In my pre-engineering school days, I always wondered why the Pratt & Whitney PT6 turboprop had its intake in the rear, and its exhaust in the front.

I also could not understand why this arrangement, which has the airflow reversing course was so popular. as it seemed to add a lot of complexity, as well as losses into the system.

After the my time in engineering school, I actually understood that this.

The reverse airflow scheme allowed for the use of a free turbine, where the meant that the power turbine is not attached to the compressor, etc.

It makes for a simpler layout. You don’t need any concentric shafts, and starting the engine requires much less “oomph”.

Well, it now looks like a very similar arrangement for advanced airliner configurations. (Yes, this is a few months old. I came across this while doing digital housecleaning)

Not bad for an engine design that is over 50 years old:

As designers of future airliners look increasingly beyond traditional tube-and-wing configurations to meet the high efficiency goals of the 2030s and beyond, new territory is being carved out in the critical area of airframe-engine integration.

Unusual features ranging from recessed inlets to pylon-mounted upper-surface engines have become familiar sights in wind tunnels, but even seasoned researchers are surprised by a new engine architecture proposed by Pratt & Whitney. The concept not only physically separates the propulsor from the gas generator, but also mounts the core backward and at an angle. This novel arrangement is aimed at overcoming installation challenges in new configurations like the D8 double-bubble airliner concept under study by NASA and the Massachusetts Institute of Technology (MIT).

Aimed at NASA’s N+3 performance goals for an airliner that could enter service around 2035, the D8 is designed to burn at least 60% less fuel than the current generation of narrowbody airliners. The secret behind this leap in performance is a configuration that clusters the engines together atop the wide tail of a flattened fuselage. Besides providing a clean high-aspect-ratio wing for low drag, this enables the engines to reenergize to slow-moving boundary layer flow over the fuselage, increasing efficiency.

But such a configuration creates several issues. The engines lie so close to the upper surface of the fuselage their fans must be sufficiently robust to cope with flow distortion from ingesting the boundary layer. Fan size will also be large because the engines envisioned for the D8 will have a bypass ratio of at least 20:1, and be targeted at extremely low noise levels of -52 EPNdb below current Stage 4 limits. Scale tests conducted at NASA of a distortion-tolerant fan developed by United Technologies Research Center show the boundary-layer challenge has been met, but other key questions remain.

Because engine cores are becoming more efficient and operating at higher pressure ratios, they are also shrinking and becoming disproportionately small compared to the propulsor section as bypass ratios increase. This leads to blade heights of 0.5 in. or less at the exit of the high-pressure compressor. At this small scale, tip clearances not only become harder to maintain, but there is little space within the core through which to run the driveshaft connecting the fan to the low-pressure turbine. Additionally, because the core is proportionately longer and thinner, designers face the issue of backbone bending which further affects clearance control.

“So that’s when we had the breakthrough idea of turning the core backward,” says Pratt & Whitney Technology and Environment Vice President Alan Epstein. Air enters the engine through the fan as normal, but instead of continuing directly into the compressor, it is ducted around the side and back of the core to enter from the opposite direction. In an arrangement similar to Pratt & Whitney Canada’s PT6, in which air flows forward through the engine, hot gas will be discharged forward through a power (low-pressure) turbine connected to the fan via a gear system. The turbine, gearbox and fan will be connected via “a really short shaft, and because the core is not connected to the power side, you can take the core off easily for maintenance,” Epstein explains.

The concept also overcomes another challenge. The idea of nested engines, as in the D8, does not meet current FAA certification criteria under the “1 in 20” rule. This states that there should be only a 1 in 20 chance of debris from an uncontained engine failure causing a second engine to fail. However, because the core and propulsor are no longer mechanically linked, “the designers have come up with an extraordinarily clever arrangement in which the cores are angled relative to each other,” Epstein says.

“We cant them at around 50 deg. and the exit from the core turns via a 50-deg. duct to go into the power turbine. So now they are more than 90 deg. off from each other. It’s simple geometry,” he says. “It enables you to have a large bypass ratio, and you are not turning much of the airflow if you are turning just the core flow, so pressure losses are low.”

I love it when advanced technology goes all retro.