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What Motor Please?

Old 07-05-2021, 06:27 PM
  #1  
The Professor
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Default What Motor Please?

Hi all,

In another post, I did say I would ask for help rather than making (more) stupid mistakes

So I am about to start building a Dumas Tiger Moth, and am reading through the destructions. It does say I need the following motor
  • GWS "A" or MPI, EPU 6 Gear Drive Unit (5.85:1 ratio)

I have googled this, and I cannot seem to find the set as listed above.

Could someone advise me as to what to buy please. I am asking early on, so I can then get the right ESC and associated bits.

I would be really grateful for any advice.
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Old 07-05-2021, 08:52 PM
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ron_van_sommeren
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That advice is obsolete now. Nowadays it's brushless: lower weight and higher efficiency.
Don't forget: you need a brushless controller for a brushless motor. (And a brushed controller for a brushed motor.)


Below an excellent quote about motor selection.
From
brushless motors Kv?.
Originally Posted by scirocco View Post
While an absolutely critical part of the system ...
... Kv is actually the item one should choose last.
  1. Decide your peak power requirement based on the weight of the model and how you want to fly it:
    Magic numbers for modellers.
  2. Pick a preferred cell count (voltage) and pack capacity for how to deliver the power.
  3. Pick a prop that will a) fit on the model and b) fly the model how you want - often as big as will fit is a good choice, but if high speed is the goal, a smaller diameter higher pitch prop will be more appropriate.
  4. Look for a size class of motors that will handle the peak power - a very conservative guide is to allow 1 gram motor weight for every 3 watts peak power.
  5. Then, look for a motor in that weight range that has the Kv to achieve the power desired with the props you can use - a calculator such as eCalc allows very quick trial and error zooming in on a decent choice. For a desired power and prop, you'd need higher Kv if using a 3 cell pack compared to a 4 cell pack. Or for a desired power and cell count, you'd need higher Kv if driving a smaller diameter high speed prop compared to a larger prop for a slow model.
The reason I suggest picking Kv last, is that prop choices have bounds - the diameter that will physically fit and the minimum size that can absorb the power you want. On the other hand, combinations of voltage and Kv are much less constrained - at least before you purchase the components.

So Kv is not a figure of merit, in that higher or lower is better, it is simply a motor characteristic that you exploit to make your power system do what you want, within the constraints you have, e.g. limited prop diameter, if it's a pusher configuration, or if you already have a bunch of 3S packs and don't want to buy more, and so on.

Minor lay-out changes by RvS

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Old 07-06-2021, 02:45 AM
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Thank you so very much for your help. Also thank you for the link to the calculator.

I am ever so grateful. 🤝
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Old 07-06-2021, 09:29 PM
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ron_van_sommeren
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Hold yer horses!
Some useful and I hope instructive insights first, before you start plugging in numbers. To get some feeling/insight, and to get rid of a few possible misconceptions.

The 'full' story.
Even a seemingly small change/error in setup can result in quite an unexpected increase.
Therefore always measure current and power in a new or changed setup.
Because current and power drawn are proportional to
  • pitch
  • #blades
  • voltage≤ respectively voltage≥
  • kv≥
  • diameter⁴
So even a small change/error in setup can have huge effects.
Increase in current with one or two cells added, simple table - RCG

Expensive examples.
Going from 2s to 3s, a 50% increase, would (3/2)≤=2.2 fold current, more than double, 120% extra.
A 25% increase in velocity konstant kv≥ would (1.25)≥=2 fold current, 100% extra.
A mere 10% increase in diameter would (11/10)⁴=1.1⁴=1.4 fold current, 40% extra.

Extreme examples.
Doubling voltage would 2≤=4 fold current.
Doubling velocity konstant kv≥ would 2≥=8 fold current.
Doubling diameter would 2⁴=16 fold current.

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Old 07-07-2021, 01:49 AM
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quorneng
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The Prof
Your Dumas Tiger Moth will not need a lot of 'modern' power. The original recommended motor and 5A speed controller would only give a maximum of 40W of power. Brushed motors with gearboxes would at best only deliver 75% of that to the prop.
The advantage of brushless motors is they are not only more efficient (85%) but are also about 1/2 the weight. There is thus little penalty to installing a bit more power than you need to just to fly. You simply use the throttle to set the power level you want..
You could do worse than buy a complete package like this.
On a 2s LiPo that motor will turn an 8 x 4 prop and could deliver 80W generating more thrust than you will need, at least to start with. Don't forget the full size Tiger Moth was fully capable of aerobatics.

I can vouch for that particular motor. I have one in a rather bigger and probably a bit heavier plane than the Dumas Tiger Moth. It turns an 8x4 prop from a 2s LiPo and has done so now for quite a few hours. .. .


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