Patent GB-834245-A (GB)
Title: Electrical analogue computer applicable to determination of the angle of attack of an aircraft
Inventor: SHANK CLIFFORD ASBURY
IPC: G06G7/80
Filed: 1956-06-07
Abstract: 834,245. Electric correspondence control systems. GARRETT CORPORATION. June 7, 1956, No. 17591/56. Class 40 (1). [Also in Group XXXVI] An electric analogue circuit for computing the angle of attack alphaFRL of an aircraft between the fuselage reference line and the direction of flight from the equation alphaFRL=WN#/SPti + #, wherein W = aircraft weight; N = acceleration normal to the fuselage reference line; S = wing area; Pti = indicated total ram pressure; 0 = wing lift slope as a function of Mach or pressure ratio; # = zero lift coefficient as a function of Mach or pressure ratio; M = Mach number comprises a main circuit receiving an-electric signal voltage from a radar unit and includes plural resistance multiplying sub-circuits which vary the signal voltage in response to the variable factors of the equation independent of pressure ratio or Mach number but dependent on other flight variables, and a further subcircuit or sub-circuits comprising variable resistance multiplying means operated by a servo system sensitive to pressure-responsive means so as to be varied in accordance with a function or functions of the ratio of ambient differential pressure to ambient ram pressure and to introduce into the computation a variable or variables dependent on pressure ratio or Mach number for the aircraft operational conditions. In Fig. 3, a self-balancing bridge energized from winding 121 of power transformer 123 comprises potentiometer 120 with a fixed tap 125 connected to a contact 126 slidable in response to total ram pressure Pti to energize a potentiometer 127 with a contact 128 slidable in response to differential pressure Qc so that the effective bridge resistance varies in accordance Qc with the pressure ratio --- (or with the Mach Pti number of the aircraft velocity). The unbalance voltage between the slider 128 and the slider 119 of bridge balancing potentiometer 118 is transformer coupled to three-stage amplifier 132 driving the grids of triodes 147, 151 whose anodes are energized in antiphase from a winding 154 of transformer 123 through windings 142, 143 of relays whose contacts 168, 170 and 169, 171 are normally open and contacts 140a, 138; 141a, 139 are normally closed, while contacts 138, 140b and 139, 141b are normally open, so that field coils 136, 137 of reversible D.C. motor 33 are both de-energized at bridge balance. Unbalance develops a grid voltage corresponding in phase sense, so that either triode 147 or 149 operates corresponding relay 142 or 143 to energize the field winding 136 or 137 for motor rotation in the sense appropriate to drive the slider 119 into a position of balance; the remaining field winding being meanwhile dis. abled, and the operated relay releasing when balance is attained. Limit switches 145 in series with the motor fields open to prevent excessive excursions of the rebalancing slider while the grids of triodes 147, 151 are negatively biased from potentiometer 162, 163 energized by rectifying the output of transformer winding 158, and hunting is prevented by superimposing on the standing bias voltage a further negative bias from potentiometer 166 over contacts 168, 170 or 169, 171 closed on energization of the appropriate control relay, so that after a time delay set by capacitance 172, resistors 163, 164, 162 and potentiometer 166, and by the magnitude of the unbalance signal the conducting triode will be biased to cut off and will reconduct after a time delay on release of the appropriate relay, so that the motor drives slider 119 towards the balance point in a series of impulses whose duration and frequency are determined by the unbalance magnitude, as described in Specification 687,154. The motor rotation is dependent on the ratio Qc -- (proportional to M) and actuates cams 32, Pti 321 (Fig. 2) to adjust potentiometers 28, 28<1> alternatively receiving input signals to be corrected for angle of attack, derived e.g. from a radar, over selector switch 29, in correspondence with different values of # as a function of Qc Mach or of the pressure ratio ---. Switch 29 Pti allows compensation for two distinct sets of operating conditions (e.g. the use or non-use of wingtip fuel tanks). Cathode follower triode 25 couples the output to potentiometer 47 adjustable in accordance with the weight of the aircraft, and its output is coupled over feedback amplifier 40, 41, 42, 43 and cathode transformer 50 to potentiometer 58 having a slider 61 adjustable in response to the indicated total ram pressure Pti, an intermediate tap 60 connected to the slider, and a grounded tap 59 ; the lower end and the slider being connected to energize potentiometer 62 having slider 64 adjusted in response to the resolved acceleration N, which develops a signal energizing cathode follower 53 having a load potentiometer 66 whose slider is adjusted in accordance with wing area S to energize feedback amplifier 54, 55, 56, 57 with a signal proportional to the product of the input NW# and a factor --. SPti The input is also supplied to transformer 70 whose output energizes, over series variable resistor 72 adjusted in accordance with an added correction factor K representing the fixed angle between the angle of attack and the centre line of a rocket discharger attached to the aircraft; a potentiometer 73 having a slider 74 adjusted by cam 75 driven by motor 33 in accordance with the zero lift coefficient # as a Qc function of M or --, and the resultant signal Pti proportional to the input by factor (K + #) is applied through cathode follower 89 to one end of the primary of transformer 100 for algebraic combination with the output signal of cathode follower 57 applied to the other end. Spaced taps 76, 77 of potentiometer 73 are alternatively connected to earth by switch 79 ganged with switch 29 for operation by solenoid 84 and control switch 86 to compensate # for the abovementioned two distinct sets of operating conditions affecting #. The output signal of transformer 100 representing the input multiplied by factor energizes variable resistor 108 adjustable in response to rocket muzzle velocity V0 as a function of rocket temperature, in series with resistor 110 and variable resistor 105 adjustable by cam 107 driven by motor 33 in response to true airspeed VA. The junction of resistors 110, 105 is connected to feedback amplifier 102, 103, and cathode follower 104, whose output represents the input signal multiplied by a factor Ground check of calibration is effected by shorting the slider potentiometer 73 over presser switch 175 to remote # from the computation, after which the computer input and output are determined by known methods, and potentiometer 66 is varied to correct the computor thereto. With switch 175 released, the value of K may be corrected by adjustment of resistor 72.