Cobra Aero reimagines the combustion engine cylinder with physics-driven design

Cobra Aero completely redesigned the air-cooled cylinder of their UAV drone engines for Additive Manufacturing. Using conformal lattice structures, they developed an engine cylinder that is manufacturable in one piece with minimal support and weighs only 420 grams — a fraction of the weight compared to the lightest cylinder among their competitors.

Instead of fins, Cobra Aero opted for a lattice structure as the heat transfer medium. To optimize its performance they combined results from multiphysics simulation to spatially vary the thickness and density of the lattice.

The result was an engine cylinder that was more efficient, met weight requirements, needed minimal support during manufacturing, and performed exactly as the simulation models predicted.

Technical takeaways

• Advanced heat exchanger design: Use lattice structures to create compact heat exchangers with Additive Manufacturing.
• Design for additive manufacturing: Follow DfAM best-practices to achieve higher levels of performance and lightweighting.
• Directly drive geometry from simulation: Generate geometry controlled by multiphysics simulation results with field-driven design.

Business value

• Utilize manufacturing equipment: Grasp the full benefits of Additive Manufacturing by designing for the processes.
• Adapt to market demands: Adopt flexible design processes to expand your offering and move fast when an opportunity arises.
• Develop better products: Stay ahead of your competition by developing high-performing products that meet multiple requirements.

Key statistics

• Cylinder weight: 420 grams
• Material savings: 50% less wasted material
• Part consolidation: From 6 parts to 1 component
• Experimental validation: Performance according to specs
• Manufacturing process: Renishaw AM500
• Material: AlSi10MG

Introduction

Cobra Aero is a Michigan-based SMB with 25+ years of history offering engineering expertise, product-development facilities, and manufacturing services of engines and related components. The company initially focused on the design, development, and manufacturing of off-road motorcycle engines, but spun out to the specialty aerospace markets to adapt to market demand. Today, they produce roughly 2,000 engines per year primarily for Unmanned Aerial Vehicles (UAV) and drones.

To stay competitive, Cobra Aero invested in metal additive manufacturing — a Renishaw Direct Metal Laser Sintering (DMLS) system. Overcoming the design restrictions of casting to produce an engine cylinder with cooling fins that were more densely packed was a quick win. But the post-processing required to remove the support structures added considerable costs and wasted material.

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The team knew that redesigning the part for AM would yield many advantages — but, at the beginning of their journey, they didn’t realize how much more we could achieve. nTop’s field-driven design and simulation capabilities played a key role in developing this new product following DfAM best practices.

“By the time we were done, our models had evolved to the point where they were simply beautiful! Apart from the lattice, we soon realized there were many other advantages to using nTop besides no longer needing support structures. We were able to integrate the cooling duct with the cylinder itself, consolidating parts into a single piece. Overall the design is just cleaner, simpler, a tighter package that prints perfectly and presents itself a lot nicer on the engine.”

— Sean Hilbert, President of Cobra Aero

Lattices as an alternative to fins

Cobra Aero already had a successful AM-adapted finned cooling system in commercial production. However, the initial design required a lot of manual post-processing to remove the support structures which consumed as much material as the end part itself.

Looking for alternatives to fins, Hilbert’s team became intrigued by the lattice structures they saw being used across a variety of industries for advanced heat exchanger design. By hollowing out a solid aircraft bracket and filling the space with a lattice, honeycomb, or gyroid structure, weight can be decreased and strength improved. As a bonus, lattices are self-supporting — they don’t require any support structures.

“Lattice structures are very print-friendly and allowed us to tailor-fit heat transfer in a better way. The motor we were working with is designed to be used in small drones, where any extra mass can take a particularly heavy toll on payload, range, and performance,” Hilbert mentions. 

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Using nTop’s advanced modeling capabilities, Cobra Aero was able to quickly generate different sizes of lattices with varying strut thickness, using lattice infill inside their cylinder geometry and terminating it with smooth transitions. Through every configuration, the software handled all data generated by lattice iterations with ease, automatically generating fillets on the strut intersections and connections to the part skin. This distributes stress more uniformly, reduced concentrations that can lead to delamination, and promotes both manufacturability and durability.

“The possibilities that nTop's software opened up were virtually endless. The fact that the generation of hundreds and thousands of different lattice shapes is mathematical, accomplished without having to create discrete, surface-based models like you’d see in traditional CAD packages, meant we could be a lot more adventurous with our designs for 3D printing.”

— Kevin Brigden, DfAM expert at Renishaw who worked on this project

Optimizing the lattice structure for heat transfer

Hilbert notes, “the issue we were exploring is that the amount of pressure drop across the cooling duct is directly related to the amount of drag on the airframe. We needed to find that sweet spot where we’re getting enough heat pulled away from the cylinder but we’re not adding a tremendous amount of drag onto the entire structure so the UAV can fly longer, more efficiently.”

To overcome this challenge, Cobra Aero used nTop’s field-driven design capabilities to spatially control the properties of the lattice. They used a range of multiphysics simulations as input — temperature, airflow velocity, pressure drop, and mechanical stresses — to generate a highly optimized structure.

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Simply put, they tightened the lattice structure in areas where conduction was more important, while in areas where convection was more important and more airflow was needed to pull the heat away, they loosened the lattice structure.

On top of that nTop’s reusable workflows allowed designers to regenerate models without having to start from scratch each time. This allowed Hilbert’s team to iterate faster and slash the total product development time

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Manufacturing and bench testing

When the team finalized their design, they used nTop’s slicing capabilities to export a CLI file that was sent directly to the Renishaw AM500 metal system for manufacturing. This way, they bypassed the need to convert the geometry to the STL format and avoided the inaccuracies that may arise with such conversion.

During the development phase, a trio of final cylinder designs were manufactured and tested in the lab to verify their performance in real-world scenarios.

“Testing showed that the new lattice structure design with nTop was more efficient at cooling than our fin design. In every case, at every different RPM, less cooling air was required to maintain proper engine temperature. This was exactly what we were hoping for and exactly what our modeling predicted.”

— Sean Hilbert, president of Cobra Aero

What this means for the overall design of the engine is that Cobra Aero can now make a smaller inlet to the cooling duct, which in turn makes a smaller frontal area on the aircraft. In other words, they can achieve the same amount of cooling with less drag.

Moving design forward with nTop

Cobra Aero’s lattice cylinder design is currently in commercial production. “Our new lattice-cylinder design is better than our fin cylinder in every way—which is a big deal,” says Hilbert. If you want to learn more about the technical characteristics and performance of the A33N engine, Cobra Aero offers extensive documentation on their website.

But Hilbert’s team didn’t stop on the cylinder. With new tools at their disposal, they redesigned other key components of their UAV engine. A great example comes from their new engine mounts. After optimizing the topology of these components and following DfAM best practices, the new mounts now provide better sound attenuation and only weigh 160 grams — compared to the 385 grams of their lightest competitor.