Engineers and designers in Nagoya, Japan, have kept busy over the past year redrawing parts of the nascent Mitsubishi MRJ regional jet to satisfy the evolving desires of potential and existing airline customers. However, in the expectation that they will freeze the airplane’s design in the third quarter of 2010, the development team knows that the time for major changes has passed.

“What we are doing now is the detailed design, so if you look at the airplane from the outside almost nothing will be changed,” Mitsubishi Aircraft executive vice president Junichi Miyakawa told AIN. The company has frozen all the interfaces between the various systems, he explained, leaving only the elemental details within each system to settle on. Still, “we might–we might–have to come back to the interfaces [depending on] the detailed design results,” said Miyakawa.

Now scheduled for Japanese certification in late 2013, the MRJ underwent some major changes in 2009 as the company added area to the fuselage cross section, subtracted composite content from its wings and consolidated its cargo capacity into a single rear compartment.

The changes will apply both to the 88-seat MRJ90, which Mitsubishi plans to introduce first, and the 76-seat MRJ70, as well as a potential stretch of the MRJ90 that would hold as many as 100 passengers. Any stretch variant will remain hypothetical, however, until a large enough customer base surfaces to justify a formal launch, according to Miyakawa.

Meanwhile, Mitsubishi believes it has found a “sweet spot” with the changes it instituted over the summer, including a second change in the size and shape of the fuselage. “We used to have a 114-inch-diameter circular cross-section almost two years ago,” explained Miyakawa. “After we listened to some customers, we expanded the width of the fuselage by two-and-a-half inches. Now what we did was basically make our fuselage circular again, with a diameter of 116.5 inches. Again, it was simply because of the voices we heard from the customers and market.”

Although moving from the original 114-inch circular fuselage to a wider design did add some weight to the structure, the second change resulted in no further weight penalty due to the uniform stress characteristics inherent in a nearly perfectly round design.

Composites for Wing Box Scrapped
Of course, the laws of physics don’t always allow desired design elements to fall into place so neatly. That certainly appeared the case when Mitsubishi decided to scrap plans for an all-composite wing. Using composites in an airplane as small as the MRJ, Mitsubishi found, would not yield enough of a weight reduction to warrant their use, particularly given the need for metal reinforcements in parts of the structure.

“A second reason is optimization of the wing boxes across the MRJ family,” said Miyakawa. The aluminum box will allow for a shorter lead time for structural changes because Mitsubishi plans to use the same wing box across the entire family of airplanes.

Still another reason centered on Mitsubishi’s desire to lighten the wing of the MRJ70 by using thinner wing skins. “The MRJ70 has less maximum takeoff weight, so the wing doesn’t have to be so strong, not as strong as the one for the MRJ90,” said Miyakawa. Manufacturing a thinner wing skin for the smaller airplane would simply prove more difficult with composites than with aluminum, he explained.

Advisory Groups Weighed In
While industrial considerations led to the decision to switch from composites to aluminum for the wings, most of the other changes resulted from consultations with airlines during a series of advisory group meetings.

“For example, we have right now a passenger seat under development in cooperation with a Japanese automobile industry company and we’ve got many suggestions for the seat design, in terms of the height, the width, the angle…so many details,” said Miyakawa.

Mitsubishi has recruited Delta Kogyo, the company perhaps best known for the seats it manufactures for Mazda automobiles, to supply the airplane’s slim-profile seats. Rather than urethane, Delta Kogyo’s 3-D net seats use fibers woven into three-dimensional structures that act as small springs and dampers. The approach results in thinner yet stronger seats, according to Mitsubishi. In fact, although the company assumes a 31-inch pitch in its calculation of both airplanes’ typical capacity, several airlines have expressed interest in a 29-inch pitch due to the extra space the thin seats afford, said Miyakawa.

Consultation with airlines also led to the decision to combine the pair of cargo holds–one in the front of the airplane and one in the rear–to a single 644-sq-ft compartment in the rear of the airplane. To help compensate for what might have presented a center-of-gravity problem, designers moved the avionics equipment to the forward belly, where an access panel will replace the baggage door.

Another improvement related to baggage capacity took the form of expanded overhead bins, the capacity of which designers managed to increase 12 percent by adding width and changing the contour of the side that faces the fuselage wall. According to Miyakawa, the bins can hold the largest roller bags allowed by any airline.

Parent MHI To Build and Test
While Mitsubishi Aircraft carries responsibility for design, procurement, sales and support, parent company Mitsubishi Heavy Industries (MHI) will take care of manufacture and final assembly, as well as flight testing. Although it has  not formally announced the location of the final assembly facility, Miyakawa told AIN that MHI had chosen its Komaki South plant, located adjacent to Nagoya Airfield, the regional airport north of the city center.

Mitsubishi stopped building the MU-300 business jet at Komaki South after Raytheon bought that airplane’s marketing rights and renamed it the Beechjet 400 in 1985. However, MHI still assembles the F-2 fighter jet at the plant for the Japanese Self Defense Forces.

Expecting to cut the first metal for the MRJ during this year’s third or fourth quarter and fly the first prototype in early 2012, Mitsubishi hasn’t yet decided between building an entirely new assembly hall and using existing buildings. Miyakawa said a decision would have to happen in “probably a year or so.”   

Although the company hasn’t chosen all the suppliers for the MRJ, all the “major” contributors have signed on, said Miyakawa, including, of course, Pratt & Whitney with its PW1000G geared turbofan. According to Mitsubishi, both the MRJ70 and MRJ90 will use some 20 percent less fuel per seat on a 400-nm trip than the comparably sized jets now produced by Embraer. At least half of that benefit would come from the engines.

Notwithstanding its purely regional airline pedigree, the MRJ will employ Rockwell Collins avionics “equivalent to the ones used on the 787,” said Miyakawa. Other suppliers include risk-sharing partner Sumimoto Precision (landing gear), Parker Aerospace (hydraulic systems), Spirit Aerosystems (engine pylons), Hamilton Sundstrand (APU, electrical power and air management systems) and Nabtesco (flight control actuator). In January last year Taiwan’s AIDC agreed to design and build the airplane’s slats, flaps, belly fairings, rudders and elevators. Mitsubishi has not signed any Chinese suppliers.

Along with the wings and fuselage, Mitsubishi itself will manufacture the airplane’s composite empennage using a process called vacuum-assisted resin transfer molding (VARTM). While conventional RTM uses pressure to push resin into a mold held together with rigid, typically metal upper and lower sections, the VARTM method uses a vacuum to draw resin into the mold, allowing for the use of cheaper materials such as nylon tape for one side of the tool. Mitsubishi’s advanced VARTM also involves the treatment of the preform with a spray of thermoplastic particles, which allow for the use of less viscous and cheaper epoxy resins. 

By last month the MRJ had drawn a single firm order, from Japan’s ANA, for 15 MRJ90s. The airline also holds an option for 10 more airplanes. Last October Mitsubishi secured a letter of intent from Trans States Holdings of the U.S. calling for an eventual firm order for 50 MRJs and options on another 50.

Miyakawa said Mitsubishi has already begun type certification work with Japan’s civil aviation bureau, the JCAB, which coincidentally maintains an office at Nagoya Airfield. The JCAB has begun talks over a bilateral certification agreement with the U.S. FAA, from which Mitsubishi expects to gain certification for the MRJ90 in early 2014.   

Saab Signs for MRJ Technical Support

Late last year, Sweden’s Saab group signed an agreement with Mitsubishi Aircraft under which it will provide documentation services for the MRJ family technical publications. The new partners have indicated that the alliance could be extended to cover wider aspects of technical support for MRJ operators. In fact, a support contract has already been signed with an undisclosed MRJ customer.  &nbs