More than a dozen so-called working teams from Pratt & Whitney, Airbus and various suppliers continue their evaluation of the integration challenge the companies face as orders mount for geared turbofan-powered versions of the Airbus A320neo airliner, with the airframer looking to advance entry into service by six months, to October 2015.
Scheduled to enter detailed design by the end of the year, Pratt & Whitney has not had to change its engine test schedules as a result of the Airbus decision. However, according to Pratt & Whitney new generation product family vice president Bob Saia, “It puts a little challenge on us because we’ve got to work faster, but in terms of the overall integration and definition the pre-work supports it.” Schedules call for the first PW1100G–the variant designed to power the Neo–to start testing late next year.
Saia told AIN that the biggest integration chore centers on the changes associated with mounting the new engine’s nacelles to the A320’s wings. If fact, new pylons and a nacelle system will account for perhaps the A320neo’s only major new hardware.
“The aircraft is really not changing much,” said Saia. “The aircraft systems, the air conditioning, the electrical system, the cockpit…because it’s a known airplane, those elements are pretty well defined. And because we had our V2500 [on the A320], even though it was done under the IAE collaboration, we have a lot of knowledge of that airplane,” he said, also citing Pratt & Whitney’s experience installing the PW6000 on the A318.
“The big change now is the mounting and the integration of the engine on the wing,” said Saia. “Even though the engine has all this efficiency, we want to optimize that efficiency as we install it on the wing. So when we do that, there can be elements like exactly how you want to position the engine. It’s not that it’s difficult, but it’s unique to every engine and every airplane.”
Unlike the Boeing 737, whose shorter landing gear does not afford Pratt & Whitney enough ground clearance to design an engine with an optimally sized fan, the A320 presented far fewer hurdles in terms of sizing. Nevertheless, as comparatively straightforward as Pratt & Whitney claims the job of retrofitting the PurePower PW1100s to the Airbus narrowbody should prove, Saia wouldn’t deny the difficulties inherent in fitting a new engine to any existing design.
“It’s like remodeling your home,” said Saia. “If you want to bring in new appliances or change things, because the walls are already pinned you’re constrained in what you can do and how you do it. And…in our case, we want to make the engine as big as we can make it.”
Happily for Airbus and Pratt & Whitney, the A320 afforded the engine’s designers enough ground clearance to make the PW1100’s fan 81 inches in diameter–an ideal size for the thrust class required for the A320 family and the roughly 3:1 ratio between the speed the low-pressure compressor and turbine turns and the fan’s rpm. In fact, according to Saia, the proportion between the size of the fan and the rest of the engine for the Neo virtually mirrors that of the C Series, which uses a 73-inch fan.
“From our engine perspective, we’re basically building off the C Series architecture, scaling up for size and thrust, and then working some of the aerodynamics to get a little bit more efficiency because of how we drive temperature and pressure,” he said.
Fine-tuning Systems Integration
Of course, designers must account for the effects of the GTF’s higher pressures, in particular–required to aid in the efficiency of the engine–as they perform air-conditioning integration, for example, Saia said. “The question is how do we now balance where you take the bleed air from the engine so you can service the air-conditioning system in the airplane and do they need to do any minor changes to the air-conditioning system?”
In this case, changes to the air-conditioning system, for example, must remain minor because most of the integration work necessarily centers on the engine when working with an existing airplane, he explained. With a clean-sheet design, engineers enjoy the freedom to effect more significant changes to the various systems so that they work with the engine in the most efficient way possible.
Another group works on the engine’s electronic control, which, in essence, “talks” to the airplane’s cockpit. That team concentrates on “optimizing” the communications of an existing cockpit with a new engine, said Saia. “We have a team that focuses on how the element is designed–even though the aircraft is already designed, the engine-level side is going to be new–and as it’s new, ‘How do you make sure that the input coming into the engine and the output feeding back into the cockpit are properly designed so all the communication channels connect?’” Still other teams concentrate on items such as maintainability, environmental factors and program scheduling.
Increase in Efficiency
In fact, Saia insists the PW1000G series could do almost as much for the efficiency of a 737 as it can for an A320, despite the fact that the design of any Boeing retrofit would require a fan whose diameter measures some 10 to 12 inches smaller than that of the Neo’s 81-inch PW1100. The P&W executive virtually ruled out lengthening the 737’s landing gear to accommodate a larger fan because, as he put it, “The problem you have with raising the landing gear is when it comes in and tucks back into the cargo bay, the hole is already defined.
“If you move the landing gear out and you can made the [it] longer, and you can still fit it in the same hole, the problem that you still have is now the wing structure has that load of the engine and the landing gear in a different location,” he added. “So that’s the rippling effect that you have.”
Instead, any engine built for the existing 737 would likely use a 69-inch to 71-inch fan, along with a 2.8:1 or 2.9:1 speed ratio between the low-pressure side of the engine and the fan. Still, as Saia emphasized, much will depend on Boeing’s ultimate specifications.
Airbus advertises “up to” a 15-percent efficiency improvement for the Neo over the existing design. On several occasions, Boeing executives expressed doubts that a re-engined 737 could deliver even a double-digit improvement over the existing U.S.-made narrowbody.
Whatever Boeing’s decision, Pratt & Whitney appears eager to accommodate. “What I will say is, if they decided to re-engine, we can give them–on a relative basis–close to about the same amount [of efficiency as we gave Airbus] because of some things we can do with the gear,” said Saia. “If we do a new airplane, then that will allow us to open up the envelope a little bit.”