Thrust augmentation through use of contra-rotating fans is among the latest British turbojet developments.
Just revealed, the new unit, developed from the Metropolitan-Vickers F/2, is designated F/3 and develops, from the straight hot jet, 2,400 lb static sea level thrust for a weight of 1,650 lb and, with the fan augmentation, a thrust of 4,000 lb for a weight of 2,200 lb. This gain in thrust is accomplished without any fuel consumption increase, the result being that the engine has a specific fuel consumption of 0.65 lb/hr/lb thrust.
Basically the airflow is simple: Through a 9-stage axial flow compressor; into an annular combustion chamber; through a 2-stage turbine driving the compressor; through two 2-stage contra-rotating turbines driving the fans; then on out separate exhausts, the two streams joining at the jet pipe end.
Unlike most axial flow compressor casings, conventionally symmetrical in shape, the casing on the F/3 has a larger diameter at the intake than at the turbine end. It is built in four sections, which bolt together along axial flanges, with a circumferential flange at the sixth compressor stage. Stator blades 70 to a row are peened into dovetail-section slots machined into the casing.
The starter motor and oil pumps are housed in a spinner-type fairing at the intake end, with the air entering the annulus between four concentric rings attached to faired spokes to form a grid. Other accessories are mounted atop the compressor casing, the drive being from a vertical shaft set just ahead of the front thrust bearing.
Drum of the compressor is a single-piece forging with machined serrated slots in the outer surface taking the turbine blade roots, which are spaced by distance pieces and peened in place. There are 68 blades in each compression stage, all similar in design.
Bolted to a flange inside the compressor drum at the third stage is a conical diaphragm, the apex of which is attached to the front thrust bearing and which is splined to transmit power through a quill shaft to the accessories.
Also bolted to a flange at the downstream end of the compressor drum is another, but more elongated, cone which extends aft inside the annular combustion chamber, ending in an internally-splined sleeve that runs in the turbine bearing and houses the turbine shaft.
At the upstream end of the combustion chamber, a deflector ring divides the air into two annular streams, directing most of it around the flame tube, which is housed concentrically within the chamber. Both inner and outer walls of the chamber contain 80 wedge-shaped inlets which are set in two staggered rows of 20 inlets each to admit the air into the flame tube downstream from the burners. Just downstream from the deflector ring is a circumferential end plate pierced by rings of small holes to meter and set up turbulence in primary air from the compressor. The 20 burners are plain shrouded tubes set 18° apart with nozzles forcing fuel upstream at 650 psi.
The combusted gas moves through 56 turbine nozzle vanes; through the first stage of the compressor turbine which has 80 blades; then through a ring of 84 stator blades and through the second 80-blade stage of the turbine compressor.
This compressor turbine has a single disk with a T-shaped rim supporting both rows of blades, which are anchored by bulb type roots locked in place by peening.
Immediately downstream from the turbine compressor are two contra-rotating two-stage turbines for driving the fans. Disks of both fan turbines are swept forward, this to allow room for bearings at the hub and to get the blades close to the compressor turbine. Each fan turbine is carried on a pair of roller and ball thrust bearings, both sets running on a common shaft supported by a six-point tubular brace, which also supports the fan ducting and jet cone.
First row of blades of the first fan turbine are set just forward of the disk, with the fan blades being mounted on a ringed shoulder extending out from the blade tips. The second row of turbine blades is mounted on the downstream edge of an extension of the disk rim. The downstream row of the second fan turbine's blades are set in the rim right over the disk, with the upstream row being supported from the outside by a crown extending forward from the second row blade tips, this shoulder also supporting the second row of fans.
Thus the four rows of fan turbine blades are nested, with one row of each being interposed between the rows of the other. Since the blades are contra-rotating, the need for stator blades is eliminated. The first row of fan blades rotates at 2,850 rpm, the second at 2,300 when the compressor turbine is turning at its maximum of 7,600 rpm.
Air intake for the fans is an annulus, with inlet guide vanes located around the turbine compressor. Annulus overall diameter is 48 in. Air from the fans is exhausted through an annular tapered throat which surrounds the jet gas stream, but is separated from it by a double-skinned wall which keeps the two air streams isolated until they meet at the jet orifice.
Cooling air some of which is also used for lubrication is taken from the fourth, sixth, and final compression stages and from the secondary air from the combustion chamber.
Radial holes drilled in the compressor rotor at the fourth stage take air inside the rotor, where it flows through the conical extension to energize an air-oil mixer to provide an oil spray for the turbine bearing. This spray is then evacuated through the cavity between outer and inner cone extensions, to be gathered in a circumferential duct around the outside of the compressor outlet, where it is vented to the atmosphere. External piping also supplies air for a similar air-oil mixer used to lubricate and cool the front bearing, the drain to the atmosphere being through a pipe inside bottom spoke of the air-intake shroud.
Additional air is bled off at the sixth compressor stage and ducted through the tubular members of the six-point fan-support frame to the interior of the hollow turbine shaft. Part of this air supply goes through an air-oil mixer to lubricate and cool the fan turbine bearings, the remainder being used to cool the downstream face of the compressor turbine disk and the upstream face of the forward fan turbine disk. Spray from the fan turbine bearings is exhausted into the tail cone bullet, escaping into the jet stream at the jet orifice.
Air from the final compression stage is bled through a channel between outer and inner cone extension to the front face of the compressor turbine disk, where a deflector plate directs it toward the center of the disk, from where it flows radially outward into the jet stream past the blade roots.
Some of the supply of secondary air for the combustion chamber is bled off to cool turbine blade tips and both roots and tips of stator blades, and also to cool the fuel nozzles. This latter air is taken into the shroud near the root and exhausted through vent holes near the nozzles.
The F/3's fuel system is conventional, the fuel flowing from the tanks to a low-pressure filter to the suction side of the engine-driven pump, which sends it to the throttle valve. From here it is taken to a centrifugal governor and then to the main high pressure valve and the manifold. The now-usual barostat to regulate fuel flow according to altitude is used, being fed by a line from the pump. Another line, taken off just before the throttle valve, supplies a solenoid and two starting igniters set just off center at the top of the combustion chamber.
|Specifications and Data|
|Thrust, max SL, with augmentation||4,000 lb|
|Thrust, max SL, jet alone||2,400 lb|
|Weight, with augmentation||2,200 lb|
|Weight, jet alone||1,650 lb|
|Specific fuel consumption (with aug)||0.65 lb/hr/lb thrust|
|Thrust/frontal area, jet alone||321.5 psf|
|Thrust/frontal area, with aug||318 psf|
|Max speed, turbine compressor||7,600 rpm|
|Max speed, first fan turbine||2,850 rpm|
|Max speed, second fan turbine||2,300 rpm|
|Length||13 ft plus|
|Max diameter, with aug||48 in|
This article was originally published in the June, 1946, issue of Aviation magazine, vol 45, no 6, pp 66-67.
The original article includes 1 photo, 1 cutaway drawing and 1 flow diagram.
Photo is not credited; cutaway credited to Max Millar, "Redrawn from Flight; diagram credited to Flight