GB1114181A - Fuel control systems for aircraft - Google Patents

Abstract

1,114,181. Gas turbine plant; ram jet units. BRISTOL SIDDELEY ENGINES Ltd. May 27, 1965 [May 28, 1964 (2)], Nos.22194/64 and 22195/64. Headings F1G and F1J. [Also in Division G3] A fuel control system for a jet engine operating at supersonic speeds is characterized in that the fuel system gain, i. e. the ratio of change of fuel flow to engine condition error, is automatically varied inversely to variations in the engine gain, i. e. the ratio of change in combustion pressure to change in fuel flow. In Fig, 1, fuel flows via a main valve 22, 24 and metering valve 67, 70 to the burner ring 8 of a ram jet A. The flow is controlled by a unit C to maintain critical pressure recovery in the engine, i. e. to maintain the shock wave at position 14. Variations in the position of the shock wave are detected by reverse pitot tubes 16 which communicate with a chamber above a diaphragm 36 in unit C, the chamber below this diaphragm being subjected to a fraction of the ram pressure measured by a forward facing pitot tube 18, the fraction being determined by restrictions 40, 42. The diaphragm 36 actuates a flapper 34 co-operating with nozzles 28, 32 to control the position of the main fuel valve 22, 24, the flapper receiving a feedback signal representing the rate of change of fuel flow which acts on a diaphragm 37. To satisfy the requirements set forth in the opening statement, the feedback signal is modulated by a signal representing the fuel-air ratio; the significance of this being that variations in the fuel-air ratio are the main cause of variations in the engine gain. In the embodiment shown, the feedback signal derives from movements of a dashpot piston 130 connected to the metering valve 67, such movements displacing fuel through a restriction 134 which is varied by a plug 136 under the control of a machmeter M, changes in machmeter output being an indication of air-fuel ratio demand. Thus the change in fuel flow corresponding to an error in the position of the shock wave is dependent on variations in fuel-air ratio (engine gain). To regulate the engine thrust, a plug 12 is movable to control the nozzle area 10 by means of a ram 76. The ram 76 is controlled by a spool valve 78 controlled by a flapper 90 movable in response to variations in fuel-air ratio as measured by a diaphragm 92 subjected to the pressure difference across metering valve 67 and a diaphragm 96 subjected to ram pressure and a fraction thereof determined by a restriction 102 controlled by the machmeter. In the embodiment of Fig. 2 (not shown), the plug (136) is replaced by a plug (74) attached to a spool 52 which moves to restrict an orifice (134). The spool is controlled by a flapper (60) controlled by bellows (64, 68). Bellows (68) is evacuated and acted on by a fraction of the ram pressure determined by plug (74), and bellows 64 is acted on by the pressure difference across metering valve (67). The feedback signal to unit C by piston (130) is thus modulated by the air-fuel ratio which varies orifice (130). In Fig. 5 (not shown), the main valve B is in series with a gain valve D and metering valve F. Valve B is controlled by a critical control unit C, and valve D is controlled by a gain control sensing unit H which receives signals representing pressure drop across valve B and air-fuel ratio, the latter being measured by a computer E. In this system, if the air-fuel ratio varies, then the valve D moves to vary the pressure downstream of valve B to vary the gain thereof. A similar thrust control system is provided.