Patent GB-814758-A (GB)
Title: Improvements in or relating to static pressure compensator
IPC: G01P5/175
Filed: 1956-08-08
Abstract: 814,758. Electric indicating systems. NORTH AMERICAN AVIATION Inc. Aug. 8, 1956 [Aug. 16, 1955], No. 24276/56. Class 40 (1). Electrical apparatus for determining the true free-stream static pressure Ps in an aircraft from measurements of the apparent static pressure Psi and total pressure Pt derived from a pitot head comprises a computer utilizing the predetermined relationship between these quantities and the flight Mach number M as obtained by tests on the particular type of aircraft having this particular position for the pitot head, and the standard relationships between static pressure Ps, dynamic pressure #P and Mach number M. By a rearrangement of these relationships, an equation is derived relating the static pressure error #Ps divided by the true static pressure Ps and a specific function of the Mach number M, which function is exhibited on a non-linear potentiometer resistor fed with appropriate D.C. voltages on a series of taps, so that as the slider is moved in accordance with the Mach number M, the voltage at the slider is proportional to #Ps/Ps, and is used to determine the value of Ps from the indicated static pressure Psi. As shown (Fig. 2), the apparent static pressure Psi obtained from the pitot head via tube 4 is modified in a static pressure controller 6 (described below) by the introduction of a signal proportional to the static pressure error #Ps to produce the true static pressure Ps in tube 7, which is converted in pressure transducer 11 (described below) into an equivalent electrical signal. The total pressure Pt obtained from the pitot head via tube 5 is fed into another pressure transducer 15 together with the true static pressure Ps in tube 7 to give a signal proportional to the dynamic pressure #P. These two signals are fed into a Mach number computer 12 (described below), where they are combined in known relationship to give a Mach number signal which positions the slider 1 of potentiometer 2 whose voltage distribution represents the specific function of M to give a voltage at the slider 1 indicative of #Ps/Ps. This voltage is fed across a linear potentiometer 14, whose slider 13 is positioned in accordance with the static pressure signal Ps, whereby the voltage at the slider is indicative of #Ps. Finally this signal #Ps is the one introduced into the controller 6, which ensures the correct relationship between the indicated static pressure Psi, the true static pressure Ps and the static pressure error #Ps, so that the pressure in tube 7 is controlled at the true static pressure, and may be used for the accurate determination of the flight Mach number M. Pressure transducer 11, 15 (Fig. 3).-A Bourdon tube 22 is subjected internally to a pressure coupled to opening 20 and externally to a pressure coupled to opening 21, whereby the pressure difference causes the end of the tube 22 to rotate armatures 23, thereby unbalancing a bridge circuit including coils 24, 27. The bridge unbalance current is amplified to drive a motor 29 which, through gears 30, twists a torsion spring 31 also attached to the armatures 23 to reposition them and rebalance the bridge, the rotation of the motor 29 being proportional to the twist in the Bourdon tube 22, and therefore to the pressure differential at openings 20, 21, and is indicated by the corresponding movement of the slider of a potentiometer 33, so that the voltage at the slider is indicative of the pressure differential. Mach number computer 12 (Fig. 5).-Signals representing the static pressure Ps and the dynamic pressure #P energize solenoids 40, 41 respectively to position armatures controlling the angular rotation of a cam shaped so that the movement of a rack 44 is proportional to the Mach number M based on the standard relationships between these quantities. The angular rotation is transmitted to the slider 1 of potentiometer 2 by a selsyn link. Static pressure controller 6 (Figs. 6, 7).-The tubes 4, 7 carrying the apparent static pressure Psi and true static pressure Ps respectively are interconnected by an orifice 51 and a loop 50 containing a pump 52. The difference #P<SP>1</SP>s between these pressures is determined by a capacitor transducer comprising fixed electrodes 55, 56 and diaphragm'54, acting as two of the arms of a bridge 61, the unbalance current being amplified at 63 and phase detected at 64 to give a signal voltage whose magnitude and sign are indicative of the pressure unbalance #P<SP>1</SP>s, which is compared at 58 with the signal voltage #Ps from the computer 10. Any difference is amplified at 59 to control the speed of motor 60 driving pump 52 to modify the pressure Ps in tube 7 until #P<SP>1</SP>s and #Ps are equal. In an alternative pressure controller the unbalance voltage from comparator 58 is used to adjust the position of an electromagnetic valve in loop 50.