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Airplane Design: Fasteners Instead of Holes
May 7, 2015   
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Polish engineers from the PZL Mielec aircraft company and the Lublin University of Technology are working on a system of clasp-like fasteners that will be used to join together aircraft components in order to eliminate the most common cause of airframe damage: fatigue failure.

Bolts and rivets used in traditional aircraft designs require holes that frequently lead to fatigue cracks, forming as a result of stress concentration between two sections of material. An array of clasps and other fasteners designed by engineers from PZL Mielec and the Lublin University of Technology will prevent such cracks from forming, making aircraft components more durable and quicker and cheaper to produce.

The new fastener system is being developed as part of a five-year zl.12 million project called Bloster. The National Center for Research and Development (NCBR) has allocated almost zl.8 million to the project under its Innolot program. The remaining funds will be contributed by PZL Mielec, which is the largest airplane producer in Poland.

PZL Mielec CEO Janusz Zakręcki says most producers of aircraft structures and metal parts still use technology that dates back to the 1970s and ‘80s. “Many components are manufactured through forming processes, milling and turning,” says Zakręcki. “They are joined into larger structures with rivets and screws, which takes complicated machinery, time and a lot of money. Our idea is to bind components with clasps and latches. This innovative and fast method reduces production costs and, at the same time, such binds are more durable.”

Projects financed under the National Center for Research and Development’s programs like Innolot are carried out by businesses, while universities and science institutes conduct research to support the project coordinators. This is a proven form of collaboration, the NCBR says, because businesses know best what they need and they also know how to launch innovative technology.

PZL Mielec has three partners in the Bloster project and, apart from the Lublin University of Technology, they are machine tool producer Bryk and composite material manufacturer Wit Composite. The engineers and researchers are designing clasps and latches to join three combinations of materials used in aircraft components: metal with metal, metal with composite, and composite with composite. The technology requires innovatively shaped new fasteners, and the engineers also need to identify structures in which such fasteners will be used. The principal challenge is to determine the shape of components to be bound together and the sequence of technical operations that need to be performed to make sure the components and aircraft structures meet the necessary durability and safety criteria.

Clasps, latches and similar fasteners have long been used in both aviation and the car industry. They are a lot like common objects such as belt buckles, snap fasteners in electronic equipment, telephones and so on. What the Bloster project engineers are aiming for is technology to allow fasteners of this kind to be used in structures exposed to huge stress.

The Bloster project began in 2013 and will continue until 2018. Demand for the new technology originated at PZL Mielec, while the Lublin University of Technology has taken care of the project’s research aspects, coordinated by Prof. Tomasz Sadowski. His team of researchers is devising ways in which components produced by PZL Mielec can be fastened together. They also pick the best materials and try to come up with processing techniques and structures that ensure repeatable technological processes. PZL Mielec’s Zakręcki adds that, in addition to conducting research and seeking new materials and structures for the company, the Lublin University of Technology tests those in laboratories.

“What we do is put the results of their research into practice as soon as possible,” says Zakręcki. “The researchers design the materials and we use these to make aircraft parts that go on to form actual aircraft structures. Then, we promptly move on to producing and utilizing the structures. In the future, this will allow us to sell better, cheaper and more functional products and, consequently, multiply our profits.”

In addition to a technology demonstrator exhibited at invention fairs, the Bloster project will result in the development of new technology ready for commercial use. The level of technology readiness is measured on a scale of 1 to 8, where 1 means a general concept and 8 stands for mass production. The technology developed as part of the Bloster project is expected to reach at least level six, which means a fully functional part mounted in an actual airplane. While this will not be the mass production stage yet, the technology will extend far beyond a few curiosities on display in a lab.

Bogdan Ostrowski, PZL Mielec’s technical director, says the researchers and engineers are planning to make several different airplane parts, such as floors, flight control surfaces and ailerons. “The project aims to make them functional, that is, fit for being mounted in working aircraft,” Ostrowski says. “The project’s test environment is our flagship product, the M-28 airplane for 18 passengers or 2 metric tons of cargo. We know it inside out, so we know how to make innovative changes to its structure. Such new solutions will make our product cheaper, better and safer.”

The M-28 is delivered to the Polish Defense Ministry as the M-28 Bryza. Produced at PZL Mielec for 20 years, the M-28 commuter-category class aircraft has a maximum takeoff weight of 8 metric tons and performs best on short flights, delivering cargo to areas with difficult access. PZL Mielec has customers in Poland and abroad.

The Bloster project is expected to end with a slew of patents. When the new system goes into production, the resulting lower number of fatigue cracks will make airplanes cheaper to repair after many years in service. Binding methods used at present make such cracks a commonplace problem. The engineers say that after the new technology passes the necessary certification procedures, it will be put into practice without delay.

Meanwhile, on April 16, the National Center for Research and Development announced another call for projects under its Innolot program. A further zl.400 million is up for grabs to help entrepreneurs finance modern technology in Poland’s aviation industry.

Karolina Olszewska
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