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?
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.
