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Aviation History
1947
1947 - 1873.PDF
;:0CTOBER 30TH, IO47 f LIGHT The Sperry Pilot Aid Introductory Details of a Simplified Autopilot for the Lighter Types of Aircraft U P to the present time there has not existed an automaticpilot sufficiently small in size, weight and price to makeit a tenable proposition for aircraft of much below 15,000 lb all-up weight. In view of this, the Sperry Gyroscope Com-pany, Ltd., of London, decided about two years ago that there was a definite potential demand among the feeder-line, charterand small types of aircraft for a simple, light, inexpensive and reliable automatic pilot which, whilst not embodying all the_ refinements of the heavier and more elaborate autopilots de- W manded by large aircraft, would effectively supply the require-ments of aircraft in the lighter classes. Development work was started at the end of 1945 on thebroad specification that the autopilot should be capable of controlling aircraft of up to abqut 15,000 lb weight in straightflight and turns; should be simple and reliable; should weigh not more than 30 lb; and, if possible, sell for /400 or less. Asmost of the aircraft in the class envisaged employ vacuum- operated flight instruments and a D.C. electrical system, noother form of power supply for the autopilot was considered. The final experimental prototype was produced at the endof 1946, and after the first test flights an officially requested demonstration was given—with complete success—to theMinistry of Supply. The Pilot Aid, as the new device is known, incorporates anair-driven vertical gyro fitted with potentiometer pick-offs for detecting roll and pitch. Electrical D.C. signals are convertedinto the movement of pneumatic valves by means of differen- tial polarized relays. The valves admit air pressure to linearservos on which feed-back potentiometers are fitted to produce backing-off signals which ensure that the servo movement isproportional to the gyro displacement. A pick-off potentio- meter is also fitted to a standard Directional Gyro and thesignals are made to operate the roll relay valve differentially so that the aircraft maintains heading through aileron control.The power supply required is 40 watts D.C. and air pressure at 12 Ib/sq in, the latter being conveniently taken from thepressure side of a standard vacuum pump. The control system relies on the aerodynamic damping ofthe aircraft in which it is installed so that stiffness (i.e., the amount of control applied per 1 deg displacement) and,therefore, the zero sensitivity has an optimum value which cannot be exceeded without oscillation. In rough air the pilotcan increase the stiffness beyond its deadbeat damping value since a«= certain amount of oscillation is lost igx .the generalbumpiness; a greater degree of sensitivj^fi^fcmsequently obtained. «^t '—J^r The gyro relay unit is remarkably compact, as may be discerned from this view showing it with the cover removed. Thz dimensions are roughly IOinx7inx7in. The control box is neat and simple, and incor- porates the engage switch, pitch attituce convol, and "stiff- ness" control. Direc- tional control is given through the adapted directional gyro. The pilot is furnished with control whereby he can applybanked turns within 20 deg limits of roll Tand- climb and dive within pitching limits of + 10 deg. He may also alter courseby caging and resetting the Directional Gyro; the aircraft then goes into a banked turn and levels out at the new datum.An advantage of the system is that the gyro is left con- tinuously running so that there is no waiting period beforeengaging. The autopilot is engaged simply by operating an electric on-off switch. As illustrated in the schematic layout diagram, the electricalsystem for each axis of control consists basically of two bridge circuits, each of which operates a signal coil in the respectivepolarized relay. In the pitch system either the gyro pick-off or pilot's pitch trim potentiometers may produce a signalin the "pick-off" coil. The pneumatic valve is opened and the servo operated until an opposite signal is generated in thefeer?-back coil by the servo feed-back potentiometer. A small trimmer completes the "feed-back" bridge for datum trim-ming on installation. The roll system is similar except that the Directional Gyro pick-off replaces the pilot's trim. Ineach feed-back line a variable resistance is incorporated in order to provide a stiffness adjustment to suit individual air-craft. The pilot's stiffness control—for use in rough weather —is a potentiometer which drops the voltage of the positiveline to all feed-back circuits. It is desirable to have a large pick-off range coupled with amaximum signal per degree of displacement and adequate mechanical sensitivity. These requirements, however, are notcompatible, and a careful compromise had to Le effected with due regard being given tg^the performance of the polarizedrelays, valve characteristics, and the servo power required to initiate control movemeut. Electro-Pneumatic Circuits ]£ach relay unit contains two polarized relays, pneumaticvalves and a magnetic cut-out valve. The polarized relays add the signals received from pick-off and feed-back bridge cir-cuits, and transform the resulting signal into a proportional mechanical movement of the pneumatic valve piston. Sincethe signals received from the pick-off are small and the friction of the valves and bearings relatively large, a mechanical'dither" is imparted to the valve piston by applying an alternating current to the centre tapping of the relay polarizedcoil. The outlet ports of each valve are connected to the servosby suitable pipe lines. In the valve assembly two further outlets are provided so that the lines from valves to servos arespring loaded and open to atmosphere when the control is not in use. All four of these ports are grouped together and arecovered by a valve which is closed magnetically when the pilot operates the engage switch. The gyro unit houses the vertical gyro movement, vibrator,trimmers, etc., and forms the main junction box for all com- ponents. The gyro itself is a standard Mark 1 B air-drivenArtificial Horizon movement mounted on a suitable cradle and modified by the addition of pick-off potentiometers. It isdriven by pressure instead of suction, and the horizon bar assembly is, of course, removed. The pendulous body ismodified by reducing the port area and increasing the clear- ance between the vanes and body to avoid sticking due to oil
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