The clustered beneath the pi-tail of the NASA/MIT D8 design is bad for maintainability, isn't it?<p>All we learnt from early commercial jetliner like the <i>De Havilland Comet</i> (<a href="http://en.wikipedia.org/wiki/De_Havilland_Comet" rel="nofollow">http://en.wikipedia.org/wiki/De_Havilland_Comet</a>), where two pairs of turbojet engines were buried into the wings, is that such configuration increased the structural weight and complexity of the wings. Armour had to be placed around the engine cells to contain debris from any serious engine failures; also, placing the engines inside the wing required a more complicated wing structure.