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Aviation History
1921
1921 - 0134.PDF
These tests indicate that with a multiple-slot arrangementan increase in lift coefficient can be obtained of two to three times the normal value without the slot.The tests so far described have all been monoplane tests, carried out in the wind tunnel at a speed of 40 ft. per second.A further series of tests was carried out on several sections— of which aerofoil 42 is an example—to determine whether thesame effect could be obtained on a biplane. The results in Fig. 20 indicate that an increase in the lift coefficient ofapproximately 40 per cent, was obtained with a single slot, and that a normal result was obtained. Further tests \ \\ -• \ i -' ! ANGLE C FIG. 23- - >/rINCIDENCE carried out since have clearly shown that with the necessarybiplane corrections the slotted monoplane tests can be applied to biplane calculations. Centre of Pressure TestsAerofoil No. 32, the lift/drag coefficients for which have already been plotted in Figs. 9 and 10, was tested for itscentre at pressure movement, and the results are plotted in Fig. 23. At any given angle the centre of pressure withthe slot open is slightly farther back, but taking into account the decrease in lift coefficient at small angles withthe slot open, for any given value of the lift coefficient the difference is not great. The general result, however, of thecentre of pressure line being slightly behind that of the normal FEBRUARY 24, 1921 position is one that might be anticipated, as the pressure ismore evenly distributed over the whole plane, and therefore the aft portion has a greater lift. This causes the result ofthe centre of pressure to lie farther back. In commenting on the tests carried out on this section,the National Physical Laboratory reported as follows :— '' The high lift obtainable with the flap open is very remark-able, especially in view of the fact that the position of the centre of pressure is little altered. At the critical angle theC.P. is at 0.295 chord with flap open, which corresponds with its position at about 8° incidence with flap closed. Thelongitudinal balance of the machine would be approximately the same when flying at 8° incidence or landing at 22° incidence,a very valuable characteristic. Scale effect in lift and drag are both considerable, but little effect on C.P. is found." Flap Experiments with Slotted AerofoilAn increase in the lift coefficient can be obtained by the use of a plane with flaps and altering the angle of incidence ofthese flaps. A series of tests were carried out at the National Physical Laboratory, published in the Report for the year1913-14, pages in to 128. The results have been plotted in Fig. 24, compared with aerofoil No. 32 with the slot openand the slot closed. The R.A.F./19 curve shown is the envelope of the various curves, as plotted in Fig. 32 of theReport referred to above. The maximum lift coefficient on aerofoil No. 32 is approximately .943, as against .82 withthe flap, which at this value was set back at an angle of 6o°. The increase in lift coefficient by the use of flaps can beobtained with the slotted plane as with the ordinary one. A series of tests were carried out on the section shown inFig. 25, and the results are plotted in Fig. 26. With the plane inclined at 180 and 190, a progressive increase in thelift coefficient is obtained, but at 200 and 210 the plane is inclined at the critical burbling angle, and owing to this F73.25. Slotted aerofoil with flap results are somewhat unstable. Further experiments havedetermined that the rolling moments obtained with the alteration of the flap angle are of the same order as those onthe plane of ordinary cross section, indicating that full control can be obtained .by ailerons in the ordinary mannerwhen the slots are open. • Pressure PlottingReference has already been made to pressure distribution plotting on a slotted plane. These experiments were carriedout on aerofoil No. 42, this being an R.A.F./15 section with an extra nosepiece added—Fig. 27. The results are shown inFig. 28. The shape of these curves is very similar to that of the ordinary pressure plottiug, except for the break in the X34
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