Engineers at NASA’s Ames Research Center in Moffett Field, CA, needed to redesign the engine cowling of their Systems Integration Evaluation Remote Research Aircraft (SIERRA) to reduce drag and improve fuel economy. The SIERRA is an unmanned aerial vehicle (UAV) routinely deployed for environmental collection missions over remote regions. To design a more aerodynamic cowling, NASA needed an accurate 3D CAD model, consisting of engine dimensions and cowling mounting specifications. Such a model didn’t exist because SIERRA’s engine is an off-the-shelf product.

The SIERRA UAV engine assembly.
NASA engineers estimated that it would take 500 hours to build a 3D CAD model using traditional methods. This laborintensive process involves disassembling the engine, manually measuring each part, and reconstructing a CAD model of the geometry. Multiple iterations of the modeling process are usually required, due to missing or inaccurate measurements from the original parts.

To accelerate the design process, NASA contacted MACHT3 Engineering, which specializes in 3D solid modeling, finite element analysis, and mechanical design. Using a 3D laser scanner and Rapidform XOR software, MACH-T3 reverseengineered the cowling in a fraction of the time NASA originally estimated it would take them to accomplish the task.

Today’s 3D scanning technology allows engineers to generate 3D point cloud data to help them generate CAD models from existing parts. MACH-T3 used a handheld 3D laser scanner to capture the raw scan data and Rapidform XOR to convert the 3D scans into a parametric CAD model.

The Rapidform extracted solid model.
Typically, optical 3D scanners cannot pick up all data points and render a complete scan due to differences of surface texture, color, and obstructed lines of sight. Rapidform recognizes the geometry and allows engineers to extract model information, even with missing scan data. This capability makes it easy to convert the scanned point cloud data over to a CAD solid model, saving time and improving accuracy.

For example, the scanner could not capture SIERRA’s entire engine. Historically, this would have made creating a CAD model very difficult. The new paradigm introduced in Rapidform XOR, however, obviates the need for complete scans, because there only needs to be enough data to define each feature of the part.

Real-world parts are never perfect, so the engineer is empowered to choose to stay very accurate to the scan, resulting in an imperfect design, or deviate from the scan in order to idealize the model. An example of this is the heat sink on the SIERRA’s engine. The fins are almost certainly not evenly spaced, as a byproduct of the casting process. When creating the CAD model of the engine, the engineer needed to decide whether it was better to keep the uneven spacing or idealize the model with even spacing between the fins.

The entire cowling redesign project took engineers 50 hours instead of 500, allowing the MACH-T3 team to achieve success in 10% of the time it would have taken them using conventional means.

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