GB1184397A - Automatic Flare and Altitude Hold System - Google Patents

Abstract

1,184,397. Automatic control of aircraft. LEAR SIEGLER Inc. April 10, 1968 [April 19, 1967], No.17141/68. Heading G3R. The Specification describes a system for causing an aircraft to start a flare out to a predetermined hold altitude at an altitude with respect to ground which varies as a function of the aircrafts initial descent rate. Thus with a hold height of 20 ft. and a descent rate of 10 ft. /sec., the flare is initiated at 70 ft., while for a descent rate of 20 ft. /sec., the flare is initiated at say 100 ft. The system is shown in Fig.1 with relay contacts 22 and 32 in the "before flare out" position, whence the output of a barometric altimeter 31 gives a signal indicative of the rate of change of altitude with respect to sea-level e.g. 5v for 10 ft. / sec. This is combined in circuit 33 with the output from a means 34 measuring the aircraft acceleration along an axis normal to its plane. The combined output is compared in circuit 25 with output of a command system according to a predetermined approach path or to I.L.S. signals and the result used to control elevator surfaces 43 via a servosystem. The combined output is also fed to one input of comparator circuit 70. The hold altitude is set by means of a potentiometer 50 and produces a voltage e.g. 1v for 20 ft., which is compared with the corresponding voltage output of a radar altimeter 60 in comparator 55. The output of the comparator is fed to the other input of comparator 70, whence for an altitude of 70 ft. giving an output of 1À5v from means 60 and an output from comparator 55 of À5v, the output of comparator 70 becomes zero. This causes the actuation of relay 76 and the switching of contacts 22 and 32 whence the flare out commences. The rate of change of altitude input to circuit 33 is now provided by the radar altimeter and the elevator surfaces 43 are controlled by a signal combining the difference between the aircrafts actual altitude and the hold altitude, and total vertical acceleration of the aircraft. Amplifiers 71 and 72 vary the weightings of the above two components in the control signal. A roll at constant altitude will produce g forces which are erroneously interpreted by means 34 as an acceleration. Such an erroneous output is counteracted by the negation output of a (1-Cosα) function generator 86 where α is the roll angle determined by means 85. When the hold hight is selected to be very low, e.g. 2 ft. the flare out rate of descent may be too fast and a contact 97 is accordingly closed. During the pre-flare out rate this causes the subtraction from the output of altimeter 60 of a voltage representing e.g. 15 ft. and the flare out accordingly starts at a higher altitude, e. g. 85 ft. with a resulting less steep flare out. Imitation of the flare out causes a capacitor, across which the 15 ft. representative voltage is developed, to discharge, whence the flare path tends to normalise. The altitude at which flare out starts may be reduced by closing contact 98 whence a portion of the output of circuit 33 is subtracted from said output in circuit 70.