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  • MIT's "Double-Bubble" Airliner Gets a Closer Look
    Posted by Graham Warwick 10:43 PM on Mar 29, 2012

    A team of MIT, Aurora Flight Sciences and Pratt & Whitney are beginning key windtunnel tests to prove whether an innovative aircraft configuration can deliver big fuel-burn reductions without resorting to exotic airframe and engine technology.

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    Graphics: MIT

    The MIT-lead tream developed the twin-aisle "double-bubble" D8 configuration during NASA-funded N+3 studies completed in 2010. In NASA paralance, N+3 means an aircraft that could enter service around 2035. In those studies, the team concluded that the D8 could reduce fuel burn 70% relative to today's 737-800.

    Of that saving, 49% comes from the configuration, with its wide lifting-body fuselage, almost-unswept wing and reduced cruise Mach number, and rear-mounted engines ingesting the fuselage boundary layer. The rest of the 70% reduction comes from 2035-timeframe airframe and engine technologies.

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    Under a Phase 2 contract from NASA, the team will conduct a series of windtunnel tests to validate that 49% fuel saving from the configuration alone. These have begun with tests of a 1/20th-scale unpowered model of the D8 in MIT's Wright Brothers Wind Tunnel (above) to measure the aerodynamic benefit of the airframe without its integrated engines.

    Mark Drela, the MIT professor who developed the TASOPT optimization tool used to design the D8, says the aerodynamic advantages come from the lifting fuselage, which shrinks the wing; the nose-up pitching moment from the upturned nose, which shrinks the horizontal tail; and reducing Mach number to 0.72 from 0.80, which allows a lighter, more-efficient low-sweep wing.

    Next will come tunnel tests of 1/11th-scale powered models to measure the benefit of boundary-layer ingestion (BLI). This will involve testing the model first with conventional podded engines...

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    ...then with the engines integrated into the aft fuselage.
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    Nestled into the rear fuselage, the engines ingest the slower-moving fuselage boundary-later and re-energize the fuselage wake, reducing drag. Slowing the flow into the engines also increases their propulsive efficiency, reducing fuel burn. The D8's lower cruise speed helps, Drela says, as the air entering the fans has a local Mach number of around 0.60, which minimizes flow distortion.

    Other advantages of the flush-mounted engines include the ability to minimize the size, weight and drag of the nacelles by using the rear fuselage to align the flow with the engines; shielding engine noise from the ground using the fuselage and "pi-tail"; and reducing engine-out yaw moments, which shrinks the vertical tails.

    A disadvantage, says MIT professor Edward Greitzer, is the possibility of engine fratricide - an uncontained failure in engine also taking out the adjacent one. This could be overcome by extra containment or by staggering the engines longitudinally. The team is also studying small-core ultra-high-bypass engines in which the core is aerodynamically coupled via a free power-turbine to an aft fan  - an arrangement that could allow more flexibility in lining up the disk burst zones.

    The final round of tests, with a 1/4th-scale powered model in NASA Langley's 14 x 22ft subsonic tunnel, will focus on the aero-acoustic performance of the full configuration, with the goal of "traceably comparing" the D8 with the 737-800 to assess the benefits of BLI and identify any unanticipated losses from propulsion/airframe integration, such as the effects of flow distortion into the fans.

    Tags: awt, MIT, aeronautics, propulsion

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