SLEEVE-VALVE RADIAL ENGINES in production. They are nine-cylinder Bristol Perseus engines developing a maximum horse-power of 890. The advantages of sleeve-valves include the entire elimination of valve maintenance and top overhauls, and a reduction in mechanical and exhaust noises. Reduction gearing is used to drive the propeller, which may be of the controllable-pitch type.
THE normal aero engine of today works on the same principle as the ordinary motor car engine. Both are internal combustion engines; that is to say, the firing of a mixture of petrol and air in the cylinders forces the pistons, through connecting rods, to drive a crankshaft.
Early aero engines were developed from motor car engines. In the aero engine, however, better petrols have to be used than in car engines. Motor spirits go through a number of refining processes and it is possible, knowing the constituents of any particular variety, to estimate the power it will give when mixed with a certain amount of air in an engine cylinder of known bore and stroke.
Petrol evaporates comparatively easily and the vapour which it gives off lights at a comparatively low temperature, known as the flash point. The low flash point makes the spirit dangerously inflammable, but it is of great advantage for firing the mixture of petrol and air in an engine. The sparking plug supplies the necessary spark, which is produced by the magneto. As it is vital that there shall be the minimum chance of failure in an aero engine, the magneto has been developed into a most reliable instrument. Modern aero engines have two sparking plugs to each cylinder and two magnetos.
It is important that the spark should take place at the correct instant after the mixture of petrol vapour and air has been compressed by the piston. This is ensured by the platinum points in the make-and-break device that breaks the primary electrical circuit at the appropriate instant, and which can be advanced or retarded - either manually or automatically - to suit the running conditions. Because an aero engine is called upon to run at different speeds, a variable ignition timing is generally provided.
The sparking plugs used in aero engines are of a special type. The central electrode is insulated with materials such as porcelain or mica or - in the newest types of sparking plug - a material made from aluminium oxide, which has a high melting point of over 2,000° centigrade.
As a heat conductor, this new material is over fifty times as effective as mica. Magnetos, sparking plugs and their leads have to be bonded (joined together) in aircraft engines to avoid electrical interference with the wireless installation. Every metal part in an aeroplane is nowadays bonded as a safeguard against fire risk due to electrical charges.
The ratio of the volume of the cylinder to the volume of the space into which the mixture is compressed is known as the compression ratio. The importance of the compression ratio is this. The higher the ratio the greater the economy of fuel for a given power. It is not possible to increase the compression ratio much above about 6 or 8 to 1, because the petrol and air mixture detonates unless special substances are mixed with the petrol to retard the rate of burning.
Advantage has been taken in the diesel engine of the fact that compression of a combustible mixture raises its temperature to a point where it will ignite by itself. High compression ratios up to 12 or 15 to 1 are used in these engines, which are often called compression-ignition engines.
Air-cooled and Liquid-cooled
The petrol engine uses motor spirit with low flash points; diesel and compression-ignition engines use fuel - heavy oils - with high flash points. The diesel engine for aircraft is still, in many ways, considerably less developed than the ordinary petrol engine, although a considerable number of diesels are in use, especially in Germany, where a more intensive study of them has been made than elsewhere.
The diesel engine has no sparking plugs or electrical apparatus for firing it, so that the danger of fire is lessened. Compared with the use of petrol, the use of heavy oil as fuel considerably lessens the risk of fire due to accidental causes. On the other hand the diesel engine is considerably heavier than the petrol engine and, though it burns less fuel, the advantage of low fuel consumption is seen only in long-distance flights.
Modern aero engines may be classified broadly as air-cooled and liquid-cooled. In both classes there is a variety of types. Air-cooled engines may have from four to six cylinders in line; they may be upright or inverted, in powers up to about 200 horse-power; or they may be single-row or double-row radials, having five, seven or nine cylinders in each row.
Single-row radial engines have a number of cylinders arranged round a single-throw crankshaft; a double-row radial engine has a two-throw crankshaft. In a double row the cylinders of each row are staggered so that the cooling air can flow round each cylinder. Radial engines may have horse-powers as high as 1,500.
Another type of air-cooled engine is known as the H, because the cylinders are so arranged that the cross section of the engine resembles the letter H. Engines of this type have as many as twenty-four cylinders operating on two crankshafts geared together. The V engine has two rows of cylinders arranged at an angle to each other, the two rows thus making the Y; all the cylinders are connected to a single multi-throw crankshaft.
Liquid-cooled engines may be in line or V-shaped. Liquid-cooled engines in line generally have six cylinders and V-shaped engines, twelve cylinders. The engines may be cooled with water or with ethylene glycol, which has a much lower freezing point than water.
Most aero engines use poppet valves, similar to those of the ordinary motor car engine, and some employ two exhaust and two inlet valves for each cylinder. In recent years the Bristol Aeroplane Company has developed the sleeve valve engine. A cylindrical sleeve reciprocates in the bore. Specially shaped ports are provided in the sleeve and the cylinder barrel for exhaust and inlet purposes. The air-cooled sleeve valve Perseus engine of that company has an International rated output of 890 horse-power and the double-row Hercules 1,375 horsepower.
The sleeve valve engine has a number of advantages over the poppet valve type. The parts are fewer in number and cheaper to manufacture; there are no springs to fracture; the engine is easier to maintain; the mechanism enables high speeds to be obtained; greater power and economy can be obtained from a given fuel; noise is reduced and, because higher compression
ratios can be used, the fire risk is lessened owing to the lower exhaust manifold temperatures.
In general, aero engines in present production - apart from the in-line variety - have settled down to two highly specialized types, the air-cooled radial and the liquid-cooled V. Each has its own advantages and disadvantages. The air-cooled radial engine is less vulnerable, is lighter by the weight of the radiator and water system and is easier to overhaul. Although it is suitable for use in cold countries, it has the disadvantage of a large frontal area, high cylinder temperatures with local hot spots and, if it has poppet valves, a hot exhaust valve inadequately cooled. The efficiency is lower and the oil consumption higher than in the liquid-cooled engine. The sleeve valve radial engine has lessened these disadvantages, but the in-line engine will probably retain its position when the sleeve valve has been adapted to it.
THE FOUR BRISTOL JUPITER ENGINES of the Imperial Airways liner Scylla. At a height of 5,000 feet these air-cooled radial engines develop a maximum of 600 horse-power each at 2,200 revolutions a minute. Each engine is mounted in the font of a metal nacelle which is supported by two vertical tubes, one at the front and one at the back. An oil tank for each engines is contained in the streamline nacelle behind the engine; each tank holds sixteen gallons. The petrol tanks, of which there are three, are in the centre section of the upper plane; these tanks feed petrol to the engines by gravity. The total capacity of the petrol tanks is 720 gallons. The Scylla has a cruising speed of approximately 105 miles an hour at 5,000 feet.
Improvements in the air-cooled radial engine are counterbalanced by advantages in the liquid-cooled engine. The improvements in radial engines include the use of high-conductivity alloys for cylinder heads, improved cylinder finning, scientifically designed baffles to control the cooling air flow and more efficient engine cowlings to reduce head resistance. The liquid-cooled engine may have the advantage of higher rotational speeds, water cooling under pressure to maintain a high boiling temperature when flying at high altitudes, or even cooling by liquids of high natural boiling point such as ethylene glycol.
The present tendency in design is towards higher rotational speeds and exceptional power output from each cylinder. The first requirement demands light reciprocating parts such as pistons, connecting rods and valve gear, the second requirement leads ultimately to small cylinders having an approximately equal bore and stroke of about 3½ to 4½ in. The air-cooled twenty-four cylinder 800 horse-power Napier-Halford Dagger engine is a good example of the above trend.
A Schneider Trophy Example
In general, the average power output for a given cylinder capacity is being steadily increased step by step. During the period from 1929 to 1933 there has been an overall increase from 18 to about 40 horse-power per litre. One particular engine of the high-speed small-cylinder type has completed 100 hours at 47 horse-power per litre at a crankshaft speed of 4,000 revolutions a minute. The general speed increase has been from about 1,900 to over 3,500 revolutions a minute; the corresponding take-off mean effective pressures in the cylinders have been increased from 125 to over 200 lb. to the square inch. The weight per horse-power has, on the other hand, decreased from 1·8 to 1·2 lb. and the average period of operation between overhauls has been safely increased from 300 to 600 hours.
These figures represent the general and average improvements in performance during the period reviewed. In some instances, these figures have been greatly exceeded for special duty. An outstanding example is the Rolls Royce R engine fitted to the seaplane that won the 1931 Schneider Trophy race. This water-cooled twelve-cylinder 60° V engine had a bore of 6 in. and a stroke of 6·6 in., with a piston displacement of 36·7 litres. The two banks of cylinders were set at an angle of 60° to each other. The special fuel used consisted of 20 per cent petrol, 70 per cent benzol and 10 per cent methanol, with the addition of 4 cubic centimetres of tetra-ethyl-lead as an anti-detonant, that is, to prevent knocking.
The 1929 engine gave 1,900 horsepower at 2,900 r.p.m. and weighed 1,530 lb. The 1931 engine was run at 2,400 horse-power at 3,200 r.p.m. and weighed 1,630 lb. This represented a power increase of 26 per cent for a weight increase of 6½ per cent.
The oil and water-cooling systems were remarkably ingenious, virtually the whole surface of the seaplane being used for cooling purposes. The cooling water was passed from the engine cylinder jackets to special radiators within the wings and on the floats; the oil was passed through special coolers on the body and inside the tail fin, where it was sprayed through nozzles on to the inside surface. The oil left the engine at about 140° cent, and, after having cooled, returned at about 80° cent. - a reduction of 60° cent. Pure castor oil was used, the consumption being about 14 gallons an hour.
As aero engines used for military purposes have certain points of design and auxiliary apparatus peculiar to their specialized use, it will be advantageous to confine this chapter to the outstanding features of engines used in military aeroplanes. For civil aircraft the same power units may be used, with many of the chief features retained; but the aircraft operate in easier conditions and use fuels not necessarily of a quality up to the military standard.
The different functions of aeroplanes call for engines of different horse-powers in widely differing conditions. Thus some aircraft may require average powers for long journeys with great economy of fuel; others will need enormous power for leaving the ground with heavy loads and climbing quickly to operating altitude; others again will be required to travel exceedingly fast at great heights for short periods. Five classes of aeroplane duties may be distinguished by speed and cruising range. The maximum speed for single-seater interceptor fighters may be put at 400-450, and that for multi-seater fighters at 400 miles an hour. Medium bombers and heavy bombers cruise at 300 and 275 miles an hour respectively, naval aeroplanes at 220 miles an hour. The cruising range of the five classes is respectively two, two, seven, nine and ten hours.
Four Classes of the Future
A well-known authority has expressed the opinion that the engines suitable for performances such as these might-be chosen in the near future from a range of four types. These four types would develop respectively 750, 1,150, 1,550 and 2,000 horse-power for a corresponding weight-power ratio of 1·093, 1·087, 1·00 and 1·05 lb. per horse-power. Some or all of the above aeroplanes would have more than one engine up to, probably, a maximum of four located in the wings.
The problems facing an engine designer are complex, and detailed consideration requires to be given to the whole aerodynamic structure for a given duty and performance before the designer can make his choice of the many alternatives open to him.
An aero power plant may be said to comprise a cylinder unit, including pistons, connecting rods and crankshaft; a breathing unit, consisting of induction system and carburettor; an auxiliary unit, with drives for supercharger, ignition system, petrol, oil and water pumps and the like; and a drive unit, including the airscrew.
The cylinder unit comprises the crankcase, cylinders, pistons, connecting rods, main bearings and crankshaft. The chief difference between radial and in-line types lies in the design; the same materials are common to both. In radial engines there are a number of pistons operating on one crankpin. This requires a special connecting rod arrangement known as a master rod, containing the big end bearings and auxiliary rods; these have their bearings located in flanges round the master rod end.
Air-cooled cylinders are of composite construction; they have a steel barrel and a screwed and shrunk-on light alloy head containing inserted valve seats. In radial engines, the cylinders are necessarily separate. Certain in-line air-cooled engines have a number of cylinder heads attached by bolts to a common casing carrying the overhead camshaft. Special alloys of aluminium, copper and nickel are used for cylinder heads and pistons, the essential properties being good thermal conductivity, low specific weight and, for pistons, high resistance to wear.
SMALL FRONTAL AREA is a feature of the Napier Rapier engine. It has sixteen cylinders, which are arranged in H fashion. There are two rows of four above the crankcase, and two similar rows below. The Rapier Series VI engine has a maximum horsepower of 395 and uses a supercharger.
Valves are made of steel containing a percentage of carbon silicon, nickel, chromium, tungsten and manganese. To increase the cooling capacity of poppet valves, metallic sodium is used inside the hollow stem to assist in the transfer of heat from valve head to stem and thence to the valve guide and cylinder head. Sodium has a low melting point of 97·5° cent., a high specific heat and a boiling point of 880° cent., and is the most effective substance for this purpose.
Bearing materials have received much attention from metallurgists, and the present practice inclines towards either white metal or lead bronze alloys. Lead-bronze, a comparatively recent development, consists of about 70 per cent copper and 30 per cent lead. It is effective in bearings under high temperatures and pressures. Crankshafts and connecting rods are made in steel of high quality such as 65-ton nickel chrome and a similar steel containing molybdenum suitable for nitrogen hardening.
In the breathing unit are included the induction manifold and carburettor and any special means adopted to deal with the effect of altitude on engine performance. Difficulties of operation at high altitudes are due to the decrease of atmospheric pressure and temperature with height.
A standard day is a day during which the atmospheric pressure and temperature at any height conform with agreed standard conditions which approximately represent the average conditions in the Western Hemisphere. Such a standard atmosphere is known as the I.C.A.N. Standard, the initials standing for the International Committee for Aerial Navigation.
On a standard day the air density at 10,000 feet is only 74 per cent and at 30,000 feet about 37 per cent of that at sea level. The corresponding temperatures are -5° cent, and -45° cent, respectively. The main effects of these conditions on an engine are twofold. The mixture strength as supplied by the carburettor becomes richer with decrease of pressure, and the engine power decreases. The effect of reduced air density on the carburation is to reduce the weight of air passing the jet in a given time. The velocity of the air is not appreciably diminished, so that there is approximately the same suction on the jet as before.
Relatively the same amount of petrol flows through the jet and the mixture becomes richer in petrol. The tendency of the reduced air temperature is to weaken the mixture strength. The overall effect is an enrichment of mixture strength and this is corrected by a device on the carburettor known as the mixture control.
The mixture control is operated by a lever in the pilot’s cockpit and consists of means for reducing at will the fuel flow through the jet. This may be effected either by a variable jet or by a “pressure balance” system, in which a controllable air leak is provided between the carburettor float chamber and the choke tube. This control is sometimes operated by an automatic device controlled by an aneroid capsule which responds to changes of atmospheric pressure. The effect of reduced air density on the power developed is due to the reduced weight of air inhaled in a given time. The effect of reduced air temperature, on the other hand, tends to increase the power. The overall effect is a decrease of engine power with altitude. This is one of the unsatisfactory features of the normal or naturally aspirated engine. One way of offsetting this disadvantage is to use forced induction or supercharging, a means for supplying a relatively greater weight of air to the cylinders than would normally be inhaled.
Three general systems of supercharging aero engines have been proposed of which the first two are in practical use. These are the exhaust-driven centrifugal blower, the gear-driven centrifugal blower, and the positive displacement type of blower, gear-driven.
A DOUBLE-ROW TYPE OF RADIAL ENGINE. This is similar to a single row radial, but with each alternate cylinder offset in relation to the two adjacent cylinders to give a compact design. The engine illustrated is an Alvis Pelides Major which has fourteen cylinders, is fully supercharged and develops a maximum of 1,110 horse-power. Direct or geared drive can be fitted for the propeller.
The exhaust-driven blower was developed during and after the war of 1914-18, but various practical difficulties prevented the success which theoretical considerations had suggested. The exhaust gases from the cylinders are collected in a nozzle box,from which they pass at about 850° cent, through nozzles to a Rateau type turbine wheel coupled directly to the blower. The Rateau turbine is of the single wheel impulse type and was invented by the French scientist Professor Rateau. Efficiencies up to the recent times have been low and large back pressures have been imposed on the exhaust. Modern developments in exhaust turbine driven superchargers have raised the overall efficiencies into the region of 40 to 60 per cent. In theory an overall efficiency of 25 per cent should suffice for a turbine and compressor unit combined, because the energy in the exhaust gases as they leave the engine is some four times that required to maintain sea level induction pressure in the engine.
The gear-driven type of centrifugal fan is the only one that has reached the stage of universal application on engines. The gear ratio is of the order of 10 to 1 and rotational speeds of 15,000-25,000 r.p.m. are common. Although the gear-driven type, in contrast to the exhaust-driven type, absorbs some of the horse-power of the engine, it can be used up to 17,000 feet. The exhaust-driven blower has the advantage of the equivalent of an infinitely variable speed gear by virtue of its controllable exhaust flow, so that at ground level only the required supercharger speed need be developed by the gases. On the other hand the gear-driven supercharger necessarily runs at full speed, which entails a large temperature rise in the air, although the air supply is throttled to give the required supercharging pressure.
The positive displacement blower of the Roots or Powerplus type produces its compression by delivering air into a capacity chamber. Blowers of this type have the great advantage that unwanted air may be valved to atmosphere with a corresponding reduction in the power required to drive them. Modern centrifugal superchargers can use a “suction carburettor” system, so that petrol-air mixture passes through the impeller. Alternatively they may use a “pressure carburettor” system in which the carburettor choke-tubes are placed in the air supply line between the cylinders and the supercharger. This has an advantage that only air is compressed; a further advantage is that the freezing troubles that may occur in a “suction carburettor” system can be avoided. When large blower compression ratios are involved, however, it becomes desirable to use an air intercooler between the blower and the carburettor if power loss due to increased air temperature is to be avoided as well.
Three disadvantages are inherent. First, a pressure carburettor requires to be specially constructed against the fire risk of air and petrol leaks. Secondly, the petrol supply pressure must be maintained at the proper value (generally 2 lb. or 3 lb. per square inch to the float chamber), above the choke tube pressure in all conditions of flight, to ensure the correct continuity of flow. Thirdly, the intimate mixing of fuel and air is less efficient and, therefore, the distribution to the various cylinders tends to be less uniform than in the suction system, in which the fuel and air are well mixed in their passage through the supercharger impeller.
FULL SUPER-CHARGING is used on this nine-cylinder air-cooled radial engine. It is a Bristol Mercury VIII designed for high-altitude flying, and intended primarily for fighters and light bombers. The maximum horsepower is 840 at 2,750 revolutions a minute at 14,000 feet. The capacity is nearly twenty-five litres.
For a given rotational speed the height to which a supercharger will maintain the sea-level or other predetermined air pressure will depend on its compression ratio. The compression ratio of the supercharger is the ratio of the delivery pressure or “boost pressure” to the intake pressure. This decides what is called the “supercharge height” or “rated altitude” and is defined as the height in the standard atmosphere to which the engine will maintain the required boost pressure at its normal rate of revolution. As there is also a maximum permissible engine speed in level flight which, generally, is never used for more than five minutes, there will be a slightly higher altitude to which the same boost pressure will be maintained because of the correspondingly higher rotational speed of the supercharger. This is called the “maximum power height of the engine” and is generally two or three thousand feet higher than the “rated altitude”.
In the matter of fuel consumption, it is well known that the mixture strength ratios that can be used in single-cylinder tests vary from about 15 per cent rich for maximum power to 15 per cent weak for maximum economy. In a multi-cylinder engine the average ratio is not much weaker than the correct one and for maximum power some greater enrichment than 15 per cent is generally necessary to lessen the dangers of overheating the cylinders. In general the fuel consumption for naturally aspirated and supercharged engines varies from 0·475 to 0·6 lb. per brake horse-power per hour, depending on the conditions of operation.
As the fuel economy of an engine for a given power output is improved by increasing the expansion ratio, this implies an increased compression ratio. To get much more power from this engine it becomes necessary to burn a greater weight of air in the same time and it has been shown how this is done by supercharging. With certain grades of petrol, however, there are limits to the amount of supercharging that can be used, because of the high temperatures reached at the end of compression in the cylinder. This leads to the charge detonating rather than to steady combustion, and gives rise to the characteristic knocking sound associated with the condition.
Detonation or knocking has been defined as “a noise caused by a blow delivered against the cylinder wall by a wave of high pressure travelling at great speed through the gas”. Pre-ignition, on the other hand, is caused by firing of the mixture before the spark has occurred. These phenomena are quite distinct from each other, pre-ignition occurring before the spark and detonation after the spark has passed. Detonation is extremely harmful to engines as it leads to very high cylinder temperatures with consequent damage to valves and piston.
Development of Special Fuels
In order to take full advantage of high compression ratios without detriment to the engine, it is necessary to use fuels having what is known as “high anti-knock” properties. The development of such fuels has become a matter of some urgency in the effort to keep pace with the great advances in engine performance.
Two pure hydrocarbons, differing only in their chemical structure, are used at present as a scale of degree of detonation or anti-knock value. They are called heptane and iso-octane; heptane knocks worse, iso-octane knocks less, than any known petrol. In general, the anti-knock value of any fuel is obtained from its performances in a special test engine and is defined in terms of the percentage of octane in a special blend of heptane and octane giving the same degree of detonation. This percentage is called the “octane number” for the fuel.
No definite relationship can be stated between the octane number of a fuel and its permissible compression ratio: this depends entirely on the cylinder design. As a dope for treating existing fuels to reduce or accelerate the rate of burning, which is a contributing factor in detonation, there are tetra-ethyl-lead used as an anti-knocking agent in petrol engines, and amyl-nitrate which can be used as an accelerating agent if required in compression-ignition engines. Another useful anti-detonant is water, which can be introduced into the cylinder by special means and has proved most effective.
THE CYLINDERS ARE ARRANGED IN THE FORM OF A V in the Rolls-Royce Merlin engine. There are twelve cylinders in all, six on one side of the V and six on the other. High-temperature liquid cooling is used, the cooling medium being ethylene glycol. The two rows of cylinders are set at an angle of 60 degrees to each other. The capacity of the engine is 27 litres and the maximum horse-power is 1,050 for the fully supercharged model.
The auxiliary units must now be described. The present practice is to situate all the engine auxiliary units at the rear of the engine, grouped round the supercharger and carburettor. Here are found the magnetos in duplicate, the automatic mixture control, and the automatic boost control, which saves the pilot the trouble of continually moving his throttle lever to keep the boost pressure constant. An aneroid capsule and mechanism provide the necessary relay control, and the carburettor butterfly throttle is actuated by an oil or air servo piston and linkage.
Other auxiliaries are the petrol, water and oil pumps, all of specialized type and construction, and a number of other pumps such as hydraulic units to actuate the retractable undercarriages, air pumps to operate simultaneously blind flying instruments and a certain type of wing de-icing device known as the Goodrich de-icer. Another engine-driven auxiliary is an electrical generator of 500 or 1,000 watts output to charge the battery and supply power for radio communication and, up to the introduction of cabin aeroplanes, the electrically heated clothing worn by the crew.
Not the least of the auxiliaries is the engine-starting mechanism. This may be electrically or manually operated. The electrical types comprise suitable starting motors of adequate power run off the starting battery; in the inertia type a small flywheel is spun up to a high speed and clutched in to the engine crankshaft to provide the necessary turning energy. Another method is gas starting, where combustible mixture is pumped by a small engine into the cylinders, where it is fired by hand-operated starting magneto. There is also an effective cartridge starter now in general use. This starter fires a cartridge in a breech and the expanding gases operate a piston mounted on a quick pitch thread. The axial motion of the piston provides rotary motion of a starting dog in engagement with the crankshaft. This is a successful and convenient method of starting large engines and will, no doubt, be in great demand for the much larger types of the future.
The Drive Unit
Finally the drive unit must be considered. The reduction gear units are generally of two types - the straight spur gear and the concentric type. The concentric type permits the driving and driven shafts to be co-axial and involves a stationary gear member giving a torque reaction on the casing. The airscrew shafts are more or less standardized to take the various types of airscrew having either taper or parallel splined hubs. The wooden fixed pitch airscrew is now being replaced by the all-metal variable pitch type having usually three blades at 120°.
Well known British variable pitch airscrews are the De Havilland Hamilton two-pitch and constant speed infinitely variable pitch, hydraulically operated by a governor and oil pump unit or by oil supply from the engines; the Hele-Shaw-Rotol hydraulically operated constant speed airscrew; and the Curtis-Rotol, which has electrically operated variable pitch mechanism. These airscrews enable high engine speeds to be used for take-off and permit more uniform engine operation throughout all the flight conditions. The control of the variable pitch mechanism will eventually be fully automatic and the various settings at which different constant engine speeds are required will be decided merely by the position of the pilot’s throttle lever, ranging from full power to economical cruising.
The aerodynamic qualities of aeroplanes steadily improve as a result of the vast amount of research work now proceeding in most countries. Theoretical considerations serve to indicate the limits of performance which - in the absence of some appropriate invention or aerodynamic discovery - will be reached within the next few years.
For example, airscrew efficiencies are seriously affected by blade tip speeds approaching the speed of sound in air, approximately 1,120 feet per second. Therefore, much ingenuity is exercised to provide airscrew designs for the required purpose without approaching too closely this limit of speed. A speed of 900 feet per second is seldom exceeded.
Similarly the maximum speed of flight will soon approach the same limit at which compressibility effects of the air become operative. This means that the resistance to motion, or air drag, of the aeroplane becomes exceedingly large and enormous engine powers would be necessary to propel the aeroplane at such speeds.
TYPICAL OF IN-LINE ENGINES is this De Havilland Gipsy Six Series I engine with its six cylinders one behind the other in a straight line. The engine is known as an inverted type, that is, the cylinders are arranged below the crankcase ; air cooling is used. The engine has a maximum output of 200 horse-power for a capacity which is just over nine litres.
