stunthanger.com
Electric Stunt => Gettin all AMP'ed up! => Topic started by: frank williams on October 03, 2014, 07:00:09 PM
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Ok.
So I got a motor that has a Kv of 487 (pretty low). It has a max power of 1500 watts, continuous current of 50A and max of 70A.
So, if I run a six cell battery and about a 13x6 prop (probably 8500 or so) will it work ok?
The current and power are plenty hefty enough, but am I too close to the limits.
Its a Thrust 50 sold by Precision Aerobatics.
Show me the calculations .... I know there is and 80% in there somewhere.
Frank
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KV is the maximum rpm that the motor will produce (with no prop) per volt of battery power.
Here goes the calculation:
KV X Voltage = max rpm (487 X 22.2 = 10,811)
Since we need to put a prop on the motor to fly that pattern we usually take 80% of the max rpm to be what we call "usable rpm" (10,811 X .8 = 8,649). Note, the 80% is an approximate value that we normally use to come close to the loading that the prop puts on the motor.
In my considered opinion that's way too close for safety. If you are planning on using a KR governor don't as you will have absolutely no "head room" for the variable gain to work with.
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Frank,
Earlier this year I was doing some experimenting with different motors and found out really quickly that when you start approaching the limit it will show up really quickly starting with the hourglass. The last three or four manouvers really start getting iffy. Under ideal conditions I could just nurse it around but in heavy weather I'd have bailed. Keep your Kv up there.
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That's a humongous motor for stunt -- 1500W implies a 12 pound airplane if it's the continuous rating. Even if it's peak power it still implies a 9 pound airplane. (That's going by the 7W/oz continuous, 11W/oz peak rule for sizing electrics, which applies if you're using something like a Hubin or a Renicle timer).
That doesn't mean that it won't work for CL stunt, but the motor will be much heavier than it needs to be, and will probably be noticeably less efficient.
I'd see if you could trade it for a motor more in keeping with your model weight (unless you intend to fly a 10 pound stunter), with a higher Kv.
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Thanks guys for your replies. I figured that it wasn't a good combination ..... just hoping that maybe there was a magic bullet somewhere.
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Maybe try an 13-8?
Anyone try high pitched props? Seems pretty common in RC-lectrics. Should turn better, and be more efficient. Probably be lacking in acceleration though???
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I have a sport model with the Eflite E-60 that comes in at 1200 watts and flies a 80" span, 12 lb model with ease, maybe 1/2 power for cruise. It flies with an 6S cell battery with a 16" prop
I am not flying any aerobatics with this model but it gives some idea of the power these bigger motors have
Fred