CRJ1000 avoids hiccups in run-up to first flight
As analysts and pundits debated the merits and launch prospects of Bombardier’s C Series airliner, the Canadian manufacturer quietly went about its business readying yet another version of its CRJ series for first flight. Plans called for the only CRJ1000 prototype to take to the skies by the end of last month or early this month, in time to allow a 14-month flight test schedule to culminate in certification and delivery to launch customer Brit Air during next year’s fourth quarter.
By now Bombardier has accumulated considerable experience certifying CRJs, the latest accounting for the fourth major variant of the now 16-year-old platform. So when virtually no news about possible delays surfaced between the time Bombardier launched the program in March last year and the days leading to expected first flight, the accomplishment went largely unnoticed.
Measured Design Approach
But while the popular perception that paints the latest CRJ as “just another stretch” of a now 16-year-old platform no doubt contributed to the relative lack of first-flight fanfare, lengthening the CRJ airframe for the third time involved much more than just inserting a pair of fuselage plugs. New wings, stronger main landing gear, a new carbon brake system, fly-by-wire rudder controls and structural reinforcements required to extend the airframe enough to carry 100 passengers in fact all carried some potential to force a schedule slip. However, Bombardier managed to mitigate much of the risk with its characteristically measured approach to design changes.
The CRJ1000’s reinforced main landing gear, for example, needs to support 7,300 more pounds at 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 CRJ900. “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.”
Thankfully for Bombardier, tests proved the original forging strong enough for the job. “The lead time now for these high-strength, special metal forgings is really getting very difficult,” added Blondin. “The whole industry is suffering with the heavy demand on these exotic materials.
So for us it was a must that we were able to achieve that.”
Originally designated the CRJ900X, the CRJ1000 is 9 feet, 8 inches longer than the biggest Bombardier airplane now in production. Using a 2-percent more powerful version of the GE CF34-8C5s on the 86-seat CRJ900, the newest CRJ will fly as far as 1,580 nm under optimal conditions, compared with the CRJ900LR’s 1,675 nm. 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.
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. GE expects the enhancements to cut maintenance costs by up to 3 percent compared with the standard CF34-8C5.
Further mitigation of performance penalties comes from some obvious changes to the CRJ1000’s wings, including 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.
Although the CRJ700 and CRJ900 both use resin transfer molding 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 CRJ900 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.
Exhibiting similar pragmatism during the design of the CRJ900, Bombardier accepted a small weight penalty in that airplane by using the identical wings that support the 700. This time, however, the CRJ1000’s weight requirements would simply prove far too demanding to borrow the same wing again. “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. “Wing skins, wing spars, ribs et cetera have all been re-stressed to the new weights.”
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. “It all has to do with establishing pilot commonality in the handling of the airplane.”
Another challenge to commonality related to a switch from a mechanical rudder system to Bombardier’s first-ever application of a fly-by-wire system from Germany’s Liebherr. Blondin explained that the company experienced some trouble making the mechanical rudder travel limiter work “in an efficient manner” at certain aircraft speeds and configurations. “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 of 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 the fly-by-wire rudder system and coupled autopilot system gain certification on the CRJ1000, Bombardier plans to offer it on CRJ700s and CRJ900s as well.
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. On the negative side, they will cost more to replace once it comes time for refurbishment.
However, according to Blondin, the carbon brakes should last longer than steel, notwithstanding their tendency to cool more slowly. “This is going to be part of the training that we’re going to provide with the new airplane,” said Blondin. “I understand from a brake wear point of view, you have to have a technique where you don’t drag on the brakes. You try not to use carbon brakes lightly. So on a landing, our crews would be trained to do a good, firm brake application until the airplane has decelerated, then do three or four little shots of brake application.”
Another difference in the operational characteristics of the CRJ1000 will reveal itself during takeoff, when pilots will have to cope with an angle-of-rotation limitation due to the airplane’s length. “The 900 was not body-angle limited, but this airplane will be,” said Blondin. “In fact we are working on a flight director that will provide active rotation guidance to pilots so that we don’t get into any tail contact.”
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.
“We expect that there will not be any major issues with the airplane from a handling point of view in any axis when compared to the 900,” said Blondin.
Bombardier plans to certify the airplane to the same type rating as the CRJ700 and CRJ900, allowing pilots to transition among any of the three after a two- or three-day “differences training” course. Although he said he expects authorities to waive the requirement for mandatory simulator training, Blondin said most pilots will likely undergo a single simulator session to get comfortable with the airplane.