Patent GB-735296-A (GB)
Title: Improved de-icing control
IPC: F02C7/047
Filed: 1952-03-19
Abstract: 735,296. Gas turbine plant. GENERAL MOTORS CORPORATION. March 19, 1952 [March 30, 1951], No. 7067/52. Class 110 (3). [Also in Groups XI and XXIX] An apparatus for sensing icing conditions in an inlet to an aircraft gas turbine comprises a pair of openings in the inlet so disposed as to be subjected to different pressures as a result of air flow through the inlet, a differential pressureresponsive means biased by the difference of the pressures and a restricted bleed orifice between the openings so that obstruction of one or both of the openings terminates the differential pressure on the responsive means. In the form shown (Figs. 2, 3 and 4), a pitot tube comprising a pair of tubes 34, 35 having hollow fairings 37, 38 projects through the wall 12 into the annular inlet to the axial-flow compressor. The fairing 38 has a forward facing scoop 40 fitted to it and an opening between the scoop and the interior of the fairing and the fairing 37 has an opening 41. The interior of the fairing 38 communicates by means of passages 42, 43 with a chamber 28 and the interior of the fairing communicates by means of a passage 44 with a chamber 33, the chambers 28 and 33 being separated by a flexible diaphragm 26. An adjustable snap-action switch 32 is fitted within the chamber 33 and bears upon a pressureplate 59 cemented to the diaphragm 26 and the chambers 28 and 33 communicate via passages 45 and 46 having a restricted bleed orifice 50 (Fig. 4). Thus when the pitot tube is ice-free, the over-pressure in the chamber 28 keeps the diaphragm 26 in the upper position and the switch 32 open; when one or both openings 40, 41 are obstructed by ice, the pressures in the chambers 28 and 33 are equalized, via the bleed orifice 50, and the diaphragm 26 drops on to a boss 29 thus allowing the switch 32 to close. The switch 32 may be connected in a circuit such as that shown in Fig. 7. In this circuit, when the change-over switch 74 is in the " Auto " position, closing of the switch 32 due to icing of the pitot tube lights a signal lamp 79 and energizes a solenoid 82 which puts the duct de-icing system into operation. Since the pitot tube may remain iced-up after icing conditions have passed, a push-button 83 is provided which closes a normally-open, solenoid-controlled switch 86-88, embodying a hold-on circuit, the switch energizing heaters 64 in the pitot tube, which melt the ice and thus open the switch 32. If icing conditions still obtain, the pitot tube is quickly iced-up again and the switch 32 reclosed. When the change-over switch 74 is in its manual position the switch 32 is not used and the de-icing plant is in operation continuously; also, if the push-button 83 is operated, the switch 86 ... 88 will remain closed and the heaters on until the switch 74 is put off. In another form of pitot tube, Fig. 6, an outer tube 103 contains a pressure tube 109, the end 111 of which is forward facing, and has an opening 115 on the trailing side shielded by sleeve 116. The pressure and static lines are connected to 113 and 114 respectively. The bleed orifice is in the form of a small hole 117 in the tube 119. A heating element 118 fed from a line 119 is fitted.