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Old Mar 24, 2014, 10:39 AM
Wanted for breaking OHM's law
Dennis Sumner's Avatar
United States, MI, Canton
Joined Sep 2002
1,573 Posts
Quote:
Originally Posted by oeren View Post
Hello,

I'm building a FT Spitfire and I finally got together the electronics and tested them. But after running the motor on full throttle for about 5 seconds, the motor was pretty hot to the touch. I've heard of the 5 second rule, and it wasn't so hot that I couldn't hold it for longer than that. I was just wondering if that's a bad sign and that I should change something in my set up.

I'm running

-8x4.5 propeller
-Exceed RC Rocket 1400kv
http://www.hobbypartz.com/86ma01-2205-1400kv.html
-Dynam Detrum 25A ESC(3A BEC)
http://www.hobbypartz.com/60p-dye-1002-25a-esc.html
-Sky Lipo 2200mah, 11.1V 3S, 20C cont. / 40C burst

I don't have an ammeter/wattmeter yet, so I'm not sure how much current I'm pulling. And I haven't had any luck figuring out the online power system calculators. From the information and specs posted with the products, it looked like everything would be fine.
I looked at the specs for your Flight Test Spitfire and it looks like they are using a similar "370" class motor and the all up weight for the Spitfire is 15-16oz. The motor you purchased is good for 90watts and 12A max current. As others had said, it is best that you invest in a wattmeter and check your current draw with the 8x4.5 prop. Not sure how the Spit would fly with the 6x3 prop....
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Old Mar 24, 2014, 10:58 AM
Registered User
Staffs, UK
Joined Nov 2003
10,396 Posts
Quote:
Originally Posted by oeren View Post
Is there a rule of thumb when it comes to prop size and kv/watt rating on a battery? I can't imagine just doing trial and error with a watt meter. That'd waste time and money, I'd at least want an estimate to make sure I'm purchasing the somewhere around the right product.
Most of us will use either manufacturer recommended prop sizes, what people here recommend or alternatively use one of the many *calc programs to give us a reasonable starting point. If you ask before destroying anything you'll get a few good answers and after a while you'll get a feel for what sort of prop size is reasonable.

Steve
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Old Mar 24, 2014, 11:09 AM
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DustBen's Avatar
United States, NE, Kearney
Joined Dec 2011
2,232 Posts
Quote:
Originally Posted by Dennis Sumner View Post
I looked at the specs for your Flight Test Spitfire and it looks like they are using a similar "370" class motor and the all up weight for the Spitfire is 15-16oz. The motor you purchased is good for 90watts and 12A max current. As others had said, it is best that you invest in a wattmeter and check your current draw with the 8x4.5 prop. Not sure how the Spit would fly with the 6x3 prop....
a 6x3 prop spinning at 16,800 rpm is going to make power about like a Cox GoldenBee/BabeeBee. Not a ton of power.
I'd have to take a look at putting the 7x3.5 someone else suggested... and see how much power it sucks.
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Old Mar 24, 2014, 02:55 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
The Netherlands, GE, Nijmegen
Joined Feb 2001
10,607 Posts
Quote:
Originally Posted by oeren View Post
When choosing a motor, I thought the only thing I look at was Kv and maybe watts ...
Kv is not a rating, Kv is not a figure of merit, the way e.g. max current or max power is.

Kv says nothing about a motors maximum and actual power, rpm maximum and actual current, torque, efficiency, temperature, quality, magnets, voltage. Achieving a certain high or low Kv is no big deal, just a matter of less or more turns of copperwire in the motorcoils (or changing magnet strength).

A motors Kv constant is not a unit of measurement (kg, s, ampere, meter, newton) it is a physical property (mass, time, current, distance, force, ...). It is, by definition, impossible for a motor to have 1000Kv. Correct: the Kv constant of motor is 1000rpm/volt, or, Kv = 1000rpm/volt.

600Kv motors don't exist. It's like saying a motor weighs 100mass, can handle 50current, can do 12000speed and costs 75money.
A motors Kv constant is a physical property (generator constant in the world of grown-ups), not a unit of measurement. Kv is measured/expressed in rpm/volt.
Therefore
Kv (mass, time, current, pressure ...) = 600rpm/volt (kg, second, ampère, pascal ...)


Now if only manufacturers and shops would understand that and change their poor/bad Kv notation.


Vriendelijke groeten Ron
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Last edited by Ron van Sommeren; Mar 26, 2014 at 03:05 AM.
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Old Mar 24, 2014, 07:47 PM
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Lnagel's Avatar
Moab, Utah, USA
Joined Apr 2003
5,906 Posts
With a 3s battery your referenced motor is pretty much maxed out at 99 watts with a 7x4e prop. Your 8x4.5 prop, which is probably also a slow fly,will cause it to draw way more than maximum allowable power with a 3s battery. Unfortunately, going to a 2s battery is not feasible with that motor because in order to reach the desired power level of 100 watts the motor would have to pull 13 amps which exceeds its maximum continuous current rating. Since the FT Spitfire recommends an 8x4 prop, your best bet would be to go to a 1000 to 1200 Kv motor weighing around 40 grams, an 8x4e prop and a 3s battery.

Larry
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