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For original paper see Khalilov et al. ibid., vol.6, p.193 (1990). The authors comment on a recent paper by Khalilov and Khruslov. They show that the matrix generalization of the modified Korteweg-de Vries equation presented by them is related to simpler generalizations of the modified and unmodified Korteweg-de Vries equation given by other authors.

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Many species of owl has ability of fly silently. To explore the mechanism of owl’s silent flight, the kinematic character of the silent flight owl have been studied preliminarily on 3D motion capture technology. The paper has compared the flight motion between the long eared owl(Asio otus) and sparrow hawk(Accipiter nisus). According to the kinematic data, the flight trajectory, flapping frequency, flapping cycle, wrist angle, upstroke and downstroke percentage of cycle were computed and analyzed. The results show that wing tip, wrist and primary feather P10 of long eared owl and the eagle owl have obvious trajectory in flapping flight. Especially the wing tip trajectory in upstroke shows double“8”type. However, the wing tip trajectory in upstroke of sparrow hawk shows single“8”type. The wrist angle of silent owl keeps in downstroke and decreases in upstroke. But the wrist angle of sparrow hawk has increased first and then kept in downstroke, and the angle has begun decrease in upstroke. The above results show the difference of wing kinematics between silent owl and sparrow hawk, and will be useful for bionic design and silent fly technology.