I had my throttle servo failed(just stopped working fortunately on the ground) on a used plane I bought. Had a HS635HB. Not much experience with gas engines so I am wondering what servo I should use? How much torque? Metal gear, karbonite, etc. Any tips appreciated. I do not want do overkill, but do not want to undersize it either.

I have had 3 good years so far from hitech 645mg and 5645mg, right or wrong, that's what I use and they haven't failed.

Futaba 3003, $9.00 will last a life time and all the servo you need for a throttle.

The HS-635HB is more servo than you need for any throttle. Even with the throttle spring connected the load on the servo is minimal. I wouldn't put an MG on a throttle if you gave it to me. The Karbonite gears will have less slop to begin with than a MG servo and will stay tight for a long, long time. An MG servo, not so much
Pete

Thanks for the input. I thought the 635 may be little overkill.

I have been using futaba s3004 on all my gassers and so far no failures in 8 years

635's and 645's are way overkill.

Some have brought up that the Karbonite gears are fairly brittle and that the vibration from a solid throttle link will wear them out. They suggest the nylon gear servos, like the 3003's, can do a better job with vibration.

Others will point out that a servo with very little backlash and deadband are critical for repeatable idle settings.

Then there are the people who will invest big bucks in a throttle servo and install a sloppy linkage.

Here are the bullet points:
<ul>[*]You don't need a lot of torque. Cheap standard servos have plenty of torque.[*]Sloppy linkages negate your investment in expensive digital servos[*]Vibration is a factor to consider (I know plenty of people who have not had any problems with Karbonite geared servos). [*]If you want a dependable idle you want a servo with very little backlash and relatively little deadband (in other words, very repeatable positioning). But your financial investment must be matched, or exceeded, by your mechanical setup.[/list]Paul<br type="_moz"/>

Most any servo should work. The trick is to not use a stiff throttle pushrod wire, as it transmits the vibrations back to the servo then. But some servos have more fragile servo gears in them too. So the guys going with metal gear servos isn't a bad idea. But a standard Futaba S3003 or S3004 servo would work fine too. Also make sure the servo is not jamming up against the stops and WOT or low speed too. If the servo stalls out at the extreme ends for too long, it tends to burn out the motor controller inside the servo.

Also the longer arm you have on carb...that makes it more easy for sevo to do its job. Any standard servo with not much free play should work fine.

What he said. Ialso get the Tower TS-53 servos when they toss them in for the Super Saver's Club and use those. The throttle is pretty low force and speed is non-critical.

Just keep the servo 12" from the magneto/ignition module.

I'm using my older servos up. Stuff that really doesn't have the ability to run a control surface on a gasser. Currently using up a group of Futaba S130's I pulled from a heli! Old stuff for sure! I'm careful with end point adjustments (no binding at either end of the travel!), and installing proper mechanical differential. Not having any throttle servo trouble at all - ever.

ON my Edge 540 I'm using a Futaa S3051HV servo on throttle. I prefer a decent digital on throttle for the better repeatable positioning of the servo and the postive effect his has on the throttle control on a gasser'carb.
In my warbids I use pretty much any std or mini analogue

Power HD HD-1501MG

I don't understand going cheap on the throttle servo and risking hundreds of dollars.?
I use the old reliable hitech 645mg on about everything.
They cost about $30, going with a lesser servo just doesn't save that much.

My experience with throttle control troubles comes from other peoples' experiences since I have not had any problems so far. I have Futaba S148 or S3003 servos for throttle. Most of what I have seen with gassers is that either the throttle linkage or the servo mounts fail, causing loss of throttle control. As a safety requirement, our club insists on having optical kill switches linked to one of the radio channels so that the engine could be shutoff during flight in case of throttle control loss.

I have always used the cheap futaba 3003's or 3004's on the throttle using nyrod or golden rod for the linkage with ball links on both ends..I have used this setup on many gas planes without any problems and using the nyrod -golden rod keeps any vibration that is caused by the engine reaching the servo..

I usually make an extended carb arm to increase the leverage of the servo, and also to match the servo arm length to the carb arm length. I don't like trying to adjust the little short Walbro throttle arms with a longer servo arm. The holes in the carb arm will usually tap out to 4-40 or 3mm threads. Then cut out a piece of aluminum the shape I want and mount it to the carb.

This is exactly my setup and these maybe cheap servos but they work great and i have never had one fail.

You really don't need to increase the leverage on a throttle servo. But it will keep your end points from being ridiculously low therby decreasing servo resolution

For me, the higher the money investment I have in a plane will in turn cause me to use higher quality components such as hardware, servos, etc. Even though high end components can fail, I just don't feel comfortable risking a high dollar plane with cheap gear. I wouldn't put $125.00 servos in a .40 trainer and I wouldn't put $25.00 servos in a $1,500.00 plane. That's how I look at it.

I wish I was the owner of the hobby shop where you buy your servos.