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Aviation History
1937
1937 - 0513.PDF
FEBRUARY 25, 1937 11 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT 198s on AT-IVE MERITS OF \AROJS A1R-Q00LED ENGINE UftOUTS A wealth of information is available at a glance from this chart. equivalent conventional form of cowled radial engine appeared to suffer more than the in-line engine in this respect,, a con tributory factor being a boundary layer breakaway over the airscrew spinner which was the centre of the duct entry. Tests carried out in N.A.C.A. wind tunnels on radial air- cooled installations showed that for a given nacelle-wing combination, the maximum propulsive efficiency was obtained with the airscrew located at 30 per cent, of the chord ahead of the leading edge, and tests in this country confirmed this for similar geometric arrangements. Research at the R.A.E. also showed that the radial engine nacelle drag was a function of the ratio of wing thickness to engine cowl diameter. From a consideration of pitot entries in the leading edge of the wiag, and provided the internal wing structure was arranged for duct cooling and accessibility, the drag due to spoiling loss and nacelle surfaces could be practically elimi nated by the retraction of the radial engine within the leading edge oii aircraft with wing thickness equal to the engine diameter. Existing test data indicated that if the airscrew were required to work within 10 per cent, of the chord its propulsive efficiency would not be appreciably affected. This arrangement would only be possible on aircraft of at least Short Empire boat size and proportion. Fuselage interference effects and in many cases the housing of a mechanically retracting undercarriage needed careful con sideration in deciding upon the design of a nacelle and its position in relation to the wing if the minimum drag was to be achieved. A compromise generally had to be made after consideration of all the factors involved. Research as has been carried out on full-scale installations to date indicated a form drag with nacelles irrespective of the type of engine enclosed, and by the reduction of this drag in common with the development of the technique of surface finish there was an appreciable gain to be derived. Until a series of full- scale tests was carried out it would be impossible to assess with complete accuracy the relative drag cost of representa tive power units mounted on a wing. There was some evidence to show that on a radial air- cooled engine an improvement was to be obtained by deleting the conventional controllable gill exit and ducting the cooling air to the trailing edge of the wing. A controllable flap at the exit would be required to induce flow through the duct tor engine running on the ground and in climbing flight. ihe passage areas were arranged in a representative double- row installation so that the air velocity did not exceed 100 rn.p.h. In discussing the drag equivalent of power plant weight Mr. moen recalled that he had already endeavoured to outline me mam characteristics affecting engine weight and its effect ua ampacity, maintenance and prime cost of the power plant, aern^ "ed that weight mu*t also be reviewed from the lent nf T" aSpe,Ct as Prod"cing an additional drag equiva- the wLrTi c?mPlete Power plant. The relationship between the narfll 1 S and cowl diameter, he recalled, influenced of sain T-,,^Tg< and wilh increased wing thicknesses the rate favour the radial, since the improvement was the reduction of duct entry loss. radials have h A P°mt ln mind' small-diameter double-row ,,arv types anrT",.C!!ye!0ped for hi8h-speed twin-engined mili- the last Paris show. He h^vTsotnV'ere C°mmon thought to this development and had explored at the merits of the two-row radial with the single bank in this category with regard to mechanical layout, weight and cost. Capt. Barnwell had examined the com parative performance of two twin- engined single-seater fighters having double-bank and single-bank radial engines respectively, and for the purpose of comparison the wing loading of the two aircraft was constant so that the landing speeds were the same. It was seen by figures provided that the top speeds of the two machines were approxi mately equal, the drag of the increased wing surface, due to the extra weight of the small double-row engine, off setting the increased nacelle drag of the larger but lighter single-row type. Mr. Fedden thought it would be fair to state that on comparable aircraft designed around radial and in-line engines, little advantage could be claimed by either in regard to the relative wetted surface and over all drag. There did not appear to be any appreciable advan tage with any one type of engine layout, so that the final verdict would have to be obtained from the consideration of other qualities, such as first cost, production in time of emer gency, installation simplicity, ease of maintenance and repair and fuel consumption. Mr. Fedden thought that low power/ weight ratio would also be of fundamental importance and it appeared that the only justification for departing from this characteristic would be the introduction of specialised engines completely buried inside the wing. Standardisation of Complete Power Units The standardisation of all aircraft components was one of the most pressing needs of the industry to-day. Progress has been achieved along lines which facilitated standardisation due to the localisation of power units on monoplane wings. Not only would there be a saving m time and cost of the design staff, together with the simplification of stores equipment, but by the use of the standardised power unit a saving in time during which an aircraft would be out of commission when it was necessary to change engines. Air-cooled power plants generally and particularly air-cooled radials were especially suited to standardised unit construction. In a typical modern installation, the exhaust system, the removable cowling and controllable gills were standardised units built into the aircraft by the aircraft constructor in collaboration with the engine manufacturer. This was only the commencement of what could be accomplished now that modern aircraft were stabilised into a more concrete series, and it was suggested that the scope should be considerably extended, the engine constructor providing a quickly remov able engine mounting plate together with a standardised frame structure between it and the fireproof bulkhead. Within this space a number of engine components would be installed in a manner be^t suited for their proper functioning, accessi bility and to facilitate easy power unit removal. All multi-engined aircraft would have their power units in nacelles mounted centrally or dropped relative to the wing according to design requirements. Wing chords would vary with different designs. Installations employing conventional controllable gills, should be arranged so that the gill exit had a reasonable clearance ahead of the leading edge. The engine could be located near to the wing when ducted installations were used, and although a standard mounting structure was not incom patible between the two systems, the latter might restrict the disposition of attachment points on the normal fireproof bulk head line. The principal dimensions and anchorage points must be common to all of these alternative arrangements, viz. (1) the distance between the normal bulkhead and the engine; (2) the four attachment points on the plane of the bulkhead. The following design considerations would decide the position of these four points: (a) the type of construction and form of the after body nacelle—monocoque or strut-braced; (b) undercarriage and wheel housing; (c) oil tank and oil cooler installation; and (d) provision of sufficient space for cooling air flow in the case of the completely ducted installation.
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