Pushing Boundaries

MIT sets out to prove its unconventional airliner delivers the benefits promised

Aircraft all look the same these days because the traditional tube-and-wing shape works well, both technically and operationally. To convince manufacturers to change direction would take a compelling demonstration of the benefits of a different approach.

A team led by Massachusetts Institute of Technology (MIT) created a stir in 2010 when it unveiled an unconventional «double-bubble» airliner concept, claiming it could reduce fuel burn by 49% relative to today’s Boeing 737-800 without resorting to any advanced technology, and by a whopping 70% if it incorporated all the exotic airframe and engine technologies expected to be available by 2035.

The D Series configuration was developed by MIT and teammates Aurora Flight Sciences and Pratt & Whitney under a NASA contract to study concepts for so-called N+3-generation airliners that could enter service around 2035. Boeing’s answer was a hybrid turbine/ electric-powered airliner with slender low-drag, truss-braced wing; Northrop Grumman concluded that a tube-and-wing design packed with cutting-edge technology could approach NASA’s target of better than 70% fuel savings.

But key to MIT’s approach was that so much of the fuel savings came from the configuration itself, with its lifting-body fuselage, reduced cruise Mach number, unswept wing and flush-mounted aft engines. On paper, the design looked much better than today’s best airliners, so NASA awarded the team a Phase 2 contract to conduct large-scale wind-tunnel tests to validate and «bookkeep» the benefits from the configuration’s advanced features and compare its performance to the 737-800.

Under NASA’s N+3 study, MIT produced two D Series designs for a twin-aisle, 180-seat, 3,000-nm-range airliner: the current-technology, aluminum-air-frame D8.1 and advanced-technology, composite-airframe D8.5. The 150-ft.-span D8.1 was optimized for Mach 0.72 at 40,000 ft., and the 170-ft.-span D8.5 for Mach 0.74 at 45,000 ft. A key feature of both versions was the location of the engines: embedded in the tail and immersed in the airflow over the fuselage.

Compared with the 737, the D8’s fuselage with its double-bubble cross section generates more lift, which shrinks and lightens the wing; the upturned nose generates a nose-up trimming moment that shrinks the tail; the twin-aisle cabin is shorter, reducing weight; and the twin-fin «Pi-tail» is lighter. Reducing Mach number and wing sweep saves weight, while integrating the engines into the rear fuselage allows use of minimal lightweight nacelles, reduces engine-out yaw moments and tail size, and increases propulsive efficiency by ingesting the fuselage boundary layer.

Boundary-layer ingestion (BLI) is a major contributor to the fuel savings claimed for the D Series, and quantifying its advantages and disadvantages is the focus of Phase 2. «The BLI benefit is 8-9 percent, and comes from reengergizing the wake and deenergizing the [engine] jet,» says Ed Greitzer, professor of aeronautics and astronautics at MIT. «But there are possible aerodynamic and mechanical issues from flow distortion, so Phase 2 will look at airframe-propulsion integration and what are the unexpected unknowns.»

The research will include back-to-back tests of large-scale powered wind-tunnel models of the D8, first with a conventional podded engine, then with integrated BLI engines. «We have never done back-to-back testing before. It will be a reality check,» says Mark Drela, the MIT professor who developed the configuration. «BLI works. In the aircraft business it is uncommon, but any ship has BLI—the prop is always in the wake and it’s very effective,» he says. «But there are a lot of reasons not to want it on an aircraft—engine stress, noise, surge margin—so it will be a tradeoff. We will see where it ends up.»

The BLI benefit comes in two ways. «First is that the propulsor works in the wake versus clean flow,» says Drela. «The wake equals drag and in a conventional aircraft the propulsor has to overpower it. That is wasted energy that is left behind the aircraft. With BLI the propulsor doesn’t overpower the wake— it tries to cancel it and leave behind still air.» The second is because less thrust is needed to overcome drag, so the engine can be smaller and lighter. This is a «significant secondary effect,» he says.

But there is a problem. «The engine fan wants uniform flow, which is directly counter to BLI,» says Drela. «Non-uniform flow reduces aerodynamic efficiency; it reduces surge margin, moving the fan closer to the stall; and it increases cyclic loads on the blades, as the wake is one-sided.» The solution is to design the aircraft so the airflow into the fans «is as well-behaved as possible to reduce the downside as much as possible.»

Flying slower helps a lot when shaping the fuselage to guide the boundary layer into the engines without flow distortion, says Drela. «The fan wants to see air at Mach 0.6. At normal cruise speed, that means you have to slow the boundary layer down from Mach 0.8 to Mach 0.6. Flow doesn’t like deceleration, so you get distortion,» he says. «When you fly at M0.7, the deceleration is a factor of two less and there is an enormous reduction in complexity.»

The D8 is designed around turbofans with a bypass ratio of 20:1, compared with 5-6:1 for today’s CFM56; but because the configuration is so efficient, it requires much less thrust than a 737. The combination of high bypass and low thrust means the mass flow and blade size in the high-pressure core are dramatically reduced, introducing aerodynamic, thermal and mechanical challenges. So another part of the Phase 2 work, involving Pratt & Whitney, is looking at architectures and technologies for high-efficiency, high-pressure-ratio, small-core engines.

Phase 2 centers on tests of three wind-tunnel models of increasing scale and fidelity. The initial l:20th-scale model, without engines, is being tested in MIT’s Wright Brothers’ Wind Tunnel as a rehearsal for the later test. This will be followed by a 1:4th-scale powered model to be tested with podded, then integrated, engines in the MIT tunnel, then a 1:4th-scale model with BLI propulsion that will undergo aeroacoustic testing in NASA Langley Research Center’s 14 X 22-ft. subsonic tunnel.

One reason for a phased approach is to prove out the MIT team’s strategy for building wind-tunnel models, which involves fused deposition modeling, better known as three-dimensional printing. «It’s cheaper and faster, so we can do rapid iterations, but we have to go through additional safety steps before we give the model to NASA to test in their tunnel,» says Greitzer.

Isolated propulsor tests for the 1:11 model are underway at MIT, to map the fan and test distortion, and the back-to-back podded/integrated tunnel tests are planned for the summer or early fall, says Drela. The model is sized around the largest electric ducted fans available for radio-controlled aircraft, the 6.5-in.-dia. TF8000. The team will design its own fans for the 1:4 model based on results from the 1:11 tests. Because of a tunnel scheduling conflict at NASA, the large D8 model test «will not be this year,» he says.

The back-to-back comparison of podded and integrated propulsion poses a measurement challenge. «The big problem with BLI is that you cannot take a standard aircraft, put it on and see what happens. That is not a good measure,» says Drela. «Defining what is thrust and what is drag is no longer the same, so how do you quantify the benefit?» Thrust and drag definitions are ambiguous for highly integrated configurations, says Greitzer, because the boundary layer, which is airframe drag, is reenergized by the propulsor and because of pressure-field effects from non-uniform flow into the propulsor.

«How much fuel per mile is the ultimate measure,» says Drela. «But efficiency is ill-defined and fuzzy, and has been fudged in the past, so we will look at power, which has a lot less ambiguity.» The analysis will consider sources and sinks of power—mechanical energy added to the flow through propulsor shaft power and viscous dissipation in boundary layers, wakes, jets and vortices. «You compare the two to end up with how much energy you have to supply, and you don’t have to define a thrust or a drag.»

While wind-tunnel testing gets underway to quantify the pluses and minuses of BLI, study work is continuing on innovative engine architectures to mitigate the component-technology challenges of small cores. «In very efficient aircraft, the power required is reduced, so core size is reduced,» says Greitzer, adding that compressor blade heights can be less than 0.4 in. This creates issues with aerodynamic effects, tip clearances, manufacturing tolerances, high temperatures, thermal/mechanical fatigue and engine bending.

«We need to think about unconventional engine architectures,» says Greitzer. Pratt has come up with an engine that combines a two-spool gas generator with a free turbine, or aft fan. In this «offset centerline» architecture, the fan driveshaft does not go through the engine core. Instead the gas generator is aerodynamically coupled to the fan. Without a shaft running through its center, the core can be reduced in diameter. «That enables a smaller compressor flowpath with reasonable clearance-to-span ratios,» he says.

Assuming an advanced turbofan with a bypass ratio of 20 and an overall pressure ratio of 50, compared with around 30 in the CFM56, the D8’s projected thrust requirement of just 8,000-12,000 lb. puts the core into the size range normally associated with axi-centrifugal compressors, which are typically used in turboshafts but seldom in turbofans. «But if we take the shaft out of the core, we can continue to use all-axial,» he says.

The new architecture could help overcome one of the challenges of the D8 configuration—complying with the FAA’s «1 in 20» certification requirement, which stipulates that the probability of an uncontained failure in one engine also taking out the other engine must be less than 1 in 20. The D8 configurations produced for Phase 1 had three engines side-by-side in the tail, the center one set farther back, but the Phase 2 design has two engines mounted cheek-by-jowl. «The offset-centerline core helps, giving us some flexibility in lining up the disk burst zones,» says Greitzer.

Over the next year, the MIT team will focus on the clearance and manufacturing issues with small cores as they produce an initial engine layout for the D8 configuration. They will complete fan design and isolated-propulsor tests for the l:4th-scale wind-tunnel model and move closer to proving whether the double-bubble D8 is as good as it seems.

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