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Aviation History
1935
1935 -2- 0786.PDF
SUPPLEMENT TO FLIGHT 36 THE AIRCRAFT ENGINEER NOVEMBER 28, T935 cowling tests were carried out on the Rapier Bulldog and these tests will now be described. Tests on Rapier Bulldog The first series of tests made on the Rapier Bulldog was with a cowl arrangement that had been flown full scale. These tests were made to establish the cooling drag and also to furnish a basis of cooling flow on which the subse- quent controlled cooling experiments could be considered , the flight tests with this particular cowl arrangement having shown that the cooling flow was adequate. The method of tests followed closely on the lines of the tests already outlined in the case of the Gauntlet. Details of the cowl are shown in Fig. 7. ••• ... r«ONT ELEVATION FLATS OVM CYLINDER HEAO. Fig. 7. Diagrammatic sketch showing internal baffle systemand cowl as originally fitted for cooling tests on Rapier- engined Bulldog. — * Flight tests had shown that the engine was adequately cooled on the climb at 95 miles per hour and from the flow measurements it was deduced that 0.32 cu. ft. (0,0241b.) ol air per second per horse-power was required to cool the engine at maximum power. The cooling drag at level flight attitude was i2.2lb. at 100 feet per second corresponding to 6.2 per cent, ot the b.h.p at the top level speed of 150 m.p.h. and the flow through the cowl was 28 per cent, in excess of the actual quantity required to cool the engine. Passing now to the controlled cowling tests, the cowl was redesigned, first by incorporating controlled exits, and secondly by adding fairing pieces to the entries (Fig. 8). The points in the new design are that the flaps allow the exit air to discharge without undue disturbance to the outside flow. The entry has been carefully shaped to prevent breakaway for the flow over the range of forward speed required. Wool tuft exploration was found of great assistance in developing the entry shape. With the flaps set to give adequate cooling the level flight cooling drag is now 3.0 lb. at 100 feet per second, i.e., at 1.5 per cent, of the b.h.p. ot the engine. The corresponding figure deduced from flow measurements for the power absorbed in the cowl was zk per cent., which seems to indicate a negative spoiling dra?;. The nose fairing used to obtain the zero for the drag tests may not have been perfect, but a considerable portion of the resistance of the aeroplane is due to the boilv and the projection behind the engine and their drag may have been less in the de-energised air. The effect ol opening the flaps on the flow and drag may be seen from the following table and the large increase in drag when the flap* are opened to large angles will be noted. TABLE SHOWING COOLING DRAG AND QUANTITY OF AIR PASSING- THROUGH THE COSVL.' Quantity of airpassing through cowl in cu. ft. at1 ooft. per sec. I^evel Climbattitude, attitude. Drag due to cooling , lb. at 100 ft. per sec. Level attitude. 12.2 Climb attitude. 10.8 Cowl detail.Cowl No. 1 (as flown] 12.2 10.8 '72 68 Controlled coolingcowl with faired entry scoops inletarea o.01 ft. — Exit area : 1.91ft. "• .. .. 16.4 17.2 69 — 1.06ft.2 .. .. 4.5 6.7 59 — 0.55ft.2 .. .. 2.5 4.5 52 — 0.23ft.2 .... 2.4 — 48 — Tests were made also to determine the drags of the external exhaust and external oil-cooling systems fitted to the Rapier Bulldog. These tests showed that the drag due to the exhaust pipes was about 51b. at 100 feet per sec. and of the external oil-cooler 1.71b. at 100 feet per sec. Both these losses should be eliminated on a high speed aircraft. • '•'•• *: ' "•.% .'• FROMT ELEVATION 5:DE ELEVATION - SlDC PLATE Pa? FLAP OPENINQ 7-65'. SECURING FUAP (MCASURED AT 4'25* THROAT) PLAN Fig. 8. Cowling tests on Rapier-Bulldog. Details of faired entry used in conjunction with controlled exit. Reviewing the cost of cooling results for the Rapier Bulldog and Gauntlet installations it appears that "at maximum level speed, using fixed cowls, the cooling drag of the Rapier installation is costing about 6 per cent, of the b.h.p., while for the Gauntlet the cost is about 10 per cent. It has been shown from the Rapier that by using a cowl with adjustable exit this loss can be reduced by about 5 per cent. ; for the Gauntlet a reduction of about 9 per cent, is estimated. The actual minimum drag cost is a little arbitrary as it depends on the nose shape relative to which the drag differences have been measured, but is clearly very low. The Gauntlet result is interesting in showing that this low cooling drag is possible without the addition of an internal baffling to the cylinders. Exit Conditions The conditions to be satisfied at exit are pretty clear apart from the possibility of the " tired " air reacting on the main flow. It is obviously very bad practice to dis- charge de-energised air at any point where the flow has small stability, e.g., on the upper surface of a wing or
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