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Old Jul 31, 2015, 09:33 PM
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Question
Determining motor kv.....

Hey guys,
I have an old ArtTech 1000mm w/s F4U that I'm planning to use at our clubs' warbird races. It is an "el cheapo" & I won't fret if I total it since it has a very brittle foam construction. I finally maidened it a couple weeks ago & it flew fine, albeit quite slow for the warbird pylon racing. I removed the 11 X 6 X 3 stock plastic prop & replaced it with a Xoar wood elec. prop 12 X 8 X 2blade. The stock 20A ESC read 17.5A full throttle with a 35c 2200 3S battery & somehow still felt like it was lacking "pulling power". I put in a 4S just to "feel" the difference & definitely was better, although the motor & newly installed Turnigy Plush 30A ESC were both hot after 3min full power.

Ok, there are no markings on the motor at all. How do I determine, if possible, what KV the motor is or can't that be calculated? The only data I could find was that it is a C4038-15. Here's the link for the plane listing. Thanks for any help...

Denny

http://www.nitroplanes.com/at-21148-f4u-arf.html
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Old Jul 31, 2015, 11:28 PM
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read here : http://www.bavaria-direct.co.za/constants/

scroll down until you see: " The Drill Press Method"


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Old Aug 01, 2015, 04:32 AM
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Very simple method if you have a digital tachometer and wattmeter, voltmeter or even a lipo voltage indicator:
- have your power system ready and connected (Rx, ESC and motor)
- put a strip of metallic tape outside the rotor. This tape will reflect the tachometer laser and give rpm readings.
- Remove any prop or adaptor from motor and fix it in a vise (I usually just hold it in my hands)
- Turn on the Tx
- Plug the battery to the ESC
- power up to full throttle an check your rpms and voltage
- motor kv=rpm/voltage

Here is a link for a test I did (the lamp is used to help tachometer readings)
https://drive.google.com/file/d/0B77...RRWjduUTQ/view
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Old Aug 01, 2015, 06:00 AM
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The Electric Motor Constant Kv
AKA The Generator Constant, RPM/volt, Annotated as Kv

http://theampeer.org/Kv/kv.html
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Old Aug 01, 2015, 10:28 AM
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Was this a static run of 3 minutes? Of course it will get hot! How hot? Too hot to touch?
A 12/8 could very possibly pull far more than 17A on 3S. On 4S, I'd drop back to a 10/8 and test max static current briefly to establish max numbers, then fly it and recheck motor/esc heat. Esc/ motors are not designed to be stationary and must have adequate air flow to live.
Doug B
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Old Aug 02, 2015, 08:42 PM
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Thanks for the responses guys. I'll fly it on 3S & try a 4S in the air to see how it does. I have a nice assortment of replacement motors so I'll check to see what will improve the planes' speed a bit. I'm not looking for a speed demon.....just something competitive.

Denny
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Old Aug 04, 2015, 04:23 PM
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Tim Lampe; Hobbico R&D
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When I want to do a quick kV check I connect the motor to a data logging ESC (like most Castle Creations ESCs). I connect a battery, run the motor at full throttle for two or three seconds (no prop of course), then read the voltage and rpm in the data log from that run and do the math (rpm divided by voltage = kV).

I've read about the drill method and of course reading the rpm with a tachometer and the voltage with a Watt meter (which I've done, but also subject to a little human error and interpretation), but I wonder how much more accurate those are than reading the data from an ESC?

Tim
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