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Aviation History
1921
1921 - 0816.PDF
~ DECEMBER 8, 1921- A NEW SILENCER FOR AERO ENGINES The Birger " FOR some months past there have been rumours of a new Swiss silencer for aero engines, which was said to have given excellent results, both as regards silencing the engine and also from the point of view of absorbing little power. This Silencer is now being introduced in this country by Sir William Maxwell, K.B.E., of 6, Broad Street Place, London, E.C. Sir William, it may be mentioned, is one of the directors of Titanine, Ltd., the well-known dope manufacturers of Empire House, 175, Piccadilly. The new silencer, which was put through preliminary tests at the R. A.E., Farnborough, is the invention of M. J. Birger, a Swiss engineer, and was demonstrated at Stag Lane aerodrome last week. Fitted to a D.H.9C, with Siddeley " Puma " engine, the " Ad Astra " silencer proved remarkably efficient as regards its silencing properties, and one can well believe the various technical reports which s"tate that conversation between pilot and passenger is easily possible at full throttle. Personally we were unable to test this, as a thick fog prevented flying on the day of our visit to Stag Lane, but a run on the ground, although not a fair test on account of the fact that the silencing is then less complete, indicated that in this respect the new silencer is very effective. The demonstrations were witnessed by representatives of several aero engine firms. M. Birger was there to give any technical information desired, Mr. Ortweiler, of Aerial Derby fame, acting in the dual capacity of interpreter and pilot. As, at the moment, no sectional drawings of the " Ad Astra " silencer arc available, it is a matter of some difficulty to do more than indicate th± general principles upon which the silencer is designed. It consists, essentially, of two portions, a fixed silencer in which the exhaust gases are made to follow a zig-zag path, and a rotor in which the gases are cooled before being allowed to escape into the air. The exact details of the silencer are not definitely known, but generally speaking they do not differ greatly from those in use on motor-cars. The collector pipe from the cylinders leads the gases into the rear end of the silencer body, and one gathers that the usual perforated plates and deflectors are fitted, which reduce to a certain extent the velocity of the exhaust gases. The latter are finally allowed to escape through the front of the silencer into a streamline nose-piece which is rotated rapidly by the slip stream of the propeller. This rotor, which runs on ball bearings, has an aluminium covering with spiral corrugations, and is, moreover, provided with internal blades, running, one gathers, radially from the central spindle. These blades act in a certain measure as a fan, but their main function is to throw the gases out against the rotor covering by centrifugal force. The covering being of aluminium, and therefore a good conductor of heat, the gases are cooled rapidly and consequently contract, and it is claimed that this feature not only decreases back pressure, but actually slightly increases the power. We have had the privilege of reading extracts from a report on tests on one of these silencers at the Swiss Federal Aerodrome at Dubendorf, where a comparison was made between the engine running with free exhaust and with the " Ad Astra " silencer. On these tests it was found that there was no loss of power (this was confirmed at Stag Lane, where the number of revolutions was the same when the silencer was cut out as when it was working) and that there was a saving of 4J to 5 per cent, in fuel consumption. It is thought that with an improved type (the one under tests is stated to have been of fairly primitive construction) a saving in fuel Ad Astra";; of about 8 per cent, may be effected. It was also found that, although a smaller quantity of cooling water was used, the temperature was lower when running with the silencer than when running with free exhaust. When the engine was examined after the tests it was found that the sparking plugs and valves were remarkably clean. The silencer fitted to the D.H.9 for the demonstration weighs, we understand, 22 kilograms, but as it is designed for a higher-powered engine the weight, it is claimed, can be materially reduced. As regards head resistance, we are informed that this is equivalent, for the silencer shown, to that of a flat disc of 12 cm. diameter. The question of resistance is, as M. Birger points out, to a great extent a question of choosing a good streamline shape. As regards the adaptability of the " Ad Astra silencer " to different types of engines, the inventor claims that there is no difficulty about the Vee type, and that the silencer can be THE " AD ASTRA" SILENCER : View of the apparatus fitted experimentally to a Siddeley "Puma " engine on a D.H.9 biplane. placed in any desired position. In the case of a Vee it would probably be placed above the engine, between the two rows of cylinders, although it would have to be»placed so high that the exhaust gases could escape over the top plane of a biplane. Fitting it to a " broad arrow " type might be a matter of greater difficulty, although in that case there would presumably have to be a larger silencer serving two banks of cylinders and a smaller on the opposite side serving a single row. It cannot be said thatthe " Ad Astra " increases the good appearance of a machine, but if all the claims made for it can be sub- stantiated this is a matter of small importance. The addition of about o • 1 lb. weight per h.p. of the engine, and a small increase in the resistance of the aeroplane, is a small price to pay for engine silence, especially when, as is claimed in this case, this is accompanied by a saving in fuel, and greater cleanliness of plugs and valves. T23& • - -HP •*•" • • " ~ • ' -'• •••' " LONDON TERMINAL AERODROME Monday Evening, December 5 BAD flying weather has interfered again with the regularity of the London-Paris service. Several days last week were blank from an air transport point of view, and today there was again no flying owing to dense fog on the coast. On Tuesday Mr. Uwins arrived on the Bristol io-seater, en route from Bristol to Martlesham. The double under- carriage has been replaced by one of the single Oleo type, and the engine-starter has been removed. With these alterations, I understand that the tests at Martlesham were very satis- factory. With a load of 12 passengers and 200 pounds of goods, and with full tanks, the machine got off the ground in 180 yards, a testimonial to the reserve of power in the Napier " Lion " engine. With the same load the machine cruised comfortably at 100 miles an hour. " Ferry " Fees for Pilots. " " - " APROPOS my remarks last week as to the price paid aviators for delivering aeroplanes to Brussels for the Disposal Co., I understand that this has now been increased, so that the pilot makes at least £8 8s. on the trip. Rumours to the effect that one pilot took over a machine at the old price while negotiations were proceeding is, I am given to understand, incorrect, the pilot having been promised the rate which was decided upon when negotiations were completed. The staff of the Disposal Co. are holding a competition in connection with the dressing of dolls, prizes being offered for the best-dressed dolls in different classes. In one class, for instance, the doll must not cost more than twopence, it being the ingenuity in the dressing that counts. No entrance fee is charged, and the competition is open to 8l6
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