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Software, electronics, and materials propel airliner design

Software, electronics, and materials propel airliner design

Fact: with or without the acquisition of Douglas Aircraft, Boeing Commercial Aircraft Group (BCAG) is the planet's biggest, baddest, and arguably best manufacturer of passenger airliners. In 1996, BCAG announced 717 new commercial airplane orders, 64% of the worldwide total for all manufacturers. It employs roughly 87,000 people and offers the broadest range of aircraft available. Boeing produces no fewer than 17 versions of the 737,747,757,767, and 777. Eight further variations (typically differing in fuselage length, capacity, and range) are under various stages of consideration.

At the end of June, the company rolled into the sunshine its latest creation, the Next Generation 737-800, an almost completely re-engineered, stretched-fuselage 189-passenger version of the 30-year-old 737 design, which includes a larger wing, new engines, a redesigned tail, an advanced cockpit, a new interior, and hundreds of other significant advancements. In 1994, to much fanfare, the all-new 777 took to the skies as the first completely digitally defined airliner.

But what's most interesting about BCAG isn't any single design. Except for the easily recognized 747, all Boeing aircraft have a cylindrical fuselage, two wing-mounted engines, and a conventional tail in the back. From a distance, you can be forgiven for confusing a 767 with a 777.

No, what makes BCAG interesting is the steady march of engineering improvements and technologies affecting the latest designs and those soon to come. "It used to be you could focus almost exclusively on aircraft performance, because the step from propeller driven aircraft to jets, and then jets to jumbo jets were so dramatic that raw performance offered the biggest payoff," says Robert Spitzer, BCAG's vice president of engineering. Today, performance is a given, and the most engineering effort is being paid to safety, maintainability, product life, decreasing the development cycle, and cutting costs for both acquisition and operation. Some examples:

  • Focus on the man/machine interface. As engineers perfect the mechanics of the aircraft itself, human factors increasingly become the weak link.

  • Knowledge-based engineering. Capturing engineering rules and automating the design of repetitive components saves time, money, and makes for a better product.

  • Evolution of the "virtual" airplane. CAD and FEA tools allow all design, analysis, and increasingly even tests to be performed on electronic versions of a design.

  • Improved flight-safety systems. Predictive Wind Shear warns pilots of impending wind shear events in time to properly prepare for them.

  • Quieter, more community-friendly aircraft. Boeing's advanced, patent-pending noise analysis techniques are helping to make aircraft more acceptable neighbors.

  • Application of advanced materials. "Boutique" aluminum alloys have kept an onslaught of composite materials at bay.

Predictive windshear uses Doppler weather radar to give advance warning over current reactive windshear system.

These trends have also had an interesting impact on the role of the engineer at Boeing. "We're asking our designers to be broader than ever before and have a perspective of the total business," says Spitzer. "They have to be jugglers." Not only are the products benefiting from new engineering, but the process of building them and the designers themselves are being re-engineered as well. Here's how.

Information advantage. While the 777 was celebrated for its thorough application of solid modeling and digital preassembly, digital-information revolutions of several other kinds were kindled behind the scenes. The most significant of these was the push for more knowledge-based engineering (KBE).

Knowledge-based engineering is the process of capturing engineering rules, design parameters, material properties, and manufacturing criteria for the design of specific components and incorporating them to a specialized software program--in this case, ICAD from Concentra Corp. (Burlington, MA). The program then crunches through the information--including FEA results, weight and balance studies, fastener spacing, etc.--and outputs 3-D CATIA models of those components, as well as finished 2-D drawings and wire frames for manufacturing.

Using ICAD, engineers automatically generated solid models and complete detail and assembly drawings of repetitive structure on the Next Generation 737.

"For repetitive structures, like fuselage frames and wing stringers, we can embed 90% of what it takes to design these in ICAD," says Keith Wiege, chief engineer, structures. "It allows us to go through a repetitive design process much quicker with higher quality. And the design criteria is captured so that we know it is always the same." He's quick to note, however, that KBE does not replace engineers, it just frees them from the tedious work. "Without good engineering," he says, "it would just make fantastically mediocre designs very quickly."

Wiege estimates that KBE can cut the cycle time by 50% for those components to which it can be applied. As an example, he describes a 777 redesign of the aft fuselage that occurred as the affected empennage components were being fabricated in Japan. "We were able to regenerate the stringer drawings using KBE within a few weeks," he notes. "Prior to that it would have taken months and not been as precise."

Traditional computer-aided engineering tools--such as CATIA CAD software and finite element analysis software--have received recent boosts as well. For the 737 Next Generation, engineers applied for the first time Transparent Digital Pre-Assembly. This process automatically compared components to those near it on a nightly basis and reported interferences to the engineer in the morning. "You no longer have to go look for problems, it will just tell you," says Carolyn Van Horne, deputy chief project engineer on the new 737.

CATIA information found more application in the manufacturing area as well. Not only was the aircraft designed 100% digitally, that same data was used to create the tools. The benefit? Parts just fit.

The Next Generation 737 also marked the first time that a "strain survey" replaced a full static test to meet FAA certification. Instead of dedicating a complete airframe to be bent to limit load or even failure, engineers took the first production airframe to 60% of limit load and compared the readings from thousands of strain gauges to the FEA analysis.

The results correlated within 1-2%, and always on the conservative side. "This saved the cost of an airplane," says Gary Prescott, chief engineer program tech support. "Instead of destroying an airframe in tests, we're delivering it."

To perform the FEA, engineers used ELFINI, a finite element modeler and solver produced by Dassault Systemes, producer of Boeing's CATIA software. Prescott notes that today's sophisticated nonlinear elements provide superior results to FEA than a decade ago, especially with post buckling computations.

Safety first. "Nothing is better than 12,000 engineers thinking about safety," says George Dial, manager of airplane safety engineering. "It's intimately a part of everything we do." And well it should be, and not just for reasons of basic societal responsibility. While air travel is rightfully advertised as incredibly safe, the sheer increase in the amount of travel and the attention given to any accidents that do occur make it paramount that safety receives top priority.

Interestingly, about 70% of aircraft accidents are attributed to human error. The quality of the man/machine interface can have a significant impact, and engineers are doing their part to make Boeing aircraft more human friendly in the cockpit as well as in unexpected areas, such as production and maintenance tasks. "We can improve safety and efficiency for the production people," says Ramzy Boutros, human factors engineer, "and it turns out that you get a higher quality, better flying airplane as well."

The electronics equipment (EE) bay on the 737 Next Generation provides an excellent example. Increased flight-deck functions meant more avionics and connectors in an already crowded location. This 100% digitally defined aircraft, however, didn't have a physical mockup to test accessibility and maintenance routines. So Boutros again turned to CAE software tools to perform his analysis.

He leveraged a Boeing-developed design-visualization program called FlyThru. It uses Silicon Graphics Onyx work-stations to pull hundreds or thousands of CATIA solid-models from a digital-definition database and display them in their final assembled locations. In this environment is placed a human model called Safework developed by Les Consultants Genicom (Montreal) which is then manipulated into position to judge access to the EE bay. "With FlyThru Human Model we are actually able to do the human interface with a high degree of fidelity," say Boutros. Verdict: mechanics couldn't reach all the new connectors.

Boutros' study led to a complete redesign of the EE bay in which it was moved to the forward cargo bay. There, mechanics and manufacturing personnel can sit and easily reach everything. It's producible, maintainable, easier to troubleshoot," he says.

CAD also plays a role in performing risk and spatial analysis for other tasks. Tire bursts, for instance, can be visualized from points of view in the wheel well that would be impossible otherwise. "You can project a cone of damage and see what systems are affected," says Dial.

Cockpit of the future. Other attempts to improve the man/machine interface are occurring in the cockpit. An unlikely source of usable information is a byproduct of new rules to increase the number of flight data recorder parameters from 44 to 88. Though the change officially takes place over five years, Boeing will begin delivering airplanes that meet it by April '98.

How can data intended for accident reconstruction assist before the accident? "Many airlines are buying a combined flight data-acquisition/data-management recorder," says Scott Pelton, manager of avionics, displays and maintenance. Data stored on optical drives or tapes lets airlines do trend analysis of fuel burned or oil consumption or inefficient work patterns.

At landing, pilots will soon benefit from new multi-mode receiver landing systems. They address limitations with current Instrument Landing Systems (ILS), which increasingly face interference from FM radio on adjacent frequencies and multipath reflections from tall buildings near airports. Besides, ILS is scheduled to be phased out by 2010.

These multi-mode landing systems will be modular units that physically fit in the space of current ILS-only packages, but will include one, two, or three of the following: FM immune ILS, microwave landing capability, and GPS landing capability. Systems bought with only one of the modules can be upgraded by simply plugging in additional electronics cards in the future.

The microwave system is being studied by European carriers, and has the capability of guiding the aircraft in for a category IIIb landing--one with zero decision height such that the ground isn't visible at touchdown. GPS cannot currently offer such precision, but "it's where the future of en route navigation is, and where landing navigation is as well," notes Pelton.

Airline customers can choose from three suppliers: Allied Signal, Collins, and Sextant. First delivery of a Boeing with multi-mode capability will be in October 1997 on a Next Generation 737.

Other cockpit innovations being implemented by Boeing engineers include a Predictive Windshear System (PWS) which uses airborne Doppler weather radar to analyze horizontal wind velocities, and a new Enhanced Ground Proximity Warning Systems (EGPWS). Rockwell's Collins division and Allied-Signal Bendix both have PWS units which will first appear on new Boeings next spring. They increase the advance warning time over previous methods by as much as 60 seconds. "Current systems are effective, but don't tell you about the windshear until you are actually in it," says Craig McMillian, flight crew operations engineer.

The EGPWS systems use an onboard terrain database in conjunction with GPS to prevent the number one cause of aircraft accidents--controlled flight into terrain. See DN 7/7/97 for a thorough look at these systems.

Incremental improvements. Engineers often equate the aerospace industry with great leaps in innovation. Once true, the barnstormer days of aviation are over. Both Super Jumbo airliners and the High Speed Civil Transport are on the drawing board, but decades of technological, financial, infrastructure, and political hurdles must be cleared before either makes a first flight.

Instead, the improvements will be behind the scenes and under the skin. Boeing will continue to make its design and manufacturing more efficient and even more dependent on computers and digital product definition. And expect to see a mad rush towards in-flight entertainment systems--fueled by the consumer electronics industry--and further introduction of "free flight," GPS-based navigation systems.

"There is a limited amount of work to be done in some classic areas, like aerodynamics, where the potential of marked improvements is slim," says Spitzer, engineering vice president. "What's hot is composites, which could ultimately save cost and improve performance, and matching up to manufacturing."


Do you see what I hear?

A patent-pending phased array microphone systems developed at Boeing should help engineers better identify and reduce noise sources on future aircraft. Invented by Robert Dougherty and Jim Underbrink, the system addresses bandwidth issues of previous phased arrays that limited them to detection over an unacceptably narrow frequency range.

Phased arrays consist of an arrangement of multiple microphones that do not have to be physically moved or scanned, as is necessary with a single microphone system, to pinpoint noise sources. Typical arrangements spaced 64 microphones uniformly along the axes of a cross formation or over a planar square pattern. These designs, however, only operate over a narrow wavelength, because, if the sound waves are short enough to fit several between each microphone position, significant alias sound sources will be generated that make identifying the location of the actual source impossible.

Dougherty solved this problem by arranging the 64 microphones in a single-arm logarithmic spiral which contains no repetitive microphone spacing. Underbrink furthered this concept with a multi-arm spiral possessing even better spurious sound-source suppression.

"It prevents situations where pairs of microphones are fooled in the same way," says Dougherty. Usable bandwidth extends from 1900 Hz past 40,000 Hz compared to a single specific frequency for standard filled-square grids.

Primary application of the technology has been in wind tunnels to study scaled models of aircraft or portions of aircraft. Using two arrays--one on the side of the tunnel and the other on the bottom--engineers can create three-dimensional computer images of sound sources. NASA Langley adopted the multi-arm spiral as a standard. Out on the tarmac, engineers have used a 16 sq-ft array mounted on a forklift for studying such things as 777 engine noise.

Design changes prompted by such acoustic analysis are still a few years away. What's next for Dougherty? "My goal is to take fly-over measurements and compare actual noise to model noise," he says. "They are not always the same thing."


Materials menagerie

It's been decades since 'miracle' composites were envisioned as the future material for all aircraft. But in the commercial airline market, metals and metal producers have proved to be a moving target. Composites have exhibited some expense and maintenance issues over the long haul, while new "Boutique" aluminum alloys have made this familiar material ever more appealing.

New 'Botique' aluminum alloys offer engineers specific improvements over old-generation alloys.

The greatest trends have been away from focusing on pure performance at any cost--where composites have an edge--to concentrate on increasing corrosion, fatigue, and toughness properties. Even more recently, engineers have pushed for materials that cost less, have shorter cycle times, and are more easily machined.

A prime example is stretcher-leveled 7050 plate for producing the big monolithic machined wing ribs that replaced the built-up assemblies on the Next Generation 737. It has proven a cost effective alternative to forgings, which tend to warp and distort when machined.

Most improvements in one property of an alloy usually trade with decreased properties in another. The crusade is to find a metal that is both tough, corrosion resistant, machineable, and very strong. "We're still looking for basically a stainless-steel aluminum," says Brian Smith, unit chief in metals technology at Boeing. The best candidate is Aluminum-Scandium, a material investigated more in Russia but recently supplied in various forms by US producers, such as Alcoa. It is both strong and corrosion resistant.

"Aluminum alloys are not stagnant technology," says Smith. "The industry has been very aggressive about pushing forward."


Commercial

Boeing Commercial Aircraft Group pumps out almost 40 jetliners a month, accounting for (currently) 75% of the company's total sales.

Projects include: the next-generation 737; multi-mode landing systems; and a Predictive Windshear System.

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