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Futaba 9152 Mod
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If you are using the Futaba 9152 servo for your plane, you will need to read this section.

The Futaba scale servos, like the 9152, use a larger output shaft…about 7.5mm versus the standard 6mm. At the time I am writing this, there are only 4 servo arms being produced for this servo. Three are nylon versions made by Futaba, and the last is an aluminum series manufactured by AirWild. Unfortunately, all of these styles use reinforcement ribs which prohibit their being connected to the Direct Drive horn without modification.

Since the AirWild arm is pricey, and we are going to have to modify the arm any way, it makes sense to use the inexpensive Futaba arms. We can use the modification to our advantage.

 

 

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These are the arms available through Futaba

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This is the arm you want - with 6 fingers

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We will need to remove the reinforcement ribs on the top

Start by using the servo arm that has 6 fingers. We’ll talk about why you want this arm in a minute. We will need to machine the ribs on the top of the arm to remove them completely. Continue the machining in towards the mounting screw until the remaining boss is exactly 3/8” - or the same as the inside diameter of the Direct Drive horn. This is a precise procedure, best performed on a lathe. If you do not have access to a lathe, send the arms to us and we will machine them for you. If you pay the postage in both directions, we will perform the modification for free.

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The top rib is removed to match the horn

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The arm can now be pressed flat onto the top of the Direct Drive horn.

Now, the reason we use the 6 finger arm...every servo centers differently…an unfortunate but true fact. We normally reset the center with Direct Drive using the sub trim function of the radio. The down side of sub trim is that it subtracts from the available travel the servo will have in one direction. The amount could be as much as of one notch on the servo shaft. This is also why, on occasion, you cannot get the full 60 degrees of servo travel in each direction. We are going to fix that with our modification, though.

The 6 finger servo arm is designed so that as you rotate the arm on the shaft to line up each consecutive finger, the angle of the fingers change by 1/6th of a shaft notch. Pretty neat! To minimize the sub trim we will need to use, we are going to match each servo to the arm finger it best lines up with. Do this by plugging the servo into the receiver and turning everything on. Make sure the trim and sub trim are set to zero. Make sure the end points are the same, and that no mixes are altering the center point of the servo. Now rotate the arm on the servo shaft until you find the finger that lines up the closest to center. Remember that for an elevator installation the finger will extend to the long end of the servo, but for all others the finger will extend towards the short end. Remove the other 5 fingers using tin snips and smooth the nubs with sandpaper.

Be sure to mark the servo and arm so they will remain a set from now on. Sand the surface of the remaining finger that will contact the horn to make it rough.

 

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View showing the hole locations

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Finally, you will need to mount the horn to the arm using glue and two screws. Install the arm onto the horn and align them.  Use the farthest servo arm hole as a guide to drill a 1/16" hole through the horn. Drill a 1/16" hole through the servo arm using the inside horn hole as a guide.  Separate the horn from the arm and open the hole you drilled in the horn to 1/8" diameter.  Tap the two holes in the arm with a 4-40 tap.

Secure the horn to the arm with two 4-40 button head bolts. Nuts are not necessary, but the photo shows how you can install one for insurance.  Permanently lock the arm and bolts in place with thin CA. That’s it!

A little extra work, but you will now have a full 60 degrees of control throw if you want it.

 

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