3156 centering problem

[yellowbillycat]

So I've been scrubbing many forums on many sites and I'm at a bit of a loss....

I test flew a T-Rex 450 S CF the other night with 3 Futaba S3156 servos on the swash and had nothing but fun and excitement. Or rather terror and panic.

Seems the servos do not center well at all, they have a difference of about 5 degrees (according to the MK. I Eyeball) depending on which direction they approach center from. And this isn't deadband, backlash, or binding, this is 5 degrees PAST the center point, not short of it.

So in the air, if you pull aft cyclic to level the heli and release when level, the heli will immediately start rotating forward. Do the opposite, push forward cyclic, and the heli will rotate aft when you return the stick to center.

Don't even get me started on what this does to collective! :shock:

And this is not a balance or trim issue. You can see the centering error in the servo arms when the heli is parked on the table and idle. You can even see it if the linkages are disconnected from the servo. Happens whether you move fast or slow also.

I've only seen one other post about this issue, but I've also only seen one other post about someone using the 3156 servo, for that matter. I think that guy was 0 for 7 on 3156 servos.

So has anyone else encountered this, with the 3156 or anything else, or heard stories, anecdotal evidence, etc.?

Any advice at all? HELP!

I've contacted Futaba via email and forums. Still waiting on email reply. Forum response was mostly about checking that the load on the servo was appropriate.

Thanks,

Erik

[yellowbillycat]

Others with issues:

Here at HF:
http://www.helifreak.com/showthread.php?p=627078&highlight=s3156#post627078
http://www.helifreak.com/showthread.php?p=616272&highlight=s3156#post616272
http://www.helifreak.com/showpost.php?p=740758&postcount=2

RcUniverse:
http://www.rcuniverse.com/forum/m_7540537/anchors_7540537/mpage_1/key_3156/anchor/tm.htm#7540537

I have found some posts by happy users, but without much time on the servos.

Are there folks out there that have had some run time with the 3156 and like it?

[Don McCullough]

A few days ago I got three S3156s for collective on a T-Rex 450. Of the three, one exibited the over-drive at centering. On the largest horn it had about 1/8" overshoot -- too much to even attemp flying. My LHS is returning it to Futaba (they sold out of the last shipment). Will be interesting if it is fixed and what they say.

At least I know not all Futaba S3156 servos have this problem. The other two are really great with good solid centering and consistent travel. I bolted a Hitec HS65 horn top to top of the weak Futaba servo horns -- at least that part is solved. The stock servo horns are only good on very light weight planes, no use in any heli.

If Futaba can fix the quality problems and offer a thicker/longer horn (see the Hitec HS65) they would have a real winner in the 400-450 size heli market.

Don

[yellowbillycat]

So I sent the 3156 servos back to FuTowerCo, Inc and all 3 were determined to be faulty and we to be replaced under warranty. However, since I had already purchased and installed the stalwart hs-65mg's, I called and asked for a Tower merchandise credit which was granted.

I didn't like the servos at all, but The Hobbico Empire was both fast and accommodating in getting this resolved quickly.

[Wyn]

Interesting, I have 2 out of 3 S3156 servos with the over-centering problem. In one of them it's very minor, in the other it's very pronounced.

It seems to me like a control system with hysteresis (slop) in the feedback (potentiometer). Imagine every time the output shaft changes direction there is some lost motion that it has to "take up the slack" on, so it appears to over-travel.

I'm in the middle of putting a heli together, so I think I'll take it apart and see what I see.

[Wyn]

It's slop (hysteresis) in the feedback pot:

There's a brass shaft with milled flats on either end, one end goes into the output and the other into the feedback potentiometer. The end that goes into the output had about 5 degrees of slop.

Decided to clean it and put a drop of thick CA in the hole in the output that receives the milled flat of the brass shaft. Back together; it now centers the same from either direction. Although it seems to work fine, I'm not recommending this to anyone since any given glue may, or may not hold.

[Don McCullough]

The latest batch of S3156s seem fine. I have six now in two 450s and all are perfect. Much better responce.

Don

[juniorrex]

The latest batch of S3156s seem fine. I have six now in two 450s and all are perfect. Much better responce.

Don

I people, I have read this thread and I want to get these servos, S5065's from HITEC are to slow to arrive. This centering stuff is scarry!!! What if the hobby store has old stock??? Your up the creek!! (I can only find them states side I live in Canada :o

May I ask what most of you are using for the arms as I am comming to realize the stock are crap. Futaba rep told me (today) that the 3103 and 3107 are compatible but then again they look week also!! Any ideas?

PS. what is the spline count on these?

Thanks

JR

[chaosman]

Bought 7 S3156 over the last couple of weeks. Running absolutely perfect on my T450ers.
Using them on one with vstabi virtual flybar system with 166hz digital update rate.
No problems at all!
The servo horns suck, but hopefully I will receive HD-2213 or HD-2216 horns soon. There are reports that they are just fine as a replacement for the scrappy Futaba ones.