The pilot's control member 10 of an aircraft controls two power units 25, 125 which each execute movements corresponding in amount and direction to the movement of the control member and thus, through a differential gear 31, drive a torque shaft 32 which drives the irreversible gearing 50 associated with the control surface, and means are provided to prevent reverse rotation of one power unit, should it fail, by the other. The control member 10 is connected by a chain drive 11, shaft 12 and chain drives 13, 14 to two selsyn transmitters 16, 116 adapted to control the power units 25, 125 respectively. Each power unit comprises a constantly rotating constant speed electric motor 26 driving a variable delivery pump 27 which provides pressure fluid to drive a hydraulic motor 28, and each includes a mechanical lock normally held out of engagement by fluid pres‐sure, but spring‐loaded to lock the motor should it fail. The pump 27 normally docs not deliver fluid, but the delivery is controlled by a lever operated by follow‐up gears 33 in turn controlled by the output of the motors 28 and by the selsyn receiver 29 (see also Group XXIX) to provide a correspondence control. The pumps 27 and motors 28 are of the variable stroke rotary type to provide control in two directions of rotation. The motors 28 each drive one sun‐wheel 30, 130 of a differential gear 31, the planet carrier of which drives the torque shaft 32, and also the shafts 65 associated with the follow‐up gears 33 through gearing 55, 56, 64. The torque shaft 32 drives, through bevel gearing and shafting, jacks 50 each comprising a worm 49 engaging a worm‐wheel on a nut 75 engaging a screw 76 to move the surface 24. The surface 24 is in independent sections, each having its own jack, and the worms 49 are secured to the driving shaft 46 by frangible pins which break if the corresponding sec‐tion of the surface jams, so permitting continued operation of the rest of the surface. ‘Desynn’ (Registered Trade Mark) transmitters 34, 37, associated with the control member 10 and this surface 24 respectively, control indicators 35, 36 in the pilot's cabin. A device 20 for simulating ‘feel’, such as described in Specifications 619,987 and 619,988, and a trimming device 21 both operate through a differential gear 18 and chain drive 17, while the automatic pilot is connected to the shaft 12. A further differential gear associated with the shaft 32 may provide for operation of the surface by a reversible electric motor if both power units 25 fail. In an aircraft with a pressure cabin 114, the torque shaft 32 passes through a seal 45, the jacks 50 being outside the cabin and the power units 25 inside, but not necessarily in the pilot's compartment. Specification 577,496 is referred to.
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