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WOT4 Becomes Racing Car

A bit of a head scratcher.

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Andy4808/02/2018 16:20:46
1333 forum posts
1 photos

Great stuff!

Not a clue what went wrong though. Did you try doing exactly what you did down at the field, see a duplicate model and delete it. Maybe you deleted the wrong one?

Martyn K08/02/2018 16:38:44
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4538 forum posts
3162 photos

I have no idea either. I cant reproduce it with the original setup. I wonder if I had restarted the Tx (power off/on) that may have cleared the problem as that was the only thing that changed by the time I got home and tested it.

Concious that we are hogging BEBs thread here. Sorry BEB

Denis Watkins08/02/2018 17:55:40
2661 forum posts
136 photos

This can be .problem for 40/46 size models on wet grass

Martin Harris08/02/2018 18:04:38
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7495 forum posts
187 photos

I witnessed an interesting and similar phenomena a few years ago when a clubmate was doing touch and goes on a cold misty day with a more than adequately powered Mahers 3DoubleD (Moki 150). Power was applied at the appropriate time, the engine responded with its usual sound and revs but the model refused to fly away - luckily ending up in the next field undamaged.

The cause? Propeller icing - the prop was simply beating the air with an inefficient profile.

Edited By Martin Harris on 08/02/2018 18:05:09

Biggles' Elder Brother - Moderator08/02/2018 19:18:40
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Hi,

lots of interesting ideas here. I am away from base just at the moment and on my phone, so I'll do a full response later this evening when I get home. Thanks again.

BEB

john stones 108/02/2018 19:34:33
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9241 forum posts
1413 photos

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

Andy4808/02/2018 21:12:32
1333 forum posts
1 photos

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

MaL09/02/2018 10:38:37
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94 forum posts
12 photos

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

FlyinBrian09/02/2018 12:02:36
464 forum posts

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 - Moderator09/02/2018 14:13:07
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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 Fry09/02/2018 14:50:04
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2345 forum posts
30 photos

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 Moyler09/02/2018 16:11:14
128 forum posts
56 photos

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 - Moderator09/02/2018 16:22:01
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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 Walby09/02/2018 16:45:41
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447 forum posts
68 photos

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 Cotsford09/02/2018 16:58:13
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7161 forum posts
405 photos

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 - Moderator09/02/2018 18:27:48
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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 Walby09/02/2018 19:08:23
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447 forum posts
68 photos

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 Fry09/02/2018 19:11:44
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2345 forum posts
30 photos

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

Andy4809/02/2018 20:01:21
1333 forum posts
1 photos
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 Walby09/02/2018 21:45:59
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447 forum posts
68 photos

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!

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