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Aviation History
1949
1949 - 1981.PDF
FLIGHT, 8 December 1940. 747 AERODYNAMIC CLEANNESS : : : small. It followed that the suction-wing principle must be associatedwith the flying-wing layout if it was to be truly advantageous. Referring to the aircraft types which he had tabulated earlier,the lecturer said that D and E were the direct corollary to the above conclusion, the Type D not showing the full possible gainbecause of the increased drag with the lower wing-loading. Fig. 6 showed that nominal payload over 4,000 n.m. range on Type Ewas actually increased almost fourfold from that of the conven- tional type of tractor aircraft. As demonstrated in Fig. 7, move-ment of transition further along the chord involved difficulty in dealing with the quantities of air which needed to be absorbedfrom the boundary layer on Type C and, to a lesser extent, on Type E; Types F and G, which had suction wings only in thoseregions where depth was needed were, therefore, worthy of examina- tion. _ There was very little loss in payload from this change iftransition was at the slot, and the aircraft was a practical pro- position if transition moved forward. * 8ENGINES l | i | i i i i 100 150 200 250 300 20 40 60 80 EQUIVALENT AIR-SPEED (kt) TRANSITION POSITION (per cent chord) fig. 7. Suction quantities for orthodox aircraft with high- aspect-ratio, small-area, thick suction wings. Left, variation of suction quantity with speed and height for transition at 30 per cent, chord. Right, variation with speed and transition position at 20,000ft. " This suction-wing type is, in fact, a world-beater if transition can be maintained to the slot," said the lecturer, pointing out that the nominal increases obtained with the various types were as follows: — (A) Orthodox tractor, 25,500 lb. Orthodox pusher, with transition at 0.5 chord, 50,000 lb. (B) Tailless aircraft pusher, with transition at 0.5 chord, 62,000 lb. (C) Orthodox, with thick suction wing and transition at 0.71 chord, 53,500 lb. (E) Thick suction wing, tailless, 93,000 lb. (F) Thick-centre-section suction wing, tailless, 91,000 lb. Still more substantial gains would have been obtained if the slot and the assumed transition-position had been taken to occur at 0.80 or 0.85 of the local chord. The figures should encourage designers who had hitherto considered this type of wing as an aerodynamicist's dream. Discussing less favourable aspects, Mr. Richards said that if the required laminar flow could not be achieved and transition at only 40 per cent of the chord was possible, the gain practically vanished in comparison with the normal flying wing. Even if laminar flow to the slot could be achieved, a similar (although less serious) con- clusion was reached when considerably more suction mass flow was required to deal with emergency conditions that caused transition to be brought forward (e.g., rain, flies or mud). It would be essen- tial to incorporate means of guaranteeing that transition did not move forward in such circumstances. The research scientist should appreciate that these, m fact, were his largest problems and that he should aim primarily at extending laminar flow both by a structural development and by modification of the Griffith wing, either by the addition of auxiliary slots near the leading edge or by fitting porous inserts. Reviewing various Ameri- can approaches to the prob- lem, Mr. Richards said that it had been shown that by increasing the aspect ratio (without serious weight penalty) from 12 without suction and with a normal aerofoil section, to 20 with suction and a 40 per cent thick section, an increase of Fig. 8. Drag results obtained with N.A.C.A. 64A. 0H0 aerofoil section with and without suction through a surface of sin- tered bronze. Fig. 9. Typical plat- ing schemes (maxi- mum plate size, 12ft. x 8ft.). A = Brabazon wing. B =AII wing normal sec- tion. C — All-wing, thick-suc- tion centre-section with normal out- board positions. D -V.C.4 wing. maximum L/D of30 per cent was ob- tained, even with a0.010 parasite drag allowed for. Herewas a most promis- ing application ofthe suction principle which could well befollowed up in this country. In Fig. 8 the lec-turer showed the variation of totaldrag and wake drag at a Reynolds num-ber of six millions on a wing fitted with a porous surface. Boundary-layer surveys indicated that almost com-plete laminar flow was obtained over the centre porous surface of the model, and that very low drag coefficients were measured for smallsuction powers. The surface waviness on the model was poor and the sealed drag without suction was considerably greater than thatof a similar but wave-free model. Thus, by using a porous surface, the adverse effects of waviness could be overcome and laminar flowachieved over surfaces that were practicable for production. To indicate gain in performance by use of a normal thickness porouswing, Table VI showed the payload that could be carried on the large pusher-type as compared with the basic tractor and basic pusher typeshaving laminar flow at 50 per cent chord. The figures suggested that porous suction could only be a means of allowing the maintenanceof extensive laminar flow under practicable conditions. TABLE VI : PAYLOAD (LB) CARRIED OVER 4,000 NAUTICAL MILES STILL-AIR RANGE ,. -• • ~ - • - . Type ,, Tractor, orthodox Pusher (laminar flow to 0.5chord) Pusher (porous suction) EmptyWeight 168,800 173,300 192,300 Actual Payload 25>5oo 49,000 40,000 Payloadif empty weight issame as basicaircraft 25.500 53.5oo 64,500 401 SUCTION CQ QUANTITY COEFFICIENT Going on to discuss the practicability of obtaining extensive laminarflow over surfaces of production aircraft, Mr. Richards said that N.P.L. tests on the allowable limits to steps at sheet joints on wingsurfaces suggested that at high Reynolds numbers (of 20 to 40 million) steps of moie than 0.0005m could not be tolerated. The lecturer then discussed the very real difficulty of matching onesheet to another to obtain continuity of profile; he was forced to con- clude that the problem of producing multi-sheet laminar-flow wingswould not be solved until new methods of approach were developed. On the whole, the civil-aircraft designer must at present be satisfiedwith one sheet length of laminar flow, the joint being made at or near the leading edge, where the favourable pressure gradient was large.Sheet size was thus a determining factor. Taking the largest practi- cal sizes at present available, the lecturer showed in Fig. 9 some sheetgeometry for various wings. It would be seen that, because of the trimming losses arising from the sweep of the leading edge, theBrabazon and V.C.4 could be designed to give transition to 40 per cent chord, or with the bigger sheets to 60 per cent, but that the normaltailless and suction tailless types could not be expected to offer transition further aft than 15 to 20 per cent (or at the most 30 percent) with the bigger sheet. Returning to the subject of emergency conditions (ice, rain, mud,flies, etc.), the speaker said that such circumstances made it impera- tive for future suction wings to include auxiliary suction slots orregions of porous suction forward of the main slots. Experiments to simplify the mud-and-flies problem were being madeby the R.A.E. and seemed promising; the other difficulties of plate joints and Reynolds number, however, were so acute that the useof the " porous region " still showed great advantages. Mr. Richards concluded with a detailed summing-up, outstandingpoints of which we gave in our preliminary notice of his lecture (page 671, Flight, November 24th).
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