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Old Jul 14, 2014, 02:22 PM
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Compass Knight 3D Conversion

Hi guys, need a little help from the department of guesswork.

Here's the deal:

Recently acquired a Knight 50 3D airframe off e-bay for a silly (very silly) price.
The intention is to convert it to electric power.
I have a good idea how to mount the motor and other bits and pieces, but what battery set up to use is the question.
As some will know, the Knight airframe is quite tall but very narrow, so where to place the batteries is a problem.
So far, thoughts tending to one of two options.

1) 2 x 4s 4000Mah in series. One on its side where the fuel tank used to be, the other on the front platform.

2) 2 x 6s 3000Mah in series. Same places.

Given the model only has an 87t main gear, either of these would need a low Kv motor. I am aiming for a h/s around the 2200 mark, so the 8s set up would need 600Kv and the 12s 500Kv.

I have the 3000 6s batteries for my 500, but they are only 20c so 60a max.
The 4000 I have in mind are 25c, so 100a max.
I will have to invent custom mounts for whichever batteries I choose, so I really need to know which before I start cutting anything.
I am leaning to the 12s set up, but I m not sure whether 60a will be enough.
Anybody running a 600 size on 12s can give me an idea of current consumption?

Otherwise, any other thoughts would be welcome.

Over to you

Borderlord
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Old Jul 14, 2014, 11:19 PM
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A couple of thoughts....
with that gearing, it will not be easy to achieve your desired head speed at 12S. As you go up in cell count, the total current used may decrease, but head speed rises, unless you plan to run in an esc governed state.
The difficulty may be finding a pinion with a low enough tooth count to work with an 87T main gear. The smallest I have seen to suit 6mm shaft motors is 10T, and even using that at 12S gives a lot higher head speed than you want, even with a 510kv motor.
Here is a screenshot of the hs calc with those parameters, (12S, 10T-87T, 510kv)...
Attachment 6932237
Another aspect is the batteries you already have. Perhaps it might be prudent not to expect them to output at 100% of the manufacturer's rated spec, i.e. for the 6S 3000mah 20C, safer to give them an 80% efficiency, particular if already used and charged a few times. Anyway, we all know they "exaggerate" the C ratings.
So that would give you ~ 48-50A as a consistent output at 6 or 12S.
If you look at the specs of the 510kv motor that I selected randomly, it looks to need a bit more than those batteries can supply. Of course there are variables and it might run happily on a lot less, but projected maximums are what successful hardware configurations are usually based on. For a conversion like this, allow yourself a bit of headroom for those projected maximums, so that you're not running anything on the border line.
There are possibly other lower wattage motors that might do the job too, don't know, that main gear ratio seems a bit limiting though. If you could find a larger main gear, from what I can see it could help to make the job a bit easier. Otherwise, higher C rated batts (if you really want 12S).
However, I think 12S is a bit much for this.
According to this thread, 8S with 760kv motor is usable . I would have expected a bit lower than that for suitable motor kv at 8S.
With these type of conversions, someone ends up being the ground breaker (so to speak), rather glad it's not me.
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Old Jul 16, 2014, 11:46 AM
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Hi Mono

Thanks for your thoughts.
Unfortunately, I couldn't access your screenshot. Got a 'link invalid' message.
Interested to see what it comes up with, seems to differ from my findings.
I am using Mr Mel's calc, and putting 510Kv,12s and 10t pinion gives 2545 rpm.
This is a little high, but still in the ballpark.
Out of interest, the 600E (dedicated electric version of the 3D) uses the 760Kv motor, supplied in the kit, and on 8s 10T gives 2528.
Much the same as 12s 510Kv.
8s, 600Kv and 11t gives 2195, so we're still close.
Point taken about max current from the 6s packs. They have spent most of their life towing my 500 through hover and ff, and this pulls only about 20a.
Having said that, if,as you suggest, I might only get about 50a from them, that's still 2.22Kw. Should be enough.

Actually an interesting choice of motor you found at random. I have been looking at the SK3 Competition series, but that looks a good alternative.
Not quite sure what you mean by 'it looks to need a bit more than those batteries can supply'.
Thought the motor took only what it needs for the model.

Anyway, jury still out.

Finally, some time ago I converted a Raptor 30 to electric. Mostly guesswork and asking advice, but the resulting model flew (still flies) an absolute treat and is one of my favourites.
I like being a groundbreaker.

Cheers

Borderlord
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Old Jul 16, 2014, 10:05 PM
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The screenshot was just info from hs calc, so it was nothing you don't already know.
As you have already successfully converted a Raptor, I'm sure you have a better idea of what you need to do, than I do. The points I mentioned were just a couple of things that popped into my head when I read your post.
Quote:
Not quite sure what you mean by 'it looks to need a bit more than those batteries can supply'.
Thought the motor took only what it needs for the model.
Yes, it does only take what it needs. It's just that particular motor has the potential to pull far more...
Max Cont. Current: 75A
Burst Current (5 sec): 125A
Max Power: 5500W @ 44.4V
... and that must always be taken into consideration.
Just as you would not pair it to a 50A esc, same applies with batteries that are limited to 50A, even though it might only be pulling 40A under normal running, must allow for the 'not normal', to be safe.
I guess it's the old fashioned electrician in me that likes to see power systems designed to hold up, regardless. But it is also normal procedure for rc component selection.
The SK3 motors are lower wattage (3100-3770W) so less demanding and might be better suited for your purpose.
Just one other thing that occurred to me, as you will be putting batteries in different spots (custom mounts), is the problem of getting the CoG correct and the extent to which it affects your choice of components and their layout. Of course, it might not be an issue at all, I'm just curious.
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Old Jul 17, 2014, 08:59 AM
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Sooooooo........... Is it done yet ? Looks like a interesting project . I know JR sold a nitro to electric kit for their .90 , wonder if it would work for yours , or at least give you some ideas of what to make .
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