Since it consists of little more than a stick at one end and a couple of ailerons at the other, I thought that hooking up the roll control system of my homebuilt would not have been much of a problem. But then you wouldn't think anything would be much of a problem, until you try it.
The pilot's end of the system is a sidestick that swings 30 degrees to the right and left, turning a torque tube that runs aft along the left side of the airplane to a point behind the pilot's seat and above the rear spar. There, a crank and pushrod convert the rotary motion of the stick into a vertical, linear motion.
At this point there was, until a couple of weeks ago, a gap. Think of it as one of those blurry areas the television networks put over suspects' faces and indecent body parts.
Jumping to the other end of the system, we find the ailerons. Their motion of about 30 degrees up or down becomes, through the intervention of a single bellcrank, an in-and-out motion of about one inch in slender pushrods that run along the rear spars and pass through small holes into the fuselage. There, until a couple of weeks ago, they ended at…the gap.
The problem was how to close this gap, which had persisted for years because, principally, of my tendency to procrastinate. You would suppose that the ends of the two aileron pushrods could simply be connected to a single bellcrank that would convert the up-and-down motion coming from the sidestick into a right-left motion of that common pushrod. And that would be that. Unfortunately, the rear spar has a couple of bends inside the fuselage, and so these pushrods have to do some acrobatics just to get near each other. And then there is an additional complication: differential.
Aileron differential is the mechanical arrangement, found in practically every airplane, that causes ailerons to deflect upward more than they do downward-twice as much, typically. Its purpose is to reduce adverse yaw-the tendency of the nose to swing out of a turn because of the excessive drag of the downgoing aileron. Adverse yaw can be controlled with rudder, but airplanes with little or no adverse yaw are more pleasant to fly than those that have a lot. We speak kindly of airplanes that stay coordinated "feet on the floor"-as if we had chronically tired feet.
To achieve aileron differential we take advantage of the geometry of circles. Think of a clock face. The tip of the minute hand moves a greater distance horizontally than vertically between the hour and five after; but it moves farther vertically than horizontally between ten and fifteen after. Using a crank that rotates the same 90 degrees, we can exploit this characteristic to produce a linear motion that is much greater in one half of the crank's arc (aileron up) than in the other (aileron down). It should be obvious (don't you love people who say that?) that the largest differential that can be obtained from a single bellcrank making a 90-degree arc is .7071 divided by .2929, or about 2.4:1. Since control surface deflections are usually limited to about 30 degrees, this means 30 degrees up and 12 or 13 degrees down.
The predictable behavior of circular-arc linkages breaks down when the components are close together, and, because my habitual design procedure for irksome details is to leave some empty space and worry about them later, I found, when I was forced finally to do the job, that I had very little choice about where to put the various components. I had created a situation in which various parts of the aileron linkage, a flap actuator, and a cable trolley intended to keep the right and left flaps in step with each other all had to be stuffed into a small, inaccessible corner of the fuselage behind the pilot's seat. Thus, what looked on paper like a simple exercise in bellcrank geometry turned out to be a struggle with complication, contortion and perspiration instead.
