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Aviation History
1934
1934 - 1410.PDF
SUPPLEMENT TO FLIGHT MARC H 29, 1934 THE AIRCRAFT ENGINEER ENGINE COWLING By J. D. NORTH, F.R.Ae.S., M.I.Ae.E. (Concluded from page 14) Townend Rings on Pusher Engines A Townend Ring will effectively reduce the resistance caused by an engine fitted at the rear of an aircraft body. For use under these conditions the ring chord angle requires to converge fairly rapidly to the rear, whereas for the more usual type of installation the con- vergence is almost invariably in the opposite sense. 4 ring used on such an installation is subject to a largft down-wind force, whereas on the normal installation there is a large up-wind force. A reduction in drag in this case must be associated with large increases in the pressure on the rear end of the body within the ring, but the essential characteristic of the Townend Ring—that of producing a downwash which prevents the flow from breaking away—remains unaltered. In tandem installations Townend Rings have been used on both front and rear engines. It appears that in these cases the reduction in drag which is possible' is mainly that due to the front ring. In certain cases the use of a Townend Ring on the front engine of a tandem pair is stated to have very considerably improved tl.ie cooling of the rear engine—a point of very considerable importance. In view of the effect of the ring in pre- venting a breakaway of flow behind the engine, this effect is not altogether surprising. Model Tests and Full-Scale Results on TownendjRings The Townend Ring owes its origin to investigations carried on in the wind channel with small-scale models. There is inevitably some doubt as to the direct applic- ability of the results of such tests to the prediction of performance for the full-scale aeroplane. Full-scale tests of Townend Rings have now been made in sufficient numbers to place it beyond doubt that the effect shown by small-scale models also occur in the full- size aeroplane, and there is enough evidence to show that the magnitude of the saving in resistance shown by model tests is of the same order as that indicated by full-scale performance tests. Models used for Townend Ring investigations require to be a fairly large scale. Our own experience indicates that a scale of one-fifth with a wind channel speed of 60 ft./sec. is sufficient for most purposes, but that it is dangerous to go to a much smaller scale. It is very common practice to represent the engine on small-scale models by a conventionalised dummy engine which is arranged to give the known or assumed drag of the real engine. Such conventionalised models cannot safely be used with a Townend Ring on any scale, be- cause the change in drag caused by a Townend Ring depends on the exact pattern of the air flow caused by the engine, and not by the engine's absolute resistance. The representation of engine installations by conven- tionalised models is usually extended to cover cowling details, and air passages through cowlings are either not represented at all or are replaced by a few passages drilled through the body block. Tests which hare been carried out comparing the resistance of a representa- tive model of a complete engine with the normal type of cowling accurately represented, invariably shows very much higher drags for the normally-installed engine than are given by the usual t3pe of conventional repre- sentation. Many comparisons between the results of model tests made by my firm and of full-scale tests showing the effect of Townend Rings are available. Where the full- scale tests show directly the change in performance due to the fitting of the Townend Ring, the correspondence between model result and full-scale test has generally been very satisfactory. Where estimates of the effect of fitting Townend Rings have had to be made from the estimated drag of the unringed machine, no check on the real drag being available from a full-scale test with the ring omitted, the maximum speed obtained has frequently fallen short of the designer's hopes, but this characteristic is not confined to aircraft with Townend Rings, and should not be taken as evidence of their failure to produce, full scale, the saving in drag shown by model tests. In a figure shown earlier, the model results of varying the chord angle of incidence of the Townend Ring were shown. In order to obtain if possible full-scale con- firmation of these results, a variable angle Townend Rang, precisely similar to the model, was tested full scale. Fig. 20 shows the estimated variation in performance with ring angle, and that measured full scale. It should be noted that the full-scale results plotted are the mean of six speed readings for each ring chord angle. The total estimated change in speed between no Townend Ring and Townend Ring at its most effective setting is 15.5 m.p.h. The variation between the vari- ous measured full-scale speeds at each individual ring angle vary between 2 and 7 m.p.h. I do not regard these particular tests as showing any abnormal degree of variability. Total Power Plant Drag with the Townend Ring It may be interesting to compare estimates of the drag necessary for cooling with results which have actually been obtained with the Townend Ring. The estimate made by Pye, which has been earlier men- tioned, that at 150 m.p.h.. about 1.5 per cent, of the total engine output must necessarily be used in pro- viding engine cooling when the temperature difference available for heat transfer is 180 deg. C, relates only to ideal conditions which we can scarcely hope to approach in the air-cooled engine, even if the suggestion which has once been made of using streamline sections instead of circular ones for engine cylinders were found to be practicable. McKinnon Wood, in a paper to which reference has also been made, has suggested a method of cooling which is certainly practicable, involving the enclosure of the cylinders in ducts which confine the cooling air flow to a path closely following the cylinder contour and em- bodying a fan so that the cooling is not dependent on the air speed of the aircraft. The use of the fan is indicated as preferable on account of the claimed self- regulating characteristics of this cooling system, and if aircraft of much higher speeds than are at present common are to be attained with engines having charac- teristics not greatly different from those of existing 306/
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