December 17, 2012
Credit: Photo Credit: Pratt&Whitney
Guy Norris Los Angeles
A full moon hung over Pratt & Whitney's test site the night its Airbus A320NEO engine ran for the first time. Whether this was a good omen or not, the company knows only a successful test effort will prove if its pivotal gamble on geared turbofan technology was correct.
Pratt began its march back into the single-aisle mainstream at 11 p.m. on Nov. 28, when the first PW1100G NEO engine fired into life on the company's West Palm Beach, Fla., test stand. “The first time we put power to the igniter it lit,” says the engine maker's Next-Generation product family Vice President Bob Saia. Since then, the engine has been taken through a 'break-in' cycle from idle to full thrust, ensuring the mechanical integrity at various power levels before clearing the PW1100G for the remainder of the forthcoming test effort.
Although it is too soon to know whether the engine is on track to meet critical fuel-burn targets, the overall performance from a temperature and speed perspective “is right on pre-test predictions,” says Saia. “We haven't yet got a true calibration in terms of fuel efficiency,” he adds. Mechanically, the engine has completed the break-in cycle testing with “no anomalies. We're really pleased with what we got.”
The initial work allows Pratt to make some design tweaks to optimize the engine at the early stages if needed. It also incorporates lessons learned from the two forerunner geared turbofan (GTF) developments, the PW1500G for Bombardier's CSeries airliner and the PW1200G for the Mitsubishi Regional Jet (MRJ). “On the PW1500G, for example, we had an unexpected rub in the last stage of the low-pressure compressor. This wasn't due to the way we broke in the engine, but because of the way it was designed,” says Saia, who adds that a quick change corrected this “subtlety.” Similarly, the initial runs also indicated that designers had allowed too much clearance in the PW1500G high-pressure compressor, which had to be tightened.
Testing now is focused on measuring the effectiveness of the engine's secondary flow system to adequately cool and seal the turbine, and other hotter running stages, as well as the various cavities. “So this first engine is involved in a lot of first-of-model testing, and evaluations of the fundamental architecture. So if we uncover anything, we will have time to adapt the design,” Saia says.
The second and third engines are in assembly with the second expected to start tests around mid-February. This will be used for operability and performance trials, as well as fan work that will include crosswind testing to check the ability of the inlet and compressor to handle various simulated angles of attack. For “fan-mapping” tests, the engine will be fitted with flapper valves to form a rudimentary variable area fan nozzle to alter the pressure ratio. “We also want to characterize the structural element of the fan for operability as well as for stress or flutter,” Saia adds. The same engine will be flown in the second quarter of 2013 on Pratt's Boeing 747SP flying testbed with a production-representative variable nozzle.
The third and fourth test engines will be heavily instrumented “stress-test” units aimed at gathering strain-gauge measurements of the high- and low-pressure spools, respectively. The sheer amount of internal wiring required for these measurements means the stress-test engines enter the build cycle much earlier, says Saia. Here again, Pratt hopes to take advantage of work already performed on the PW1500G. “We have completed all of the certification testing on things like aerofoil stress (on PW1500G), so before the PW1100G starts testing we've already got the fundamentals of it characterized. There may be differences in the lengths of the blades and such, but it takes a lot of risk out of the early engines. That is not to say we still may not break a motor now and then.”