What exactly killed my big seagull

[Matt Halton]

Interesting reading there Tim, hopefully you have identified the cause. Thanks for posting your findings.

 

Cheers

 

Matt

[Mal Quelch]

Tim, one symptom that has not been addressed as yet is the 'impossibe' high Voltage measured.

 

Did somehow he UBEC become added in series with the flight battery? As you say it is really an impossible voltage, and without knowing how the installation was connected I cannot image how it could happen. It would need a connection between UBEC and battery that would not be there in normal useage. The fact that it remained there constant (and recorded) is also confusing.

 

Can you illustrate the wiring used and where your logger was connected as to me at least its very interesting.

Mal

[kc]

Wouldn't a normal Nicad or NiMH Rx battery have saved the day? Is weight so critical on this model that a normal Rx battery cannot be used?

If an impossibly high voltage occurs surely one should suspect the data logger might be giving a faulty reading?

 

Last month a clubmate had a brand new ESC go up in smoke on the first flight ( this was an ARTF glider with ESC & motor supplied which lived up to it's name ) Result no response to the RC, fortunately being a glider it just circled, missed the trees by feet and made a good landing on it's own. Lucky!  I think I will fit a separate Rx battery to my models!

Edited By kc on 22/03/2011 10:58:40

Edited By kc on 22/03/2011 11:01:04

[Tony Smith 7]

I'm having trouble understanding the interconnection of the devices. Does the logger sit between the Lipo and everything else, ie positive and negative from battery to logger, then positive and negative from logger to ESC? How does the UBEC connect?

 

Also, where was the 5A going after the accident? Or was that another misreading from the logger?

[Steve W-O]

The logger fits between the battery and everything else, the is also an option to measure the current being used by the servos, to measure RPM, throttle position, temperatures, airspeed, altitude, GPS position, rate of climb/descent. And an OSD unit for FPV

 

Must say, I was wondering about that 5A to, but it could be (though I doubt it) lack of calibration.

Edited By Steve W-O on 22/03/2011 11:38:22

[Former Member]

[This posting has been removed]

[kc]

I am no expert on electronics and most of the other people here have obviously more knowledge than I have......but .....to me it seems obvious that the data logger is the suspect.. It gave a false 29 volts and a suspect 5 volts and it sits between everything else. What happens if it goes haywire even for just a moment or two?

An unnecessary complication?

[Kozmyk]

Posted by Biggles' Elder Brother - Moderator on 22/03/2011 01:07:18:

OK this calculation is based on pure metal/metal systems - there is bound to be a contact resistance in the connector's mechanical contact. Experimental evidence I've checked up on suggests that for gold plated contacts the contact resistance is better than 0.0001 ohms. So that wouldn't explain it either as it is still 100 times lower than the resistance we would need.

 

BEB

 

Wayne Giles has done some interesting work regarding connector resistance.

 

Connector Resistances

 

Interesting how much better solid bullets are than cage/banana types, excluding XT60s that is.

 

Connectors become complicated little beggars when it comes to resistivity/conductivity.

With a body commonly of some form of brass,solid or otherwise, a layer of nickel, a layer of gold; then either a solder joint or a crimp joint to the copper cable.

[Tony Smith 7]

Posted by kc on 22/03/2011 12:15:17:

I am no expert on electronics and most of the other people here have obviously more knowledge than I have......but .....to me it seems obvious that the data logger is the suspect.. It gave a false 29 volts and a suspect 5 volts and it sits between everything else. What happens if it goes haywire even for just a moment or two?

An unnecessary complication?

To be fair I would think that the only bit of the logger that's actually "between" would be a shunt which is a passive device, and of course the connectors.

[Steve W-O]

Posted by Tony Smith 7 on 22/03/2011 14:16:53:

To be fair I would think that the only bit of the logger that's actually "between" would be a shunt which is a passive device, and of course the connectors.

 

Yes, straight through, not even a shunt, it uses a Hall effect sensor

Edited By Steve W-O on 22/03/2011 14:43:29

[Tim Mackey]

Sorry, been out all day. trying to address all the points raised.......

Gerhard - I will closely examine the cables later ( indoor flying in half an hour ) - 12 AWG multi-strand copper, silicon ins.

Danny - 515Kv Scorpion 2kw, and the 14 x 10 prop ( which worked superb BTW )

Steve, sorry mate, its just when you said they were essential in every model, I didnt think you meant the ET loggers ! HK price for a genuine Eagle Tree unit - £36 ??

I paid twice that a few years ago Did you want me to send you FDR files then ?

Mal The high voltage reading has been explained- its an oddity of the logger when its output lead is suddenly disconnected whilst input remains steady - just a glitch, and only happens on that scenario - under normal use its usually reliable. The logger just interconnects between main battery leads and ESC input.

KC Yes, probably, but in fairness the UBEC itself was fine. I prefer UBECs to separate batteries, only because its one less battery for my old brain to rememberr to charge. However very valid point - and maybe I might also re-think things now.

TS7 - see above.

Steve again ( and Eric ) - Calibration - nope, all recalibrated at time of install, with comparative adjustments made against a DMM. I think the wayward 5A after impact was another glitch due to the way it was suddenly disconnected at WOT. The GUI also shows the throttle as remaining WOT even though I shut it down on impact. This was all replicated in the bench tests yesterday.

KC again - the logger is fine IMO - as Steve said earlier, its effectively passing current straight through between the main poles - Hall effect sensors do the data collection. I don't consider it any more of a risk than say the ESC itself.

Kozmyk - I will take a look at that link later

[Biggles' Elder Brother - Moderator]

VA - you're absolutely right - in one sense my rough calculation is a "worse case" scenario because I have made no allowance for the effect of any cooling by air currents in the fuselage. There would probably be (we can't be certain - but its likely) convective heat transfer away from the joint. But that of course would work in our favour - cooling the joint, so it just makes solder melting caused purely by the current even more unlikely as the cause.

 

Danny - yes my calcs assume that the joint was sound. The fact that they don't explain what happens is why one possible explaination for what happened is that in some way the joint wasn't sound IFSWIM! Tim, no slur to your soldering capabilities intended, but, for example,an internal void in the solder is something outside of your control to some extent - it could happen if the wire was not pushed all the way home in the hollow for instance? But in reality the extra resistive heating could easily have come from somewhere else - such as a very small piece of debris caught between the spring element and the body of the plug. Or small scale local corrosion in that region not visible on inspection. Remember, the space shuttle was brought down by a single frozen 'O'-ring! It doesn't take much sometimes.

 

Kosmyk - yes you are right of course. The real contact resistance is very difficult to know because of the different metals involved - and if the theory result was out by say a factor of 2 or 5 I would conclude it doesn't tell us anything conclusive because our uncertainty in the contact resistance could be that big. As I said this is only a "back of fag packet" analysis - not a really detailled one. But as its out by a factor of 100 on the worse case scenario. Personally I think that's too big a descrepency to be explained in terms of uncertainty over the contact resistance.

 

I stick my my conclusion - its likely that there was some other effect at play that caused the joint to fail.

 

BEB

[John Privett]

Posted by Tim Mackey - Administrator on 22/03/2011 08:37:11:

Air void in the solder / simply a dodgy joint looking likely then - I am very experienced and if I say so myself, pretty good at soldering, but, hey no-ones perfect.

 

 

I had on "odd occurrence" with my my first electric plane some years ago involving soldering 4mm gold connectors. Like you, I reckon I can solder reasonably well - certainly well enough to recognise a duff soldered joint, or so I thought!

 

This was back in the days of 400 brushed motors and brushed speed controllers. Eager to try out this "new technology"(!) I tested the battery/ESC/motor combination with the leads simply held in place, and after establishing that it worked then soldered on the 4mm connectors for the battery - I think the motor was soldered direct to the ESC output.

 

It wasn't until a week or so later when it was assembled in the model that I tested it again - and got no response at all. Inspecting the connections gave no clue as to any problem and I decided that I must have somehow damaged the ESC earlier. A new ESC "proved" the point, and the old one was discarded into a dark corner somewhere.

 

It wasn't until a year or more later that curiosity got the better of me and I had a closer look at the "duff" ESC. A bit of probing with a voltmeter established that power was not reaching the ESC from the battery. Re-soldering the connectors cured the problem - the ESC was fine after all!

 

Somehow an apparently perfectly good-looking soldered joint had been so far from perfectly good as to be completely ineffective. And I've still no idea how...

 

Now clearly your case is slightly different, as it did work for the first flight. Perhaps something in-between a good connection and the non-connection that I had could work well enough to fly the model, but with enough resistance to heat up and come apart.

[Tim Mackey]

JP says...

Now clearly your case is slightly different, as it did work for the first flight. Perhaps something in-between a good connection and the non-connection that I had could work well enough to fly the model, but with enough resistance to heat up and come apart.

 

Yup, thats my best guess too - a dodgy soldered connector perhaps as result of too hot an iron, ( I use my 80Watt large tip job on anything bigger than 2mm gold) and LMP solder - could have caused "boiling" of the solder inside the solder bucket, creating air pocket.

It obviously held on for the maiden flight, but was doomed to let go sometime.

 

Two thing I have done as a result

1) Ordered these connectors for all setups of 50A+

2) Shall reserve my heavy iron for cell soldering only - 30 watt should be plenty for cabling.

 

Jury is still out on whether UBEC or separate battery is best for radio.

 

 

[Tim Mackey]

Kozmyk - just read the stuff in your link. I have bought several batteries recently that have those EC3/5 connectors, but being a separate gold bullet man, cut them off

The other thing that I like about separate connectors, is that in some installations I have the pos and neg cabling running down separate sides of the fuselage /cowl whatever, and also, it s easier to fit an antispark system on a single lead setup.

I think the non caged 6mm system I linked to in the above post look good.

Ill see how they prove to be in reality.

[Kozmyk]

Posted by Tim Mackey - Administrator on 23/03/2011 00:11:40:

I think the non caged 6mm system I linked to in the above post look good.

Ill see how they prove to be in reality.

 

For sure Tim, I think those 6mm. 5.5mm and 5mm connectors are pretty bullet proof (sorry) for the higher current jobs.

I'm happy enough with EC3s for the crates I'm using them on.

 

Any heavier current and it'd be solid bullets for sure.

I particularly like the 8 slot type:

5mm Bullets

5.5mm Bullets

6mm Bullets

[James M]

Hi All

 

Just an idea on how the volts could have climbed to 29volts+ and why current was still being drawn from the battery ...................

 

......... Battery was around 25volts at the start and the bec outputs about 5 volts give or take. If you add the two voltages together you get about 29 volts.

 

So if the Ubec and battery are combining there voltages, and as electricity takes the path of least resistance, it would kill the radio gear and it will also give you the steady current draw off the battery that maintains the voltage to the ubec.

 

The only bit I don't know is how the ubec was wired to the battery and connected to the receiver to allow this to happen......................

 

..............the maths and theory are good in principle................

[Tim Mackey]

As I explained , the high 29V reading was a glitch in the logger due to the way its was suddenly interrupted. The UBEC is merely hard wired in parallel with the ESC input leads, and therefore, as the negative input lead connector failed, all power to the UBEC was lost too, and it was therefore "out of the equation".

I have replicated it several times with / without UBEC connected, and its the same result.

[Simon Chaddock]

Tim

A fascinating piece of detective work that would do justice to the AAIB!

It also shows why they are so keen to retrieve the black box recorders.

 

The AAIB make recommendations to reduce the chance of a re-occurrence.

What would yours be?

[kc]

Why is the "jury still out" about using a UBEC or separate battery?

Surely your plane would have been in one piece still if just the motor had failed. Except for a reduction in weight I can see no reason for scrapping the Rx battery. You just remember to charge the Rx when you charge the TX!

Am I overlooking some safety advantage of a UBEC?  The BMFA handbook page 41 seems to recommend against the use of BEC except for lightweight models. It says " should not use BEC .....where servo drain might be high ...or with 4 or more servos ".  OK it does say " the decision is yours but if you have any doubts then you should use a separate battery " 

Edited By kc on 23/03/2011 10:41:03

[Tim Mackey]

Simon, thanks. My recommendations ?..... Take up golf

No seriously, I guess they would be

 

1) Double check - and really apply force to any soldered joint.

2) Ensure any components such as cabling and connectors meet specs required.

 

I'm still mulling over the radio power thing, but am leaning towards a battery back up system on any decent sized / valuable model. Perhaps some sort of auto switching unit that transferred supply from an independent battery if the primary ( either BEC, UBEC, or battery ) failed.

I know of course of "regular" dual supply type systems, but need to dig further into whether they could be used in that scenario IYSWIM, and am also a little reluctant to introduce yet more complication and potential failure points into the installation.

[Tim Mackey]

Posted by kc on 23/03/2011 10:31:12:

Why is the "jury still out" about using a UBEC or seperate battery?

Surely your plane would have been in one piece still if just the motor had failed. Except for a reduction in weight I can see no reason for scrapping the Rx battery. You just remember to charge the Rx when you charge the TX!

Am I overlooking some safety advantage of a UBEC?

 

Well I can only say that having another battery on-board to remember to charge is my main concern - may not be valid, but its my thoughts

The model may have survived if it had used a separate battery - but that's certainly not guaranteed - an instant motor cut could still have resulted in a crash, if it happened in the wrong place, especially with a heavy wing loaded model, with the gliding performance of a house brick.

[David Ashby - Moderator]

...but at least you still have half a chance Tim (which is better than nowt) and the opportunity to steer the model away from people and property.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Edited By David Ashby - RCME Administrator on 23/03/2011 11:13:43

[kc]

I think we should all thank Tim for his honest & comprehensive research into the cause of the crash. We can all learn from this.

[Geoff Smith 1]

Hiya Tim, just a question from a newby. What does IYSWIM mean, I've seen it crop up a few times and have now got very inquisitive. Cheers mate.

 

 

Geoff