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Throttle servos

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Old 12-09-2005, 02:15 AM
  #1  
Brian Soltis
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Default Throttle servos

I'm not sure how many times this has been asked but I am curious if there is any reason to use a standard servo for the throttle. It seems to me that a micro servo would be fine as I can't imagine there being any reason to need the torque that the "average-sized" servos have.
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Old 12-09-2005, 03:05 AM
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flyierjon
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Default RE: Throttle servos

Hi Brian, Depending on what size airplane your flying it is ok. If its any bigger than a 40 size airplane i would say go with a standard servo but i have used mini servos for throttle before in my 40 size airplanes and smaller. And what kind of push rod are you useing? Sometimes the type of push rod matters if your going with a cable pushrod rather than a 4-40 metal rod.
Hope this helps!
Jon
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Old 12-09-2005, 08:10 AM
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Default RE: Throttle servos

I am curious if there is any reason to use a standard servo for the throttle.
There actually isn't any compelling reason at all to use any particular size servo over any other. The only really important consideration that matters is what was just mentioned, how hard is it to move the pushrod when it's connected to the carb.

Once upon a time, mini servos were a new deal and cost more than standard ones. There certainly isn't a good reason to use a more expensive servo even if it'll do the job and you don't need to save weight. So there wasn't much reason for anyone to suggest it back then. And right now, almost nobody seems to worry with the idea because, except for the lightly powered wet models and the electrics, there isn't a compelling reason.

But if you read a lot of magazine articles about models that were plan built or kits, back before the magazines went almost entirely to ARF reviews, you did see some authors/designers mention using lighter servos for the task to save weight. But they didn't make a big deal about it.

And with the bigger models that're the rage these days, weight means little to most. And truth is, the idea to use a heavy throttle servo is just as useful to balancing most models if you're smart enough to ignore the designed location on the servo tray (that's most often behind the CG) and move the sucker up front, nearer the engine and in front of the CG. Since getting back into the hobby, I've stuck together 5 ARFs, and every one had the throttle servo back on the tray with the rest and the tray behind the CG. I put that servo just in front of the CG on all of them, and every one was lighter for it.

Nowadays, with ARFs being what a bunch of the flyers have flown since they started, nobody even seems to think about such an idea as using one lighter than the rest. Well, except a few of the sharper electric writers. But then, most of the electrics are using ALL micros. And probably fewer still would think of putting a servo somewhere other than where the ARF designer has it.

Hey, your idea is sensible and solid. It just isn't widely thought of. And with mini's as cheap as standards, and some cheap BIG dudes out there, it just makes good sense to use what does what needs to be done. Mini's will drive any throttle I've ever seen that didn't have a poorly installed or designed connection system and a maxi in a big dude that's probably going to come out tail heavy can help you solve that CG with less added dead weight.
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Old 12-09-2005, 01:33 PM
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Default RE: Throttle servos

Use the micro. I have used a HS-55 and HS-81 as throttle. No friction to speak of and so long as you don't have any cable binding, a small servo is good to use.
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Old 12-10-2005, 09:13 PM
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Default RE: Throttle servos

WEIGHT IS EVERYTHING!

I use digital micro throttle servos, futaba 9203 I think? But I fly bigger airplanes. Weight it everything, even big airplanes, the lighter they are, the better they fly. I can't emphasize how much weight matters!
Back on the subject, on my smaller models where I don't need the precision I use HS-81's, and even they are a little big.

Coffeegrinder

(anything to save weight)
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Old 12-11-2005, 10:28 AM
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Default RE: Throttle servos


ORIGINAL: darock


And with the bigger models that're the rage these days, weight means little to most.
I was going to say, you obviously haven't been reading the GP Ultimate thread. Then coffegrinder piped up. This obsession with shaving weight has really gone out of the box, IMO. Sure, it is a good idea. But when a fellow talks about using mini-servos on the ailerons of a 15-16 pound bipe, just save a quarter-ounce.. sorry, that's gone beyond obsessive and into the realm of idiotic.

Brian, there IS one downside to using mini-servos on throttle. If you use a buddy-box for training, a mini-servo will usually NOT respond identically to the same ATV/endpoint settings of a "regular" servo on two different radios. IOW, when you give control to the student, throttle response is now different.

This has happened to me twice, now, and is a real PITA. Other than that, I see no issues.

But I have always wondered WHY the manufacturers will not offer a moderately-priced, light, low-torque, HIGH-SPEED servo strictly for throttle use??? Such would sell like hotcakes. I'd buy one for every model.
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Old 12-11-2005, 11:00 AM
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daven
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Default RE: Throttle servos

Personally I wouldn't use a micro servo, but quite often use minis.

The hitec 80, 81, and 85 all work great for me.
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Old 12-11-2005, 03:39 PM
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Default RE: Throttle servos

HT 5245 mini digital is one that I like for throttle. 72 oz is plenty and it's MG.
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Old 12-11-2005, 06:50 PM
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Default RE: Throttle servos

I've used some HiTec HS81s for throttle servos with no problems. The one thing I worry about when using minis on throttles is the gear train. In some cases (like in a twin) the throttle servo is very close to the engine, and sometimes linked by a metal control rod. I worry that the vibration from the engine might wreak havoc on the little tiny plastic gear teeth in a mini.

On big four-strokes, I make a "servo saver" arrangment with a couple of small wheel collars and some fuel tubing to isolate the servo from the big vibes. It might not be necessary, but it makes me feel better...

Good flying,
desmobob


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Old 12-12-2005, 03:50 AM
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Default RE: Throttle servos

Hello guys,

I'm too concerned about the vibration with small gears issue, but the indirect drive system can help here: with it, the servo pots is isolated from direct vibration, at least... All the good servos have indirect drive, but the minis?

Hitec 225 are indirect drive, while HS-81 and HS-55 are not. They are a bit heavier though, being 27g against 17g of the HS-81 and 8g of the HS-55. Also, here in Europe Robbe makes a very fast lightweight and ball-beared mini servo with indirect drive, the FS 501 BB.
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Old 12-12-2005, 09:11 AM
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Default RE: Throttle servos

On larger planes (Gas) the small servos do not stand up to the vibration. I have tried three times & have had two HS-225's fail after about 30-50 flights. This is not a gear failure but a burnt out servo motor. The HS-81-85 would almost guarantee a failure in this size plane.
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Old 12-12-2005, 10:42 PM
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Default RE: Throttle servos

I've had several hs81mg develop weird twitching problems after 30 flights on four strokes (saito 82). As much as I hate to add the weight I am using a standard this time.
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Old 12-12-2005, 11:18 PM
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4 stroken ron
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Default RE: Throttle servos

Desmobob: I agree with you. I have an OMP 65" YAK profile with a SAITO 180. The first picture shows how I orginally had the servo linkage mounted and the second shows how I changed it.
Ron
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Old 12-13-2005, 12:24 AM
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Default RE: Throttle servos

Ron,

That looks like something I'd like to try. It's kind of hard to see. Is it just a piece of fuel tubing connecting the two ends without anything inside the tubing?

So obviously it never falls off, comes loose, slips, or is otherwise "soft". Do you put a drop of CA on it or anything or just the tubing?

Balin
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Old 12-13-2005, 09:24 AM
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4 stroken ron
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Default RE: Throttle servos

What I had there is ball links on both ends of a 2/56 rod. I thought that might tranfere too much vibration from the big pounding SAITO to the small fragile HiTec servo. So I took out the 2/56 rod and just slid small fuel tubing over the ball links. It is a very tight fit. Does not slip. No more vibration to the servo. I did not use any CA, but it won't hurt.
Ron
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