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Propulsion Systems

All our aircraft have been propeller-driven either using internal combustion engines or electric motors. Both tractor and pusher configurations have been used, and in some cases hybrid designs with both engines and motors have been employed. Currently, four-stroke gasoline or JP8 kerosene-fueled engines give the best range and endurance to aircraft, but they do need fuel supplies to be kept in mind; they can also cause vibration problems with on-board sensors unless careful consideration is given to antivibration mountings (of both engines and sensors).

Fitting an aircraft with electric motors for propulsion is relatively straightforward. A wide range of relatively inexpensive motors is readily available, and by adopting rare-earth magnets, very good power to weight ratios can be achieved. They tend to be very smooth running and extremely reliable since the main rotor and its bearings are usually the only moving parts. The Achilles heel of electric propulsion is the energy density of the batteries currently avail- able.[1] LiPo batteries are readily available and reasonably priced, but using them to lift useful payloads typically restricts endurance to less than 2h, often dramatically so. The advent of lithium-sulfur batteries may mitigate these problems to some extent, as would modestly priced fuel cells - currently neither technology is viable for low-cost UAV systems.

  • [1] Typical energy densities for LiPo batteries are currently around 0.36-0.95 MJ/kg, while gasoline has an energydensity of 46.4 MJ/kg. Even allowing for the greater efficiencies of electric motors compared with gasoline engines,this is a massive disadvantage when range or endurance is important.
 
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