a significant area of current research is to develop aircraft which are unmanned. more specifically, some researchers are focusing on hovering type aircraft, or vertical take-off and landing (vtol) aircraft. since these types of aircraft do not need to be constantly in motion to maintain lift, they have certain advantages over fixed-wing aircraft. suitable applications could involve aerial photography, atmospheric measurements or surveillance and defense. the most common type of vtol aircraft is the helicopter. one disadvantage of this aircraft is the degree of danger involved due to the exposed rotor blades. this is one motivating factor behind the 'ducted fan' system. this type of system uses a rotor which is enclosed in a aerodynamically shaped duct.