Anyone know what this motor can do? - RC Groups
Thread Tools
Mar 06, 2012, 09:39 PM
Too much to do not enough time
BobaFett1138's Avatar

Anyone know what this motor can do?

I just picked up a motor from a seller here, a 4258-400kv BL outrunner. It's what comes in a starmax 63 inch P-47.

From the seller and other things I've read, it swings a 4-blade 16-inch prop and draws around 65 amps on 4S. Cool and everything, but I'm looking to use it in a different application. Looking to use it as a power system for a 3D plane, approximate weight 4-5 pounds. I'd also be putting either a 2 or a 3 blade prop on it. Here's my big concern:

Can it handle 5 or even 6S lipos?
What is its max amp rating?
What kind of wattage/thrust is this motor capable of producing?

I would test it myself, but I would only be able to test it on 3S. And my neighbor who flys RC doesn't have an ESC or batteries which could handle this thing. I'm designing my own airplane, and trying to build it around the powerplant. I'll be working on buying the ESC and suitable batteries later. For now, I'm just trying to get the airframe designed and built. Can anyone enlighten me about this motor's specs? Despite almost an hour of searching, what I have here is about all I know. There is a Turingy motor from Hobby King which is a 4258 at 400KV, but ti doesn't appear to have the same ability that the Starmax one does. Thanks for any help you can provide!
Sign up now
to remove ads between posts
Mar 06, 2012, 09:40 PM
Registered User
power it up with your 'lightsabre' ?
Mar 07, 2012, 06:56 AM
Registered User
Easy answer / rule of thumb: weigh it. Multiply weight in grams by three and there is a ballpark max input in watts. Divide that by the voltage of your proposed batteries and you have a ballpark max amps for the motor with those batteries. You will get different max amps for 3S, 4S, 6S.

When talking watts on a motor, we are talking watts input, not watts output. Most of us don't have the instrumentation to measure watts output.

Working with your numbers, the motor should weigh around 320 gms or 11 ounces. If it weighs considerably less, I would begin to doubt the 65 amps or that it runs cool at that level. Can't think of any reason why a Starmax OEM motor would be any more capable than a Turnigy.

4 blade, 16 inch prop is nice info, but what was the pitch on that blade?

I'm a novice, certainly no expert.

I'm sure other, more knowledgable people will chime in shortly.
Last edited by Pete Miller; Mar 07, 2012 at 07:13 AM.
Mar 07, 2012, 07:30 AM
Registered User
Air Hobby RC's Avatar

Sometimes you need to be careful when doing an estimate. MicroDan motors are small but pack a powerful punch depending on the prop and LiPo. A 58g motor can do 1185w on a Sport APC 6x4 prop. Also Watt meters are really easy to pick up and set up. If your going to build a plane around your motor I suggest you do the extra leg work on the motor and pick one up. Before you start your plane designing.
Mar 07, 2012, 08:45 AM
Registered User
Originally Posted by Air Hobby RC
Sometimes you need to be careful when doing an estimate. MicroDan motors are small but pack a powerful punch depending on the prop and LiPo. A 58g motor can do 1185w on a Sport APC 6x4 prop.

But I don't think the quality of the OEM Starmax motor is quite equal to the quality of a Micro Dan.

And definitely get a Watt Meter. You're "flying blind" without one.
Mar 07, 2012, 09:12 AM
Too much to do not enough time
BobaFett1138's Avatar
Thanks pete/airhobby. Guess I'm just going to have to test it out myself. I was just hoping an owner of one of these motors had done a bench test on it..... and now in retrospect, I probably should have posted in the starmax P-47 thread..... oops. Thanks for the advice though, and I guess a wattmeter is next on my list of purchases. I know the basics, like how to plug in the ESC wires into the motor and what my watts per pound should be. I'm just trying to learn more by actually doing a project. Thanks for teh info, I appreciate it! (But if anyone else wants to chime in, please do!)
Mar 07, 2012, 09:26 AM
Registered User
Dr Kiwi's Avatar
It would not surprise me to find that the Starmax is simply a Turnigy with a different label!

At 261g it ought to be capable of ~800W... on 4s that's just under 60A......on 4s you could expect to spin your prop at ~4500rpm. Since you want it for a 3D plane you need a prop with as much diameter as possible to get you maximum thrust... so that probably precludes 5s and 6s. .... but limiting yourself to 4s means high amp draw [due to I^R, amps are what kills you in terms of waste heat].
Last edited by Dr Kiwi; Mar 07, 2012 at 09:33 AM.
Mar 07, 2012, 10:13 AM
Too much to do not enough time
BobaFett1138's Avatar
Well, looking at just sheer ground-clearance, the biggest thing I could bolt on would likely be a 15 inch 2-blade. I also read somewhere that for 3D, the pitch should be half the length, so a 15X8X2? If the 800 watts figure is correct, that means I can pull about 4.5 pounds worth of plane. Again, this merits bench testing, but speculation: do you think it would be possible to run 5S with the 14 inch? As far as amp draw goes, it's not an issue. The ESC I'm looking at ATM is good for 85-90. My main concern is can the MOTOR handle it?

Now, I looked at the Turnigy motor from HK. The casing looks pretty different, but I only get 1 crappy view. So that's up in the air. Assuming it IS the same though, I've read that some people ARE using it on 6S.
Static thrust being slightly less than 5 pounds, that's approximately what your specs are suggesting. However, despite the extra 7.4 volts, the power appears to be approximately the same, just less amperage. Seems somewhat counter-intuitive..... but electrical engineer I am not.

But regardless, thanks for the info Kiwi!
Last edited by BobaFett1138; Mar 07, 2012 at 01:51 PM.

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Help I can't find any info on this engine anywhere can anyone help? wet banshee Engines 6 Mar 11, 2012 10:11 AM
For Sale What do you think i can get for this? gasman1129 Boats (FS/W) 1 Mar 05, 2012 07:04 AM
Help! Anyone know what kind of motor this is? ABADGTP Electric Heli Talk 8 Dec 16, 2011 10:41 PM
Discussion Can Anyone Explain What Is This Voice What Comes From Motor RcLento Power Systems 14 Sep 08, 2011 03:12 PM
Discussion Anyone know what a JR 9303 can do that an Evo 9 can't? Wildewinds Radios 28 May 02, 2006 06:46 AM