability

This report is a sequel to the earlier report titled, Aeroelastic Effects in Multi-Rotor Vehicles with Application to Hybrid Heavy Lift System, Part 1: Formulation of Equations of Motion (NASA CR-3822). The trim and stability equations are presented for a twin rotor system with a buoyant envelope and an underslung load attached to a flexible supporting structure. These equations are specialized for the case of hovering flight. A stability analysis, for such a vehicle with 31 degrees of freedom, yields a total of 62 eigenvalues. A careful parametric study is performed to identify the various blade and vehicle modes, as well as the coupling between various modes. Finally, it is shown that the coupled rotor/vehicle stability analysis provides information on both the aeroelastic stability as well as complete vehicle dynamic stability. Also presented are the results of an analytical study aimed at predicting the aeromechanical stability of a single rotor helicopter in ground resonance. The theoretical results are found to be in good agreement with the experimental results, thereby validating the analytical model for the dynamics of the coupled rotor/support system.

Venkatesan, C.↗
NASA NTRS1976

An experimental investigation of the flap-lag stability of a hingeless rotor with comparable levels of hub and blade stiffness in hovering flight

An experimental investigation of the flap-lag stability of a hingeless rotor in hovering flight is presented and discussed. The rotor blade and hub configuration were selected such that the hub and blade had comparable levels of bending stiffness. Experimental measurements of the lag damping were made for various values of rotor rotational speed and blade pitch angle. Specifically at a blade pitch angle of 8 deg at three-quarters radius, the lag damping was determined over a range of rotational speeds from 200 RPM to 320 RPM and also over a range of blade pitch angles from 0 deg to 8 deg.

Curtiss, H. C., Jr.↗
NASA NTRS1972

Investigation of the stability of the tip vortex generated by hovering propellers and rotors.

The objective of this experimental and theoretical investigation was to determine what factors and mechanisms are involved in vortex interaction and instability and how these phenomena manifest themselves. To answer these questions, the schlieren method of flow visualization was used to observe the wakes generated by two- and four-bladed model propellers and rotors. A concurrent free-wake analysis was conducted for comparative purposes. Schlieren pictures showing wake asymmetry, interaction, and instability are presented. Various factors and mechanisms believed to be responsible for these are discussed along with the effects produced by the number of blades, collective pitch, and tip speed. Free-wake calculations that qualitatively confirm those factors responsible for wake asymmetry and interaction are also presented.

Tangler, J. L.↗
NASA NTRS1981

Full-scale wind-tunnel test of the aeroelastic stability of a bearingless main rotor

The rotor studied in the wind tunnel had previously been flight tested on a BO-105 helicopter. The investigation was conducted to determine the rotor's aeroelastic stability characteristics in hover and at airspeeds up to 143 knots. These characteristics are compared with those obtained from whirl-tower and flight tests and predictions from a digital computer simulation. It was found that the rotor was stable for all conditions tested. At constant tip speed, shaft angle, and airspeed, stability increases with blade collective pitch setting. No significant change in system damping occurred that was attributable to frequency coalescence between the rotor inplane regressing mode and the support modes. Stability levels determined in the wind tunnel were of the same magnitude and yielded the same trends as data obtained from whirl-tower and flight tests.

Warmbrodt, W.↗
NASA NTRS1982

VTOL and VSTOL handling qualities specifications, an overview of the current status

The highlights of a comparative analysis between the current helicopter and VSTOL specifications and four representative rotary wing aircraft are presented. Longitudinal, lateral, and directional control power and dynamic stability characteristics were analyzed for hovering conditions. Forward flight static and dynamic stability were analyzed for the longitudinal and lateral-directional axes. Results of the analyses in terms of the applicability/utility of the MIL-H-8501A criteria are presented for each of the above areas. The review of the MIL-H-8301A criteria against those in MIL-F-83300 and AGARD 577 indicate many areas in which MIL-H-8501A does not give adequate design guidance.

Goldstein, K. W.↗
NASA NTRS1975

Importance of helicopter dynamics to the mathematical model of the helicopter

A mathematical model of the helicopter requires appropriate representation of the constituent elements of rotor dynamics. General-purpose programs that model a variety of configurations for a broad range of operating conditions result in varying and incompatible levels of sophistication. Analysis of specific dynamic problems facilitates the identification of configuration parameters which determine system behavior. For the present analysis, the nonlinear equations of a torsionally rigid hingeless rotor are linearized about an equilibrium condition to determine flap-lag stability characteristics in hover. A collocation method was used to obtain the coupled natural frequencies and modes. These modes allow exact treatment of the effect of elastic coupling which more than compensates for the destabilizing inertial coupling. The sensitivity of damping to the number of modes was found to be small, and reasonable accuracy was obtained the first flapwise and edgewise coupled modes. The range of destabilizing precone was found to be small.

White, W. F., Jr.↗
NASA NTRS1988

A comparison of the various helicopter mathematical models used in the methodology assessment

Various features of the computer codes used in the helicopter industry and by government agencies for rotorcraft aeroelastic stability analysis are compared. Mathematical rigor in modeling rotorcraft is given primarily to the rotor-system dynamic behavior; the aerodynamic modeling is still limited to strip theory and to uneven application of corrections for stall, reversed flow, yawed flow, radial flow, and unsteady aerodynamic effects. The forward-flight regime analysis is included in five of the 11 codes surveyed. However, only two of these codes are capable of a Floquet analysis for aeroelastic stability. For the hover regime, nine of the 11 codes use eigen-analysis approach. The remaining codes perform a harmonic analysis of the transient response of system.

Stephens, Wendell B.↗
NASA NTRS1992

Optimum design of high speed prop rotors including the coupling of performance, aeroelastic stability and structures

An optimization procedure is developed for the design of high speed prop-rotors to be used in civil tiltrotor applications. The goal is to couple aerodynamic performance, aeroelastic stability, and structural design requirements inside a closed-loop optimization procedure. The objective is to minimize the gross weight and maximize the propulsive efficiency in high speed cruise. Constraints are imposed on the rotor aeroelastic stability in both hover and cruise and rotor figure of merit in hover. Both structural and aerodynamic design variables are used.

Chattopadhyay, Aditi↗
NASA NTRS1961

Effects of Gyroscopic Cross Coupling Between Pitch and Yaw on the Handling Qualities of VTOL Aircraft

Gyroscopic cross coupling between pitch and yaw was simulated with a variable-stability helicopter while hovering in a yawing maneuver to ascertain the effect of cross coupling on handling qualities. Pilot ratings of the controllability of cross coupling were obtained for various combinations of longitudinal control power, angular velocity about the yaw axis, and simulated engine angular momentum. A theoretical investigation, supplemented by simulator data, was undertaken to determine the effect of longitudinal damping on the coupling controllability. Also, a comparison was made between flight and simulator data. The results indicated that for an aircraft with otherwise satisfactory longitudinal handling qualities, the level of cross coupling is satisfactory when less than 30 percent of the available longitudinal control will trim out the largest gyroscopic coupling moment which might be encountered. Increased longitudinal damping resulted in a significant increase in the controllability of pitch-yaw gyroscopic cross coupling.

Garren, John F., Jr.↗
NASA NTRS1990

Flight investigation of variations in rotorcraft control and display dynamics for hover

Results are presented from a flight test of experimental control/display systems for the U.S. Army/NASA CH-47B variable-stability research helicopter in hovering maneuvers, supplementing the preliminary report of Eshow et al. (1987). The display format was a simplified version of the pilot night-vision system for the Apache AH-64, and the tests measured pilot performance in hover-pad capture and hover-pad tracking tasks (under simulated zero visibility) using displays based on (1) gainlike and (2) integratorlike controlled-element responses to control inputs. The implementation of the vehicle-response dynamics through an explicit model-following control system and the derivation of the display laws are described, and the test results are presented in extensive tables and graphs. Display type (1) was found to be preferred by the pilots and to give generally better performance than type (2).

Eshow, Michelle M.↗
NASA NTRS1987

Calculated performance, stability, and maneuverability of high speed tilting proprotor aircraft

The feasibility of operating tilting proprotor aircraft at high speeds is examined by calculating the performance, stability, and maneuverability of representative configurations. The rotor performance is examined in high speed cruise and in hover. The whirl flutter stability of the coupled wing and rotor motion is calculated in cruise. Maneuverability is examined in terms of the rotor thrust limit during turns in helicopter configuration. Rotor airfoils, rotor hub configuration, wing airfoil, and airframe structural weights representative of demonstrated advanced technology are considered. Key rotor and airframe parameters are optimized for high speed performance and stability. The basic aircraft design parameters are optimized for minimum gross weight. To provide a focus for the calculations, two high speed tiltrotor aircraft are considered: a 46-passenger civil transport and an air-combat/escort fighter, both with design speeds of about 400 knots. It is concluded that such high speed tiltrotor aircraft are quite practical.

Johnson, Wayne↗
NASA NTRS2001

Aeroelastic Stability of A Soft-Inplane Gimballed Tiltrotor Model In Hover

Soft-inplane rotor systems can significantly reduce the inplane rotor loads generated during the maneuvers of large tiltrotors, thereby reducing the strength requirements and the associated structural weight of the hub. Soft-inplane rotor systems. however, are subject to instabilities associated with ground resonance, and for tiltrotors this instability has increased complexity as compared to a conventional helicopter. Researchers at Langley Research Center and Bell Helicopter-Textron, Inc. have completed ail initial study of a soft-inplane gimballed tiltrotor model subject to ground resonance conditions in hover. Parametric variations of the rotor collective pitch and blade root damping, and their associated effects oil the model stability were examined. Also considered in the study was the effectiveness of ail active swash-plate and a generalized predictive control (GPC) algorithm for stability augmentation of the ground resonance conditions. Results of this study show that the ground resonance behavior of a gimballed soft-inplane tiltrotor can be significantly different from that of a classical soft-inplane helicopter rotor. The GPC-based active swash-plate was successfully implemented, and served to significantly augment damping of the critical modes to an acceptable value.

Nixon, Mark W.↗
NASA NTRS2001

Aeroelastic Stability of a Soft-Inplane Gimballed Tiltrotor Model in Hover

Soft-inplane rotor systems can significantly reduce the inplane rotor loads generated during the maneuvers of large tiltrotors, thereby reducing the strength requirements and the associated structural weight of the hub. Soft-inplane rotor systems, however, are subject to instabilities associated with ground resonance, and for tiltrotors this instability has increased complexity as compared to a conventional helicopter. Researchers at Langley Research Center and Bell Helicopter-Textron, Inc. have completed an initial study of a soft-inplane gimballed tiltrotor model subject to ground resonance conditions in hover. Parametric variations of the rotor collective pitch and blade root damping., and their associated effects on the model stability were examined. Also considered in the study was the effectiveness of an active swashplate and a generalized predictive control (GPC) algorithm for stability augmentation of the ground resonance conditions. Results of this study show that the ground resonance behavior of a gimballed soft-inplane tiltrotor can be significantly different from that of a classical soft-inplane helicopter rotor. The GPC-based active swashplate was successfully implemented, and served to significantly augment damping of the critical modes to an acceptable value.

Nixon, Mark W.↗
NASA NTRS1999

Aeroelastic Stability for Straight and Swept-Tip Rotor Blades in Hover and Forward Flight

Two isolated, hingeless rotors with identical fiber-reinforced composite root flexures were tested in hover and forward flight at realistic tip speeds. The rotors differed in blade planform and structure. The first blade set was of rectangular planform. The second set had blades of stiffer construction with swept-tips. The purpose of these tests was to measure the isolated rotor aeroelastic stability of bending/torsion coupled rotor blades over a range of flight conditions. The two soft inplane rotor systems were designed with low first torsion frequency to emphasize the effects of torsion. Regressing lag mode damping is shown in hover with precone and collective pitch variations and in forward flight with airspeed, shaft angle, precone and collective pitch variations. Correlation with comprehensive analysis for a number of test conditions is shown. Agreement in hover is excellent and forward flight correlation is reasonably good.

REGRESSING LAG MODE DAMPING↗
NASA NTRS1989

Structural optimization of rotor blades with straight and swept tips subject to aeroelastic constraints

The main conclusions obtained in the present study are summarized. Their application to the structural optimization of a helicopter blade should be limited by the assumptions used in obtaining the numerical results presented here. The optimum design procedure described here is very efficient, and can produce improved designs with a very limited number of precise analyses. The method of constructing the approximate problem is such that previously conducted aeroelastic analyses can be reused in a new optimization problem. For example, if an optimization study is preceded by a parametric study in which the effect of various combinations of blade design parameters is examined, all the aeroelastic analyses performed for the parametric study can be reutilized in the optimization study. This is not possible when the approximate problem is built from Taylor series expansions. The results of the optimization are quite sensitive to the aeroelastic stability margins required of the blade. In the optimization of case 2, changing the aeroelastic stability constraints from simply requiring that the blade be stable in hover, to requiring that the stability margins be maintained during the course of the optimization, reduced the gains in n/rev vibration levels by more than 50 percent. The introduction of tip sweep can reduce the n/rev vertical hub shears beyond the level that can be obtained by just modifying the mass and stiffness distributions of the blade.

Friedmann, Peretz P.↗
NASA NTRS1992

Aeromechanical stability of helicopters with composite rotor blades in forward flight

The aeromechanical stability, including air resonance in hover, air resonance in forward flight, and ground resonance, of a helicopter with elastically tailored composite rotor blades is investigated. Five soft-inplane hingeless rotor configurations, featuring elastic pitch-lag, pitch-flap and extension-torsion couplings, are analyzed. Elastic couplings introduced through tailored composite blade spars can have a powerful effect on both air and ground resonance behavior. Elastic pitch-flap couplings (positive and negative) strongly affect body, rotor and dynamic inflow modes. Air resonance stability is diminished by elastic pitch-flap couplings in hover and forward flight. Negative pitch-lag elastic coupling has a stabilizing effect on the regressive lag mode in hover and forward flight. The negative pitch-lag coupling has a detrimental effect on ground resonance stability. Extension-torsion elastic coupling (blade pitch decreases due to tension) decreases regressive lag mode stability in both airborne and ground contact conditions. Increasing thrust levels has a beneficial influence on ground resonance stability for rotors with pitch-flap and extension-torsion coupling and is only marginally effective in improving stability of rotors with pitch-lag coupling.

Smith, Edward C.↗