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Aviation History
1917
1917 - 1169.PDF
NOVXUBXX 8, z 917. NOTES ON THE RUNNING ENGINES,OF PILOTS. By IT has frequently been the experience of the writer that a large proportion of flying officers are, very naturally, handicapped by a lack of knowledge regarding the vagaries of the engines fitted to modem aircraft. It is the aim of the following notes to give a few hints and tips, in genuinely non-technical language, which, it is hoped, may be of some occasional service. The writer has seen the majority of the handbooks published by the flying services, and issued by the OF AIRCRAFTFOR THE USE ••GNOMAD." plugs, or clean and heat the ones removed before replacing them. What may be very valuable minutes will be lost by making a second attempt to start up, even if the reason of the failure to start be such as failure to have the switch at " contact," or a wrong position of throttle. When once the com- pressed air has been admitted to the cylinders, water will be found to have collected on the plugs, unless the heat of the plug has been sufficient to evaporate it. It will not be out of place here to mention that the manufacturers, and one point seems to be common nuisance of plug changing may be greatly diminished to both, i.e., the absence of " running tips," whereas great importance is given to such details as clearances, tolerances, nature of metals, and so forth. These may be of the greatest value to engineer officers and air-mechanics, but are of comparatively small use to pilots. It may be taken that the flying officers' interest in The same remark, of course, applies to the changing his engine starts at the moment when he approaches of a set of plugs. Twelve plugs (hot) under a closed cowl have been removed and new plugs (hot) inserted by 4 A.M.'s in under 50 seconds from the moment when the engine ceased to revolve ! The threads of spare plugs by training air-mechanics in a " plug changing stunt.' In engines which may be required at short notice for urgent duty, a set of plugs which have been tested in that engine, and kept heated, can be in- serted in an astonishingly short time by three mechanics who have been trained to work togther. his machine in contemplation of a flight, and when " revs." are the one important point to be gained. It may not be out of place to remark that few of the " hints and tips " here given have not proved of value during two and a half years' experience of should be kept slightly greased, and should invariably nearly every aero engine in use in the two Flying have been tried in their respective cylinders to ensure a quick change. Too much attention cannot be paid to the gaps between the plug points. Inequalities in the gap may result, under certain conditions, in vibration and, of a certainty, destroy synchronous firing, even when it may not be noticeable to the pilot or engine hands. It is to be trusted that the reader will not deem some of these precautions meticulous. The writer once witnessed the failure of five out of eleven machines to start, owing to troubles which would not have occurred had the above precautions been taken, and owing to the particular circumstances of the case in point, this failure might have been attended by disas- trous results. Another point regarding plugs may be mentioned, viz., the effect of heat on plugs. A great many rotary engines are started and almost immediately are opened "full out," and it is beyond doubt that, owing to the sudden heating of the plug points, strains are imposed on the plug which would not occur if the heat were more gradually raised. This has particular bearing on the gaps in certain types of plugs; gaps which have been most carefully set before insertion will be seriously dis- turbed by being suddenly subjected to their maximum temperature. As is well known, the bottom plug in a rotary engine should be removed after flight, to prevent the collection of oil in the bottom cylinder when the engine is stationary, but a point that is frequently missed is that an engine that has been run until the oil is hot enough to become thin, is often stopped for an appreciable time before being run again, and it will often happen that during this time, sufficient oil will run into the bottom cylinder, or the adjacent one (which adjacent one depends on the direction of rotation, and plug position), to oil up the plug; hence it is advisable to turn the engine by means of the prop, in the time between the cessation of cne flight and the beginning of the next. - (To be concluded.) Services. Plugs. It is generally admitted that plugs are one of the chief sources of small trouble to-day, and these troubles will not diminish as the efficiency of engines is increased. One is frequently asked " Which is the best plug ? " and this question implies, in the writer's opinion, a misconception on the enquirer's part of the points met with in the plug problem. There is no " best plug"; a plug that will " stand up " in one engine is quite unsuitable in another, and this fact has been known to apply, even in the case of two engines of identical make and power. A no h.p. Clerget engine has been flown at full revs, when fitted with a set of is. 6d. motor cycle plugs, and after I hour and 40 minutes showed no signs of the in- dignity to which it had been subjected. It has been thoroughly proved that inserting heated plugs in an engine, immediately prior to flight, eliminates a very large proportion of cases where the engine refuses to start. The heating of the plugs ensures that no water can be there, it helps the firing of a mixture which may not be perfect, and thick oil which comes in contact with a hot plug is not so liable to oil it up as would be the case if the plug were cold. The above remarks apply particularly to rotary engines and to large water-cooled engines which are started by compressed air. In the latter instances, should the engine fail to start at the first attempt, condensation will almost invariably put several plugs out of action. This trouble may, to a great extent, be obviated by the use of heated plugs, which must only be inserted just before the engine is to be started, for, although the body of the plug may lose its heat very quickly on contact with the cold cylinder, the electrode will retain a very considerable amount of heat for some time. If, for any reason, the plugs should have lost their heat, it will always saye time to remove them immediately after the first failure to start and, either replace them with a set of fresh hot 1169
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