Laser Scanning Aircraft for CFD Analysis

Last months contribution to LiDAR News featured our use of 3D laser scanning technology on the original Wright Brothers 1903 airplane to help NASA and other experts figure out how the Wright Bros. developed flight without the use of modern day computers. See this link to review that article.

This month we come full circle and discuss our use of similar 3D laser scanning technologies for the capture of modern day aircraft but for mostly the same purpose to help engineers figure out how and why things fly the way they do. They do that with the help of CFD software computational fluid dynamics which shows the reaction analysis on digital CAD models subjected to the forces of flight airflow, lift, drag, gravity, etc. This digital wind tunnel software process is less expensive than real wind tunnel testing and far less expensive than running real flight tests. However the quality of the tests is, among other things, subject to the accuracy of the 3D CAD models of the aircraft. Hence the use of 3D laser scanning of aircraft by our expert engineers at Direct Dimensions, something weve been doing for over 15 years.

Scanning NASA Drydens Modified F-15 Test Plane
As a laser scanning and reverse engineering company, we are constantly challenged by our customers in various industries to develop new solutions to complex 3D problems. One industry that certainly pushes the limits of our capabilities is the design and manufacture of aerostructures. In November 2006, for example, engineers at NASA Dryden Flight Research Center in the Mojave Desert at Edwards, CA, were conducting research aimed at lessening the destructive effects of supersonic flight. The LANCETS research, (Lift and Nozzle Change Effect on Tail Shock) measures the benefits of redistributing aircraft lift to reduce shock-wave pressure, thereby lessening the sonic boom.

In order to obtain in-flight data, they would need to modify and fly a certain NASA-owned F-15 test plane. However measuring pressures on flight surfaces on a plane traveling at supersonic speed from a chase plane some 200 feet away is expensive – and risky. So before committing to the expensive aircraft modifications, NASA wanted to see if advanced computational fluid dynamics (CFD) software could simulate these flight conditions accurately enough to predict how these relatively minor flow surface adjustments would reduce supersonic noise levels. This approach would allow modifications to the plane to be made digitally, and then the CFD analysis would test the changes without the costs and time associated with actually flying the airplane.

Accurate CFD analysis would require a precise digital model of this particular F-15 which had been previously modified from its original design for other NASA test programs. Traditional 2D sectional drawings of the nominal plane geometry were available, but these would not work in this case. Plus this particular test airplane had been hard landed a few times in its life as a test plane, twisting the airframe enough that the dimensions differed fractionally from the original design. To do this CFD analysis, NASA needed a dimensionally accurate as-built 3D CAD model of this specific F-15 airplane.

Locating an Experienced 3D Team
Through a Google search, the engineers at NASA Dryden found our firm, Direct Dimensions, Inc. (DDI), a 3D metrology firm located in Owings Mills, MD, with specific expertise in this type of challenging work. Founded in 1995, Direct Dimensions specializes in a wide variety of 3D scanning and reverse engineering technologies for a wide range of industries and applications.

Plus with over 25 years of direct experience in the aerospace field solving 3D manufacturing problems, and as the founder and chief engineer of Direct Dimensions, I felt confident that our team could handle this aircraft 3D measurement problem. While working in the late 80s as an engineer at what became a Lockheed Martin aerostructures manufacturing facility in Baltimore, I recognized the need for better portable 3D measurement tools to solve complex problems like this. I was one the original developers and the first industrial customer for the Faro Arm portable coordinate measuring machine, the now ubiquitous 3D measurement tool used extensively around the world.

With many aerospace-related projects under our belt, my team at Direct Dimensions was eager to take on this new NASA challenge. Since the companys inception, aerostructures work has been at the core of our business. Early projects, for example, involved digitizing aircraft cockpits for human factors analysis with the U.S. Navy, scanning large aircraft fuselages to accurately design flight simulators, and once, for a major accident investigation the exterior of a 29-passenger turboprop with only two Faro Arms, something that proved extremely challenging. Even with new computerized 3D measurement technology, these projects often took days to capture all of the critical characteristics required for these projects.

In fact, as the nature of these measurement challenges evolved, so too did the 3D technology needed to accomplish these projects. By 1999, for instance, our Direct Dimensions engineers digitized and modeled the exterior of a large BAC-111 commercial airliner for flight test modifications design. For this project the engineers utilized the then relatively new portable laser tracker to capture the flow contours of the entire aircraft. While state-of-the-art at the time, the effort still took over eight working days with the tracker just to capture all of the required 3D data.

Travel to NASA Dryden at Edwards, CA
After the F-15 project was defined and NASA authorized the work, our engineers Dominic Albanese and Glenn Woodburn flew to California armed with our newest 3D measurement technology at the time – the FARO LS 3D Laser Scanner. Being portable and designed to capture the shape of large objects, this fast new 3D scanner could acquire up to 120,000 points per second over ranges of up to 80 meters. The FARO LS was the ideal technology at the time to quickly and accurately capture the exterior shape of this jet. With accuracy of +/-6 millimeters, this scanner can take a digital shapeshot – like a snapshot but in full 3D – of very large objects, such as airplanes and buildings. The raw 3D scan data is actually a high-resolution 3D point cloud of laser reflected spots off the surfaces. This dense 3D point data can then be digitally processed and modeled into various CAD formats for a wide variety of applications, which is our absolute specialty.

Upon arrival at NASA Dryden, Albanese and Woodburn set up the LS scanner in the hangar and devised their scanning plan. Noticing the bright desert sunlight streaming through the skylights, the engineers rearranged the schedule to perform the bulk of the scanning in the evening and into the night. Bright desert sunlight on the glossy surface of an aircraft would cause noise in the scan data. Over the course of a single long evening, the team scanned the entire aircraft. Care was taken to capture the plane from all angles to assure even fine details were properly represented.

In the end over 50 individual scans from different positions yielded over 50 million data points for post processing. Upon return to the DDI facility in Baltimore, the engineers began to process these huge raw data sets to reverse engineer the F-15.

Reverse engineering the F-15
One of the more impressive aspects of using 3D scanning technology is the amount of data captured within even a single scan. In fact much of this data is not needed, such as the walls of the hangar, equipment in the area, and even people walking through the room during a scan. Consequently one of the first tasks in the modeling process was to digitally remove all this extraneous data. The individual scans were then aligned and knitted together using Innovmetrics PolyWorks software to ultimately create a mesh surface using all the data points. With the polygon mesh complete in STL format, the once immense 50-million raw point file was reduced (called decimated) to a somewhat less-staggering 1-million triangle mesh file by removing redundant data within a certain tolerance.

The next phase of the process was to establish the craft’s primary geometry and construction lines such as its main fuselage center line, certain axes of rotations, wing sweep profiles, and various cross sections and contour curves. These geometric elements were then imported into SolidWorks where they were ultimately stitched together and reverse engineered to create a complete solid CAD model of the F-15.

The final digital model was sent to NASA Ames Research Center in Hampton, VA where it was imported into their CFD package. Running the advanced software with the digital model of the F-15 as a virtual wind tunnel, NASA engineers were able to test their experimental modifications on an essentially perfect digital recreation of their F-15 jet.

Upgrading our 3D Scanning of Aircraft
The F-15 project for NASA, while challenging, provided our engineering team at Direct Dimensions with another opportunity to refine and streamline our process for laser scanning and digitally replicating aircraft OML (Outside Mold Lines) for CFD analysis. In fact, after completing the F-15, our team was immediately asked to digitally recreate an F-16 Fighting Falcon for the same group of engineers at NASA. We have since kept busy modeling aircraft and components for analysis for a variety of aerospace customers. While Direct Dimensions created a thorough and refined process that worked for the creation of models for CFD, that process is consistently fine-tuned with the advent of new technologies and tools.

More recently in the fall 2008, our team received another opportunity to scan a plane, this time a Gulfstream test plane – again for NASA in conjunction with Texas A&M University, and also for CFD analysis. In the two years since the F-15 project in 2006, an entirely new laser scanner had entered the market that weve used and tested for over a year by then. The Surphaser HSX mid-range spherical laser scanner could collect up to 800,000 points per second and has an accuracy of less than a millimeter. Our engineers at Direct Dimensions were among the first in the country to adopt this new technology for scanning aircraft for CFD analysis. The new technology allowed for full scanning of the much larger Gulfstream plane with significantly increased accuracy and resolution and in much less time than it took for the smaller F-15.

Scanning technologies have been used by other firms to capture aircraft. In 2005 engineers from the scanning firm Berding 3D used a Cyra2500 from Leica and a Vivid 910 from Konica Minolta to capture a Saab A-35 Draken aircraft. Like us at Direct Dimensions, they also processed the scan data with PolyWorks to deliver the final CAD models to their customer for use in similar aerodynamic testing.

We also know groups at Boeing in St. Louis and Wichita that perform 3D aircraft scanning with tools and methods very similar to ours at Direct Dimensions. We are working now to collaborate with this team and will share ideas at the upcoming Coordinate Metrology Systems Conference in Phoenix, Arizona (see for details).

We are very proud of our accomplishments for applying advanced 3D laser scanning technologies to the measurement and modeling of complex aerostructures. To learn more about these capabilities, see our Direct Dimensions website page specific to this topic.