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Uninhabited Air Vehicles: Enabling Science for Military Systems Executive Summary U.S. Air Force (USAF) planners have envisioned that uninhabited air vehicles (UAVs), working in concert with inhabited vehicles, will become an integral part of the future force structure. Current plans are based on the premise that UAVs have the potential to augment, or even replace, inhabited aircraft in a variety of missions. However, UAV technologies must be better understood before they will be accepted as an alternative to inhabited aircraft on the battlefield. The U.S. Air Force Office of Scientific Research (AFOSR) requested that the National Research Council, through the National Materials Advisory Board and the Aeronautics and Space Engineering Board, identify long-term research opportunities for supporting the development of technologies for UAVs. The objectives of the study were to identify technological developments that would improve the performance and reliability of “generation-after-next” UAVs at lower cost and to recommend areas of fundamental research in materials, structures, and aeronautical technologies. The study focused on innovations in technology that would “leapfrog” current technology development and would be ready for scaling-up in the post-2010 time frame (i.e., ready for use on aircraft by 2025). To date, UAVs have been considered advanced-concept technology demonstrations, with an emphasis on mission payloads. Therefore, the design of the systems has been outside of the U.S. Department of Defense’s procurement process for weapon systems, which has enabled developers to aggressively use available advanced technologies. Although this approach has been effective for meeting near-term goals, it will provide only limited opportunities for fundamental technology development because it favors the adaptation of available technologies. The committee recommends that the USAF establish a research and development program to develop technologies that will advance the use of UAVs either
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Uninhabited Air Vehicles: Enabling Science for Military Systems by enabling unique missions or by providing significant cost savings. The following steps are recommended for establishing a research program for UAV technologies: the establishment of requirements for a range of missions and system attributes, with a focus on key air vehicle concepts the identification of technologies that could meet requirements the development of technology forecasts and trends for relevant technology areas the initiation of research that could provide the necessary technologies Both fundamental research and technology development will be required to improve available technologies and develop military UAVs with significantly lower system development costs. Because of the wide variety of possible configurations and missions, the committee used “notional vehicle types” to identify technical areas of need. Three notional vehicle types were identified as indicative of the range of technologies that would improve the USAF’s capability of designing, producing, and fielding generation-after-next UAVs. The notional vehicle types represent classes of vehicles, not conceptual aircraft designs suited to any particular mission. The three vehicle types were: high-altitude, long-endurance (HALE) vehicles, to provide a focus on long-term technical advances for reconnaissance and surveillance aircraft high-speed, maneuverable (HSM) vehicles, to emphasize the potential for a highly survivable, second-generation combat UAV very low-cost vehicles, to highlight performance-cost trade-offs Based on analyses of the notional vehicle types, the committee identified technical needs and opportunities in research and development for major UAV subsystem technologies. The committee considered the following five technology areas: aerodynamics (and vehicle configuration); airframes (especially materials and structures); propulsion systems; power and related technologies; and controls. VEHICLE DESIGN ISSUES Two issues related to system design—(1) human-machine science and (2) manufacturing and design processes—will strongly influence the design of future UAVs. Both issues should be considered in the selection and prioritization of research opportunities. Human-machine science includes (1) integration of human-machine systems (e.g., allocation of functions and tasks and the determination of the effects of automation on situational awareness), (2) human performance (e.g., human decision-making processes and methods for defining and
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Uninhabited Air Vehicles: Enabling Science for Military Systems applying human performance measures in system design), and (3) information technologies (e.g., effects of human factors on requirements for information content and display). Manufacturing and design processes include (1) designing for low-cost fabrication (e.g., reducing vehicle size and modular design and construction) and (2) low-cost product realization (e.g., new approaches to product design, low-cost manufacturing processes, and consideration of cost as an independent variable). GENERAL RESEARCH OPPORTUNITIES The committee identified opportunities for research on crosscutting vehicle subsystem technologies that could benefit all types of UAVs. The committee recommends that the USAF long-term research program focus on four areas: (1) computational modeling and simulation; (2) propulsion technologies for small engines; (3) integrated sensing, actuation, and control devices; and (4) controls and mission management technology. Computational Modeling and Simulation The low cost and short design cycles that will be necessary for UAVs will require changes in design practice, especially an increased reliance on computational modeling, simulation, verification, testing, and training. The committee recommends that the following research opportunities in this area be pursued: development, validation, and application of computational tools for major subsystem design, including unsteady, nonlinear, three-dimensional aerodynamics models; structural analysis and aeroelasticity models; aerodynamic modeling concepts for designing vehicle control systems; propulsion system models; and simulation models for assessing control laws validation of manufacturing process models for UAV components clarification of the role of uncertainty in computational analysis integration of models and simulations to provide a “virtual mockup” for testing and evaluation of the total system Propulsion Technologies for Small Engines In the past, development costs have been a major factor in the development of UAV propulsion technologies. To meet program budget constraints, the practice has been to adapt existing devices, usually at the expense of both performance and reliability. To address this concern, the committee recommends that research be focused on technologies that could enable the development of small, low-cost turbine engines. The following topics should be considered:
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Uninhabited Air Vehicles: Enabling Science for Military Systems low-cost, high-temperature materials and coatings cooling schemes to reduce the need for costly air-cooled parts technology and approaches to reduce leakage through clearances between stationary and rotating parts bearing and lubrication systems that would be more reliable after long-term storage small, low-cost accessories (e.g., fuel pumps, engine controls, and electrical generators) Integrated Sensing, Actuation, and Control Devices Minimizing the weight and volume of sensors, actuators, and other subsystems will be critical for UAVs, which will have stringent size and payload limitations. Emerging microelectromechanical system (MEMS) technology can provide transducers as small as tens of microns. Potential MEMS-based sensors include inertial sensors, aerodynamic sensors, structural sensors, and surveillance sensors. Innovative uses for MEMS-based transducers include: structures that respond to load variations, controls of aerodynamic flow, and improvements in situational awareness (e.g., collision avoidance and detection of biological and chemical agents). Controls and Mission-Management Technology The optimal utility and effectiveness of UAVs will require exploiting the capabilities, and recognizing the limitations, of controls and mission-management technologies. The committee envisions that UAVs will operate in integrated scenarios with the following features: several vehicles with specified missions; communication links among vehicles and between vehicles and remote human-operated control sites; and the capability to use sensors and information-processing systems located on the vehicle, on other vehicles, and at ground sites. Important areas for research in controls for UAVs include: rapid (automated) design and implementation of high-performance control laws, robust vehicle management functions (e.g., to carry out mission sequence), and mission-management technologies, including real-time path planning and control of dynamic networks. RESEARCH OPPORTUNITIES FOR SPECIFIC VEHICLE TYPES In addition to the general research just described, the committee identified research opportunities that would support the development of each notional vehicle type. As the long-range plans and priorities for UAVs emerge, the USAF should include the applicable opportunities in its long-range research program. Key subsystem technologies that will enable the development of HALE UAVs are listed below:
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Uninhabited Air Vehicles: Enabling Science for Military Systems vortex drag reduction (e.g., lifting systems and tip turbines) laminar-to-turbulent transition for low Reynolds numbers aeroelastic controls high-compression operation of gas turbines or piston engines alternative propulsion systems (e.g., fuel cells, solar cells, and energy storage systems) materials and designs for aeroelastic tailoring low-rate manufacturing technologies for ultra-lightweight airframe structures The following key subsystem technologies will enable the development of HSM UAVs: nonlinear, unsteady aerodynamics simulation of flow fields for complex configurations modeling tools for propulsion-airframe integration stiff, lightweight structures for highly-loaded propulsion systems fluid seals high-load, long-life bearings probabilistic structural design methods for a high-speed, high-g environment automated manufacturing processes for high-performance structural materials high-temperature composite materials1 Finally, the following key subsystem technologies will enable the development of very low-cost UAVs: very low Reynolds number aerodynamics bearings for long-term storage low-cost accessories for propulsion systems (e.g., fuel pumps, engine controls, and electrical generators) structural design criteria for expendable, low-use systems expanded suite of structural materials (including low-cost, commoditygrade materials) modular designs for low-cost manufacture 1 Some important research and development programs in composite materials and structures, such as the National Aeronautics and Space Administration’s High Speed Research Program, have recently been discontinued.
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Representative terms from entire chapter: