Landing Gear Prepared for Flight

FDM mock-up key in evaluation and testing of Sukhoi Superjet 100 main gear.

Aircraft landing gear is an extremely complex system comprised of mechanical, electrical and hydraulic assemblies. Retracting and extending on every flight, the range of motion for these assemblies presents unique design challenges that are further complicated by the punishment that landing gear endures, which makes it one of the most maintenance intensive parts on an aircraft.

Figure 1:  Superjet 100, Sukhoi's first regional jet, features Messier-Dowty's landing gear. photo courtesy of Sukhoi Company (JSC)	Figure 2: Assembled in Russia, the Superjet 100 would receive its landing gear just a few weeks prior to the test flight. photo courtesy of Sukhoi Company (JSC)

To minimize aircraft downtime and ensure reliability of the gear, Messier-Dowty, the world leader in landing gear systems, uses sophisticated tools for design, analysis and testing. In preparation for the test flight of the new Superjet 100 regional aircraft, which is manufactured by Sukhoi Civil Aircraft of Russia, Messier-Dowty Toronto validated the performance of the main landing gear with both digital and physical mock-ups. To make the 16-foot -high physical mock-up, Messier-Dowty turned to RedEye for rapid prototyping with its fused deposition modeling (FDM®) process.

Figure 3: Both digital and physical mock-ups were used for validation and testing of the main landing gear.  Messier-Dowty used CATIA for digital design and analysis and fused deposition modeling (FDM) for the physical mock-up. image courtesy of Messier-Dowty	Figure 4: The 16-foot tall landing gear mock-up included components measuring up to 18 x 24 x 75 inches (keyboard on lower left corner illustrates scale).

Preparing for Flight
As the date for the Sukhoi Superjet 100’s test flight approached, schedules grew increasingly aggressive, and the global nature of the project added challenges to Messier-Dowty’s performance evaluations of the gear. Components for the production landing gears were coming from international locations, and many would be delivered just days before system testing commenced. Also, the landing gear would arrive at Sukhoi’s assembly facilities in Russia just a few weeks before the aircraft’s test flight. To  better facilitate meeting these scheduling challenges, Messier-Dowty used mock-ups early in the development process to validate both function and durability.

Dressings—the hydraulic, brake and electrical systems in the landing gear—are critical components, which is why they are the primary focus of design validation and testing. As the gear folds and unfolds from the aircraft’s fuselage, the dressings are subject to chaffing, binding and pinching that can contribute to system failure. Although Messier-Dowty uses digital mock-ups for analysis of stresses, motion and kinematics, they are unsuitable for emulating the dynamic motion of the hoses and electrical harnesses. That is why Messier-Dowty employs physical mock-ups to evaluate the dressings.

Figure 5: The RedEye team delivered the 29-piece landing gear in less than one month.	Figure 6: Following assembly, Messier-Dowty used the mock-up for review and analysis of dynamic motion, fit, ease of assembly and servicing, and test rig design. photo courtesy of Messier-Dowty

Marianna Lakerdas, senior designer for systems installation, stated, “We know from past experience that a full-size, physical mock-up with all the dressings is an invaluable tool. The mock-ups let us see exactly what the hoses and harnesses will do.” Mock-ups also help the engineering team confirm clearances between mechanical and electrical components. Chris Brookfield, principal designer, stated, “Seeing actual, full-scale parts puts everything in perspective.” Lakerdas agrees, “People want to see it, feel it and touch it. You can’t do that with a digital mock-up.”

Physical Mock-up
Historically, Messier-Dowty’s mock-ups were machined from wood. However, the company discovered the advantages of rapid prototyping in an earlier project. With this past success,  it decided to use the FDM rapid prototyping process for the Superjet 100, which quickly delivered precise prototypes with more detail  than the machining alternative.

Chris Brookfield contracted the rapid prototyping services from RedEye. In less than one month, Messier-Dowty received 29 landing gear components, with the largest measuring 18 x 24 x 75 inches. “If we would have had these machined, it would have taken two to three times longer, even though design details would have been eliminated to expedite delivery and keep costs down,” stated Brookfield. “RedEye actually delivered all of the landing gear components, with every design detail, faster than we could use them.”

Jeff Hanson, sales manager at RedEye, noted that the four-week delivery spanned both the Christmas and New Year holidays. He said, “Because of the size of the components, most had to be sectioned, and our model shop had to join them. In all, we processed 58 individual pieces to make the 29-component landing gear. In spite of the holidays and all of the part assembly required, we delivered a very accurate prototype of the landing gear to Messier-Dowty’s schedule.”

To start the project, Brookfield simply uploaded STL files to RedEye. He stated, “In a matter of minutes, our active involvement in the project was complete. Instead of spending time detailing CAD data and making engineering drawings, our design engineers were working on other important tasks.” Brookfield continued. “To be able to just hand over files and have RedEye replicate exact duplicates is phenomenal.”

The part list included the landing gear’s piston, main fitting, linkages and trunnion. “With the exception a few off-the-shelf hardware items, the entire mock-up was made with FDM,” Brookfield commented. He also included parts from the aircraft, such as the rear spar and bay doors, in order to perform on-site retraction tests.

According to Jeff Hanson, “For the mock-up, we used polycarbonate, because of its strength and durability. As with the five other thermoplastics we offer, polycarbonate is dimensionally and mechanically stable over time. This ensured that Messier-Dowty would have a functional mock-up for the duration of the design review.” Hanson noted that all of the components were constructed on the company’s FDM Titan™ machines.

Figure 7:  Although it has sophisticated CAD tools, Messier-Dowty finds that the physical, full-scale mock-up is an essential tool in its product qualification.  photo courtesy of Messier-Dowty

Delivered just four weeks after transmitting files, Messier-Dowty assembled the main gear and started its review.

Prior to receipt of the dressings, the design team performed clearance, retraction and serviceability checks. For these applications, Marianna Lakerdas commented, “Nothing beats seeing actual parts, in full scale, right in front of you.” She noted that in one instance they found a 0.20-inch clearance between a bracket and the belly fairing.

“We were surprised by how tight the clearance was. We went back into our CAD system and confirmed that the gap, as designed, was that small,” she said. “Zoomed in on the part in CAD, the clearance appears much bigger. While the gap is acceptable, seeing it on the full-scale model gave us a real sense of how tight the fit really is.” She also noted that the accuracy of the FDM parts was 0.010 inch. “The FDM mock-up is very accurate. We are very happy with it,” Lakerdas stated.

The limitations of digital mock-ups also affect design decisions that can cause difficulties with assembly and maintenance. “There is nothing better than getting designers in front of the FDM mock-up to see how the design impacts assembly and servicing. It helps them grasp how difficult it will be to get a cotter pin or bolt onto the subassembly,” said Brookfield. “It is also a great tool for junior designers. The mock-up gives them a much better, practical sense of how to design landing gear components,” he continued.

In light of the tight schedule, Messier-Dowty also used the FDM mock-up to verify the design of the test rigs for fatigue and system assessments. “Production parts will be delivered just days before the tests need to be started. If we found a problem, there wouldn’t be enough time to correct it without impacting the schedule,” stated Lakerdas. Mounting the FDM mock-up on the test rigs allowed the design team to detect problems and make changes well before testing started.

Beyond Design
Full-scale main gears, both mock-ups and production units, are in high demand. “Everyone is vying to get their own gears,” said Lakerdas. But they will have to wait for production units because Messier-Dowty has other plans for its rapid prototyped mock-up. Lakerdas stated, “We sent the mock-up to the 2007 Paris International Airshow and MAKS 2007, the Russian airshow, to demonstrate why Messier-Dowty is the world leader in landing gear systems.” She continued, “We needed a robust mock-up, and that is why we turned to RedEye. Having a good looking mock-up that will be the showpiece is really a bonus.”

Brookfield stated, “The cost of a full-scale, 16-foot tall mock-up is pretty expensive. But if you look long term at what it saves down the road, there is a huge benefit.” Although the cost of the FDM mock-up was comparable to a machined, wood model, Brookfield notes that the actual expense was much less. He commented, “With machining, we spent a lot of time creating drawings, discussing project and part details, reviewing suggested changes and fielding questions. That was all eliminated when we worked with RedEye. Taking all of that labor out of the project saved us money and made us more productive.” Brookfield concluded, “Considering all of the other advantages of the FDM rapid prototypes, we will continue to use them wherever possible.”

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