Refugee from Compromise

Flying wings-a goofy name, really; "tailless aircraft" is better-are not merely an obsession of cranks, however. They appear periodically among the meditations of even such organizations as Boeing, which, in spite of a brief delusional period characterized by persistent repetition of the phrase "sonic cruiser," is on the whole crank-free. Giant transport planes consisting of nothing but wing are proposed; they are called "blended wings" or "spanloaders." The latter name hints at one of the characteristics that make them desirable. The idea is to lighten and simplify the airplane by putting the weight where the lift is, in the wing, rather than concentrating it in a slender tube at the middle.

This is a wonderfully simple idea, and like most simple ideas it is suspect. The fuselages of high-altitude airplanes are round not just to make them roll downhill better, but also because round is the natural shape of an inflated object. (When metal fatigue started showing up in the skins of old 747s, it was in the flat sides of the two-story portions that it appeared.) Pressurizing a wing is such a difficult structural problem that it would send designers scurrying after circular airfoils.

Other great names in aviation have had their flings with flying wings. We currently have, obviously, the Northrop Grumman B-2 bomber, thought by some to make up for the unjust fate of the magnificent YB-49 and other Jack Northrop flying wings of yore. Until recently we had the extraordinary 250-foot Helios, a featherweight sun-driven plank, propelled by 14 electric motors, that had climbed to nearly 100,000 feet. Sixty years ago the Germans had the Lippisch-designed Messerschmitt Me-162 Komet, the rocket-powered interceptor that was praised for its wonderfully docile flying qualities by those of its pilots who were not incinerated in fueling accidents. In the Fifties our Navy, briefly impressed with the possibilities of taillessness, was operating Vought F7U Cutlasses from carriers, and after them the ageless Douglas F4D Skyray-but that was a delta wing, and a slightly different story.

It is reasonable to wonder why, if airplanes can fly without tails, so many have them nevertheless; or, to put the question another way, if airplanes have tails in order to provide them with longitudinal stability, how can any airplane do without one? And why do we find long, high-aspect-ratio flying wings, like the YB-49, surprising-looking, while we take equally tailless narrow deltas, like Concorde, for granted?

I was traumatized at an early age by the unruly behavior of balsa wings when launched without fuselage and empennage. They are mere whirligigs, tumbling leading-edge-over-trailing all the way to the ground.

If I had been more persistent in my investigations-my laxity in such matters is but one of many reasons that my name shall not be inscribed among those of aviation's greats-I would have found that it is, in fact, possible to make a simple rectangle of balsa wood glide. You just have to weight its leading edge so that it balances a little ahead of its quarter-chord point, provide some sort of rear-mounted vertical fin and turn up part of the trailing edge. A little experimentation is necessary to discover the proper combination of nose weight and trailing edge flip; and then the simple wing will glide very creditably.

The upturn of the trailing edge is a hallmark of flying wings. When it is built into an airfoil it is called reflex, and it is needed to counter the so-called "negative pitching moment"-the tendency to tuck nose-under-that characterizes normally cambered airfoils.

Call the portion of the trailing edge that you have bent upward an elevator. Its function is to overcome the slight noseheaviness that you introduced with the weight. Now, the amount of noseheaviness is constant, since the weight and the location of the center of gravity don't change. The aerodynamic force on the elevator, on the other hand, varies with speed. The faster the wing goes, the more the elevator pushes down, lifting the nose, increasing lift, making the wing climb and thus slowing it down. The slower it goes, the less downward push there is on the trailing edge, and so the nose drops and the wing picks up speed. This is longitudinal stability; the wing seeks a certain angle of attack and returns to it after being disturbed.

When we apply this principle to a full-scale airplane, the first difficulty that arises is that the force that the elevator can produce is quite limited; for one thing, it is operating in the wake of the forward part of the wing, and for another it is close to the CG, and so has very little leverage. Consequently, the amount of noseheaviness that can be tolerated is quite limited as well. (No tailheaviness at all can be tolerated, by the way. Think it through: any tailheaviness would require an upward force at the trailing edge and a downward-deflected elevator-not an elevator at all, in fact, but a depressor. Increasing speed would then force the nose downward, pushing the speed higher and higher. Rather than return to its trimmed speed, the wing would diverge from it.)

Limited CG range can be a serious handicap of tailless airplanes with short chords-that is, ones with wings of conventional aspect ratio. The problem grows as the size and variety of potential payloads increase. To design a one- or two-seat flying wing for recreational use is easy, because its occupants can be put right on the CG; there their weight can change without altering the balance of the whole. But four seats in two rows present a problem; they can't all be on the CG. To handle a wider range of CG locations, you need an elevator with a longer lever arm than the wing's chord allows.

At this point, if you're not a zealot of taillessness, you just move the elevator two or three chord lengths aft, you add some structure to connect it to the wing, and you have a conventional airplane. But if you are resolute in wanting to avoid adding an empennage, you cast about for different solutions. One is to sweep the wings. This helps in two ways; wing sweep improves directional stability, and it allows you to put the elevators (and rudders) farther aft. If the elevators are at the tips of the wings, they may also serve as ailerons, and in that case are called elevons.

The most extreme instance of sweep increasing the leverage of the elevators is the delta configuration. The dominant dimension of the delta wing is its chord, not its span-its aspect ratio is less than one, in other words-and the elevators are far enough away from the center of lift to allow a fair amount of CG travel-provided that the CG always remains ahead of the center of lift.

A handicap afflicting deltas and flying wings in general is the limited maximum lift available for landing. Recall that most high-lift devices are variations upon the theme of deflecting a portion of the wing's trailing edge downward. But slowing the flying wing down requires raising the elevators, and so the effect is the opposite of that desired from landing flaps.

The impossibility of putting powerful flaps on a flying wing means that for a given landing speed, the wing area must be made quite a bit larger than if the wing had flaps. As the total surface area-the "wetted area"-of the airplane increases, so do the drag and weight. But reducing drag and weight was the whole point of the flying wing in the first place. It seems that we've strayed into the Realm of Compromise, where aeronautical engineers spend a large and vaguely unhappy portion of their lives. Above the gate is carved in dark letters, "Leave all hope, you who enter!"

Oh, no, I was mixed up-that's Hell. The Realm of Compromise is just a dull place, ruled by a committee, where the weather is always mild and partly cloudy. There flying wings acquire tails, supine pilots rise up and sit in chairs, and hawks are equipped with rotating beacons and fixed landing gear. It's where airplanes come from-except the ones called "uncompromising." Those are the ones we admire from afar, but hope that someone else will fly.