Nano and Micro Aerial Vehicles

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Nano Aerial Vehicles and Micro Aerial Vehicles are primarily used for military applications which include Intelligence, Surveillance and Reconnaissance (ISR missions), and used for both indoor and outdoor missions, which represents a challenging environment factor to overcome. The use of MAVs and NAVs reduce human risk as it does not require an on-board pilot for the mission, they also tend to improve the assessment of danger and have visibility in the dark when equipped with infrared cameras. Other payload equipment include numerous tactical and strategic advantages integrated with the use sensors for high risk applications to locate and analyse biological and chemical gases as well as nuclear radiation and other threats as stated by Petricca, Ohlckers and Grinde (2011). Historically, the development of unmanned aerial vehicles is credited to advances of model airplanes throughout the 19th and 20th century; Mueller (2011) states that the 19th century brought about the development of model sized airplanes which lead to the 20th century integrating its technological advancements in radio receivers and propulsion systems to develop radio controlled airplanes. Moreover, it should be noted that propulsion systems were enriched drastically from simple rubber band propellers to liquid fuel internal combustion engines and electrical motors powered by batteries and fuel cells. However, in order to comprehend the various types propulsion systems employed on board the Unmanned Aerial Vehicles (UAVs), certain parameters and configurations of Nano and Micro Air Vehicles must be explored along with its aerodynamic constraints. Firstly, NAVs and MAVs are subclasses of UAVs; it is critical to note that NAVs are limited to being less than or equal to 7.5 cm in length, width or height, while maintaining a weight which is less than 10 grams as defined by Defence Advanced Research Projects Agency (DARPA) in 1997 . Whereas, MAVs are limited to having a maximum wingspan length of 15cm while maintaining a weight which is less than 20 grams as outline by DARPA. Figure 1: Reynolds Number for Air Vehicles sourced from T. J. Mueller, “Aerodynamic measurements at low raynolds numbers for fixed wing micro-air vehicles,” Tech. Rep., University of Notre Dame, Notre Dame, The Netherlands, 2000. With the current technology and understanding of aerodynamic factors for these miniaturized air vehicles, the configuration of the Nano and Micro Aerial Vehicles tend to determine or rather limit the range of applications they are used for, there are four main configurations; fixed wings, rotary wings, flapping wings and passive. However, the latter configuration will be excluded from this report as it does not have a propulsion system on-board to analyse, but rather uses a crude method of being dropped or hand launched. Figure 1, on the right depicts one of the major aerodynamic challenges with the use of NAVs and MAVs (Mueller 2000). It can be seen that for a large commercial aircraft such as the 747 the Reynolds number is around 2,000,000,000 whereas the NAVs and MAVs have low Reynolds numbers; In cases where there is a low Reynolds number (typically less than 100,000), it is proven that the aerodynamic efficiency (L/D ratio) decrease at a more rapid rate. Other challenges include system integration of the airframe, communication, processor, sensors and power for operational function-ability with in the specific weight limits to keep consumption of power to a minimum. The fixed wing configuration is designed for high speed flight where hovering and flying at low speeds are unnecessary; it is essential for the fixed wing configuration to have a thrust to weight ratio of less than 1, as it requires less power to fly since the configuration of the wings impart an additional lift component. Since the fixed wing configuration is limited to high speed flight, it is typically used for outdoor applications. It should be noted that there are two subclasses; rigid...
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