Technicalities: On Balance

We have ways to make you roll.

Technicalities Ailerons

Technicalities Ailerons

** Clockwise from top left: servo tab (Fouga
Magister); horn balance (American Yankee);
overhung nose (Cirrus); beveled trailing
edge (Mooney)**

The ailerons of my homebuilt aircraft, Melmoth 2, are quite similar to those of a 1918 Fokker D.8.

You would think that a great deal might have changed in nine decades, but apparently not. Perhaps it’s true, if not in biology then at least in aviation, that “ontogeny recapitulates phylogeny” — that is, the development of the individual revisits some of the stages in the evolution of the species.

The D.8’s very simple style of aileron — basically, a blunt circular-arc nose centered around a hinge halfway between the upper and lower surfaces of the wing, with very narrow gaps between it and the wing — found its way into many high-performance airplanes, including the P-51 Mustang. Despite this distinguished pedigree, however, my ailerons are bound for the junk pile. The stick forces in roll are fine for cruising but too high for fun.

Everybody praises airplanes that are “light on the controls.” Nevertheless, mine isn’t the first airplane to be heavier in roll than one would wish, and it won’t be the last. How does one fix this problem?

The first thing that affects the effort required to bank an airplane is the basic relationship between the size of the aileron and that of the stick, wheel or yoke — I’ll say “stick” to mean all of them, since Melmoth 2 has a stick. Many people suppose stick forces can be changed by fiddling with the bellcranks and pulleys between the cockpit and the ailerons, but that doesn’t work. Only by making the stick longer or increasing its throw can you gain an appreciable amount of mechanical advantage. Melmoth 2 has a sidestick that’s just 6 or 7 inches long and swings about 45 degrees from side to side. That can’t be changed.

If you can’t change the stick, you have to change the aileron. To get light stick forces, you need to reduce the torque needed to rotate the aileron, which is called the “hinge moment.” Lots of things affect the hinge moment, first of all speed. Like lift and drag, control forces increase with the square of speed. Double the speed and the ailerons feel four times stiffer. Even before Melmoth 2 first flew, the French designer René Fournier, an old friend, looked at its ailerons, shook his head and warned in somber tones that the stick forces might get so high at very high speed — over 200 kias — that it would become impossible to move the ailerons at all. I don’t know whether this is true — my VNE is 190 kias, and I don’t go even there — but at a normal cruising speed of 145 kias, I can’t comfortably deflect them more than 3 or 4 degrees. And so I have been missing the great pleasure of terrifying passengers by rolling the airplane with no warning.

But speed is a given, and so I had to look elsewhere. Hinge moments also depend on the chord of the aileron and the shape of the wing’s airfoil, which in turn determines the shape of the aileron. The chord of my ailerons is quite short, and so if the hinge moments are unexpectedly high it may be due in part to the cusped shape of the trailing edge of the laminar airfoil. Straight-sided ailerons deflect more easily, and to really get forces down you can thicken a straight-sided aileron and provide it with a wedge-shaped trailing edge. This so-called “beveled trailing edge” was used on Mooneys and on late Mustangs, among others, with success. It works because airflow bending over the corner of the bevel tugs on the aileron with a mechanical advantage about three times that of the force that resists deflection. (The effect can occasionally be so strong that rather than resist going to full deflection, the aileron begins to pull toward it. This phenomenon, called “aileron snatch,” is disconcerting to a test pilot but is always bred out of certified airplanes.)

Another possible approach is the servo tab, which looks like a trim tab but is linked to the wing in such a way that when the aileron deflects downward, the tab deflects upward, and vice versa. Like the beveled trailing edge, the servo tab gets its power from being farther from the hinge than the center of pressure on the aileron.

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.