GB893565A - Improvements in or relating to computing apparatus - Google Patents

Abstract

893,565. Measuring airspeed. KELVIN & HUGHES Ltd. May 20, 1960 [June 3, 1959], No. 18972/59. Class 40 (1). [Also in Group XXXVI] The true airspeed of an aircraft is computed by an automatically - balanced Wheatstone bridge having diametrically opposite arms varied in accordance with functions of Mach number and stagnation temperature respectively, the latter being driven by the rebalancing means of a further bridge. The Mach number signal is preferably derived from measurements of pitot pressure (P) and static pressure (S) in force balance systems as in Specifications 889,594, and 889,595, giving outputs representing S and (P - S). These systems are shown at 10 and 11 in Fig. 1, the mechanical outputs being transferred by log cams 16 and 17 to a differential synchro 20 which thus gives an electrical P-S output representing log - A further S synchro 22 (with mechanical feed-back) and cam 28 (to compensate for errors characteristic of the particular aircraft) provides a mechanical input representing Mach number (M), and this drives one of the variable resistors in the A.C. bridge 31 (details in Fig. 2, not shown). A further automatically-balanced bridge contains a temperature probe 35 (e.g. platinum resistance thermometer) and two ganged variable resistors driven by the rebalancing motor, whose degree of movement therefore represents the square root of the stagnation temperature (Ti). This mechanical signal drives the second variable resistor in the bridge 31 and the output at 39 is thus proportional to the product of the two inputs, i.e. the square root of stagnation temperature and the desired function of Mach number provided by the cam 28. This output synchro or potentiometer to give an indication of airspeed, or can be fed into a mechanical integrator to show distance flown. The function of Mach number (M<SP>2</SP>/1 +M<SP>2</SP>K) can be controlled by arranging a replaceable fixed resistor in parallel with that driven by the cam 28 (Fig. 3, not shown), in which case the bridge has two opposite arms simultaneously adjusted for rebalance, so that it computes the airspeed as the square root of the product of stagnation temperature and function of Mach number squared. According to the Provisional Specification the temperature can be sensed by a thermocouple.