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HT 81/85 on 4 stroke throttle

Old 07-12-2005, 02:32 PM
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Default HT 81/85 on 4 stroke throttle

I am having 2nd thoughts about the throttle servo and would like your input please. I picked up a HT85 and when I pugged it in it went pop pop and jumped all over the place.--Did not have a servo arm on it and it was not mounted in the plane when I did it. Need to return or exchange the bad 85 to the LHS and now have a 81 in there.

As you can see the servo is just under the motor box. Only the rubber mounting grommets are contacting the servo(other than the servo arm to throttle) Linkage is direct. The motor is a SA100. I am a bit worried about the vibration to the servo.

1) What is the lightest servo you recommend for the Job?

2) Is a MG servo needed? I have herd that the MG servos wear out very fast on the larger 4 stokes?

Thanks for your great customer service. It is one reason I choose and recommend HT over others.

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Old 07-13-2005, 12:05 PM
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Default RE: HT 81/85 on 4 stroke throttle

The HS-81/85's have been used succesfully as throttle servos but I have heard of the nylon version stripping. Metal gears would wear but not that fast. The 225BB is also another option.

Old 07-13-2005, 05:05 PM
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Default RE: HT 81/85 on 4 stroke throttle

I use mini-micro servos for throttle on a lot of my planes (even the 4-strokers) and the only one that's ever stripped is the 225 (so I'd have to disagree with Mike and recommend that avoid that particular servo).

Right now I'm using a couple of HS81 servos on throttles and even have an 8-gram servo on a 4-stroker with no problems.

The secret is to add a vibration-absorbing link to the throttle linkage. This can be done by using a section of thin-guage piano wire that is formed with a series of right-angle bends to make a kind of square-wave shape. Support this at each end (so it doesn't vibrate) and it will absorb virtually all the vibration from the engine, while still transferring the push/pull needed to operate the throttle arm.

If you need pictures I'll take some tomorrow and post them if you like.

For 3D stuff I *love* the speed of the Hitec HS81 servo on a 5-cell pack and I see little point in carrying around an extra ounce or so of weight in the form of a standard-size servo for throttle use. You don't need metal gears either -- the nylon ones work just fine and are cheaper.

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