FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1946
1946 - 0817.PDF
APRIL 25TH, 1946 FLIGHT 421 METRO-VI CK to form a nozzle ring to the movingturbine blades. These last art- mounted in two rows of 80 each onthe turbine disc and are separated by a ring of 84 stator blades inter-posed between them. This, then, comprises the first turbine, the solepurpose of which is to drive the compressor, and the maximumspeed of which is 7,600 r.p.m. Immediately rearward (down-Stream) of the first turbine is a second turbine, composed of twocontra-rotating wheels, which oper- ate in the gas stream exhaustedfrom the first turbine. This second turbine is for driving the fans, alsoarranged in -two banks which counter-rotate, and, as the tworings of turbine blades on each wheel or disc are nested together,with one row of each interposed be- tween the two rows of its neigh-bour, the unit can justifiably be regarded as a four-stage component. the nested blades counter-rotate, the necessity for fixedrings of stator blades is eliminated. The rotational speeds of the fan turbine wheels are much lower than that of thecompressor-turbine, the first fan wheel rotating at 2,850 r.p.m and the second wheel at 2,300 r.p.m. ; both thesespeeds correspond to maximum static thrust. Fan Blade Mounting' Having passed through the four blade stages of thesecond turbine, the gas efflux flows down the roughly constant-velocity tail pipe to the outlet orifice. The idealsectional profile for the tail pipe would, of course, be pro- gressively curved in conformity with the expansion of thegases for a uniform velocity of flow, but as this raises difficulties in production, the tail pipe cone profile is madeas a fixed-angle cone and, for all practical purposes, the flow characteristics can be regarded as conforming toconstant-velocity laws. The ducted fans are arranged in an extremely ingeniousmanner, and the structural aspect of the blade mountings is so brilliant as to take on the semblance of technicalcourage. This is not actually the case since so much re- search has gone into the design as to provide very surefoundations, yet at first sight the method adopted is such as to strike one as far fetched. As may be seen from Max Millar's 'drawing, the fanblades are mounted on rings carried on the blade crowns of the second turbine. The " torque " loads on the turbine Front view ot two-stage compressor turbinewheel. Note the unusual " M " form splines on the drive shaft ; presumablythis is a measure for reduction in stress concentration. Additionally, since CAS TURBINE and fan blades compensate one an-other, and, since the turbine blades are designed to " windmill" andthe fan blades to "screw," this is not particularly difficult; but thecentrifugal loads and bending loads cannot be cancelled-out and thesemust have occasioned the designer no small trouble. The matter isfurther complicated by the fact that labyrinth seals have to be intro-duced between fan and turbine blades to preclude untoward pres-sure losses as between the turbiiK- stream and fan stream ; and whenit is realised that the temperature difference between these streams isof the order of 600 deg C, the in- genuity and quality of the designmay be appreciated. The intake for the fans is anannulus surrounding the gas tur- bine/compressor part of the unit, Turbine and compressor rotor assembly showing the cone extension member which transmits the turbine drive to the compressor. Detail of turbine blading showing " bulb " anchorage andpeen locking. The provision of rubbing strips on the blade crowns is taken from steam turbine practice. and the overall diameter isapproximately 48 inches; from the fans the augmenterair stream is thrust rearward in an annular tapered throatisolated from, yet surround- ing, the jet gas stream, untilit reaches the outlet orifice in the same transverse plane asthe jet orifice. There is thus no confluence of the twostreams until they have leit the unit. On the subject of thrustaugmentation, as previously stated this will fall off withincrease m flight speed, since the limitation imposed on thefan stream velocity by the
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
