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Aviation History
1931
1931 - 0596.PDF
ISUPPLEMENT TO FLIGHT 42 JUNE 19, 1931 THE AIRCRAFT ENGINEER Results of experiments on actual machines are always more convincing than those obtained in the wind tunnel. It is, therefore, very interesting to compare the L/D X K,_ curves of a complete aeroplane for the normal wing and for various flap settings, as shown on Fig. 2. These curves are constructed from data given in R. and M., No. 1085.* This point is illustrated by Fig. 3, which is replotted from the diagram published in " Flight " of March 7 1930, page 270. Assuming that a wing incidence of 17°' may be obtained by a machine of normal proportions during a three-point landing, a slotted wing would have its KL increased from 0.54 to 0.61 at this incidence i.e., a 13 per cent, increase, whereas an increase to 0.87' ,,, • t 7/ / ,'/ i / / fj /V/ 1 1 Ij 7 s s X FLAPS N ORMAL ^FLAPS DCS! \ \ " \ i i ii i FIG.2 VN 8" \ i i i •- PUHCM.HH This report gives figures of carefully-measured lift and drag of the complete aeroplane with propeller stopped, at flap settings of —5.1°, 0°, 8° and 15.7°. It is thought that an intermediate setting between — 5.1° and 0" may have given a curve in about the position shown by ? ? P. It will be seen from the abo\*e that the effect of flaps on the maximum lift of the original aerofoil is to increase this by about 20 per cent. For a comprehen- sive discussion of the present state of development of variable camber the reader is referred to Capt. Macmillan's very able and interesting article which was published in THE AIRCRAFT ENGINEER of June 20, 1930. The object of the present article is to consider the advantages of variable camlier from different points of view, and to suggost new avenues for research and development work. In the case of modern designs of aircraft where a high degree of aerodynamic efficiency is being obtained, the wing drag is now a greater proportion of the total drag of the machine than was formerly the case. Any improvement in the characteristics of a wing will there- fore result in a greater proportion ate improvement in the performance of a modern aircraft than would be achieved in the case of an obsolete design. Another important advantage of variable camber flaps in the case of modern highly efficient aircraft is their effect in coirsening the extremely flat gliding anglo, which is characteristic of these machines, when coming in to land, so that apart from their effect in reducing landing speed, they give a useful effect as air brakes. A combination of variable camber wings with lead- ing edge slots gives very great advantages, inasmuch as the full benefit of slots in reducing landing speed is only obtained at angles so large as to be unattainable in normally proportioned aircraft, whereas the two in combination result in a high lift being obtained at the normal attitude the machine would take in an ordinary three-point landing. The lift obtained by the combina- tion of flap and slot is, moreover, higher than that obtainable with slots alone. 1-2 1-1 10 •9 •8 •7 •6 •5 •4 •3 •2 •1 - ""V SLOT OPEN AP DOWN 20' / y i— 5 10 15 0 / FLAP NORMAL ,R A F. 28 FIG 3 [X = 28* / 3PEN NORMA AEROH SECTIO I 31L N 20 25 30 • Lift and Drag of the Bristol Fighter with Fatrejr variable camber wings. representing a 61 per cent, improvement, is inherent in the wing but cannot be used, the required wing inci- dence being 28°. When the slot and variable camber flap are combined, however, the KT/ at 17° is increased to 0.98, representing an 85 per cent. gain. It is obvious that no form of varinhle lift wing is worth adopting unless lateral control is both light and effective at and beyond stalling speed; in this connec- tion it is pleasing to note that in the case of variable camber wings recently tried it was found that the lateral control was effective throughout the speed range of the machine and at all variations of wing section, and that the ailerons felt even lighter with full camber than with flaps normal. It must also be borne in mind that whatever be the devices used to augment the lift of the wing, these must be so arranged that for high- speed conditions the profile of the wing is not interfered with to any appreciable extent. This object may be achieved by careful attention to detail design. In order to obtain the maximum possible benefit from the use of variable camber, this should be regarded not as a means for reducing the landing speed of existing machines, but as a means for reducing their overall size and weight, and increasing performance and manoeuv- rability, while still retaining a reasonably slow landing speed. To illustrate this new point of view we will make a comparison between two aircraft, both designed foi tne same purpose, one with fixed wings, which will herein- after be referred to as the F.W. machine, and the other with variable camber wings, which will be referred to as the V.C.W. machine, on the basis of the same land- ing speeds for both designs. The wing section selected for both cases will be R.A.F. 28. For the purpo?" <>t this example a hypothetical specification will "e adopted, the leading requirements of which are: — (1) Stalling speed, 55 m.p.h. (2) Maximum speed at 10,000 ft. to be not less tian 160 m.p.h. (3) Military load, 1,200 lb. , (4) Duration, 6$ hours at a cruising speeo 130 m.p.h. (5) Span not to exceed 44 ft. of 5566
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