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.

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We analyzed a small coaxial helicopter developed for entertainment in 2002. The upper and lower rotors are rigid rotors. The upper rotor is connected to a stabilizer bar. The cyclic pitch of the upper rotor is controlled by the stabilizer bar when the attitude of the helicopter is varied. The angle between the upper rotor and the stabilizer bar is 41. The cyclic pitch of the lower rotor is controlled by servo motors and the inputs of the cyclic pitch from the servo motors are at the azimuth angles of 45 , 225 and 135 , 315 . This paper clarifies how the angle between the upper rotor and its stabilizer bar and the azimuth angles of the inputs of the cyclic pitch to the lower rotor are determined.

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.

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