Bombardier’s CRJ1000 not just another stretch

 - July 12, 2008, 4:13 AM

Bombardier expects the largest airplane it has ever built–the 100-seat CRJ1000–to take to the skies for the first time this month, on schedule and on budget. Plans call for the only CRJ1000 prototype to embark on a 14-month flight test regime expected to result in certification and first delivery to launch customer Brit Air in the fourth quarter of next year.

The fourth major iteration of the company’s immensely successful regional jet line, the CRJ1000 has quietly taken shape over the course of the last 15 months at the company’s Mirabel, Quebec production facilities–devoid of much of the fanfare typically associated with the creation of a new airplane. So when virtually no news about possible delays surfaced between the time Bombardier launched the program in February last year and the days leading to first flight, the accomplishment went largely unnoticed. 
 
But while a perception that paints the latest CRJ as “just another stretch” of a now 16-year-old platform might explain the unusually subdued build-up to first flight, lengthening the CRJ airframe for the third time involved much more than just inserting a pair of fuselage plugs.

The CRJ1000’s reinforced main landing gear, for example, needs to support 7,300 more pounds (mtow) than the gear on the CRJ900. Using completely new forgings, however, might have required an irreconcilable expense of time and resources. So Bombardier decided to strengthen the gear’s leg by shaving less metal off the same forging used for the CRJ700 and 900. “When we first analyzed that, it was one of the major pacing items for us to maintain the schedule we had,” explained CRJ program director Jean-Guy Blondin. “With a new forging we would have seen possibly a six- to eight-month delay.”

The CRJ1000 will measure 9 feet, 8 inches longer than the 86-seat CRJ900. Using a 2-percent more powerful version of the GE CF34-8C5s on the CRJ900, the newest CRJ will fly as far as 1,580 nm under optimal conditions, compared with the CRJ900LR’s 1,675. At a maximum takeoff weight of 91,800 pounds, takeoff field length increases to 6,995 feet in the CRJ1000 from 6,379 feet in the CRJ900, and landing field length jumps to 5,923 feet from 5,582 feet.

Improved Field Performance
For customers wanting better field performance, GE will offer an optional 5-percent takeoff thrust increase in a version of the CF34-8C5 designated the CF34-8C5A2. Software modifications to the engine control will supply the added thrust, and new coatings and airfoil design modifications will help compensate for the extra demands placed on the high-pressure turbine.

Changes to the CRJ1000’s wings include a 7.5 percent trailing edge extension and a 26-inch wing tip extension, larger composite flaps and ailerons and an extension to slat Number 3, located on the outboard end of the wing. “Even though it is basically the same design as the 900’s, at the part number level, there probably isn’t one part in the wing that is exactly the same,” said Blondin.

Although the CRJ700 and CRJ900 both use resin transfer molding (RTM) composites in their flaps, the CRJ1000’s new wings will for the first time allow Bombardier to realize some weight advantage from the switch from metal. Blondin explained that a move to lighter flaps in the CRJ700 and 900 would have required Bombardier to flight-test and recertify them, hence the reason the composite flaps in the NextGen version of those airplanes, introduced last year, weigh the same as the metal ones in the original version. 
 
One of the last tasks performed by engineers in Mirabel in preparation for first flight involved installing the rigging for the wings’ new flap system, said Blondin. Other late work involved preparing the new gearing on the elevator, modified to give the airplane the extra elevator authority needed to compensate for the longer fuselage. “The gearing change allowed us to increase, for the same column movement, the position of the elevator so that someone [accustomed to] the 700 and 900 would get the same type of rotation from the same type of movement,” said Blondin.

Meanwhile, explained Blondin, Bombardier experienced some trouble making the mechanical rudder travel limiter work “in a very efficient manner” at certain aircraft speeds and configurations. Hence, the switch from a mechanical rudder system to Bombardier’s first-ever application of a fly-by-wire system from Germany’s Liebherr would also help maintain handling commonality. “Usually Bombardier doesn’t like to take an inordinate amount of risk in developing new technology, [but] this is something that Liebherr had been working on, they offered it to us, we saw an opportunity to simplify the build and at the same time improve reliability,” said Blondin. 

The switch to fly-by-wire rudder control will not change the design of the cockpit, said Blondin, as pitch and roll remain electromechanically controlled. In fact, the cockpit in the CRJ1000 will look almost exactly the same as the CRJ900’s, except for a button on the autopilot control panel that will allow for vertical nav capability–a so-called coupled approach. Once certified on the CRJ1000, Bombardier plans to offer the fly-by-wire rudder system and coupled autopilot system on CRJ700s and CRJ900s as well.

New Carbon Brake System
Although Bombardier has gone to great lengths to make the CRJ1000 flying characteristics virtually mirror those of the CRJ900, once on the ground CRJ1000 pilots might notice a difference in the airplane’s braking action thanks to a new carbon brake system–another first for the CRJ line. The new brake stacks, supplied by Saywell International subsidiary ABSC, will weigh some 300 pounds less than the alternative steel stacks and occupy less space. However, they will also cost more to replace.

Another subtle difference in the operational characteristics will reveal itself during takeoff, when pilots will have to cope with an angle-of-rotation limitation due to the airplane’s length. “We are working at a flight director that will provide active rotation guidance to pilots so that we don’t get into any tail contact,” Blondin said.

Specifications call for a reduction in body angle by roughly 1 degree–not a big difference, said Blondin, but one that a pilot accustomed to flying the CRJ700 and CRJ900 will definitely notice. In terms of general handling, however, Bombardier expects no discernable departure from the feel of the CRJ900.