Photos: Renishaw

Cooling unmanned aerial vehicle (UAV) engines often requires airflow – the faster the aircraft moves, the easier it is to cool. So, when a customer asked about cooling a UAV that didn’t move quickly, Cobra Aero President Sean Hilbert was momentarily stumped.

“We were brainstorming one day on how to develop a cylinder for a UAV aircraft that didn’t have a lot of airspeed, one that hovered. We knew we needed to cool that engine in a way that was going to be really difficult with the current architecture,” Hilbert explains. “Additive manufacturing (AM) allowed us to design a cylinder unique to that application that we couldn’t have made any other way.”

Michigan-based Cobra has produced small motorcycle engines since 2003 and aero engines since 2012. Hilbert says the company continues to use various traditional machining methods, but he envisions a day when all of its parts will come off of metal 3D printers.

Hilbert initially worked with Renishaw on a test product to explore the capabilities of AM, quickly realizing the technology’s potential. Cobra invested in a Renishaw AM 400 system to increase its in-house AM capabilities.

To optimize the design of its engine cylinders and to gain expertise in AM, Cobra Aero visited a Renishaw Additive Manufacturing Solutions Center.

“Staying at the cutting edge of manufacturing is important to us,” Hilbert says. “Investing in AM allows us to develop tools and new products for high value, small-volume applications, speed up the manufacturing process, and produce designs that would not be possible using conventional subtractive machining.”

The original additively manufactured cooling system with fins (left) was redesigned by Sean Hilbert’s team into a lighter structure using a honeycomb lattice structure.

Design changes

As Cobra engineers became accustomed to AM processes, they moved away from traditional designs and started working on features that could not be produced by any other process. That design for AM (DfAM) journey started with the engine cylinders the company produces.

“That’s where most of the design changes happen rapidly, that’s where the highest tooling costs are, and it’s really where the performance is locked in,” Hilbert says.

An early cooling system, made on the Renishaw AM machine from a traditional design, used cooling fins, rigid structures, and external plastic ducts to move cool air across the cylinder. Hilbert’s team replaced that design with a lighter version using a honeycomb lattice structure instead of fins.

“The air flows through, and the lattice structure conducts air out away from the engine,” Hilbert explains.

Instead of using a plastic duct, and the challenges of sealing interfaces, engineers designed the duct into the 3D build process, integrating it into the part.

Another cylinder-cooling device takes advantages of two capabilities unique to AM – feature consolidation and complex internal channeling. Hilbert says Cobra prints the cylinder cooler as a single piece instead of bolting a head to a liner, simplifying assembly and removing assembly points that could fail. Internal channels in the liquid-cooled device allow tailored engine cooling to direct more heat mitigation to the cylinder head, where temperatures are the highest, rather than in the cylinder walls.

“We decided to redesign the cylinder because it would improve the overall performance of the engine. Modifications to this high-value part were evolving rapidly, which is why we chose AM to manufacture this component,” Hilbert says.

“By using metal additive manufacturing, Cobra Aero was able to design a part that was unique to the application,” adds Stephen Anderson, AM business development manager at Renishaw Inc. “Thanks to our laser powder bed fusion technology, they were able to produce a single part with complex lattice structure that performs better than one produced with conventional manufacturing techniques.”

Cobra Aero
http://www.cobra-aero.com

Renishaw 
https://www.renishaw.com/additive