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Unexplained crash

When a model stuffs in for no obvious reason

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AVC19/06/2018 12:16:41
539 forum posts
147 photos
Posted by Don Fry on 19/06/2018 12:10:42:

I struggle to see any other cause than a loss of signal issue

I agree, although a loss of signal per se doesn't make the aileron servos to turn to the same position (i.e. banking) twice. The F/S would just hold the last position, unless is set.

That's why I keep thinking on the fail safe setup, specially in this transmitter with "dual" F/S (signal and battery)

alex nicol19/06/2018 12:36:26
245 forum posts
12 photos

the failsafe was not set, and on default. if I switch the tx off first the throttle closes, nothing else moves. I can't tell how the throttle servo was at the best of my knowledge it went in at full chat and the fuselage is mangled all the way back to the win t/e

the model has a 2 piece wing which attaches to the fuselage sides and a joining tube. The wings ripped two lumps out of the fuselage spilling the radio gear and the section from wing l/e to firewall is just mush

my initial feeling was to throw the whole lot in the bin bag, but as I was gathering the bits common sense kicked in. Admittedly I did look at the carb and the trottle was closed, but I suspect this was due to the crash as opposed to servo action

having found a servo meltdown post crash is a possible cause but I'm not 100% convinced, it could also be tx fault or external interference and it's how to eliminate these that's concerning me

AVC19/06/2018 13:00:56
539 forum posts
147 photos

I understand your concerns. I would make a simple test: discharge the battery down to 1 volt per element (assuming is NiMH), connect all servos, rx and battery, and switch it on, to see how the servos react to a low voltage. If there's no reaction, then the interference is more likely.

And sorry for being a pain, but I'm quite sure that the default position when failsafe is not set, is "hold" which means that the servos will remain in the last position. Have you tested that in your case, without setting the F/S, the throttle will close? If this is the case, then you have it set.

alex nicol19/06/2018 14:55:01
245 forum posts
12 photos


I've Just checked the flight pack voltage (Futaba battery checker) and its still showing 6.3v. with regard to the f/s if the throttle is open at all it closes on Tx power off all other servo's remain static. Upon power up they give a very quick & very small glitch, nothing that'd raise any concern.

I've got an old precedent fun-fly airframe which I'm thinking of kitting out with a known good flight set & see what happens......of there's any grief I'd need to consider looking at interference or Tx issue.

To All,

Thanks for all your contributions, certainly a lot to consider. If I find anything other than the "hot" servo I'll post it

On a separate note - how do I post a picture or create an album on here

Allan Bennett19/06/2018 20:17:33
1518 forum posts
38 photos

Many years ago I had three unexplained crashes within a period of a few months. The second model used the same receiver as the first crashed model, so I blamed the receiver (having run out of any other ideas). But the third crash was using a different receiver, so I binned the transmitter, and have never had an unexplained crash since.

The original transmitter was a 35MHz Futaba, I think a 6-channel one, so I upgraded to a Futaba 10CP with a synthesised 35MHz module.

alex nicol22/06/2018 19:28:35
245 forum posts
12 photos

Quick Question,

In preparation for Sunday's test, I'm in the process of dotting i's and crossing the t's. I have an old and somewhat tatty precedent fun fly airframe. with a known good set of gear fitted plus new battery and switch. Everything has been checked out static several times. One question is what sort of range should I get with the arial removed from the tx ( futaba 12z on 35mhz)

Denis Watkins22/06/2018 19:42:24
3675 forum posts
173 photos

Alex, you can definitely damage the innards of your TX, transmitting with no aerial

You have a load, with nowhere to go, so internal components will heat up

alex nicol22/06/2018 20:23:39
245 forum posts
12 photos

Thanks, I wasn't aware of that.

Martin Harris22/06/2018 20:42:59
8549 forum posts
214 photos

Not sure with the 12z but some Futaba transmitter aerials were connected via a screw tag at the bottom which could work loose, allowing the connector to drop off. If that was the case, it's quite conceivable that a range test could appear OK from the signal emanating from the connecting wire but erecting the aerial would effectively leave it in range test condition - fly a few yards further than the range test and ... splat!

alex nicol22/06/2018 21:08:06
245 forum posts
12 photos

I'm not sure of the physical arial connections, but I've tried aerial up and Arial down range checking. I checked aerial up at twice the distance of the crash with not a glitch......and that included the use of mobile phones for coms to ensure signal given and reaction were good......all performed perfectly. The only thing I haven't tried is range test aerial up with engine running.

At this stage my thinking is

1. the elevator servo that failed shortly after the incident

2. tx fault

3. interference

I've tried everything I can think of and can't get the fault to re occur or system failure. It's test time on Sunday afternoon so fingers crossed and praying for cause 1

but in the mean time happy to consider all suggestions

Denis Watkins23/06/2018 08:00:26
3675 forum posts
173 photos

Clubs usually have a 35mhz frequency scanner that was used through the day to confirm and check frequency's switched on, and crystal accuracy.

You could do with the use of this simple scanner as you can test channels and your flight switch for on and off

And check that other unknown users are not switching on their sets uncontrolled outside your circle

Peter Christy23/06/2018 09:36:31
1431 forum posts

Clutching at straws a bit here, as there doesn't seem to have been a definitive answer yet, but have you checked the "buddy" switch?

I ask, because I used to take a spectrum analyzer down to my old club once a year, and check the transmitters of anyone who wanted a check. One transmitter showed an intermittent loss of modulation, which we traced to the buddy switch! The switch was noisy, and would occasionally try and take its input from the buddy lead, which, of course, wasn't connected! Result: total loss of signal!

You should be able to check it fairly easily. Just waggle a stick while flicking the switch. The servo should stop when in the "buddy" position and come back immediately in the normal position. Any jittering or erratic behavior here means a dodgy switch.

Probably not the fault, but simple to check and eliminate....



Biggles' Elder Brother - Moderator23/06/2018 10:59:08
15748 forum posts
1460 photos

Hi Alex,

sorry I'm a bit late with this I've only just spotted your question on inserting photos above.

For tutorials on this go here, you'll also find and area we call "The Sanbox" there where you can test out if you have it all working OK and it doesn't matter if it all screws up!


alex nicol23/06/2018 12:09:17
245 forum posts
12 photos

I'l try and answer all 3 in the one go


we're just a small club with about 20 members. I believe there is a club scanner about and am hoping we can use it this weekend. When the incidents happened there were only 2 of us at the site and the other members gear whilst not switched on is on 84, mine is 76


Haven't checked the buddy switch, but good call, will give it a try


Thanks for the info, I'll give it a go

Denis Watkins23/06/2018 12:34:00
3675 forum posts
173 photos
Posted by alex nicol on 23/06/2018 12:09:17:

I'l try and answer all 3 in the one go


we're just a small club with about 20 members. I believe there is a club scanner about and am hoping we can use it this weekend. When the incidents happened there were only 2 of us at the site and the other members gear whilst not switched on is on 84, mine is 76


Haven't checked the buddy switch, but good call, will give it a try


Thanks for the info, I'll give it a go

Right Alex, have already posted that your even number crystals are usually apportioned to glider pilots on the hills

So glider guiders could be operating at your frequency, where you think you are alone, and your not!

For example, he should be on 85, and you on 75 to clear them from interference, power models on odd numbered crystals

And the scanner will give you a definitive on and off signal indicator, and that the frequency is well defined

All this is to reinforce the safe use of the equipment

gangster23/06/2018 15:53:36
923 forum posts
16 photos

If you want to do a check for modulation failure as detailed above you probably won’t need anything as elaborate as a spectrum analyser. An ordinary fm broadcast receiver tuned to vhf broadcast band will hear the harmonic

onetenor24/06/2018 01:53:07
1900 forum posts

The hot servo was probably "arcing" internally.As we all know don't we ???? arcing creates radio/wireless signals and i'm sure that this was the cause of the problem and why the fault was exactly the same each time. Had you been on modern 2.4 gear it might not have happened. However the problem might have caused a fire or meltdown so it's a good job you found ityes

alex nicol24/06/2018 07:15:27
245 forum posts
12 photos

Hi Dennis,

no glider pilots anywhere near, the nearest suitable hills are miles away. I've flown on 76 for 15years plus without as much as a glitch.

Hi Onetenor

I'm fairly sure the root cause was down to the failing servo. I'm struggling to see how frequency could effect the outcome.....for the amount of heat and current generated could potentially cause random Rx function.

all of that said, it's test time this afternoon with a new switch, battery ( power cycled and timed a few times)and a known good flight pack in an old ( bit tatty but structurally sound) airframe, fingers crossed

Frank Skilbeck24/06/2018 07:36:39
4383 forum posts
101 photos

A couple of years ago we were setting up a friends electric model and every time we plugged it in the Rx wouldn't work and the ESC got hot, we tracked it down to a faulty (new) servo, it was pulling so much current it was overloading the BEC in the ESC. If you were using a high capacity AA pack there's a chance that a failing servo pulling a high current could cause the voltage to dip and cause the Rx to "brown" out.

Piers Bowlan24/06/2018 07:46:13
1770 forum posts
42 photos

A servo 'meltdown' would suggest it has either shorted out for some reason or stalled. Either way the current drawn would become very large and the battery voltage drop rather rapidly. In the case of a straight short (which would create enough heat to melt plastic) not only would the servo in question fail to move but the subsequent drop in voltage would cause all of them to stop working (according to my schoolboy physics). The question is, would the very large current caused by a short, if that is what it was, damage your receiver? I know that you said that your receiver works without fault now - good.

If you still have any doubts about the reliability of your transmitter however you could always send it away to be checked.

ps you beat me to it Frank smiley


Edited By Piers Bowlan on 24/06/2018 08:16:24

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