Patent GB-797937-A (GB)
Title: Stall preventing means for aeroplanes
IPC: B64C1/00
Filed: 1956-12-06
Abstract: 797,937. Aircraft stall preventing means. SPERRY RAND CORPORATION. Dec. 6, 1956 [Dec. 29, 1955], No. 37348/56. Class 4. [Also in Group XXXVIII] An aircraft stall preventing means comprises computing means for deriving, from measures of the dynamic pressure and the aircraft mass, a signal representing the maximum permissible load factor, accelerometer means supplying a signal representing the actual load factor, and means operative to increase the air speed when the actual load factor signal approaches in value the maximum permissible load factor signal. One or other of the two signals may be modified by a safety factor. In the embodiment described, the maximum lift/weight ratio is computed as 9SC L /W, where 9 is the dynamic pressure, S the wing area, C L  the lift coefficient, and W the weight. A bellows 2 subject internally to pitot pressure and externally to static pressure operates an inductive pick-off 10, the output of which controls and I.A.S. indicator 16<SP>1</SP> and is applied through potentiometer 18 across potentiometer 22. The former is manually set to suit the individual aircraft, and the wiper 28 of the latter is actuated together with the wing flaps 20, to correct for changes in C L . The signal applied to the primary of a transformer 61 thus represents 9SC L . The weight of the aircraft is derived by a differential 48, one input of which is manually set to the gross take-off weight. The other input effects deductions due to fuel consumption and bomb releases, being driven by a servo motor 46 which also drives the wiper of a potentiometer 50 deriving a positional feed back signal as one input to the amplifier 44 controlling the motor. The control input to amplifier 44 is derived by a potentiometer arrangement, of which one section 34 is controlled by a fuel level indicator 37, and the other sections 42 are shorted out by switches 40 as the bombs are released. The differential output drives a weight indicator 56 and the wiper 60 of the variable turns primary of transformer 61, whereby the output on leads 64, 66 represents 9SC L /W. The actual lift/weight ratio is measured by an accelerometer comprising a weight 68 normally balanced by a spring 74, and connected to rotate the rotor 76 of a synchro 78 relative to the stator 80, to derive a signal proportional to the vertical acceleration. This is increased by, say, ten per cent by a transformer 86 as a safety factor, and then algebraically subtracted from the signal on lines 64, 66 through transformer 94. The resultant is fed to a demodulator 96 and a rectifier 98, so that if the modified actual lift/weight ratio exceeds the maximum lift/weight ratio, a signal proportional to the excess is fed through amplifier 100 to a motor 102 which advances the throttle 104 at a corresponding rate, thus producing a gain in airspeed to avoid stall. The excess signal is also passed through a rate network 106 to the elevator servo amplifier 107, so that as the excess signal rises, the elevators are depressed, the resulting dive giving a gain in airspeed more quickly than the throttle advance. When the excess signal falls off as the maximum lift /weight ratio increases with airspeed, a reversed rate signal raises the elevators back to the original position.