Patent GB-951441-A (GB)
Title: Flight instrument correcting system
IPC: G01K7/20
Filed: 1960-04-25
Abstract: 951,441. Aircraft instruments; Mach number meters; indicating-apparatus. KOLLSMAN INSTRUMENT CORPORATION. April 25,1960 [May 6, 1959], No. 14461/60. Headings G1J, G1L and G1N. The invention is described in connection with an aircraft indicating system in which the pilot and co-pilot have separate instruments each of which can be operated from either of two static pressure sources. Each set of instruments comprises (1) a combined indicated airspeed and angle-of-attack meter, (2) an altimeter, (3) a Mach number meter, (4) a true-outside-temperature indicator, (5) a true-airspeed indicator, and (6) a ram-air-temperature indicator. Instruments (4)-(6) are operated by signals from an external thermometer and from a computer supplied with the outputs of instruments (1)-(3) two alternative computers being provided, either of which can be used with either set of instruments. Two independent static pressure sources are available for operating instruments (1)-(3) and either source can be connected to either set of instruments, the corrections necessary with each source being automatically switched into each computer. A separate sensor is provided for altitude and Mach number for each set, but there is a common angleof-attack sensor and external thermometer for the two sets. Each altimeter has its own scaleerror corrector, and the corrected readings of the two altimeters can be compared with one another to check for faulty operation. Airspeed and angle of attack (Fig. 1). The pilot's instrument 20 and co-pilot's instrument 22 each contain a dial with three indicators, two in the form of pointers 26, 28, and one as a movable segment 30 (further details in Fig. 2, not shown). Pointer 26 shows the maximum permissible speed and is operated by a diaphragm 31 subjected to external static pressure through a conduit 32. The linkage between the diaphragm and the pointer can be designed so that the limitation on permissible speed can correspond to the factor which is most important at any particular range of altitudes. Pointer 28 shows indicated air speed and is operated from diaphragm 34 subjected to the Pitot pressure internally and static pressure externally. Segment 30 is driven from an angleof-attack sensor 42 which is common to the pilot's and co-pilot's system. The sensor passes a signal to the stator of a synchro 44, whose rotor passes an error signal to a motor 48 which turns the segment 30 and also drives the synchro stator into a null position. The segment has lines corresponding to approach speed and stall speed, and is read in conjunction with the airspeed pointer 28. Altimeter (Fig.1). Each instrument 50, 52 contains a pointer 56 and drum driven from a diaphragm subjected to external static pressure through a conduit 62. The appropriate scale corrections are made by a servo system (see below), and a spring 60 returns the instrument to uncorrected pneumatic operation when electrical power is removed. The altimeter is the subject of Specification 871, 042. Mach number meter (Fig.1). Each instrument 70, 72 has a diaphragm 74 subjected to external static pressure through a conduit 76 and a second diaphragm 78 responsive to the difference between this pressure and that from a pitot tube 36. The diaphragm 78 (Fig.3) moves a sliding bracket 86 carrying an arm 88 which engages a further arm 90, so that expansion or contraction of the diaphragm causes rotation of a shaft 92 against a spring 98. This rotation is communicated by means of arms 100,102 to a further shaft 104 which is also biased by a spring 108. Shaft 104 carries a gear sector 106 meshing with a pinion 110 on the shaft of the pointer 112 (82 Fig. 1). The static pressure diaphragm 74 moves an arm 116 (Fig.3) to cause rotation of an arm 118 which engages the end of the shaft 92 and moves it axially. Expansion or contraction of the diaphragm 74 thus alters the point of engagement of arms 100 and 102 and modifies the action of diaphragm 78 on the pointer so that the latter is operated in accordance with the correct function of the two pressures to indicate Mach number. Computer (Fig. 1). Each unit 120, 122 is supplied with signals from the angle-of-attack sensor and the corresponding Mach number meter, and derives inputs for instruments (4)-(6) above, and also provides static error corrections for the altimeters. Each computer contains two cams 124, 126, each comprising a disc 300 (Fig.4) profiled on its upper surface in accordance with the flight characteristics of the aircraft, as described in Specification 772, 588. The angular position of the disc corresponds to some function of Mach number, while the radial position of the follower 304 is determined by some function of airspeed. The vertical position of the follower thus gives an indication of the required correction. The primary static pressure system has inputs 128, 130 for the pilot's and co-pilot's instruments respectively, and the alternative static pressure system has corresponding inputs 132, 134. By means of selectors 136, 138 the pressure inputs to the above described instruments can be joined to either pressure system, and the change-over valves are ganged with switches 144, 146 which select the outputs of the appropriate cams 124, 126. The scale-correctors for the altimeters 148,150 each contain a cam 152 calibrated in conjunction with the particular altimeter and adjusted by a motor 154 operated from a synchro 156 attached to the corresponding altimeter pointer. The cam follower 160 adjusts a differential synchro which adds an error signal from the cam 124 or 126 in the computer and causes a motor 165 to rotate the case of the altimeter to apply the necessary correction, as described in Specification 869, 886. The cams 124 and 126 are rotated by the signal from a synchro 114 on the shaft of the Mach number meter via a further synchro 161 and motor 164. The cam followers 172, 174 are similarly positioned by a motor 168 energized from the angle-of-attack sensor 42 via a synchro 166. The rotations of the cams 152 in the pilot's and co-pilot's systems should be equal, and these rotations are compared in a device 260 which operates a warning light 262 if there should be an error in either system. The shafts of cams 124 and 126 in the two computers also adjust the sliders of resistors 264, 266 and 268 which are graded to supply output signals which are the particular functions of Mach number necessary to operate the remaining instruments described below. Selector switches 270, 274, 276 connect each one of these instruments to either computer 120 or 122. True outside air temperature indicator (Fig. 1A). An external thermometer bulb 194 is in parallel with one arm of a bridge 196 which also contains the computer resistor 264 via the selector switch and lines AB. Any unbalance energizes a motor 202 which rebalances the bridge at 208 and adjusts the indicator 190 accordingly. Synchros 204 and 210 carry the signal to the co-pilot's indicator 188. The indication will be T/(1 + 0.2K 1 M<SP>2</SP>) where T is the temperature measured by the bulb 194, M is the Mach number and K 1  is a constant. True airspeed indicator (Fig. 1A). A bridge 220 has in one arm a resistor 214 which is set by the motor 202 in accordance with the square root of the true air temperature, and in the opposite arm the computer resistor 266 via the selector switch and lines CD. Any unbalance energizes a motor 226 which rebalances the bridge at 218 and adjusts a digital indicator 230 accordingly. Synchros 228 and 232 carry the signal to the copilot's indicator 234. The indication will be 38.94M#T 1 , where M is the Mach number and T 1  is the true air temperature. Ram air temperature indicator (Fig. 1A) A bridge 240 has in one arm a resistor 216 which is set by the motor 202 in accordance with the true air temperature, and in the opposite arm the computer resistor 268 via the selector switch and lines EF. Any unbalance energizes a motor 246 which rebalances the bridge at 242 and adjusts an indicator 250 accordingly. Synchros 248 and 252 carry the signal to the co-pilot's instrument 238. The indication will be T 1  (1 + 0.2K 2 M<SP>2</SP>) where T 1  is the true air temperature, M is the Mach number and K 2  is a constant. Specifications 951,442, 951,443, 951,444 also are referred to.