Without smooth, non-binding control-surface movements, our planes would be nearly impossible to direct. Linkage systems, including the servo, servo arm, pushrod and control horn, are often overlooked, but they are the keys to smooth, strong, equal control-surface deflections. Take the time to install the linkages properly and securely to guarantee control surfaces that work properly for the life of the plane. Let’s look at these vital links and see where we can improve them, starting with the servo.
When the servo arm travel is not parallel to the pushrod, you have extra stress on and less control of the pushrod and servo arm. With everything set in line, you have less binding and maximize the servo movement.
First, make sure that the servo arm travels parallel to the movement of the pushrod so that it won’t cause undue friction in the linkage (see Figure 1). Don’t allow the servo arms to contact other servo arms or bind with the clevises at full rates. Now you can concentrate on maximizing servo power and control.
With programmable radios, we have the ability to control the amount of servo throw; this allows us to use all of the potential power and efficiency of the servo’s full range of travel. When you use the full 120 or so degrees of motion from the servo arm, it exerts a more precise and powerful force on the control surface. For example, if two control surfaces travel 35 degrees in each direction, and one setup has the servo arm moving 30 degrees in each direction while the other has the servo arm moving 60 degrees in each direction, the setup with 60 degrees of motion will spread the load out across the servo’s full range of travel. This will provide maximum power and resolution to that control surface compared with the one with 30 degrees of servo-arm motion (see Figure 2). Every time we reduce (or restrict) servo throw, we give up bits of information that represent lost motion, and that can translate into less power and precision on the control surfaces.
By having full travel to drive the control surface, you maximize the servo’s precision and power. This setup works best with the pushrod in the outer servo-arm and control-horn holes.
If you use less servo throw, you have to connect the pushrod closer to the control surface on the control horn. The linkage setup will have less leverage to overcome the weight of the control surface and airflow as it moves over the control surface.
Ideally, at neutral stick, the pushrod geometry will look like this.
To maximize servo throw and resolution, we must take great care to set up our linkages properly. First, we must make sure that our servo output arms are carefully set so that they are perpendicular to the control rods and parallel to the hinge line. Do this by switching the servo arms around until the spline alignment is correct; try not to use the radio programming to achieve this. The numbers at the base of the arms on the multiple-horn servo arms represent the degrees from zero of alignment on the servo spline. By rotating the multiple-horn servo on the servo spline, you’ll find that the servo-arm positions will be slightly different with respect to the control rod. These differences are just a matter of a few degrees and are specified by the numbers on the servo arms.
Using this mechanical process instead of the programming method allows you to arrive at the perpendicular angle to the pushrod and have equal movement on both sides of center. This way, the transmitter is left at zero with an equal amount of electronic signal in each direction of servo throw. Keep in mind that any programmed offset on either side of the servo’s center affects the total travel when using all of the available servo travel. Ideally, at neutral stick, the pushrod geometry will look like that shown in Figure 3.
If you require more throw than your setup allows, use a longer servo arm to give you the throw you need. Avoid moving the pushrod closer to the surface on the control horn; moving it would reduce the leverage applied by the servo. At the other end, if you find that you need less throw, move the pushrod closer to the center of the servo arm. Avoid using the dual rates or endpoint adjustments for this whenever possible. Make large adjustments mechanically, and use the radio programming to make fine adjustments.
Most setups have the pushrod at 90 degrees when at neutral stick, but when it’s pushed to full deflection, note that there is more of an angle and not the best linkage geometry when the most pressure is exerted on the control surface.
With this setup, the linkage geometry is angled when there is the least amount of pressure on the control surface. But when at full stick deflection, the linkage is straight and providing the best angle and power to the control surface.
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.
There you have it! By setting up the mechanical advantage and the linkage geometry first, you’ll end up with a plane that flies more efficiently. You’ll feel more in tune with the plane because of the better transmitter-stick resolution, and the control surfaces on the plane will move more easily with more power and authority. And you will still have plenty of radio programming to fine-tune the plane’s flight performance. As they say, “Try it; you’ll like it.”