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General Purpose Servo

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Engine Doctor28/10/2020 09:52:06
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2673 forum posts
44 photos

Whatever you decide on make sure they are not fakes/ copies. Buy from a lms or a known supplier. Futaba copies that are plain dangerous are still being sold on an online auction site, usuall prices 4 for £10 or 5 for £12 ish. They even have a Futabe name sticker that's blurred out in the pic on the site. There are now even copies of Tower pro servos ? Beware ,if the price looks too good to be true it usually is.

Nigel R28/10/2020 09:55:46
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4291 forum posts
715 photos

I'd go 422 or 425 elevator/aileron/flap, 485 on the rudder. Throttle, anything really.

 

edit: second the above about buying from a proper shop, I've been stung with a few fake Hitecs

Edited By Nigel R on 28/10/2020 09:57:29

John Wagg28/10/2020 10:19:03
139 forum posts
22 photos
Posted by Engine Doctor on 28/10/2020 09:52:06:

Whatever you decide on make sure they are not fakes/ copies. Buy from a lms or a known supplier. Futaba copies that are plain dangerous are still being sold on an online auction site, usuall prices 4 for £10 or 5 for £12 ish. They even have a Futabe name sticker that's blurred out in the pic on the site. There are now even copies of Tower pro servos ? Beware ,if the price looks too good to be true it usually is.

+1 - I recently came back to RC and bought some Futaba 3003 (?) of Ebay. They work but the centering is very poor and are noisy. The same for some mini servos which I replaced with some from 4-MAX. The 4-MAX ones centre spot on and cost only a few pence more.

Lesson learnt.blush

Martin Harris28/10/2020 12:07:24
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9596 forum posts
258 photos
Posted by Doc Marten on 28/10/2020 09:21:33:

I've only had 2 Corona servos and the mounts crumbled on both when I was using them to drive the ailerons. I also found them very noisy.

I now stick with genuine Futaba 3001.

I do tend to use 3001s on most models - short of larger high speed aerobatic models with large control surfaces they seem quite adequate for average models, despte their modest claimed output torque and probably more importantly, they have been very reliable.

However, my Coronas have worked very well too and I actually reclaimed some with failed lugs by aralditing some aluminium mounts to their cases:

img_1131[1].jpg

P.S. Regarding mounting errors, how many people put their metal "top hat" spacers in the wrong way up i.e. with the screw head contacting the "brim"? Almost every second hand model I've encountered has them this way up allowing the spacer to bite into the mounting surface, ruining the controlled pre-loading of the rubber mount!

Edited By Martin Harris on 28/10/2020 12:10:56

Nigel R28/10/2020 12:28:26
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4291 forum posts
715 photos

"despte their modest claimed output torque and probably more importantly, they have been very reliable."

I believe Futaba's torque claims (and the rest of their specs, too). Many, how should I say, cheaper brands? I do not.

Matt Carlton28/10/2020 14:12:42
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131 forum posts
12 photos

It's also worth pointing out that the published torque figures may not be what you get, depending on the geometry of the control horn and servo horn.

 

Edited By Matt Carlton on 28/10/2020 14:13:01

Doc Marten28/10/2020 14:25:49
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1005 forum posts
19 photos
Posted by Martin Harris on 28/10/2020 12:07:24:
Posted by Doc Marten on 28/10/2020 09:21:33:

I've only had 2 Corona servos and the mounts crumbled on both when I was using them to drive the ailerons. I also found them very noisy.

I now stick with genuine Futaba 3001.

I do tend to use 3001s on most models - short of larger high speed aerobatic models with large control surfaces they seem quite adequate for average models, despte their modest claimed output torque and probably more importantly, they have been very reliable.

However, my Coronas have worked very well too and I actually reclaimed some with failed lugs by aralditing some aluminium mounts to their cases:

img_1131[1].jpg

P.S. Regarding mounting errors, how many people put their metal "top hat" spacers in the wrong way up i.e. with the screw head contacting the "brim"? Almost every second hand model I've encountered has them this way up allowing the spacer to bite into the mounting surface, ruining the controlled pre-loading of the rubber mount!

Edited By Martin Harris on 28/10/2020 12:10:56

That is a very neat, functional 'bling up' you've done there Martin, one to go in the reference bank for me.

I also mount the copper grommet tubes (whatever they're called) with the collar down, against the mount which seems incorrect but isn't, the other way around allows the grommet to be over 'squished' by being forced into the mount.

Ron Gray28/10/2020 14:43:40
2459 forum posts
996 photos

Liking the ally plates there Martin, another excuse for firing up the 3D printer!

Bob Cotsford28/10/2020 15:18:43
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8847 forum posts
496 photos

I found that the Corona 339 body will fit into the lower case of most 'standard' sized servos. As I have a habit of hoarding damaged/reclaimed odds'n'sods of course I had a couple of 148 style cases floating around so I glued the lugless servos into those.

3001/148 HS425 size should be fine for a .60 size model though I'd probably look for a 5kg.cm metal gear servo for the rudder, especially if it's driving the tailwheel too.  That's just me though!

Edited By Bob Cotsford on 28/10/2020 15:21:32

Frank Skilbeck28/10/2020 16:20:29
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4871 forum posts
110 photos
Posted by Matt Carlton on 28/10/2020 14:12:42:

It's also worth pointing out that the published torque figures may not be what you get, depending on the geometry of the control horn and servo horn.

Edited By Matt Carlton on 28/10/2020 14:13:01

The servo torque figure is the rotational force generated at the servo output, yes the actual linear force and control surface torque will vary with the control and servo horn geometry but the servo torque is unchanged.

Matt Carlton28/10/2020 22:20:12
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131 forum posts
12 photos

Frank - That's what I meant really. A novice could assume that a 3kg torque figure will translate to a 3kg force applied to the control surface, so as a word of caution, just because a servo spec states "3kg torque", care must be taken with horn lengths etc to make the best use of the torque available.

Edited By Matt Carlton on 28/10/2020 22:20:58

Ace29/10/2020 09:59:40
349 forum posts
23 photos

Correct me if I am wrong but all torque figures quoted are at just 1cm from the center spline. Increase that to 2cm and it halves the torque available, increase again to 4cm and it halves again. So a 3kg at 1cm = 1.5kg at 2cm and at 4cm just 750g

Nigel R29/10/2020 10:15:05
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4291 forum posts
715 photos

Torque is specified in that way, yes. Nothing unusual. As we (normally) hook the servo up via a rod to a surface which also rotates about a hinge, it kind of all comes out in the wash.

kc29/10/2020 10:31:31
6780 forum posts
174 photos

An article about servo and servo arms by Dave Burton appeared in RCME June 2012 page 48, if you want to know more about the force on control surface etc. Should be in the archives.

Really just a list of what servos would be suitable for which type of model is all we really need.

Edited By kc on 29/10/2020 10:59:14

Dickw29/10/2020 10:33:25
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788 forum posts
103 photos
Posted by Ace on 29/10/2020 09:59:40:

Correct me if I am wrong but all torque figures quoted are at just 1cm from the center spline. Increase that to 2cm and it halves the torque available, increase again to 4cm and it halves again. So a 3kg at 1cm = 1.5kg at 2cm and at 4cm just 750g

to be completely accurate, and as Frank said earlier, torques is a rotational force at the servo shaft and doesn't change with distance. What does change is the linear force it produces at the end of the servo arm. Torque is specified as force times distance (e.g. kg.cm) so as the distance increases the linear force (push/pull) available will decrease, but the torque remains the same.

As you say, the linear force for any length of servo arm can be calculated from the torque figure but don't forget the effective length of the servo arm will change as the servo rotates.

Dick

Ace29/10/2020 10:34:43
349 forum posts
23 photos

So providing the pivot distance at the servo and hinge are the same = status quo. However if either are varied a +/- will be introduced? eg, 1cm at the servo with 2cm at the pivot horn would double the force applied with a penalty of only 1/2 the distance moved. (geared 2:1)

Ok Dickw clarified wile typing thumbs up

Edited By Ace on 29/10/2020 10:36:29

Doctor Chinnery29/10/2020 12:03:21
90 forum posts

And from a practical point of view Y-O-Y-O-Y :-

Why can't servo manufacturers agree on a standard diameter and number of splines on the drive shaft thus making all servo arms interchangeable between similar sized servos?

Frank Skilbeck29/10/2020 12:14:11
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4871 forum posts
110 photos
Posted by Doctor Chinnery on 29/10/2020 12:03:21:

And from a practical point of view Y-O-Y-O-Y :-

Why can't servo manufacturers agree on a standard diameter and number of splines on the drive shaft thus making all servo arms interchangeable between similar sized servos?

+1 for that, I have stacks of servo arms from various servo manufacturers and even then I sometimes can't find one that fits!

Nigel R29/10/2020 13:29:02
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4291 forum posts
715 photos

"Really just a list of what servos would be suitable for which type of model is all we really need."

FYI Hitec provide a recommendation on their individual servo info pages.

from e.g. HS-311 info page

Radio Control Applications
Parkflyer N / A
Sport Plane 50 - 64 oz.
Sailplane N / A
3D Performance N / A
Scale 50 - 64 oz.
Electric Helicopter N / A
Gas Helicopter N / A
On Road 1/8th
Monster Truck 1/8th
Buggies 1/8th
Truggies 1/8th
Short Course / Truck 1/8th
Crawlers 1/8th
Sail Boat < 1 meter
Power Boat 1/10th
Geoff S29/10/2020 14:18:17
3907 forum posts
62 photos

That Hitec information is vague hardly useful if they don't also say what function the servo takes. eg if a HS-311 is used on a 1 meter (sic) sailboat is it as rudder or as a sail winch - functions needing very different torques?

I suspect many of us (me included) over spec our servo requiremnts just to be on the safe side. I tend to prefer flying gentle aeros etc and make very limited demands on my models (I learned to fly late in life) but others flying extreme 3D make heavier ones. It's amazing how well the small, cheap 9gm servos supplied ready fitted to a lot of medium sized foamies flying on 2200 maH LiPos stand up to use. Most failures I've had are damage to plastic gears due to 'ground' or operator handling and for that reson try to go for metal gears and ball bearings where I can.

geoff

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