Date: Jan 15, 2020 Author: Lynn Manning Source: Auvsi (
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When a fast-growing company designs and manufactures some 2,000 small engines per year, it likes to stay abreast of the latest digital-engineering tools. Yet Sean Hilbert, president of Cobra Aero, is also adamant that the final proof of the integrity of every power and propulsion product he creates for his drone and motorcycle customers is always the test bench.
“In my experience with moving back and forth between the computer-aided engineering and the testing realms, honing in on that final design always requires a real-world check,” Hilbert says. “You need those digital-to-physical iterative loops in order to calibrate your models and make a great design.”
Speeding up those iterative loops is essential to staying ahead of the competition. Hilbert has been exploring the synergies between design software and additive manufacturing — 3-D printing — to deliver the benefits of lightweighting, part consolidation and material savings in less time than traditional methodologies. To that end, over the past 18 months Cobra Aero has been deeply engaged in several product-development projects with advanced computational-modeling software company nTopology and additive manufacturing (AM) equipment manufacturer Renishaw.
Test results on one of the team’s collaborations, the A33N drone engine, have just come in: its 3-D-printed air-cooled engine cylinder — with a novel interior lattice structure designed using nTopology software and manufactured on a four-laser Renishaw RenAM 500Q machine — significantly outperformed Cobra’s current workhorse cooling-fin design.
“Testing showed that the new lattice structure 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," Hilbert says. "What this means to design going forward is that we can now make a smaller inlet to the cooling duct, which in turn makes a smaller frontal area on the aircraft, so we have less drag on the aircraft for the same amount of cooling — exactly what we were hoping for.”
The finned cooling system against which the latticed one was measured is itself an AM-adapted, commercially successful design that is being flown now.
“The finned part that we’re currently shipping works wonderfully and prints beautifully," Hilbert says, "but it requires a lot of post-processing because it has almost as much support structure on the cooling fins as material in the part itself. And all those support structures have to be manually removed after 3-D printing. We knew we could improve on that design.”
Cobra Aero president Sean Hilbert holding a 3D-printed engine cylinder with a unique interior lattice structure designed with nTopology computational-modeling software.
Cobra Aero president Sean Hilbert holding a 3D-printed engine cylinder with a unique interior lattice structure designed with nTopology computational-modeling software. At far right is a finished A33N test engine with air-cooled lattice cylinder on top. Photo: Cobra Aero
Designing for printing
When they began looking for design alternatives to fins, Hilbert’s team became intrigued by the internal lattice structures they saw being used across a variety of industries, from aerospace to medical devices. By hollowing out a solid aircraft bracket, or a human joint implant, and filling the space with a lattice, honeycomb or gyroid structure, weight can be decreased and strength improved. The bonus: lattices are self-supporting so don’t require any support structures during the AM build.
“We put two and two together and said, ‘hey that looks like an interesting technology to use on our engine cylinders for a couple of reasons,’” Hilbert says. “One, a lattice structure would be very print-friendly and two, it might allow us to tailor-fit heat transfer in a better way.”
Kevin Brigden of AM machine-maker Renishaw had been supporting Cobra with the development of their 3-D-printed finned design following Cobra’s purchase of a Renishaw system. He recommended they take a look at nTopology.
“We thought we could use lattice structures available in nTopology’s software to simultaneously improve the printability of the component and also increase its performance,” he says. “That really got Sean’s attention because these motors are designed to be used in drones, where extra mass can take a particularly heavy toll on payload, range and performance.
“It became immediately apparent to myself and the guys at Cobra that the possibilities that nTopology’s nTop Platform software opened up were virtually endless,” says Brigden, whose specialty is design for additive manufacturing. “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.”
The team began using the software to generate different sizes of lattices and varying wall thicknesses of struts, running some computational fluid dynamics bulk properties simulations to give them a rough idea of their target lattice densities.
“The issue is that the amount of pressure drop across the cooling duct is directly related to the amount of drag on the airframe," Hilbert says. "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.”
Testing demonstrated that the new lattice structure was more efficient at cooling the drone engine than the previous fin design.
Testing demonstrated that the new lattice structure was more efficient at cooling the drone engine than the previous fin design. Image: Cobra Aero
“The computational modeling technology of nTop Platform really unshackles the previous geometry limitations that many of our aerospace customers have run into with conventional CAE tools,” says Jonathan Harris, lead application engineer for nTopology. “They’ve been trying to get every last thickness-to-weight ratio or performance metric into a part and they can’t go the full mile."
The software allows rapid consideration of different lattice structures, automatically filling in fillets to broaden and smooth strut intersections and connections to the part skin. This distributes stress more uniformly, reducing concentrations that can lead to delamination, and promoting both manufacturability and durability.
"By the time we were done, our models had evolved to the point where they were simply beautiful!” says Hilbert. “Apart from the lattice, we soon realized there were many other advantages to using nTopology software 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.”
With their cylinder test results in hand, Cobra Aero is now in a position to take further advantage of other advanced capabilities available in nTop Platform as they refine their design towards commercial production.
“We’ve definitely come to the conclusion that, in the laboratory anyway, our new lattice-cylinder design is a better mousetrap than our fin cylinder—which is a big deal,” says Hilbert. “We now know we’re in the ballpark in terms of lattice density. From here on out it’s a matter of using nTop Platform to fine-tune all the design parameters we need and make the final tweaks of what we want to go to production with.”