David K. Schmidt

On the conflict frequency at air route intersections

Abstract Past studies have developed models for the expected number of perceived overtake and crossing conflicts between aircraft based on route geometry, separation criteria, aircraft velocities, flow rates and controller perception considerations. The frequency of overtake and crossing conflicts, furthermore were assumed to be approximately Poisson distributed. In this paper previous results have been extended, and the number of crossing conflicts, at route intersections or route transitions, is shown to be expressable as a random sum of correlated random variables. This allows for a detailed derivation of the variance and an approximate probability distribution for the number of crossing conflicts. Results indicate that the variance of the conflict frequency may be considerably larger than that obtained by the Poisson assumption, a fact that could explain some of the appreciable variability in air traffic controller work load. A numerical example is included with results compared to those of a numerical simulation of intersection conflicts.

An analytical method for ride quality of flexible airplanes

A new and easily used method of aircraft ride quality analysis is developed, and its use is illustrated by a numerical example using DC-8 airplane equations of motion. The method readily allows any type of stability augmentation system to be included for good handling qualities. A particularly powerful method of specifying and achieving all roots of a desired closed-loop characteristic equation by use of full state variable feedback is described and applied to the DC-8 example. ! 3 However, the need for much research remains before design criteria and Federal Aviation Regulations Part 25 certification standards can be set, bringing this technology to production application. Relaxed static stability (RSS) is the CCV function most likely to result in the largest, most immediate payoff and first- reach production status for future large transports. RSS allows more aft e.g. location and reduced empennage size, with associated drag and weight reductions, by use of stability augmentation systems to restore acceptable handling qualities. These advanced airplanes must be optimized for good ride, as well as performance, and no design criteria exist to achieve this in other than a trial and error fashion. An investigation is being conducted into the relationships and tradeoffs between handling qualities and ride quality for large, highly elastic airplanes with RSS implemented, but with no active elastic mode suppression control system in use. An important question to be answered is how sensitive the ride quality is (in turbulence) to the type of stability augmentation used, and to variations in handling qualities parameters such as short-period and dutch-roll damping ratios and undamped natural frequencies. The commonly used ride quality parameter is the vertical and lateral normal acceleration rms responses at selected fuselage stations. In terms of perturbation of flight path angle 7; pitch angle 6; roll-angle , and £/; and antisymmetric shapes and coordinates <£,- and £,-, the vertical and lateral normal acceleration load factors as a func- tion of fuselage station 4 (positive forward of the e.g.) are

Analysis of airframe/engine interactions - An integrated control perspective

Techniques for the analysis of the dynamic interactions between airframe/engine dynamical systems are presented. Critical coupling terms are developed that determine the significance of these interactions with regard to the closed loop stability and performance of the feedback systems. A conceptual model is first used to indicate the potential sources of the coupling, how the coupling manifests itself, and how the magnitudes of these critical coupling terms are used to quantify the effects of the airframe/engine interactions. A case study is also presented involving an unstable airframe with thrust vectoring for attitude control. It is shown for this system with classical, decentralized control laws that there is little airframe/engine interaction, and the stability and performance with those control laws is not affected. Implications of parameter uncertainty in the coupling dynamics is also discussed, and effects of these parameter variations are also demonstrated to be small for this vehicle configuration.

Cooperative synthesis of control and display augmentation

The cooperative control synthesis technique, previously developed to synthesize control augmentation so as to optimize the human operators subjective rating, is extended to the synthesis of display augmentation for closed-loop manual control tasks. The procedure allows for simultaneously augmenting the display dynamics as well as plant or controlled element dynamics via feedback, and explicitly includes task-related and pilot-centered requirements in the design objectives. Connections are made between the proposed technique and classical manual control theory, and the new methodology is demonstrated by considering a compensatory tracking task for a generic controlled element. Analytical evaluations of the various control and display augmentation results and comparisons with the results of previous experimental studies are discussed.

A Comparative Study of Multivariable Robustness Analysis Methods as Applied to Integrated Flight and Propulsion Control

Wee multivariable robustness analysis methods will be compared and contrasted. The focus of the analysis will be on system stability and performance robustness to uncertainty in the coupling dynamics between two interacting subsystems. Of particular interest is interacting airframe and engine subsystems and an example airframelengine vehicle configuration is utilized in the demonstration of these approaches. The Singular Value (SV) and Structured Singular Value (SSV) Analysis methods will be compared to a method especially well suited for analysis of robustness to uncertainties in subsystem interactions. This approach is referred to here as the Interacting Subsystem (IS) Analysis method. This method has been used previously to analyze airframelengine systems, emphasizing the study of stability robustness. However, performance robustness is also investigated here, and a new measure of allowable uncertainty for acceptable performance robustness is introduced. The IS methodology does not require plant uncertainty models to measure the robustness of the system, and will be shown to yield valuable information regarding the effects of subsystem interactions. In contrast, the SV and SSV methods allow for the evaluation of the robustness of the system to particular models of uncertainty, and do not directly indicate how the airframe (engine) subsystem interacts with the engine (airframe) subsystem.

Preliminary Design of a Tornado Probe

An interdisciplinary design team at Purdue University, Lafayette, Ind., is developing a system for probing tornadoes. The objective of the program is to obtain horizontal profiles of temperature, pressure, wind velocity, and magnetic field in and around a tornado vortex. The present plan for achieving these objectives includes an aircraft to search for tornadoes on tornado days. Upon sighting a tornado the aircraft will fire a 2.75-inch rocket into the vortex. This 2.75-inch rocket will be outfitted both with instrumentation for measuring pressure, temperature, magnetic field, and wind velocity and with a telemetry system for relaying data back to the launch aircraft. The launch aircraft will contain equipment for recording the telemetry information, and photographic equipment will be used to make indirect measurements.

Limitations of block-decentralized control with implications for ASTOVL aircraft

Limitations of block-decentralized control laws are investigated for an Advanced Short Take Off and Vertical Landing (ASTOVL) vehicle. The vehicle configuration selected is similar to those proposed for the Joint Strike Fighter (JSF) program. For the model considered, no block-decentralized control law can achieve acceptable performance as defined herein. This result follows from necessary conditions for the existence of a block-decentralized control law that achieves specified performance. One of these conditions is violated for the case study. To achieve the required decoupling, block-decentralized control requires an engine feedback gain that is larger than actuation constraints allow. The performance specifications and actuation constraints are considered realistic estimates for this class of vehicles, and the case study demonstrates possible limitations of block-decentralized control for ASTOVL vehicles, in general. Introduction Advanced Short Take Off and Vertical Landing (ASTOVL) aircraft configurations are of current interest. These aircraft configurations frequently exhibit significant airframe/propulsion subsystem 8-9 dynamic interactions. Fig. 1 displays a generic ASTOVL vehicle. The airframes lift must be generated by the propulsion subsystem during low speed flight and hover. The lift is generated by re-directed aft thrust and thrust from an auxiliary lift system. Some proposed designs call for the lift system to possess both fan and turbine, and possibly even a burner. However, the primary thrust of the lift system is typically provided by either main engine by-pass and/or core flow. Secondary thrust is generated by airflow from intakes located on top of the fuselage. Nozzle vectoring and Reaction Control System (RCS) jets will generate t Presented at the AIAA GNC Conf., San Diego, 1996, AIAA Paper No. 96-3921. t Research Associate, Student Member, AIAA. * Prof, and Director, Associate Fellow, AIAA. Copyright

Analysis of airframe/engine interactions in integrated flight and propulsion control

An analysis framework for the assessment of dynamic cross-coupling between airframe and engine systems from the perspective of integrated flight/propulsion control is presented. This analysis involves to determining the significance of the interactions with respect to deterioration in stability robustness and performance, as well as critical frequency ranges where problems may occur due to these interactions. The analysis illustrated here investigates both the airframes effects on the engine control loops and the engines effects on the airframe control loops in two case studies. The second case study involves a multi-input/multi-output analysis of the airframe. Sensitivity studies are performed on critical interactions to examine the degradations in the systems stability robustness and performance. Magnitudes of the interactions required to cause instabilities, as well as the frequencies at which the instabilities occur are recorded. Finally, the analysis framework is expanded to include control laws which contain cross-feeds between the airframe and engine systems.

Cooperative synthesis of control and display augmentation for a STOL aircraft in the approach and landing task

Application of the Cooperative methodology to synthesize control/display augmentation systems for the piloted longitudinal landing task with a modern, statically-unstable, fighter aircraft is considered. Starting with a control augmentation law which yields augmented vehicle dynamics that meet Level I handling qualities specifications, the effect of time-delay in the Head-up-display is studied using model-based criterion. This evaluation showed that even with good conventional dynamics, a realistic value of display time-delay will cause significant deterioration in pilot workload and piloted-system performance. Application of the Cooperative methodology to control augmentation alone resulted in augmented vehicle dynamics which provide direct control of the flight path from the pilot stick. Analytical evaluation of these dynamics indicates that such dynamics might lead to improved pilot ratings over conventional dynamics, especially in the presence of time-delays in the system. Also, application of the methodology to simultaneous synthesis of control augmentation and flight director laws revealed that it might be advantageous to consider the control/display trade-off in the early design stages.

Experimental investigation of control/display augmentation effects in a compensatory tracking task

The effects of control/display augmentation on human performance and workload have been investigated for closed-loop, continuous-tracking tasks by a real-time, man-in-the-loop simulation study. The experimental results obtained indicate that only limited improvement in actual tracking performance is obtainable through display augmentation alone; with a very high level of display augmentation, tracking error will actually deteriorate. Tracking performance improves when status information is furnished for reasonable levels of display quickening; again, very high quickening levels lead to tracking error deterioration due to the incompatibility between the status information and the quickened signal.