fuel consumptions in the neighbourhood of 0-45 Ib. per B.H.P. hour may be attained.
Supercharging for High Flying. Since the power is proportional to the weight of petrol-air mixture taken in per cycle, and since this weight depends on the density of the surrounding atmosphere, the power falls off with the height reached. Tests show that in the aver- age engine the power is sensibly proportional to the atmospheric pressure. The law of variation with atmospheric density varies slightly with the type of engine, but may be taken approximately
B.H.P. is proportional to p" where P is the density, and n varies from i-l to 1-3. increasing slightly with the height. At different heights the power developed by a 2OO-H P. engine at a constant engine speed is thus approximately as follows :
Height, feet
o
5,000
10,000
15,000
20,000
25,000
444
30,000
Density .
I-O
869
7H
613
527
360
58
B.H.P. . .
200
171
135
in
90
73
B.H.P. as % of ground B.H.P.
I-O
8SS
673
552
45
365
290
Since the resistance to the motion of an aeroplane diminishes directly as the air density, other things being unchanged, the level speed should only diminish slightly with an increase in height. This diminution in speed is, however, rendered more pronounced by the fact that the angle of incidence of the planes requires to be increased in order that they may support the same weight in air of reduced density, and this increases the head resistance.
The climbing speed of the aeroplane is reduced in a much greater degree, since the energy to be expanded in lifting the dead weight of the machine through a given height is independent of the density and remains constant at all heights; and at some definite height, depending on the design of the aeroplane and the power of the engine, the latter is only sufficient to overcome the head resistance when flying level at the minimum safe speed of the aeroplane with the increased angle of incidence of the planes, without leaving any surplus lifting capacity. This height is termed the " ceiling " of the
Any device which would enable the power of the engine to be main- tained at height would not only increase the level speed, but more especially the rate of climb and the height of the " ceiling."
Three such devices have shown promise. In the first the engine is fitted with differential pistons. Air is drawn into the space between these on the outward stroke of the engine, compressed on the return stroke, passed through a cooler, and forced into the cylinder through a series of ports uncovered by the piston slightly before the end of the suction stroke. By this method the weight of mixture in the cylinder is increased. The degree of " supercharging " may be adjusted by a regulating valve so as to keep the power constant over a range of heights up to about 10,000 feet. This scheme has not as yet been very successful owing mainly to mechanical defects.
In the second system a centrifugal blower is geared to the engine. The discharge from this is passed through the carburetter on its way to the cylinders which are thus fed with mixture under an increased pressure. The system is, of course, an added complication and involves the maintenance of very high-speed gears and bearings. As the induction system is under pressure, any leaky joints will derange the operation of the engine, and lastly, since the speed of the blower is constant at constant engine speeds, the amount of supercharging falls off with height, while, near the ground, air must be blown to waste through a bypass valve.
In the third system the engine exhaust is discharged through a single-wheel high-speed impulse turbine of the Rateau type. This turbine is direct coupled to a centrifugal blower feeding the car- buretter, and delivers sufficient air to the engine to maintain its power at all heights up to about 15,000 feet. This method is partially automatic in that if the pressure in the induction pipe is maintainec constant, the pressure of the exhaust gases will be constant, anc since the pressure on the exhaust side of the turbine diminishes with height, the pressure available for driving the turbine increases with height to an extent which compensates for the increased demand for power by the blower. A valve for bypassing the whole or part of the exhaust gas directly into the atmosphere is provided to enable the output from the blower to be regulated.
Here also the induction system is under pressure. The weight complete for a 2OO-H.P. installation can be cut down to about 60 Ib and at 15,000 ft. the gain in power is about 80 H. P. for an expenditure of only 0-75 Ib. per H.P. thus gained.
The increased complexity of the installation, the work thrown on the pilot, and the risk of breakdown will all retard the introduction of such schemes. Moreover, the additional weight may alternatively be devoted to increasing the size of the cylinders, leaving the crank- case and crank-shaft, etc., sensibly unaltered. Such a " light " engine would not withstand being opened out fully near the ground, and special precautions would require to be taken to prevent this happening. At height, however, it could be fully opened up, and the increased power corresponding to its increased cylinder diameter taken advantage of. Such a unit has the advantage of simplicity. Many of the latest and most powerful engines are really in a modified
degree " light " engines, in that they cannot be run for more than a very few minutes " all out " near the ground.
Other methods of reducing the drop in power with height are possible. One such method is to design the engine with a compression ratio too high to permit of ground operation, and to reduce this near the ground by a cam giving a late closing to the inlet valve. As the height is increased the inlet valve would be closed earlier in the stroke until, at some predetermined height, normal timing would be attained. A second method which has been suggested consists in admitting a proportion of cooled exhaust gas to the cylinder with the working mixture. This reduces the tendency to detonation and enables a higher compression ratio to be adopted than would other- wise be possible. As the height increases the proportion of exhaust gas would be reduced, until, at the predetermined height, the engine would be working on a normal mixture.
Engine Starters. The operation of starting an aeroplane engine by swinging the airscrew by hand has always been dangerous, and to remove the necessity for this, several types of self-starter have been devised. An electric motor geared to the crank-shaft through a clutch achieves this, but the number of starts possible with one charge is limited by the accumulator, and the weight and bulk of the installation restrict its sphere of usefulness. A compressed-air starter is lighter. Here a high-pressure cylinder supplies air to the correct cylinders by means of a distributor operated from the crank- shaft of the engine.
The most usual starting system consists of a supplementary magneto placed in the cockpit and rotated by hand by the pilot when the crank-shaft has been brought into the correct position. For success one or more of the cylinders must contain an explosive charge and therefore the crank-shaft is rotated slowly by hand, drawing a charge of petrol vapour from the carburetter as in normal operation. The plan is, however, unsatisfactory in cold weather, and starting is facilitated by admitting coal gas or hydrogen into the induction pipe from a small container, while the crank-shaft is being rotated.
One modern device, still (1921) in the experimental stage, consists of a small two-stroke single-cylinder engine which is started by hand and drives a compressor which draws an explosive mixture from its induction pipe and forces this through a distributor into the appro- priate cylinders of the main engine. This charge is then fired in the usual way.
Future Development of the Aero Engine. The development of the aero engine must increase its reliability, its useful life, its efficiency and its output in horse-power per unit weight, especially at height. Experience gained in the operation of existing types, by a process of survival of the fittest, slowly leads to the elimination of those details in design which are in the main responsible for breakdowns. The reduction of bearing loads and the improvement in bearings, in- creased perfection in balancing, better design of valve springs and of valve gears, of pistons and piston rings and of lubrication systems, will all add to the useful life, while improvements in carburation, in cooling and lubrication, induction systems, and in sparking plugs, will lead to increased reliability of operation. Efficiency will be enhanced mainly by such modifications in cylinder design or by the use of such fuels as will admit of higher compression pressures.
It seemed possible in 192 1 that the Diesel cycle might be developed for aero-engine work, and the Junker engine of this type was said to have attained a fairly advanced stage of development in Germany. In view of the heavy cylinders required a sufficiently light Diesel engine, however, appears to be very difficult of attainment. Failing this, the direct injection of fuel into the cylinder during the suction stroke, using moderate compression ratios, may have possibilities. This is a modification of the method used in the early Antoinette engine, where fuel was injected by a pump into the inlet pipe of each cylinder. The method has the advantage of eliminating the car- buretter and induction system and, in theory, of enabling a uniform mixture to be given to all the cylinders. Promising experiments on single-cylinder engines were in progress in 1921.
Outside existing designs in 1921 there appeared to be scope for an engine working on the two-stroke cycle, and for a double-acting- line engine with cylinders in tandem. It is true that attention had already been paid to both these types without, as yet, successful results. Still, many of the initial difficulties had been surmounted, and there was every reason to hope that a successful design would ultimately be evolved. Such an engine would have excellent pros- pects of fulfilling the ideal conception of I Ib. per B.H.P. which is at present the dream of the aero-engine designer. In view of the immense progress in the design of aero engines during 1911-21, it seemed probable that the aero engines of the future might well show as much improvement as those of 1921 did as compared with the machinery to which the early fliers entrusted their lives.
(A. H. Gi.)
VI. AIR NAVIGATION
Historical. Navigation is the art of selecting the course which a craft should take in order to proceed from any one position on the waters to any other. For guidance in the building- up of air navigation centuries of experience of the sister art ot
