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Types of propulsion systems

The UAV’s propulsion system provides the necessary power to propel the aircraft for forward flight or hover. The main propulsion system types are gas systems and electric systems. Both systems can be used to drive propellers or ducted fans. Each system has its own advantages and disadvantages and is best suited for a specific UAV application.

Gas propulsion systems store energy in the form of fuel, and use an engine to drive the propellers or other propulsion devices. Various types of engines exist for UAVs, including two-stroke or four-stroke internal combustion engines running on nitro or gasoline fuels. In general, fuel systems are more difficult to setup, use and maintain than electric systems. They are also noisier, less reliable, and cannot be controlled as precisely as electric systems. However, they can provide significantly longer flight times, and can be quickly refueled. For these reasons, fuel power systems are often used in airplanes and helicopter UAVs that require longer flight times. In case of autonomous UAS, fuel propulsion system is not desirable as automatic refueling would present big challenge for implementation.

Electric propulsion systems store energy using a battery, and use electric motors to drive the propellers or other propulsion devices. Electric systems are easier to operate and produce less noise than gas systems. Electric systems are more reliable, minimizing the possibility of crash due to motor shutdown or failure. The main drawback of electric systems is their effect on total UAS flight time. Although electric systems are more efficient than fuel systems, batteries can hold less energy per weight compared to fuel, and thus electric systems provide significantly less flight time. Also, batteries require significant recharging time when depleted, while fuel tanks can be refueled very quickly.

Another new alternative propulsion option for UAS is fuel cells. Fuel cell is an electrochemical cell that converts the chemical energy from a fuel into electricity through an electrochemical reaction of hydrogen fuel with oxygen or another oxidizing agent. Fuel cells are different from batteries in requiring a continuous source of fuel and oxygen (usually from air) to sustain the chemical reaction, whereas in a battery the chemical energy comes from chemicals already present in the battery. There are few advantages of using fuel cell over battery. For instance while batteries need long time to recharge, fuel cell could be refueled in couple of minutes. Fuel cells can also provide far more power and could therefore potentially extend UAV’s flight times. However this is relatively new technology used in UAS and it is still in development, meaning it still needs many improvements before its implementations in commercial use. As it is fast growing technology, in the future it could potentially present new solution for increasing UAV flight performance.

In application of autonomous power line monitoring, automatic recharging is required. Based on currently available solutions, battery propulsion systems present best solution for given application.

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