Patent GB-1196588-A (GB)
Title: Improvements relating to Turbo-fan Aircraft Engines
Inventor: WOTTON JOHN
IPC: F04D27/02
Filed: 1966-11-09
Abstract: 1,196,588. Gas turbine jet propulsion plant; axial-flow fans. BRITISH AIRCRAFT CORP. Ltd. Feb. 5, 1968 [Nov. 9, 1966], No. 50249/66. Headings F1C; F1J. [Also in Division G3] A gas turbine ducted fan aircraft engine has variable pitch fan blades 10 controlled by a pneumatic motor 15 which rotates with the fan and is supplied with motive fluid from the engine by way of a control valve 19. The engine is of the three-spool type, Fig. 1, and the fan blades 10 are driven through a connection 96 from a frusto-conlcal drum 9 carrying five stages of inwardly projecting turbine blades 91 to 95. The air supply for motor 15 is tapped from the low pressure compressor 2 via a bleed 151 and an internal passage 152, the latter communicating with a space 153 and thence via a hollow turbine blade 91 and a line 20 with the motor control valve 19. A signal air supply is also fed to valve 19 from bleed 151 via line 154, valve 155, line 156, chamber 158, hollow turbine blade 95 and line 23. The motor 15 is mounted on a bracket 97 attached to connection 96 and can adjust the pitch of the fan blades 10 by rotating a ring gear 13 relative to the fan annulus 12 via a worm 16 and pinion 17. The gear 13 is integral with a ring 14 mounted on _the annulus 12 and connected to each blade 10 by a link 18. Valve 19 has an internal spool, Fig.3, having pistons 24, 25 of unequal diameter at opposite ends co-operating respectively with chambers pressurized by a branch 21 from line 20 and by line 23. During normal forward thrust operation, valve 155, which is manually controlled by the pilot or linked to the engine power control, remains closed so that the spool of valve 19 is held in its lowest position, shown, under the action of the air pressure in line 21. In this position the lands 28, 29 on the valve spool permit air to flow from branch 22 of line 20 via port 27 to motor 15 so that the latter drives ring 14 until lug 141 on the ring abuts a stop 37. The ring 14 is held in this position by the air-pressure tending to rotate the motor further. When engine thrust is reduced to, say, 50% valve 155 is manually or automatically opened so that the spool moves upwards under the action of the pressure in line 23 to a central position, Fig.4 (not shown), in which the air pressure forces on the spool balance. The centrifugal force acting on a bob-weight 35 now acts to position the spool via a bell crank 33, link 32, beam 31 and extension rod 30 of the spool, so that land 29 covers port 27 and land 28 uncovers port 26 to allow air from line 22 to drive motor 15 in the reverse direction. Thus the blades 10 move towards a coarse pitch position, slowing the fan and reducing noise. When the blades 10 reach the desired position the follow-up action of beam 31 returns the spool to its central position so that further adjustment is prevented. When the engine speed falls to near zero r.p.m. the centrifugal force on the bob-weight is overcome by a spring 36 acting on the bell crank 33 so that the crank moves clockwise to the Fig. 5 position (not shown), causing the spool to rise and uncover port 26 permitting the motor 15 to move the blades 10 towards the feathered position. When that position is reached the follow-up action of beam 31 prevents further adjustment. Should reverse pitch of the blades 10 be required, valve 155 is manually fully opened so that the increased pressure in line 23 moves the spool upwards to the maximum extent thus fully opening port 26 so that motor 15 moves the blades into the reverse pitch position, Fig. 6 (not shown), with lug 142 abutting stop 38.