Patent GB-666540-A (GB)
Title: Improvements in and relating to control apparatus for automatic pilots for aircraft
IPC: G05D1/06
Filed: 1946-01-31
Abstract: 666,540. Automatic control systems for aircraft. SPERRY CORPORATION. Jan. 31, 1946 [Jan, 31, 1945], No. 27750/49. Divided out of 666,471. Class 38 (iv). An automatic control system for aircraft, arranged to effect an adjustment of a control surface, e.g. elevator, to maintain a predetermined magnitude of lift despite disturbances such as changes of bank, load, or relative air speed, comprises means, namely strain gauges, for defining a measure of the desired lift of the aircraft and arranged to produce signals dependent on the deviation of the vertical component of instantaneous lift from the desired lift, these signals being applied automatically to adjust the control surface to counteract the deviations and restore the lift to the desired value. Four strain gauges 50 ... 53 are provided, respectively at the upper and lower surfaces of each wing, adjacent to the body of the aircraft, so that when the wings are loaded the upper gauges 50, 51 are in compression and the lower gauges 52, 53 in tension. The strain gauges are preferably of the type comprising a continuous solid filament of electrical conducting material bonded throughout its effective length to the surface of the strained member, so that its length and electrical resistance vary with the strain. The four gauges are connected in a Wheatatone bridge circuit fed from an A.C. source 55, and the resultant signal, which is proportional to the total lift, is fed through an amplifier 62 to the primary winding of a transformer 64 the secondary of which is rotatable with the fore- and-aft gimbal axis 68-68. The signal transformer 64 is arranged so that its output voltage V3 is equal to the input voltage V2 multiplied to the input voltage V2 multiplied by the cosine of the bank angle as given by the vertical gyro, and is thus proportional to the vertical component of the lift. The voltage V3 is applied to one winding of the threewinding transformer 65 of a follow-up system, the other two windings being connected respectively to a velocity follow-up amplifier 67 and the divided secondary winding of a transformer 66. The amplifier 67 is phase sensitive and connected to feed a two-phase motor 78 driving a generator 77 which is connected to provide a high negative feed-back signal proportional to the motor speed, and the outer ends of the shafts of the motor and generator are associated with couplings controlled by a manually-operated link 9 whereby the system angularly displaces alternatively, either the primary winding of transformer 66 or the primary winding of a pitch signal transformer 86 of the Selsyn type. To produce a datum signal corresponding to the weight of the aircraft the latter is first set on normal level flight and the linkage 9 operated to engage the clutch connecting the rotary winding of transformer 66 to the coupled shafts of motor 78 and generator 77, with the result that motor 78 turns in one direction or the other until the voltage V4 is made equal to voltage V3. Automatic control is then introduced by operating the clutch linkage 9 to disengage the signal transformer 66, which is now held by a friction brake 11, and engage the rotor of pitch signal Selsyn 86 which, through three lines 87, is connected to a receiver Selsyn 88 with rotor mounted on the pitch axis 79 of the vertical gyroscope 69. The output signal from Selsyn 88 is fed through an amplifier 28 to a servo-motor 16 for deflecting the elevation or pitch control of the aircraft. If, through banking or downward pitching, the vertical component of the lift should fall, the voltage V3, which is a measure of the said vertical component, becomes smaller than the pre-set voltage V4, with the result that motor 78 operates the rotor of Selsyn 88 to change its signal output from its normal zero value, so that the amplified signal fed to the servomotor 16 causes operation of the elevator 29 to correct the angle of attack until the voltage V3 is again equal to V4. Specification 666,471 is referred to.