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Aviation History
1947
1947 - 1666.PDF
366 FLIGHT SEPTEMBER 25TH, 19 <^ A New Autopilot ..... fore, a degree of slip-out or slip-in will be present. Todetect this, a third pendulum monitor is mounted on the gyro-unit base-plate, this sending a signal to the rudderservo-motor to produce the requisite rudder correction. Before leaving the pilot's controller it must be men-tioned that the "stick" self-centring action and head- depressed contact are such that when the required climbor dive attitude is attained and the controller '' stick is released and allowed to self-centre itself, the aircraftwill continue flying in the obtaining attitude. To resume level flight from a given pitch attitude opposite '' stickmust be applied. When, however, an autopilot-controlled turn is being made, the controller must be held in the appro-priate position so long as the turning action is required, for on release of the knob back to the self-centred posi-tion, the autopilot will bring the aircraft back to level flight. Servo-Motor Details Each servo-motor is an identical unit and is composedof two parts1, (i) a motor, clutch and reduction gear assembly; and (ii) a torque shaft assembly. Of these, thelatter is a "permanent" aircraft fixture, whilst the former can be changed as a sub-unit for periodic overhaul. Themotor element is novel in that it is an hysteresis motor, the rotational direction and speed of which is determinedby phase displacement. These motors have high torque coupled with low inertia,which gives very high response and so contributeslargely to the sensitivity of th6 system. The motor drive is appliedthrough reduction gearing, in which an electricallyoperated clutch is incor- porated, to the torque shaftfor operation of the control surface. Each servo-motoris capable of supplying an output torque of 60 lb ft atthe torque shaft, which BRASS latter normally operatesthrough approximately 4- 60 degrees, although a larger or smaller angular range can be used. The transmission linkage between torque shaft and control surface can vary considerably, the torque shaft Break down detail of servo-motor clutch showing stack of plates, locking slots, return spring, energizing coil, etc. Amplifier unit showing ai bottom ieft the quick-release knob. When this is pulled, the whole unit can be unshipped from its mounting. Gyro unit showing anti-vibration mountings and screw-jack point for levelling. having a flange-terminal' coupling for versatile linkage. In tandem with and driven by the motor is a miniaturegenerator which, according to the direction and speed of the motor, produces anappropriate feed-back out- put to give proportionalitybetween the input signal and velocity by backing-OSthe input, this being effec- tively an electric follow-upsystem. The magnetic clutch con-sists of a stack of alternate brass and steel annularplates mounted axially and capable of being squeezedtogether by an electro-mag- net housed inside the plateassembly. Operation of the clutch is instantaneous, andsafety is assured in the event of disruption or failure of the power supply since disengagement is automatic withbreaking of the circuit. The reliability of the clutches has been proven to be equal to that of manually operatedsystems. > -~ •" Vj ; Amplifier and Junction Box The amplifier unit is, of course, necessary to boost andtranslate the minute signals received from the gyro unit in order to provide sufficient working power to energizethe servo-motors. The main amplification is attaiped by means of three magnetic amplifier sub-units, one for eachcontrol circuit. The great attraction of this svstem, and indeed, its novel aspect, is that the amplification is obtai'through an inert means. There are no moving parts, t units are quite small, and there is no maintenancerequirement. The junction box is simply what its name suggests butlias the refineinent that it embodies potentiometers whereby the magnitude of control response may be adjusted to suitthe requirements of the particular aircraft control charac- teristics. That is to say, the relative harmonization betweenthe. "weight" of the respective surfaces can be adjusted at the junction box. Additionally, the embodiment ofreversing links allows the standard box to be used for any aircraft, the relation between the gyro rotation and rote"tion of servo-motor being antipathic or sympathetic as required for the particular aircraft. Thus, for example,if the rudder servo is to rotate anti-clockwise for a given surface movement due to the linkage employed, this canbe fitted in with the uni-directional gyro rotation by c°n" necting the reversing links one way or other. Another
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