Slaying the Draken21 Nov, 2006 By: Jack Thornton
Berding conquers huge 3D scanning job to help Lockheed Martin learn from aerodynamics of 50-year-old fighter plane
Built by Saab in Linkoping, Sweden, the Draken was designed in 1949. It was Europe's first supersonic combat aircraft. Between 1955 and 1974 more than 600 were built for Sweden, Denmark, Finland and Austria. A late-model Draken (Swedish for "dragon") could fly at Mach 2 -- twice the speed of sound -- and had a flight ceiling of 65,000 feet, comparing favorably with many newer aircraft. The Draken was the first aircraft designed with full-sized wind tunnel models rather than one-eighth scale (or smaller) models.
A Saab A-35J Draken in flight.
The Draken was designed to meet a variety of criteria. It had to be capable of short takeoffs and landings at small airfields close to battle zones -- sometimes even using ordinary roads. It had to offer an optimized combination of high-speed and low-speed performance. It had to be capable of re-arming in minutes between missions. And the aircraft had to be modular -- bolted together so its four segments could be individually replaced, sent to service or upgraded easily and quickly. Surprisingly, a few late-model Drakens still fly today, all with civilian pilots.
When aerospace/defense powerhouse Lockheed Martin wanted to research the aerodynamics of this 50-year-old Swedish-built jet fighter, it turned to Berding 3D Scanning, a digitizing and surveying firm, for help with processing point-cloud data using PolyWorks from InnovMetric Software. Utilizing long-range, low-definition and short-range, high-definition scanning systems, Berding 3D Scanning specializes in developing digital representations of physical objects, ranging from small component parts to entire plant layouts. Berding faced a huge challenge, but thanks to the right software tools, it answered Lockheed Martin's call with great success.
Accurate Data Map Needed
Because no current data was available regarding the Saab Draken's aerodynamic performance, the Lockheed Martin M&FC (Missiles & Fire Control) unit wanted to develop a very accurate data map of the potential test aircraft that could then be imported into engineering analysis tools.
The mission for Berding technical manager Mat Cappel was to generate complete, accurate surface data of the entire aircraft, plus high-resolution scans of weapons and gun bays, which could then be fed into Lockheed Martin's simulation systems in Orlando. The scanning was done at the private Inyokern Airport and aircraft graveyard in California's Mojave Desert, where six of the remaining Drakens were refurbished and are now stored.
Saab A-35J Draken aircraft at Inyokern, Calif., that was scanned by Berding 3D for Lockheed Martin Missiles & Fire Control.
"The technical challenge was creating aerodynamic surface data files by merging high-definition and low-definition scans," said Cappel. "The differences in resolution were enormous. Points in 3D space in the high-definition scans were about one micron apart," he explained, "and points in the low-definition scans were about a millimeter apart." That meant the high-definition scans were 10,000 times -- or five orders of magnitude -- as dense as in the low-definition scans. "We did a total of about 250 separate high-definition and low-definition scans that added up to 4.6GB of data," Cappel said.
The Lockheed Martin project stretched Berding's capabilities. "What they needed from us," Cappel said, "was high-definition information for model development plus low-definition data for a framework of constraints and to locate the high-definition scans of critical areas both one to another and to the geometry of the Draken as a whole." They also needed a way to merge the information.
"PolyWorks is one of the best, if not the best, at combining both long- and short-range data, and may be the only package that can do this efficiently," Cappel explained, noting that without the mix of high- and low-definition scans, digitizing the entire aircraft might have been computationally impossible because of the jet's large size -- the Draken is more than 50' long with a 31' wingspan and a rudder that stands nearly 14' high.
"Other 3D point-cloud processing packages probably could produce a reasonable representation of such a huge model," Cappel said. "But they might not be able to accurately handle 4GB of data we gathered, and I seriously doubt any of them would have been anywhere near as fast."
The PolyWorks IMAlign module aligned the 250 point-cloud scans -- millions of 3D data points each -- into a single model. After the scans were aligned, the point-cloud model was transformed in PolyWorks' IMMerge into a polygonal model. The PolyWorks alignment technique eliminates with the need for targets or markers on the surfaces being scanned; the scans are aligned to each other with their geometrical shapes.
The aligned model, created with PolyWorks.
"PolyWorks is incredible at handling so many aligned scans and combining the data using feature-based alignment as opposed to targeting," Cappel continued. "We did, however, use some targeting as well as feature-based alignment on this project. Not having to use a lot of targets dramatically speeded up the scanning process for us."
PolyWorks completed the formatting by generating NURBS (nonuniform rational B-splines) surfaces that could be imported into almost any CAD system. Cappel pointed out that PolyWorks lets users determine and extract critical feature curves to create the curves network on which the NURBS are fitted. "We find this is a great help in obtaining high-quality NURBS surfaces."
NURBS surfaces for import into CAD, generated by PolyWorks.
Two other big factors directly linked to the quality of the NURBS surfaces, Cappel said, were the high-quality of PolyWorks' polygonal model underlying the NURBS surfaces and PolyWorks' unmatched array of polygonal editing tools.
For all this digital effort, the final deliverable from Berding to Lockheed Martin was a relatively small file: 200MB uncompressed.
A second point cloud analysis system was also used -- Leica's Cyclone software for orthographic, sharp-edged geometry -- to scan and digitize the landing gear. "This was less than 10% of the total job and the Cyclone data fit very nicely into PolyWorks," Cappel said.
From Scans to NURBS to CAD
"Working with our [UGS NX] system, we turned around the data very quickly," Cappel said. "With PolyWorks, this only took two-and-a-half weeks with three-and-a-half people working a lot of overtime. Lockheed Martin technicians assembled final scans into a digital representation of the entire aircraft using CATIA from Dassault Systemes. They also performed NX-to-CATIA translations.
"Nothing but PolyWorks could have done the job," he added. "PolyWorks is so smooth when blending the high- and low-resolution scans. It's quick, simple and precise and it's very good software for organic shapes such as aerodynamics. We rely on it to analyze, sort out, constrain and format all our point clouds every day."
He continued, "It also helped that we could react right away. We got the call for the job one week before Thanksgiving 2005 and we were on a plane to Los Angeles three days later." The scanning was complete in two days and our two-man team was home for Thanksgiving.
"Lockheed Martin was in a hurry because it faced 90 days of nonstop data crunching in its flight simulation software," Cappel said. "It needed initial data in a week and we got it to the company." He added, "All of the Lockheed people we worked with told us they are absolutely thrilled with the quality and comprehensiveness of the data. There have been no glitches and no restarts," which can destroy simulation schedules.
When Berding took on this monumental task, its goal was to help Lockheed Martin's gain a better understanding of the aerodynamics of a potential test aircraft from a commercial company, and that was accomplished. The Draken's aerodynamics was revolutionary for its time and is still impressive today, half a century later.
About the Author: Jack Thornton
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