Patent GB-544226-A (GB)
Title: Improvements in or relating to means for giving warning of the formation of ice on aircraft
IPC: B64D15/20
Filed: 1940-06-11
Abstract: 544,226. Moisture indicators. BRITISH THERMOSTAT CO., Ltd., and SHERLOCK, J. E. June 11, 1940, No. 10170. [Class 37] Apparatus for detecting and giving warning of conditions favourable to ice formation on aircraft consists of an electrical detecting device sensitive to deposition of water on an insulating surface, means for causing such deposition to occur at a relative humidity of less than 100 per cent and means actuated by the change of resistance caused by deposition of moisture to operate an electrically actuated signal. A number of annular silver discs 1, Fig. 1, are mounted coaxially and insulated from one another by thin annular discs 2 of transparent cellulose. The discs 1, 2 are held in a two-part housing 3 of insulating material, the parts being clamped together between annular metal plates 5, 7 at the ends of a casing 6 which encloses the discs 1, 2 and the housing 3. The silver discs 1 have tabs 8, 9, Fig. 3, which project outwardly through the insulating housing 3, the tabs of alternate discs projecting in opposite directions. The element is housed within the casing 6 and mounted on a bracket 10, to which is attached a tube 11 carrying a front orifice plate 12. The bracket 10 also carries a socket 13 for entry of conductors 14. The casing 6 carries a tubular body 16 the closed end of which is pierced by opening 17 covered by a spring loaded relief valve 19 which limits the air pressure that can be produced in the pocket 18 by the relative wind velocity. Small holes 47 facilitate escape of condensate. A bi-metallic strip 20, Fig. 2, is mounted in the casing 6 and is connected by a conductor 24 with the tabs 8 projecting from one side of the disc assembly. A second bi-metallic strip is mounted at the other side of the assembly and connected to the other tabs 9. A contact 22 carried by the bi-metallic strip 20 engages a fixed contact 25 to which is connected one of the conductors 14, the other of which is connected to a similar contact similarly related to the second bi-metallic strip. The two alternating sets of silver discs 1 thus constitute a pair of terminals when the two pairs of switch contacts are closed, the terminals being insulated from one another by the insulating discs 2. The tubular pocket 18 is arranged so that its open end in the orifice plate 12 faces the wind. The wind pressure causes a static pressure above atmospheric and thereby increases the humidity and thus anticipates conditions in which deposition of ice is likely to occur by causing deposition of moisture on the internal edges of the disc assembly 1, 2. Current can therefore pass through the circuit if both thermostat switches are closed. One of the thermostatic switches is set to open when the temperature falls below the operating range, whilst the other is set to open when the temperature rises above this range. The circuit connections are shown in Fig. 8. The detector A is connected to relays B, C operated from a battery D. Included in the circuit is a thermally operated flasher E, which operates continuously to energize a commutating relay B at intervals as long as the main switch F is closed. The polarity of the detector A is thus periodically reversed, preventing polarization through electrolytic action. When atmospheric conditions are such that both thermostat switches are closed and the humidity is sufficient to cause condensate on the inner surface of the disc assembly 1, 2, the resistance is lowered sufficiently to cause the main relay C to be energized so that an indicator lamp G is illuminated and gives a flashing signal owing to the operation of the relay B. In a modification the silver discs 1 have their connecting tabs projecting radially inwards. The thermostatic switch consists of two bi-metallic strips carrying contacts which are normally closed by and adapted to be opened by a fixed stop between them when the strips flex in either direction due to temperature changes. The relief valve is omitted, holes being provided for the escape of air and condensate. The Provisional Specification describes a modification in which two insulated wires are wound on a former so as to produce a double helix. This winding is introduced into a tubular steel support and the inside of the winding is polished so as to expose the wires. The metal supporting tube may be formed on the winding by spraying its external surface with an atomized metallic spray. The winding may be painted or sprayed with insulating material before the metal is sprayed on. Instead of increasing the air pressure around the detector, this may be artificially cooled by adiabatic expansion of air in a small wind tunnel mounted on the aircraft so that the slip stream blows through the tunnel.