Crash every second flight ??? !!!

[Nick Farrow]

Hi,

I'm after some advice.

I fly with with 35mhz and have one model that I seem to loose control on every 2nd flight., over the last 3 weeks. I fly over very long grass weeds so the crashes are not fatal! The model has a corona dsp rx and has been rock solid prior to the last 3 weeks.

The symptoms are first flight rock solid, the flight battery is down from 12.6 to 11.7/11.8 for the second flight. On the second flight its all very gradual loss of control. One time it gently rolled onto its back and nosed in on take off, with about half throttle.

Today, on the first battery rock solid first flight, second flight did a circuit where is just felt hyper sensitive//verge of no control so ditched in long grass. After 15 mins searching I found it in one piece, no damage, as but with the battery connected it appeared to be powered down,no response to Tx.

Next flight on a new flight pack, perfect. Second flight (batt @11.8v) on launch very little up. aeleron response was perfect. I just had to land at 200m distance with no up. Smashed nose, broken tails the servos that were still intact were still responding to the tx.

The only thing that has happened over this time is the loss of a couple of inches of Tx aerial. Other models flown with the tx in this state are fine (new aerial on order).

I dont feel safe flying this model on a second flight with the same flight pack. Other than change everything, suggestions on proving this would be welcome

Thanks

nick

[extra slim]

I would imagine the low voltage cut off in the ESC is kicking in. With radio on, and motor running the volts will be pulled down further towards the cut off. I must admit, I dont usually use a lipo for more than 1 flight at a time...unless I know the setup very well and have flight tested it, knowing exactly how long a full battery lasts for...but even that is very rare.

FYI...if it happens again, you can usually drop the throttle right back, and let go off the sticks, and after a split second you get "some" control back in order to level it up..

Just my thoughts.

[Former Member]

[This posting has been removed]

[The Wright Stuff]

Posted by extra slim on 22/06/2016 14:07:34:

I would imagine the low voltage cut off in the ESC is kicking in.

Yes, I really can't think of any other plausible explanation. Out of interest, how are you measuring the 11.8 volts? Is the ESC cutoff set for the right kind of battery?

[ken anderson.]

hello nick-I would fly one battery per flight..not have a flight with the battery and then later use the same battery again....I think that's how you may be having a problem...the cut off is kicking in.....

ken Anderson...ne...1...... lvc dept.

[Nick Farrow]

Hi,

Thanks, a bit of a consensus!

I'm not sure I can check the low battery operation of the esc, but I'll see if I can find the manual somewhere.

The behaviour is the same with different batteries and I check the voltage with a little plug in cell monitor after each flight. I had casually wondered about low voltage but assumed that as I was flying with a low throttle (after launch) this was not a likely cause.

[Denis Watkins]

Hi Nick, it would be useful to know the % capacity as well as the voltage in this case. When I was teaching, I always explained the issue of voltage simplistically as "the size of your bucket", but the "weight" of electricity is important too. A pack may claim 2200ma 3cell, but that does not mean that it can hold that capacity for long enough. A degrading pack will soon loose its clout as you find.

[Dave Bran]

Engaging Tongue In Cheek mode.......

Well, if you are insistent about continuing to fly a plane that clearly should have been fully investigated and the obvious faults cleared by ground testing after the first incident, might I suggest fitting a recovery buzzer?

Disengage TIC mode

[Bob Cotsford]

If the low voltage cut-off is kicking in surely it's the motor power that gets cut first, not the radio supply? If the motor was still running when it failed on take-off at half throttle then I would say the ESC is faulty, possibly going into over-temperature shut down. How long are you leaving between flights on the same battery?

[Tony Smith 7]

I would suspect the BEC function on the speed controller. By which I mean maybe it's cutting or limiting power to the received - maybe enough to drive the rx but not properly drive the servos.

Do you have another ESC you could try, or use a separate SBEC for the radio stuff? Or do one flight, then extensive bench testing with the battery in that "second flight" state.

[Cuban8]

A multitude of reasons that could be causing the problem. I'd be inclined to run the model (restrained) on the ground and try to recreate the symptoms - don't forget the requirement that one should be reasonably confident that a flight can be performed safely!

Perhaps you know someone with some electrical expertise who can take some measurements and eliminate a few obvious faults and maybe pin the cause down?

FWIW, I'd start by monitoring the BEC voltage for stability (usually 5V) with motor running and waggling the sticks. Servos will soon slow and do all sorts of odd things if the RX supply is collapsing for whatever reason.

Edited By Cuban8 on 22/06/2016 18:26:52

[Bob Cotsford]

Another thought - is there any cooling airflow to the ESC other than when you remove the battery hatch to change batteries?

[Allan Bennett]

If I understand the opening post correctly, you're starting your second flight with the battery showing 11.7 or 11.8v: That's only half full and, since it's not recommended to go down below about 20% full, your second flight only has about 30% of usable battery capacity available. So, as someone else has suggested, you should start each flight with a full battery.

Perceived gradual loss of control could possibly be caused by the motor losing power and you not noticing that the model is slowing down to stalling speed but, in the light of no radio response after one of your crashes, I think you need to look at the receiver as well as the ESC. A failing BEC in the ESC wouldn't, I believe, result in gradual loss of power, but rather a sudden loss of all control.

[Chris Bott - Moderator]

Have a first flight Nick, and then check the temperature of the ESC. Your description smacks of something that cycles over quite a long period.

If the ESC is hot, then there's every chance that it will get even hotter on flight 2 and a BEC can shut down on over temp, at which point your radio will fail.

Of course on another day it starts off cool and everything seems fine again.

[Daren Graham - Cambria Funfighters]

Just a thought. What model, how many servos and what current do they draw and what Tx?

[Rosco]

I don't know why you keep flying your model if you know that there is a consistent problem. As had been mentioned, Why are you doing two flights on one battery? I understand that you may need to land for some reason but common practice is to use one battery per flight.

Do you have any other models with this problem?

I'd run the setup you have and replicate the second flight on the ground (without actually flying it) and then individually swap the electrics to see if it has the same problem/s.

Just a trial of elimination process really.

Rosco

Edited By Rosco on 23/06/2016 00:19:18

[Robert Armstrong 2]

I was told some time ago that the ESC sets the overall LVC value on startup. It decides from the initial input voltage, as it does not 'know' the actual cell count of the attached battery, but deduces it from that initial voltage. That seems reasonable as the ESC isn't connected to the balance lead (well, not on any ESC that I have seen myself) and the overall LVC value is related to the LVC per cell set for the EXC.

It therefore seems possible that the ESC may be setting the LVC value too low if the initial voltage is low for the actual cell count, so you are getting a brownout before LVC occurs. Have I expressed that clearly? Second the proposal for a controlled ground test.

Are the individual cell voltages reasonably balanced? If you have one duff cell, the voltage may drop suddenly as the duff cell drops off.

I always start with a charged battery. Well, try to anyway. I now check battery capacity before flight and after, following an unexpectedly short flight.

[Bob Cotsford]

But even if LVC was setting itself for 2S Li or 8S Ni, surely the BEC voltage wouldn't be falling dangerously on take-off with cells at around 11.8v! I'm assuming that voltage is off-load as Nick doesn't say, but that's still 3.9v per cell and enough power for a take-off.

Again, if the ESC is working and not overheating LVC will cut power to the motor long before the BEC voltage drops far enough to affect the radio. It would be helpful if Nick could supply some more details, eg motor/prop/airframe, how many of what size servo, what ESC?

[Nick Farrow]

Hi,

Thanks for all of your helpful comments.

I have 3 turingy 2200/3s 25c (or the like).pushing a keda 2837/? and have a 10x8 prop. the model is a 40 inch depron P51 flitetest swappables scrathc built), 4 x 9g towerpro servos. The ESC is a 40amp robotbirds unit, the manual has these features

• 1.5amp BEC

• Low torque ‘soft start’

• Auto shut down on signal loss

• Temperature protection,

shuts down at 110 ̊C

• Voltage cut off selectable to:

4.8v, 5.6v, 6v, 7.2v, 8.4v, 9v, 11v, 12v

I have suffered LV before (no with this speed controller or plane) and it normally just shows as a performance degrade on the motor. I usually check this by just pulsing the throttle every now and then and if there is no real response in the motor revs, I land. This normally works quite well for me! Even when this happens (as others have said) there servo response is good.

On the face of it, does not 'feel' like that. On the second take off there is ample pull and no motor fade. After the first flight, the motor and ESC are warm but not hot.

I'll repair the plane to the point where I can run up the motor and put my watt meter on it. I'd like to try and prove what is going on! I'm wondering if its the 'autoshut down on signal loss', given the missing last 2 inches on the tx perhaps a range check at 11.8v.

I'll keep you posted!

nick

[bouncebounce crunch]

Me Idiot and me have rubbish time with electric models. but i did have this experience. Model charged and reading of 96% full on Prolux battery tester. take off and into one circuit I hear motor pulsing, cut the lap short to land cross field and _____ zip--- nought---watch model slowly descend at a good rate from a good height and crunch.

what did i find? my charger was only charge balancing 2 of the 3 cells. so one cell was always below what it should have been. the balance plug was repaired by a club mate and the battery is still fine today 3 months later.

Thank you to my club mate as i would never have seen what was wrong; i blamed the charger but it was the balance plug.

Balance plug leading from charger, not the battery lead.

Edited By bouncebounce crunch on 23/06/2016 12:59:46

[Nick Farrow]

Hi,

Just checked with the battery that was used yesterday and my wattmeter.

The battery with no load (just the watt meter) showed 11.6

Running the motor flatout for 30 secs and the meter recorded:

30.6 amps, 10.68volts, 335 watts. Servos all worked fine, so I'm thinking its not a low voltage issue (assuming its set for 9v)?

Thanks

nick

[Simon Chaddock]

I assume the battery was not recharged after the flight?

Were the above figures the 'peak'? If so they would have occurred at the start of the run.

The important ones would be those right at the end of the run when the battery was nearing discharge..

LVCs are usually set to the equivalent of 3.2V per cell so 9.6 for a 3s.

[Nick Farrow]

Hi Simon,

Correct, the 11.6v was the initial no load voltage (with the same 'crash' battery in the same charge state), when pulling the 335 watts it dipped to 10.68 and after the 30 seconds of full power recovered to 11.4 ish.

nick

[Tony Smith 7]

What was the output voltage from the BEC during those tests?

[Chris Bott - Moderator]

Nick have you eliminated the ESC over temperature possibility?