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Author Topic: Puffer Wiggle  (Read 6043 times)

Bunkerbarge

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Re: Puffer Wiggle
« Reply #25 on: August 29, 2009, 09:58:47 am »

I'm not sure if we are now going up the garden path with this and getting a bit bogged down with irrelevencies.  If the model was sailed with the rudder disconnected and locked and only the ESC powered up then the problem is surely not with the electrical side of things.  Colin has rightly identified that the issue is with the hull and rudder design or combination and how they behave hydrostatically.

One thing I haven't seen mentioned, although I may have missed it, is what motor and therefore how fast is this model going?  Don't forget that a true speed for one of these vessels was no more than about nine knots however most models seem to progress across the ppond considerably faster than that!  Going too fast with the model down to it's marks will quite often cause instability, especially with a hull having the hydrodynamic efficiency of a brick!  In conjunction with this is rudder size and integrity of the linkages.  Any slack in the rudder, no matter how insignificant it may seem will lead to instability with such a hull as this down to it's marks so that should be checked out as well.

If this effect does not come to like at slower speeds I would do no more than run her slower by either controlling it or adjusting the range on the ESC channel of your radio.  You don't really want to play around with changing the rudder or removing ballast or even the motor, but if it is more stable at a slower speed, then run it at a slower, more scale speed.

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FullLeatherJacket

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Re: Puffer Wiggle
« Reply #26 on: August 30, 2009, 08:41:15 pm »

Just back from a model show and seen the pictures. The tiller arm looks very very short, even if it's true scale. Any play in the linkage will be more significant the shorter the tiller arm and take-off point on the servo disc are/is/am. I'd suggest you double the length of the tiller and move the pushrod further out on the servo disc.
It may well be another "irrelevency" to the problem in hand but it won't hurt.
Wasn't there a thread a while back about a canal boat which did similar loopy things? Did the bloke ever solve that prob?
FLJ
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ianb

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Re: Puffer Wiggle
« Reply #27 on: September 05, 2009, 06:16:59 am »

Update on the problem.

I went to the pond this morning with a friend to follow derekwarner decoy's suggested tests. Results; radio connection is reliable and positive with the Tx antenna down to a distance of about 200 feet. Over that distance and the motor shut down, with the rudder holding its last position. This is the way it should work according to JR.

Next, I tried the puffer at a slower speed, about 2/3, to see if the twitch re-occurred. It did, but not as much as before and not as often.

As to the questions from Bunkerbarge; the motor is a Model Motors Direct 555 and the "full" speed is less than walking speed, perhaps 2 mph. Definitely not a hotrod! There is less than 1/32 rudder free play at the aft end furthest away from the hinge.

An idea came to mind, maybe completely wrong, but then I am usually so.

The pond is man made and about 20 inches deep. I know that there are many small rocks, about 4-5 inches round, here and there on the bottom. I wonder if the prop wash goes down, hits a rock and is deflected back up which affects the steered course just a little. Or the fact that the puffer is quite heavy with a very unstreamlined hull shape may make her very sensitive to water flow changes reflecting from the pond bottom. What think you?

If you tell me I'm waaaaay off base, I won't be offended.

Ian
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derekwarner

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Re: Puffer Wiggle
« Reply #28 on: September 05, 2009, 08:40:41 am »

Ian......considering all of the comments from experienced members on Mayhem...  ;D :-)) & your comments & tests....

Forget the size of the rocks in the pool, forget the 1/32" radial arc at the rudder extremity, forget the hydrodynamic "built like a brick" of the hull....& my understanding of JR R/C equipment is that it is second to none  ..........

so  >>:-( I suggest that irrespective of the motor suppression installed ............the motor is the culprit  <*< >>:-(...in that it is providing a spurious intermittent EMF signal that is not being filtered out...& hence the Rx jitter  O0 ....Derek
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Derek Warner

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Martin [Admin]

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Re: Puffer Wiggle
« Reply #29 on: September 05, 2009, 08:47:36 am »

Any chance of a video clip Ian?
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ianb

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Re: Puffer Wiggle
« Reply #30 on: September 05, 2009, 12:00:23 pm »

Sorry Martin, no chance at all.

I don't possess a video camera, couldn't operate it if I had one, couldn't upload a video to save my life, and can't even post pictures without you having to correct my efforts.

No, video is way beyond my limited technical ability.

Ian
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GG

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Re: Puffer Wiggle
« Reply #31 on: September 05, 2009, 04:34:51 pm »

Time for my two pennyworth's?
All the supplied information suggests,
1) the rudder servo and linkage are not to blame
2) no evidence of motor interference
3) the rudder is completely, or almost so, covered by the propwash
4) the model has a very bluff bow shape

The third point would mean that the the rudder is unaffected by small changes in heading.  The water flows past the rudder just the same and so it exerts no corrective action.
The last point suggests that the models "centre of resistance" lies ahead of the models "center of gravity", possibly moving further forward as the speed and associated bow wave increases.  This makes for an unstable model which will not hold any heading when disturbed.
I suspect that the as the model builds up speed, it becomes directionally unstable and starts to weave from side to side.  This slows the model and reduces the instability allowing the model to run straight again.  The the whole process repeats again.  This ties in with a description of the model's behaviour.
One way to test this theory is to enlarge the rudder.  Extending the rudder rearwards will not take it out of the propwash and might be of limited value.  Extending the rudder downwards, and thus out of the propwash, is like to be more effective.  It will now be in the free flowing water stream and thus notice as soon as the models heading changes.

I cannot help but think that a lot of confusion has arisen through the use of the word "twitch" which to me usually means a violent motion caused by sudden rudder movement.  Hence, my use of the word "weave"
GlynnG
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ianb

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Re: Puffer Wiggle
« Reply #32 on: September 06, 2009, 06:23:27 am »

GlynnG,

Thank you for your very succinct post. You are quite correct, the word "weave" is a far better choice than "twitch". Mea culpa.

The cause of the weave may be exactly as you have so carefully reasoned. If so, then there is nothing I can do in a practical sense to alleviate or cure it ie "its in the nature of the beast".

The best thing to do may be to just accept it, and make small corrections to the steering as required in order to maintain an approximately straight course.

Thank you to everybody who contributed many valuable ideas and much good advice. Much appreciated.

The Mountfleet Puffer has been a challenging but very satisfying build. She really looks great on the water. I'm happy, and isn't that why we build model boats?

Ian
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GG

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Re: Puffer Wiggle
« Reply #33 on: September 06, 2009, 02:28:56 pm »

Glad to help Ian,
              If nothing else, this thread illustrates the difficulties of trying to sort out problems on an Internet Forum.  If the problem is not described with 100% clarity (not impossible but never easy!) it can encourage people to launch into their own pet theories.  A good example is external interference, often claimed when the true problem can lie closer to home.
I'd be tempted to try adding a temporary rudder extension to your model, perhaps some thin material held in place with waterproof tape.  This might confirm the problem and possible solution, perhaps too late for this model but handy in future?
Just returned from sail my latest model which has this problem.  I quickly learnt to sail at less than full throttle and keep applying small rudder corrections.
GlynnG
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ianb

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Re: Puffer Wiggle
« Reply #34 on: September 20, 2009, 09:56:18 am »

Final update on my problem if any body is interested.

I installed a good piezo electric gyro, left over from the days when I tried to fly model helicopters, at the approximate CG of the puffer. 50% gain at the rudder neutral was put in.

What a difference! Almost a straight course without any correction from me. No weaving that I could see.

I am coming to the conclusion that the hull in it's present form, ballast, speed, rudder size etc., or the combination, is just not stable in direction when on the water. Why? I don't know.

So, my problem seems to have been solved with an item from my parts box.

Thank you to everybody who contributed ideas and suggestions.

Ian
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Colin Bishop

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Re: Puffer Wiggle
« Reply #35 on: September 20, 2009, 12:27:54 pm »

Thanks for the feedback ian. Some hulls are just like that - the Battleships Rodney and Nelson were full size examples with poor low speed steering characteristics.

Colin
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farrow

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Re: Puffer Wiggle
« Reply #36 on: September 23, 2009, 11:09:43 pm »

Many years ago, I skippered a motor barge with a similar shape to your model. An yes she was a COW to steer, you never knew which way she would sheer, but when manoeuvring she would turn on a sixpence. I ended up with large biceps and frayed nerves, as when you was going up a river you never knew if she would veer off course and hit something.
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