The last time the aviation industry saw tremendous growth and evolution in aero engines was the 1980s. This was the time when GE joined forces with SNECMA to create CFM. P&W joined with Rolls-Royce, MTU and other to create IAE. Those were heady days. The CFM engine took to exclusive spot on the 737, and never looked back. P&W had success with their PW2000 on the 757, GE left that segment, though Rolls-Royce stayed to compete for every 757 sale. It was the time when the engines saw rapid progress.
Meanwhile P&W and GE started working on new ultra high bypass engines with sweeping open fans. The race was on to radically lower fuel burn and these “propfans” were thought to be the answer. These engines offered lower power and speed, but they burned 50% less fuel. During the first oil crisis, saving fuel costs was paramount.
There were two primary options, a GE solution and an P&W/Allison solution. Both engines were tested on a McDonnell Douglas MD-80. Boeing also tested the GE engine on a 727. Take a look at this video from GE explaining the influence the GE36 had on today’s GE engines. Because of their design, the engines were all rear mounted. Rolls-Royce was working on a shrouded engine called Contrafan – and of the offerings, this was the only design that could fit under a wing.
As this image clearly shows, the rear mounted engines may have been small but the blade sweep was very large. In other words, these engines, even if they saved lots of fuel, were limited. At the time only McDonnell Douglas was developing more rear engine designs. Boeing was focused on under wing engine layouts as was the nascent Airbus.
As the image illustrates, propfans don’t work well under a wing. But they can, and do, work well on high wing designs like the Antonov AN70. These engines, while being great on fuel burn still have to overcome a big issue – noise. Take a look at this video of the AN70 in action and note the noise it makes. For a military aircraft this is simply not an issue. But for commercial airplanes, this is a growing and serious matter. Particularly as residential areas crowd airports.
Besides the airplane design limitations, oil prices spiked when the PW/Allison and GE programs were being tested and prices were on their way down. Financial pressure dissipated and airlines went after growth and less concerned with fuel costs.
Airlines are now once again sensitive to oil shocks. The recent flirtation with $140/bbl oil was a rude reminder that airlines need to maintain focus on ensuring the lowest fuel burn possible. This is one of the prime drivers behind the current fervor in thinking about re-engine programs. But as we have shown, even if unducted fans (UDF) were to make a comeback, their usefulness is limited since the current designs are all under wing powered. They simply have no place in any re-engine program.
Of the big engine makers, Rolls-Royce is talking about a UDF solution as the future; its RB282 and RB285 two-stage and three-stage conventional engines are not gaining traction. No surprise then that they pooh-pooh the LEAP-X and GTF. Especially the latter, since they could have been a part of that program under the guise of IAE. P&W felt strongly that the geared solution was the right one, it took the risk and went ahead without IAE.
GE/CFM are also talking about the open rotor as the future of engines, dismissing the architecture of the GTF in the process.
P&W’s wilderness days are coming to an end as its GTF technology is proven appropriate and GE races to develop its own new generation engine, the LEAP-X. A recent demonstration of how quiet the GTF is going to be means that it will be a good neighbor engine too. Rolls-Royce bet wrong with the Contrafan during the previous round (but got other programs like Trent exactly right) and might be well advised to rapidly update that idea now. Otherwise it will have to wait out another cycle and then have to try sell an idea which remains limited even with new airplane designs planned for 2020 or later.