Nature is divergent. Darwin in the Galapagos encountered an outpouring of species, no two precisely alike. Aeronautical engineering seems to go the other way. All the species in a given niche eventually resemble one another. It practically takes a specialist to distinguish one business jet from the next.
It was not always so. The early jets -- Sabreliner, JetStar, Lear, Hansa, Jet Commander, Mystère 20, DH.125, all born in the early 1960s -- had quite distinct personalities. The JetStar had four engines and slipper tanks on its wings, the Jet Commander had a straight mid wing, and the wings of the Hansa Jet swept forward, the Lear had huge tip tanks and a particularly ruthless-looking windshield. Horizontal tail placement was up for grabs: The Sabreliner's horizontal tail was mounted in the fuselage, barely above the engine exhaust; the Jet Commander's was just clear of the fuselage; the JetStar, Mystère and 125 used cruciform tails (low, middle and high respectively); and the Lear and Hansa went all the way with T-tails. (Oddly, I don't remember a high-wing business jet, not even from the very imaginative Russian design bureaus, which, after the demise of the Soviet Union, mailed new batches of capitalist-oriented "concepts" to Jane's All the World's Aircraft almost weekly.)
Today, most of the quirks are extinct. The differences in appearance among small to medium-sized jets are minor. There is still some freedom in horizontal tail placement, Dassault has a three-engine model, and Honda (not yet actually delivering airplanes) has gone out on a limb by placing the engine pylons on the wings rather than the fuselage -- quite a fantastical arrangement, given the extreme uniformity of almost all the other designs. But to a casual observer, they look very much alike.
There has been lately, however, a curious new trend among bizjets. They're getting pregnant.
You cannot have failed to notice. The newest products are sprouting what appear like bush-plane cargo pods. The wing root fairing threatens to swallow the fuselage. The Beech Premier looks like one airplane that has landed on top of another. Cessna's Citation X and Columbus, viewed head-on, appear to be only halfway out of the boxes they came in. What's going on?
If you go back and look at pictures of the early jets, you will find that almost without exception their wings intersected the circular portion of the fuselage. Their undersides were smooth. The fuselage was notched or perforated and the wing box passed through it.
The single exception then was the 125. It was set up differently, and looked rather odd at the time. The wing passed below the fuselage, making a bump (echoed, by the way, by another weird bump over the pilots' heads). At the time, the 125 was one of the more ungainly-looking small jets, and the reason was the way it seemed to have been cobbled up out of a bunch of pieces that didn't quite fit together.
But the 125's baby-bump proved to be prophetic.
Fast forward to airplanes in current production, and you will find that the embedded wing is almost gone -- only Learjets still have them, I believe, though I am willing to be corrected on this and, if my past experience with making broad statements here is any guide, should fully expect to be.
Why the change?
Being myopically fixated upon engineering and aerodynamics, I had always assumed that the reasons for putting the wing outside the fuselage were basically structural. The fuselage is a thin-skinned tube that swells under pressurization loads. A circular cross-section stays circular under pressure -- that's why bubbles are round -- and so the lightest pressure vessel is one that has a round cross-section. Its surface is in pure tension, the most efficient way for most structural materials to carry loads. Cutting a notch in the belly to allow a wing to pass through would involve a lot of extra structure, not only because flat surfaces need to be elaborately stiffened to carry pressurization loads, but because the transition from the thin-skinned tube to the more rigid notch would create fatigue-prone stress concentrations -- areas where a more flexible component is joined to a less flexible one, and their relative motions under stress lead to cracking.
So, I thought, by keeping the fuselage round and running the wing under it, you saved a lot of structural complexity and a certain amount of weight, and, since the wings of jets tend to be thin, you didn't pay much of a penalty in added bulk and surface area. A collateral benefit, in airplanes with small fuselage diameters, would be a flat floor for the full length of the center aisle.
But recent developments made that explanation seem less and less adequate. The wing centersections were getting so deep, and the fairings so enormous, that no weight saving seemed to justify them. So I asked some Cessna engineers why they use the external wing. (The original Citation 500 had an integrated wing; since 1989, when the first CitationJet appeared, the wings of all Cessna jets have passed under the fuselages.) The answer, it turned out, was not structural at all. It was first and foremost a matter of manufacturing convenience.
