GB765802A - Improvements in or relating to control means for electro-thermal de-icing systems - Google Patents

Abstract

765,802. Deicing aircraft. ROTOL, Ltd. June 14, 1955 [June 16, 1954], No. 17740/54. Classes 4 and 114. An electro thermal aircraft deicing system applicable to surfaces or propellers comprises a heating current control including a control switch 35, and actuating means 41 operable by one signal pulse to move said control switch 35 from the " on " position to the " off " position, and by at least one further signal pulse to move said control switch back to the " on " position, a temperature responsive switch 28 located in a zone at ambient temperature and including an electrical heater 31 and a switch 27, 30 closed only at switch temperatures above a selected temperature higher than 0‹C., first circuit means for supplying current to the heater 31 when the control switch 35 is " on ", second circuit means including a switch 33, 34 which is closed when the control switch is " on", for transmitting signal pulses to the actuating means when the switch 27, 30 is closed, an ice detecting device 11 for transmitting signal pulses to the actuating means 41 at a frequency dependent on the rate of ice formation on a part of the device, and switch means 37, 38 for connecting said signal pulse transmitting means with said actuating means only when switch 35 is " off". The ice detector comprises a compartment 18 with holes 22 facing upstream and projecting beyond the aircraft surface. The compartment 18 communicates through tube 19 with a space 17 on the right hand side of a diaphragm 12 in a chamber 11, the space 17 being supplied with fluid under pressure through conduit 20. Under icing conditions, the holes 22 become blocked, and the diaphragm is urged to the left, against spring 15, making contacts 13 and 14. This supplies current to a heater 23 located close to compartment 18, which is thus freed from ice, whereupon diaphragm 12 moves back again, and current to the heater 23 is cut off. This cycle repeats continually, at a frequency depending on the severity of the icing conditions. With switch 35 in the " off " position shown, each time contacts 13, 14 are bridged, current is fed through contacts 37, 38 to a solenoid 40, which pulls down an armature 65 to advance a pawl 42, rotating a cam surface 43. The amount of rotation depends on the throw of the armature, which is variable by a screw 51. After a number of signals from the ice detector, the number being determined by screw 51, the cam 43 moves switch 35 to the " on " position. Signals from the ice detector now no longer reach solenoid 40. Contacts 45 and 46 supply current to a relay 61, which in turn supplies current to heaters 62 located on the aircraft surface, e.g. the propellers. In addition, contacts 45 and 46 supply current to a heating element 31 in the temperature responsive switch 28. After a certain time, depending on the ambient air temperature, a bi-metallic strip bridges contacts 27, 30, to transmit a signal pulse to the solenoid 40 through contacts 33, 34, which then places switch 35 in the " off " position. A variable resistance 49 is placed in series with the heating element 31, to vary the time delay of switch 28 for a given ambient temperature. A heating element 55 located close to a bi-metallic strip 52 is placed in series with the heater 23, so that when contacts 13 and 14 are bridged, heater 55 heats the strip 52, which is adapted to bridge contacts 53 and 54, to operate a relay 67 supplying current to heaters 70 located in breaker strips adapted to separate ice formed on the aircraft surface into separate areas.