MJet’s Wing Decision Raises More Questions about Composites

 - October 30, 2009, 6:38 AM

Resorting to worn-out business-speak to describe the merits of any new aircraft program has become customary, but Trans States Holdings president Rick Leach’s description of the Mitsubishi MRJ as a “game-changing regional jet” might actually apply this time. After all, Mitsubishi has changed its own game plan rather late in the development process by deciding to forego composite wing boxes in favor of more traditional aluminum, enlarge the airplane’s cabin and consolidate its two cargo holds into a single 644-cu-ft aft compartment.

One might have wondered what precipitated the changes–particularly of the wing structure–given that Mitsubishi distinguished the MRJ concept from its competition largely with its plans for extensive use of composites. Coincidently, Mitsubishi makes the composite wings for the Boeing 787, a project that has experienced its share of troubles–most notoriously with delamination of the wing skins near the end of the structures’ stringers during static testing. Of course, one wouldn’t expect Mitsubishi to declare that its decision to switch to aluminum wing boxes in any way reflects its experience with the 787. In fact, it might not. But the prospect has raised some profound questions about the readiness of composite technology for use in such load-bearing components.     

Realistically, a number of factors likely played a part in the decision to use an aluminum wing, including MJet’s assertion that it found that a composite wing simply wouldn’t result in enough weight reduction to warrant its use in the MRJ. “We concluded that on ‘smaller’ aircraft like the MRJ, the composite materials would not yield the reduction in weight that we anticipated, as we would have to reinforce parts of the wing, which effectively adds weight,” said a Mitsubishi spokesman. “So this point is completely unrelated to the 787.”

Nevertheless, the bending tests performed on the 787’s wing determined the need for reinforcements in that structure as well. Boeing believes the resulting weight penalty will prove minimal, however. 

“A second reason [for the change to aluminum] is optimization of the wing boxes across the MRJ family,” the Mitsubishi spokesman added. “Aluminum…better lends [itself] to optimizing the wing boxes for all members of the MRJ family–the MRJ70, the MRJ90 and the newly announced 100-seater, which has been added as a concept and is being considered as a future addition to the family.”

Meanwhile, input from customers such as Trans States, which this month signed a letter of intent that calls for an eventual firm order for 50 airplanes and options on another 50, likely played a large role in the decision to reconfigure the fuselage design. The latest configuration incorporates a resizing of the fuselage to increase head clearance by about 1.5 inches and overhead bin space by 12 percent. 

All the changes have delayed projected first flight of the 92-seat MRJ90 by six months, from late 2011 to the second quarter of 2012. MJet now expects to deliver the first MRJ90 during the first quarter of 2014. Development of the 78-seat MRJ70 trails that of its larger sibling by about a year.