Florida Jets 2020

5 Great Servo Installation Tips

5 Great Servo Installation Tips

After you install your servos according the manufacturer’s directions, you might find that when the servo arms are placed on the splines, they aren’t at a perfect 90-degree angle to the servo case and control linkage. Or, after you’ve hooked up the various linkages, you discover there is too much or not enough travel throw when a certain control surface is deflected. How about adjusting those throttle linkages to get that carburetor barrel either wide open or fully closed when the throttle trim is lowered? Here are 5 tips that will help you to achieve great radio- and servo-setup success.
1 First, check that your servos are properly installed. Unless you’re flying a foamie or small electric in which the servos are glued into place, use the rubber grommets and brass eyelets that come with your servos. Install them so the wide brim of the eyelets are under the grommets (between them and the servo tray). Tighten the screws until their heads meet the brass bushing’s top edge. The rubber grommet will be compressed a bit, but that’s OK. The object is to have a secure, shock-mounted servo installation that won’t move when the servo arm is deflected. If the eyelet is installed with the wide end up, the grommets will be compressed so much that they won’t isolate the servo from the source of vibration.

2 This is a crucial setup check and should be done before any linkages are hooked up. Does the control surface move in the correct direction relative to the transmitter’s stick input? Start with one servo and place the servo arm on the spline. Don’t concern yourself with whether it is exactly 90 degrees to the case. Turn on your transmitter and receiver and move the stick (top) that corresponds with that channel. If you see that the arm is moving in the wrong direction required for the correct control surface movement (middle), use the servo reversing menu and hit select “norm” to “rev” so the servo responds in the correct direction (bottom). Now go one by one through the remaining servos and correct their directions if necessary.

3 First, all servos should be centered with the transmitter sticks and the control trim levers centered, then place the servo arm on the spline (mechanical portion). Move the arm’s position on the spline to get it as close to 90 degrees to the servo case as possible then, if necessary, use the sub-trim menu to adjust the arm’s position. Do the mechanical adjustments first; don’t rely on the subtrim function only. This can affect the servo’s overall control throws and end points.
For most elevator, rudder and aileron servos, the servo arm should be at a 90-degree angle to the case.

4 Because the servo placement is usually pre-determined in an ARF, you need to mechanically (i.e. no programming) set the control linkage at 90 degrees to the servo arm. Determining which hole to use in the servo arm is simple: if you want more throw on the control linkage, place it in the hole farthest from the servo’s center; closer if less throw is desired. Different size models will have various linkage setup requirements, so consult the instruction manual for the proper linkage setup. With the linkage disconnected to the servo’s arm, there shouldn’t be any binding when you move it by hand.

5 The control surface’s linkage connection depends on the type and size model you’re flying. If you want to achieve maximum surface deflection, connect the clevis to the control horn using the hole closest to the surface. For large-scale and 3D airplanes, connect the linkage to the outermost hole (farthest from the surface) for maximum leverage; this also helps to prevent flutter. This photo (below left) shows threaded rods for control horns with plastic connectors to which the clevises attach. Note that they are at the end of the rod rather than close to the surface. It is usually best to have a straight line from the pushrod linkage’s fuselage exit to the hole in the surface’s control arm/horn. Sometimes a slight bend in the rod (top right) after it exits the fuselage is needed to relieve servo and linkage binding.




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  1. When putting the servo arms onto the servo, if it is not 90 degrees, take the arm off and turn it 90 degrees to the next arm. They are slightly off set so the 4 arms align differently. You can usually find one of the four that are right on or st least very close to where you want them.

  2. Before cutting off servo arms, try each one for 90° alignment. The arms pictured originally had 4 arms each. Due to the way they are splined, the different arms will align differently. This can greatly reduce the amount of subtrim required to get 90° alignment at neutral.

  3. Drilling out the servo horn holes is often necessary to get the pushrod to fit. You don’t need to drill all the holes because this will weaken the horn, and anyway you won’t need all the holes. Often you need just the outer hole so that as much movement as possible is transferred to the control surface. If that gives too much surface movement, its preferable to use a longer horn at the surface. Reducing the throw at the servo end just adds slop into the system so should be avoided where possible. Don’t over-drill the horns either, there should be as little space as possible for the pushrod to move around because that will all the surface to flap uncontrolled, something sometimes referred to as ‘flutter’.

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