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Aviation History
1935
1935 - 0755.PDF
APRIL 4, 1935. FLIGHT. 361 A NEW POBJOY ENGINE The "Niagara 11" : Many Detail Improvements and Direct Electric Starting POBJOY aero engines are well known as being among the lightest and most efficient radial air-cooled units on the market. The new model, the " Niagara II," is basically the same as the Niagara I, two of which were in the Monospar S.T.10 which won the King's Cup Race last year, and which was described in Flight of February 1, 1934. The latest version, however, incorporates several interesting fea tures, including direct electric starting, drive for an electric generator, positive oil feed to the exhaust rockers of all cylinders, a redesigned rear cover and induction cover, and redesigned cylinder heads, giving better cooling by an in crease in fin area. Rotax are responsible for the whole of the electrical equip ment—the starter, generator and magnetos, the latter being of the four-pole type, running at half engine speed. The new induction cover is a particularly interesting Elek- tron casting, so designed that the mixture from the carburetter is led upwards and backwards through the centre of the cover to a common chamber encircling a hot-spot to which heat is supplied by the exhaust gases. From this chamber separate passages conduct the mixture to each cylinder. The excellent distribution achieved by this method is claimed to result not only in increased efficiency, but also in exceptionally easy starting. Two further points of interest are drives arranged one on each side of the rear cover, and available for two engine- driven fuel pumps, or, as in this first engine, for one Tecalemit fuel pump and one sealed indicator which records the number of hours the engine has run. These hours, incidentally, are calculated at a running speed of 3,100 r.p.m., so that a pilot who habitually flies at more than this specified cruising speed actually gets a smaller number of hours out of his machine than recorded on the counter; in view, however, of the extra wear entailed by the higher speed, this is an equitable arrange ment, as the maker's guarantee, which extends for 250 hours running, is taken from the reading of the counter. The Rotax starter motor weighs gib. complete, and the dog engages direct on the rear end of the crankshaft. The This view of the "Niagara II " with its cowling shows the clean external appearance. motor is geared 60:1, and is sufficiently powerful to turn the engine over at no r.p.m. when cold; starting is therefore an easy matter. The Rotax generator is mounted vertically beneath the engine, in front of the Claudel-Hobson carburetter, and is driven from a shaft which at its upper end serves to drive both magnetos. Between the shaft and the generator itself is a very neat shock-absorbing unit. The generator is suffi ciently large to keep an aircraft battery charged over and above the requirements for night-flying equipment, radio, and engine starting. The aluminium cylinder heads now have deeper and larger fins, and, of course, carry the patented Pobjoy type of manganese-chrome steel valve seats, which are screwed and shrunk into the head, and are so designed that a lip on the upper side can be peaned over, thereby entirely eliminat ing any possibility of the seats working loose and falling into the cylinders. The mounting of the brackets carrying the valve rockers is another interesting Pobjoy fea ture ; the arrangement prevents alteration of the valve clearances due to unequal expansion between the steel cylinders and the aluminium heads; the brackets are virtually bolted direct to the steel cylinder barrel, in such a manner that the head can expand irrespective of the bolts. The high-pressure oil feed, which is now led to each exhaust Section of a cylinder head, showing the steel seating ring and the valve seats. The rear cover and induction cover ensure even distribution of the heated mixture from the carburetter. (Above) All exhaust rocker boxes now have direct high-pressure lubrication. (Right) Assembly of the shock- absorbing drive to the generator. 1
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