Every part of an airplane can take many forms, and to the extent that comparative testing yields rankings along one scale or another, there appears to be a best engine, a best airfoil, a best placement of wing or empennage, and so on. Young professional designers, the keen edge of their ambition as yet undulled by friction with management or with practical realities, and amateurs, who are more like artists than engineers, instinctively long to create an airplane that unites all the best features. To this impulse add another: the desire to recapture a certain look that somehow or other got embedded in the designer’s psyche when he or she was 12 or so. And so you get things like ducted fans, mid-engine pushers, flying wings and what have you — in short, all the stuff that still makes sketching on napkins fun. But none of that finds its way into manufactured products.