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Aviation History
1956
1956 - 1020.PDF
166 FLIGHT, 27 July 1956 The First D.H. Engine By EDWARD A. GOUGH SINCE de Havilland aero engines have successfully poweredaircraft over the greater part of fifty years now, they canfairly claim to have had the longest career in British aviation. Although the name de Havilland was not generallyassociated with engines until the famous Cirrus family of the 1920s, the first D.H. unit was designed as early as 1908. Bearing in mind England's late start in aviation, this was aremarkable achievement. Even in the most progressive aviation circles the idea of producing an "aeroplane engine" was barelyeight years old; before 1900, aviators—apart trom a few who attempted to adapt motor-car engines—were still prone to thinkin terms of manually operated paddles, flapping wings or steam- engines. Credit for the idea of producing the first lightweight petrolengine specially for the purpose of propelling a flying machine must go to Professor Samuel Pierpont Langley. Around 1900,ably assisted by one Charles Manley, he designed and built the world's first aero engine—a surprisingly advanced five cylinderwater-cooled radial. Langley's attempts to fly have been written down as one of the classic failures in aviation. His "aerodrome"(as he called his flying machine) ended up in the river Potomac on the two occasions that he tried to rise into the air. This wasno fault of the Manley engine, which might otherwise have won the universal fame it deserved and had considerable influence onthe course of later design. However, the arguments in favour of the petrol motor as the most practical power unit for an aero-plane were conclusively demonstrated when the Wright brothers made their historic flight in 1903.* Only five years after this momentous event the young Geoffreyde Havilland entered aviation and designed his own aeroplane, engine and propellers. With the assistance of F. T. Hearle [whois today a director of the parent D.H. company] he also built the aeroplane and its propellers. It was a unique achievement andestablished a tradition for which the firm is justly famous today. The first de Havilland engine, a horizontally opposed flat four,was of similar layout to several car engines of the day. Geoffrey de Havilland's experience as designer to certain Mid-land car manufacturers, notably Wolseleys, had suggested to him the advantages of this arrangement. But the specialized applica-tion of an engine to power an aircraft called for revolutionary thinking in terms of power and weight, with the consequentlightening of every possible component. How far one could carry this process was a matter of trial and error, or happy inspiration,for there was no precedent to guide a designer who in automobile practice would normally pile on a few inches of metal where anyweakness might be suspected. When the drawings were completed they were put out to theIris Motor Company of Willesden, who undertook the construc- tion of the motor. The de Havilland Iris, as it came to be known,was one of the earliest British examples of a flat four aero engine. The French manufacturers Clement, Darracq and DutheilChalmers had already exploited the horizontally opposed type quite successfully, but the Iris owed little to them. The mainfeatures of the layout—the central mounting of the carburettor and H.T. magneto, with the flywheel at the rear—are seen in theheading illustration. Less obvious was the offsetting of the opposed cylinders, to allow individual attachment of each con-necting-rod to the crankshaft. Cast-iron cylinders were employed and a copper water-jacket fitted to the outer section of eachcylinder for the thermo-syphon cooling system. The paired cylinders were held in place by a triangular yoke which was pulleddown onto the cylinder heads by three long studs passing into the aluminium crankcase; the drilled Meccano-like yoke illustratesthe way in which lightness was achieved. One of the features of the Iris which put it in the forefront of engine design was themechanical operation of both inlet and exhaust valves. Mechani- cal inlet valves were still something of an innovation, for thespring-loaded automatic-type inlet valve had had a good run of spluttering success to recommend it. The valves themselves weredriven from a single camshaft mounted in a separate crankcase compartment and were of the opposed type, this time vertically,the inlet being placed above the exhaust and forming a kind of inspection point for it. Diagonal exhaust push-rods "nicelycarried out" (according to a contemporary report) passed through the crankcase between the connecting-rods to operate the valves * On the return of the original Wright biplane and engine to the U.S.A.—a return which was delayed by the controversial claim for the first powered flight for Langley's "aerodrome"—apprentices of the de Havil-land Technical School made replicas, which are now in the Science Museum. The 12 h.p. engine ran successfully on several occasions beforeit went into the National Aeronautical Collection. via bell-crank levers. These push-rods were hollow and the cam-shaft gear train also was "lightened out as much as is consistent with strength." The carburettor was of the float-feed type, witha celluloid-treated cork float; it was placed centrally on the crank- case, and long manifold arms each supplied two inlet ports. The pistons were made in two parts; the crown and outer partcarrying the rings were turned from solid vanadium steel accur- ately machined for uniform thickness, whilst the inner sectioncarrying ihe gudgeon pin was a steel casting. In a detailed de- scription of the engine in Flight for May 21, 1910, it was notedthat Hoffman ball-bearings were used for the big-ends, mains and end-bearings and that Mr. de Havilland placed very greatconfidence in them as a result of his experience with them under extremely trying conditions. Articles that appeared in the French magazines L'Automotorand Genie Civil in September 1909, gave a general description of the main features and performance of the Iris, and it is interest-ing to note that at that time it developed 25 b.h.p., a rating which was subsequently doubled with no increase in r.p.m. Finally thearticles mention the promising tests made with an engine in- tended for use in an aeroplane, concluding . . . "Les essais d'unsemblable moteur, destine a un aeroplane, ont, parait-il, donne de tres bon resultats." This was the engine fitted to Geoffrey deHavilland's biplane No. 1 which was nearing completion. However, the trials of the No. 1 in December 1909 were un-successful. This is hardly surprising, for knowledge of even the most elementary kind was virtually non-existent and flying ex-periments in Britain were barely a year old. With enthusiasm undiminished, work on a second biplane was put in hand, thistime using a stronger wood for the spars—American whitewood had proved inadequate. The undamaged Iris engine from theNo. 1 biplane was recovered and fitted to the successful No II biplane. It was not until six months later, at the Olympia Aero Showin March 1910 that the D.H. engine attracted interest in England. An example of the Iris engine was exhibited on the HandleyPage stand and a brief mention was given in The Aero. But soon full recognition of this important addition to the handful ofBritish aero engines available came with the comprehensive articles in Flight dealing with de Havilland biplane, engine andpropellers. A warm welcome was extended to the young aviator for his enterprise in producing both his own aeroplane and apowerful light-weight motor to propel it. Performance details of the engine were quoted originally as40-45 b.h.p. at 1,500 r.p.m., the weight being 250 lb and bore and stroke 4^in and 4Jin. However, in the week following the articleFlight published a letter from Geoffrey de Havilland in which he gave some additional and corrected information on the engine'slatest performance and pointed out: "With reference to the article in Flight re the de Havilland engine, Inotice that the power of the engine is given as 40-45 h.p. at 1,500 r.p.m. Since the article was written, however, the usual adjustments requiredin a new engine have been made, with the result that 52 h.p. is developed at 1,500 revs, continuously, and 40 h.p. at 1,050. The weight is 200 lbs.in running order, but without fly-wheel, the latter weighing 30 lbs." Only a few Iris engines were built, three being ordered by theGovernment and one installed in the Royal Aircraft Factory airship Gamma. But, as with aeroplanes, theoretical knowledgeof aero engines, backed by practical experience, was in its infancy. Speaking in 1946, Sir Geoffrey de Havilland reflected on theproblems of his first engine and said that "It suffered from a fundamental weakness in crankshaft design which resulted incrankshaft failure after a certain number of hours. The engine was of the flat-four type, popular today in light aircraft, but thecrankshaft was two-throw and not four-throw as in modern engines, and the balance was therefore not good." In spite of this the Iris gave of its best on September 10, 1910,when Sir Geoffrey made his first flight in the No. II biplane. For many weeks afterwards, too, it could be heard purring awayhappily over the Hampshire downs. In the New Year of 1911 it again performed magnificently at the War Office acceptance trialswhen both biplane and engine were bought and their designer offered an important post at the Royal Aircraft Factory underMervyn O'Gorman—a long and fruitful association which pro- duced some notable aircraft.
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