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Aviation History
1958
1958 - 0263.PDF
FLIGHT, 28 February 1958 DART UP TO DATE . . . 277 The seven combustion chambers, while again generally littlechanged, show a small performance-gain over those installed in the RDa.6. The flame-tubes, already exceeding lives of over3,000 hr without repair in production engines (the earliest Dart tubes were changed at 700 hours or less), have been slightlymodified for the RDa.7, and are of optimum development design and therefore acceptable for production. The outlet-diameter hasbeen increased and the pressure-loss through the can has been reduced by increasing the number of tertiary air holes in thecombustion chamber. As shown in the drawing overleaf, the number of Clark coolingrings has been increased to three. This is the first production engine to be so equipped, although Rolls-Royce have beendeveloping three-ring tubes since late 1954, and two cooling-strip flame-tubes were fitted to Mk 510 engines in production. A certainamount of discretion has to be exercised in fitting these cooling strips, for at lower chamber pressure ratios they are less effectiveand so reduce can life; the present design is matched to the increased flame temperatures of the Dart 525 tubes, but it cansafely be assumed that flame-tube development will continue throughout the life history of the Dart. The most important change made to the RDa.7 engine, andcertainly the primary distinguishing feature, is the addition of a third turbine stage. This is not a complete innovation for theDart, as a three-stage turbine was married to a Dan 510 com- pressor for the Mk 21 engine developed for the Breguet Alize. The third turbine stage is, in fact, an addition with whichRolls-Royce have been experimenting for some years: a three- stage turbine was envisaged for the still-born Dart RDa.5, andthe development work for this engine—and a good many hours of cold air rig and engine testing—provided useful experiencewhen it was needed for the RDa.7. Introduction of the three- stage turbine has partly been adjusted to suit airframe needs; itwas more logical to develop the 505 and 506 two-stage turbine engines to give additional power in Mk 510 form than it was tobranch out at once with a more complex design. The nozzle box and first two turbine stages of the RDa.7 aresubstantially similar to those of earlier Dans, but the increased combustion temperature and higher gas-stream energy resultingfrom the additional mass-flow necessitated the additional turbine stage. An immediate result has been a reduction in the loadingper stage and an overall increase in component efficiency of the turbine as a whole. The first two turbine stages carry the triple-tipseals introduced in the two-stage turbine of the Dan 510, and Increased efficiency and higher powers have been obtained by the addition of a third turbine stage. Also risible in this photo- graph are the concentric turbine and reduc- tion-gear drive shafts. changes have also been made to the blade material. The two-stage engines used Nimonic 80A high- and low-pressure-stage turbine blades (although Nimonic 90 blades have been introducedon the first H.P. stage) and the new turbine uses Nimonic 95 on the H.P. stage blades, 90 on the I.P. stage and 80A on the L.P.stage. With the additional turbine disc the method of attachment tothe mainshaft assembly has been changed. Where a central locking nut was previously used, there are now ten tapered-shankbolts locating the discs radially; five pass through the H.P. and I.P. discs and a further five run through all three discs, whichoverhang from the drive-shaft flanges and abut against the mating faces of the bolt bosses. Both high-pressure air tapped from the second-stage compressorand low-pressure air from the intermediate casing is used for turbine cooling. High-pressure air is ducted into the annularspace on the front face of the H.P. disc, where it is separated from L.P. air by a labyrinth seal carried from the ten disc-mountingbolts. A second labyrinth seal permits a limited airflow outwards across the front face of this disc, while the remainder passes alongthe annulus between the interstage sleeve and the central bore of H.P. and L.P. discs for cooling the interstage disc faces beforejoining the main gas-flow at the blade platforms. The near face of the L.P. disc is cooled by low-pressure air bled through holes inthe drive shafting and ducted along the interstage cooling-air sleeve. Another feature unique to the Dan RDa.7 is that the driveshafts have been divided, so that from the turbine there are quite separate shafts driving the compressor and the reduction gears.Although this change must represent a certain weight penalty in comparison with the single-shaft engines, it provides a secondstage of defence against possible failure—and is therefore directly in line with Rolls-Royce design philosophy. In the event of afailure of sufficient magnitude to fracture the reduction-gear shaft, Combustion, cooling and pressurization airflows are shown in this diagram. Arrow size is roughly proportional to mass flow; high-pressure air is indicated by means of light tone and low pressure air with dark. AIR INTAKE AIR SEALS THREE STAGE TURBINE AIR SEALS AIR SEALS COOLING AIR OUTLET TO PROPELLER SHAFT AIR SEAL OUTLET TO HOT AIR GATE VALVE
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