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Aviation History
1921
1921 - 0844.PDF
DECEMBER 22, 1921 DEVELOPMENTS IN AIRCRAFT DESIGN BY THE USE OF SLOTTED WINGS o^S:^-. BY F. HANDLEY PAGE [AMONG the many papers read before the International Air Congress, which was held in Paris concurrently with the Aero Show, few were as interesting as that presented by Mr. Handley Page, dealing with the applications of slotted aerofoils. By the courtesy of Mr. Handley Page, we are able to commence publication this week of his highly informative paper, which, it will be seen, treats the subject from a point of view somewhat different from that of the paper read by Mr. Handley Page before the R.Ae.Soc. in February (published in full in FLIGHT of February 24, 1921). In the following the paper is published practically in full, with here and there a few passages condensed so as to economise space—ED] In his introductory remarks, Mr. Handley Page referred to the part which improvements in aero engines will have in reducing the cost of air transport. He also mentioned some minor improvements which can be effected in the design of airscrews and in careful streamlining of the fuselage. The main portion of his paper, however, dealt with improve- ments in the wings themselves. On this subject Mr. Handley Page stated that it was impossible to get a good ratio of lift to resistance and a high maximum lift with the same section, and that to get both combined in one plane it is necessary to have some variable device which will admit of two different effects on the resulting air flow. The lecturer another consideration, namely the provision of a slot of such an area that sufficient air can be sent through it to clear out the dead air that tends to form on the upper or suction surface of the plane and, by preventing this formation, preventing burbling. The investigation of the right shape of slot, its anglff relatively to the main plane, and the size of the openings on the upper and lower surface have been the subject of an exhaustive research to obtain the best result. So far an increase in maximum lift up to 70 per cent, has been obtained with a single forward slot, up to 100 per cent, with a slot and slotted flap at the rear, and up to 250 per cent, with a multi-slot construction. Open slots, however, though increasing lift, increase resistance whilst open. Closed, their resistance is little more than full section. Therefore a closing device is necessary for best high speed. The slots are now designed to be closed by the pilot, either by movement of the auxiliary plane itself, or by a device for closing- the slot in a similar way to a valve ; the latter con- struction is easier and better from an economical point of view. With the upper surface so closed that the exit is completely stopped, little increase in the resistance is found by leaving the lower portion open, and the characteristics of the slotted plane with the upper exit closed, and the top surface unbroken, are practically the same as those of an unslotted plane. FIRST FULL-SCALE EXPERIMENT WITH HANDLEY PAGE SLOTTED WING : On the left the ' ,,. fitted with slotted wings, and on the right a close-up view of the arrangement of the slot. D.H.9 »• mentioned that a similar form of device to that invented by him -was devised independently by Herr Gustav Lachmann in Germany, and that at present further research work on this type of aerofoil is being carried out in England, U.S.A. and Germany. The theory of the slotted wing, he pointed out, is not yet susceptible of complete mathematical treat- ment, but the effect may be dealt with qualitatively. Mr. Handley Page then gave a qualitative argument deduced by Professor A. Betz, as follows :— - " If the simplest form of one plane is considered with a single slot through the forward portion, splitting this plane into two portions, an auxiliary and a main plane (see Fig. 1, middle diagram), the auxiliary plane, lying as it does with its rear edge on the upper surface of the main plane, tends to have the air at its rear edge accelerated owing to the region of accelerated air flow in which it is placed. In consequence the velocity change between the front and rear edges of the upper surface of the auxiliary plane is diminished, the pressure gradient is reduced, and the tendency to burble lessened, in a similar but reversed way the front portion of the main plane, lying as it does in the region of the reduced air flow of the rear edge of the auxiliary plane, has the velocity of the air flow reduced. There is, therefore, a lesser change of velocity from front to back of the main plane, and therefore a lesser tendency to burble." The above explanation deals only with the position of the planes relatively to one another, and their mutual effect on one another. It is necessary, however, to take into account A complete experimental plame built with the slots is shown in the accompanying photographs. In one view the forward slot is open and the gap between the auxiliary and the main plane can clearly be seen. In another the slot is shown closed. The opening and closing of the slot are effected by a lever control in the pilot's cockpit. Some actual test results will now be given. The lift co- efficient is measured in absolute units^ From it the lift can be obtained from the formula:— Lift in lbs. = Ky.p/g.A.V.2 ••..•-:-, Where Ky = lift coefficient (absolute). p = Density of the air in lbs. per cubic -ft. - - .- g = 32-2. A = Area of plane in sq. ft. V = Velocity in ft. per sec. The tests are on four different plane sections known as RAF/15, RAF/6, RAF/19 and Airscrew 4. The sections tested are shown in Figs. 1, 2, 3 and 4, respectively. RAF/15 is a thin high-speed section and the variation of lift with angle of incidence with a Jront slot only, as well as with a slot in front of the flap, is shown in Fig. 5. An increase in lift is obtained not only by increasing the angle of the flap, but also by placing at the forward end of the flap a slot of similar section to that in the front (see the cross section at the bottom of Fig. 1). For RAF/6 a very complete set of tests is given, the results of a test on 3 model carried out at the National Physical Laboratory, the actual figures for which are given in Table I.844
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