898,509. Measuring electrically. BRISTOL SIDDELEY ENGINES Ltd. Nov. 30, 1960 [Dec. 1, 1959], No. 40798/59. Class 40 (1). In an apparatus for estimating the gross thrust of an aircraft jet propulsion unit a signal is derived which is a function of (KPr-Pa), where K is a constant dependent on the jet nozzle geometry, Pr is the static pressure on the upstream side of the jet propulsion nozzle and Pa is the ambient pressure. This signal may be multiplied by the same function of the area of the jet nozzle at its outlet plane (An) if this area is adjustable. In addition, in order to obtain a signal which varies less with changed altitude and forward speed, the gross thrust signal may be divided by the appropriate function of the impact pressure, Pi, due to the forward speed of the aircraft. In the preferred embodiment a force proportional to (KPr-Pa) is applied to a lever 13 by the arrangements of bellows 15 and 16, bellows 15 being evacuated, and bellows 16 being connected at 18 to the upstream side of the jet propulsion nozzle 11. Bellows 16 is K/K - 1 times the size of bellows 15. The bellows 21 and 23 provide a rebalancing force on the lever 13 proportional to the impact pressure Pi as sensed by the pitot tube 19 and communicated to bellows 21, bellows 23 being evacuated. The lever is kept in balance by sensing deflections of the lever 13 by switches 32 and 33 which energize motor 29 and turn lead screw 28 to move the pivot 26 and hence the point of application of the rebalancing force derived from Pi to the position for equilibrium of the lever 13. It is shown that the position of the pivot point 26 then bears a square root relation with the KPr - Pa desired function of The position of Pi the pivot 26 is signalled by potentiometer 34 to a multiplier 36 whose other input is a signal derived from potentiometer 35 coupled to the nozzle mechanism so as to give an output proportional to the square root of the nozzle area. The indicator 37 may then be suitably calibrated.
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