Technicalities

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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.

It's easier and cheaper to build the wing and fuselage as separate components. That is the primary driver. All manufacturers seem to agree about this point. Cessna happens to have evolved a large and varied line of jets by mixing and matching wings and fuselages, but manufacturers of only a single model keep the wing and fuselage separate too.

Of course, that is not the end of the story. Once the decision has been made to place the wing box -- that is, the main spanwise structural core of the wing, minus leading edges and the flap- and aileron-related stuff behind the rear spar -- outside the pressure vessel, the designers' task is to make the best of the arrangement.

Structurally, it is convenient. The wing is attached to the fuselage by a small number of links -- five or six -- each of which is designed to carry a load in one axis, but to flex or rotate freely in the others. The wing-to-fuselage attachment actually looks like a foreshortened version of the trusswork connecting the fuselage to the wing of a parasol airplane, or to the upper wing of a biplane. The reason for this approach -- rather than, say, bolting the wing spars solidly to the fuselage frames -- is that the wing and the fuselage are subject to different kinds of loads. The fuselage expands and contracts under pressurization loads, but the wing doesn't. The wing, on the other hand, flexes under the weight of the fuselage, so that its centersection, where the wing-to-fuselage attachment is, changes shape slightly, the middle moving down when the wingtips move up. If the wing were solidly secured to the fuselage, those distortions would be communicated to the fuselage members, stressing them in unnatural ways. The system of a few simple links, each carrying loads in one axis and free to move in others, handles each stress in the most efficient way without inducing unintended secondary strains.

Having placed the wing outside the fuselage, one must fill the gap between them. On large, thin-winged airplanes, the wing-fuselage fairing is not very conspicuous, and one has to look twice to notice the external wing placement at all. But it's desirable for wings to get thicker near the root for a variety of reasons, and the fairings consequently get deeper. On slower airplanes -- under Mach .80, say -- a fairing can be relatively short and blunt without an excessive drag penalty. The faster the airplane, however, the more important area-ruling becomes.

Area-ruling is the tailoring of the airplane's shape to avoid abrupt changes in cross-section, which increase the transonic drag. A wing suddenly sprouting from a fuselage obviously produces a sudden increase in cross-sectional area. By lengthening the fairing ahead of the wing and behind it, the rapid change in cross section at the wing is smoothed. A problem crops up, however, behind the wing, where the engines produce another sudden increase in cross section. The deep wing-root fairing can't be suddenly truncated, because flow would separate behind it; so the fairing is carried smoothly back to the tail and the sides of the fuselage are pinched inward, often to a startling degree, at the engines.

By the time you have a wing root fairing that is several feet deep and extends almost the full length of the fuselage, you have added a great deal of surface area and weight, which are bad, and internal volume, which is good. Nothing can be done about the surface area. It is accepted as a necessary evil, and can be dealt with by appropriate additions of thrust, which is available these days in more or less unlimited quantities. But the internal volume is put to work for additional fuel tankage and for controls and plumbing which, running outside the pressure vessel within a nonstructural shell, become much easier to install, inspect and maintain.

As I mentioned last month in talking about wing sweep, parasite drag, for airplanes operating above Mach .8 or so, consists of subsonic and transonic components. Once the transonic component, which is related to the formation of shock waves, begins to increase, it rises very steeply -- much more steeply than the better-behaved subsonic component. Increased surface area raises the subsonic drag, but careful area-ruling keeps the steep transonic drag rise at bay. Ultimately, you can afford the added bulk of the huge wing-fuselage fairing, so long as you get the shape right. And that is why the Citation X -- one of the well-along pregnant Cessnas -- is the fastest civil jet, with a high-cruise speed of Mach .92.

You used to see in airplane workshops an amusing chart that showed how an airplane would look if various departments in the company had their way with it. The variations were ludicrous exaggerations, of course, but the point was that the particular obsessions of each specialty conflicted with those of every other. It was the business of project managers to balance competing demands and spread the grumbling evenly throughout the organization. This process, now aided by computers, normalized to the corporate bottom line, and called "multidisciplinary optimization," is ultimately responsible for the strange shapes of modern jets -- and also, as it turns out, for their remarkable performance.