aero-engines: piston and turbine
aero-engines: piston and turbine defined in 1939 yearaero-engines: piston and turbine - AERO-ENGINES: PISTON AND TURBINE;
aero-engines: piston and turbine - Captain Norman Macmillan, M.C., A.F.C. This article records their development through forty years, from the experiments of the Wright brothers to the days of jet propulsion. See also Internal Combustion Engine; Jet Propulsion.
The production of the Wright brothers' first petrol engine - a four-cylinder water-cooled 12 h.p. unit driving two propellers by means of open chains, one of which was crossed to make the two air-screws turn in opposite directions to cancel out uneven torque - was the essential element in the world's first aeroplane flights on Dec. 17, 1903, for the brothers had previously solved the problem of control over flight by glider experiments: but until they made their own engine, none existed which could convert their glider into an aeroplane.
The evolution of the aeroplane has followed the same course ever since, and the aircraft designer of today still awaits the production of prime movers which will enable him to improve his products in speed, range, and load-carrying.
The Earliest EnginesThe Wrights first engine weighed a little over 200 lb., a ratio of weight to power of about 17 lb./h.p. Their next step was to increase engine power and reduce the ratio of weight to power. Their second engine developed 16 to 17 h.p. and their third 20 h.p. Both weighed 240 lb. The effect was at once noticeable.
Their first machine weighed 745 lb. with pilot and flew at 30 m.p.h. Their second machine weighed 900 lb. and flew at 34 m.p.h. Their third machine weighed 925 lb. and flew at 38 m.p.h. The weight to power ratios of the three machines were 62 lb., 53 lb., and 46 lb. per h.p. By comparison the Lancaster bomber's ratio was 11.8 lb./h.p., while that of the American Thunderbolt fighter's was reduced to as little as 7 lb./h.p.
The second Wright engine had a weight to power ratio of 15 lb./ h.p., and the third 12 lb./h.p. Their fourth engine developed 25 h.p. for about 10 lb. per h.p. The Wright " Cyclone 18 " air-cooled radial engine of 1943 developed 2,000 h.p., its dry weight was 2,200 lb. - a weight to power ratio of 1.1 lb./h.p.
During the 40 years which elapsed between the first Wright engine and the "Cyclone 18 " a great progressive variety of engines flew the skies, while the research for greater power and the reduction in the weight to power ratio proceeded.
Air-cooled engines were soon in competition with the water-cooled engines of the Wrights.
The greatest early development of aero-engines took place in France. It was there that the 3-cylinder fan-shaped 25 h.p. aircooled Anzani engine was made with which Bleriot flew the English Channel on July 25, 1909. Another notable French engine of that early period was the 24 h.p. water-cooled V8-cylinder Levavasseur engine used in the Antoinette monoplane with which Latham endeavoured to emulate Bleriot's cross-Channel flight.
One of the best British engines of that period was the- 35 h.p. Green 4-cylinder water-cooled engine. In May, 1920, using a ten-year-old 35 h.p. Green engine, Hinkler flew non-stop from England to Turin in 9½ hours.
Early aircraft speeds were slow, and cooling was therefore inefficient for the first air-cooled engines. Bleriot's 38 minutes flight across the Channel was the longest ever made by the 25 h.p. Anzani, which usually overheated after about 20 minutes. But flying through a rain shower cooled Bleriot's engine; his success was really a triumph for water cooling.
Rotary EnginesThe search for more efficient air cooling produced the rotary engine. This engine had a fixed shaft around which the crank-case revolved, so that the radially disposed cylinders were cooled not only by the forward motion of the aircraft but also by their own movement.
The first rotary engines were French. The earliest model was the 34 h.p. 5-cylinder Gnome. The next model was the 50 h.p. 7-cylinder Gnome which drove Paulhan's winning. Farman aeroplane in the 1910 London-to-Manchester flight for the Daily Mail Â£10,000 prize. Then followed a 7-cylinder 80 h.p. model. These engines had a mechanically operated exhaust valve in each cylinder head, and an automatically operated inlet valve in each piston head.
Fuel was fed into the crank-case through the fixed shaft and the gas found its way to the combustion chamber via the somewhat unreliable piston valve. Then came the 9-cylinder 100 h.p. Gnome Monosoupape engine, which retained the principle of one valve per cylinder head, but used inlet ports in the cylinder walls instead of the piston valve.
The 80 h.p. and 110 h.p. 9-cylinder Le Rhone, the 85 h.p. 7-cylinder Clerget, and the 110 h.p., 130 h.p. and 140 h.p. 9-eyIin-der Clerget engines all used two valves per cylinder head, one for inlet and one for exhaust. These rotary engines brought the weight to power ratio down to about 3 lb./ h.p., and were widely used in the First Great War. The 80 h.p. Le Rhone that engined the Sop-with Pup weighed 240 lb. and could develop 93 h.p. at 1,200 r.p.m. The Monosoupape used in the Avro 504 and the D.H.2 developed 100 h.p. at 1,200 r.p.m. and weighed 300 lb. There were two 14-cylinder double-row7 rotary engines, the 120 h.p. Le Rhone, and the 140 h.p. Monosoupape; the second engine weighed 2-46 lb./h.p.
British DesignsThe 160 h.p. Bentley 1 and the 240 h.p. Bentley 2 were the two British-designed rotary engines. The weight of the B.R.I, was 405 lb., giving it a weight/power ratio of 2.6 lb./h.p. The B.R.2 weighed 498 lb., with a ratio of 2.08 lb./h.p. The Bentley 2 marked the limit of development of the rotary engine, because of the heavy stress due to the rotation of the major part of the rotary engine's weight.
Rotary engines required frequent dismantling for overhaul. The permitted running time was: Le Rhone 30 hours, Gnome 40 hours, Clerget and Monosoupape 60 hours, and Bentley 90 hours. Even their great ease of removal from and refitting to aircraft could not compensate for the frequency of overhaul periods, and soon after the First Great War the rotary engine died out.
first great wak developmentsThe water-cooled engine was enormously improved during the First Great War both in power and in weight to power ratio. The Hispano-Suiza V8-cylinder water-cooled engine weighed 490 lb. dry and developed 200 h.p. at 2,000 r.p.m. Other water-cooled engines developed during the First Great War were the Rolls-Royce 6-cylinder 100 h.p. Hawk; the 120 h.p. and 160 h.p. 6-cylinder Beardmore; the 230 h.p. 6-cylinder B.H.P., later called the Siddeiey Puma; the V12-cylinder Rolls-Royce 250 h.p. Falcon; and the 300 h.p. V8-cylinder Hispano-Suiza. The Rolls-Royce Eagle VIII water-cooled V12-cylinder engine developed 372 h.p. at 2,000 r.p.m. and weighed 947 lb. dry; its gross weight to power ratio was but 3.1 lb./h.p.; two of these engines powered the Vickers-Vimy aircraft that made the first non-stop transatlantic flight in 1919, and the first London-Australia flight later in the same year. The American Liberty V12-cylinder water-cooled engine developed 405 h.p. at 1,650 r.p.m. with a dry weight of 820 lb., giving a ratio of little more than 2 lb./h.p.
Air-cooled stationary engines were again temporarily eclipsed. The 70 h.p. and 80 h.p. V8-cyliuder Renault engines, which were so successful in the early part of the First Great War, were replaced by the 100 h.p. Raf 1a VS-cylinder air-cooled engine and then the 140 h.p. Raf 4a V12-Cylinder air-cooled engine; all were relatively heavy engines - 5.66, 5.78, 4.61, and 4.85 lb./h.p. respectively.
Succeeding them came the air-cooled radial engines, which, like the rotary engines, were first developed in France in the Gobron, Anzani, and Salmson engines. Among the earliest British radials were the A.B.C. 9-cylinder Dragonfly which developed 385 h.p. at 1,710 r.p.m. with a weight of 540 lb. This engine proved unreliable and was subject to mechanical failures. Contemporaneous with it was the Cosmos Mercury, a double-row 14-cylinder air-cooled radial, built, like the Dragonfly, to satisfy the British Air Board Scheme A issued in 1917. This Mercury developed 347 h.p. at 2,000 r.p.m. and weighed 582 lb. The later Cosmos Jupiter 9-cylinder air-cooled radial developed 450 h.p. at 1,800 r.p.m. and weighed 502 lb. These engines brought the weight to power ratio below 1.5 lb/h.p.
the interim periodAeroengine development after 1919 was for a long time slow, after the expenditure on research and production fell. By 1923 the Siddeiey Jaguar 14-cylinder, two-row air-cooled radial engine developed 360 h.p. and weighed 710 lb.; its maximum running speed was 1.650 r.p.m. By 1925 the Jupiter (then a Bristol engine) developed 485 h.p. By 1930 horsepower had risen to about 600-800 h.p. for normal Service engines, although special racing engines had been built to put forth two to two-and-a-half times that power for short periods. Among the air-cooled radial engines then were the 625 h.p. two-row 14-cylinder Armstrong Siddeiey Panther (2,400 r.p.m.); the 750 h.p. 9-cylinder Bristol Pegasus (2,525 r.p.m.); the 800 h.p. two-row 14-cylinder Pratt & Whitney Twin Wasp (2,400 r.p.m.); and the 735 h.p. 9-cylinder Wright Cyclone (1,950 r.p.m.); their weights were 980; 1,000; 1,162; and 945 lb.
The production of the ultralight aeroplane in 1923 brought motor-cycle engines into use for aircraft. Converted engines were the 2¼- h.p. A.B.C. and the 3½ and 6 h.p. Douglas flat-twin, and the 5 h.p. Blackburn and J.A.P. V-twin engines. Then came the 20 h.p. Bristol Cherub flat-twin specially built for aircraft. These engines were succeeded by the 4-cylinder in-line air-cooled 70 h.p. Cirrus made by the Aircraft Disposal Co., Ltd., from parts taken from the 70 h.p. Renault engines left over from the First Great War; this powered the first Moth in 1925. The Cirrus Minor and Major engines made later by Blackburn Aircraft Ltd., developed 90 h.p. and 150 h.p. and were inverted 4-cylinder inline air-cooled engines. De Havilland made their first Gipsy engine in 1927: the 1944 range included the 90 h.p. Gipsy Minor, the 130 h.p. Gipsy Major I and 140 h.p. Gipsy Major II, all with the same lay-out as the Cirrus. Four-cylinder De Havilland engines were followed by the 200 h.p. Gipsy Six, the 205 h.p. Gipsy Six II, and the V12-cylinder 525 h.p. Gipsy Twelve, four of which powered the D.H. Albatross airliner. All the later D.H. engines were inverted and air-cooled.
About the beginning of the Second Great War aero-engines in the lowest-powered class included the 33 h.p. Carden water-cooled 4-cylinder developed from the 10 h.p. Ford car engine; the 35 h.p. air-cooled V-twin Anzani made by Luton Aircraft Ltd.; and in the U.S.A. the 40 h.p. flat-four Continental, the 40 h.p. Aeronca flat-twin, and the 50 h.p. Menasco flat-four, all air-cooled. These five engines weighed 130 (dry), 110, 154, 121, and 164 lb., all showing a marked advance on the earliest Wright engines.
modern enginesAt the outbreak of the Second Great War the Bristol Pegasus (with the same capacity as the Jupiter of 1925) could produce 1,000 h.p. at 2,600 r.p.m. for a weight of 1,135 lb., while the newer 14-cylinder two-row Hercules radial gave 1,375 h.p. at 2,750 r.p.m. The 14-cylinder Twin Wasp R-1830 radial gave 1,200 h.p. at 2,700 r.p.m. for a weight of 1,420 lb. The Wright Cyclone-14 GR-2600 double-row radial gave 1,600 h.p. at 2,400 r.p.m. for a weight of 1,900 lb.
By 1943 the Hercules had been further developed to give 1,650 h.p.; Pratt and Whitney had a Double Wasp R-2800 18-cylinder two-row radial giving 1,850 h.p. at 2,600 r.p.m. for a weight of 2,280 lb.; and Wright had produced the Cyclone-18 GR-3350 giving 2,000 h.p. for a weight of 2,200 lb. These last two engines made possible the construction of aircraft like the Martin Mars 70-tons flying boat, and the Boeing B - 29 Super - Fortress bomber, which bombed Japan from China and Saipan.
During the first decade after the First Great War the Napier Lion was the most prominent British water-cooled engine. This 12-cylinder three-row broad arrow design developed 450 h.p. in 1920 and produced 700 h.p. about ten years later. As a racing engine the Lion VII-D was boosted to over 1,250 h.p. for the R.A.F. Schneider Trophy racing aircraft in 1927-29.
The Diesel or compression ignition aircraft engine has now been outclassed by the high efficiency petrol engine, but about midway between the wars Diesel efficiency for long-range flight exceeded that of petrol engines. The subsequent introduction of 87, 90, and 100 octane fuels reversed the position. Bristol in the U.K. and Packard in the U.S.A. experimented with Diesel engines. but the only Diesel engine produced in quantity was the German Junkers Jumo 205 of 700 h.p. at 2,500 r.p.m. and 1,257 lb. weight. Napier had a licence to build them in Britain. This German Diesel engine with six cylinders, twelve pistons, and two crank-shafts geared to a common air screw shaft, gave many maintenance problems in service. Nevertheless, the Diesel engine may some day be used for long range freight aircraft where optimum speed is not as important as economy of operation. It uses a heavier fuel than the petrol engine, with a lower fuel fire-risk, and dispenses with spark plugs.
rolls-royce seriesThis modern series of liquid-cooled engines began with the F.11. No. 1 first flew in Aug., 1927, piloted by Capt. Norman Macmillan. That engine was the prototype of the Kestrel, and from the Kestrel were developed the Buzzard, Merlin, and Griffon. The Merlin engine won the Battle of Britain in 1940, proving its superiority over German engines used by the Luftwaffe in that most gruelling of tests.
The Kestrel engine was made in a large series, with slightly varying characteristics, but typical examples were the moderately supercharged Marks VII, VIII, and IX which developed 680 h.p. at 2,900 r.p.m. for a weight of 975 lb. The Buzzard developed 955 h.p. at 2.300 r.p.m. for a weight of 1,540 lb. The 1929 Schneider Trophy "R" engine developed from the Buzzard gave 1,900 h.p. at 2,900 r.p.m. and the 1931 engine gave 2,300 h.p. at 3,200 r.p.m. for respective weights of 1,530 and 1,630 lb.
The 1939 Merlin II developed 1,030 h.p. for a weight of 1,335 lb. and the Merlin X gave 1,145 h.p. with a two-speed supercharger, both at 3,000 r.p.m. The later Merlin XX gave 1,260 h.p. for a weight of 1,450 lb. The Merlin LX1, provided with a two-stage (two-blower) supercharger, maintained ground level power to 40,000 feet, where the charge was compressed to six times atmospheric pressure.
A supercharger is an air blower or compressor built into the engine's induction system to raise the air pressure within the system and so force more air into each cylinder during its intake period. More fuel must be metered to balance the additional air. Greater power can be obtained from the higher gas pressure of a supercharged engine than from a non-supercharged engine of equal size. In practice supercharging is largely used (1) to increase power for taking off and (2) when flying at a height where power would normally fall owing to the lower density of the air. Superchargers are variously driven by gears from the engine shaft, by the exhaust gases (turbo-superchargers), and in the Daimler Benz turbo-supercharger engine by a fluid flywheel drive that compensates variations in air density as the aircraft changes height.
All the Rolls-Royce engines were V12-cylinder designs, liquid-cooled, not by water alone, but by a mixture of ethylene-glycol and water which has a greater temperature range between freezing and boiling points.
engine developmentIn 1903 the Wright engine developed 3 h.p. per cylinder. In 1913 just over 18 h.p. per cylinder was considered good. In 1918 the Liberty engine gave 37.5 h.p. per cylinder. In 1923 the Jupiter gave 50 h.p. per cylinder. In 1939 the Pegasus gave 111 h.p. per cylinder. The early Griffon gave 146 h.p. per cylinder and before the end of 1944 produced 170 h.p. per cylinder. Penultimately the V12-eylinder engine (according to Sir Roy Fedden, 1944 Wilbur Wright Memorial Lecturer before the Royal Aeronautical Society) may produce about 2,400 h.p. or 200 h.p. per cylinder.
The two-row 18-cylinder radial engine can produce over 3,000 h.p., but when the penultimate power of this typo of piston engine came in sight, with the demand for more power still growing, attention was given to other types of piston engines. Rolls-Royce developed the single-crankshaft 24-cylinder "X" engine which gave 1,845 h.p, at 3,000 r.p.m. The Allison Division of General Motors Corporation, biggest makers of liquid-cooled aircraft engines in U.S.A., produced a 2,200 h.p. 24-cyhnder double-crank-shaft double-V engine. But, in both cases, the initial experiments were soon discontinued, perhaps because of the urgency of war conditions.
Napier have for long been developing the "H" type engine, which began with the Rapier, a 16-cylinder air-cooled engine with eight upright and eight inverted cylinders each in two rows of four, and developing 305/385 h.p. at 3,500/3,900 r.p.m. for a weight of 720 lb. Next came the Dagger, with 24 cylinders similarly disposed in rows of six, and weighing 1,280 lb. with a power output of 630/705 h.p. at 3,500/4,000 r.p.m. The Mark VIII Dagger was developed to give 925/955 h.p. at 4,000/4,200 r.p.m. for a weight of 1,390 lb. The Dagger was air-cooled. Then came the 2,200-2,400 h.p. Sabre 24-cylinder "H" engine, with horizontal instead of vertical cylinders, and liquid-cooled, with sleeve valves instead of the poppet valves used in the earlier engines. All these "H" engines have two crank-shafts, geared together to drive a common air-screw shaft.
" H " engine development may produce engines of up to 6,000 h.p. Other piston engine developments may result in 28-cylinder four-row radial engines producing up to 4,000/5,000 h.p.; 42-cylinder six-row radials producing about 8,000 h.p. are regarded as the final power development of piston petrol engines. Beyond that power output it is thought that the piston engine will not be able to compete with the gas turbine. It is also regarded as possible that the limit of airscrew conversion of power may be reached with units of about 10,000 h.p.
Gas Turbines and Jet Propulsion UnitsThe gas turbine as a prime mover has long been sought, but its employment in aircraft was achieved only after the beginning of the Second Great War. Four nations - Italy, Britain, U.S.A., and Germany - employed the gas turbine prior to 1945. The gas turbine principle is simple. A compressor is fed with air as the aircraft moves through the air. After compression the air passes into a combustion chamber into which fuel is metered, and there the mixture is fired, at first by a sparking plug and subsequently by spontaneous combustion due to the non-dissipated heat. The burning gas, under increased pressure, passes rearward and drives a turbine. The turbine is coupled to the compressor by a shaft and provides the power for the compressor. After flowing through the turbine, the gas, still at high pressure, is emitted through a suitably ducted passage and orificed jet. To start the engine an electric motor drives the compressor to feed air into the combustion chamber.
With high speed vehicles, such as aircraft, propulsion can be provided by the reaction of the jet. But jet propulsion efficiency equals and exceeds the normal airscrew only at high speeds and altitudes. At 20,000 feet the optimum propulsive efficiency of the propeller is reached at 300 m.p.h. Above that speed jet efficiency improves, equalling the propeller at 540 m.p.h. and surpassing it at higher speeds. At 30,000 feet the jet equals the propeller at 450 m.p.h. These figures indicate that in the widest field of aviation the two methods of propulsion are complementary, with each having its most efficient speed range. This is further borne out by the comparison of fuel consumption. Compared with the piston engine operating at its most efficient fuel economy, the cruising consumption of fuel for gas turbine jet propulsion is from two to three times greater; this should not, however, be considered final, and better consumption should be realized as development continues.
Future developments of the gas turbine may seek to combine the properties of airscrew and jet propulsion by making the turbine drive not only the compressor but also an airscrew. In this case the power developed by the prime mover may be apportioned in the ratio of about 75 per cent to the airscrew and about 25 per cent to the jet.
The advantages of gas turbines over piston engines may be summarised as lighter weight to power ratios, fewer and less complicated moving parts, more compact design rendering mounting within the wing a simpler process, more even torque reducing vibration to the minimum and making the unit suitable for shaft drive to remote airscrews, greater power per unit, and the use of cheaper fuels having a lower crash-fire risk than high octane petrols.
It is considered in aeronautical circles that not until between 1950 and 1960 will the gas turbine engine be developed into a satisfactory prime mover for long range civil aircraft. When this is accomplished the gas turbine will bring into being the true flying wing aircraft which, by dispensing with the orthodox tail unit, and by burying the engines within the wing, will enable faster speeds to be obtained than are today possible for normal transport aircraft.
Meanwhile rocket assisters are increasingly used to give additional acceleration to aircraft taking off in confined spaces or when very heavily laden. During flight additional emergency power has been obtained from piston engines by (1) water or water-alcohol injection and (2) super fuel such as iso-octane.
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