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12-10-2010, 12:56 PM

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Micro Air Vehicles (MAVs) offer the promise of affordably expendable, covert sensor platforms for a range of close-in situational awareness activities. Since 1996, the US Naval Research Laboratory (NRL) has been developing technologies that will enable Navy-relevant missions with the smallest practical MAVs. This effort includes the development and integration of sensors, avionics, and advanced intelligent autopilots for flight control, with aerodynamic technologies. The NRL Micro Tactical Expendable (MITE) air vehicle is a result of this research. The operational MITE is a hand-launched, dual-propeller, fixed-wing air vehicle, with a 9-inch chord and a wingspan of 8 to 18 inches, depending on payload weight. The 14-inch MITE 2 can carry a oneounce analog camera payload for mission flight durations in excess of 20 minutes, at air speeds of 10-20 miles/hour. While the MITE is presently a remote controlled air vehicle, both miniature .traditional. autopilots and also .advanced. autopilots, based on visual and spectral imaging techniques, are being developed. Autonomous MITEs will provide inexpensive, covert, highly portable sensor platforms for distribution and use in remote or urban environments. Multiple MITEs will provide distributed networks of roving and fixed sensor systems.
The Office of Naval Research has sponsored the Naval Research Laboratory (NRL) to conduct a Micro Air Vehicle (MAV) exploratory development research program. The goal of this MAV research is to develop and demonstrate MAV technology that supports Navy-specific applications and is complimentary and supplementary to the Defense Advanced Research Projects Administration (DARPA) MAV Program. Additional MAV requirements beyond the DARPA effort include Navy-specific applications, electric propulsion, non-Global Positioning System (GPS) navigation, and airframe size commensurate with operating conditions. NRL developed a baseline design called the Micro Tactical Expendable (MITE) which consists of a low aspect ratio flying wing with dual, counterrotating propellers mounted at the wing tips. This configuration was selected based on a tradeoff analysis that included aerodynamic performance, payload interface, launch and recovery methods, and compact storage. Specifically, the low wing aspect ratio is necessary to provide sufficient wing area within a compact wingspan. Although the low aspect ratio increases the induced drag of the wing, the increase in wing chord raises the Reynolds number, which improves the boundary layer characteristics, hence performance, of the airfoil. The dual propellers provide slipstream flow over nearly the entire wing for enhanced lift at low speeds. By counter rotating in the direction opposite of the wingtip vorticies, the Zimmerman Effect is produced, which reduces the induced drag to a value noticeably lower than expected for the low aspect ratio of the wing. Another benefit of counterrotation is the balancing of torque and slipstream rotation effects, allowing high lateral stability at low speeds to enable easy hand launching of the MITE. As a flying wing, MITE uses elevon controls, which have enhanced effectiveness since they are also immersed in the slipstream. Payload location is in an unobstructed nose, which is ideal for imaging and accommodating a range of sensors. Despite being a flying wing with no geometric dihedral, the overall combination of design features results in a simple configuration possessing inherent stability about all three axes, good performance, ease of hand launch, and simple landing by gliding to the ground. Wing Aspect Ratio vs. Reynolds Number for Minimum Drag Fifty percent of the total drag for a welldesigned airplane, cruising at its best speed for range, is induced drag. When flying at best efficiency for endurance, the induced drag component is an even greater percentage of the total drag. The profile drag of the airfoil shape is a relatively small component of the overall airplane.s drag. Consequently, while airfoil designers strive to minimize profile drag, airplane designers must minimize induced drag to maximize range and endurance. The induced drag of a wing is a result of a spanwise component of the airflow near the wingtips caused by the finite span of a wing. Induced drag is a function of the square of the lift coefficient and inversely proportional to the wing aspect ratio. Since the optimal cruise lift coefficient is a function of the airfoil, reducing induced drag requires increasing the aspect ratio, i.e. increasing the wing span-to-wing chord ratio. For a given wing loading, increasing the wing aspect ratio results in a reduction of the wing chord. Typically, the wing aspect ratio is as large as structural considerations allow, since reducing the chord reduces the wing thickness in proportion, while at the same time increasing the wing root bending moment since the wingspan is increased.

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