Patent GB-1021544-A (GB)
Title: Improved variable pitch propeller control system
Inventor: TIMEWELL PETER WALLACE
IPC: B64C11/42
Filed: 1961-08-12
Abstract: 1,021,544. Variable pitch propeller control. DE HAVILLAND AIRCRAFT CO. Ltd. Aug. 9, 1962 [Aug. 12, 1961], No. 29199/61. Heading B7V. A selectible pitch system for the control of a variable pitch propeller comprises a piston 114 (Figs. not shown) displaceable in a cylinder 108 and operatively connected to control, by the position of the piston, the pitch angle of the propeller (not shown), a decrease pitch line 50 communicating with the cylinder on one side of the piston for the supply of liquid thereto to displace the piston in one direction in the sense to decrease the propeller pitch, an increase pitch line 48 communicating with the cylinder on the other side of the piston for the supply of liquid thereto under pressure to displace the piston in the direction opposite to said one direction to increase the propeller pitch, stop means 216 limiting displacement of the piston in said one direction, a third line 104 for the supply of liquid under pressure, a stop withdrawal valve 200 connected to said third line and operable by a high pressure in said third line to render said stop means inoperative, an hydraulic pitch lock valve 130 connected between said increase pitch line and said cylinder and operable selectively to connect and disconnect said increase pitch line to said cylinder, a non-return valve 124 by-passing said pitch lock valve and arranged to open only for the flow of liquid from said increase pitch line to said cylinder, a pitch lock servo-valve 146 operable selectively to connect said pitch lock valve to the decrease pitch line (via line 140) to open the pitch lock valve and to the third line to close the valve, said pitch lock servo-valve being operable by the pressure in said third line to connect said pitch lock valve to said third line only upon the occurrence of a low pressure in said third line, a source of liquid under pressure connected (via line 90) to said third line, a pressure relief valve 96 connected between said third line and a low pressure source (20b) and capable of assuming an open condition to provide said low pressure in said third line, a closed condition to provide said high pressure in said third line and a partly closed condition to provide an intermediate pressure in said third line which intermediate pressure is sufficient to maintain said pitch lock servovalve 146 the position in which the pitch lock valve 130 is connected to the decrease pitch line and is insufficient to operate said stop withdrawl valve 200, said pressure relief valve 96 comprising a hollow first valve member 94 the interior of which is open (holes 100) through the wall thereof to said low pressure source, a valve seating interposed between said third line and said low pressure source and engageable by said first valve member to close the pressure relief valve, a first spring 63, urging said first valve member towards said seating, a chamber 65 in said pressure relief valve opening to the interior of said first valve member 94, a second valve member 71 displaceable in said chamber, a head 73 to said second valve member engageable with said first valve member 94 to isolate the interior thereof from said chamber 65 and a second spring 75 urging the head of said second valve member into sealing engagement with said first valve member, the effect of said two springs on said first valve member being such as to cause the pressure relief valve to assume the partly closed condition, a selector lever 238 adjustable to select a pitch angle for the propeller, a first cam 240 angularly adjustable by adjustment of said lever 238 and defining a selected range of pitch angles corresponding to displacement of said piston 114 in said one direction beyond the limiting position defined by the stop means 216, a main stop withdrawal valve 64 connected between said source of liquid under pressure and the chamber 65 of said pressure relief valve 96 and operable by said first cam 240 when angularly adjusted within said selected range to supply liquid under pressure from said source thereof to said chamber to urge said first valve member 94 into engagement with the associated seat to cause said pressure relief valve to assume the closed condition, a follow-up mechanism 236 connected (via linkage 224 and lever 246) between said piston 114 and said second valve member 71 of the pressure relief valve 96 and responsive to the departure of the piston in said one direction from a position selected by said lever 238 by an amount greater than a predetermined value to displace said second valve member 71 from engagement with the first valve member 94 to relieve the latter of the pressure of said second spring 75 and to place the chamber 65 in communication with said low pressure source through the interior of said first valve member 94 thereby causing the pressure relief valve 96, to assume the open condition, means (i.e. a groove) on said second valve member 71 operable by displacement of said second valve member to isolate said chamber 65 from said main stop withdrawal valve 64 an hydraulically controlled clutch 254 interposed between said follow-up mechanism 236 and said second valve member 71, a second cam 241 angularly adjusted by adjustment of said lever 238 and also defining said selected range and a clutch operating valve 68 connected between said source of liquid under pressure and said clutch and controlled by said second cam to supply liquid under pressure to said clutch to disengage the latter for lever positions outside said selected range and to disconnect said clutch from said source of liquid under pressure to cause said clutch to engage for lever positions within said selected range whereby said follow-up mechanism is operatively connected to said second valve member only for lever positions within said selected range.