Patent GB-1184779-A (GB)
Title: Control for Ducted Fan Engine
IPC: F02K1/18
Filed: 1968-04-18
Abstract: 1,184,779. Gas turbine jet propulsion plant. UNITED AIRCRAFT CORP. April 18, 1968 [April 21, 1967], No. 18379/68. Heading F1J. [Also in Division G3] A gas turbine by-pass engine having a variable area propulsion nozzle at the exit end of the bypass duct has means controlling the nozzle actuator in response to the difference between actual and desired ratios involving pressure, the actual ratio including measurement of at least the difference between the inlet and outlet total pressures of a ducted fan of the engine. The purpose of the invention is to maintain the corrected air mass flow through the engine constant once the cruising speed of the aircraft (10, Fig. 1, not shown), in which the engine is mounted has been selected, by control of the nozzle 38 without further adjustment of the variable area intake (14). In one embodiment, a function representative of the actual air mass flow through the engine is computed in unit 42 by measurement of the ratio of total pressure rise across a fan 18 as sensed by probes 44, 48 to the difference between fan exit total and static pressures as sensed by probes 48, 52. This function is compared with a function representative of the desired air mass flow, involving the rotational speed N of the compressor 20 and the engine inlet temperature T 2 , and the difference effects adjustment of the nozzle rams 40 to restore the mass flow to the desired value. The fan 18 and compressor 20 are mounted on separate shafts driven respectively by turbine sections 26, 24. The computing unit 42 includes a housing 62 containing a bellows 68 which measures the fan total pressure rise (P t3f -P t2 ) and a bellows 70 which measures the difference between the fan exit total and static pressure (P t3f -P s3f ). The summation of the signals produced by the bellows is transmitted through a pivotal lever 90 to control fluid leak-off through an orifice 92 at the end of a pipe 94 supplied with pressurized fluid by a pump 100, thus controlling the pressure drop across a restriction 96 in the pipe. The controlled fluid-pressure is in turn transmitted through line 101 to chamber 102 to act on a piston 63 at the end of a rod 64, the latter carrying a feedback cam 104 which co-operates with a plunger 106 carrying rollers 84. The repositioning of rollers 84 changes the effective moment arms of levers 86, 80 in such manner that when the forces produced by the bellows are balanced the leak-off area of orifice 92 resumes its null position and the new established position of piston 63 will be indicative of the ratio of the signals sensed by the bellows. This ratio signal, representative of the actual air mass flow, is then applied to an adding bar 66 via rod 64 where it is compared with the desired mass flow represented by the profile of a three dimensional cam, the latter being positioned as a function of compressor speed and inlet temperature. If there is a discrepancy between the actual and desired signals, the adding bar 66 operates a spool valve 110 to adjust nozzle rams 40 until the mass flow is restored to the desired value. In a modification, the total pressure at the discharge of the by-pass duct 30 is measured instead of fan discharge static pressure. When, however, the duct 30 is provided with burners 32, the total pressure is measured in the region of the burners rather than at the duct discharge.