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Post by Dillzio on Feb 14, 2011 17:46:05 GMT 1
Hi all, had a rather unpleasant experience during my last flight, one of my aileron servos died. Rather than dying in the neutral position, which would have left the plane fairly flyable, it dyed while one of the ailerons was all the way down, so all the plane wanted to do was spiral out of control.
Luckily it was in pretty close when it happened, and I just barely managed to bring the cub in for a 'landing'. It seems that no damage was done during the crash landing, but i'll have to keep looking to be sure. I've taken the servo out to diagnose the problem, and it seems the servo just abruptly died. There was nothing wrong with the connection, there were no wires come loose inside the servo, and as soon as I replaced the servo with another it worked fine.
The experience has got me quite worried, I frequently fly my cub really far away, and if this happened while the cub was 500m away I don't know if I even would have managed to get the thing back again. Is it a common thing for servos to just suddenly die mid flight? Has anyone else experienced it before?
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Post by pauliepilot on Feb 14, 2011 18:47:56 GMT 1
That's bad news lucky u wasn't to far away like u said...i had both rud/elv servos come out the foam that was lucky cuz i was in a dive 15ft away n 9ft high just didn't pull up...n i did think the same 5mins later,what would i have done if i was 300ft high??? Not a nice thought.
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Post by john66 on Feb 14, 2011 18:49:51 GMT 1
What Brand/model were the servos?
John
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Post by flydiver on Feb 14, 2011 19:21:22 GMT 1
I have had 1 (HXT 900) die in flight with no provocation. No prior crashes, plane was probably a year old. Other servos have gotten the 'jitters' and been retired before failing.
Sorting out servos is tough since they get used and abuse so many different ways. You find hoard of folks loving the cheap HXT 900, and the detractors saying anything but Hitec HS-55 is garbage.
I've had better luck with the HXT 900. There are just dozens of different models and they seem to be proliferating fast.
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Post by toff on Feb 14, 2011 22:18:47 GMT 1
Apart from nylon gears stripping their teeth in micro servos (SG50's - and it was my fault for being clumsy), I 've never had a problem like this at all - touch wood!
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Post by Dillzio on Feb 15, 2011 2:30:59 GMT 1
Thanks for all the responses guys!
The servos are Corona 928BBs from HobbyKing, I made a video comparing them to the HXT900s a little while ago. I've had them strip gears before or break the servo arms, but that was after crashing, they've never just died like that on me before. I made the plane in July 2010, so it's only 6 or 7 months old - wouldn't expect them to have just worn out by now.
Maybe I should work out a program of rotating servos the same way you would tyres in a car? Every six months you put a new pair of servos in for the rudder/elevator, move the elevator/rudder servos to the ailerons, and move the aileron servos to the flaps?
One other note on the quality of the servos, it may have been an isolated incident, but one time one of the servos arrived DOA. One of the wires had been 'fractured' where it was stripped, and had snapped off the circuit board. I had to strip, tin then resolder the wire back on but after that it worked fine.
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Post by flydiver on Feb 15, 2011 4:19:50 GMT 1
Like lots of electronics in this sport (ESC, BEC, brushless motors) there is more going on there than initially meets the eye.
There is a mechanical device (motor and gears) hooked up to an electronic device that has a bunch of parts.
A critical component is the potentiometer (POT). It is kind of like a phonograph needle on a disk. The needle feeds back the position on the disk. This is how it knows where it is and where to be/go depending on input from the TX-a feedback mechanism. This mostly is physical/analogue (there are some new magnetic POTs that have recently come out).
In cheap servos this POT can be pretty lame. The mechanism also 'centers' (goes back to home/neutral) to the same spot all the time. So "home" starts to wear out. Usually this is the source of jittering around home (no input). The servo will be fine in action but hunt around with no input because the input area is getting worn out.
Coupled with all that is a mini-brain to deal with all the input/output. So, those tiny little devices are pretty complex.
Why does one just up and die without stripping gears (crash) or showing signs of wear by jittering? I really have no good idea.
Some servos as a statistical group seem to just be designed better and have less problems than others. But in any given group you probably can have a failure.
Case in point. I got a slope plane called a Weasel. It is pretty much the top of it's class for small slope action. The selling website recommends Blue-Bird BMS-380MG servos (~ $15) -metal gear. I have been sloping a small benign hill for a year with the same HXT 900 and had LOTS and LOTs of crashes so chose to get the slightly cheaper non-metal gear Blue-Birds. One crash stripped one. So I got metal gears to replace the stripped gears only to find out that the servo itself was bad. Turned out to have garbage in the motor.
Conclusion - I think Blue-Bird servos are expensive and crap.
Most folks conclusion - Blue-Birds are great servos. Very few people using that plane with those servos have anything bad to say about them.
That's a long way around saying this is a difficult area to sort out. When you decided to go with the Corona 928BB I was curious and watching. I continue to watch. There are a BUNCH of Coronas. Are some good, some bad? I don't know but would like to. Congratulations, you've just become one more data point.
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Post by ginginho on Feb 15, 2011 10:14:45 GMT 1
[snipped long way round] That's a long way around saying this is a difficult area to sort out. When you decided to go with the Corona 928BB I was curious and watching. I continue to watch. There are a BUNCH of Coronas. Are some good, some bad? I don't know but would like to. Congratulations, you've just become one more data point. Me too as I was the one who suggested trying them out.  I'll stick to these as the servo of choice for this sort of size/price bracket. They certainly are better than the TP-SG90/HXT900's that I have. Dillz, if you are flying a long way out regularly and are concerned about servo reliability, perhaps you should consider fitting a premium brand: Hi-Tec 55/56/65, MPX Nano S, JR NES375, Futaba S3113 etc. Of course you'll pay a high price for peace of mind. |
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Post by geoff2474 on Feb 15, 2011 11:05:44 GMT 1
Hi Guys
While we are on the topic of servos. Is it OK to mount servos across the plane instead of length ways on? It just so happens that the servos I have will fit just right this way on. I can't see it as a problem. But then I know nothing.
Geoff
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Post by john66 on Feb 15, 2011 12:06:24 GMT 1
[snipped long way round] That's a long way around saying this is a difficult area to sort out. When you decided to go with the Corona 928BB I was curious and watching. I continue to watch. There are a BUNCH of Coronas. Are some good, some bad? I don't know but would like to. Congratulations, you've just become one more data point. Me too as I was the one who suggested trying them out. I'll stick to these as the servo of choice for this sort of size/price bracket. They certainly are better than the TP-SG90/HXT900's that I have. Dillz, if you are flying a long way out regularly and are concerned about servo reliability, perhaps you should consider fitting a premium brand: Hi-Tec 55/56/65, MPX Nano S, JR NES375, Futaba S3113 etc. Of course you'll pay a high price for peace of mind. Hmmm I had a TP SG90 that bought my plane down I believe, on inspection it had a throw of about 270 degrees. Whens all said and done and our modified planes are up there having cost prob about £300, and thats without counting all the hours put into it. I think I would like some servos I can rely on. Im considering MG ones, as one of the main probs seems to be that as a relative newb with the associated flying/landing skills, that the abuse nylon gears take they strip quite easy. Can anyone recommend any, I understand that this is very expensive but nonetheless I would be gutted if I lost my plane due to a £2.60 servo. John |
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Post by john66 on Feb 15, 2011 12:14:33 GMT 1
Hi Guys While we are on the topic of servos. Is it OK to mount servos across the plane instead of length ways on? It just so happens that the servos I have will fit just right this way on. I can't see it as a problem. But then I know nothing. Geoff I too wouldnt have thought that was a prob if you have enough room in the fuse for the arms to be placed accordingly but with two Im not so sure that would work out, space wise, hmmm  John |
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Post by john66 on Feb 15, 2011 12:23:26 GMT 1
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Post by ginginho on Feb 15, 2011 14:26:03 GMT 1
Hi Guys While we are on the topic of servos. Is it OK to mount servos across the plane instead of length ways on? It just so happens that the servos I have will fit just right this way on. I can't see it as a problem. But then I know nothing. Geoff Geoff, the main thing that you need to take into account is that the arm at rest (i.e. midpoint) is at (or as near to) 90 degrees to the body of the servo. This ensures that the effort to move the pushrod (and hence the control surface) is not exasperated. That's my understanding of servo installation, anyway. ;D |
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Post by ginginho on Feb 15, 2011 14:29:44 GMT 1
John, No personal experience but I have read a fair few posts on forums that rate Savox servos. BTW, you can get them a bit cheaper at Steve Webb's site (www.servoshop.co.uk) and a large enough selection of different servos to make your head hurt and your eye to water. |
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Post by john66 on Feb 15, 2011 14:35:15 GMT 1
BTW, you can get them a bit cheaper at Steve Webb's site (www.servoshop.co.uk) and a large enough selection of different servos to make your head hurt and your eye to water. Cheers for the heads up Nick, half a servo cheaper for four, cool  Thanks, John |
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