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Aviation History
1947
1947 - 1043.PDF
JUNE 26TH, 1947 Electric accessories, inlet trunks and throttles areshown above with, top right, sleeve porting and, right, cooling, baffling, and exhaust trunking. FEDDEN POWER UNITS reduced considerably: a very important point in a small unit. The design of the turbine casing is arranged so that, contrary to normal practice, the torque reaction stresses are not transmitted through the nozzle blades. The construction of the turbine provides for easy removal of the assembly as a complete unit, and on removal the flame tubes are fully exposed for inspection and replacement without removing the shroud casing. The reduction gear ratio of 14.785: 1 is believed to be the highest yet attempted. A compound epicyclic type reduction gear is employed, and a special feature of the design is that there are no highly stressed intermediate shafts susceptible to torsional vibration. The high-speed gears have double helical teeth, and they are mounted in a novel manner so as to ensure symmetrical loading both of the gears and their mountings. The compressor shaft is splined to a doubled sun wheel which meshes with three fixed planet pinions: these, in turn, mesh with an internally-toothed annulus carried in a bell housing. This comprises the first half of the reduction gear. The bell housing has a splined hub to transmit drive to the sun wheel of the forward, or second, half of the reduction gear. In this part of the transmission the sun wheel meshes with three planet pinions which are carried on the airscrew shaft, reaction being through their engage- ment with a fixed enclosing annulus. Rotation of the airscrew is opposite in direction to that of the rotor system, and this has the effect of reducing the torque and gyroscopic loads on the engine mounting by 12 percent and 33J per cent respectively. A hydraulic torque meter is embodied in the reduction gear to register torque-failure warning and, at the same time, to institute automatic feathering of the airscrew. The <Tlat Six Like the Cotswold, the 5.3-litre flat-six Fedden engine many interesting design features, perhaps the chief of which is that it was conceived from the start to be com- pletely submerged in a wing. In actual fact, the wing pro- file was imposed before a single line was made on the drawing boards. This principle requirement has been the guide in the whole design, and in terms of general layout has imposed strictures which have left little freedom to the designer. Conversely, this procedure being followed from scratch has enabled the production of a simple layout with excellent accessibility to all accessories and other items re- quiring checking or adjustment. The engine has been designed basically to drive a pusher- propeller through a suitable extension shaft. A single spar wing layout had to be adopted for the aircraft. Although the main stresses are carried by the substantial single spar a certain proportion of the torsional stress has to pass through the engine cowling. This is of stainless steel to ensure sufficient resistance against possible fires, and special quick-acting fasteners have been developed which permit the cowling to act as an integral part of the wing shell. It was intended to use the present design as a basis for further developments, notably for an exact replica of this engine fitted with British accessories. Further plans were prepared for this engine to be used in a simple tractor form mainly for fuselage installation. A geared version of a substantially increased rating was also envisaged. Detail Design Features The main detail design feature of the engine is the vertical finning of the cylinder barrels. The cardinal prerequisite for a successful air-cooled cylinder, particularly when sleeve valves are used, is a uniform temperature distribution over the barrel, and from this viewpoint it is now generally ack- nowledged that vertical finning presents considerable ad- vantage. In this engine the improvement obtained was striking, and the bearing pattern of the sleeve in the cylinder is very much better than that obtained with con- ventional finning. In addition to this the carefully dis- tributed air flow ensures very low barrel temperatures with minimum cooling air consumptions. Hundreds of hours of single cylinder running have confirmed the expectations. The cylinder barrels are cast in low expansion silicon aluminium alloy and are each attached to the crankcase by six studs. The three cylinders of each bank are held together at the base by minia- ture turnbuckles, and each set of three cylinders is normally handled like an inte- gral block and is not disturbed during as- sembly or stripping of the engine. This in- genious design proved to be a success from the beginning. The three inlet ports and two exhaust ports are so designed that they can be fully machined from the outside of the barrel, and some interesting work has been done on air flow rigs to refine the port design in both barrel and sleeve to achieve the very best gas flow conditions. A special small banjo casting is inserted between the opposing side inlet ports of adjacent cylinders, being held in plac'e by small dowels. The cylinder head is a single-piece aluminium alloy die casting, also in low expansion silicon aluminium. The flame crown of the cylinder head is quite flat and the sparking plug positioning has been spread as much as pos- sible to ensure an efficient and unobstructed air flow. Detonation-free running has been obtained at an 8 :1 com- pression ratio on 90 octane fuel with very good powers and consumptions. The cylinder head is fixed to the barrel by means of ten studs, and spigots with a small collar into the barrel, the fixing flange being a separate dural forging. Serrated bear-ing cap faces are used.
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