10
AVIONICS AND CONTROL

INTRODUCTION

The 1980s saw a significant change in the nature of commercial air transportation and military aircraft operations as a consequence of remarkable growth in application of new avionics. These included widespread implementation of fly-by-wire systems and significant advances in fully electronic displays ("glass cockpits"), digital flight control and flight management, ring laser gyro-based inertial navigation, and full-authority digital engine controls.

These innovations provided much-increased functional capability without adverse impact to the weight of aircraft. In fact, despite the proliferation of avionics, they have accounted for approximately 1 percent of the airplane weight for the last 20 years.

However, advances in avionics have brought a new set of problems. For example, demand has increased for coordination and standardization in areas as diverse as microwave landing systems, software standards, electromagnetic vulnerability standards, and certification and testing requirements. Other problems include generally inadequate testing and validation to ensure that such systems meet all requirements when they are introduced, and the often massive cost and schedule overruns resulting from problems in software development and validation.

The digital systems introduced in the 1980s included box-for-box replacements or additions to existing functions. This created a proliferation of black boxes and consequent challenges to system integration, validation, reliability, and cost. There were also many technology developments occurring in other sectors that failed to find their way into aeronautical applications in a timely manner. For example, fiber optics, which have extensive applications in communications, have not yet seen significant application in aircraft. In short, although avionics and control technologies have produced continuous advances in aircraft systems, there is still significant opportunity for greater efficiency, enhanced functionality, and better integration of systems. This is particularly true for systems that reduce the burden on the crew of flying the aircraft and systems that allow for increased capacity of the global air traffic management (ATM) system. It is important, however, that system and component developers



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Aeronautical Technologies for the Twenty-First Century 10 AVIONICS AND CONTROL INTRODUCTION The 1980s saw a significant change in the nature of commercial air transportation and military aircraft operations as a consequence of remarkable growth in application of new avionics. These included widespread implementation of fly-by-wire systems and significant advances in fully electronic displays ("glass cockpits"), digital flight control and flight management, ring laser gyro-based inertial navigation, and full-authority digital engine controls. These innovations provided much-increased functional capability without adverse impact to the weight of aircraft. In fact, despite the proliferation of avionics, they have accounted for approximately 1 percent of the airplane weight for the last 20 years. However, advances in avionics have brought a new set of problems. For example, demand has increased for coordination and standardization in areas as diverse as microwave landing systems, software standards, electromagnetic vulnerability standards, and certification and testing requirements. Other problems include generally inadequate testing and validation to ensure that such systems meet all requirements when they are introduced, and the often massive cost and schedule overruns resulting from problems in software development and validation. The digital systems introduced in the 1980s included box-for-box replacements or additions to existing functions. This created a proliferation of black boxes and consequent challenges to system integration, validation, reliability, and cost. There were also many technology developments occurring in other sectors that failed to find their way into aeronautical applications in a timely manner. For example, fiber optics, which have extensive applications in communications, have not yet seen significant application in aircraft. In short, although avionics and control technologies have produced continuous advances in aircraft systems, there is still significant opportunity for greater efficiency, enhanced functionality, and better integration of systems. This is particularly true for systems that reduce the burden on the crew of flying the aircraft and systems that allow for increased capacity of the global air traffic management (ATM) system. It is important, however, that system and component developers

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Aeronautical Technologies for the Twenty-First Century Recommendations General NASA should play a major role in the development and validation of the key technologies in avionics and control, including system development and integration, simulator and/or experimental flight validation, and serving in a technical advisory capacity for industry and other agencies of the government. Specific NASA should work with the FAA, academia, and industry to produce advances in flight path management; pilot/vehicle interface (i.e., establish a cognitive engineering effort in this area); avionics and controls integration; control function applications; and aircraft power and actuation. include a significant degree of upgrade capability to avoid obsolescence brought on by this rapid pace of technology development. Current research and development in avionics and controls is expected to result in operational solutions to many of the problems anticipated because of air traffic growth. Techniques are being developed for the more effective use of computer automation to manage air traffic, but integration of that technology into the national or worldwide system is a major challenge. A primary hope for meeting the challenge of increasing congestion in the air and on the ground is full-scale implementation of the Global Positioning System (GPS) and differential GPS. Emerging technologies also offer significant enhancement of the pilot's situational awareness, which is fundamental to improved safety and mission effectiveness. Again, implementation of the GPS is key to providing enhancements in this area. Panoramic pictorial presentations of the aircraft flight situation, voice-interactive communications with automated systems, and tactile augmentation for control manipulators are examples of expected advances over the next generation in pilot/vehicle interfaces. These advances in capability will be brought about through advances in integration and fusion of multi spectral sensors; large, flat-panel, color displays; miniature optics and related multiple laser projection arrays; and continued growth in computational/image processing power. However, a major issue is the ability to use this technology in a manner that truly enhances, rather than complicates, a pilot's ability to manage the aircraft and its mission. This problem is addressed in detail in Chapter 11. The remainder of this chapter outlines in list format the key technologies that have been identified by the Committee as vital for maintaining continual advancement in the state of the

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Aeronautical Technologies for the Twenty-First Century Benefits of Research and Technology Development in Avionics and Controls Aircraft Operations Enhanced functionality Engine control Aerodynamic actuator control Greater situational awareness Smaller crew Enhanced safety Reliable automated systems Enhanced communication On-board position determination/collision avoidance On-board flight path management On-board health monitoring Enhanced controllability and maneuverability Aircraft Design and Development Integrated systems Technology validation art in avionics and controls. The roles that the National Aeronautics and Space Administration (NASA) can be expected to play in the development of these key technologies are identified by acronyms in the lists and have been categorized, for brevity, as follows: R Fundamental research SD&I System development and integration V Simulator and/or experimental flight validation TA Technical advisory Fundamental research (R), of course, implies that NASA should be the primary agent for advancing the base state of the art in areas where the technology is not yet mature enough to warrant full-scale development of components or systems. The designation of system development and integration (SD&I), however, indicates that NASA should proceed, in conjunction with industry, universities, the Federal Aviation Administration (FAA), and other appropriate agencies to develop and, where appropriate, implement full-scale systems. The major new issue is system integration. Only when a full-scale system is put together are integration tasks adequately addressed. Similarly, the simulator and/or experimental flight validation (V) designation implies that NASA should be heavily involved, again in conjunction with industry and others, in providing the experimental ground facilities and flight test aircraft to prove the utility of specific technological concepts. Finally, the designation of a technical

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Aeronautical Technologies for the Twenty-First Century advisory (TA) role indicates that although NASA may have significant expertise to offer in the development of a particular technology, it should be limited to a support role. The Committee has identified five categories in which to organize the key technologies. From most general to most specific, they are as follows: Flight path management; Pilot/vehicle interface; Avionics and controls integration; Control function applications; and Aircraft power and actuation. In the sections that follow, these categories and the designators described above are correlated to show how NASA can be expected to contribute to advancing avionics and controls technologies for future aircraft. The boxed material summarizes the primary recommendations that appear throughout the chapter and the benefits that can be gained through research and development aimed at advanced avionics and controls. FLIGHT PATH MANAGEMENT Under the general category of flight path management, the Committee has identified four specific areas of concern: (1) navigation, guidance, and performance/mission management; (2) communications; (3) collision avoidance; and (4) bad weather detection and avoidance. The Committee also considered several unique requirements for rotorcraft. These are discussed in the following sections along with very specific descriptions of the technologies that are needed to address the associated problems. Navigation, Guidance, and Performance/Mission Management Global Cooperative Airspace Management. The greatest problem in global airspace management has to do with technology integration involving an overall system architecture, appropriate algorithms, and air and ground communications technology. True four-dimensional control of aircraft will be needed to enable the necessary large gains in airport and airspace capacity. Figure 10-1 shows the complexity and scope of the integration task; Figure 10-2 summarizes air traffic automation aids being pioneered in NASA research that could realize major benefits by the 2010•2020 era. TECHNOLOGY NEEDED NASA ROLE Integration of individual communication elements in such a way as to ensure optimal operational capability and overall system fault tolerance (e.g., use of GPS and airborne downlink data to aid in air traffic control surveillance and traffic management) V, TA

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Aeronautical Technologies for the Twenty-First Century FIGURE 10-1 Integration of elements needed for cooperative global airspace management. Operational All-Weather Landing and Takeoff Systems. As traffic density increases and use of the GPS becomes a reality, the need for maximizing airport runway availability will also increase. To simplify traffic management, aircraft designers must seek technologies that not only are properly integrated with other system elements but also provide capabilities for the airplane to continue operating in an autonomous fashion when faced with potentially dangerous environmental conditions. Airborne navigational capabilities that integrate elements in such a way as to ensure optimal operational capability and overall system fault tolerance (e.g., airborne downlink data to aid in air traffic control surveillance and traffic management) V, TA (specifically for autoland as well as navigation)

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Aeronautical Technologies for the Twenty-First Century FIGURE 10-2 Taxonomy of ATM automation aids. TECHNOLOGY NEEDED NASA ROLE Precision runway guidance required to ensure that aircraft have the capability for autonomous operation as a backup to ground-based systems SD&I, V, TA Precision runway guidance sensors, integrated with on-board landing guidance system and data base of landing site information, to enable accurate synthetic vision displays SD&I, V, TA Integration of fuel optimization flight path with ATM metering system R, SD&I, V, TA Automatic aircraft flight path monitoring (on-board) versus aircraft configuration for takeoff and landing V, TA

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Aeronautical Technologies for the Twenty-First Century Communications Automated Digital Data and Voice Communications to ATM System. When used in conjunction with digitized high-speed communications technology, satellites offer a solution to many of the current problems encountered in today's flight path management. To realize the potential of these new and complex technologies, more attention to questions of proper integration is required. Aircraft manufacturers are preparing to take full advantage of the new technologies; however, modern aircraft already have operational capabilities that are not, or cannot be, realized fully in today's operating environment. TECHNOLOGY NEEDED NASA ROLE Satellite communications uplink and downlink for interchange of ATM system data V, TA Satellite communications downlink for on-board weather sensor (e.g., radar), video (i.e., wide bandwidth) data to support extended weather data advisory system V, TA Integrated very high frequency (VHF) radio communications and satellite communications with automatic link establishment transparent to crew V, TA Satellite communications and/or data link for transmission of in-flight diagnostics to ground-based maintenance facility TA Integrated antenna and radio frequency signal processing for radio-communications, satellite communications, GPS, distance-measuring equipment, and air traffic control transponder TA System integration V Collision Avoidance Integrated Ground/Air-Based Collision Avoidance Systems with Appropriate On-board Situational Displays. Collision avoidance will become a more critical issue if air traffic density increases as forecast. The technologies to deal with it are available now, but validation and systems integration are major concerns.

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Aeronautical Technologies for the Twenty-First Century TECHNOLOGY NEEDED NASA ROLE Downlink to ATM system of Terminal Collision Avoidance Systems (TCAS)-detected threats and integration of these data with flight path management SD&I, V, TA Independent airborne resolution of traffic alerts, comparing onboard TCAS data with ground-based data link transmissions of potential threats SD&I, V, TA Integrated Airport Ground Traffic Management. A highly publicized, aircraft ground collision in Los Angeles in 1991 illustrates the need for improved situation awareness. Traffic and potential threat status must be available, in real-time, to airborne and ground traffic as well as to the control tower. TECHNOLOGY NEEDED NASA ROLE Integration of airport ground traffic control data by using surveillance radar and aircraft runway steering sensors V, TA Integration of onboard differential GPS/inertial position/velocity reports for ground traffic surveillance data and for automatic threat resolution SD&I, V, TA Bad Weather Detection and Avoidance Airborne and Ground-Based, Real-Time, Weather Threat Displays and Alerting Systems. The promise of microwave landing systems, coupled with the forecast traffic increase, underlines the need for accurate detection and charting of weather threats to avoid excessive flight delays. TECHNOLOGY NEEDED NASA ROLE Optical and Doppler radar sensors for clear-air turbulence detection R, SD&I, V Automatic downlink of severe weather and turbulence-detection graphical data, preprocessed onboard with detailed position coordinates SD&I, V Improved optical and radar detection of windshear and heavy rain, with quantitative threat estimation capability on-board R, SD&I, V

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Aeronautical Technologies for the Twenty-First Century Integrated windshear detection and avoidance. R, SD&I, V Unique Rotorcraft Requirements Automated Nap-of-the-Earth Guidance for Rotorcraft. A leading cause of civil rotorcraft accidents is collisions with uncharted obstacles such as power lines. This is a major contributor to the fact that helicopters on emergency missions have high accident rates. The hazard potential of obstacles in nap-of-the-earth navigation increases significantly in low visibility and adverse weather conditions. Solutions lie in improved sensor technology and appropriate coupling of this information to aircraft flight controls and head-up displays. TECHNOLOGY NEEDED NASA ROLE High-resolution obstacle detection sensors, including wire detection R, SD&I, V Obstacle avoidance guidance R, SD&I, V PILOT/VEHICLE INTERFACE Fundamental to increased safety in the commercial and military airspace of 2020 will be optimization of the pilot's situational awareness and spatial orientation. The Committee has identified simulation, cockpit display and control technologies, and synthetic vision/virtual reality as key to providing this capability. A truly integrated cockpit with intelligent automation is evolving, but significant steps must still be taken and many emerging technologies must be considered and exploited properly. Simulation Simulation has become recognized as an increasingly economic, effective, and safe means to design and validate systems. All simulations require validation in order to predict performance. TECHNOLOGY NEEDED NASA ROLE Development of techniques and specifications to accelerate simulator validation R, SD&I, V Cockpit Display Technology Spatial orientation is enhanced through improvement in the display media used in the visual presentation of aircraft attitude and motion data. The traditional visual interpretations of spatial orientation are reinforced through the use of other human senses. Virtual auditory and

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Aeronautical Technologies for the Twenty-First Century display systems will allow an ''open cockpit'' awareness of aircraft attitude; rates; normal/abnormal aircraft system operation; and relative orientation of other aircraft, the ground, and weather. There will be less reliance on voice communications in the ATM system. TECHNOLOGY NEEDED NASA ROLE Wide field-of-view optics allowing single-panel panoramic instrument panels and synthetic vision windows SD&I, V Improved clarity of field of view of head-up display symbology through color, contrast, perspective, and enhanced effective optical focus at infinity, as well as use of the windscreen as the combining glass SD&I, V Helmet-mounted display hardware improvement allowing light weight, and full field of view SD&I, V Eye and head tracking technology SD&I, V Direct writing on the retina R, SD&I, V Virtual auditory systems that provide sound orientation to the airplane and the external environment SD&I,V Enhanced voice synthesis techniques with advances in computational rates and clarity R, SD&I, V Displays for nonaudio ATM system communications R, SD&I, V Techniques for enhancing display resolution and development of new display media R, SD&I, V Cockpit Control Technology The increased variety of methods for pilot control of aircraft cockpit functions will complement the development of display technology and will be made necessary by the accelerating complexity of the civilian and military environment. Voice Control. Enhancements will allow the pilot to command and query the aircraft through structured sentences. Voice control will allow the pilot to transfer control of the aircraft to automated systems during incapacitating emergencies.

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Aeronautical Technologies for the Twenty-First Century TECHNOLOGY NEEDED NASA ROLE Improved algorithms for voice recognition and parsing of words and syntax R, SD&I, V Compensating techniques for variations in human speech (e.g., pilot/copilot) and for individual variations due to factors such as stress SD&I, V Hand Gesturing. Control will be necessary for "virtual reality" systems, in which cockpit hardware is replaced by a visual representation. Motion of forearms, legs, will be used in military aircraft to supplement existing hand motion control. TECHNOLOGY NEEDED NASA ROLE Development of reliable mechanisms for tracking body motions and flexure SD&I, V Design of Fiber-Optic Control Sticks, Transducers, and Switches. Fiber optics complement all optical aircraft. TECHNOLOGY NEEDED NASA ROLE Optical force transducers, toggles, and buttons requiring no electrical-to-optical conversion SD&I, V Unique Synthetic Vision/Virtual Reality Considerations Synthetic vision replaces, or augments, the cockpit windows by superimposing sensor data (television, infrared, microwave) on the normal visual scene. Virtual reality extends the synthetic vision concept further by synthesizing the entire cockpit and aircraft external environment through the combination of sensor data, previously stored data, and real-time data received through aircraft data links. Virtual reality technology means that the pilot's point of view need not be tied to the pilot's eye location. Replacing Cockpit Transparencies. Aircraft sensor data will require enhanced capabilities from those sensors. Sensor suites not only will create a visual telepresence but will provide weather detection, clear-air turbulence detection, obstacle avoidance, wake vortex avoidance, and reduced vestibular and visual illusions due to cloud decks, window reflections, and ground lights. Head motion will be minimized by the fusion of all sensor data into one head-up or helmet-mounted display. Infrared and remote television sensors will allow the crew

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Aeronautical Technologies for the Twenty-First Century to monitor the aircraft structure, wheel trucks, and proximity of the gear to taxiway edges and ground obstructions. The level of on-board database-enhanced detail will be controllable by the crew, for example, high cultural detail for landing and ground operations. Details critical to safety (traffic, ground obstacles during low flight) would be basic at all times. TECHNOLOGY NEEDED NASA ROLE Increased sensor resolution and sensitivity R, SD&I, V Development of sensor systems and combinations of sensors less prone to atmospheric or other interference SD&I, V Investigation of sensor placement on the aircraft structure; consideration of pilot's field of view, collocation of sensors, effects due to aircraft structure and surrounding air, conformal antenna placement, and electronic scanning SD&I, V AVIONICS AND CONTROLS INTEGRATION To enable many of the benefits promised by the introduction of digital electronics into avionics, advances are needed in hardware and software design techniques and performance evaluation methods. Introduction of new devices and components can improve performance and reduce cost; parallel processing can provide the computing power to take advantage of new capabilities; software design techniques can reduce software errors and improve performance; proper use of fault tolerance techniques can improve reliability and safety; and verification and validation methods can ensure that system designs meet their performance and reliability requirements and also improve the certification process. Hardware Photonics. Photonics technology is needed to enable optically-based systems that will simplify testing and certification against high-intensity radiated fields and reduce the weight needed to shield electrical systems. TECHNOLOGY NEEDED NASA ROLE Network configuration evaluation tools; testing methods SD&I, V Communication protocols; optical sensors; optical signal conditioning SD&I, V

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Aeronautical Technologies for the Twenty-First Century Parallel Processing. Although anticipated commercial avionics applications can be accommodated with a state-of-the-art single processor system, the fault tolerance required by flight-crucial systems adds substantial overhead that significantly reduces the effective throughput of avionics computers. Parallel processing is a promising technique to provide the necessary computing power to accommodate fault tolerance overhead. TECHNOLOGY NEEDED NASA ROLE Network topologies; synchronization techniques R, SD&I Passive Cooling. The use of smart sensors/actuators increases the presence of electronic components in locations where active cooling is inappropriate or infeasible. TECHNOLOGY NEEDED NASA ROLE Integrated electronic design/thermal management tools; high-temperature electronics SD&I, V Devices and Components. Reduction in failure rate and increased reliability are needed. TECHNOLOGY NEEDED NASA ROLE Integrated failure rate estimation tools; high-temperature electronics; thermal environment analysis tools R, SD&I, V Software Computer-Aided Software Engineering. Design and analysis tools that include requirements, design, code generation, documentation, analysis, test, configuration management, and operational support are needed to improve reliability in software development and ensure software integrity. TECHNOLOGY NEEDED NASA ROLE Integrated design, analysis, and reuse tools R Reuse Technology. Software reuse can have a dramatic impact on development, testing, and reliability. A consistent approach to software reuse is needed to reduce cost, improve quality, and reduce development time.

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Aeronautical Technologies for the Twenty-First Century TECHNOLOGY NEEDED NASA ROLE Cataloging, retrieval, and certification methods R Parallel Processing. Effective utilization of parallel machines demands major advances in recognition of parallelism in algorithms, partitioning schemes, compiler technology, and operating systems. TECHNOLOGY NEEDED NASA ROLE Distributed operating systems; partitioning techniques R Expert Systems. Diagnostic, health, and status monitoring systems are required to reduce maintenance costs. TECHNOLOGY NEEDED NASA ROLE Wider application of expert systems organized-domain knowledge development of inference engines SD&I Data Compression. The ability to handle large amounts of data with reasonable memory and interface systems is required. TECHNOLOGY NEEDED NASA ROLE Data compression algorithms, including "wavelet" technology engines R, SD&I Neural Networks. Pattern recognition of faults and faulty manufacturing actions in real-time may be possible due to extremely high-speed computation and learning of neural networks. TECHNOLOGY NEEDED NASA ROLE Theoretical basis; validation techniques R Functionality System and Software Reliability. Design methods and software reliability techniques and analysis tools are needed to support design for testability and validation.

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Aeronautical Technologies for the Twenty-First Century TECHNOLOGY NEEDED NASA ROLE Modeling techniques; software instrumentation; performance/reliability trade-off techniques; and techniques to estimate and increase mean time between failures R Architecture Fault Tolerance. Schemes are needed for making trade-off analyses of different topologies for optimizing weight, power consumption, performance, maintenance costs, and reliability. TECHNOLOGY NEEDED NASA ROLE Techniques for managing redundant computing resources; definition of fault classes R Verification & Validation Formal Methods. Techniques are needed that use mathematical logic to demonstrate the consistency between specifications and implementation. TECHNOLOGY NEEDED NASA ROLE User-friendly theorem provers; formal specification languages; mathematical verification methods R, SD&I Integrated Tool Set. Design and assessment tools must be integrated to provide improved productivity in development of systems. TECHNOLOGY NEEDED NASA ROLE User-friendly interfaces; interface parameter definition R, SD&I CONTROL FUNCTIONAL APPLICATION To increase the functional capability of the mechanisms by which aircraft flight is controlled, the Committee has identified controllability and maneuverability, load alleviation and ride control, engine control, aerodynamic flow control, and noise reduction as areas in which NASA must play a significant role. The following sections describe in detail how NASA research, development, and validation can play a part in bringing specific key technologies to fruition.

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Aeronautical Technologies for the Twenty-First Century Controllability and Maneuverability Relaxed Static Stability. Relaxed static stability or static instability (in tandem with center-of-gravity control) allows maneuverability improvements and trim drag reductions. Stability is provided by the flight control system. Fuel consumption improvements on the order of 5 percent are expected for conventional subsonic transports. The additional flexibility in center-of-gravity location and even greater fuel burn reduction are particularly important to tailless flying wing designs, allowing the use of more wing volume. Relaxed static stability will also significantly enhance subsonic performance of supersonic aircraft, which exhibit different inherent pitch stability characteristics in subsonic and supersonic flight. The major issues to be resolved are the provision of these functions at the needed levels of reliability. TECHNOLOGY NEEDED NASA ROLE Adaptive fault detection, isolation, and reconfiguration techniques, and architectures to accommodate sensor, actuator, structure, surface, and processor failures or damage R, SD&I Integrated Controls. Integration of flight and propulsion control systems enhances the optimization of steady-state and transient performance. Integrated control may be used to reduce fuel burn and extend structural life, reduce pilot work load, and improve accident avoidance capability by "closing the loop" around aircraft performance with coordinated control inputs; it can also enhance safety and reliability through reconfiguration following damage or failures. This is important in advanced subsonic aircraft, especially rotorcraft and tiltrotors. Emphasis needs to be placed on practical methods, because the gap between theoretical approaches and application has been too large in the past. TECHNOLOGY NEEDED NASA ROLE Robust control design methods for multi-input/output with broad tolerance of system uncertainty R, SD&I Adaptive fault detection, isolation, and reconfiguration techniques and architectures to accommodate sensor, actuator, structure, surface, and processor failures or damage R, SD&I Adaptive control design methods for real-time application R, SD&I, V Real-time multivariable system optimization techniques R Control law partitioning methods for decentralized architectures R

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Aeronautical Technologies for the Twenty-First Century Integration of lift/flow, maneuvering, and stability control with load alleviation through adaptive filtering and very wide bandpass actuation R Load Alleviation and Ride Control Active Flight Controls. Alleviation of loads and rigid body and structural mode excitations (resulting from turbulence, gusts, maneuvers, buffeting, and flutter) with active flight controls allows the use of lighter structures and higher aspect ratios or more highly swept wings. Control surfaces are deflected to reduce aircraft response to atmospheric disturbances, redistribute lift to reduce critical structural loading, or damp wing and body structural modes. Aerodynamic flow control (described subsequently) is another method of achieving the desired distribution control forces and moments. Improved handling qualities, extended fatigue life, and improved ride quality and secondary loading are direct benefits. Applications to both advanced subsonic aircraft and rotorcraft must be addressed. TECHNOLOGY NEEDED NASA ROLE Improved nonlinear computational fluid dynamics models of unsteady aerodynamic forces and aeroelastic interactions R Intelligent structures providing local sensing of load, acceleration, and damage conditions, and distributed actuation for aerodynamic performance optimization and load alleviation R, SD&I, V Engine Control Active Inlet Distortion Control. Active control of individual inlet guide vanes, based on measurement of local pressure distribution, could dynamically adjust compressor distortion tolerance. A design stall margin of 10–20 percent is possible. TECHNOLOGY NEEDED NASA ROLE High-frequency sensors and actuators SD&I Control laws R Active Combustion Control. Low nitrogen oxide (NOx) burners required by High-Speed Civil Transport (HSCT) could exhibit combustion instabilities in the form of blowout. Active control techniques that sense the presence of burning via noise measurement might allow achievement of low emissions via high-frequency fuel flow modulation. Similar techniques may be used to eliminate afterburner screech and rumble.

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Aeronautical Technologies for the Twenty-First Century TECHNOLOGY NEEDED NASA ROLE High-frequency, reliable fuel-metering actuators SD&I High-frequency, dynamic pressure sensors capable of measuring burner pressure fluctuations SD&I Control laws R Active Compressor Surge and Rotating Stall Control. Active control can be used to reduce or eliminate the design constraints and performance penalties imposed by compressor surge and stall. Some of the benefits that might be expected from active compressor stabilization are the potential for shorter, lighter engines; increased fuel efficiency; and simple inlets. TECHNOLOGY NEEDED NASA ROLE High-frequency sensors and actuators with associated signal processing SD&I Rotating stall detection and suppression with associated signal processing R Active Turbine Clearance Control. Active control of turbine blade tip clearance based on real-time measurements of blade clearances, rather than open-loop scheduling would allow engine running with tightest practical clearances in the compressor and turbine, achieving the maximum available efficiency. TECHNOLOGY NEEDED NASA ROLE High-temperature, reliable clearance control measurement SD&I Control laws that would preclude rub R Magnetic Bearings. Magnetic suspension bearings, in which the rolling contact is replaced by magnetic suspension of the rotating assembly, could use feedback control to maintain positional stability. In fact, this feedback can be used to control shaft dynamics. This would reduce engine maintenance, because vibrational coupling to the airframe often dictates engine removal before the engine itself demands it. Magnetic bearings might also permit removal of the oil systems—a tremendous practical advance.

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Aeronautical Technologies for the Twenty-First Century TECHNOLOGY NEEDED NASA ROLE High-power electronics and magnetics in the hostile engine environment SD&I Control algorithms for center-of-mass rotation, gyroscopic maneuver, and gust loads that preclude bearing rubbing R Aerodynamic Flow Control Laminar Flow Control. Boundary-layer control on wing, nacelle, and tail surfaces can be used to maintain laminar flow across the wing, drastically reducing skin friction. Laminar flow is maintained by sucking air out of the boundary layer through slots or perforations in the wing surface. Total aircraft drag reductions on the order of 20 percent are possible with extensive use of laminar flow control. The application may differ between cruise and climb/descent operations. TECHNOLOGY NEEDED NASA ROLE Intelligent structures capable of detecting laminar/turbulent transition location, and of detecting and canceling incoming boundary-layer disturbances R, SD&I Manufacturable/maintainable suction system, including surface perforations and distributed suction SD&I, V Variable Wing Camber. A variable wing camber allows in-flight adaptation of wing geometry to local ambient conditions and maneuvering requirements. Chordwise pressure profile control via variable camber allows wing performance (lift/drag) optimization, whereas spanwise lift distribution control can be used to reduce wing bending stresses during maneuvering. TECHNOLOGY NEEDED NASA ROLE Continuous variable camber wing SD&I, V Intelligent wing structures sensing local load, pressure, and acceleration R, SD&I Noise Reduction Active Noise Control. Reduction of noise—both near-field (cabin) and far-field (community) noise—is possible through the use of active noise control. Antinoise concepts

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Aeronautical Technologies for the Twenty-First Century adjust the output of single or distributed secondary acoustic or vibration sources (based on input from direct measurements of the acoustic field or the use of coherent measurements) to achieve destructive interference and noise attenuation. This technique is particularly useful at low frequencies where passive acoustic treatments are generally ineffective. Rotorcraft far-field noise, as well as local vibration levels, can also be reduced by active control of rotor pitch. Benefits of active noise and vibration control include reduced weight of passive airframe and engine acoustic treatments, and improved engine performance due to relaxation of design constraints to reduce noise levels. Both acoustic and structural actuation methods need to be considered. TECHNOLOGY NEEDED NASA ROLE Durable, high-power, compact, efficient acoustic sources and drivers R, SD&I Fast, adaptive control algorithms for broadband and random noise cancellation with an uncertain plant and complex noise field R Improved numerical and analytical models characterizing noise propagation and structural/acoustic coupling for control, detection, and secondary source design R Control techniques for minimizing radiated three-dimensional noise fields R Individual rotor blade control systems to achieve higher harmonic control for vibration and noise reduction R, SD&I, V AIRCRAFT POWER AND ACTUATION The Committee believes that development of new and more efficient techniques for generating, storing, and distributing power would find great application in future aircraft systems, as well as in other areas such as automobiles and spacecraft. Thus, NASA is encouraged to aggressively pursue the elimination of complex secondary power systems on aircraft and to develop and validate simpler, more reliable actuation systems. Power Sources New Secondary Power Supplies. Power for the hydraulic, electrical, and air systems currently comes from the main engine. These systems extract horsepower and penalize engine fuel consumption on the order of 3 percent directly, while adding complexity to engine systems with attendant penalties in fuel efficiency. This added complexity occurs in a hostile

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Aeronautical Technologies for the Twenty-First Century environment and degrades the reliability of the aircraft's essential power source for flight. New secondary power sources can reduce the shaft horsepower extraction from the engines and result in a 1 percent savings in specific fuel consumption. TECHNOLOGY NEEDED NASA ROLE Develop battery and fuel cell technologies for reliable, airborne applications R, SD&I, V Develop cogeneration concepts, such as turbines, driven by cabin exhaust air or wingtip vortices R, SD&I, V Application of high-temperature superconductivity as appropriate SD&I, V Power-by-light concepts to take advantage of fiber optics R, SD&I, V Actuation New Actuators. The current control surface actuation is accomplished by hydraulic actuators fed by a series of hydraulic lines coming from fluid reservoirs and powered by hydraulic pumps that are driven by the main engines. On a typical twin engine this transport system weighs approximately 2,500 pounds. These systems are also a leading cause of commercial aircraft dispatch delay because of their mechanical complexity and consequent low reliability. TECHNOLOGY NEEDED NASA ROLE Electrohydrostatic actuator—a local hydraulic system that would reduce cost and improve maintainability R, SD&I, V Electromechanical actuator—an electric motor with reduction gears that would improve reliability R, SD&I, V Micromachine applications SD&I, V Supersonic Aircraft Actuators. Supersonic aircraft have very limited space for actuators. Higher-pressure systems in military use are available, but other concepts are needed to avoid the surface fairing that would encompass large actuators.

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Aeronautical Technologies for the Twenty-First Century TECHNOLOGY NEEDED NASA ROLE Develop hinge line actuation concepts TA