General Electric Discussions Talk about topics related to e-powered RC flying

Whats the best motor to use?

Old 06-30-2020, 10:50 PM
  #1  
Skyler_darkwolf
New Member
Thread Starter
 
Join Date: Jun 2020
Posts: 2
Default Whats the best motor to use?

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?
Skyler_darkwolf is offline  
Old 06-30-2020, 11:23 PM
  #2  
quorneng
Super Contributor
 
quorneng's Avatar
 
Join Date: Jul 2008
Location: Cheshire, UK
Posts: 1,932
Default

You will have to provide a bit more getail. Like what battery you intend to use (capacity, number of cells) and the likely total plane weight.
quorneng is offline  
Old 07-01-2020, 01:56 AM
  #3  
Skyler_darkwolf
New Member
Thread Starter
 
Join Date: Jun 2020
Posts: 2
Default

My best guess would be close to 4 pounds and I was thinking of putting in dual 4500mah 3s Lipo 60C batteries.
Skyler_darkwolf is offline  
Old 07-01-2020, 04:30 AM
  #4  
Turbojoe
Mountain Models Minion
 
Turbojoe's Avatar
 
Join Date: Aug 2005
Location: Mesa, Arizona
Posts: 1,255
Default

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
Turbojoe is offline  
Old 07-01-2020, 11:04 AM
  #5  
quorneng
Super Contributor
 
quorneng's Avatar
 
Join Date: Jul 2008
Location: Cheshire, UK
Posts: 1,932
Default

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.
quorneng is offline  
Old 07-01-2020, 11:12 AM
  #6  
solentlife
Super Contributor
 
solentlife's Avatar
 
Join Date: Oct 2010
Location: Ex UK Brit now in Latvia west coast - Ventspils
Posts: 12,522
Default

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 ...
solentlife is offline  
Old 07-01-2020, 11:13 AM
  #7  
ron_van_sommeren
homo ludens modelisticus
 
ron_van_sommeren's Avatar
 
Join Date: Jul 2005
Location: near Nijmegen, Nederland
Posts: 1,206
Default

Originally Posted by Skyler_darkwolf View Post
... dual engine ...
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
ron_van_sommeren is offline  
Old 07-03-2020, 02:56 PM
  #8  
AEAJR
Community Moderator
 
Join Date: Aug 2005
Location: NY, USA
Posts: 5,842
Default

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

AEAJR is offline  
Old 07-04-2020, 11:33 AM
  #9  
ron_van_sommeren
homo ludens modelisticus
 
ron_van_sommeren's Avatar
 
Join Date: Jul 2005
Location: near Nijmegen, Nederland
Posts: 1,206
Default

Below an excellent quote, a step by step instruction for 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 mass of the model and how you want to fly it:
    Magic numbers for e-flight - WFF
  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

Vriendelijke groeten, en wees voorzichtig, Ron
• Without a watt-meter you are in the dark ... until something starts to glow •
e-flight calculatorswatt-metershigh power motor tips&tricksCumulus MFC
ron_van_sommeren is offline  

Currently Active Users Viewing This Thread: 2 (0 members and 2 guests)
 
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -

Copyright 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.

Page generated in 0.06948 seconds with 10 queries