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Author Topic: need to get this right  (Read 2413 times)

warspite

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need to get this right
« on: August 10, 2008, 10:14:05 PM »

Some of you will notice i have posted quite a few things regarding a Plastic fantastic airfix vosper MTB.

Is this what the modelling world now generically refers to the plastic kit conversions?

Anyway, after much searching, i found the thread relating to the use of servo's to power control these types of kit, finally.

The upshot of the search and the testing of the drive train, suggests that the higher the speed you need to achieve the more you will need to use a generic ESC, the slower more plod along boats which can quite happily run with the servo motor rpm can use the servo.

So i just need to get this right as i have 8, yes 8, plastic fantastic boats to fit out, 2 vospers fully sealed against water ingress but to cruise at a scale speed of about 20 knots min. 1 german 'S' boat to refit (again) and another to build (possibly another to resurect from my childhood) a Revell pt109, the running gear in this may be easier to fathom out as it is deeper, a LCT and a battleship at 1/600, the other 3 boats not included in the plastic fantastic size range, are a refit of a matchbox flower class (yes i know it is still classed as a plastic fantastic, but at 3ft it's not exactly got a space problem, a cardboard shell to complete and seal etc, based upon a castle class corvette shape and a GRP tug about 9-12" long.

For the vosper MTB's i have a motor from maplins and in the test it works quite well, whether it does the scale speed of 20 Knots i am unsure, though when it gets put in the water (bath) i doubt it will preform anywhere near what i expected, i think i have got the amp setup right and was wondering if this picture confirms that this is ok, not really sure what the amps are but the meter shows 2 notches past the 50/10/2 marking on the dial when the selector is on 0.1/.05 - any idea on what amps it was drawing at full throttle (sounded like it was the fastest it would go, setup was no grease in prop tube (forgot) and free running in air, will be a pig to fit in the boat.
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malcolmfrary

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Re: need to get this right
« Reply #1 on: August 11, 2008, 10:19:09 AM »

With this kind of meter, set the switch to one of the Amp ranges (preferably the highest), plug into COM (Black lead) and Amp (red lead)(usually). Use the ammeter in place of the positive lead from the battery, red to +ve.  With a needle type meter you have to be careful about connecting the right way round, and not over-doing the reading - both result in a curly needle.  I have got used to my digital meter, much easier to set up and read.
Picking the scale to read usually involves a bit of mental arithmetic, again, thats a benefit of a digital meter.
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maninthestreet

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Re: need to get this right
« Reply #2 on: August 11, 2008, 11:56:02 AM »

The picture isn't very clear, but I would say you were measuring DCV (DC volts), and not amps (current).
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RickF

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Re: need to get this right
« Reply #3 on: August 11, 2008, 12:39:13 PM »

You also need to check the stall load of the motor. Make sure you have a fully charged battery, connect the circuit as described by Malcolm, then, when the prop is running flat out, stop the motor by gripping the shaft. Watch your fingers and note the maximum amps reading. This will be the stall current. Add 10-15% (ish) and that is the max current your ESC will need to handle.

Rick
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warspite

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Re: need to get this right
« Reply #4 on: August 11, 2008, 08:40:34 PM »

from the meter the top left corner (which the wife spilt paint on) states DCV, the dial is actually set at 0.1 on the DCV and .05 on DCmA, top right in red is ACV bottom right is ohm's in blue.

From TECHBOTS.co.uk the motors they refer to as 'A' type, have the same dimensions and are stated at having a load current at 1.1A, 13100rpm and running on a voltage of 1.5v-4.5v, with the 4.8v battery pack running through the receiver and then through an ACTion P52 i believe i need to go up to the P78 for 2A, though do i need a bec then, or could the P52 handle the full throttle?
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warspite

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Re: need to get this right
« Reply #5 on: August 11, 2008, 08:54:45 PM »

I also thought about the siting of the batteries, here's a thought, a Ni-Cd 4.8v 600mAh is basically 4 x 1.2v batteries stuck side by side opposite ended to it's neighbour, with soldered contacts connecting pos to neg - is it possible to solder connect 4 uniross rechargeable 800mAh's pos to neg to each other, so that they surround the motor on three sides, ie two bow end of motor and one abreast each side of the motor but further astern, so as to spread the weight, as the motor sits where the batteries would ideally go, would there be frequency suppression issues etc, could they be recharged as if they were just as the 4 pack systems
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malcolmfrary

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Re: need to get this right
« Reply #6 on: August 11, 2008, 09:37:29 PM »

I usually create a sausage string of cells, either 4 or 5, AA or AAA and plant them as required for weight distribution.
When siting the aerial, that just goes as far away from the motor as possible - usually not very far in a boat a foot long but every little helps.
A fellow club member has a scratch 1:60 PT boat.  This rides a bit deep, but looks good on the plane.  The smaller ones need to be built light.  I tested an Airfix Vosper, and the plastic bits floated at waterline, so I never got to load them further with batteries and electronics.  Ideally, the hull would need replacing in a lighter construction.
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warspite

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Re: need to get this right
« Reply #7 on: August 11, 2008, 10:57:11 PM »

As for how the hull floats, from memory, the full kit does run deep as a kit.
So just ran another water test using a 4.8v 600mAh Echo NiCd and an overlander 4.8v 700mAh for waterline weight tests

On the rear of the kit are 6 mouldings of rungs, underside of bottom one is flush with the underside of the keel, the 6th is one step up from the deck, at the front of the vessel where the keel curves up to the keel line (no idea what it is actually called - see the earlier photo's for the models bow).

Unladen the hull on it's own, sits the depth of a rung underneath the keel line at the bow and just below the lowest rung on the stern.

With the 4.8v x 600 (PX60AAMCx4 - i.e. AA's) and weight moved to give the waterline under the keel line, it reaches flush with the top of the 3rd rung.

With the Overlander, although it has a greater charge, the cells are about a 1/4 bigger on dia and about 1/2 the length compared to an AA cell, balanced again to the underside of the keel line, the rear was about half way through the 3rd rung.

With both battery sets, the rear was about the 5th rung and the front had the waterline above the keel line.

All in all, the use of another overlander, split and wired to distribute the weight and keeping the weight of the other equipment down to a minimum, i might just get away with it running with the weight of 4xAA's, that is to say, reduce what weight i can by losing plastic where i can by 'thinning' areas not on show.

Places like under hatch covers, by removing the deck within the square etc, under the (carly?) floats, including the inside of the bottom one, try to compartmentise the inside and seal it so that the electrics can be fitted into a plastic card box that may be lighter than the P52 abs box.

any comments? :)
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warspite

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Re: need to get this right
« Reply #8 on: August 11, 2008, 10:59:57 PM »

Sorry the photo's are on the thread ' here one i found earlier---MTB'
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warspite

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Re: need to get this right
« Reply #9 on: August 11, 2008, 11:45:43 PM »

these picture site the components, servo rudder, motor fitted where shown etc overlander split and spraed around the motor the on off switch will be a micro version with a lead fitted to the black electrical connector fitted under a curved cowling to hide it, the feather to one side of the shaft - as far back as possible and the ESC on the other side, weight loss will need to include some of the plugs and sockets.

The next water trial will need to have the motor fitted and the rest of the equipment positioned as shown, if all goes well, ie fully charge battery and run to check amp's by linking black croc to - neg on battery, pos on battery to -com on ammeter, + on ammeter connecting to positive of motor, if this gives a figure i will ask what it is, as i read on techbots, this type of motor should only go to 1.1amps at 4.5v, will see.

Then if ok, will fit the electric stuff and position the batteries once the top has been built, final trim will be decided upon after fixing top, once this is closed up i will not be going back into it, unless i can think of a way of sealing it without fixing the top on, any comments?
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warspite

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Re: need to get this right
« Reply #10 on: August 12, 2008, 12:21:20 AM »

These pictures site the components, servo rudder, motor fitted where shown etc overlander split and spread around the motor, the on off switch will be a micro version with a lead fitted to the black electrical connector(shown on top of the battery) fitted under a curved cowling to hide it, the feather to one side of the shaft - as far back as possible and the ESC on the other side, weight loss will need to include some of the plugs and sockets.

The next water trial will need to have the motor fitted and the rest of the equipment positioned as shown, if all goes well, ie fully charged battery and run to check amp's by linking black croc to - neg on battery, pos on battery to -com on ammeter, + on ammeter connecting to positive of motor, if this gives a figure i will ask what it is, as i read on techbots, this type of motor should only go to 1.1amps at 4.5v, will see.

Then if ok, will fit the electric stuff and position the batteries once the top has been built, final trim will be decided upon after fixing top, once this is closed up i will not be going back into it, unless i can think of a way of sealing it without fixing the top on, any comments?

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malcolmfrary

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Re: need to get this right
« Reply #11 on: August 12, 2008, 11:11:09 AM »

Provided it floats at an acceptable level (its military, commercial rules dont really apply) it will be OK.  Personally, I've never been a fan of sealing parts in that might at some time need attention, but then again, I know that most of my stuff WILL need some attention sooner or later.  The big problem with a plastic deck is that over time a sheet of shaped plastic left to its own devices will warp, and never in a beneficial direction.  The answer might be to find a substance to use as a gasket sealant at the deck edge that could be easily broken without damaging the plastic to allow access for servicing, but I have yet to find one.
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warspite

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Re: need to get this right
« Reply #12 on: August 12, 2008, 08:50:55 PM »

Thanks FLJ for the info this morning, will trial the new motors with the view to going back to twin prop, although i would assume that this does not mean there would be any significant increase in speed, splitting the overlander so that the batteries run up the middle with the two matchies down the side etc.

Will need to do a build thread soon - after the tests of course. O0
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warspite

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Re: need to get this right
« Reply #13 on: August 14, 2008, 08:30:45 PM »

Mainly for FLJ sake, after comparing the matchie to the speed of the other motors, it appears to be as fast if not slower than all the others, the diference is that the quality is far superior to the others and it is smaller that the ones i was going to use, the general thought is that i will use this motor shown below, after doing a heath robinson rpm test the final conclusion is, i will have to stick with the largest motor, and a single prop, know any suppliers of small 3 bladed racing props?, doesnt look like prop shop do them - small anyway, unless i was looking in the wrong place.

I will attempt to run the boat on the 5 blades at the next test stage, the matchies i can use on other vessels.

I went on an american site when searching for slot car motors, and they had these 15-20 package motors, generally the same size as the maplins type, but their speeds and torques were exceptional, one or two in excess of 18000rpm and 50+ grammes etc, really powerful, some of them were the same as the lens motors found in cd drives, so they were generally a supplier for the industry.
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malcolmfrary

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Re: need to get this right
« Reply #14 on: August 15, 2008, 11:47:21 AM »

For small size and efficiency, have you considered brushless?  The electric flyboys have some potentially light and powerful gear, but they tend to lack reverse.
With care and attention and some very basic tools, it is possible to make your own small props.  Manufacturers don't have a magic wand, and when you chase the train of events back, it always comes back to somebody sitting down with a sheet of brass and a file.  A manufacturer has to set things up so that his products can be turned out simply and consistently.  The individual modeller generally just has to get it rright once.
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warspite

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Re: need to get this right
« Reply #15 on: August 16, 2008, 12:25:16 AM »

Unfortunetly, I do not have the time / patience / resources and aptitude to be able to manufacture my own (why do you think i do plastic fantastics  {-)), I only intend to do three very small vessels two vospers and an LCT, so two reasonable fast vessels and a plod along.

With the space restraints, acheiving a cruise speed on direct drive is the only goal, even that may be a pipe dream, will see. :)
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