Pratt & Whitney is pleased with the bird-strike performance of the “hybrid-metallic” fan blades used on the PW1000G family of geared turbofan engines.
The PW1000G’s blades feature a titanium leading edge on an aluminium blade, says Alan Epstein, vice-president of technology and environment at the engine maker. He spoke with Flightglobal during a visit to Singapore.
“In terms of bird strikes, it has behaved very well,” he says.
He notes that the reduction gear in the PW1000G series means the tips of the fan blades travel at “barely sonic” speeds, which could be a factor in the blades’ ability to withstand bird strikes.
Epstein estimates that the PW1000G picks up one percentage point of engine efficiency through the use of aluminium blades.
“When we started with geared turbofans, we knew the blades had to be composite,” he says. “But some young guys thought they had a better idea. They were given some money, and they showed that they could use aluminium. Not only that, but the aluminium was much more efficient, because you can make it thinner.”
He says carbonfibre does not scale down well for use on smaller engines.
“[Carbonfibre] is fine for 70,000lb thrust engines. But because the fibres are fixed in size, when you get down to single-aisle size aircraft, you make a compromise where a blade gets thicker than it wants to be dynamically. We realised we could pick up a [percentage] point in efficiency in not using carbon.”
He adds: “The only thing that does any work on the blade is the titanium leading edge. It chops up the birds and ice and all that stuff, and the rest of the blade is along for the ride.”
Epstein also touched on the decision in 2012 not to certificate the Bombardier CSeries PW1524 engine with a variable engine fan nozzle (VAN). Early in the programme, there was concern that flutter could result from an in-flight engine shutdown, hence the inclusion of the VAN.
“We took the VAN off because the low speed fan is much better than we thought,” says Epstein.
“We were worried about things such as having a giant engine diameter. What happens if you shut it down in flight? Will it have too much drag? Turns out it has less, not more. This is one of the advantages in the design. The windmilling operation was much better than any legacy engine would do.”