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Old Sep 01, 2009, 06:40 PM
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Motors for dummies?

Where do we go to learn the basics? I consistantly see people talking about using a 11, 12, or 13t gear on their motor to spin the main gear on their heli, but what I need to understand is how to calculate which gear you will need to obtain the head speed you desire. I understand the conversion of taking the main gears number of teeth and dividing it by the motor gear's teeth, and that gives you the rotation ratio (Say 10-1 for example), but if I want a 3,000 RPM head speed, how do I know what motor I need and what battery it needs backing it to make it run at 30k RPM? How do I know what the amperage draw will be on the motor when it's running at 30k RPM, so I don't get an undersized ESC and smoke it? I'm looking into getting a T-rex 450 with a 430XL motor, as it seems to be highly recommended, but I'd like to understand why it's highly recommended, rather than just blindly trusting other's opinions. How do I know how much leeway I have (IE: How do I know if I'm running 3k RPM head speed on that motor with only 30% power, so I have a huge amount of torque I haven't accessed, so when I put it under load, it will maintain a 3k head speed? Obviously once I purchase it, I'd learn that simply by seeing what head speed it has when I spin up the blades, but I want to know how to calculate this stuff ahead of time so I can make educated decisions. I'm a nitro guy, still new to the electric erra, so I still don't know why I'd use a 2850kv motor over a 3500kv motor. How I know one will be more powerful than I need, and I should downsize a little to give me longer flights etc...

Lots of questions, I know. I'm sure someone probably has a link that will explain it all.
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Old Sep 02, 2009, 01:41 AM
If it flies, I will crash it
mxspode's Avatar
USA, CO, Westminster
Joined Jan 2008
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That is a lot of questions. I find this calculator to be very helpful when setting up a heli. Have a look and see if it answers at least some of your questions.

Rob...
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Old Sep 02, 2009, 08:40 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
The Netherlands, GE, Nijmegen
Joined Feb 2001
10,791 Posts
* Get a current/Watt-meter, it will save you money and will more than pay for itself!
3$ diy Watt-meter:
http://www.rcgroups.com/forums/showthread.php?t=949923

* E-flight calculators, a compilation
http://www.rcgroups.com/forums/showthread.php?t=606703

* Choosing a power setup
http://www.rcgroups.com/forums/showthread.php?t=739069
http://www.wattflyer.com/forums/showthread.php?t=27019


For a rainy day ...

* Presentation: de-mystifying Electric Flight
http://www.rcaircrafters.org/The%20Electric%20Show.pdf

Get a current/Watt-meter

* http://www.ampaviators.com
-> Beginner Guide

Get a current/Watt-meter

* E-book: Everything You Wanted To Know About Electric Powered Flight
http://homepage.mac.com/kmyersefo/ev...ng-e-power.pdf
Corresponding discussion:
http://www.wattflyer.com/forums/showthread.php?t=31071

Get a current/Watt-meter

* Several Wiki's
http://www.wattflyer.com/forums/vbglossar.php

* Get a current/Watt-meter

* http://www.ezonemag.com
-> Faq

* Get a current/Watt-meter

* System wiring diagrams:
http://scriptasylum.com/rc_speed/_wiring.html
* LiPo pack and balancer cable wiring:
http://scriptasylum.com/rc_speed/_lipo.html

* Get a current/Watt-meter

* Monthly Ampeer newsletter, on paper and online available. Excellent articles in the archives by e-flight pioneers Ken Meyers and Keith Shaw a.o.
http://homepage.mac.com/kmyersefo

Get a current/Watt-meter

* Brushless motor animations and simulations. Mostly outrunners but inrunners are just outrunners turned inside out, nothing fancy:
http://www.rcgroups.com/forums/showthread.php?t=216928

Get a current/Watt-meter

* Current, voltage, Watt, battery-types and -C-rating explained
http://www.rcgroups.com/forums/showthread.php?t=417868

Get a current/Watt-meter

* 'Dry' testing brushless motors after a mishap:
http://www.wattflyer.com/forums/showthread.php?t=35216
http://www.rcgroups.com/forums/showthread.php?t=240993

Get a current/Watt-meter

* How to repair a brushless motor
http://www.rcgroups.com/forums/showthread.php?t=1079423

Get a current/Watt-meter

* Motor_rewinding_101, about poles, winds, delta, star
http://www.gobrushless.com
-> knowledge base
-> basic overview (1-5)

* And I almost forgot, get a current/Watt-meter
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Old Sep 02, 2009, 09:49 AM
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Joined May 2009
76 Posts
Quote:
Originally Posted by mxspode
That is a lot of questions. I find this calculator to be very helpful when setting up a heli. Have a look and see if it answers at least some of your questions.

Rob...
That has some nice tools. How do you calculate how much amperage you'll be using? From what I read, that motor's max amperage is 35, but it's not going to be pulling 35 amps when it's hovering, but instead when it's doing heavy 3d. So how do I calculate what it's draw will be at hovering so I can get a better idea of what my flight times will be?
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Old Sep 02, 2009, 07:41 PM
If it flies, I will crash it
mxspode's Avatar
USA, CO, Westminster
Joined Jan 2008
3,024 Posts
That calculator does this for you. Look at the upper right section. I have no idea how accurate those numbers may be. I set the timer on my DX6i for 6 minutes and land it then. That is not a full pack but enough for me. After that I need a break.

Rob...
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Old Sep 03, 2009, 07:29 AM
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That portion couldn't be accurate.. it says with a T-rex 450, running a 430XL 3500kv at 3030 RPM head speed with a 13t gear, on 3s 2200mah batteries, I'll get 10 minutes... Everyone on here is getting 4-6 mins, and my other bird that's running a less powerful motor gets 6 mins at 80% with only a 2400 rpm head speed.
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Old Sep 03, 2009, 09:01 AM
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Chicago 'burbs
Joined May 2006
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Quote:
Originally Posted by Maverick3n1
That portion couldn't be accurate.. it says with a T-rex 450, running a 430XL 3500kv at 3030 RPM head speed with a 13t gear, on 3s 2200mah batteries, I'll get 10 minutes... Everyone on here is getting 4-6 mins, and my other bird that's running a less powerful motor gets 6 mins at 80% with only a 2400 rpm head speed.
There is no way a calculator is going to be able to predict this stuff (at the level you want), unless someone fed in actual real world measurements of your exact setup. Even then I would expect differences.

So you should do it the way most people (I think) do it.

Take off and fly conservatively for ~2 to 4 minutes. Land and see what the battery has left. That will give you a starting idea. Then build up gradually. In the end you want to leave the battery with 20% reserves.

Or stick with what your buddies are telling you. That should already be in the ballpark of what to expect.
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Old Sep 03, 2009, 08:36 PM
If it flies, I will crash it
mxspode's Avatar
USA, CO, Westminster
Joined Jan 2008
3,024 Posts
Exactly. The calculator I linked is a tool and as I mentioned I am not to sure how accurate the results are. The tool gives you approximations based on a set of standard measurements. I have used it to figure out gearing and how much battery to use several times and I find it handy for that. Alan's point is well taken. I would never be careless enough to just go out and try to fly for ten minutes because a calculator say's I can.

By the way regarding Maverick's comment, I think the 10 minutes is based on draining your battery 100%. I usually only draw mine down 70-80%.

The calculator is a handy tool and good for basic info. I find it very useful.

Rob...
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Old Sep 07, 2009, 05:21 PM
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That was with an 80% draw. It has the option to choose what percentage you want to draw your batteries to.

How about this portion?

What is the difference between a 2 turn/winding, and a 3 turn/winding? How does that effect the performance of a motor?

Also, I have a motor that is 4 pole, and another that is 6 pole. They are both approx 2500kv, so what is the difference? Does one have more torque than the other?
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