What auto can learn from aero: simulating without prototypes

Automotive Design & Production, Oct, 2003 by Lawrence Gould

For the 777, computations regarding aerodynamics were double checked by "conventional" computational "fluid dynamics (CFD) and a number of wind tunnel tests to determine whether the results were reasonable, says Jean Jacques Chattot, professor of mechanical and aeronautical engineering, and director of the Center for CFD at the University of California (Davis, CA). "Indeed, they were quite good."

Fact is, CFD and wind tunnel test are Complementary, and they don't necessarily Produce the same results. When discrepancies do occurs, explains Smith, "there would be quite a bit of assessment to determine what should be the right answer than the other. I wouldn't say that either one is automatically accepted."

Up to the Boeing, maybe 25% of an aircraft design was produced with CAD-drafting for the most part,no 3D. The 3D solids modeling was used for special studies. For instance, Boeing used solids modeling to figure out a problem with the landing gear was in the mockup for the 767. The 3D model twisting a "tiny bit."

For the 777, some systems and subsystems require specialized modeling-and mockups. One mockup was for the 777 nose to check critical wiring. A second mockup, an "iron-bird," was a complete working physical prototype of the aircraft's internal dynamic systems. This proved out the integration of electronics and hydraulics, which were simulated separately though not integrated with the 3D solid modeling tools.

FIRST FLIGHT, AND TEST

Designing and simulating the 777 consumed about 2,200 workstations linked to a four-IBM mainframe cluster in Puget Sound, plus four more mainframes in other locations, and 3 terabytes of data. This work validated the tooling and assembly plans lot approximately two million parts, a validation that usually occurs once assembly begins in the plant. The work paid off for the Boeing "I77, resulting in these benefits: * Elimination of more than 3,000 assembly interfaces, without any physical prototyping

* 90% reduction in engineering change requests (6,000 to 600)

* 50% reduction in cycle time for engineering change request

* 90% reduction in material rework

* 50x improvement in assembly tolerances for the fuselage

Regarding that last item, the fuselage is 200-ft long. Alignment was off by 0.023 in.--about the thickness of a playing card--while for most other airplane parts in previous aircraft the alignment is to within a half inch of each other. Likewise, the wing tip on that very first Boeing 777 out of production was oil by 0.001 in. By comparison, the wing tip on the Boeing 747 was off by 4.0 in.

That first Boeing 777 out of production flew. It went through flight test. It was then refurbished and delivered to a customer. It was the next five or so airplanes out of production that went through non-destructive tests, such as jet engine checks for particular aircraft models under test. Also, there were structural tests that bent and flexed the wings and cabin, and pressure tests that compressed the cabin--numerous fatigue tests that run through an enormous number of cycles to both simulate actual performance in service, such as takeoffs and landings, and to show the state of the aircraft after 20-plus years of use. Boeing's modeling methods have been so good that the FAA has accepted them in lieu of certain physical destructive structural tests on finished aircraft.