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Servos moving on power up?

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Keith Miles 228/06/2019 22:02:52
303 forum posts
6 photos

Is there any way to stop servos moving briefly when powering up despite surfaces being in a neutral position?

Up to now, this has not been a problem with any of my aircraft due to some free movement, in both directions, of the control surface or throttle barrel (yes, I’m an IC die-hard!).

However, I am currently assembling a model with flaps that have a hard stop in the “UP” position and are fitted with surface mounted hinges on the underside.

Because of this, it seemed sensible to do an initial set up with servo links detached to avoid putting strain, albeit brief, on the servos.

Thus far, despite extensive efforts and with both servo travels set to “0” with servos at “Flaps Up” position, the servos still briefly move slightly further “up” when Rx is switched on before returning to a null position.

Other than this issue, both servos are operating as required.

Tx is Spektrum DX9. Rx is Spektrum AR8000. Servos are Hitec HS325HB.

Whilst radio in this model is to be powered by a 2s Lipo regulated to 6 volts, previous models have all been 4.8 volt NiMH powered and, as stated, they give an initial “twitch” as well.

Is this just a feature of all analogue servos and do I need to go digital to solve the issue?

Martin Harris28/06/2019 22:11:08
9263 forum posts
245 photos

I've got a vague recollection that this behaviour has been linked to failsafe positioning? Might be worth experimenting...

Chris Walby28/06/2019 22:31:31
1199 forum posts
299 photos

Don't like the idea for anything else, but you could fit a switch in the flap servo positive wire. Then power up the RX as normal and once its bound to the TX select flaps middle position and switch power to the flap servos.

Could try it out and see if it works as a trial

PS I had a model there if you powered the RX up with he gear selected down (model on its underside) it would ripe the bell cranks and make a right mess. On solution was to have a tag plugged into the RX battery plug with "do x, y and z" with TX switches before powering the RX up...

Chris Bott - Moderator28/06/2019 23:53:41
6810 forum posts
1427 photos
1 articles

Keith how much do they move?

I set my flaps so that with servo centred I get half flap.

I mechanically set the linkages so the full up and full down flap are pretty much at either extreme of servo travel.

So unless your servos are glitching beyond normal travel range, there wouldn't be a problem, would there?

Keith Miles 229/06/2019 00:53:52
303 forum posts
6 photos

Chris Bott,

The problem is that there is no relief angle at the flap leading edge/hinge line. It is 90 degrees therefore preventing the flap from physically moving upwards, relative to the wing, beyond the zero flap position. To put it another way, with the wing inverted and servo links detached the flaps drop under gravity to what would be the “fully up” position and cannot physically drop any further. The hinges are surface mounted to the underside of flap and wing not centre mounted like ailerons, elevator or rudder. The initial “twitch” of the servo on power up, however slight, is nevertheless, therefore, going to try to apply a force to an immovable object, albeit only briefly. Foreseeing this, I elected to do the initial set up with links disconnected.


Cannot comment on “failsafe” but I suspect that this is not the issue but probably a feature of servos finding their initial position. Happy to experiment, perhaps.

Chris Walby,

Yes, I can imagine! Sounds like the same issue.

It has occurred to me, since posting, that I could power up and down with flaps down selected on the Tx switch. It just means needing to develop a routine for that model!

Thanks guys for your input!

Simon Chaddock29/06/2019 10:15:39
5692 forum posts
3026 photos


Yes the servo will 'twitch' on power up but it will not go beyond the electronic max travel, normally 125%.

If you set the linkage so the 'up' stop occurs at 125% servo travel then it would never exceed the stop. You would of course have to set the flap up travel to 125%.

Alternatively assuming the linkage was strong enough I am pretty sure a metal geared servo would not be harmed by the very brief servo "stall" that might occur if the start up twitch did try to exceed the stop limit.

Steve J29/06/2019 10:20:19
1841 forum posts
53 photos

As a general rule, Spektrum receivers move their outputs (except for channel 1) to the bind position on power up.

I have some servos which twitch on power up. It's never been an issue so I haven't done any testing.


David P Williams29/06/2019 10:35:09
902 forum posts
309 photos

I found with my Spektrum gear that if I made changes to the setup on the TX after the initial bind, then I would get the servos twitching on powerup. Rebinding when the setup is complete stopped this happening. Maybe try a re-bind and see what happens?

Keith Miles 229/06/2019 17:33:29
303 forum posts
6 photos

Thanks guys.

Some more interesting ideas, there!

The re-bind idea sounds interesting and kind of fits in with the earlier comment about “failsafe”. Might rebind an existing model to see what happens!

A club member has also suggested a means of setting the flap switch differently in the “Flap System” menu which he thinks might also work.

I was beginning to believe that the initial “twitch” was unsolvable but now I think about it, not all of my fitted servos, even those of the same type, in the same model, exhibit the same degree (excuse the pun!) of initial movement before settling so maybe that’s a clue?

Don Fry29/06/2019 17:53:22
4557 forum posts
54 photos

Steve and David, thank you. That is a puzzle solved. Some twitch, some don't.

One goes from full up flaps, to full down, then back. Spectacular, does not break anything, but it's nice to know why.

Steve J29/06/2019 19:42:09
1841 forum posts
53 photos

I usually rebinding after trimming out a model, but I still have some servos that twitch on power up.


Keith Miles 201/07/2019 15:36:24
303 forum posts
6 photos


Just rebound my Wot 4 and it might have made a difference, although I’m not absolutely certain.

All I can say for sure is that, after rebinding, the Hitec 325HB aileron servos on a Y-lead do not move at all whilst the three Futaba S3003 servos (throttle, elevator, rudder) all move just once briefly and in unison by a few degrees before settling.

Have also rebound, on the bench, the two Hitec 325HB servos intended for flap on the model under assembly (Seagull Super Chipmunk).

Again, an apparent change. Movement on power up is now much reduced with both in unison, however movements are not quite equal and, unlike their counterparts on the Wot 4 ailerons, they do still move albeit slightly.

So, rebinding does seem to have some effect but does not appear to be a definitive cure as other factors, as yet undetermined, seem to be playing a part.

As life is short, I’m beginning to favour the suggested option of powering the Rx on and off with Tx flap switch set to flaps down but I will be trying it on the bench first!

Don Fry01/07/2019 16:04:52
4557 forum posts
54 photos

If you go down that route, I recall the DX9 will support "switch in xxxxx position" warnings on startup, and won't start transmitting until the switch is correct. Warning menu in system setup?

Keith Miles 202/07/2019 00:09:46
303 forum posts
6 photos
Posted by Don Fry on 01/07/2019 16:04:52:

“If you go down that route, I recall the DX9 will support "switch in xxxxx position" warnings on startup, and won't start transmitting until the switch is correct. Warning menu in system setup?”


Don’t think I’ll be doing that, Don!

After further experiments, none successful, I gave up trying to totally eliminate the start up “twitch” on the servos and came up with a different solution to reduce its effect. One servo actually now moves slightly in the “safe” direction only while the other moves slightly more the other way first i.e. two small “twitches”.

So, to compensate, the servo arms to horn link angles have been adjusted by (a) angling the servo output arm as far as possible towards the control horn; (b) using the innermost hole on the servo arm and outermost hole on the control horn and (c) by making some further minor adjustment to sub trim. Flap travels were then set to the recommended 20mm and 30mm with flap switch set 0, 66% and 100% with timing set at 6 seconds.

The increase in link angle and the sub trim adjustment has given just enough free play to eliminate any potential excess force being applied to the servo and control horn on powering up whilst keeping free movement of the flaps to an acceptable minimum. It has also eliminated the tendency for the substantial flaps to rotate the servos with the power off and the wing the “correct” way up!

As for the model generally that’s another tale of woes which, in due course, I might just report on in another thread!

And for those still trying to visualise the flaps on this particular model, think of a drop-leaf table and you will understand why there is limited “up” travel and a defined “up” position!



PS. I did try the “flaps down”, Rx off, Tx off idea. On switching back on, servos went to flaps up and switch then  no longer did anything!


Edited By Keith Miles 2 on 02/07/2019 00:11:00

Edited By Keith Miles 2 on 02/07/2019 00:19:30

Steve J02/07/2019 07:54:22
1841 forum posts
53 photos
Posted by Don Fry on 01/07/2019 16:04:52:

I recall the DX9 will support "switch in xxxxx position" warnings on startup

I use that feature for a couple of models to make sure that I have the gear switch in the down position when I turn the transmitter on.


Keith Miles 202/07/2019 09:13:18
303 forum posts
6 photos
Posted by Steve J on 02/07/2019 07:54:22:
Posted by Don Fry on 01/07/2019 16:04:52:

I recall the DX9 will support "switch in xxxxx position" warnings on startup

I use that feature for a couple of models to make sure that I have the gear switch in the down position when I turn the transmitter on.


I have a Corsair with air retracts and currently have a piece of yellow fuel tubing on the switch to remind me! When I get time, I might check out that idea. Thanks, guys. I am now wondering, however, if only certain Rx channels have certain features. You wouldn’t, for example, need a “safe” mode for aileron, rudder or elevator would you? The only one that I am currently familiar with is the preset “throttle high” warning!

As an aside, it does seem to me that modern Tx capabilities are so vast as to warrant exclusion of detailed information and guidance in the manual with forums like this one and YouTube filling in the knowledge gaps!

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