Breguet’s range equation is a cruel tyrant. The fuel-efficiency conscious airframe designer has only three levers — weight, thrust and aerodynamics — to pull, and yanking one often complicates things for the other two. At the concept stage, however, it still helps to tug upon one of Breguet’s levers, just to see what happens.
So it appears with the Boeing design concept named “mid-wing aircraft“, which is revealed as a patent application published on 28 June.
Boeing obviously set a challenge for itself: How to incorporate an ultra-high bypass ratio turbofan engine — either a geared direct drive system or an open rotor — into the 747's classic airframe, augmented by a full stretch of the upper-deck. In Breguet-speak, this is about tugging as hard as possible on the thrust lever, at the expense of weight and aerodynamics.
Ultra-high bypass ratios are achieved by significantly enlarging the fan diameter, but that adds considerable weight and more drag. Perhaps to counter the effect, Boeing in this concept reduces the 747's traditional four engines to a twin-jet.
As the fan diameter increases, the 747's classic low-wing attachment to fuselage becomes impossible. So Boeing attaches the wings into the middle of the fuselage, then carves out space for a full payload within the wingbox structure. It’s hard to imagine how this works in practice, but as a thought experiment it’s an interesting idea.
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