Calculating a rudder servo?

[FighterCat57]

Any sort of guide for sizing up a rudder servo?

This one's for a 41" cat, I've been curious for some time about how to calculate the right servo...

[sailr]

High Torque, metal Gear. The Hitec HS645-MG is my servo of choice.

[Fluid]

The 645 works fine, but for a stronger self-centering action go with the digital 5645 like here: http://servocity.com/html/hs-5645mg_digital_torque.html

There is a $15 premium for the digital servo, but they will hold much stronger on center. I had handling problems with a non-digital servo in my F-41 Stryker cat until I went digital.


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[FighterCat57]

Digital Metal Gear across the board without a doubt.

What I'm curious about is the OZ/in estimation by rudder/hull size?

[Fluid]

It's more complex than that, it includes the speed and hull weight too. Anything over 150 oxxin should be enough for most 10S FE boats. Always use 5 cells of at least 1200 mAh - weak cells will not power the servo and it will not reach its rated performance.



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[FighterCat57]

It's more complex than that, it includes the speed and hull weight too. Anything over 150 oxxin should be enough for most 10S FE boats. Always use 5 cells of at least 1200 mAh - weak cells will not power the servo and it will not reach its rated performance.



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That's what I'm looking for. What are the complicating factors?

AFAIK the determining factors are;
Rudder size
Hull size / Type
Speed

I just don't know the calculations to come up with oz/in req's. I'm trying to come up with some guidelines to follow.

[properchopper]

I've been through about a half-dozen HS 645MG's ( @ 34-40 bucks ea.) and a 5645MG on my OPC Tunnel [which , due to the extended outdrive kicks back hard on the servo, stripping the metal gear]. The rebuild kit is & 18.00 and the metal gear that always strips is literally as thin as a piece of paper.

Then I started using these :

EXI D-226F
180 0z/in
Digital
Coreless
ball bearing
metal gears [big ones]
centers like a guillotine

Here's the kick : HobbyPartz - $ 14.70 - free shipping !!

Now my tunnel pulls out of turns & goes straight with no bobbles & my P-Mono with a big rudder loves it.

[FighterCat57]

Well... I'm choosing between 10kg (140 OZ/In, 14kg (190 OZ/in) and 24kg (350 OZ/in)

It sounds like just use the one with the most torque. I use the 10kg servos in all the 36" and 32" cats without trouble. I use the 24kg (1/5th scale) servo in the big gasser. Guess I'll use the 24kg DMG in the Sprint Cat.

[sailr]

After removing the o-ring, do you still need a minimum 3amp bec?

[Fluid]

According to what guys flying BIG planes say, this servo draws a lot of amps even without the o-ring. Drawing a lot of amps does not mean the servo is "powerful", it may just be inefficient. These may be decent sport servos, although some buyers have noted that they did not have all metal gears, but rather a mix of metal and plastic. Most users recommend removing the o-ring and greasing the gears.




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[properchopper]

Inside of D226F. The only non-metal gear doesn't absorb any counterforce on the servo arm rotating mechanism - if I'm reading it correctly it is just a position-indicating gear. The O-ring on this one doesn't create what seems like a significant amount of drag on the shaft as far as I can tell, and it's job water-sealing the shaft is something I'd like to retain.

Recently one of mine quit on my tunnel (which is rough on servos). I wrote HobbyPartz for a RMA. I got a nice response and TWO new servos, N/C, no questions asked.