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Aviation History
1986
1986 - 2152.PDF
Aerodynamically speaking Traditionally, Airbus uses smaller wings than its competitors, demanding high usable maximum lift, and the A320 is no exception. Not that this will limit the A320's potential for growth, chief engineer Andre Bord is quick to point out: "If the A320 meets or exceeds its predicted buffet limit there will be growth potential." Wing design was driven by the fact that trie A320 is a short-haul aircraft with long legs, requiring aerodynamic efficiency with good fuel volume: "3,000 n.m. is a long range for a small aircraft," says Bord. Studies trading weight against drag favoured an aspect ratio of around 9-5 (Airbus partner and wing designer British Aerospace selected 9-4), while the demand for a com petitive cruise Mach number of 0 • 76 to 0 • 78, allied with good low-speed behav iour, resulted in a 25° sweep. Similarly, cruise Mach number dictated wing thickness/chord ratio, which at 10-8 per cent is similar to A310's. The A320 aerofoil section is signifi cantly different to the A310's, however. Airbus could not simply scale down the A310 profile, says Bord, because there was then insufficient room aft of the rear spar in which to instal the flap system. British Aerospace therefore developed a new profile which, for the same absolute thickness, is 30 per cent thicker aft of the rear spar than A310's. easing flap installation. Aero dynamically, the result is a slight improvement in drag-rise Mach number, says Bord. While Bord believes that the A320 wing will have the highest CLma:* (maximum lift coefficient) in the market, Airbus could have done better, he admits, but adds the requirement was for "good, but not extraordinarily good high-lift performance with simple design". Simpler than the A310's, the A320's single-segment Fowler flaps have a rack-and-pinion action hydrau- lically driven via rotary actuators. The full-span leading-edge slats posed something of a problem. The engines are closer to the wing on the A320 than on the A310, and the pylons therefore interrupt the slats. Some local reshaping of the engine pylons was required to provide a degree of continuity, slats extended, without incurring a penalty at high speed. Although the A320 wingtip design is improved over that of the A310, Bord now believes that wingtip fences, successfully introduced on the extended-range A310-300, can be usefully applied to the A320. Wind- tunnel tests are under way, Bord reveals, and it is planned to fit fences to the higher gross-weight A320-200. Winglets by any other name, Airbus' fences are designed to minimise the off- design penalties of these drag-reducing devices, and particularly the risk of a winglet stalling during a crosswind approach. The A320 tailplane is really a small wing, says Bord, and has a soph isticated inverse profile, the result of a similar optimisation process to that used for the wing itself. The A320 fuselage presented its own aerodynamic challenges. The demand for a full six-abreast final seat row followed by a seven-trolley galley, with full containerised baggage underfloor, extended the "cylindrical" fuselage section well aft. To minimise drag, Airbus partner MBB "spent a lot of money" optimising the rear-fuselage shape using a sophisticated method for measuring drag on the rear part of the fuselage only, using models, says Bord. As a result, he says, the A320 rear- fuselage design, with its distinctive "waisted" planform, is significantly better than that of the A310 and is likely to be used on the long-range A330/340 family. Bord explains the value of such an effort: friction drag accounts for 60 per cent of total drag, and the fuselage accounts for 30 per cent of that (18 per cent of the total). This can be varied by 10 per cent (1-8 per cent of total drag)—"that is worthwhile". would one pilot know what the other was doing? In fact, there is a "psychological" coupling, says Corps. Correcting or opposing inputs from the other pilot are perceived as a need to apply more stick to achieve the desired result, so the other pilot's corrective action can be "felt". This is particularly important when one pilot is under instruction. A similar debate surrounded what would happen should one pilot become incapacitated and fell across his sidestick, jamming it, or if he decided the time had come to end it all and aimed the aircraft at the ground, refusing to relinquish control. Again the argument is turned on its head by Airbus. A control jam in a conventional aircraft requires an extremely high effort to overcome. In the A320, however, the only possible mechani cal jam is within the sidestick assembly itself, and the pilot can simply "kill" the jammed stick by pushing a takeover button on his own stick. Ironically, the electronic coupling logic which Airbus originally devised for the sidesticks, and which worked well in the simulator, was proved unsuitable on the first-ever flight-test of the A300 testbed equipped with two sidesticks. Originally, within a 75 per cent stick- movement cone, simultaneous inputs from both pilots were summed alge braically, allowing for corrective action during training. The first pilot to move his stick outside the 75 per cent cone, however, took command while inputs from the other stick were phased out over 3sec—unless the other pilot moved his stick outside the cone and so regained command. On the first flight-test the copilot got into a pilot-induced oscillation involving large stick displacements. Despite attempts by the pilot to take command, the copilot retook control every few seconds when his stick moved outside the 75 per cent cone. The copilot had to be left to sort things out. The electronic coupling that will be used on the A320 is much easier for pilots to understand, Corps admits. Each side- stick will have a takeover button. In normal flight, simultaneous inputs from both pilots will be summed algebraically, but priority will be given to the pilot who depresses, and holds depressed, the take over button on his stick. A green light on the glareshield tells the pilot he has priority, while a red light tells the non- priority pilot that his stick is dead. He can then retake control by depressing his take over button. If he elects instead to release his stick, the green light in front of the priority pilot will go out. The priority pilot can then release the takeover button, extinguishing the red light and returning the aircraft to normal flight control. If the non-priority pilot does not release his stick, however, because he is incapaci tated, the priority pilot keeps his takeover button depressed for 30sec and then the dead stick is electronically latched. As soon as the obstruction is removed and the dead stick centred, it is automatically delatched. Fly-by-wire improves integration of the automatic flight control sytem (AFCS), FLIGHT INTERNATIONAL, 30 August 1986
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