The MC-21, from the outside, looks pretty average; not much different from any other narrowbody aircraft. But there is one part of the Russian aircraft that is unique and which could pioneer new developments in the aviation world. The MC-21 is the first in the world to fly with a composite wing built using out of autoclave (OOA) techniques. Here’s why that matters.
What’s new about the MC-21?
Russia’s much-anticipated narrowbody aircraft, the MC-21, is set to achieve certification within weeks. At first glance, it’s pretty run-of-the-mill in terms of its design and aerodynamics, but there’s something special about the wing of this aircraft.
The MC-21 is set to become the first commercial aircraft in the world to use a composite wing, manufactured in a process known as ‘out of autoclave’ (OOA). The process is unusual for commercial aviation, and has allowed the manufacturer to make the wing lighter, slimmer and stronger.
Irkut explained in a statement,
“The use of rigid and lightweight composite materials made it possible to develop a wing with a high aspect ratio and improve the aerodynamics of MC-21, which in turn allowed increasing the diameter of the fuselage to make it more comfortable for passengers. The wing is manufactured using unique vacuum infusion technology, patented in Russia.”
Composite wings are nothing unusual
Many aircraft use composites in their construction, sometimes a little, sometimes much more. For example, carbon fiber-reinforced polymer (CFRP) wing box structures have been around for decades. The first to use CFRP in a center wing box was Airbus, with its A380. It was combined with metal components, but of the 8.8 total tonnes of the structure, 5.3 tonnes were made from CFRP.
For the wings themselves, the Boeing 787 Dreamliner, the Airbus A380, and the Boeing 777X all use CFRP. The military A400M also does. But all these wings and components are produced using autoclave curing, where the resin is baked in high-pressure ovens to harden and fuse the item.
But creating autoclaves large enough for the aerospace industry can be an expensive process. As an example, NASA explored the idea of building an autoclave of 40-ft by 80-ft (12m by 24m) autoclave to cure 10m/33-ft diameter launch vehicle barrels and found that it would cost roughly $40 million to build and another $60 million to install, according to Composites World.
Out of autoclave (OOA) manufacturing removes the need for these giant, hot ovens to be constructed. Instead, the resin is cured using a combination of vacuum, pressure, and heat. The process is known to be much cheaper and easier to do, and results in a more high-quality product, thanks to its effective elimination of voids in the material.
Despite the time and cost savings, OOA is rarely used for large aircraft parts. In fact, the only aircraft that has really embraced the OOA process to date is the Lockheed Martin X-55 Advanced Composite Cargo Aircraft (ACCA), an experimental freighter with no plans to go into production. That is, it was until the MC-21 came along.
The unique wing of the MC-21
The wing of the MC-21 is essentially a standard design with upper and lower skins, stringers on the inside, and ribs and spars for strength. Apart from the ribs, all the parts are composite. The man manufacturing the wings, AeroComposit, chose to produce these composite parts using an OOA infusion method.
This was a new step for Russian aerospace. In an industry where autoclave manufacturing is the norm, the choice to forgo this legacy technique in favor of a method that is yet to be really tried and tested was a brave one. But the company had help, as Jeff Sloan detailed in Composites World.
“AeroComposit had help not only from Solvay, but also, initially, from partners Diamond Aircraft (general aviation), aerocomposites manufacturer FACC (Ried Im Innkreis, Austria), automation specialist MTorres (Torres de Elorz, Spain) and infusion equipment specialist Stevik (Cergy, France),” he said.
The firm looked not only to use OOA infusion for the MC-21 wing, but also wanted to employ as much automation as possible. It wasn’t a straightforward process, but through a combination of trial and error and by making multiple design modifications, AeroComposit got there in the end.
A unique and patented process
The use of the OOA process has allowed Irkut to utilize a wing that is thinner and lighter than would have been possible with traditional methods. It has also allowed the MC-21 to have a wider cabin than any of its competitors. Anatoly Gaydansky, general director at AeroComposit, told Sloan for Composites World that the process has ended up being a unique and patented procedure. He commented,
“The technology developed by our company involves the installation of two membranes. Besides, we are practicing our own patented scheme of vacuum bagging. One might as well say that we have created still another version of infusion technology, specially adapted for building large-size primary structural elements.”
According to the report, the serial production of the composite parts for each set of wings has been taking around two months to complete. But as the production ramps up, this will be driven down to just one month in the future. With the assembly process to follow, the wings will be completed in around four months altogether.
Due to sanctions placed on Russia in 2018, Irkut was unable to source the composite thread from overseas. As such, Russia has developed its own composite thread manufacturing facilities, and the first all-Russian set of wings was completed and delivered in May this year. With Irkut and AeroComposit effectively developing a blueprint for OOA applications in primary structures, this could be the start of a new era in composite use in aviation.
For more detail on the process used to make the MC-21 wing, and the pioneering work of AeroComposit, we highly recommend this long read on Composites World.