Patent GB-718659-A (GB)
Title: Fluid stream direction indicator
IPC: B64D45/00
Filed: 1952-07-24
Abstract: 718,659. Electric indicating systems. GARRET CORPORATION. July. 24, 1952 [Aug. 31, 1951], No. 18743/52. Class 40 (1). Apparatus for determining and indicating the direction of flow of a fluid (e.g. about an aircraft) comprises a pivoted sensing head positioned in a fluid stream and having angularly positioned fluid flow openings communicating with an interior duct of the head which openings contain temperature responsive resistor elements in opposite arms of a Wheatstone bridge whose temperatures are differentially variable by the fluid flow through the openings and duct, when the head axis is misaligned with the direction of stream flow so as to unbalance the bridge and produce a signal representing the misalignment which operates an indicator and also a servosystem restoring the head axis to alignment with the stream direction so that the bride is rebalanced. In Figs. 1, 2, 4 a spherically-shaped sensing head 14 is pivoted on ballraces about a shaft 16 carried by a tubular member 13 attached to a boom 11 to present its exposed curved surface 17 to the direction of fluid flow and the interstices between the head and the tubular member are sealed by a gasket 28. The surface 17 has openings 18, 19, symmetrically angularly positioned on opposite sides of the axis of the head which communicate with an internal duct 21 of the sensing head and contain thermally responsive resistor sensing elements 22, 23 (e.g. wire coils) connected in an A.C. energized Wheatstone bridge 38 whose unbalance output is amplified and fed to a servocontrol 42 actuating a reversible servomotor 29 which rotates the sensing head through flexible bands 31 passing over pulleys 30, 34, 35 in accordance with the direction of unbalance of the bridge circuits. Misalignment of the axis of the sensing head with the flow direction sets up differential pressures across the openings 18, 19 and consequent circulation of fluid therethrough whereby the resistor elements are unequally heated and the bridge is unbalanced so that the servomotor is actuated to realign the axis and the flow, and to produce a deflection indicating the angle of fluid flow with respect to the axis of the instrument which is transmitted to a visual indicator 49 by a selsyn system 47, 48. The unbalance output of the bridge may actuate a signal generator 50 incorporated in an automatic pilot system, and increased sensitivity is achieved by inserting an H-shaped heater coil 26 in the fluid flow through the duct 21 between . the openings 18, 19. A second heater coil 27 may be positioned in the sensing head between the two openings to prevent icing and for the fluid velocities the sensing head may have a conical or ogival shape (Fig. 5 not shown.) Angular misalignment may be indicated directly by a galvanometer in the bridge unbalance circuit while the sensing head may be universally mounted and provided with additional sensing elements and servosystem for two dimensional realignment with flow direction. The apparatus is applicable to the indication of yaw, pitch and direction of airflow of an aircraft to control an automatic pilot and to define fluid flow over wind tunnel models, air foils, cascades and ducts.