A380 Update - New technology
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A380 Update - New technology
A380 Fights Weight Growth With New Technologies
By Michael A. Dornheim/Aviation Week & Space Technology
18-Jun-2001 12:29 PM U.S. EDT
The A380 is facing an uphill battle against weight, yet is supposed to have 20% lower operating costs. How can it be cheaper if it's going to be heavier?
The higher empty weight per passenger comes from engineering basics--the square-cube law--and new certification rules that do not apply to its nearest competitor, the Boeing 747-400. But it turns out that weight is not that important to direct operating cost, at least by Airbus' calculations.
Nonetheless, Airbus is introducing novelties such as a 5,000-psi. hydraulic system with electrically powered backup to keep weight in check. The structure, the systems and the aerodynamics are pushed with this goal in mind. The A380 also will have a variable-frequency AC electrical system, reduced aerodynamic stability and a configuration tweaked to reduce wake vortices.
Weight growth has occurred as customers have let their needs be known over the last year. The early objective was to carry 555 passengers for 7,650 naut. mi., which required a 540-metric-ton (1,190,000-lb.) maximum takeoff weight (MTOW) for the 356-metric-ton (784,800-lb.) maximum zero fuel weight (MZFW), said Robert Lafontan, vice president of engineering and product development for the Large Aircraft Div. here.
After the first marketing campaign, the range was increased to 8,150 naut. mi. in April 2000 and MTOW went up to 560 metric tons. Lower cargo hold length was increased to accommodate another pallet and MZFW went up to 358 metric tons.
Last summer, airlines started signing memorandums of understanding and wanted lower noise to avoid airport curfews. The resulting September 2000 design had a better high-lift system, including a droop-nose device on the inboard leading-edge flap, to improve the climb trajectory. There were big changes to the engines--fan diameter was increased to 116 in. from 110 in., the sound-absorbing nacelle inlet was lengthened 17 in. and the fan exit was lengthened 5 in., and the core exhaust pipe was extended. This increased weight and MZFW went up to 361 metric tons, but MTOW stayed at 560 tons and range dropped to 8,000 naut. mi.
The payoff was that estimated noise dropped to the London Heathrow "Quota Count 2" (QC2) level from QC4 (a lower number is quieter). The 747 is at best QC4. Other QC2 aircraft are the General Electric GE90-powered Boeing 777 and Airbus A330.
Fuel tankage has not been a concern. The basic A380-800 (formerly called the A380-100) does not use any center wing tank fuel, and the A380-800R longer range version uses only half of the center tank.
A380 final design and supplier selection is still underway. Detail definition isn't due to be completed until a year from now. Operators are still giving their design input in 14 customer focus groups set for 2001, on items such as maintainability, systems routing and preferred suppliers. The number of people working on the A380 within the Airbus system is ramping up to about 4,500 by the end of the year. The number of engineers and managers at Airbus headquarters here should reach 250 at year-end from about 150 now.
Earlier A3XX designs had a 780-sq.-meter (8,400-sq.-ft.) wing, but this was increased to the current 845 sq. meters (9,100 sq. ft.), partly so that control surface actuators would fit in the rear spar without fairings, Lafontan said. The weight penalty was negligible when the beneficial effects of a larger wing, such as better takeoff and landing performance, were included, he said. The large area was not needed for cruise, but the design was driven by a goal of reaching a 35,000-ft. initial cruise altitude in 30 min. or 200 naut. mi., which benefits from more wing area. Also, a large wing makes it easier to stretch the aircraft.
The goal was to have the same takeoff and landing distance as a 747, or about 11,000 ft., Lafontan said. By taking advantage of the larger wing, single-slotted flaps give slow enough takeoff and landing speeds, and heavier and more complex double- and triple-slotted flaps are not needed. Approach speed is predicted to be about 145 kt. at maximum landing weight.
Airbus undertook a special effort to keep the A380 wake vortex no stronger than the 747 so other aircraft wouldn't require extra in-trail separation from it. Engineers reviewed NASA, European and Russian TsAGI studies. They noticed that the two-engine Airbus A330 and four-engine A340 have different vortex patterns even though they have the same wing, owing to changes in location of the flaps and engines. They also observed that the A320 and A321 have different patterns, apparently because one model has single-slotted and the other, double-slotted flaps.
Lafontan said the location of the flaps, ailerons and engines on the A380 was adjusted to minimize the wake vortex, and it is estimated to be a few percent stronger than the 747-400's. Some of the design data came from a catapult tunnel in Lille, in northern France, with a 30 X 33-ft. section and 100-ft. length. A 50-lb. model was catapulted through a smoke sheet in the tunnel, and the evolution of the vortex pattern was observed to help correlate the near-field pattern measured in a normal wind tunnel with the far-field effects felt by a trailing aircraft.
The economic objective of the 555-seat A380 is to have a 20% lower cash operating cost per seat than a 417-seat Boeing 747-400, Lafontan said. "Cash operating cost" is similar to U.S. direct operating cost, and does not include the aircraft purchase price or insurance. So far, Airbus estimates the A380 has an 18% lower per-seat cost and a 9% higher trip cost, based on a 6,000-naut.-mi. route. On a 4,000-naut.-mi. trip, the estimated advantage drops slightly to 17%, said Christopher W. Stonehouse, head of the Fulfill Customer Order group for the A380 program
What are the important parts of this claimed per-seat cost, and what do they have to do with weight per passenger? The largest part of the reduction, a 38% share, is lower crew costs, which has little to do with weight or technology. It is the simple arithmetic of the same number of highly paid pilots flying a larger number of passengers. Airbus assumes the A380 pilots will be paid more because they're flying a larger aircraft.
Maintenance accounts for 30% of the reduction and comes partly from having more passengers to absorb relatively fixed costs, and partly from the promises of new technology. Estimation of maintenance costs can be ephemeral; for many items like avionics, they are not related to weight.
FUEL IS THE THIRD major cost reducer at 28% share, and Airbus believes the fuel burn per passenger will be 11.9% less. This depends upon engine and aerodynamic efficiency, and the empty weight per passenger of the aircraft. "Engine specific fuel consumption is only a temporary advantage because the competition will soon get it," Lafontan said.
Aerodynamic efficiency is harder to obtain for the airframe manufacturer and is a medium- to long-term advantage, he said. Airbus estimates that the A380's lift-to-drag ratio will be more than 10% better than the 747-400's. Since many elements of the 747's aerodynamic design date back more than three decades, this should be within grasp. The A380 is designed with a Mach 0.85 long-range cruise speed, but also with a broad range of efficient speeds to adjust arrival times into noise-sensitive areas. Boeing has historically conducted more than twice as many wind tunnel tests as Airbus and had a more systematic approach of checking results, Lafontan said. "We didn't have the people or money."
Weight is the final element of fuel burn and is the area where the A380 faces the greatest struggle. Larger airplanes are heavier per passenger, by a factor of two from the Boeing 737-400 to the 747-400 (see table).That is because they usually have more range and must carry more fuel, and they fight the square-cube law, where areas such as the cabin floor and wing grow as the square of length, while the weight grows more rapidly as the cube of length. The square-cube law is not strictly true for aircraft, but the tendency is there.
The double deck of the A380 is a tactic to delay the weight growth, but the fact that the A380-800 is the first of a model makes it relatively heavier than the stretched-out Boeing aircraft listed in the table. Lafontan noted that while the estimated weight per passenger is heavier than the 747-400, it is about the same as the unstretched 747-200B.
With a 49% larger floor area, the A380 gives about 10% more space per passenger than the 747-400, but it may be paying for that in weight and drag. One way this is used is to make each seat and aisle 1 in. wider.
For the A380, Airbus did studies of how passengers perceive the cabin, Lafontan said. He noted that some first-class seats can weigh up to 200 kg. (440 lb.) while engineers sweat to save smaller amounts on structure, and that a better understanding of passenger perception could make a more efficient design.
In 1998, a short mockup of the A380 upper cabin was taken to several major cities, and more than 1,200 frequent fliers were interviewed. Airbus found that cabin perception changes with culture, but did conclude that the floor should be lowered 2 in. and the ceiling line reshaped. From this basis, nine designs were made, and four were built into long mockups.
Using the advice of 150 Airbus employees of varied backgrounds, the choice was narrowed to what Airbus calls "flat" versus "traditional" styling. By then the airlines had become involved. Two mockups reflecting these styles have been built, and the final concept is to be selected this month.
Weight will be increased by the latest certification rules that the A380 has to meet. An example is the evacuation slide standards. The upper deck doors are more than 30 ft. above the ground, and slides have to reach the ground at a safe angle even if the aircraft is tipped up; however, extra slide length is undesirable if the sill height is normal. Previously, slides just had to touch the ground in the tipup case, even if it meant a straight 90-deg. drop, Lafontan said. Slides are also becoming stiffer. And because passengers are reluctant to leap onto the slide when they can see how far down the ground is, the trend is to put blinder walls at the exit and a curve in the slide to mask the ground--more weight added.
"By meeting the latest requirements, we have twice the slide volume, more weight, they won't fit on doors, and it costs more to have fuselage cutout panels for putting slides under the floor," Lafontan said. "Intelligent" slides that vary length depending on fuselage tipping may be required. Airbus plans to certify the two decks as independent, each with its own slides and rafts. Even though a large staircase connects the two levels, it is not required for evacuation certification.
APPLICATION for certification was started about two years ago with the FAA and European Joint Aviation Authorities, said Wolfgang Didszuhn, Airbus vice president for product integrity. The French DGAC certification agency--acting as part of the JAA--will be the primary certification authority, and the FAA may delegate certification to the JAA, with some exceptions.
A draft of special certification conditions should be completed by this summer, Didszuhn said. Interpretation of the rules may be needed for new technologies such as integrated modular avionics. The touchdown sink-rate requirement for the landing gear may be increased to 12 fps. from the normal 10 fps., as experience shows that touchdown rate grows with gross weight, Didszuhn said. That would make the structure heavier.
The landing gear for a huge aircraft promises to be heavy, particularly when runway loading limits must be observed. The configuration is a pair of body gear with six-wheel trucks and wing gear with four-wheel trucks and a track of about 14.5 meters. The usual "ACN" method of rating runway loading doesn't work well for six-wheel trucks, and Boeing modified it for the 777, said Michel Comes, director of systems engineering for the Airbus Large Aircraft Div.
At Toulouse-Blagnac airport, Airbus built four flexible types of instrumented pavement and tested them last year with a real 777 and 747, and a 20-wheel weighted mockup of the A380 main landing gear. About 5,000 passes were made over the pavement samples, and the data were used to optimize gear design. "The tests showed we could use a shorter bogie beam length, and we saved more than 300 kg.," Lafontan said. The bogie was shortened again recently to save another 150 kg. The pavement loading is less than that of a 777, he said. Tests on rigid pavement will be conducted this summer.
The rear wheels of the six-wheel trucks are steerable, and the goal is to make a U-turn fit within the roughly 185-ft. U-turn diameter of the 747.
Estimated weight is under 20 metric tons for all the landing gear parts, including brakes and wheels, or under 3.6% of MTOW. "This is heavier than the 747 as a percentage of weight, but it meets newer requirements such as runway fatigue," Lafontan said.
The brakes have a different backup hydraulic configuration that may save 20 kg. per wheel, or 400 kg. on the aircraft, Comes said. Normally they would have two sets of clamping pistons powered by two hydraulic systems, but the "single-cavity" design has just one set of pistons powered by one hydraulic system with a local electric pump as backup. New-technology tires may also save 400 kg., Lafontan said.
AIRBUS' NOVEL HYDRAULIC system design relies on electricity for redundancy and employs 5,000-psi. pressure instead of the typical 3,000 psi. for airliners. The electric backup configuration saves 400-500 kg. compared with having three standard hydraulic systems, and 5,000 psi. saves 1,200 kg., Comes said.
Because of engine rotor burst concerns, the airplane has to survive with one hydraulic system remaining, and having four conventional systems is oversized and costly, Comes said. The A380 has two standard central systems, each supplied by four engine-driven pumps on one side of the wing and an electric motor pump for ground use only. There are also two electrically powered systems with "electrical backup hydraulic actuators." Each EBHA has an integral electric pump and operates as an electrohydrostatic actuator. EBHA performance is less than a normally powered actuator, except the aileron EBHAs have equivalent performance.
The result is that there are four circuits--yellow and green central systems, and left and right EBHA backups. An advantage of electricity is that it can be routed above the cabin floor without worrying about hydraulic fluid leaking in the cabin. This allows the electric wires to be more widely separated, reducing the chance of simultaneous damage in a rotor burst. The copper wiring is also lighter than hydraulic lines, Comes said.
The A380 has three ailerons per side, each with two actuators. At least one actuator is powered by a central system; the other is either an electrically powered EBHA or runs off the other central system. The other primary control surfaces have a similar mix of power sources. There are seven spoilers per side, each with one actuator. The rudder has two separate panels, each with two actuators, and the elevator has two panels per side, also with two actuators apiece. Stabilizer trim has one electric and yellow and green hydraulic actuators. The leading-edge slats are driven by green and an electric system, while the flaps are yellow and green. The wing landing gear retraction and brakes are powered by green, and the body gear runs from yellow. The thrust reversers are all-electric actuators separate from the hydraulic system; only the inboard engines have reversers.
There have been difficulties making 5,000-psi. systems reliable, but Airbus believes they can work. "We need to demonstrate that it's feasible and not risky," Comes said. A test rig was built here to test fluids under contaminated and accelerated life conditions. The result was no fluid degradation, and "we didn't see any erosion in the actuators," Comes said. Airlines use "Type 4" hydraulic fluid now, and Airbus is looking at higher temperature Type 5 fluids, one of which has been qualified by Boeing, he said. "It has to use normal fluid or else it would be a showstopper," he said.
Suppliers have told Airbus they can make 5,000-psi. components work if they design them from scratch and perhaps use titanium instead of aluminum or steel. Test results on a new 5,000-psi. fitting were presented to airlines late last year. With 3,000 psi., some high-pressure pipes were 8-10 cm. (3.1-3.9 in.) in diameter, and Comes said the smaller 5,000-psi. components should be easier to maintain.
THE VARIABLE-FREQUENCY electrical system is innovative for a transport. Normally, a variable-ratio drive keeps the alternator at a constant speed as the engine rpm. varies, but half of the drive and generator failures involve the constant-speed drive, Comes said. Eliminating the drive and letting frequency vary removes this headache.
The problem is that the electrical equipment has to tolerate alternating current ranging from about 380-760 Hz., instead of a constant 400 Hz. Cruise would be about 500 Hz. Tests have been conducted and specifications given to electrical consumers, such as the fuel pumps. "Most consumers, like the galley and inflight entertainment, are satisfied," Comes said.
The weight impact is not clear. Power generation will be lighter but power consumers may be heavier. Honeywell, Sundstrand and TRW Lucas are competing for power generation.
Airlines want a generator loss to be a non-event. Cruise consumption is about 400 kva., and there will be four 150-kva. engine generators so one can die without load-shedding, Comes said. Ground consumption is expected to be 200-250 kva., and there will be two 120-kva. generators on the auxiliary power unit plus four 90-kva. ground connectors.
Structural materials include the latest light aluminum alloys similar to those in the 777, Lafontan said. The 2524 alloy can be stressed 5-10% more than conventional 2024, he said. Some fuselage skin will be Fokker's Glare aluminum-fiberglass composite, and the fixed leading edge will be a Fokker thermoplastic composite (see following story). Laser welding of aluminum will replace riveting to attach stringers on some parts of the lower fuselage.
The wing center section will be carbon composite, a first in commercial aviation, Lafontan said. The center section is more than 2 meters tall and would normally weigh more than 10 metric tons. It will be monolithic composite except where the design forces use of honeycomb. Honeycomb is avoided because it is hard to repair and it might trap fluids.
Tail surfaces are large on the A380--the horizontal is about the same size as a 2,360-sq.-ft. A310 wing, and the vertical is bigger than a 1,320-sq.-ft. A320 wing. These surfaces will be made of carbon composite, as will the rear pressure bulkhead, fuselage keel beam, floor beams and unpressurized tail cone that holds the stabilizer.
To reduce the control authority demands on the tail and keep its size, weight and drag small, the A380 center of gravity (CG) will be located about 6% aft of other aircraft, just in front of 50% mean aerodynamic chord. "It's statically stable but not dynamically stable," Lafontan said.
Aft-CG tests were conducted in the No. 1 A340 last year. "With experience, we can reduce the stability margin," Comes said. "It's not unstable. In certain maneuvers it may become unstable, like all aircraft."
The fly-by-wire system will give a normal feel to the pilot, and will have failure modes the same as the A320, A330 and A340, Comes said. If it reverts all the way to "direct law," fuel can be transferred to move the CG forward as required.
The final backups to Airbus' fly-by-wire system have been changing. The A320 has manually powered stabilizer trim and a mechanically signaled rudder. "These backups go beyond regulatory requirements," Comes said. "We demonstrate safety with the standard system." The A340-500/600 replaces the mechanically signaled rudder with a backup electrical signaling system that runs off the hot battery bus. It has an independent gyroscope and accelerometer to maintain yaw damping and bypasses the standard flight control system. The A380 includes this change, and also replaces the manual stabilizer trim with a backup electric motor.
Original link http://www.aviationnow.com/avnow/new.../ta3800618.xml
By Michael A. Dornheim/Aviation Week & Space Technology
18-Jun-2001 12:29 PM U.S. EDT
The A380 is facing an uphill battle against weight, yet is supposed to have 20% lower operating costs. How can it be cheaper if it's going to be heavier?
The higher empty weight per passenger comes from engineering basics--the square-cube law--and new certification rules that do not apply to its nearest competitor, the Boeing 747-400. But it turns out that weight is not that important to direct operating cost, at least by Airbus' calculations.
Nonetheless, Airbus is introducing novelties such as a 5,000-psi. hydraulic system with electrically powered backup to keep weight in check. The structure, the systems and the aerodynamics are pushed with this goal in mind. The A380 also will have a variable-frequency AC electrical system, reduced aerodynamic stability and a configuration tweaked to reduce wake vortices.
Weight growth has occurred as customers have let their needs be known over the last year. The early objective was to carry 555 passengers for 7,650 naut. mi., which required a 540-metric-ton (1,190,000-lb.) maximum takeoff weight (MTOW) for the 356-metric-ton (784,800-lb.) maximum zero fuel weight (MZFW), said Robert Lafontan, vice president of engineering and product development for the Large Aircraft Div. here.
After the first marketing campaign, the range was increased to 8,150 naut. mi. in April 2000 and MTOW went up to 560 metric tons. Lower cargo hold length was increased to accommodate another pallet and MZFW went up to 358 metric tons.
Last summer, airlines started signing memorandums of understanding and wanted lower noise to avoid airport curfews. The resulting September 2000 design had a better high-lift system, including a droop-nose device on the inboard leading-edge flap, to improve the climb trajectory. There were big changes to the engines--fan diameter was increased to 116 in. from 110 in., the sound-absorbing nacelle inlet was lengthened 17 in. and the fan exit was lengthened 5 in., and the core exhaust pipe was extended. This increased weight and MZFW went up to 361 metric tons, but MTOW stayed at 560 tons and range dropped to 8,000 naut. mi.
The payoff was that estimated noise dropped to the London Heathrow "Quota Count 2" (QC2) level from QC4 (a lower number is quieter). The 747 is at best QC4. Other QC2 aircraft are the General Electric GE90-powered Boeing 777 and Airbus A330.
Fuel tankage has not been a concern. The basic A380-800 (formerly called the A380-100) does not use any center wing tank fuel, and the A380-800R longer range version uses only half of the center tank.
A380 final design and supplier selection is still underway. Detail definition isn't due to be completed until a year from now. Operators are still giving their design input in 14 customer focus groups set for 2001, on items such as maintainability, systems routing and preferred suppliers. The number of people working on the A380 within the Airbus system is ramping up to about 4,500 by the end of the year. The number of engineers and managers at Airbus headquarters here should reach 250 at year-end from about 150 now.
Earlier A3XX designs had a 780-sq.-meter (8,400-sq.-ft.) wing, but this was increased to the current 845 sq. meters (9,100 sq. ft.), partly so that control surface actuators would fit in the rear spar without fairings, Lafontan said. The weight penalty was negligible when the beneficial effects of a larger wing, such as better takeoff and landing performance, were included, he said. The large area was not needed for cruise, but the design was driven by a goal of reaching a 35,000-ft. initial cruise altitude in 30 min. or 200 naut. mi., which benefits from more wing area. Also, a large wing makes it easier to stretch the aircraft.
The goal was to have the same takeoff and landing distance as a 747, or about 11,000 ft., Lafontan said. By taking advantage of the larger wing, single-slotted flaps give slow enough takeoff and landing speeds, and heavier and more complex double- and triple-slotted flaps are not needed. Approach speed is predicted to be about 145 kt. at maximum landing weight.
Airbus undertook a special effort to keep the A380 wake vortex no stronger than the 747 so other aircraft wouldn't require extra in-trail separation from it. Engineers reviewed NASA, European and Russian TsAGI studies. They noticed that the two-engine Airbus A330 and four-engine A340 have different vortex patterns even though they have the same wing, owing to changes in location of the flaps and engines. They also observed that the A320 and A321 have different patterns, apparently because one model has single-slotted and the other, double-slotted flaps.
Lafontan said the location of the flaps, ailerons and engines on the A380 was adjusted to minimize the wake vortex, and it is estimated to be a few percent stronger than the 747-400's. Some of the design data came from a catapult tunnel in Lille, in northern France, with a 30 X 33-ft. section and 100-ft. length. A 50-lb. model was catapulted through a smoke sheet in the tunnel, and the evolution of the vortex pattern was observed to help correlate the near-field pattern measured in a normal wind tunnel with the far-field effects felt by a trailing aircraft.
The economic objective of the 555-seat A380 is to have a 20% lower cash operating cost per seat than a 417-seat Boeing 747-400, Lafontan said. "Cash operating cost" is similar to U.S. direct operating cost, and does not include the aircraft purchase price or insurance. So far, Airbus estimates the A380 has an 18% lower per-seat cost and a 9% higher trip cost, based on a 6,000-naut.-mi. route. On a 4,000-naut.-mi. trip, the estimated advantage drops slightly to 17%, said Christopher W. Stonehouse, head of the Fulfill Customer Order group for the A380 program
What are the important parts of this claimed per-seat cost, and what do they have to do with weight per passenger? The largest part of the reduction, a 38% share, is lower crew costs, which has little to do with weight or technology. It is the simple arithmetic of the same number of highly paid pilots flying a larger number of passengers. Airbus assumes the A380 pilots will be paid more because they're flying a larger aircraft.
Maintenance accounts for 30% of the reduction and comes partly from having more passengers to absorb relatively fixed costs, and partly from the promises of new technology. Estimation of maintenance costs can be ephemeral; for many items like avionics, they are not related to weight.
FUEL IS THE THIRD major cost reducer at 28% share, and Airbus believes the fuel burn per passenger will be 11.9% less. This depends upon engine and aerodynamic efficiency, and the empty weight per passenger of the aircraft. "Engine specific fuel consumption is only a temporary advantage because the competition will soon get it," Lafontan said.
Aerodynamic efficiency is harder to obtain for the airframe manufacturer and is a medium- to long-term advantage, he said. Airbus estimates that the A380's lift-to-drag ratio will be more than 10% better than the 747-400's. Since many elements of the 747's aerodynamic design date back more than three decades, this should be within grasp. The A380 is designed with a Mach 0.85 long-range cruise speed, but also with a broad range of efficient speeds to adjust arrival times into noise-sensitive areas. Boeing has historically conducted more than twice as many wind tunnel tests as Airbus and had a more systematic approach of checking results, Lafontan said. "We didn't have the people or money."
Weight is the final element of fuel burn and is the area where the A380 faces the greatest struggle. Larger airplanes are heavier per passenger, by a factor of two from the Boeing 737-400 to the 747-400 (see table).That is because they usually have more range and must carry more fuel, and they fight the square-cube law, where areas such as the cabin floor and wing grow as the square of length, while the weight grows more rapidly as the cube of length. The square-cube law is not strictly true for aircraft, but the tendency is there.
The double deck of the A380 is a tactic to delay the weight growth, but the fact that the A380-800 is the first of a model makes it relatively heavier than the stretched-out Boeing aircraft listed in the table. Lafontan noted that while the estimated weight per passenger is heavier than the 747-400, it is about the same as the unstretched 747-200B.
With a 49% larger floor area, the A380 gives about 10% more space per passenger than the 747-400, but it may be paying for that in weight and drag. One way this is used is to make each seat and aisle 1 in. wider.
For the A380, Airbus did studies of how passengers perceive the cabin, Lafontan said. He noted that some first-class seats can weigh up to 200 kg. (440 lb.) while engineers sweat to save smaller amounts on structure, and that a better understanding of passenger perception could make a more efficient design.
In 1998, a short mockup of the A380 upper cabin was taken to several major cities, and more than 1,200 frequent fliers were interviewed. Airbus found that cabin perception changes with culture, but did conclude that the floor should be lowered 2 in. and the ceiling line reshaped. From this basis, nine designs were made, and four were built into long mockups.
Using the advice of 150 Airbus employees of varied backgrounds, the choice was narrowed to what Airbus calls "flat" versus "traditional" styling. By then the airlines had become involved. Two mockups reflecting these styles have been built, and the final concept is to be selected this month.
Weight will be increased by the latest certification rules that the A380 has to meet. An example is the evacuation slide standards. The upper deck doors are more than 30 ft. above the ground, and slides have to reach the ground at a safe angle even if the aircraft is tipped up; however, extra slide length is undesirable if the sill height is normal. Previously, slides just had to touch the ground in the tipup case, even if it meant a straight 90-deg. drop, Lafontan said. Slides are also becoming stiffer. And because passengers are reluctant to leap onto the slide when they can see how far down the ground is, the trend is to put blinder walls at the exit and a curve in the slide to mask the ground--more weight added.
"By meeting the latest requirements, we have twice the slide volume, more weight, they won't fit on doors, and it costs more to have fuselage cutout panels for putting slides under the floor," Lafontan said. "Intelligent" slides that vary length depending on fuselage tipping may be required. Airbus plans to certify the two decks as independent, each with its own slides and rafts. Even though a large staircase connects the two levels, it is not required for evacuation certification.
APPLICATION for certification was started about two years ago with the FAA and European Joint Aviation Authorities, said Wolfgang Didszuhn, Airbus vice president for product integrity. The French DGAC certification agency--acting as part of the JAA--will be the primary certification authority, and the FAA may delegate certification to the JAA, with some exceptions.
A draft of special certification conditions should be completed by this summer, Didszuhn said. Interpretation of the rules may be needed for new technologies such as integrated modular avionics. The touchdown sink-rate requirement for the landing gear may be increased to 12 fps. from the normal 10 fps., as experience shows that touchdown rate grows with gross weight, Didszuhn said. That would make the structure heavier.
The landing gear for a huge aircraft promises to be heavy, particularly when runway loading limits must be observed. The configuration is a pair of body gear with six-wheel trucks and wing gear with four-wheel trucks and a track of about 14.5 meters. The usual "ACN" method of rating runway loading doesn't work well for six-wheel trucks, and Boeing modified it for the 777, said Michel Comes, director of systems engineering for the Airbus Large Aircraft Div.
At Toulouse-Blagnac airport, Airbus built four flexible types of instrumented pavement and tested them last year with a real 777 and 747, and a 20-wheel weighted mockup of the A380 main landing gear. About 5,000 passes were made over the pavement samples, and the data were used to optimize gear design. "The tests showed we could use a shorter bogie beam length, and we saved more than 300 kg.," Lafontan said. The bogie was shortened again recently to save another 150 kg. The pavement loading is less than that of a 777, he said. Tests on rigid pavement will be conducted this summer.
The rear wheels of the six-wheel trucks are steerable, and the goal is to make a U-turn fit within the roughly 185-ft. U-turn diameter of the 747.
Estimated weight is under 20 metric tons for all the landing gear parts, including brakes and wheels, or under 3.6% of MTOW. "This is heavier than the 747 as a percentage of weight, but it meets newer requirements such as runway fatigue," Lafontan said.
The brakes have a different backup hydraulic configuration that may save 20 kg. per wheel, or 400 kg. on the aircraft, Comes said. Normally they would have two sets of clamping pistons powered by two hydraulic systems, but the "single-cavity" design has just one set of pistons powered by one hydraulic system with a local electric pump as backup. New-technology tires may also save 400 kg., Lafontan said.
AIRBUS' NOVEL HYDRAULIC system design relies on electricity for redundancy and employs 5,000-psi. pressure instead of the typical 3,000 psi. for airliners. The electric backup configuration saves 400-500 kg. compared with having three standard hydraulic systems, and 5,000 psi. saves 1,200 kg., Comes said.
Because of engine rotor burst concerns, the airplane has to survive with one hydraulic system remaining, and having four conventional systems is oversized and costly, Comes said. The A380 has two standard central systems, each supplied by four engine-driven pumps on one side of the wing and an electric motor pump for ground use only. There are also two electrically powered systems with "electrical backup hydraulic actuators." Each EBHA has an integral electric pump and operates as an electrohydrostatic actuator. EBHA performance is less than a normally powered actuator, except the aileron EBHAs have equivalent performance.
The result is that there are four circuits--yellow and green central systems, and left and right EBHA backups. An advantage of electricity is that it can be routed above the cabin floor without worrying about hydraulic fluid leaking in the cabin. This allows the electric wires to be more widely separated, reducing the chance of simultaneous damage in a rotor burst. The copper wiring is also lighter than hydraulic lines, Comes said.
The A380 has three ailerons per side, each with two actuators. At least one actuator is powered by a central system; the other is either an electrically powered EBHA or runs off the other central system. The other primary control surfaces have a similar mix of power sources. There are seven spoilers per side, each with one actuator. The rudder has two separate panels, each with two actuators, and the elevator has two panels per side, also with two actuators apiece. Stabilizer trim has one electric and yellow and green hydraulic actuators. The leading-edge slats are driven by green and an electric system, while the flaps are yellow and green. The wing landing gear retraction and brakes are powered by green, and the body gear runs from yellow. The thrust reversers are all-electric actuators separate from the hydraulic system; only the inboard engines have reversers.
There have been difficulties making 5,000-psi. systems reliable, but Airbus believes they can work. "We need to demonstrate that it's feasible and not risky," Comes said. A test rig was built here to test fluids under contaminated and accelerated life conditions. The result was no fluid degradation, and "we didn't see any erosion in the actuators," Comes said. Airlines use "Type 4" hydraulic fluid now, and Airbus is looking at higher temperature Type 5 fluids, one of which has been qualified by Boeing, he said. "It has to use normal fluid or else it would be a showstopper," he said.
Suppliers have told Airbus they can make 5,000-psi. components work if they design them from scratch and perhaps use titanium instead of aluminum or steel. Test results on a new 5,000-psi. fitting were presented to airlines late last year. With 3,000 psi., some high-pressure pipes were 8-10 cm. (3.1-3.9 in.) in diameter, and Comes said the smaller 5,000-psi. components should be easier to maintain.
THE VARIABLE-FREQUENCY electrical system is innovative for a transport. Normally, a variable-ratio drive keeps the alternator at a constant speed as the engine rpm. varies, but half of the drive and generator failures involve the constant-speed drive, Comes said. Eliminating the drive and letting frequency vary removes this headache.
The problem is that the electrical equipment has to tolerate alternating current ranging from about 380-760 Hz., instead of a constant 400 Hz. Cruise would be about 500 Hz. Tests have been conducted and specifications given to electrical consumers, such as the fuel pumps. "Most consumers, like the galley and inflight entertainment, are satisfied," Comes said.
The weight impact is not clear. Power generation will be lighter but power consumers may be heavier. Honeywell, Sundstrand and TRW Lucas are competing for power generation.
Airlines want a generator loss to be a non-event. Cruise consumption is about 400 kva., and there will be four 150-kva. engine generators so one can die without load-shedding, Comes said. Ground consumption is expected to be 200-250 kva., and there will be two 120-kva. generators on the auxiliary power unit plus four 90-kva. ground connectors.
Structural materials include the latest light aluminum alloys similar to those in the 777, Lafontan said. The 2524 alloy can be stressed 5-10% more than conventional 2024, he said. Some fuselage skin will be Fokker's Glare aluminum-fiberglass composite, and the fixed leading edge will be a Fokker thermoplastic composite (see following story). Laser welding of aluminum will replace riveting to attach stringers on some parts of the lower fuselage.
The wing center section will be carbon composite, a first in commercial aviation, Lafontan said. The center section is more than 2 meters tall and would normally weigh more than 10 metric tons. It will be monolithic composite except where the design forces use of honeycomb. Honeycomb is avoided because it is hard to repair and it might trap fluids.
Tail surfaces are large on the A380--the horizontal is about the same size as a 2,360-sq.-ft. A310 wing, and the vertical is bigger than a 1,320-sq.-ft. A320 wing. These surfaces will be made of carbon composite, as will the rear pressure bulkhead, fuselage keel beam, floor beams and unpressurized tail cone that holds the stabilizer.
To reduce the control authority demands on the tail and keep its size, weight and drag small, the A380 center of gravity (CG) will be located about 6% aft of other aircraft, just in front of 50% mean aerodynamic chord. "It's statically stable but not dynamically stable," Lafontan said.
Aft-CG tests were conducted in the No. 1 A340 last year. "With experience, we can reduce the stability margin," Comes said. "It's not unstable. In certain maneuvers it may become unstable, like all aircraft."
The fly-by-wire system will give a normal feel to the pilot, and will have failure modes the same as the A320, A330 and A340, Comes said. If it reverts all the way to "direct law," fuel can be transferred to move the CG forward as required.
The final backups to Airbus' fly-by-wire system have been changing. The A320 has manually powered stabilizer trim and a mechanically signaled rudder. "These backups go beyond regulatory requirements," Comes said. "We demonstrate safety with the standard system." The A340-500/600 replaces the mechanically signaled rudder with a backup electrical signaling system that runs off the hot battery bus. It has an independent gyroscope and accelerometer to maintain yaw damping and bypasses the standard flight control system. The A380 includes this change, and also replaces the manual stabilizer trim with a backup electric motor.
Original link http://www.aviationnow.com/avnow/new.../ta3800618.xml
