They are vertical takeoff and landing aircraft which use two opposing rotors enclosed in a circular shroud for propulsion.

In the late 1980s, Sikorsky Aircraft flew a small UAV named "Cypher", with coaxial rotors inside a torus-shaped airframe.

It led to a true flight prototype Cypher that weighed 110 kilograms (240 pounds), had a diameter of 1.9 meters (6.2 feet) and was powered by a compact, 40 kW (53 hp) Wankel engine.

The Cypher can carry a sensor package on struts above its hull, or can transport loads weighing up to 50 lb (23 kg).

The Cypher II is similar in size to its predecessor, but has a pusher propeller in addition to its rotor and can be fitted with wings for long-range reconnaissance missions.