A small battery-powered helicopter with a total weight of about 200 g and a rotor diameter of about 35 cm has been developed. Some indoor flights were performed without a human operator using a transmitter. The helicopter is capable of autonomous hovering flight near walls when IR range finders mounted on it are used to measure distances to walls and the floor. Four mounted photodetectors permit two maneuvers. In the first maneuver, the helicopter can follow a moving light. So it can be controlled simply by moving a light, and can follow a robot or a person carrying a light. In the second maneuver, the helicopter flying over a first light can fly towards a second light, when the first light is turned off and the second light is turned on. The position of the helicopter can be controlled by successively switching (on and off) lights in a row.

During the 20th century, aircraft were only used for transportation. If aircraft can be made small and lightweight, however, they can become tools to assist in everyday life. We developed a small, lightweight co-axial helicopter with a rotor diameter of about 30 cm. The mechanisms for varying cyclic pitch of the upper and lower rotors, which are used in the coaxial helicopter for entertainment, are adopted in our develop helicopter. Our developed helicopter is equipped with a flight control system for the attitude and position, which is composed of a micro computer and some sensors. And the helicopter can make autonomous hovering flight just measuring the height and the distances from the walls. The weight of the helicopter is no more than 200 g and this helicopter is the lightest helicopter for an autonomous hovering flight among the helicopters where all control systems are onboard, as far as the authors know.

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Thrust and torque generated by a model rotary wing were measured at an ultra-low Reynolds number, Re ¼ 4 Â 10 3 , for various aspect ratios with and without linear blade twist. The measured characteristics were compared with those calculated by the method which is well known to be effective for analyzing a rotary wing at a high Reynolds number. The method combines annular momentum theory and blade element theory. This calculation method can give the quantitative explanation of the effects of the aspect ratio and of linear blade twist on the characteristics of the rotary wings. The calculation results also indicate that the present calculation method has the capability of giving an accurate quantitative estimation of rotary wing performance with the blade aspect ratio larger than 10, operating at the ultra-low Reynolds number.