When Back Becomes Front

The workings of an usually-placed rudder.

Defiant

Defiant

** Never averse to oddness, Rutan tried
putting the Defiant's rudder at the front. But
didn't that make the airplane want to swap
ends?**

A ready asked me the purpose of the odd-looking fin projecting downward from the nose of the Defiant on the cover of our August 2011 "Rutan Retrospective" issue.

It’s the rudder. Rutan called it a “rhino rudder,” because in versions he tried on the VariEze and Long-EZ it was on top. But what was the rudder doing on the front of the airplane?

In an obscurely related development, I had just had a rather fractious exchange of e-mails with a person who was incensed that a certain Wikipedia article made reference to a “canard stabilizer,” citing in a footnote a 1992 article of mine. He had attempted to edit the Wikipedia entry, only to have it changed back again, Whac-A-Mole fashion, by the original author.

That one of my articles should be cited as an authoritative source in Wikipedia is flattering, but it somewhat shakes my faith in Wikipedia. It reminds me of Groucho Marx’s comment that he would not want to belong to any club that would have someone like him for a member. On the other hand, Wikipedia can be pretty ferocious about sources, which are the fig leaves of scholarship. Once, when I corrected some biographical information in a Wikipedia article about my own great-grandfather, the editors pounced upon my emendations as being inadequately sourced.

Nevertheless, “canard stabilizer” is a misnomer, and I would hate to think that I used the phrase, especially 20 years ago, when I knew so much more than I do now. Accordingly, I asked my computer to search all of my articles from 1850 onward. I was relieved to find that the words “canard” and “stabilizer” could be found in various degrees of proximity, but never adjacent to one another. I think the writer responsible for the Wikipedia footnote must have something in common with the authors of those promotional blurbs for films, and California ballot propositions, in which quoted words are cherry-picked, or ingeniously linked with ellipses, in such a way as to exactly reverse their original meaning.

But about that rhino rudder ...

The vertical tail surfaces of most airplanes — there were a few exceptions back when Baron von Richthofen was still with us — consist of a fixed component, the fin or “vertical stabilizer,” and a movable one, the rudder. To some extent they cooperate, but the fin’s main function is to keep the airplane’s fuselage aligned with the direction of its flight, while the rudder’s is to allow the pilot to adjust that alignment. Now, the rudder’s job can be done in other ways; on the B-2 bomber, for instance, both fin and rudder are dispensed with because of their radar reflectivity, replaced by spoiler-like split ailerons that manage yaw by tugging at a wingtip. On the Defiant, the rudder was placed below the pilot’s feet, eliminating all the guides and pulleys and long runs of steel cable that would have been required had there been rudders where you would expect to find them, on the big fins at the wingtips.

A directionally stable airplane is like a weathercock, with the pivot point at the center of gravity. It naturally tends to align itself with the wind — although there are some airplanes that, while they have no tendency to swap ends, will fly along in a steady sideslip, feet on the floor, because of excessive control system friction or minimal fin area. Any area behind an airplane’s center of gravity is stabilizing, and any area ahead of it is destabilizing. The rhino rudder therefore was destabilizing, but the vertical fins at the wingtips were much larger than the rudder and handily overcame its destabilizing influence.

Not all surfaces are equally effective at producing a side force that increases with sideslip angle. To look at an MD-80, you would think it directionally unstable. But the airfoil section of the vertical fin is much more effective at producing a side force than the cylindrical fuselage is, and so the MD-80 is directionally stable despite its long nose and short tail. A yaw damper helps, to be sure.

The same principle applies when you look at an airplane in plan view — that is, downward from above — but now we’re talking about “longitudinal stability” rather than “directional stability.” Surfaces ahead of the center of gravity are inherently destabilizing. That includes any canard surface. Hence, the term “canard stabilizer” is misleading.

So how did it find its way into Wikipedia, supposedly by way of an article of mine?

Although a horizontal tail surface provides both stabilization and pitch control, people often just call it the stabilizer. (Some, especially nonpilots, making a complementary error, call the complete vertical surface the “rudder.”) Thus, when talking about, say, a VariEze, one might carelessly toss off some phrase such as, “Rutan moved the stabilizer to the front of the airplane,” merely intending, by “stabilizer,” to signify “the auxiliary wing usually found at the back end.” One could say this without meaning to imply that in moving to the other end of the airplane, the horizontal tail took its stabilizing function along with it. For that matter, we often speak of canards as “tail-first” airplanes, even though, when the auxiliary surface is found in the front of the airplane, it has surely forfeited its right to be called a “tail.”

Static longitudinal stability is the airplane’s tendency to maintain a certain trimmed angle of attack and to return to it after a disturbance. Basically, it requires that the center of gravity be ahead of the center of lift of all of the lifting surfaces — wing, canard, horizontal tail, whatever — taken together. That way, if, say, an upward gust increases the lift, the nose tends to rotate downward, reducing it.

If that simple rule were all that a designer had to worry about, there would be no stability specialists in this world. Actually, maintaining stability gets a lot more complicated, because wings, stabilizers, fuselages, nacelles, propellers and jet wakes influence it, and one another, in ways that vary with angle of attack and power setting, and also with the shapes and sizes of bodies and lifting surfaces.

For example, a wing deflects air downward. As the airplane slows down and the wing works harder, that “downwash angle” increases. Now, think of how that looks to the horizontal tail. If the wing produced no downwash, and the airplane pitched up from zero to 10 degrees angle of attack, the horizontal tail would see a 10-degree increase in its own angle of attack and push upward proportionately to lower the nose. But if the angle of the wing’s downwash at the tail is four degrees, the horizontal stabilizer sees only a six-degree change in its angle of attack, and so its stabilizing contribution is much smaller.

When the stabilizer leapfrogs the wing and becomes a canard, the wing becomes a stabilizer. But they continue to interact. Because the span of the canard surface usually is smaller than that of the wing, only the inboard portion of the wing has to cope with its downwash, but the wing produces an upwash ahead of it that affects the entire canard. A further complication, important near the stall, arises if the canard’s tip vortices strike the wing. Canard surfaces have to be placed above, or on a level with, wings, in order to allow their tip vortices to pass above the wing when the nose is up; placing the canard below the level of the wing risks an early wing stall triggered by the upward-flowing outer edge of the vortices.

Just getting an airplane to fly straight and level requires juggling a surprising number of elements. It’s even more complicated than saying things in a way that does not create more misunderstandings than it dispels.