PURPOSE:To enable highly accurate detection of an airspeed by a method wherein a Karman's vortex is generated with a rope body stretched on an object desired to detect an airspeed and the rope body is made to resonate to the karman's vortex by adjusting the tension thereof to determine a frequency in the resonation of the rope body. CONSTITUTION:A hard steel wire 1 is mounted on a pulley 4 and a fixing bracket 9 at both ends thereof to stretch and fixed on a strut 2. A microphone 3 set in the strut 2 picks up a frequency of vibration of the steel wire 1 as caused by a Karman's vortex which is generated when an air current hits the steel wire 1 to be amplified and the frequency is converted into a voltage signal with an f-v converter to be inputted into an arithmetic device. The arithmetic device computes a tension of the steel wire 1 from an angle of rotation of a motor 7 which rotates a pulley 4 and compares a primary resonance frequency of the steel wire 1 to be obtained from the tension with a frequency of vibration of the steel wire 1. When both are equal, an air speed is determined from this value by a computation. When the primary resonance frequency of the steel wire 1 is larger (or smaller) than the frequency of the vibration, the pulley 4 is rotated with the motor 7 to loosen (or intensify) the tension of the steel wire 1 thereby making the both equal.
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