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Matching ESC to motor & prop

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Applewood23/03/2018 14:59:00
35 forum posts

I've acquired a model and hoping to build 8S setup. Its got an AXI 5330/18 motor, and I've selected via ecalc the 18/10 prop. giving 12.5C, 8.1min flight, 1578W, 70DEG, 1.4:1 RATIO, 110 km/h. Looking at the motor results its says current 43.9A optimum and motor @ maximum 56A.

If I allow 20% headroom 1.2x56 = 67. So would a minimum 70a ESC be ok?

I'm looking at 8S HV ESC and they are very expensive in comparison to no HV. Looking to do this on budget.

Also if the battery load is only 12.5C , will that mean I don't need a really high C lipo??

Any advice would be appreciated. Thanks.

Callsign Tarnish23/03/2018 15:07:08
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70 forum posts
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You'll probably get many views, my own one is that I allow plenty of headroom, generally an ESC +50% of my expected max load and a high C rating for cells. This helps to keep these components of the power train cooler and less stressed.

Edited By Callsign Tarnish on 23/03/2018 15:07:40

Dickw23/03/2018 15:47:32
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394 forum posts
54 photos

What is the model? Chances are you would rarely be running at full throttle in most models, but some you might be.

Motocalc agrees with your ecalc figures and I would be happy with a 70a ESC on that setup.

HV ESCs usually are more expensive, but this one seems a reasonable price and spec:-**LINK**

The C rating required depends on load and battery size, so I am guessing that your 12.5C comment is based on ~4000mah cells.

I would have no problem in using C ratings as low as 20 to 30 on 4000 mAh cells at 50 amps in most models.

One thought is that a larger ESC size and higher battery C rating would give you more room to experiment with different props if that is a possibility – e.g a 20x10 prop would put you closer to 80 amps.

Dick

Edited By Dickw on 23/03/2018 15:48:41

Simon Chaddock23/03/2018 15:53:32
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5289 forum posts
2779 photos

Applewood

Ecalc will give you a good indication of the likely maximum current but there is always a range. The only way you can be really sure is to use a Watt meter to actually measure the maximum current or more important it allows you to stop before you exceed it.

You are playing with quite a high power set up which usually means the consequences of exceeding limits is that much more expensive. An appropriate Watt meter is not expensive and certainly less than a 70A HV ESC. wink 2.

Battery C ratings are rather subjective. A battery may well be able to achieve the claimed figure but if you actually use it what does it do to the performance life of the battery? A rule of thumb is to keep within half the claimed C rating..

Applewood23/03/2018 15:56:41
35 forum posts

Thanks Dick, Its a Tony Nijhuis 72inch mk5 Spit. weighing in at 4760g. Appreciate your advice.

MattyB23/03/2018 16:13:47
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1859 forum posts
28 photos
Posted by Dickw on 23/03/2018 15:47:32:

What is the model? Chances are you would rarely be running at full throttle in most models, but some you might be.

Motocalc agrees with your ecalc figures and I would be happy with a 70a ESC on that setup.

HV ESCs usually are more expensive, but this one seems a reasonable price and spec:-**LINK**

The C rating required depends on load and battery size, so I am guessing that your 12.5C comment is based on ~4000mah cells.

I would have no problem in using C ratings as low as 20 to 30 on 4000 mAh cells at 50 amps in most models.

One thought is that a larger ESC size and higher battery C rating would give you more room to experiment with different props if that is a possibility – e.g a 20x10 prop would put you closer to 80 amps.

Dick

The advice above from Dick is excellent; I concur that 70A ESC should be fine. The only thing I would say is that whilst Dick is right in his calculations that a 20x10 would pull ~80A, I can't see any reason that you would need that - you already have ~150W/lb on the 18x10 which should be extremely "adequate" as Rolls Royce used to say of their cars!

PS - Using a low C battery may well be fine, but they will get hotter for a given amp pull and will probably have a shorter life as a result. They do tend to be significantly cheaper though!

Edited By MattyB on 23/03/2018 16:15:03

Dickw23/03/2018 16:34:51
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394 forum posts
54 photos

A Spitfire - like it - just building a smaller one myself.

So now that we know what it is I agree with MattyB that you shouldn't need any more power as thrust is already likely to exceed weight. You will probably only be using full throttle in shortish bursts e.g take off and climb out and a few aerobatics, with most of the flight throttled back. Your planned setup seems fine to me.

Dick

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