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Aviation History
1981
1981 - 1770.PDF
1716 Despite other differences, the A300 and A310 wings have the same sweep- back and were designed to fly at about Mach 0-8. The A320 will fly at about Mach 0-76, and sweepback will be less pronounced. Airbus does not have the choice of high speed for the A320, even if it was considered necessary. The higher aspect ratio design to which the com pany is committed would be immen sely expensive to produce unless the thickness-to-chord ratio was increased. A thick (top-to-bottom) wing cannot fly at high Mach numbers without incurring huge shock wave drag, even though it would have a supercritical profile to reduce "peaky" pressure distribution and consequent shock waves. The high thickness/chord ratio, however, has a payoff in better low- speed characteristics, allowing simpler and cheaper high-lift devices. Airbus accountants will raise a cheer if the wing is good enough to need no lead ing edge devices, but the company is saying nothing. Weight saving through the use of composites will be a big part of the programme, though certification hurdles may still prevent their use in the major load-bearing structures of early A320 models. The fin may be a complete carbonfibre structure, and the horizontal tailplane may make it in time, too. By changing the tail- plane's job from its present duty of holding up the nose, two things are achieved at once. Extra weight is taken off the wings, and the tailplane ceases to be a major load-bearing structure, thereby making use of com posites at the back end much more viable. This is where the old-but- scarcely-used idea of relaxed stability comes in. Traditionally, fail-safe pitch stability has been assured by keeping the air craft's e.g. forward of the centre of lift, the required nose-up moment pro vided by negative lift at the tail- plane. Canard stabilisers are the tradi tional answer to the illogical idea of intentionally producing negative lift These wing plans of the A3Z0's bigger forbears show the direction wing design is taking—at Airbus anyway. From an already high A300 aspect ratio and wing loading, the A3I0 wing increased both of those values. And the A320 wing will have a higher-still aspect ratio, but wing loading may remain the same or even decrease Wing area 219m FLIGHT International, 6 lane 1981 anywhere on the aircraft. The idea now is to position the resolved lift and weigh centres at the same point; but this makes the aircraft harder to fly manually—it becomes "twitchy"—and it would no longer have a natural tendency to pitch nose down at the stall, thus recovering. But who will fly these new-genera tion aeroplanes manually, an over confident electronics engineer may ask? The pilots will say that they want to be able to, and if the potential passenger were to hear that only a machine stood between his arrival or non-arrival at his destination, Airbus would have a job marketing its pro duct. But the relaxed stability aeroplane will come, and the A320 will be as close to relaxed stability as technology and safety allow. Britain's Boyal Air craft Establishment has done quite a lot of work on the idea by moving sandbags about in the cabin of a BAe One-Eleven, and it says that flying the machine manually is not the problem it is made out to be. There remains, however, the problem of what it is like to fly manually in turbulence, or when other things have gone wrong and crew stress is high. What Airbus will probably do is leave the e.g. slightly ahead of the centre of lift and use a smaller, lighter composite tailplane. In fact the final tailplane, unlike the one shown in this feature, may look slightly odd to traditional aviators' eyes, because it will be "out of scale" from the rest of the machine. The relaxed stability idea can obvi ously be used about the yaw axis too, with an autopilot taking out the twitches. A matter of balance There is an alternative to sandbags for moving the e.g. to the required position—fuel. But Airbus will not move fuel about in the A320. It costs too much to fit the systems. Though the engineers are there with the ideas, the accountants still hold sway, and Airbus is proving very astute at get ting the cost/design compromise right for the market. There is another cost/weight/effec tiveness compromise Airbus is con sidering for the new type—the double- bubble fuselage. It looks as if this shape will be the final choice. The drawbacks are increased production cost and weight. The aerodynamic penalties of the double-bubble com pared with the oval section are virtu* ally zero. The advantages are that the freight bay will take containers, and automated loading will be that much easier. Apart from the increas ing importance of squeezing every ounce of possible revenue out of aero planes, containerisation could reduce the effects (and perhaps the likeli hood) of labour problems with loading personnel. Lockheed is fitting active ailerons to its TriStar 500s to alleviate gust
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