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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1974

Lifting-surface theory for a semi-infinite wing in oblique gust

An unsteady lifting-surface theory is developed for the calculation of the airload on a semi-infinite-span thin wing in a compressible flow due to interaction with an oblique gust. By using the solutions obtained for a two-dimensional wing, the problem is formulated so that the unknown is taken to be the difference between the airload on the semi-infinite wing and that on a two-dimensional wing under the same gust conditions. Since this airload difference is nonzero only near the wing tip, the control points need be distributed in the tip region only; this significantly simplifies the numerical procedure. Results are presented for a wing with rectangular tip. The implication for noise and unsteady loads due to blade-vortex interaction for helicopter rotors is discussed.

Chu, S.↗
NASA NTRS1975

Optimal control alleviation of tilting proprotor gust response

Optimal control theory is applied to the design of a control system for alleviation of the gust response of tilting proprotor aircraft. Using a proprotor and cantilever wing analytical model, the uncontrolled and controlled gust response is examined over the entire operating range of the aircraft except for hover: helicopter mode, conversion, and airplane mode flight. Substantial improvements in the loads, ride quality, and aeroelastic stability are possible with a properly designed controller. A single controller, nominally optimal only at the design point speed (160 knots here), operated efficiently over the entire speed range, with the possible exception of very low speed in helicopter mode. Kalman-Bucy filters were used as compensation networks to provide state estimates from various measurements in the wing motion, rotor speed perturbation, and tip-path-plane tilt.

Johnson, W.↗
NASA NTRS1975

A Study of Gust and Control Response of Model Rotor-Propellers in a Wind Tunnel Airstream

One-ninth-scale model hingeless and gimballed rotor-propellers were tested at an advance ratio of 0.7 in the Wright Brothers Wind Tunnel at MIT, in the presence of sinusoidal longitudinal and vertical gusts produced by a gust generator of novel design. The gimballed rotor was also subjected to sinusoidal collective and cyclic control inputs. Model test data in terms of blade inplane and out-of-plane bending, longitudinal and lateral gimbal motion, wing vertical and chordwise bending, and blade and wing torsion are presented and compared with theory.

Ham, N. D.↗
NASA NTRS1976

Application of the aerodynamic energy concept to flutter suppression and gust alleviation by use of active controls

The effects of active controls on flutter suppression and gust alleviation of the Arava twin turboprop STOL transport and the Westwind twinjet business transport are investigated. The active control surfaces are introduced in pairs which include, in any chosen wing strip, a 20-percent chord leading-edge control and a 20-percent chord trailing-edge control. Each control surface is driven by a combined linear-rotational sensor system located on the activated strip. The control law is based on the concept of aerodynamic energy and utilizes previously optimized control law parameters based on two-dimensional aerodynamic theory. The best locations of the activated system along the span of the wing are determined for bending-moment alleviation, reduction in fuselage accelerations, and flutter suppression. The effectiveness of the activated system over a wide range of maximum control deflections is also determined. Two control laws are investigated. The first control law utilizes both rigid-body and elastic contributions of the motion. The second control law employs primarily the elastic contribution of the wing and leads to large increases in the activated control effectiveness as compared with the basic control law. The results indicate that flutter speed can be significantly increased (over 70 percent increase) and that the bending moment due to gust loading can be almost totally eliminated by a control system of about 10 to 20 percent span with reasonable control-surface rotations.

Nissim, E.↗
NASA NTRS1976

A complete second-order theory for the unsteady flow about an airfoil due to a periodic gust

A uniformly valid second-order theory is developed for calculating the unsteady incompressible flow that occurs when an airfoil is subjected to a convected sinusoidal gust. Explicit formulas for the airfoil response functions (i.e., fluctuating lift) are given. The theory accounts for the effect of the distortion of the gust by the steady-state potential flow around the airfoil, and this effect is found to have an important influence on the response functions. A number of results relevant to the general theory of the scattering of vorticity waves by solid objects are also presented.

Goldstein, M. E.↗
NASA NTRS1977

Dynamics of ultralight aircraft: Motion in vertical gusts

Gust load calculations are extended to the range of conditions encountered by ultralight aircraft such as hang gliders. Having wing loadings of the order of 5 kg/sq m, these gliders acquire a substantial fraction of the motion of a gust within a distance of 1 or 2 m. Comparative loads and displacements for a small powered airplane having a wing loading of 50 kg sq m and for a commercial jet with 500 kg sq m are shown.

Jones, R. T.↗
NASA NTRS1977

A comparison of the results of dynamic wind-tunnel tests with theoretical predictions for an aeromechanical gust-alleviation system for light airplanes

Dynamic wind tunnel tests have been conducted on a 1/6-scale model of a general aviation airplane equipped with an all-mechanical gust alleviation system which uses auxiliary aerodynamic surfaces to drive the flaps. The longitudinal short period motions were studied under simulated gust conditions in order to verify the mathematical model used in a previous study to predict the performance of the full scale system and determine the amount of normal acceleration alleviation which could be attained. The model responses were measured for different configurations with the system active and without the system active for comparison. The tests confirmed the general relationships between the experimental variables and the model responses predicted by the mathematical model, but there were significant differences in the magnitudes of the responses. The experimental results for the model were used to estimate a reduction of 30 percent in the rms normal acceleration response of a similar full scale airplane in atmospheric turbulence.

Stewart, E. C.↗
NASA NTRS1977

Optimal control alleviation of tilting proprotor gust response

Optimal control theory is applied to the design of a control system for alleviation of the gust response of tilting proprotor aircraft. Using a proprotor and cantilever wing analytical model, the uncontrolled and controlled gust response is examined over the entire operating range of the aircraft except for hover: helicopter mode, conversion, and airplane mode flight. Substantial improvements in the loads, ride quality, and aeroelastic stability are possible with a properly designed controller. A single controller, nominally optimal only at the design point speed (160 knots here), operated efficiently over the entire speed range, with the possible exception of very low speed in helicopter mode. Kalman-Bucy filters were used as compensation networks to provide state estimates from various measurements in the system. Efficient control requires the measurement of the wing motion, rotor speed perturbation, and tip-path-plane tilt.

Johnson, W.↗
NASA NTRS1978

Turbulent transport model of wind shear in thunderstorm gust fronts and warm fronts

A model of turbulent flow in the atmospheric boundary layer was used to simulate the low-level wind and turbulence profiles associated with both local thunderstorm gust fronts and synoptic-scale warm fronts. Dimensional analyses of both type fronts provided the physical scaling necessary to permit normalized simulations to represent fronts for any temperature jump. The sensitivity of the thunderstorm gust front to five different dimensionless parameters as well as a change from axisymmetric to planar geometry was examined. The sensitivity of the warm front to variations in the Rossby number was examined. Results of the simulations are discussed in terms of the conditions which lead to wind shears which are likely to be most hazardous for aircraft operations.

Lewellen, W. S.↗
NASA NTRS1978

Tracking error of 100-m antenna due to wind gust

A procedure is shown to derive root-mean-square tracking error for an antenna system in response to wind gust loading. Example calculations are illustrated for the design of a proposed 100 m antenna. The effect of wind gust correlation is also considered.

Massoudi, M.↗
NASA NTRS1980

The design, testing and evaluation of the MIT individual-blade-control system as applied to gust alleviation for helicopters

A type of active control for helicopters was designed and tested on a four foot diameter model rotor. A single blade was individually controlled in pitch in the rotating frame over a wide range of frequencies by electromechanical means. By utilizing a tip mounted accelerometer as a sensor in the feedback path, significant reductions in blade flapping response to gust were achieved at the gust excitation frequency as well as at super and subharmonics of rotor speed.

Mckillip, R. M., Jr.↗
NASA NTRS1980

Applications of NASTRAN in Gust Response Analysis at Northrop

A comprehensive gust response analysis was performed on a complete model of an airplane using the NASTRAN aeroelastic package. The analysis investigated random response to atmospheric turbulence and transient response to a discrete gust, including control system dynamics in both cases. It is shown that NASTRAN is an effective tool for aeroelastic analysis and active control system design.

Singh, A. K.↗
NASA NTRS1979

Application of stochastic optimal reduced state feedback gain computation procedures to the design of aircraft gust alleviation controllers

A stochastic linear model that accounts for process parameter and initial uncertainty, measurement noise, and a restricted number of measurable outputs was used to determine feedback gains useful for reducing the vertical acceleration which results from the presence of a vertical wind gust. Considered in the study were the influence of various feedback configurations, the effects of sensor noise, flight condition changes, and initialization procedures. Results showed that for sixth order linearized longitudinal motion, a controller with feedback on three states could be designed for effective gust alleviation taking into account both sensor noise and flight condition variation.

Sobel, K.↗
NASA NTRS1980

Aerodynamic response of an airfoil with thickness to a longitudinal and transverse periodic gust

The unsteady lift of an airfoil with thickness subject to a two-dimensional periodic gust is analyzed using the recent theory of Goldstein and Atassi. It is found that to properly account for the coupling between the steady potential flow and the unsteady vortical flow, one has to consider the contribution of order alpha-squared (when alpha is steady state disturbance) to the potential flowfield. A closed form analytical formula is then derived for the lift function. The results show strong dependence on the wave members of the gust.

Hamad, G.↗
NASA NTRS1980

Design for active flutter suppression and gust alleviation using state-space aeroelastic modeling

An analytical design technique for an active flutter-suppression and gust-alleviation control system is presented. It is based on a rational approximation of the unsteady aerodynamic loads in the entire Laplace domain, which yields matrix equations of motion with constant coefficients. Some existing rational approximation schemes are reviewed, and a new technique which yields a minimal number of augmented states for a desired accuracy is presented. The state-space aeroelastic model is used to design a constant gain, partial-feedback control system, which simultaneously assures stability and optimizes any desired combination of gust response parameters throughout the entire flight envelope.

Karpel, M.↗
NASA NTRS1981

Transonic flutter and gust-response tests and analyses of a wind-tunnel model of a torsion-free-wing fighter airplane

The paper reports the results of analytical and exploratory wind-tunnel studies of the transonic flutter and gust response characteristics of a 1/5.5-size complete-airplane version of a torsion-free-wing (TFW) fighter aircraft. The critical flutter mode for the TFW-free configuration was found to occur at M = 0.95 and had the rigid-TFW pitch mode as its apparent aerodynamic driver. However, the minimum dynamic flutter for the TFW-free case was only about 20% lower than for the TFW-locked; therefore the present TFW is a viable design concept with respect to flutter. The present TFW was not effective as a gust alleviator.

Ruhlin, C. L.↗
NASA NTRS1983

Calculation of wing response to gusts and blast waves with vortex lift effect

A numerical study of the response of aircraft wings to atmospheric gusts and to nuclear explosions when flying at subsonic speeds is presented. The method is based upon unsteady quasi-vortex lattice method, unsteady suction analogy and Pade approximant. The calculated results, showing vortex lag effect, yield reasonable agreement with experimental data for incremental lift on wings in gust penetration and due to nuclear blast waves.

Chao, D. C.↗
NASA NTRS1983

Calculation of wing response to gusts and blast waves with vortex lift effect

A numerical study of the response of aircraft wings to atmospheric gusts and to nuclear explosions when flying at subsonic speeds is presented. The method is based upon unsteady quasi-vortex-lattice method, unsteady suction analogy, and Pade approximate. The calculated results, showing vortex lag effect, yield reasonable agreement with experimental data for incremental lift on wings in gust penetration and due to nuclear blast waves.

Chao, D. C.↗