By Major C. F. Abell OBE
Flight magazine 25 March 1920
A PAPER under the above title was read by Maj. C. F. Abell, O.B.E., before the Royal Aeronautical Society on March 17, 1920. Owing to lack of space we are unable to publish the paper in full and must confine ourselves to giving the last part, containing the conclusions which Maj. Abell has drawn from the data dealt with in the first part of the paper. The object of the paper, the lecturer said, was ” to set out in simple form a description of the various types of machinery used during the War in airships, so that the very wide experience gained in the 3,000,000 miles flown, largely no doubt in small ships, may be of some value to those interested in the design of ships of more ambitious lines.”
Maj. Abell dealt with the troubles experienced in the different types of airships, from the original “Blimps” (the S.S. type) through tbe Coastal, North Sea, S.S. Zero, S.S. Twin, Rigid R. 9, R 23, R 23 X, R 31 and 32, to the R 33 and 34.
The lecturer then stated the following requirements in engines and installations as they appear to the airship engineer, the point which be desired to emphasise being that airships have peculiarities of their own, and that engines which are perfectly satisfactory for other services are not necessarily suitable for airship work.
This should be a single line engine of six cylinders, the reason for this demand for a single row of cylinders being that an engine of this type gives a greater degree of accessibility; that the exhaust piping arrangement is simplified; and that the space available in a gondola of a certain size is more with this type than with a “Vee engine.”
Cylinders – These sbould be made separately and should all be interchangeable. In case of a collapsed piston and cylinder badly scored, the engine with separate cylinders can obviously be overhauled in less time than one having cylinders cast in pairs. There should be no water pipes connecting the cylinders, as these interfere with their removal.
Valves – These should be fitted in the head of the cylinder and operated by overhead rockers, which in turn are operated by push rods from the camshafts. With this construction the timing is not interfered with when cylinders are removed.
Crank-case top-half – Should be fitted with inspection doors at each side, so that big ends can be rebedded without much difficulty. If possible inspection doors should be fitted big enough to allow pistons and connecting-rods complete to be removed without disturbing the cylinders, but in cases where the cylinders are made easy to detach, there is less need for such large inspection doors. They should be big enough, however, to allow big ends to be refitted from the side, as distinct from the top of the crank-case. The camshafts should be fitted in the top half of the crank-case, one on each side of the engine, and the design of the case should be such that the crankshaft journals are supported in this half with bearing caps, and not in the lower half of the crank-case as in some engines. Cylinders should be secured by readily accessible bolts. Special spanners should not be required. There should be no oil ducts cast in the crank-case. The main oil lead should be bolted to the outside where it can be seen. The oil ducts supplying the main bearings spould each be fitted with a gauze filter easily detachable.
The lower half should carry the oil pumps in an easily accessible position, but otherwise should be more or less an oil splasher.
Magnetos – Should be driven from any convenient point, preferably from the camshafts. They should all be arranged to rotate in the same direction, to simplify the carrying of spares in the ship. There should be two magnetos per engine. A flexible coupling should be fitted, provided with a method of quick adjustment of the timing.
Lubrication system – Should be of the “dry base” variety. There should be two pumps for draining the base and one supply pump. Tbe scavenging pumps should be fitted one at each end of tbe oil base-chamber, so as to give efficient drainage when the ship is at a steep angle. The supply pump should also be fitted in the base and could be driven by the same gear as the other pumps, but must be readily accessible.
All crankshaft and connecting-rod bearings should be oiled under pressure.
Water Pump – Should be centrifugal,and so fitted that it can be instantly replaced if necessary. The packing gland should be arranged, of of course, on the suction side and so that it can be repacked with engine running if required; it should not be possible for any leakage from this gland to find its way into the engine interior.
Carburettors and Induction Pipes – Carburettors should be fitted in a high position on the engine so as to interfere as little as possible with the access to inspection doors. Induction pipes should be fitted with efficient flame baffles. All carburettor controls should be integral with the engine. Carburettor drains and overflows should be carried overboard well aft.
Safety Devices – Should be employed whereby either lack of oil pressure or excessive speed would stop the engine at once by cutting off the fuel supply at the jet. It is important that this control should operatc on the fuel and not on the ignition, as when an engine which is working under heavy load is switched off it carries on self-igniting and eventually comes to rest after running astern violently for a few revolutions. The effect of this on the transmission might be very serious, and in addition the running asternl would fill the gondola with exhaust fumes, which might easily gas the mechanic if he happened to have the windows shut.
Starting Gear – A starting gear of some description will, of course, be necessary, and this problem becomes one of difficulty when a big bore engine of few cylinders is contemplated. The electric starter, employing a small high-speed motor, run from a battery, driving the engine round very slowly, through a reduction gear might be used, but there is the weight to be considered which, if the battery is big enough to give sufficient turns to the really refractory engine, will be considerable. Further, the engine sbould be turned over suffiently quick to pick up fuel from the jets.
The starting apparatus recommended is the suction-pump starter as fitted to the engines in the German airships. This apparatus has many advantages. It is simple and effective, light, requires no external doping, is safe, as it operates by suction and, therefore, the risk of petrol vapour escaping and settling in the bottom of the gondola is reduced to a minimum, and it does not set a limit to the number of starts. A brief description may be of interest. By very simple mechanical means all tbe valves in the engine arc lifted together about 3/32 in. from their seatings, and simultaneously the exhaust pipe is closed to the atmosphere and opened up to a large diameter hand-operated suction pump, the working of which draws air and petrol from the carburettors into all cylinders. After a few strokes of the pump, the valves are reseated and the engine may be started by hand magneto, the valve in the exbaust pipe having been reopened to atmosphere.
The exhaust manifold should, of course, be efficiently cooled. Water cooling has disadvantages as regards weight, and the fact that the radiator has to be increased in size to such an extent to provide the additional cooling required. An air-cooled exhaust system might be satisfactorily arranged by providing suitable ribs on the manifold, enclosed in a fairing, and utilising the exhaust outlet from the silencer to create an induced draught.
All exhaust joints should be recessed, and the exhaust washers should fit these recesses accurately, so that a washer blown out is quite impossible. Further, in a suction starting system as described above, it becomes increll.singly important to have sound exhaust joints.
As regards future requirements in engines, it is thought that the possibility should be considered of using engines of 500 to, say, 1,000 h.p., driving propellers at a distance of, say, 50 ft. as a maximum. An arrangement such as this would enable the heavy gondolas to be carried in that position on on the hull, which the constructional engineers consider to be the best, and the propellers would be driven through shafts at various heights up the side of the hull.
The engine seating in the gondola should be arranged to give free access to oil pumps and to the bolts securing the lower half of crankcase, and there should be sufficient room in the well under the cngine to allow the bottom half to be lowered and drawn out from forward. The gear-box should be situated well away from the stern of the gondola, so as to be accessible, and to allow the gondola to be streamlined off.
The radiator should be fitted aft, as close to tbe propeller as possible, so that adequate cooling is obtained at the slowest air speeds of the ship. This position will be found to enable the size of the radiator to be reduced considerably.
The transmission shafting should, of course, be well provided with flexible joints, of some very simple form. The spherical splined type is suggested as these are very simple and reliable, and are very noisy if any periods are encountered in the engine or transmission, which noise will tend to prevent an engineer holding on to a period any longer than absolutely necessary.
The reduction gear-box should be of plain spur-wheel type provided with pump lubrication. Provision should be made for the easy detachment of driving or driven wheels for replacement. In reversing boxes, the oil pump should, of course, he driven from the driving-shaft, so that oil pressure is maintained when going astern.
The propeller brake should be fitted on the tail shaft, so that the propeller is under control even when the gear is in neutral.
The practice of using two engines to drive one propeller through gears is thought not to possess sufficient advantages to justify it. No doubt the big diameter slow-running propeller which it is possible to use with this arrangement is more efficient, but it is a very debatable point whether the gain in efficiency is worth the extra complication entailed. Further, the running of two engines geared together in this mauner is by no means alway5s satisfactory, as it is so difficult to synchronise them, and extended running of them not synchronised leads to vibrations and strains being set up, broken pipes, etc. One engine in each gondola is sufficient, for if two are used, the size of the car at once becomes prohibitive, at least if accessibility to the engines is fully considered.
All filters should be, of course, in duplicate, so that one may be cleaned while the other is in use.
Valves should be fitted in both suction and outlet water pipes ncar the engine, so that in case of breakdown the engine may have its cylinders removed without losing a great deal of water.
A petrol flowmeter should be fitted so that the engineer can see at a glance his fuel consumptiou, and this will help him in stopping loss of petrol overboard if a carburettor float punctures.
All thermometers, pressure gauges, and other instruments should be grouped together on a board fitted to the side of the gondola. As little as possible should be attached to the roof, as this should be easily detachable for the purpose of shifting engines, etc., when required.
ENGINE-POWER AND SPEED
No suggestion has heen made as to the horse-power or r.p.m. required of the engine. Broadly speaking the former should be as high as possible, the latter as low. It is thought that 400 h.p. at 1,400 revolutions would be a useful all-round engine.
Of course, the engine-speed at 1,400 mentioned above, almost definitely calls for a reduction gear if the propeller is to be reasonably efficient, but the sooner the gear-box is discarded the better.
Too much stress cannot be laid on the importance of being able to carry out quite extensive refits in the air. Repairs of the order of fitting a new connecting-rod should be possible in, say, a period of four hours while under way.
Close attention should be given to the design of the engine seatings and gondola generally so that the fitting of a complete new machinery installation is, at the outside, no more than the work of one day. As an instance of this, it was found possible to change an engine in the N.S. class of ship in six hours’ work. It is, of course, a far more complicated job to change a set of machinery of a rigid ship, with its gearboxes, etc., but nevertheless it is thought that if universal joints and other details in the transmission and engine and gear-box seatings are of suitable design, it should not be necessary to do any lining-up at all.
In the engille design, more importance is thought to attach to fuel economy at all speeds than weight per b.h.p., because of the length of flight the ship may be called upon to carry out, estimated at 100 hours.
The question of recovery of water from the exhaust gases is a most important one, and it is thought that no expense should be spared in the carrying out of experiments on this subject.
It is understood that an engine of large output at a low speed of revolution is now in its experimental stage. In the event of this proving a success, it would appear that the abolition of the reduction gear-box is in sight. Attention should, therefore, be given to the question of a satisfactory form of propeller with swivelling blades. A satisfactory reversible propeller would have further advantages in the ability to adjust the pitch for varying conditions.
With regard to the question of the fuel arrangements,ear that the present system, which entails a great deal of manual labour, is not satisfactory, for several reasons. It is slow in operation, both in re-fuelling the ship, and in trimming during flight, and it definitely calls for extra men in the crew, who could be dispensed with if the system were made morc automatic in its action. The suggestion is made that experiments be carried out in connection with propeller drive petrol pumps, several of which could be fitted along the keel on hinged arms so that they could be swung into the ship when not required.
With regard to the type of fuel used, it is thought that the question of paraffin should be thoroughly investigated, since by its use the danger of fire would he reduced to the minimum, and also the question of its cost renders it attractive. It is admitted that the weight of a 500 h.p. engine designed for paraffin would at the present day appear excessive, but for future requirements with, say, a ship of 10,000,000 cub. ft., it might be found practicable to employ machinery much heavier tham is at present possible.
This article was originally published in Flight on 25 March 1920.
Major C. F. Abell, OBE, FRAeS, MIME
Educated at Sherborne and Leeds University, from 1915 to 1919 Charles Francis Abell served in the RNAS and RFC. After the war he joined the editorial staff of The Automobile Engineer, leaving in 1922 to become works manager of Sheffield-Simplex, Ltd. He joined the Bristol engine division on January 1st, 1927. For some years manager of the experimental department, he later became service manager, an appointment he held until his retirement.
One of the first tasks undertaken by Frank Abell for Bristol was to supervise the installation of the Mercury engine in the Short Crusader for the 1927 Schneider Trophy race. Before he retired, he had seen the Proteus turboprop installed in the Britannia and the Olympus turbojet in the Avro Vulcan.