GB502060A - Automatically protected pitot static tubes - Google Patents

Abstract

502,060. Heating Pitot-tube apparatus electrically. KOLLSMAN, P. April 11, 1938, Nos. 11016 and 11017. Convention dates, April 9, 1937 and March 12, 1938. [Class 69 (i)] [Also in Groups XI, XXXV, and XXXVII] An electric heater B, Fig. 1, arranged to heat conductively the metallic dynamic and static pressure conduits 2, 5 of the Pitot head of'an aircraft speed-indicator in order to keep them free from ice &c. is protected from overheating by automatic means controlling the current supply to it in dependence upon the rate at which the heat is dissipated. In the form shown in Fig. 1, the Pitot tube consists of a dynamic pressure tube 2 extending within an outer shell A forming the static pressure conduit 5 and provided with static pressure openings 4. A tube 7 extends from the conduit 2 to drain any moisture which' may enter.the latter. The heater B consists of enamel-embedded wire enclosed in a copper shell which is preferably welded or soldered to the shell A. The current supply to the heater passes through contacts 17, 18 of a switch mounted on a partition 20 and comprising a bimetallic strip 19 which causes the' contacts to open at a predetermined safe high temperature. In a modification, the switch controls a relay which operates a switch in the heater circuit. In a further modification, the switch is located at a point outside the Pitot tube, and controls the heater current in accordance with the temperature of the ambient air. In another form, the current is limited by including in the circuit a resistance having a high temperature coefficient, for which purpose the heater element may be formed of nickel wire. In place of, or in addition to, the use of a temperature-responsive switch, the heater circuit may be controlled by a member responsive to the air-pressure when the aircraft is in flight, the circuit being completed only at such times. In the arrangement shown in Fig. 14, the circuit of the heater 108 is controlled by a relay 115, the energizing circuit of which is controlled both by a temperature-actuated switch 121 and by contacts 128 controlled by a pressure-actuated capsule 127 connected to the dynamic pressure duct of the Pitot tube. The heater is thus in operation only when the machine is in flight and the temperature is below a predetermined maximum.