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Aviation History
1936
1936 - 0632.PDF
MARCH 12, 1936. FLIGHT. 2/5 Four- and five-tube Burgess silencers, which have lately been tested at the R.A.E. it can be made up into individual elements which could be housed actually inside a fuselage, nacelle or wing. C. G. Vokes and Co., Ltd., have lately taken out a patent for incorporating the silencer in the leading edge of a wing where its warmth would do a certain amount of de-icing. It is also possible to incorporate it in the exhaust collector ring of a radial engine where it offers no resistance. For one bank of a Kestrel III engine a silencer of this type weighs 30 lb.—or 24 lb. if 22-gauge material is used. Results given in Table 3, were obtained with the Kestrel III silencer. Actually these tests were made without draught from a fan blast^vhich would give two to three more decibels. The temperatures are estimated. TABLE 3. No. of Elements. 4 0 5 plus interference box. 4 plus interference box. Reduction of Sound. Decibels. 20 88-27 28 22 23 Temperature of Silencer. 250 deg. 250 deg. 250 deg. 250 deg. Back Pressure. In. of Mercury. 0.555 0.11 0.8 0.555 The Burgess type tested at Farnborough during these particular experiments consisted in its simplest form of a straight tube through which the gas had an unobstructed flow. The tube was perforated and enclosed by an outer shell of considerably larger diameter, the space between being filled with a refractory packing material having sound-absorbing properties. The results given in Table 2 were obtained with a silencer having four perforated tubes in a single shell and a filling of a special form of mica. This silencer dealt with the high-frequency band by absorption and the low frequency by absorption and reflection. The Burgess H.D.F. type of silencer, the special con struction of which combines low-frequency filters with high-frequency absorption tubes, is a type in general use on heavy industrial engines. If applied to aircraft engines it could be expected to give a greater noise reduction than that obtained from the type actually submitted to test. Silencers of this type would be slightly heavier and some what more expensive to produce, and until means have been found to reduce airscrew noise, the manufacturers maintain that the simpler and lighter type of Burgess straight-through construction appears to be adequate. Reference has already been made to the R.A.E. silencer. In this each high-frequency element consists of a portion of the centre tube surrounded by a larger tube, the inter space being divided by cross plates into a series of annular compartments. A number of equally spaced radial holes are drilled through both tubular members in each com partment. The outer shell is divided by a conical baffle into two compartments. This principle of positioning a high-frequency filter at the entrance of a large expansion chamber was adopted to prevent the outer shell resonating with the high-frequency sound, an object attained in the Vokes silencer by the asbestos-lined double wall. Although they have experimented with various types of silencers for radials, the Bristol company hold the opinion that the forward type of exhaust collector ring offers the most satisfactory solution from all points of view. They maintain that instead of increasing drag it can, in its most modern form, be blended with the fuselage or nacelle in which it is incorporated to form an efficient aerodynamical entry. It can make effective use, without increasing drag, of the available slipstream for cooling purposes, thus reducing the risk of fire in a crash—a characteristic not usually possessed by the rear exhaust collector ring—as well as permitting the exhaust gases to be released at the The Bristol company has developed a very efficient single - outlet exhaust collector ring for its latest engines. Here it is embodied in the N.A.C.A. cowling of a Pegasus-Douglas *; •- -
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