RC Model Reviews

iflylilplanes wrote:Thanks Bill,
I'll have to look into those programmable digital servos. I always have problems centering servos in my scratch built models, the servo arms seem to be one spline tooth out either way and will not match up with the servo centering in the transmitter setup. Will also look into servo disks without holes and drill my own holes for the kiwk links.

The little number on the servo output arm corresponds to how many degrees the arm is offset to the servo spline. It is better to adjust the output wheel/arm manually so that the arm is perpendicular to the pushrod instead of programming the centering on the transmitter.

Now that we have maximized servo power and resolution, we can focus our attention on pushrod geometry. The connection between the servo arm and the control horn is vital to making sure that the flying control surfaces work smoothly. The linkage geometry should have direct travel between the servo arm and the surface control horn. Ideally, the linkage should have a direct straight line that is maintained throughout the travel arc of the servo and control horn. Servos that are installed so that their servo-arm travel arcs move in the same direction as the control horns’ travel arcs already have this linkage geometry.

Problems with linkage geometry often arise when a servo is mounted so that the travel arc of the servo arm is moving perpendicularly to the travel arc of the surface control horn. This often happens when aileron servos are mounted in the bottom of the wing so that the top of the servo faces outward. This setup does make it easy to remove and install the servo and arm, but it can create linkage-geometry problems.

The linkage will typically be angled to the control horn and put extra pressure on the connection at the control horn. You can install ball links at both ends of the pushrod to relieve some of that pressure, but we still have to contend with the slight angle of the pushrod during movement. If the linkage is set up like most, i.e., so that the pushrod has a straight connection to the control horn when the servo is at neutral, this will be the only time when there is a straight direct link between the two. However, to improve our linkage geometry, we could move the control horn in so that it lines up closer to the servo body than it does to the end of the servo arm. That way, there is a straight and direct link between the servo arm and control horn when it is at the end of the servo travel. This gives us two positions where the linkage is straight and direct (once at each end). Most of the pressure is exerted on the control surfaces when they are deflected at their extreme ends. It makes sense to have a straight and direct linkage at that time instead of when the control surfaces are at neutral.