Technicalities: On Balance
I didn’t like any of these trailing-edge treatments, however. Beveled ailerons are just too ugly to bear, and servo tabs involve some delicate parts that, if they come adrift, could increase the risk of flutter. Tabs also reduce the effectiveness of the aileron, since they deflect in the opposite direction.
Historically, the preferred solution has been aerodynamic balancing. This means putting some of the area of the aileron — or any control surface — ahead of the hinges so that the pressures on either side of the hinge line balance out in seesaw-fashion. The most in-your-face kind of aerodynamic balance is the external spade, which consists of a flat plate set some distance from the aileron itself, usually below it, on the end of a forward-angled arm. When the aileron is in trail, the spade is aligned with the airstream. When the aileron is deflected, the spade “digs in” and pulls the aileron in the direction it’s going. Spade balances can completely eliminate control forces, and so they are mainly used on competition acrobatic airplanes. Check for them the next time you see an Extra; some Pittses have them as well. Control surfaces with zero hinge moment have no tendency to center hands-off, however, and some pilots find that complete neutrality annoying.
The widely used Frise aileron has a somewhat spadelike quality. The hinge is set close to the lower surface and far enough back that the overhung nose of the aileron pokes down below the wing surface when the aileron is deflected upward. The airstream pulls it downward and aft, helping to push the rear portion of the aileron upward. The balancing effect of a Frise aileron occurs mostly on the upgoing aileron; but since ailerons are mechanically connected, one of them can provide balance for both. As a collateral benefit, the drag of the upgoing Frise aileron combats adverse yaw.
On the other hand, you have never seen a Frise rudder. For elevators and rudders, you need a balance that works in both directions, a style that can, of course, be used for ailerons as well. One solution is a so-called “horn” that sticks out ahead of the hinge line at the outboard end of the surface. Horns are most commonly found on tail surfaces because wingtips call for other treatments. Another is to move the hinges aft so that the aileron has an overhanging nose. Look at the ailerons of a Cirrus, which, like my airplane, has a sidestick and therefore a special respect for hinge moments. The hinge is far behind the leading edge of the aileron.
In general, for reasons laid down around the time of the Big Bang, the hinge moments of a free-flying airfoil-shaped surface, like a stabilator, become neutral when the hinge is a quarter of the way from leading edge to trailing edge. When the movable surface is close behind a fixed one, however, as an aileron is behind a wing, its leading edge is at least partly shielded and doesn’t feel the same air pressure as it would were it out in the open.
The behavior of a partly shielded surface depends on a mystifying variety of parameters, including nose shape, gap size, the angle at which the surfaces converge at the trailing edge and the way the nose projects outside the wing contour when deflected. Ideally, you’d like the hinge moment to increase in a nice, steady linear fashion as the deflection increases, but things don’t always work out that way. Around 1940, NACA researchers put considerable effort into figuring out how to predict the hinge moments of new designs and ensure they behaved well. They came up with a complicated procedure but admitted it was rather unreliable.
So I’m making a pair of aerodynamically balanced ailerons, and since it’s impossible to know in advance how they will feel, I’m equipping them with adjustable hinges. At first, the hinge line will be at the quarter-chord point. If they’re still too stiff, I’ll move the hinge line aft a bit at a time. If they snatch, I’ll move it forward a bit. Since, for flutter-prevention reasons, I want them to be almost perfectly mass-balanced — mass balance, which has to do with weight distribution, is an entirely different animal from aerodynamic balance — the nose balance weights also have to be adjustable.
If this project is successful, perhaps I will make up a little placard, similar to the passenger warning required in experimental airplanes. It will read: This aircraft is experimental and may roll unexpectedly at any time.