Patent GB-953631-A (GB)
Title: Air mass relative motion meter
IPC: G01P5/24
Filed: 1962-07-09
Abstract: 953,631. Determining motions by acoustic energy means. BOEING CO. July 9, 1962 [July 10, 1961], No. 26177/62. Heading H4D. True airspeed together with angle of attack and/or yaw angle of an aircraft are measured by an arrangement involving the difference in the transmission time of acoustic type waves between a single transmission source and at least two spaced, receiving devices. In the described embodiment the transmission source 10 which is mounted in the free airstream at the extremity of a probe 18 (Fig. 3) forwardly projecting from the nose (or, as also shown, from the wing tip) of an aircraft, comprises a pair of transmitting ultrasonic transducers 10a, 10b (see also Fig. 2) radiating rearwardly directed beams towards two receiving transducers 12, 14 carried by respective probes 20, 22. The transmitting transducers are energized simultaneously by 500 microsecond " bursts " of energy from a 100 kc/s. oscillator 26 (Fig. 2) under the control of a multivibrator 28 triggered by a free-running blocking oscillator 30, the oscillator 26 being such that each " burst " commences with pulses of the same phase and polarity. Energy received by the receiving transducers 12, 14 is fed to respective detectors 32, 34, the outputs from which trigger respective blocking oscillators 36, 38 having long recovery times to minimize triggering by noise signals. The outputs of the latter together with that of oscillator 30 are then fed to resolvers 40, 42 which operate in known manner to generate a current or voltage control function representative of the transmission times T 1 , T 2  for the propagation of the ultrasonic energy between the transmitting transducers and the receiving transducers 12 and 14, respectively. Additionally, a further resolver 46 supplied with the outputs of oscillators 36, 38 is provided for generating a control function representative of the difference in transmission time T 1  - T 2 = T. The three control functions are then fed to a known form of computer together with a control function Vo (=velocity of sound in ambient static air), derived via a resolver 48 and two further control functions d, B representative, respectively, of the distance between the transmitting transducer and the receiving transducers and of the angle between the longitudinal axis of the aircraft and the propagation path of the ultrasonic energy. From the geometry of the arrangement the transmission times are shown to be which reduce to tan α =#T. d tan B/T 1 d+T2d - 2 V0T 1  T 2  where #T = T 2  - T 1  and a is the angle of attack. This is derived as one output of the computer which also operates to solve the equation where V is the true airspeed. If the assembly 16 (Fig. 3) is rotated so that the receiving transducers lie in a horizontal plane the measured angle a represents the yaw angle and by continuous rotation both the latter and the angle of attack may be measured. Alternatively, a triangular arrangement of three receiving transducers or of two pairs in orthogonal planes may be provided. If it is desired to measure the angle of air mass relative motion in different planes simultaneously, a stationary head having a single transmitting element comprising a cluster of transducers co-operating with a cluster of receiving transducers may be employed. The transducers may comprise magneto-strictive, electromagnetic or piezoelectric elements and instead of utilizing the pulse technique for the measurement of the propagation time known frequency or phase modulation or amplitude modulation techniques may be employed.