GB704925A - Pressure modulated skid-preventing airplane landing wheel brake control apparatus - Google Patents

Abstract

704,925. Fluid pressure aircraft brakes. BOEING AIRPLANE CO. March 14, 1952 [March 27, 1951], No. 6713/52. Class 103(1) The hydraulic brakes on an aircraft are controlled in two stages, in the first of which the system is subject to a series of brake application and release cycles which continue until the pressure has been reduced to a pressure below that at which skidding of the wheels takes place and as described in Specifications 696,446 and 704,880, and a second stage in which the brake pressure is gradually increased at a rate approximately proportional to the rate of increase of. weight load of the aircraft. In the arrangement shown in Fig. 3, brake fluid is supplied to the wheel brake actuators 10 under the control of the pilot's metering valve 12 and solenoid valves 20 which are operated to prevent skidding by apparatus 28 as disclosed in the aforesaid Specifications, valve device 18 being provided for reducing the pressure actually applied to the wheel cylinders. Pressure is supplied to the metering valve 12 from the supply service pipe 14 in such a way as to carry out the second stage of the braking operation by means of flow restrictors 38 and 42 and automatic control means 32, operation being as below. When the pilot initially applies the brakes there is a surge of fluid through the conduit 14<SP>1</SP> from the outlet 50 at the bottom of the control device 32 so that the pressure drops there and also in the chamber between the top of a valve sleeve 52 and an outlet 54 connected to a return line 22. The outlet 54 from the sleeve is opposite to the lower end 48<SP>1</SP> of a control member 48 the upper face of which is subject to the pressure in an accumulator containing a spring 46 compressed by a piston 44. The pressure above the piston 44 is that in a pipe 40 between small and large flow restrictors 38 and 42. Before the commencement of a braking operation the pressure above piston 44 will be substantially that delivered to the metering valve 12. On the reduction of pressure caused by the brake application in the valve sleeve 52 control valve 48, 48<SP>1</SP> will therefore move downwardly restricting the return flow by pine 22 thereby tending to raise the pressure at the outlet 50. With, however, continued braking cycles the pressure in the accumulator device 32 will fall progressively due to the flow restrictor 42 being of greater flow capacity than the restrictor 38. After the initialperiod of cyclical braking to prevent skidding has been completed, and the brake pressure has been reduced to a level below that which will cause skidding, the pressure will gradually build up with increase of pressure in chamber 34, the rate of increase being governed by the replacement flow through the restrictor 38. The size of the restrictors 38 and 42 are chosen to suit load operating characteristics of particular aircraft and the conditions of runway likely to be encountered. Since if the runway is extremely icy or slippery so many skid prevention cycles might be made as would cause the accumulator pressure to fall below that necessary for effective braking, one modification described provides a special valve device for ensuring that the accumulator pressure never drops below a selected minimum pressure such as 100 lbs. per square inch.