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Aviation History
1951
1951 - 0898.PDF
A II May 1951 taking the form of two contra-rotating wheels, with tip-shrouded blades, the shrouds serving as mounting rings for two rows of augmenter fan blades. These contra-rotating fans operated in an annular duct, and augmented the total thrust by no less than 67 per cent. The F.5 power unit comprised basically an F.2/4 turbojet, serving as a gas generator for a contra-rotating fan of open, as opposed to ducted, type. Gases passed from the generator through a straight duct to the augmenter turbine, a four-stage contra-rotating unit essentially similar to that used in the F.3, but remotely mounted at the tail extremity of the duct in its own nacelle fairing, and carrying on the tip shrouds two rows of fan blades. These fans were essentially multi-bladed airscrews of 5ft 6in diameter, the leading unit having 14 blades and its down- stream complement 12 blades, all of fixed pitch. By comparison with the turboprop power unit, as it exists today, the open-fan augmenter has the advantage of lightness, simplicity and absence of gearing, and by comparison with the F.3 ducted-fan unit, the F.5 gave a 100-lb increase in thrust whilst being 100 lb lighter in weight. Metrovick ceased development of these units, however, in favour of concentrating work on the Sapphire (subsequently taken over by Armstrong Siddeleys) and the company later decided to withdraw from the aircraft gas-turbine field. It is, however, entirely possible that the last has not been seen of the open-fan type augmented power unit for aircraft, in that there are many indications that such units will conveniently bridge the transition stage from the operational zone of the turboprop to that of the turbojet. D. Napier and Son, Ltd., did not release full details of the Naiad turboprop until 1948, but when these details were published they aroused considerable interest by reason of the unique struc- tural form employed. From the main mounting ring (lying in the plane of the final compressor stage) subsidiary supporting struc- tures project forward to carry the front half of the engine by pick- ing up to the reduction gear casing, and run rearward to support the rear half by attachment to the turbine inlet manifold. By this means, the compressor casing and the turbine shaft housing are relieved of bending loads. 563 Metrovick F.5 open (contra- rotating) fan thrust-augmenter unit. \\J Company, who had developed a straight-through combustion system. As already mentioned, the W2B was the forerunner of the first Rolls-Royce turbojet, the Welland. Experience gained with this development work was applied to a new project, designed to replace the Welland in the standard Meteor nacelles, but with a static thrust rating of 2,000 lb. The new unit was known as the Derwent 1, and it was further developed in a number of series. The Derwent 2 gave an increased thrust of 2,200 lb, the Series 3 was a special unit for experiments with wing boundary-layer control by suction, whilst the Series 4 gave an additional increase Napier Naiad turboprop with ducted spinner. From a hollow-spinner air-intake shrouding the roots of the airscrew blades, air passes to the compressor via two kidney- section ducts, one each side of the reduction gear. A 12-stage axial compressor gives a compression ratio of 5.5 : 1, and the mass airflow at peak r.p.m. is 17.2 lb/sec, an exceptional figure for a unit of but io.§2in in diameter. Delivery from the compressor is given to five combustion chambers of conventional design, which feed the two-stage turbine. As a result of the unusual structural form of the Naiad, it is necessary for the rotating assembly to be articulated. To this end, the compressor is carried in a gimbal assembly at its rear end, and in a self-aligning bearing at its forward end, torque connection being made through annular couplings. Reference is now permissible to the latest Napier power unit, the Nomad NNm. 3, but no details are available beyond bare mention that the unit is of compound type; that is to say, of a kind in which a piston engine serves as a high-pressure gas generator for a turbine. The great attractiveness of such a design is that an exceptionally low specific fuel consumption can be attained. Rolls-Royce, Ltd., established a department for the design of gas turbines in 1938, and subsequently the company manufactured components for Whittle units, thereafter undertaking the construc- tion of a Whittle-type turbojet known as the WRi. Then, in 1943, ^search on the W2B/23 unit was taken over from the Rover (Below) Rolls-Royce Welland turbo- jet. A derivative of a Whittle design.
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