Technicalities: On Balance
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.