Showing papers by "Robert F. Stengel published in 1992"

Robustness of solutions to a benchmark control problem

Abstract: The robustness of 10 solutions to a benchmark control design problem presented at the 1990 American Control Conference has been evaluated. The 10 controllers have second-to-eighth-order transfer functions and have been designed using several different methods, including H-infinity optimization, loop-transfer recovery, imaginary-axis shifting, constrained optimization, structured covariance, game theory, and the internal model principle. Stochastic robustness analysis quantifies the controllers' stability and performance robustness with structured uncertainties in up to six system parameters. The analysis provides insights into system response that are not readily derived from other robustness criteria and provides a common ground for judging controllers produced by alternative methods. One important conclusion is that gain and phase margins are not reliable indicators of the probability of instability. Furthermore, parameter variations actually may improve the likelihood of achieving selected performance metrics, as demonstrated by results for the probability of settling-time exceedance.

Identification of aerodynamic coefficients using computational neural networks

Abstract: Precise, smooth aerodynamic models are required for implementing adaptive, nonlinear control strategies. Accurate representations of aerodynamic coefficients can be generated for the complete flight envelope by combining computational neural network models with an Estimation-Before-Modeling paradigm for on-line training information. A novel method of incorporating first-partial-derivative information is employed to estimate the weights in individual feedforward neural networks for each aerodynamic coefficient. The method is demonstrated by generating a model of the normal force coefficient of a twin-jet transport aircraft from simulated flight data, and promising results are obtained.

Stochastic measures of performance robustness in aircraft control systems

Abstract: Stochastic robustness, a simple technique used to estimate the robustness of linear, time-invariant systems, is applied to a twin-jet transport aircraft control system. Concepts behind stochastic stability robustness are extended to stochastic performance robustness. Stochastic performance robustness measures based on classical design specifications and measures specific to aircraft handling qualities are introduced. Confidence intervals for comparing two control system designs are presented. The application of stochastic performance robustness, the use of confidence intervals, and tradeoffs between performance objectives are demonstrated by means of the twin-jet aircraft example.

Probabilistic reasoning for intelligent wind shear avoidance

Abstract: A computerized intelligent cockpit aid can increase flight-crew awareness of wind shear, improving avoidance decisions. The FAA Windshear Training Aid (WTA) provides guidelines for assessing the risk of wind shear encounter from meteorological evidence. Use of these guidelines in the cockpit is complicated by uncertainty surrounding meteorological knowledge of wind shear. Bayesian network representation is discussed as a means of modeling this uncertain knowledge in a computer. A probabilistic model of the WTA guidelines using Bayesian network representation is presented. This model combines evidence from sources of varying reliability and incorporates results from meteorological studies of wind shear. The probabilistic model can provide flight crews with meaningful estimates of risk to aid their decisions, using evidence from a variety of sources and a base of meteorological knowledge.

Target pitch angle for the microburst escape maneuver

Abstract: Recovery performance of a commuter-type aircraft in a microburst encounter is studied using a constant-pitch-attitude strategy and flight path optimization. Results obtained indicate that the pitch attitude which maximized climb rate in a wind shear condition is strongly dependent on whether the aircraft is subjected to a horizontal shear or a downdraft. The pitch attitude which maximizes ground clearance depends on the altitude of the encounter, the strength of the microburst, and the initial position of the aircraft with respect to the downburst core. Best results are obtained at relatively low target pitch angles, in severe wind shear encounters at very low altitudes. A technique for maximizing ground clearance involves maintaining a low pitch attitude early in the encounter, followed by a gradual pitch-up that ceases when the wind shear has been excited.

Stochastic prediction techniques for wind shear hazard assessment

Abstract: The threat of low-altitude wind shear has prompted development of aircraft-based sensors that measure winds directly on the aircraft's intended flight path Measurements from these devices are subject to turbulence inputs and measurement error, as well as to the underlying wind profile Stochastic estimators are developed to process on-board Doppler sensor measurements, producing optimal estimates of the winds along the path A stochastic prediction technique is described to predict the hazard to the aircraft from the estimates as well as the level of uncertainty of the hazard prediction The stochastic prediction technique is demonstrated in a simulated microburst wind shear environment Use of the technique in a decision-making process is discussed >

Stochastic Robustness Synthesis for a Benchmark Problem

Abstract: Stochastic Robustness Analysis guides the synthesis of robust LQG regulators for a Benchmark Control Problem. Probabilities of exceeding allowable design limits, including stability, settling time, and control usage, are estimated by Monte Carlo evaluation Robust. low-gain, compensators that fulfill objectives are designed by numerically minimizing quadratic functions of these probabilities. The method is straightforward and makes use of uncomplicated design principles.

On the flight of a golf ball in the vertical plane

Abstract: The equations of motion for a ball moving in a vertical plane are used to calculate flight paths for a typical golf ball subjected to a variety of launch conditions, spin rates, atmospheric parameters, and wind fields. Time histories of the ball's velocity, flight-path angle, height, range, and spin rate between tee and first impact illustrate that lift induced by the ball's spin has a significant effect on driving range, actually causing the flight-path angle to increase during the first few seconds of flight. While light winds have the expected effects, heavy tailwind is shown to spoil the carry of the ball. Linearized sensitivity analysis indicates that wind uncertainty is an important contributor to impact range and time uncertainty.

Real-time decision aiding - Aircraft guidance for wind shear avoidance

Abstract: Modern estimation theory and artificial intelligence technology are applied to the Wind Shear Safety Advisor, a conceptual airborne advisory system to help flight crews avoid or survive encounter with hazardous low-altitude wind shear. Numerical and symbolic processes of the system fuse diverse, time-varying data from ground-based and airborne measurements. Simulated wind-shear-encounter scenarios illustrate the need to consider a variety of factors for optimal decision reliability. The wind-shear-encounter simulations show the potential of the Wind Shear Safety Advisor for effectively integrating the available information, highlighting the benefits of the computational techniques employed. >

Comment on 'Effect of thrust/speed dependence on long-period dynamics in supersonic flight'

Abstract: Introduction R EFERENCE 1 discusses effects of speed-dependent variations in thrust on the long-period motions of aircraft in supersonic flight. Its principal contribution is the demonstration of effects that a complex pair of zeros have on speed-tothrottle feedback. It is concluded that there are fundamental differences between thrust/speed effects in subsonic and supersonic flight. It also is stated that "thrust variations with speed (or Mach number) have practically no effect on the phugoid in supersonic flight," then that "the phugoid is not influenced at all in supersonic flight." The purpose of this Comment is to show that thrust/speed effects are indeed small but not zero in supersonic flight and that this is a kinematic effect of increasing speed rather than an aerodynamic effect of Mach number. In the process, a simplified analytical model is offered for further study. Furthermore, it is noted that pitching moment/speed sensitivity due to thrust-axis offset is likely to have important long-period effect even when the direct thrust/speed effects are small.

Optimal recovery from microburst wind shear

Abstract: Severe low-altitude wind variability represents an infrequent but significant hazard to aircraft taking off or landing. During the period from 1964 to 1985, microburst wind shear was a contributing factor in at least 26 civil aviation accidents involving nearly 500 fatalities and over 200 injuries. A microburst is a strong localized downdraft that strikes the ground, creating winds that diverge radially from the impact point. The physics of microbursts have only been recently understood in detail, and it has been found that effective recovery from inadvertent encounters may require piloting techniques that are counter-intuitive to flight crews. The goal of this work was to optimize the flight path of a twin-jet transport aircraft encountering a microburst during approach to landing. The objective was to execute an escape maneuver that maintained safe ground clearance and an adequate stall margin during the climb-out portion of the trajectory.