A Question for the Electric Cognoscenti.

[David Davis]

I have always enjoyed teaching beginners but I've never really taken to electric flight. There is a lad in our club called Ludovic who is an "advanced beginner," i.e, he can fly perfectly well but has not yet passed his A Certificate.

Last summer I said, "Ludo, why not fly my WOT 4 Foam-E on the buddy box?" He's Mode 1, I'm Mode 2. Spektrum DX9 master, Spektrum DX5 slave. I can't remember who took off but the model was in the air and flying well under his control, flying beautifully in fact. I enjoyed watching him fly for a long time, probably for too long. When I took over control to land the model, the model dived into the ground and was all but destroyed. I did not have the motivation to repair it so I gave it to a club colleague, a retired ship's engineer and he repaired it. He flies it now.

Examining the LiPo revealed that there was hardly any power left in the battery but my understanding is that if a battery is run down too much, there is still sufficient voltage left to power the reciever, so I should have been able to glide the model down to a safe landing. So why did it crash?

I am always the first to remark that nearly all crashes these days are caused by pilot error and I am quite prepared to admit that I may have become disorientated, fed in the wrong signals and that was what caused the model to crash.

However, what are the other likely causes?

The model is/was perfectly stock and fitted with an Orange receiver and 2200 3SGens Ace LiPo.

[Frank Skilbeck]

David, the ESC should have what is called a Low Voltage Cutoff (LVC) which will either cut the motor or reduce power if the voltage drops too low, but the supply to the inbuilt battery elimination circuit (BEC) is maintained, as the power required by the receiver and servos is substantially less that the motor even an almost fully discharged battery should have plenty left to land the plane. It's not a good idea to fly until the LVC cuts in as if a LIPO is over-discharged irreversible battery damage occurs and the battery looses capacity.

Edited By Frank Skilbeck on 11/02/2018 09:01:16

[Chris Walby]

One scenario tricky to achieve but not impossible for a suitably qualified pilot!

Take off and fly around with very gentle use of throttle and control input until the ESC reaches LVC.

Then power up hard (forcing LVC to cut in) and drive multiple servers quickly and long travel (bit like in flight control checks when you stab it one way just to make sure all is working okay!).

The rapid additional load on the lipo might cause the RX to brown out (like loss of signal) and then reboot/power back up. Some RX's are more tolerant of low voltage and faster at recovery than others, but you will have no control until its finished picking up the TX signal.

Questions,

Did it dive in on some/full power (if fail safe is set it would throttle back and control surfaces go to neutral (if you have it set up that way))?

PS I had a bench set up up with 3 medium size digital servos and a UBEC where if I stick banged all three at once the RX would brown out and reset. It would not do it with one or two servos or all three with gentle movements and would only be found it the "emergency stick wiggling to avoid the ground" situation so glad I found it on the bench and not the black bag!

Servo failure would normally be detect as partial loss of control

Motor and ESC (motor control) would have been detected and you would have dead stick in

Lipo failure is possible, but post crash testing should determine condition

RX failure possible and post crash testing might detect a fault, but in flight failure is very remote

Wiring fault possible but tricky to find and worth looking at the ESC to RX cables

Others might have more ideas

[Denis Watkins]

Apologies if I overwrite what you already know David.

2 accomplished flyers need to set the trims of dissimilar Speccy buddy setups

The model is flown off by the Master, the DX9, who trims the model

The Master then flicks the experienced slave in, and he then trims the model

The Student can then get hold of the DX5 in future

Brown outs are indicated by flashing receiver led., so look for this in the heap on the ground

The ESC will take care of low voltage, and give the servos priority

But not always while the throttle is High

So have it in your head training, to throttle down, then throttle back up !

Not always possible before you hit the ground

But reaquire control from your novice over 100 feet altitude, not 10 feet

[Biggles' Elder Brother - Moderator]

I'd have a good look at the failsafe setting. It's very easy to accidentally end up with a deflection on one of the control sufaces.

BEB

[Martin Harris - Moderator]

Is it possible that the LVC activated during the changeover - and at the same time you could have stalled? Hauling back on the elevator to keep the nose up in this scenario would simply hold the angle of attack over the stalling angle...

While the correct stall recovery primarily involves reducing the angle of attack, with the power to weight ratios available to a typical model, an application of power often helps resolve an incipient stall and we may get used to the less than correct practice of simply powering out of a stall - especially if 3D flying is your forte. With the LVC active this wouldn't happen unless (with most ESCs) you throttled right back first to reset it...

[Colin Carpenter]

DSM2 or DSMX ? Seen the former dive of death with no control many times ! When you get to the wreckage all's working ! Colin

[Peter Jenkins]

On my DSM2 it took 3.5 secs for the Rx to boot up following a brown out, or switch on for that matter! 3.5 secs is a long time to be without control when flying! The ground has a habit of arriving before the 3.5 secs are up - I found! Solution - I went to JRs DMSS. But that's a different story.

[Phil 9]

Electric or ic its the same age o​ld problem for the model flyer. Everything work great untill it doesn't then its a disaster. I had a similar failure with a large 6s model I changed to a separate rx battery but that is not so good for smaller models. Despite our best efforts our models are made with compromises and I would say that most models have more than one single point of failure. Frustratingly for the engineering minded it is not always possible to identify the fault from the wreckage.

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[Dave Bran]

"It's not a good idea to fly until the LVC cuts in as if a LIPO is over-discharged irreversible battery damage occurs and the battery looses capacity."

While I am not saying that running to LVC is a good idea, in my quite long term experience the widely used statement of damage and loss of capacity as a result of under voltage is NOT a given.

As just one example of many such incidents, a 3S LiPo pack was allowed to discharge to near nothing (The 3S pack was at 0.04v total output when tested). As it was 85' above ground in a tree and unrecoverable initially after a Rx failure, it was like that for over a month.

Gently revived, this pack is still in regular use on a high discharge fast wing where it continues to provide a full capacity and current. Over a year on, only its marking identifies it from other packs. HK Turnigy budget 1600mAh 3S, not anything special.

Before anyone comments, this is ONE example of MANY, I would not bother to comment on a rare event I have in the last year brought back a number of packs for fellow club mates that would not recharge normally due to "low voltage" charger cut out, and yet all are working well ongoing though naturally under careful observation for the first few uses, the point being not ONE was lost to poor capacity or other ills from their extreme discharge. This is true for every one tried, not one failed to recover or failed to continue to serve.

NOTE: I have TWICE now had the situation where a battery has departed a plane in flight, and the ESC in one case and the UBEC in the other provided power to operate servos enough to get the plane in a less damaging orientation. The last time was last summer when a much younger flying buddy gave my Dreamflight Libelle a discus launch and clipped the ground with a wing tip on the last violent spin, shocking off the canopy and defeating the battery velcro and velcro strap as it left his hand. We both saw the battery spin away from the glider, as it rocketed up steeply, but I still had control for several seconds, enough to put it into a shallow spiral and landing on the patch from which it emerged with only minor nose creasing. The time before was a powered glider which lost its battery while inverted at the top of a loop away from the patch (the velcro glue failed and the 1000mAh 3S removed the magnetic canopy). I had enough time to half roll it (two servos), turn and level it and bring it back to rough land it on the edge of the patch, an extended control time of maybe four seconds.

For the radio make snipers, one was Futaba FASST, the other FrSky.

So, even with an unexpected sudden TOTAL loss of supply, a decent set up should continue to give some control briefly. (though you have to know its happening to take advantage!!)

With regard the loss of control when a battery goes LVC, obviously as others state it should not, as the BEC should be capable of still supplying the receiver, etc.

[David Davis]

So it probably was pilot error on my part. .

Lovely lad Ludovic incidentally, builds models out of foam. Not really my cup of tea but they're cheap and they fly well. I'm not really sure whether he designs them himself or builds them from plans or whether there is a website for people people interested in making models from this material. I'll ask him the next time the weather is suitable for flying.

[Allan Bennett]

As others have pointed out, letting the battery to run down to lvc, or even lower, should not result in the receiver losing its power from the BEC.

Asuuming that it was Ludovic who handled the take off, and you had no input until the fateful landing attempt, there's a possibility of some setup error in the buddy system. Did you check all surfaces moved in the correct direction, from both transmitters, before take off?

[David Davis]

Posted by Allan Bennett on 12/02/2018 09:47:53:

As others have pointed out, letting the battery to run down to lvc, or even lower, should not result in the receiver losing its power from the BEC.

Asuuming that it was Ludovic who handled the take off, and you had no input until the fateful landing attempt, there's a possibility of some setup error in the buddy system. Did you check all surfaces moved in the correct direction, from both transmitters, before take off?

Yes Allan I always check that control movements are the same and adjust the trims on the slave transmitter to match the master transmitter. I think it likely that I took off and handed over control in mid air. After all it was my model.

[Andy48]

Hmmm. Everyone is assuming the LVC was actually set on the ESC. There is no reason to assume it was.

This is one big disadvantage of RTF electric models. It is virtually impossible to know what the ESC settings are. Usually its a no-name ESC which is difficult to get at let alone find instructions on or have a programming card for. One assumes it has been set up but...

[Keith Berriman]

Most of my flying buddies have a count down timer setting set at 5% throttle on to warn when time is getting close

[David Davis]

Thanks RC Plane flyer I don't really understand computers but I'll get a club mate to set one up on my latest electric project, a renovated Junior 60.

[Keith Berriman]

Hello David. I have various count down set for various electric models. One is 7 mins the others are 5 mins the times have been set due to batteries and models being used . The timer usually starts bleeping at 30 seconds before end time and I always recheck battery capacity incase the timer needs resetting

[Andy48]

Posted by David Davis on 12/02/2018 12:08:27:

Thanks RC Plane flyer I don't really understand computers but I'll get a club mate to set one up on my latest electric project, a renovated Junior 60.

I take it you never checked the LVC on your plane then?

[PatMc]

There's so many "ifs & buts" over the LVC etc that it's impossible to say after such a long period whether any equipment failure has taken place.
Most likely cause is pilot error - not realising how low the power was & simply stalled the model.

[David Davis]

It seemed to dive into the ground Pat but I may have become disorientated.

As for the LVC Andy I simply assumed that this was set by the manufacturers of the speed controller. How do you test it? Run the motor in the workshop until the voltage drops to the critical point, then check that you still have ailerons, rudder and elevator?

[Andy48]

One would assume that the manufacturer does set up the ESC, but they may not have done, or the LVC may have been set too low so that the receiver still cut out.

Basically use either the programming card for the make/model of ESC to program it correctly, or use the documentation for it to check the settings as a series of long/short bleeps. However, there is rarely any documentation for RTF ESCs, or to know which programming card to use.

[Nigel R]

"As for the LVC Andy I simply assumed that this was set by the manufacturers of the speed controller. How do you test it? Run the motor in the workshop until the voltage drops to the critical point, then check that you still have ailerons, rudder and elevator?"

Usually it is selectable between a few different values. But the standard default setup on most ESCs serves us well. Either way a LIPO discharged this far is not in immediate danger of damage. I'd say the model being powered by a battery which is no longer providing motive power is in far more danger of damage!

Your test is probably not unreasonable.

[David Davis]

It was the ordinary Ripmax 40 Amp brushless speed controller as supplied with the WOT 4 Foam-E. There is a little phrase on the label, "BEC - 3A at 5V." Does this tell us anything?

[Simon Chaddock]

"BEC - 3A at 5V."

All than tells you is that the BEC circuit in the ESC provides 5V (regulated) and up to 3A.

Should be quite adequate unless a servo sticks which could then over load the BEC and drop the voltage to a point that the receiver cuts out.

[Andy48]

That doesn't tell you whether a low voltage cutoff has been set. If it hasn't then the motor can drain the battery below the level the receivers or servos will operate.