 They are available for both electric and liquid-fuelled engines, although they have only become widely used with the recent improvements in electric-flight technology for model aircraft.It is possible to equip a model jet aircraft with two or four electric ducted fans for much less than the cost of a single jet turbine or large petrol or methanol engine, enabling affordable modeling of multi-engine planes, including military bombers and civilian airliners.The fan-unit is an assembly of the spinning fan (a propellor with more blades), enclosed inside a shaped-duct.