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Aviation History
1929
1929 - 0433.PDF
y O AIR-COOLED ENGINES IN SERVICE* (Concluded from p. 144.) Air-Cooled Engine Installation THE question of air-cooled engine installation is a subject ^covering a wide field which I cannot attempt to deal with hi per, but there are certain broad principles affecting engines in service to which I wish to refer, which are as follows :— (a) Cooling Capacity-—In Mr. Penn's paper, entitled " Aeroplane Engines in Flight," which he read before the ' Royal Aeronautical Society last November, he gave us some interesting data in respect of the experimental work carried out at the R.A.E. to determine the cooling capacity and radiator efficiency on water-cooled engine installations. Up to the present time no such basis has been laid down for air-cooled engines. From a theoretical point of view, each air-cooled engine should have a different fin capacity according to the aircraft into which it is installed ; this, of course, is impractical, but it is submitted that some definite schedule should be laid down for measuring cylinder and crankcase temperatures in a new installation. The same air-cooled engine is often installed in many different aircraft ; in one instance the cylinders are exposed to the slip stream and there is a free get-away for the cooling air, and in the next machine the same engine may be installed eccentrically, or where there are projections behind certain cylinders, and the air speed over the cylinders is as much as 30 per cent, less than in the first instance. I believe this problem has been masked on air-cooled engines because on the water-cooled engine, if the cooling capacity- is insufficient, trouble is experienced at once. In the case of the air-cooled installation, the engine is more patient and does not so quickly and so violently object to the way it has been installed, and it is quite possible for the engine to run for a considerable period before trouble is experienced. The Bristol Company are endeavouring to remedy this defect by supplying to their customers a simple equipment which will quickly determine whether the engine has been properly installed or not. (b) Crankcase Cooling and Lubrication.—It is quite common for the crankcase temperature on an air-cooled engine to vary as much as 50 per cent, with the same type of engine in different installations, and the effect of such a difference in temperature is obvious on the actual working clearances and the temperature of the lubricating oil. (Generally speak- ing, the slower the machine, the greater the need for adequate crankcase ventilation. Crankcase temperatures materially affect the distribution and general running of the engine. Here again, it is considered that the engine constructor should provide a more definite schedule as to the temperatures permissible. This matter should be closely investigated on each new type of machine. Fairly recently there has come to my notice a case where four identical air-cooled engines were in competition in diffe- rent aircraft. All four machines had, to the eye, a very similar type of cowling ; three required oil coolers, and the fourth functioned absolutely satisfactorily without any oil cooler at all. As the machines were built for the same competition, the difference in speed range was not sufficiently great to account for this discrepancy, and the machine in question had undoubtedly some cooling characteristics very different to the others. These remarks apply to the radial type of engine, with which 1 am familiar, but I am quite convinced that similar remarks apply to the in-line air-cooled engine, and that if in the future we are going to see large air-cooled in-line engines functioning satisfactorily, reasonable care and intelligence will have to be used to develop a scheme by means of which the necessary velocity of air is passed through the fin cells "f each individual cylinder with minimum disturbance and interference. (c) Air Intake and Exhaust Outlet Systems.—All petrol engine carburettors require some form of heating. The air- ooled engine employs either hot oil, warm air from the1 ylinders, or hot spot from the exhaust gases. Oil, owing to its iow specific heat, is a poor medium for this purpose, and some- times recourse has to be made to one of the other two methods. The problem is not so easy as it looks, because of the many lifferent installations and the divided responsibility of the 'ircraft and engine builder, but I have proved that the service • Summary of Paper read before R.Ac.S. on Feb. 14, 1929. obtained from an engine has been seriously upset owing to the incorrect fitting of air intakes. What is wanted is some system which does not require individual attention on each installation, and eliminates the rather haphazard " cut and try " method which is so often in operation, and I suggest that the solution of the matter is some definite flame damping device in the induction system, which will enable the air intake to be placed with safety inside the cowling ; that this air intake should be provided with some simple form of air cleaner, and that the induction system should be provided with exhaust heating, which can be definitely controlled from the pilot's cockpit. Exhaust heating in my opinion is absolutely necessary for reliable running and good distribution in cold climates. If the engine maker will face up to some such system as outlined above, this could be made universal for all types of aircraft, and would, I am quite sure, prolong the service obtained from air-cooled engines. For comfort and night flying the exhaust gases on an air- cooled engine require dealing with adequately, and I believe that an air-cooled engine, with a properly designed collector system, gives better service than without, but if the system is of insufficient area the cylinder temperatures of the engine are naturally increased, with serious results to the engine generally, and indifferent service is certain to ensue. The question of fire prevention also comes into this matter, and it is most important that the collector should be so arranged that it is not shielded from the slip stream on any surface and so has hot spots which would cause dangerous temperatures. (d) Ft el and Oil.—The service rendered by air-cooled engines is affected by the use of suitable fuel and oil, and it is not so very long ago that the makers of British air-cooled aero engines were criticised in respect of the grades of fuel and oil they demanded, and it was stated that the American air-cooled engines would run on any ordinary grade of fuel and oil. It is significant to note that at the Aeronautical Conference at Chicago in December last these views were modified, and that as a result of considerable experience gained from operating air-cooled engines in America it was brought out that it paid to use good fuel, and definite stan- dards are about to be drawn up for fuels for air transport in America. Fortunately in England we are provided with excellent fuels, but in some foreign countries air-cooled engines are expected to run on low grade fuels for which they have not been designed. If the carburettor heating is on the lean side the effect on the engine with this low grade fuel is far more serious, and I believe this is a further claim for variable heating control. I think, probably, the most simple solution is the use of dope, and it behoves the engine builder to see that his valves, valve seats, valve guides, etc., are capable of coping with reasonable quantities of such dope as ethyl fluid. As a result of some considerable experimental work on air-cooled engines, I have definitely proved that reasonable quantities of dope can be used without affecting the service of the engine. On the question of lubrication oils, I think there is a good deal more to be said for the American view. The best oil is the cheapest in the long run from a service point of view, but owing to storage and climatic conditions, it is impossible, in certain countries, to use compounded oil, and we must get down to pure mineral oil for air-cooled engines. Some considerable amount of experimental work has been carried out lately by the Armstrong-Siddeley and Bristol Companies, and long tests on pure mineral oil have been made with complete success, and I think within a short time we shall see air-cooled radial engines in this country operating regularly on pure mineral oil. The use of pure mineral oil will certainly help the dope question. I think we are in a position to ask aircraft constructors, as an inducement to use mineral oil, to instal, at any rate on all commercial aircraft, a proper oil cleaner. Such a cleaner will, I am convinced, considerably extend the service rendered by air-cooled engines. The Possible Lay-out of Air-cooled Engines Most of the foregoing remarks in this paper apply directly to the air-cooled radial engine, because this is the type of engine with which I have been most closely associated. During the last few months, however, a great deal of 169
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