Both the Radial and Horizontally-Opposed Engine Have Played Prominent Parts in Aviation History.
When flying was very young the static radial was one of the most successful types of aero-engines. Even in 1909 when everything pertaining to aviation was crude and very experimental the advantages of a short crankshaft, ease of mounting and compactness found in the radial were at once realised by designers and a number were produced, notably the Anzani, Buchet, Clement, Farcot Miesse and R.E.P.
Bleriot used an Anzani on his historic cross-Channel flight in 1909, the motor being an air-cooled 3-cylindered fan-type. This engine, which gave off 25 h.p. at 1,400 r.p.m. was a development of the famous Anzani motor cycle power unit and had automatic inlets and side exhaust valves.
It cannot be said that it was very efficient, its weight per horse-power being over 5 lbs., and also, because of the design—the three Cylinders placed at an angular distance of 72°—the balance was bad, necessitating the inclusion of heavy bob weights and internal flywheels. Nevertheless it had the redeeming quality of reasonable reliability and it must be regarded as one of the pioneers of a type of engine which has now found universal favour.
After the fan-type, the Anzani Company produced another three-cylinder engine known as the “Y”-type. The air-cooled cylinders in this case were disposed at angles of 120° (in the form of a Y), and the connecting rods, working on the same throw, the power impulses occurred at equal intervals resulting in a much better balance. As in the fan-type, the inlet valves were automatically operated but the exhaust valves were overhead.
The better balance of the ” Y ” permitted a considerable reduction in weight (as the flywheels were dispensed with) and the engine was far smoother and cooler-running than its predecessor. Its normal r.p.m. was 1,300 at which speed it produced 30 h.p. Because of its reliability, its compactness and simplicity the ” Y ” soon became popular in pre-war days and several famous makes of planes were powered with it.
Then, as now, the demand for more power was felt and the Anzani concern, who were the leading manufacturers of radials, responded to it with the production of a 40-45 h.p. six-cylinder engine. This new type retained the main characteristics of the Y engine, with automatic inlet and overhead exhaust valves, cast-iron cylinders and so forth. The cylinders were placed in two groups of three, the cylinders of each group being arranged at 120° apart around a two-throw crankshaft, with the throws set at 180°. Throughout, the design was distinctly clever and was a marked advance on any previous attempt at making a multi-cylindered radial. The cam-gear for operating the exhaust valves, for example, was extremely ingenious, comprising a single cam running at half engine speed which operated the tappets and push rods of one or other of the two cylinder groups, alternately in its cycle. The arrangement of the two groups of three connecting rods onto the crankshaft was also interesting. The big-end of each rod was formed as a shoe, cut with helical edges, the whole being retained on to a split bush on the crank journal by two split phosphor-bronze collars or rings. The magneto was bolted to a platform at the rear of the engine and was gear-driven from the end of the crankshaft. Situated also in the rear of the engine was a cam-operated plunger oil pump which drew its oil through a large-bore pipe and delivered it under pressure through oilways in the crankshaft and external pipes to the big end assemblies, main and thrust bearings. The gudgeon pins and cylinder walls were lubricated by the oil-fog within the crankcase. Its robust construction and simplicity combined with its moderate normal speed-1300 r.p.m., made this 40-45 h.p. a very reliable unit and that there are quite a few of them in use in various light planes at the present day is a matter of some significance. Other engines were added later to the Anzani range, notably the 50-60 h.p. six-cylinder, the 80 h.p. 10cylinder and the 100-110 h.p. 10-cylinder. These were produced in 1914 but it is curious to find that the radial engine rather faded into the background during the four years that followed and gave place to the rotary, the straight-in-line and ” Vee ” types.
The end of the War, however, witnessed a big revival of the radial not only in this country but in France, Italy and the U.S.A. Most of these modern engines were of big power though,—ranging from 180 h.p. to 400 h.p. and it was not till the Armstrong-Siddeley ” Genet ” was produced that the modern radial engine figured in light planes in England.
The first “Genet” which was made in 1926 was a five-cylinder of 75 h.p. The cylinders were of steel and had alyminium-alloy heads which housed one inlet and one exhaust valve. The heads were shrunk, screwed and retained to the cylinder barrels by lock-rings.
The pistons, as on all modern engines, were of alloy and carried two narrow rings and one scraper. The crankshaft was carried on ball races, the front one acting as a thrust as well as a main bearing. The magneto was placed at the rear and skew-driven and the carburettor was bolted to a flange on the crankcase casting, the gases passing to the cylinders through an annular chamber and separate induction pipes. The weight of the engine including carburettor, magneto, exhaust stubs and propeller hub was 168 lbs., giving 2.5 lbs. per h.p. The normal r.p.m. was 1,850 and the maximum, (at which speed 75 h.p. was given off) was 2,035 r.p.m.
The “Genet Major” (100 h.p.) is the latest addition to the range af Armstrong Siddeley engines and made its first appearance at the 1929 Aero Show at Olympia. Like the earlier ” Genet” it has five cylinders, the vertical cylinder being situated at the bottom..
The crankshaft is of ample proportions and is supported by three bearings ; two heavy roller bearings of the ” no-cage ” type. (the races being packed with rollers, giving greater carrying capacity), take the load from the crankpin and the third bearing, situated at the front end of the front cover of the crankcase, is a deep groove ball bearing, and besides supporting the front end of the crankshaft, takes the thrust load of the propeller in either direction.
The connecting rods comprise one ” master ” which is of H section and four auxilaries.
The auxiliary rods are also of H section and machined all over from steel forgings.
The pistons follow the standard Armstrong Siddeley practice and are machined all over from ” Y ” alloy forgings.
Each piston has two compression rings and one scraper ring above the gudgeon pin and one scraper ring below the gudgeon pin. The gudgeon pins are of the full floating type.
The crankcase is spherical in shape and provided with five bosses for the cylinders ; it has an integral diaphragm supporting the front main roller bearings and extends forward to enclose the timing gear.
The Timing Gear.
In the timing gear the cam drum has two series of cams, one disposed to the rear for the inlet valves and one to the front for the exhaust valves and each series having three cams. The drum is directly attached to the satellite carrier of an epicyclic train of gears and revolves at one sixth engine speed in the same direction as the crank.
Roller ended tappets are housed in pairs in tappet guides fitting in circular bosses in the crankcase.
The cylinder barrels are machined all over from steel forgings and are screwed and shrunk into the aluminium cylinder head. Each cylinder has two overhead valves, one inlet and one exhaust. They are returned to their seats by duplex valve springs, which enable a spring of small rate to be accommodated in a small space.
The valve springs and rocker arms are enclosed in covers of streamline shape, thus minimising the head resistance of these parts.
A feature of all the Armstrong Siddeley engines is the compensated valve gear. As is well known, owing to the expansion of metals when heated, the distance from the centre of the engine to the top of the cylinder differs when the engine is hot from when it is cold. With a non-compensated design the valve rocker, supported from the cylinder head, moves away from the crankcase as the engine warms up. The tappet rods, however, exposed to a cool stream of air do not expand to the same degree and a considerable sloppiness develops in the valve gear. In order to obtain correct timing when hot, the engine has to start with longer valve periods, necessitating undue overlap of the inlet and exhaust valve periods, thus adding to the difficulties of starting. By supporting the valve rockers on an external bracket anchored at some specific point on the cylinder, the variations of expansion of the different parts are compensated and the same valve timing hot and cold is obtained.
The Genet Major is constructed on this principle but as the inlet and exhaust valves differ in their working temperatures, in order to avoid duplicity of parts a mean position for the anchorage of the rocker brackets was determined. The hot clearance between the end of the valve stem and the rocker striking pin is .013″ and in order to obtain this the inlet valve clearance is set at .010″ cold and the exhaust at .020″.
Each valve with its rocker and rocker support bracket lie in the plane of the tappet push rod. There are thus no side loads and this allows the rocker bracket to be of very small section, consequently offering little head resistance.
In regard to the cylinder barrels, these screw into steel adaptors in the crankcase bosses. These adaptors are free to turn and are flanged on the inside of the crankcase. They are pegged when the correct position has been found to line up the cylinders with their ports facing rearwards. Each cylinder is screwed down on to a loose split ring having conical faces at each end. When the cylinder is screwed down and in correct alignment, the clamping screw of the split ring is tightened.
The induction casing with integral diaphragm to carry the rear roller bearing, closes the rear end of the crankcase.
A Claudel Hobson A.V. 48 C.R. carburettor mounted vertically at the end of an induction elbow supplies the mixture. This is equally distributed by a fan to the cylinders through fine partly-tangential pipes in the induction casing. The carburettor is fitted with interconnected altitude and throttle controls and is designed to have all its principle parts easily accessible.
In one piece with the induction elbow is the magneto gear housing, on the cover plate of which are mounted the two B.T.H. magnetos and the hand starter. The port magneto is fitted with an impulse starter.
Two oil pumps are secured to a flange on the underside of the front cover and are driven from a bevel gear on the crankshaft immediately in front of the timing gear. Both the pressure and scavenge pumps are of the “gear” type and are driven in tandem.
The engine is of the dry sump type. The scavenge pump draws oil from the sumps on either side of the bottom cylinder through a strainer incorporated in the Pump body and discharges the oil.to the tank. Before reaching the tank the oil passes through a chamber surrounding the induction elbow and this serve’s the dual purpose of heating the ingoing mixture and of cooling the oil.
The normal B.H.P. of the ” Genet Major ” is 100 and the normal and maximum speeds 2,200 r.p.m. and 2,420 r.p.m. respectively. The weight of the engine is 250 lbs.
The 40 H.P. Salmson.
Another light radial power unit which has come very much to the fore in recent years is the little 40 h.p. Salmson. Hitherto it has been made in France by the Societe des Moteurs Salmson, but an English company has recently been formed and it will shortly be manufactured in this country. This engine has nine air-cooled cylinders with a bore and stroke of 70 mm. x 86 mm. Of steel, the cylinders have special aluminium heads, which carry a single exhaust and inlet valve, operated by push-rods ; the valve springs are of the hairpin type. The crankcase is in two halves, held together by nine long bolts, the rear ends of which pass through the engine plate for mounting the engine in the machine. The cylinders are spigoted into the crankcase and held down on to four substantial studs. The pistons, of course, are of aluminium, short-skirted and fitted with three rings. The ignition is provided by a single magneto mounted at the rear of the engine, and driven from the crankshaft by a train of gearwheels, which also incorporates a skew drive for the oil pump. The carburettor (a 26 D.K.1 Zenith) is placed low down in the rear of the engine, the mixture being admitted to the cylinders via passages in the back plate of the crankcase and nine separate induction pipes. The carburettor is shrouded with a stream-lined shield and has a hot-air intake. With a cubic capacity of 2,979 c.c. the engine develops 45 h.p. at 2,000 r.p.m. and in spite of its comparatively low power numerous makes of light ‘planes in which it has been installed have carried out various remarkable flights of endurance, speed and altitude.
Contrary to the general impression, the horizontally-opposed aero engine is not an innovation of recent years, being in fact, like the radial, one of the earliest forms of power units for aircraft. But owing to the fact that it could not be made as a comparatively high-powered engine without having unduly large cylinders with its attendant disadvantages it was not developed to any extent. However, before it was abandoned, the h-o twin contributed quite considerably in making flying history. The famous pioneer, Santos Dumont, used such an engine in a monoplane of his own construction in 1909. This motor was a Darracq of 25 h.p. and was remarkably advanced in its design, having steel cylinders, push rod operated overhead valves and copper water jackets.
It weighed about 120 lbs. and developed its maximum of 25 h.p. at 1,500 r.p.m.
With the introduction of the small single-seater and the earlier type of light two-seater, the horizontally-opposed engine has again appeared, two notable examples being the A.B.C. and the Bristol “Cherub.” The A.B.C. company have of course specialised in this type of air-cooled engine for many years and at the present time market two versions for aeroplane work— the two-cylinder ” Scorpion ” and the four-cylinder “Hornet.” The ” Scorpion ” is an engine of 1,500 c.c. capacity and has bore and stroke dimensions of 102 m.m. x 91.44 m.m. It runs at 2,300 r.p.m. and develops 35 h.p. Machined from the solid, the cylinder barrels are very highly finished ; and the detachable heads are of cast iron, liberally finned and fixed to the barrels by eight studs. The head carries one inlet and one exhaust valve, both inclined, and both the inside of the head and the crown of the piston are of hemispherical shape, so forming a partial spherical combustion chamber. Aluminium pistons are fitted with two rings at the top and a scraper at the base of the skirt. The big end bearings are of the split floating bush type and the connecting rods are threaded onto them over the crank webs. All the other bearings in the engine are of the ball or roller type and are of ample dimensions. The thrust loads are taken by a ball race positioned between two roller bearings at the front end of the crankshaft. A special B.T.H. magneto provides the ignition and gives two sparks to each cylinder, and the carburettor is a double-choke, triple-diffuser Zenith. Oiling is on the dry sump principle and incorporates a simple feed pump (which delivers oil to the main bearings at a pressure of 40 to 60 lbs.) and a scavenger pump. The oil consumption is .04 pints b.h.p. hour and the petrol consumption, .52 b.h.p. hour and the stated weight of the engine with magneto, carburettor and propeller boss is 109 lbs.
The “Hornet “, which is the latest production of A.B.C. Motors Ltd., is a particularly interesting motor. It is a flat opposed four and might be described as a ” double-Scorpion ” as several salient features are common to both engines. The cylinder and cylinder head design, for instance, is identical and the pistons and gudgeon pins are interchangeable.
The following are the main data of the ” Hornet ” :— Normal power, 75 h.p. ; maximum, 82 h.p. ; bore and stroke, 102 mm x 122 mm (3,990 c.c.) ; normal and maximum speeds, 1,875 r.p.m. and 2,175 r.p.m. ; weight of engine complete, 225 lbs. ; petrol consumption, 0.53 pints b.h.p. hour ; oil consumption, 0.035 pints b.h.p. hour.
The Bristol ” Cherub ” engine needs scarcely any introduction, for in the days when the light plane really was ” light ” it was used on practically every machine of note and figured conspicuously in the various competitions during the years 1924-1926.
It was produced in various forms—first as the Mark I, then the Mark II and finally as the Mark III. Owing however to the manufacturers being fully engaged in making high-powered engines and planes, it is now not in regular production. The Mark III had a capacity of 1,288 c.c. and gave 36 b.h.p. at 3,200 r.p.m. The cylinders were of steel with alloy heads in which were screwed steel valve seatiugs, valves guides and inlet and exhaust passages. Aluminium alloy pistons were fitted, each having three narrow rings. They were attached to the connecting rods with hollow gudgeon pins of the floating type. The crank shaft of special alloy steel, was carried on four bearings and the camshaft was placed transversely in the crankcase and operated from the crankshaft by a train of spur wheels.
Although of such small proportions and (for an aero motor) a high speed engine, the ” Cherub ” has shown itself to be highly reliable and long-lived and many are still in use in this country, on the Continent and in America.