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Aviation History
1935
1935 - 0805.PDF
APRIL II, 1935- FLIGHT. 385 POWER for the "POU" Some Interesting Features of the 17 h.p. Aubier et Dunne Two-stroke Unit Fitted to the Mignet " Pou-du-Ciel" 510 mm These drawings dicate the compact and simple layout of the Aubier et Dunne two-stroke. Another view appears on the next page. SINCE its short-lived popularity circa 1923, little interest has been taken in the molor-cycle-type engine for light aircraft work, though spasmodic experiments have been made, mainly abroad. With the advent of the tiny Pou-du-Ciel, interest in the truly small and simple engine has been revived. When it is built in Britain this tiny French machine is likely—as announced in Flight a fortnight ago—to be fitted with a small water-cooled four-cylinder on light car lines; in France, how ever, M. Mignet and his disciples are powering their Pous with a two-cylinder two-stroke developed for the purpose by the firm of Aubier et Dunne. Though built by a concern which has specialised in small two-stroke engines for motor cycles, the unit in question can hardly be classed in this category, for it has been designed from start to finish for aero work, and it appears to fulfil its purpose satisfactorily. Of the inverted type, the engine has two air-cooled cylin ders cast en bloc in aluminium alloy, and provided with steel liners. Six bolts secure the cylinder block to the aluminium crank case. The cylinder heads, also of alloy, are separate, and each is secured by four studs. Operating on the normal three-port two-stroke principle, the cylinders have a bore and stroke of 70 mm. by 70 mm., giving a total capacity of 540 c.c, and 17 h.p. is developed at 4,000 r.p.m., which figure is reduced by gearing to 1,600 r.p.m. at the airscrew shaft. Each cylinder is provided with a compression release valve and a centrally placed sparking plug. Each piston carries three rings and has the usual two-stroke deflector top. Connecting-rods with double-row roller big- ends transmit power to a 180 deg. crankshaft built up from four parts and running in three double-row ball bearings. The crank-cheeks and bob-weights are, of course, of such a size that they thoroughly "fill" the two compartments of the crank case, thus increasing the compression ratio. At the rear end of the crankshaft is a special flywheel mag neto with windings which give independent ignition for each cylinder, and at the forward end is a duplex plunger-type oil pump, with sight feed, furnishing an oil supply direct to each cylinder wall. Actually this is an auxiliary supply, since the main lubrication system is by petrol, oil being mixed with the fuel in the ratio of 6 to 7 pei cent. A motor-cycle-pattern carburetter, with Bowden-type con trol, feeds mixture to a short induction passage cast in the cylinder block. Fuel consumption is said to be in the region of 400 grammes (0.9 lb.) per h.p. per hour. Above the crank case cover is an aluminium housing con taining the reduction gearing and carrying the two airscrew- shaft bearings, a two-row ball thrust race in front and a smaller two-row ball race at the rear. The two gear pinions give a reduction of 2.5 to 1, and an interesting feature of the larger pinion is a shock-absorber, dogs in the outer member transmitting the load to the inner member through the medium of rubber bushes which, inci dentally, are carefully insulated from oil. The provision of this amortisseur is probably necessitated by the two-stroke engine's well-known tendency to four-stroking, and conse quently irregular running, under light load. Four radially disposed bearer lugs run, fore and aft, the full length of the crank case. Though the fin area is large, the frontal area of the engine is not excessive for the power; in any case, the point is of small importance in the case of the Pou. The price of the unit in France is 4,000 francs—£32 at par and approximately £55 at the present rate of exchange. " POU-DU-CIEL " EXPERIMENTS TV/I" MIGNET, writing in Les Ailes recently, points out •"•*-. that the location of the wing spar in the drawings Riven in his book may be a little too far back, and recom mends constructors of the Pou-du-Ciel to shift the spar for ward 5 cm. There is room to do this, as the spar does not completely fill the rib depth. The new spar location then becomes 290 mm. from the leading edge. If this is done, and E 11 the fore-and-aft position of the hinge line of the front wing is arranged to be 23 cm. in front of the vertical from the centre of gravity, M. Mignet considers that the machine should be perfectly balanced. # * # Hitherto one has rather taken it for granted, that the wing arrangement used by M. Mignet in his Pou-du-Ciel is likely
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