Whats the best motor to use?
#1
New Member
Thread Starter
Join Date: Jun 2020
Posts: 2

I'm currently mapping out an idea for a dual engine boxcar design. Going to be using 3 blade prop 12x7 and the overall wingspan should come close to 70 inches.
Could use some recommendations on what would be a good choice of motor?
Could use some recommendations on what would be a good choice of motor?
#4

I always like to figure absolute minimum 85+ watts per pound to have at least a good sedately performing aircraft and preferably 100 watts per pound. Your figure of 70" and ONLY 4 pounds sounds quite optimistic! I don't see you achieving that weight at that wingspan unless it's a glider with no power system. My bare early STICK BUILT lightweight Telemaster 40 at 72" with NO covering and only wing servos is 3.6 pounds.Two motors and TWO 4500 mah batteries capable of flying something that big are going to eat up most if not all of the weight you're planning on and then some.If anything there won't be more than a few grams left for an actual airplane. Best to actually weigh out all the wood, covering, glue, hardware, electronics and batteries and THEN decide what motors/props you'll need. I have a scratch built from 200% enlarged plans twin that is only 72" wing span that will be pretty fast based on its 1/2 scale cousins performance. I built it super,super light and plan on at very minimum 600 watts and I HOPE for only 6 pounds All Up Weight!
Joe
Joe
#5

I would agree Turbojoe that two of those batteries will make a 4 lbs target very difficult to achieve as together they will weigh 26 oz (1.6 lbs). You would be up to 2 lbs just for the motors and batteries!
You are doing well if you can make an air frame that carry a battery of 25% of the all up weight which suggest an all up weight of at least 6.5 lbs and more like 7 or 8 which means 2 x 400+ W motors for 100W/lb.
Incidentally such a motor will also be able to drive a 12 x 7 three blade prop.
To ''map out" a design you can start in a number of ways for example: All up weight, battery weight, motor power. plane size, prop size but each will provide a different result.
For example on my Bombardier Q400 twin I started with a 9 x 6 three blade prop. This set the size of the plane such that it was a true scale diameter. I like to have at least 100W/lb available so everything else was sized to suit.
It proved to be a very challenging target.
You are doing well if you can make an air frame that carry a battery of 25% of the all up weight which suggest an all up weight of at least 6.5 lbs and more like 7 or 8 which means 2 x 400+ W motors for 100W/lb.
Incidentally such a motor will also be able to drive a 12 x 7 three blade prop.
To ''map out" a design you can start in a number of ways for example: All up weight, battery weight, motor power. plane size, prop size but each will provide a different result.
For example on my Bombardier Q400 twin I started with a 9 x 6 three blade prop. This set the size of the plane such that it was a true scale diameter. I like to have at least 100W/lb available so everything else was sized to suit.
It proved to be a very challenging target.
#6
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Join Date: Oct 2010
Location: Ex UK Brit now in Latvia west coast - Ventspils
Posts: 12,597

I found as model size grew .. the motor KV came down .. such that I had to make up the 'lost KV' by voltage ...
To use 3S means (this is where personal bias really comes in) a KV of around 1000 and up. Many motors to turn a 12x7 prop well are less than that.
BUT anyway .... eCalc comes to the rescue for this stuff ... for very small registration fee ... eCalc allows to put in motor / ESC / Prop / Battery as well as anything else you have to arrive at best combination.
As a starter - it's likely the motors will be about a 38xx size .... 800kv and up ...
To use 3S means (this is where personal bias really comes in) a KV of around 1000 and up. Many motors to turn a 12x7 prop well are less than that.
BUT anyway .... eCalc comes to the rescue for this stuff ... for very small registration fee ... eCalc allows to put in motor / ESC / Prop / Battery as well as anything else you have to arrive at best combination.
As a starter - it's likely the motors will be about a 38xx size .... 800kv and up ...
#7

Keep esc(s) close to battery(s), extend motorwires, not the batterywires.
too long wires batteryside will kill ESC over time: precautions, solutions & workarounds, pictures - RCG
too long wires batteryside will kill ESC over time: precautions, solutions & workarounds, pictures - RCG
#8
Community Moderator
Join Date: Aug 2005
Location: NY, USA
Posts: 5,852

May I suggest you take a look at the online book on sizing your power system which is located on Wattflyer. It will address this and the other things you will need to know.
https://www.wattflyer.com/forums/showthread.php?t=31071
https://www.wattflyer.com/forums/showthread.php?t=31071
#9

Below an excellent quote, a step by step instruction for motor selection.
From
brushless motors Kv?.
Vriendelijke groeten, en wees voorzichtig, Ron
• Without a watt-meter you are in the dark ... until something starts to glow •
• e-flight calculators • watt-meters • high power motor tips&tricks • Cumulus MFC •
From
brushless motors Kv?.
While an absolutely critical part of the system ...
... Kv is actually the item one should choose last.
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
... Kv is actually the item one should choose last.
- Decide your peak power requirement based on the mass of the model and how you want to fly it:
Magic numbers for e-flight - WFF - Pick a preferred cell count (voltage) and pack capacity for how to deliver the power.
- 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.
- 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.
- 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
Vriendelijke groeten, en wees voorzichtig, Ron
• Without a watt-meter you are in the dark ... until something starts to glow •
• e-flight calculators • watt-meters • high power motor tips&tricks • Cumulus MFC •