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Aviation History
1934
1934 - 0157.PDF
FLIGHT, FEBRUARY 16, 1934 ENGINE COWLING AT the meeting of the Royal Aeronautical Society onFebruary 1 at which Mr. J. D. North read apaper on Engine Cowling, with Special Refer- ence to the Air-Cooled Engine," Mr. C. R FairevWaS,,in, u C fr' In introducing the lecturer, Mr. Fairev recalled that they had listened recently to the story of thedevelopment of the air-cooled engine, told by Mr Fedden It was generally conceded that a great part of that develop-ment had been due to improvements in cowling and Mr- North was to tell them that evening the story of improve-ments in cowling. Mr. North's paper was a long and interesting one. In last week s issue of FLIGHT we published a summary of the first part of the paper. The second, and rather more technical, part will be summarised in THE AIRCRAFT ENGINEER (Monthly Technical Supplement to FLIGHT) on February 22. Extracts from the discussion which followed the reading of Mr. North's paper are given below. THE DISCUSSION MR. A. H. R. FEDDEN, who congratulated Mr. North upon the thorough manner in which he had reviewed the important and interesting subject of engine cowling, said he had seen the paper for the first time only that morning, and had not yet had an opportunity to consider it really carefully. Mr. North was one of the first people in this country to appreciate the possibilities of drag reduction by ring cowling on air-cooled radial engines, and had com- pleted much valuable work in this direction. Mr. Fedden understood from Capt. Barnwell's wind tunnel investigations that there was no engineering differ- ence between the forms of external cowlings known as N.A.C.A. and Townend Rings, and that all our model tests had shown that any reduction in drag due to the fitting of an external cowling round a radial engine was entirely due to a forward component of air force on this cowling, and was invariably accompanied by an increase of drag on the rest of the model. Assuming that these wind tunnel tests were correct, he felt that the N.A.C.A. cowl- ing, with suitably proportioned baffles to the cylinders, was an interesting combination in certain types of machines. Perhaps Mr. North's remarks on the N.A.C.A. cowling were somewhat severe when he had said that cooling diffi- culties arose with geared and supercharged engines. The latest Douglas high-speed passenger machine was powered by two geared and supercharged 700-h.p. " Cyclone " engines, and it was maintained that without the N.A.C.A. cowling it would be impossible to cool adequately under the particularly arduous test conditions which the Depart- ment of Commerce tests called for. This machine had to take off from an aerodrome at an altitude of 4,943 ft. with one engine out, and to climb over 8,000 ft. with manifold pressures rising as high as 3 lb. per sq. in. What had not been fully realised in this country was the importance of correctly designed inter-cylinder baffles, and the great effect of fuselage shape. The great importance of the fuselage had recently been demonstrated most convincingly by a test in which a given N.A.C.A. cowled engine cooled adequately on a machine with nicely faired nacelles ; but when the same engine and cowling were installed in another single-engined machine with an unsuitable body shape and humps, the engine overheated very seriously. The cylinder tempera- tures were about 60 deg. higher than on the twin-engine nacelles. Another point of interest was that velocity measure- ments in the neighbourhood of the cylinder barrels might prove unreliable, and in the United States it was usual to use the difference in total head in front of and behind the inter-cylinder baffle as a measure of cooling air speed. In the case mentioned, the differential head was 4 in. of water during climb at 100 m.p.h. with the correct fuse- lage, and only 1£ in. of water at the same speed on the second machine. In level flight cruising, the differential head was 12 in. of water on the first machine. It was interesting to note Mr. North's statement that struts, cylinder baffles, etc., within a Townend Ring had a neg- ligible effect on the total drag, and further information on this point would be appreciated. Mr. Fedden would be glad to feel that this was so ; but he suggested that the statement was contrary to the results of practical full- scale tests made in this country and America, where it had been found that inter-cylinder baffles improved cooling, but increased drag with a Townend Ring, but that they im-proved cooling and did not increase drag with an N.A.C.A. cowl in front of a properly proportioned body. Whilst fully appreciating the value of the Townend Ring,and acknowledging the force of Mr. North's remarks re- garding field of vision, field of fire for forward guns, etc.,it was worthy of note that, whereas Townend Rings were used to some extent in the United States, more generallyon single-engined pursuit machines, the tendency with the latest high-speed machines was to have complete N.A.C.A.cowling, especially where there were two engines mounted in wing nacelles. It was felt that the N.A.C.A. cowlingwas more valuable than the Townend Ring from the point of view of drag, and that, provided the fuselage was cor-rectly designed and suitable inter-cylinder baffles were in- stalled, the engine could be cooled more satisfactorily. With reference to Mr. North's remarks on the effect ofring cowling on carburation, it was agreed that any form of cowling round a radial engine altered the configurationof the air stream, and that it was necessary to have a . design of carburetter in which the pressure balance effectswere not altered ; this matter had been dealt with on the carburetters of recent type Bristol engines. Mr. Fedden was fully alive to, and in agreement with,Mr. North's view in regard to the importance of suitably ring cowling air-cooled radial engines, but he was hopingthat the aircraft designer would give as much attention to the body immediately behind the cowling as to the cowl-ing itself. For the future, he suggested that controllable cowling would be absolutely necessary, and it was interest-ing to note, in connection with the patent applications given in the paper, that Patent Specification No. 11315was granted to Mr. Granville Bradshaw in 1917, which provided for a cowling practically identical with theN.A.C.A. type, and with controllable flaps to meet climb- ing conditions, combined with a really close cowling withinter-cylinder baffles. MR. H. C. H. TOWNEND, discussing the Townend Ring,and the rough rule given by Mr. North in the paper for estimating what ring angle was likely to give success, putforward another point of view, which supported Mr. North's statements. He illustrated it by means of slides. The first slide showed a model tested in the windtunnel. The model was made in two dimensions, i.e.. the body was represented by a thick symmetrical aero-foil, with a row of knobs to represent the cylinders and a straight aerofoil to represent the ring. The second slide indicated the air flow around the bodyin the absence of both the cylinders and the ring. The cylinders were dotted in in the position which they wouldeventually occupy, and the ring was also shown in the position which gave minimum drag. He pointed out thatthe section was lying with its no-lift line practically along the local streamlines, so that if the engine only were re-moved, the ring would not be expected to have much influence on the flow around the body. When the enginewas introduced, it immediately put the ring up to a high lift coefficient ; and it was possible that if the streamlinescould be obtained when the model was mounted in the wind tunnel without the engine on it, a very good ideacould be obtained of the angle at which the ring ought to be set when the engine was eventually put in its properplace. With regard to the point made in the paper that thebulbous nose was a good thing, he asked if Mr. North could state his views as to the reason for that. Discussing the statement that sometimes the effect ofthe ordinary crankcase cowling was to warm up the crank- case and reduce the cooling of it, Mr. Townend asked if itdid not also probably make the cylinder heads somewhat cooler. MR. HOFMAN, after assuring Mr. North that the in-formation contained in the paper was welcomed and would be studied very carefully, suggested that Mr. North hadbeen rather hard on the N.A.C.A. cowling, particularly in his remarks as to the obvious advantages of the Town-end Ring over the N.A.C.A. cowling. The difficulty about the latter interfering with gunfire had been overcome inseveral cases by the slight readjustment of the position of the gun, and a large number of military planes had beenbuilt in the United States with N.A.C.A. cowls fitted, there being no interference with vision. With regard to cooling, Mr. Hofman referred to some 157
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