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05-20-2020 03:37 AM  7 days ago
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helitom

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Pine Grove, Calif, USA

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Servo Failure
I fly Excel and Century machines. I have a Fury Expert that is all stock with the original FB head that has a gazillion flights on it. It started life with my trusty Futaba 9Z and has been moved upwards over the years and now is flown with my Futaba 18MZ. Anyway, this is not a "once-in-a-while machine"; it always goes with me to the field in case there is a problem with the other machines that came along for the ride or I just want to fly something that has no issues.

Two flights ago, just before the Covid stay-at-home, I became a spectator when it rolled over and went in. I did all the repairs and tinkered with other stuff while waiting for our flying field to reopen. And, reopen it did. On the day of reopening, I took ol' faithful out. On the very first flight I once again became a spectator as it rolled over and went in.

After making all the repairs - again - I got out the transmitter (in my shop) and turned it on. This is my standard drill when repairing a crashed heli to make sure everything moves as it should and the amount of movement is about the same as it was before the crash. I turned on the receiver and check the controls. All looked and worked just as I expected.

At this point I got distracted by something - the shop TV, a deer heading for the flower box, one of the feral cats telling me the food bowl is empty. Anyway, I was looking at something else when I got a glimpse of the pitch cyclic servo moving to full stop in one direction. I moved the transmitter ail/elev stick around and the servo returned back to neutral. WTFO!! I sat there for another couple of minutes and it happened again. Whenever I waited a few minutes the full travel without command input repeated without fail.

I replaced the servo and fooled with the heli for another hour or so without the problem recurring. So I, by pure accident, found the reason for the two previous crashes. Of course, a new servo fixed it. But, I have been doing models and RC longer than lotsa folks on this forum have been alive and have never seen a servo failure of this sort. I took the servo apart looking for a broken wire, a burnt or damaged component on the circuit board, or anything else that could explain this failure. I couldn't see anything wrong.

Oh well, another life lesson - sh*t happens..........
The older I get, the fewer things seem worth waiting in line for.
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05-20-2020 11:23 AM  7 days ago
Rojoalfa

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Punta de Mata, Monagas-Venezuela

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helitom I took the servo apart looking for a broken wire, a burnt or damaged component on the circuit board, or anything else that could explain this failure. I couldn't see anything wrong.
/
/

I don't understand.

The servo is bad or not??

You need to test it apart to see if it is working OK.

Either way, The problem is not solve and remain onboard...

/
Saludos cordiales,
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05-20-2020 12:31 PM  7 days ago
KJH

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Interlochen, MI, U.S.A.

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He said he replaced the servo, in the model. Upon inspection of the problem servo, he found nothing obvious. Problem servo is no longer in the model. Tom, I guess all servos have a "Do Not Use" beyond this date. The trick is to find that date before bad things happen. Sounds like you have this problem resolved.-Kevin
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05-20-2020 05:43 PM  6 days ago
helitom

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Pine Grove, Calif, USA

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Test flight is tomorrow. Hope this machine doesn't go three for three.

Servos are like birth certificates, no labeled expiration date.
The older I get, the fewer things seem worth waiting in line for.
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05-20-2020 08:06 PM  6 days ago
Rojoalfa

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Punta de Mata, Monagas-Venezuela

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/
/

Ok... Servo is dead. Dead meat.

All servos are 4.8 volts, 6 volts, HV?
Your battery is according to servos spec.

/
Saludos cordiales,
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05-20-2020 08:13 PM  6 days ago
dschertz

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Jackson, MI

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Rojoalfa All servos are 4.8 volts, 6 volts, HV?
Your battery is according to servos spec.
Seriously, what are you talking about?
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05-20-2020 09:22 PM  6 days ago
rcheli808

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Mililani,Hi-USA

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Well,I can understand your frustration truly. A while back my newly put together 700 gasser rolled over on its own during a five foot hover. After days of pondering why this happen, I accidentally found the problem. It ended up being the after market extension wire connector not being crimped at all to the servo wire.
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05-21-2020 04:28 PM  6 days ago
Rojoalfa

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Punta de Mata, Monagas-Venezuela

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dschertz Seriously, what are you talking about?
/
/

Can't be more serious!!!

One of my friend had similar problem to find out that rudder servo was 4.8 volts in a system with 6.0 volts.

He use no step down, nothing.

So, he kill the servo on the bench, then installed a new one and at the end, kill the trex heli while flying...

... End of story.

/
Saludos cordiales,
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05-21-2020 06:18 PM  5 days ago
helitom

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Pine Grove, Calif, USA

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I believe I know the rudder servos to which you refer; they are the Futaba 9251/56 which are indeed very sensitive to voltage.

However my failed servo was a cyclic servo and the system is run on regulated voltage of 5.1V which I check on often. Before the LiPo/voltage regulator the heli flew with a NiCad pack which typically was 5.2/5.3V hot off the charger.

Over the years I have had my share of servos failures. I just have never seen/heard of a failure mode like this - the servo randomly going full travel in one direction without transmitter input.
The older I get, the fewer things seem worth waiting in line for.
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05-23-2020 07:46 AM  4 days ago
wjvail

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Meridian, Mississippi

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helitom I just have never seen/heard of a failure mode like this - the servo randomly going full travel in one direction without transmitter input.
Servos used to fail like that a lot more than they do today. Digital servos respond differently when failing than analog servos and PCM (and now 2.4) control changed the way servos malfunction. Years ago when AM and analog servos were king, chattering and random full-over control seemed like a weekly event. (Having registered here in 2001, you likely knew all that...)

So your failure is a little odd in today's world. Did you look carefully at the servo lead? Is it possible there was a cut or chaffing wire on your failing servo?

I noticed you haven't mentioned what servos you were using. Some servos have been more prone to failure than others. JR 8411s were so failure prone clubs were banning their use and JR recalled them for upgrade and service. By contrast I've never heard of a Futaba 9252 failing. I'm slightly embarrassed to admit this, but I looked the other day and the throttle servo in my Raptor .50 Titian (a 9252) now has over 60 hours on it. Might be time to swap that one out.
"Well, nothing bad can happen now."
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05-23-2020 10:38 PM  3 days ago
helitom

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Pine Grove, Calif, USA

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Several successful flights in the last few days. Hoo-Rah!!The older I get, the fewer things seem worth waiting in line for.
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05-24-2020 01:08 AM  3 days ago
jbjones

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Columbus, Mississippi

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helitom Several successful flights in the last few days. Hoo-Rah!!
Alright! Congrats.
J. B. Jones
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05-24-2020 05:11 AM  3 days ago
heli4fun

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TRUMP, USA

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dschertz Seriously, what are you talking about?

Yeah,there are times i can't "Decipher" what the hell he's talking about!

Maybe he should work on his "English" rather than his helicopters...
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05-24-2020 06:27 AM  3 days ago
JLF92677

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Southern California

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helitom ...the pitch cyclic servo moving to full stop in one direction. I moved the transmitter ail/elev stick around and the servo returned back to neutral.
I experienced exactly the same failure mode back in Feb with a digital servo. As you described, it was also an intermittent failure condition and because it was on FBL heli the FBL controller adjusted the other 2 cyclic servos for attitude change causing the heli to drop about 15 feet from a 25-30 foot hover, but to my luck it suddenly stabilized. I landed immediately and checked out the heli, but was not able to replicate the condition until I got back to the shop. Removed the bad servo and sent it in for repair. However, I was more interested to know what caused the intermittent failure than having the servo repaired. At first, the repair center thought it was bad pot. But it was not, it was an intermittent failing FET (Field Effect Transistor) in the servo control circuit.

Watch attached video right most servo to swash link stalling, restarting and jumping while moving the cyclic joystick in 360 motion.

Watch at YouTube

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