“Based on what we know today, nobody would develop a BLI system, but if we advance our knowledge, then people will consider taking the risk. That's why we see BLI in the N+3 time frame. We don't see it in the next 10 to 15 years; it is more like 20 years away,” he says.
There are two main parts to NASA's research. One is testing of the D8 configuration to validate the BLI benefit and characterize the flow into the aft-mounted engines. The other is work by UTRC to design and test a distortion-tolerant fan for embedded engines. A third piece is the study by Pratt & Whitney of novel architectures for small-core engines that could power a 2035-time-frame D8-configuration airliner.
MIT developed the D8 for NASA under an N+3 Phase 1 study completed in 2010. “A key feature is the rear flush-mounted engines and BLI, but it is one of the highest risk,” says Mark Drela, professor of aeronautics and astronautics at MIT and the design's creator. “Phase 2 is focused on evaluating its feasibility and performance, so the wind- tunnel tests are focused on the rear of the aircraft.”
While the double-bubble fuselage and slender low-sweep wing were defined during Phase 1, the tail was not. During Phase 2, MIT will “improve of the aerodynamic design of the tail and how the engines blend in,” notes Uranga. The same 1:11 model will be back in the Langley wind tunnel in January with a refined tail.
The goal of the Langley tests was to make a fair comparison between embedded and podded engines. “It's about measuring the right thing,” says Ed Greitzer, professor of aeronautics and astronautics at MIT and the principal investigator. “So we have swappable tails with the same propulsors, the same fans and motors, to rule out that variability.” The BLI benefit will be determined by comparing the electrical power required to drive the fan motors in the cruise condition, defined as zero net force on the model in the tunnel, says Uranga.
“In addition to obtaining force and moments data, we did extensive flow surveys at the inlet and exit of the nacelles to examine the total-pressure profiles into and out of the propulsors. The ability of the fans to handle the total-pressure variation at the inlet is a key requirement for effective BLI,” says Greitzer. The pressure profiles will be used to estimate thrust and drag and provide a second method of calculating the BLI benefit. “Ideally, the two measurements will be the same, or close.”
The drag benefit from BLI is potentially large. On the D8, the engines are positioned to ingest most of the flow over the top of the fuselage. “About 40% of the entire fuselage boundary layer is ingested, and typically the fuselage is 25 to 30% of the total aircraft drag,” explains Uranga.
“The preliminary data shows an electrical power-saving with the integrated configuration of roughly 5 to 8 percent. There are several physical origins to this saving, such as reduced overall wake plus jet kinetic energy losses and reduced nacelle wetted area,” says Greitzer. “However, BLI is just a fraction of the potential fuel-saving that the D8 configuration could actually provide, via secondary benefits in gross-weight reduction, tail-size reduction and other areas. We feel that the double-bubble concept has a big overall potential that is well worth investigating.”
Confirming the BLI benefit assumed during Phase 1 design is crucial to the D8. “If BLI does not work, the whole concept is suspect,” admits Drela. A fan that can operate while ingesting boundary-layer flow is critical also. “At NASA Glenn [Research Center, Ohio], we are looking at how a fan operating in a distorted flow can be designed or improved so you don't lose a lot of efficiency,” says del Rosario.