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Model Air Crash Investigation Experts Sought...

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Jonathan M24/01/2019 19:17:30
870 forum posts
335 photos

WHAT HAPPENED: During a 3-point spin in calm mild weather this afternoon my almost brand-new electric Wot4 Balsa ARTF lost signal and failed to respond to sticks being centred (or any other stick movements) so continued spinning into the ground:


The Lipo was by that late stage of the flight down to 45% (badly biffed and will be disposed of for safety but cell voltages still readable) and the TX was still almost fully-charged. Range-test had proved no problem. I hadn't set up the Smart Failsafe on the RX, so all it would be expected to do is shut off the throttle on signal loss (stupid me, had it been set to neutralise control surfaces it might have survived with less than this terminal damage!), but of course the throttle-stick was fully down for the spin anyway.

These photos show the servos and control surfaces were in spin-mode upon impact - up elevator, left rudder and left ailerons (the model wouldn't enter a proper spin without them):




WHAT HAD HAPPENED PREVIOUSLY: Earlier in the flight I had been practicing some large loops. At the top, after full power for the first half, the motor had failed to respond to a reduction to idle, so I'd used the throttle cut-off switch to kill it; once I was out of the loop, I reactivated the switch and moved the stick to half-revs and the motor resumed normally. The exact same happened when I tried it again, so on the third loop I only used 3/4 throttle for the first half and the throttle then cut normally at the top.

I'd previously had no problems with full-throttle, but before going up to the patch today, I'd re-programmed the ESC according to the instructions (the usual cycle of various beeping-sounds etc), to reassure myself that it matched the stick-travel correctly, gave a soft-start and knew it was being powered by a LiPo battery.

The power-train and RX setup is (was) all Overlander: Tornado 4250 800kv motor, 60A ESC (max static draw 54A with 14x7 prop), 4s 4,250mAh Lipo, new Spektrum AR610 RX.

A mate of mine has suggested that perhaps the ESC drew too much current for the motor itself during full-throttle loops (with 4s LiPos in his warbirds he always uses a separate NiMh battery to power the RX), however the other control surfaces worked fine while the motor was out of control on full welly, also this wouldn't explain the loss of signal during the spin when the motor was off...?!

One other factor to consider: the patch is about 500m away from a mobile phone mast. Could it have been that the mast was pumping out a lot of rubbish which effected the model at the height it entered the spin? (Don't know of any other unexplained crashes at the site which could have been attributed this this.)


All in all an expensive and frustrating cock-up. I'd bought the ARTF Wot4 as a winter hack and was really getting into it. The airframe is confetti, one £58 Lipo resembles Joe Bugner after a heavy fight, the servos are probably okay and the motor might or might not be, but until I've got a clearer idea of what caused all this I'm suspicious of the ESC and/or the RX.

Any suggestions, advice - or even browbeating - welcome.


Paul Marsh24/01/2019 19:23:28
4112 forum posts
1245 photos

Maybe the BEC on the ESC went into over-load, as you said, 4 servos were being used to extremes (2 Aile/1 Rud/1 Ele) and went over the limit, either causing the voltage to drop, where the servos worked or the rx only browned out and didn't recover, as the servos "stalled"...

What is the ESC Bec's limit, and the servos full current draw...?

Don Fry24/01/2019 19:33:47
4557 forum posts
54 photos

First thought, with a debris field like that, the switch, battery, and receiver is in the bin. Test the servos if you want.

Second thought, if it does not respond to controls, land it if you can. There was something not working in the throttle servo circuit/area. Shut it and try to land.

SONNY MONKS24/01/2019 19:38:27
269 forum posts

Exactly the same happened to me when i was flying spektrum,not anymore though,i have the horus x10 now,far better value for money.Again,spektrum,renowned for nothing but problems,not always pilot error fault.

Paul C.24/01/2019 20:08:27
705 forum posts
165 photos

Hi Jon, sorry for your loss sounds like your problem started with loss of motor control in the loops. At this point no signal loss as the failsafe did not engage and you were able to stop the motor using throttle cut, we're you able to tell if the elevator servo was still operating or if it had stopped in position as you were half way through the loop ? Like Paul M i suspect ESC or RX , get all the bit out on the bench connect them up and see what you have. I lost a Wot 4 a few years back , lost all control , found it was due to the motor failing and over heating the ESC until the thermal cut out kicked in.

Best of luck with the autopsy, no doubt you will be on the receiving end of the anti Spektrum brigade posters wink


Stephen Smith 1424/01/2019 20:37:33
245 forum posts
Posted by Paul C. on 24/01/2019 20:08:27:

No doubt you will be on the receiving end of the anti Spektrum brigade posters wink


Makes me laugh, there's no anti Futaba Hitec Jr Jeti Frsky or any other anti brand brigade not obvious where the problem lies, I strongly believe one day people will work it out.

Don Fry24/01/2019 20:55:48
4557 forum posts
54 photos

Sonny doesnt count. It's his mysterious one liner.

But bottom line is, when it's broken like that, it's going to remain a mystery in all probability.

Paul Marsh24/01/2019 20:57:05
4112 forum posts
1245 photos

I have a Hobbyking Canadian Vampire and also the older RAF one. The RAF one came with a seperate Ubec but on the Canadian one, they used the onboard BEC.

Been flying the RAF for years, and both are using Hitec Optima 6ch receivers. The RAF one never had any problems, but after a few flights with the Candian one, I had a lock-out, no control, then control resumed. I thought it was the wind, but happened again, the motor shut down and loss of control. Luckliy, control came back and landed it

I quarantined the model, and assumed that to save money, the Candian Vampire had its Ubec removed and used the BEC, which may not be up to the job of running all the servos and the retracts.

So, I decided to add a Ubec, as the other models have had no problems in the past (Sea Vixen, Vampire Mk1 ).

Paul C.24/01/2019 20:57:24
705 forum posts
165 photos
Posted by Stephen Smith 14 on 24/01/2019 20:37:33:
Posted by Paul C. on 24/01/2019 20:08:27:

No doubt you will be on the receiving end of the anti Spektrum brigade posters wink


Makes me laugh, there's no anti Futaba Hitec Jr Jeti Frsky or any other anti brand brigade not obvious where the problem lies, I strongly believe one day people will work it out.

Hope you enjoy your laugh 😆

Jonathan M24/01/2019 21:49:15
870 forum posts
335 photos

One theory suggested by a couple of folk at a club meeting this evening was of an almost-loose connection somewhere in the system. This wouldn't have revealed itself except in situations of extreme- or negative-G (exactly like a full-throttle loop and spin/spiral-dive!) but disappearing during normal flying.

Might never know the exact reasons, but the (expensive) lessons learnt for me are:

  • At the first sign of something amiss, land immediately and troubleshoot it fully before flying again.
  • Fully check all connections regularly.
  • Fit a 2000mAh NiMh to power the RX and servos, while using the big 4s LiPo only for the motor.

In due course I'll probably buy and fit-out another Wot4 as it suited my purposes almost perfectly (a low-fuss, year-round hack). I can test the servos, which are probably okay but cheap enough to replace if needed, but I'll replace the rest of the gubbins. In the meantime at least I still have my IC Acrowot to crack on with the flying.

Maybe it was a rare duff RX or a lousy connection or a poor installation or a dicky ESC, but I don't think its my Spektrum gear as such. I've been using the D7 G2 for three years with IC power, slope soarers and DLG stuff - with both Spektrum and Lemon receivers without any glitches.

cymaz24/01/2019 22:02:57
9463 forum posts
1246 photos

I wouldn’t discount the phone mast. At Davidstow we have a no fly time, 3pm -winter and 4pm -summer. We have a mast nearby that is blamed for two plane crashes. Is there a 2.4 scanner you can borrow? Maybe the BMFA has one.

kevin b24/01/2019 22:25:03
1969 forum posts
176 photos

With servos as shown, stuck in their last position I would suggest power failure to the receiver.

Loss of signal would have caused the failsafe to cut in and return them to neutral (was the failsafe set up properly ?).

I think that the problem was between the receiver plug and the battery. Either a connection coming apart, or failure of the ESC. With that amount of damage I would bin the ESC anyway. They are not as robust as they appear to be, as I found out myself some time ago.

Paul C.24/01/2019 22:43:20
705 forum posts
165 photos

Take a look at the 'model vs trees' thread , turned out to be a poor connection. Might help in your investigations.


Peter Beeney24/01/2019 22:55:29
1594 forum posts
59 photos

One niggling little thought is that if the problem is in the model and the throttle stick failed to the shut motor down how did the throttle cut-off switch manage to do that? It certainly does seem a remarkable coincidence that it happened twice in succession, but I’m at a loss, for the moment anyway, trying to figure out how that might not be within the tx.

I think that if the BEC cuts out for any reason the motor always stops instantly; the reason being that the BEC’s primary function is providing a 5 volt supply to the ICs that control the motor on the ESC. The motor won’t run if these ICs have no power.

Hope you manage to find some sort of gremlin lurking about in there…


Nigel R24/01/2019 23:11:10
4298 forum posts
716 photos
Big lesson is, model gives you any funny business, get it down ASAP.

I think it'll turn out to be power problems. Signal problem would have you in failsafe after a second.

I don't see the bec in overload. Sport model surfaces aren't going to cause much power draw from three or four servos at full throw unless they all bind really badly. As in proper badly. And then you'd be brown out, power back up, back to neutral, recover, go.

Gut feel something ain't connected firmly or there's a dry joint be that a wire or something internal to bec.
Bruce Collinson24/01/2019 23:12:05
576 forum posts

How cold was it?

Geoff S25/01/2019 00:22:21
3914 forum posts
62 photos

Quite honestly if I read your first post correctly (you lost control of the motor twice) I think you were really pushing your luck not only continuing the flight but also continuing it aerobatically. I hope that had the first failure happened to me I'd have landed immediately to investigate the problem.

Looking at the scattered wreckage I'd be surprised if you were able to determine the cause with any degree of certainty. My guess would be a poor connection somewhere but it really is just a guess. It could possibly be the transmitter but it seems unlikely as the motor kill switch worked and the fault would be in the RF section rather than system. I doubt the RF mast had an effect.

It was quite cold here in Derbyshire but not actually freezing and I think very low temperatures (ie lower that -10) would normally be needed to cause a failure. All my electronic temperature testing has been at the other end of the spectrum (ie >125C) and everything works OK at that and at >20,000g, too.

I'm still amazed at how well our radios etc work as it is so I'm always expecting a failure.


Ron Gray25/01/2019 07:51:50
2466 forum posts
996 photos

That’s a real bummer Jon. As others have said, after a crash like that you will be lucky to find the root cause of the problem. I had a very similar experience with my GP Revolver a while back, it suddenly failed to respond and ploughed into the field - bin bag job. In this case I was able to trace it back to a faulty switch so no matter that I had failsafes set, no power negates that. The lesson I learnt was that switches need to be checked on a regular basis and fit a power backup system, I now use Optipower Ultraguards. So now if the Rx loses power from its main supply, the Ultraguard kicks in.

SONNY MONKS25/01/2019 08:29:25
269 forum posts

Are the phone masts to blame?,do the authorities/companys who erect these,test them for interference,it may be something to do with the environment which is effecting 2.4,just saying!

Wingman25/01/2019 10:31:37
1158 forum posts
405 photos

As others have mentioned the aircraft 'spoke' to you and you ignored it so you payed the price - luckily nobody else did!

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