Patent GB-842979-A (GB)
Title: Equipment for determining the centre of gravity of an aircraft
IPC: G01M1/12
Filed: 1956-08-20
Abstract: 842,979. Centre of gravity determining. CONTINENTAL ELECTROLOG CORPORATION. Aug. 20, 1956, No. 25386/56. Class 40 (1). [Also in Group XXXVI] Equipment for determining the position with respect to a given reference datum of the centre of gravity of an aircraft having plural sets of supporting gear comprises an adjustable variable impedance means for each set of supporting means to a value corresponding to the weight on the associated supporting gear, a further impedance means for each set of supporting gear of value corresponding to the distance of the particular supporting gear from the reference datum, an additional impedance means, and a self-balancing impedance circuit associated with each said set of supporting - gear; the latter including the appropriate adjustable variable impedance means, further impedance means and additional impedance means, together with a fixed impedance means, connected so that the additional impedance means is adjusted at balance to a value representing the product of the adjustable and further impedance means and thus the moment of the weight on the associated set of supporting gear with respect to the given reference datum; there being provided additively-connected impedance means controlled by the settings of such several additional impedance means to a value corresponding to the sum of such moments, a total weight impedance means controlled by the means setting said adjustable variable impedance means to a value that corresponds to the total weight of the aircraft, and a self-balancing impedance circuit including such additively-connected resistance means, total weight impedance means, a fixed impedance means and a rebalancing impedance means, whereby the latter is adjusted at balance to a value representing the quotient of the additively-connected impedance means representing the sum of the moments and the total weight impedance means, as a measure of the distance of the C.G. from the reference datum. In Fig. 1 the nose and main wheels of an aircraft are respectively supported on platforms A, B, C of weighing scales operating multiple resistance bridge strain gauges 11 energized from secondaries of transformers 12 with the outputs of each platform group connected in series and the groups of platforms B, C also series-connected to produce voltages across terminals 15, 16 representing the nose wheel weight and across 18, 19 representing the total main wheel weight. Secondaries 25, 29 energize paralleled resistors 22, 23 and 26, 27, respectively, of which 22, 26 are centre-tapped and 23, 27 are variably tapped to derive a voltage connected in series opposition to the weight voltage across terminals 15, 16 to energize a servo-system 34, 36 adjusting the tap on resistor 23 for balance and operating a nose-wheel weight indicator 37; the reading in the absence of weight on platform A being zeroized by adjustment of the tap on resistor 27. The combined weight voltage of platforms B, C at terminals 18, 19 is automatically balanced in a similar circuit having primed reference numerals; servo-system 34<SP>1</SP>, 36<SP>1</SP>, adjusting resistance 23<SP>1</SP> for balance and driving mainwheel weight indicator 37<SP>1</SP> which is zeroized in the absence of weight by tapped resistance 27<SP>1</SP>. A mechanical differential gear 58 receives inputs from the servo-system drives to adjust variable resistance 63 to represent the total wheel weight, which is connected in series with variable resistance 66, manually adjusted by dial pointer 71 representing hypothetical gross crew and passenger weight, as one arm of bridge 65 completed by resistances 76, 77 and by rebalancing variable resistance 73 driven by unbalance responsive servo-system 81, 82 which also drives dial pointer 86 and variable resistance 85 in accordance with the gross aircraft weight. Servo-system 34, 36 also varies resistance 88 in accordance with the nose-wheel weight in series with a variable resistance 91, adjustable manually by dial pointer 93 in accordance with hypothetical crew and passenger weight on the nose-wheel, as one arm of a centre of gravity bridge 98 completed by gross weight variable resistance 85, variable balancing resistance 104, and an interchangeable resistance unit 107 corresponding to specific aircraft types (containing a fixed wheelbase representative resistance 117 and an off-level angle compensating resistance 118 variable by dial pointer 123) in series with a wheelbase correction resistance 111 variable by dial pointer 114 according to small variations. Resistance 118 is adjustable manually or by a servo system 131, 150 responsive to a level gyro 125 in accordance with the angle α of the aircraft axis A to the horizontal to a value representing (Fig. 2). The unbalance responsive servosystem 127, 128 adjusts resistance 104 for balance at a value representing the distance A of the aircraft centre of gravity from the main wheels, given by A/WB = NW/TW in which WB is the corrected wheelbase, NW is the nosewheel weight, and TW is the aircraft total weight corrected for passengers and crew. The servo system may drive a C.G. position indicator (not shown), or may correspondingly vary a resistance 134 in series with resistance 138 and balancing resistance 135 as one arm of a bridge 132 completed by resistances 145, 146 and interchangeable resistance unit 144 (representing the distance of a required reference datum from the main wheels of a specific aircraft) in series with a resistance 162 varied by dial pointer 165 in correspondence with minor variations thereof. Unbalance responsive servo system 147, 148 varies resistance 135 to balance and drives dial pointer 152 indicating the position of the C.G. referred to the required datum. In a modification (Fig. 3, not shown) the centre of gravity bridge is rearranged to compute the position of the C.G. referred to the nose wheel. In a further modification (Fig. 6) the position of the C.G. may be computed with reference to any reference datum for all types of aircraft (e.g. the nose). Servo systems 33, 33<SP>1</SP> (Fig. 1) respectively adjust resistances 197, 197<SP>1</SP> to the compensated nose and main wheel weights, in series with resistances 199, 199<SP>1</SP> adjustable by dial pointers 204, 204<SP>1</SP> to values representing the moments of hypothetical crew and passengers about the nose of the aircraft, as one arm each of respective bridges 196, 196<SP>1</SP> which are completed by balancing resistances 221, 221<SP>1</SP>, fixed resistances 206, 206<SP>1</SP> and series variable resistances 207, 208; 207<SP>1</SP>, 208<SP>1</SP>, of which 207, 207<SP>1</SP> are adjusted by dial pointers 213, 213<SP>1</SP> responsive to the respective distances of the datum from the nose and main wheels and of which 208, 208<SP>1</SP> are manually or automatically adjustable in response to the angle between the aircraft axis and horizontal. Balancing resistances are adjusted by servosystems 224, 225; 224<SP>1</SP>, 225<SP>1</SP> to values representing respectively the compensated moments of the nose wheel and main wheel weights about the aircraft nose. Resistances 227, 227<SP>1</SP> in series in an arm of C. of G. to bridge 188 are similarly varied, and the drives 33, 33<SP>1</SP> from servo systems 34, 36 and 34<SP>1</SP>, 36<SP>1</SP> representing the nose and main wheel weights are combined in mechanical differential 183 to vary resistance 186 of the C. G. bridge in accordance with the total wheel weight. Resistance 187 is connected in series with resistance 189, varied by dial pointer 195 in accordance with a hypothetical passenger and crew weight to form another arm of the centre of gravity bridge which is completed by arms comprising a fixed resistance, a variable resistance 231 driven by the bridge balance servo-system 234, 235 together with C. G. position dial pointer 236. At balance, the latter indicates the compensated distance of the C. of G. from the aircraft nose. Windage may be compensated for its effect on the measured weight of the aircraft by constructing the variable resistances representing the nose wheel and total wheel weight, the main wheel and total wheel weight, and the nose and main wheel weight respectively so that the stators may be manually adjusted to increase or decrease the effective resistance values by amounts dependent on wind direction and velocity. The variable resistances are stated to be replaceable by variable capacitances or inductances. Specification 770,311 is referred to.