WOT4 Becomes Racing Car

[john stones 1 - Moderator]

Some electrical fault being shown up through vibration rolling along floor.

[Andy48]

BEB, you have a Taranis, have you looked at the data logs of your cross-country journey?

It was a cold day, what did the battery voltage and current read for starters.

Edited By Andy48 on 08/02/2018 21:14:30

[MaL]

Is the elevator linkage a bowden cable? Have you checked for elevator 'blow back'?

[FlyinBrian]

Have you by any chance had the UC off? if so did you put it back the right way?

I had a similar situation with a foamy Wot4 when I put the UC on back to front.

Just a thought.

[Biggles' Elder Brother - Moderator]

Hi All,

sorry for not replying earlier - I was back very late last night.

Thanks to everyone who has contributed a suggestion - I will treat them all in the spirit of brainstorming. Some are more likely than others of course. I'll do my best to respond to each:

1. The elevator is reversed. Always possible of course, but I feel not likely for a number of reasons. First, I am very particular about pre-flight checks and I distinctly remember looking at the elevator then. Second the controls would have to reverse themselves and then reverse themselves back again - because they are not reversed now! Finally, in the end I was pulling more or less full back stick, if that was actually putting down elevator in I think the model would have nosed over - it didn't show any tendency to do that. So I think reversed controls is an unlikely explanation.

2.Temperature and wetness effects. Yes it was quite cold, about 4C. The runway I was using is very well drained and although it was damp I would not describe it as "wet". The grass was short and well cut. There was no discernable lack of acceleration and general "urge" in the take off run. These factors can always play a part of course.

3. Could the battery have moved the CoG forward enough to be a problem? I don't think so. I have tried pushing the battery forward and assessing what happens to the CoG. As I said the battery can only go about 1.5cm forward. My test shows that the CoG moves by a hardly discernable about - a millimetre or two. I really can't see this causing the model to totally fail to rotate. If it was very sensitive the CoG it might make the model feel a bit sluggish - but that's all. This was a totally disinterest in leaving the ground!

4. A Taranis programming issue? Always a possibility. But I've not knowingly changed anything and checking through the program in Companion, tab-by-tab, line-by-line, I can't find anything that looks dodgy! In fact of course its a very simple program of a very straight forward rudder, elevator, throttle and aileron fixed wing model.

5. An intermittent fault on the elevator servo/connection. Now that is a real possibility. True, I haven't been able to make anything untoward happen on the bench, despite much shaking, rattling and tapping, but it could be an explanation. I will change the servo, just to be sure.

6.To be honest I can't remember if the tailwheel lifted or not, I was bit pre-occupied at the time! Regarding the general CoG condition, no problem there. Normally inverted flight needs only a little push and the model is responsive without being twitchy. If the CoG was to blame - then it changed from the existing good condition and, as we have discussed, a battery shift seems to be the only way that could happen - but it can't account for the problem for the reasons given above.

7. Rates were set at max for take off, and were not altered between the pre-flight check and the take off attempt. All three rates settings work fine.

8. Was the aircraft going fast enough? Well obviously I can't be certain, but I would say we were pretty well at full throttle and acceleration was brisk. Also, the WOT4 is well know to fly at even very low airspeeds - it's one of its party-pieces! If I was slow I might expect a reluctant attempt at take off - a lot of wing waggling etc. I got nothing, it didn't even look to me like it was getting "light on its wheels"! I'm pretty sure we were going fast enough to have been able to lift off.

9. Servo tray loose? The servo tray is firmly in place, as are the servos and the linkage. The linkage is a pushrod - no chance for slop there. All linkage connections appear to be sound - fuel tube keepers in place on clevises etc. Control horns fixed down well. I don't think its a linkage problem.

10. Were the wing bolts fastened down? One of the first things I checked on getting the model back to the pits was the general integrity of the airframe - nothing was loose. Wings, tailplane and all control surfaces were firmly attached.

11. Was the take-off downwind? No, it was basically into wind - very slightly crosswind from the model's right. About half way along the runway, when I was aware it wasn't showing any sign of taking off, I let the model "have its head" and weathercock directly into wind to help it. (The runway is very wide and you can do that). It didn't help though!

12. The undercarriage has not been removed - well at least prior to this!

13. Could te telemetry help? Yes, looking at the telemetry log might well be interesting. I will do that tonight and see if there is anything suggestive there.

OK, so that's about it. Its looking to me that the possible explanations are that the servo malfunctioned in some way or the cold affected the model more than I expected. Of the two I feel the first is the most likely and, as I have said, I will change the servo just to be sure - and try again! Watch this space,.....

BEB

[Don Fry]

Don't forget to say an incantation to Gravity over spilled fuel before relaunch. Vary as necessary if a lesser breed without the law.

[J Moyler]

Just one thought, I might be wrong but I think your Wot 4 is battery powered. Lipo's can be affected by low temperatures. I find myself for normal flying at low temperatures providing I keep the batteries in the car there are no problems. Saying that there is always an exception, I did have a similar problem with a battery that was on the way out. During the summer no problems at all but when we had are first cold spell I had problems. When I checked it home the power out using my Watt meter was down by a third.

JM

[Biggles' Elder Brother - Moderator]

Hi JM,

yes I mention cold effects in point 2 and in my conclusions above. While I don't think that this was the cause it may have been a contributory factor?

BEB

[Chris Walby]

My 2p worth and I only echo what others have said from personal experience.

Cold lipo where one cell fails (collapsed voltage/short circuit under high load) - Everything checks out pre-flight and roll out is normal, but then the ESC backs off as it thinks the lipo is flat result is the more you push the lipo the worse it preforms. I have had newish, middle aged and old lipos do this to me and only common factor cold but not freezing conditions. Check out the lipo under full load conditions and watch the individual cell voltages.

My son experienced a wildly fluctuating aileron servo, but lack of movement could be another valid outcome where a servo plugged into an extension connection is faulty. Pre-flight checks were all okay, then just after take off it played up. He managed to land it, but it all worked okay. On the club test bench (suitably restrained) and following a full a very thorough visual inspection each servo lead and connector was prodded. Sure enough if you just flexed the aileron servo/extension it would play up. Here is the twist, once unplugged and plugged back in the fault has never come back, so you may never find out what the root cause was. Could replace the servo to eliminate one item, but the nature of the crash/unplugging the connector might clear the cause anyway.

Once back together you could try some take off runs without touching the elevator and see if it ground runs or self lifts off to help explain what it did.

Hope this helps, Chris

[Bob Cotsford]

One other thing that might be worth checking, do the wheels turn freely? Not only do sticky wheels slow the model down, they also pull the nose down and the more throttle you give the more the nose down effect. Cut grass wrapped round the axle, lubricant thickening in the cold, add in lower battrery voltage because of the cold?

[Biggles' Elder Brother - Moderator]

Fair point Chris - I haven't actually checked the invidual cell voltages on the battery inquestion since the incident. I charged and checked all the batteries in the WOT4's set the day before and a bit miffed to find that one did indeed have a low cell - needless to say it didn't come with us on the Sunday. But its worth a check because they all look alike of course and its possible I left the bad battery in by mistake? I'll check it.

Nice idea Bob - but wheels spin fine.

BEB

[Chris Walby]

The problem I describe with the lipo is very difficult/impossible to detect prior to the incident as it looks ok total voltage and individual voltage. Its not until you keep pasting it under high load conditions do you see one cell voltage collapse. You can still pull high current with a resistor, but here is the twist the ESC detects a low voltage and backs the current off to save loosing the RX supply.

Oh and the other thing, one of the lipos would not fault straight away so you could give it full throttle for a few seconds at full power and see no problem. Then once taxied out and starting the prolonged increase in load you could tell the more stick you gave it the less it performed, okay if you have a really long runway! but difficult to detect a reduced acceleration as the model is heading away from you.

[Don Fry]

And from a different thread, methanol burners are said to be complicated.

[Andy48]

Posted by Chris Walby on 09/02/2018 19:08:23:

The problem I describe with the lipo is very difficult/impossible to detect prior to the incident as it looks ok total voltage and individual voltage. Its not until you keep pasting it under high load conditions do you see one cell voltage collapse. You can still pull high current with a resistor, but here is the twist the ESC detects a low voltage and backs the current off to save loosing the RX supply.

Oh and the other thing, one of the lipos would not fault straight away so you could give it full throttle for a few seconds at full power and see no problem. Then once taxied out and starting the prolonged increase in load you could tell the more stick you gave it the less it performed, okay if you have a really long runway! but difficult to detect a reduced acceleration as the model is heading away from you.

Its dead easy to detect with the OpenTX gear BEB and I have. Hook up the cheap voltage sensor £12, set the appropriate verbal warning, and as soon as one cell voltage drops below the preset value, you get a low battery warning. Its a far better way of detecting a failing battery than checking the IR.

[Chris Walby]

Andy, Yes you are right telemetry is a great thing if you have the time and skill to set it up and for most applications is advantages but not for all failure modes.

My point is that up until my battery failures I had no indication that anything was a miss. Good capacity and high current delivery out of a newish well known manufacturers product. No indications on the day until I was over halfway down the runway and then it all went pear shaped!

With telemetry BEB would have got "low battery voltage" with all the other things going on at the same time, useful tool post event, but no necessarily helpful at the time!

[Devcon1]

A couple of points, one already touched on..

I once had a connector pin pull out of the servo lead socket/housing, the servo locked out in flight but then began working again on the ground. It wasn't immediately obvious until very close inspection showed the culprit.

I've certainly experienced this in the winter with cold hands, at the point my hands become cold my brain has been sending the correct signals to my thumbs to pull on the elevator and I think that's what they are doing but my eyes tell me a different story with the model reacting as though very little input has been made, I then have to very consciously pull more stick to get the desired input.

In simple terms could your fingers or thumbs have failed to physically move the stick as much as your brain was telling you it had.

Ive always landed when this happens as it's quite an unnerving feeling.

[David Gilder]

Probably a long shot but I have experienced this when flying gliders.....

Downdraft like a Microburst.

This wouldn't need to be excessively strong for a model aircraft but would prevent it lifting!

[kc]

Everthing has been covered so far ...except the elevator horn being loose in the balsa or the elev joiner being loose in the balsa meaning only half the area moves.  Or a clevis slipping on the thread.

 

Edited By kc on 10/02/2018 08:17:25

[ChrisB]

Loose servo or tray, flexing snake or pushrod, ill fitting servo horn,Tx settings, hinges?

[Cuban8]

I blame Brexit.................sorry joke!

[Geoff S]

Probably witchcraft. Did you notice any flying brooms in the vicinity?

Geoff

[Devcon1]

How about a prize or certificate for whoever identifies the cause 😂

[Trevor]

Might be worth re-calibrating the ESC, just in case full throttle isn’t. If you see what I mean.

Trevor

[Biggles' Elder Brother - Moderator]

I think I may have found the answer!!

The WOT4 has flown a few times since the incident - no problem - and its scheduled to come to the field with me tomorrow. So this morning I'm pottering around the shed checking stuff out for tomorrow and I gather the batteries for the WOT4 together to check them and if needed charge them. I realise that actually I'm a battery short, I sould really have 4, there are only 3 and have been for a while. I remember that the battery that was in the model on the day of its racing car behaviour I left at the back of the workbench. OK no probs.

So, I check the batteries and I come to the one in question. Probably doesn't really need charging, it only did a taxi out and a 80 yard run down the strip! But I put it on the battery checker,....98% - as I thought. Hang on! 13.6v? That's very low for a fully charged 4s - should be more like 16v+. So I check the cell voltages - two at 4.16V (fine), two at 3.0V! What! Looks like two cells have gone down on the battery.

Just out of interest I put it in the model with the wattmeter attached - full throttle 82W - no wonder I couldn't take off the model weighs nearly 5 pounds!

Its possible of course that the cells "went west" after the incident - but I doubt it. I think I have found the cause of the behaviour. That batterry would have just enough umph to give me a brisk taxi and apparently reasonable initial acceleration. But within a few seconds the power would drop off, add in the fact that as the speed went up the drag resistance would increase and you can see tha,t while initially all may look OK, in reality it is far from being OK!

So what can we learn from this? When I was working with the UAVs we logged every battery into and out of the drone recording individual cell voltages. I'm not so diligent as a retired hobby flyer! But it does show that just taking a few seconds to flip through the cell voltages at the end of charging, even if you don't record them, is a very good idea!

Well, that feels good. I accept those incidents where I get it wrong and stuff it in - we all do it! I accept it when fate decrees something fails despite reasonable checks and maintenance. We learn from such things. What do you learn when it all works? Nothing, except that you were right! Experience is in reality nothing more than the accumulated knowledge of many failures! So I hate it when it goes wrong and I don't know why!

Thanks to everyone who helped, suggested and encouraged. I offer this experienece to you, that you may gain from my pain!

BEB

[J D 8]

Thats good BEB,you can stop wandering now as to what happened.

I check individual cell voltages after a flight. Have found that that this is more likely to reveal a battery on the way out thanan after charge check.