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Aviation History
1917
1917 - 1192.PDF
•^ <5R6',JK£r"5i NOVEMBER 15, 1917. valve seatings, leaky piston rings, faulty valve springs, loose valve caps (of the inner and outer types), spark- ing plugs not tight, looseness being occasionally due to faulty threads, which may give a false feeling of tightness, although the copper washer is not com- y pressed, leaks in air connections ; this, of course, only applies to engines which are started by means of compressed air. The remedies for the above are all obvious, and they are enumerated with a view to reminding the reader of the various contingencies with which he may have to contend. There are few people who have had any experience with engines, who cannot remember the moments spent in trying to think " what it can be," and a few words in black and white may bring the fault to mind at the critical moment. (3) Ignition.—One of the most prolific causes of trouble with ignition is to be met with in rotary engines which have the distributor so arranged that it is a matter of the greatest difficulty to remove oil from it. It is a good plan invariably to wipe the distributor immediately on conclusion of a flight, using a series of dry cloths, remembering that petrol does not dissolve castor oil, but that it may cause distortion of the distributor ring. The writer has met with not a few cases of faulty distributor rings, in new engines and in engines which have run a very few hours. The faults are due to either faulty material or poor workmanship, and the result most often to be met with is the unevenness of the fibre and metal insets or contact pieces, the latter either projecting from, or being sunk into, the former. In both cases, the effect is to cause jumping or chattering of the carbon brush or brushes, and hence irregular firing, excessive wear and danger of fracture to the carbons. Acci- dental earthing of the " earth " wire to or from the switch will result in failure to obtain a current. If this is suspected, a temporary removal of the " earth " connection from its terminal on the magneto will conclusively prove if the assumption is correct or not. If this connection be removed it should be remem- bered that while the wire is detached, the engine cannot be stopped by the switch. In rare instances nowadays a high tension (plug) wire may be found to have grounded on some part of the engine, by reason of the rubber insulation having come in contact with hot metal. In rotary engines, the bare wires from the dis- tributor to the plugs should never be used more than once, as by being bent or twisted they become brittle and are liable to fracture. (4) Valve Tappet Clearances.—The adjustment of these clearances is of great importance, and therefore extreme care should be taken to see that the tappets are all evenly and accurately set. Considerable loss of power may result from carelessness in this respect. In a new engine or in cases where new valves have been put in, the clearances should be frequently checked, as, until the valves have thoroughly seated themselves, the tappet clearances alter considerably on account of the valve stems lengthening under the influences of heat and the hammering action of their motion. .._,-•. (5) Valve Setting.—The above heading has been included, partly as an opportunity of noting that, generally speaking, handbook instructions should be rigidly observed, but that also, and this is equally important, a very considerable latitude may be allowed, under certain special conditions, and adher- ence to handbook instructions or precedent should not be carried to a slavish degree. It may be of interest to the reader to know that in a certain 12-cylinder vertical engine, having overhead cam shafts, which were fitted with a Vernier form of adjustment allowing infinitesimal alterations in timing, and each rocker arm (actuating the valves) mounted in an eccentric bearing, which allowed minute and independent adjustments to each valve, it was found that an extraordinary number of varia- tions of timing could be made, and in the final " tuning up " the actual timing of each cylinder showed considerable variation when compared with the others. (6) Lubrication.—Very little can be said on this subject that would be suitable for inclusion in this article. It is hardly necessary to say that every pre- caution should be taken to see that the lubricating system is in order and that the prescribed amount of oil is used. In rotary engines care should be taken that lubricating oil should go into the engine and not on to the outside of the cylinders. Congealed oil, burned on to the radiating fins of air-cooled cylinders may just make the difference between a cylinder " bhieing " and keeping bright. (7) Condition of Valves.—It is extraordinary how few air-mechanics realise what really " grinding in valves " means. Most of them are quite satisfied as long as the valve seat is bright, but pay little atten- tion to the quality of the surface obtained. The seat and seating should be ground in in the ordinary way first, and then finished by careful grinding with ultra fine pumice powder or similar compound, until a per- fectly smooth finish is obtained. This greatly in- creases the time that the valve will keep clean, apart from obtaining a good seating. The working of the valves will be very greatly helped by the application of some defloculated graphite on stems and guides, and if the tops of the valves are polished and a very thin coating of this graphite applied, the deposit of carbon will be appreciably reduced. It is doubtful if the excellent qualities of the above compound are fully realised, except by very few mechanics and officers. A small amount mixed with lubricating oil has the effect of putting a face on surfaces, to be obtained in no other way. The writer has used it for some seven or eight years in a variety of aero engines and car engines with, in the latter instances, really wonderful results in oil economy. (8, 9,10 and 11) These subjects have been dealt with in the first part of the article under itemised headings. The writer has for some time wished that some medium could be found for an exchange of notes, " hints," and so forth, and earnestly hopes that any omissions, errors, or the like will be pointed out to him. Correspondence will be welcomed and replied to when circumstances permit. The King and Queen at Bristol. CERTAINLY one of the most interesting chapters in thevisit of their Majesties to the West of England was the inspection of the British and Colonial Aeroplane Factory.On arrival the King and Queen were received by Mr. Samuel White, Chairman, and the Managing Director and members of the staff had the honour of being presented. Major Ivan Davson, representing the Controller of Aeronautical Supplies, and lieutenant B. C. Bartley, Assistant Director Aeroplanes Supplies, were also presented. Their Majesties then passed through a number of the work- shops and saw the various processes of construction of Bristol aeroplanes. jr ITg2
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