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Aug 11, 2011, 06:34 PM
65 years of RC flying
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Polaris Control setups

XL Servo Setup
While building the latest XL, I took photos and notes on the servo installation, so I could do it more efficiently next time. They might help others who are building the XL (or the Polaris, allowing for the different horn length).

One particularly useful bit of information I gathered is the required length of servo arms (measured from the center to the attachment point). This is based on using the standard plywood control horns, which when installed normally in the slots, place the hole about 14mm from the control surface. Since it's made of 6mm Depron and hinged at the top, this means that the total control horn arm length on the XL is approximately 20mm. We want less angular throw on the elevator and aileron than the servo provides, so we need to "gear down" the response by using a servo arm of 10-12mm. For rudder we want a bit more throw, so 14-16mm works well. These dimensions should give all the throw you'll ever need. You should then use travel adjust (endpoint) on your transmitter to give the throws shown in the manual for the starting points. For me this meant setting throw at about 80%, but your results could vary.

Note that for the Polaris the horn place the hole about 10mm from the surface, giving a total effective horn length of about 16mm. So you need a servo arm of 8-10mm.

Please use these dimensions only as a guide in doing your own setup as not only may the mechanical arrangements differ, but different radios and different types of servo have different angular throws.

I also wanted to ensure that the ailerons had equal throw in both directions. The usual setup with a straight servo arm and angled push rods means that they have more down than up, i.e., negative differential. This doesn't seem to hurt the handling but I wanted to eliminate it, so I used an output wheel on the servo and set the attachment points so the "arm" is at right angles to the push rod. If you only have straight servo arms, don't worry about this point.

The servos I use in the XL are 16g type (Turnigy S3101, Hitec HS81, or similar). The kit is set up to use 9g servos, but I like a little more power for the bigger model (especially for the rudder, which takes some abuse on the ground). So I don't use the supplied servo mounting spacer. I find the aileron servo needs to be spaced one thickness of Depron scrap, while the rudder servo needs two thicknesses if it is to clear the fuselage top.

One point to note in the picture of the elevator servo. I'm very careful about elevator action, so want to minimize slop. You'll see that the holes on the arm I used were a bit big, so I used a bit of copper tube as a bushing.

Note that, the push rod bend at the aileron horn is not a Z, which would be almost impossible to remove, but a bend through the horn and then up or down. This allows you to insert and remove the push rod simply by rotating it. I Use Dubro Mini EZ-Connectors on the servo arms, as they never need to be adjusted once the basic setup is completed. On the elevator, I put the adjustable connector on the control horn where it's accessible.

One thing to make sure of, especially with the elevator servo, is that you have them set at neutral when you are installing the arm and setting things up. You must plug in the radio and ensure that trims and subtrims are neutral before installing the arm. Get it as close as possible to right angles to the push rod, and then use subtrim to fine tune. (If you forget to do this and close up the elevator servo, you could get a bad surprise when you finally turn the radio on.)

I hope this is helpful. The same principles can be applied to the smaller versions, with adjustments as necessary to allow for different length control horns.
Attached Thumbnails
  • 11mm servo arm. EZ-Connector is used on control horn for adjustment. 152.5 KB Views: 44
  • 15mm rudder arm, 11mm aileron arms. Note offset position of aileron push rod EZ-Connectors. 175.0 KB Views: 51
  • Push rod at control horn. Not a Z-bend but one that allows you to insert and remove by rotating the push rod (after loosening the EZ-Connector). 134.4 KB Views: 44
Last edited by Daedalus66; Jul 11, 2012 at 02:38 PM.
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Apr 16, 2015, 09:06 AM
Registered User
xhawkchief's Avatar

motor mount

where did you purchase your motor mount

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