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Old 03-28-2016, 02:33 AM
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Join Date: Apr 2008
Location: Wisconsin, USA
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Originally Posted by sabreflyer View Post
Thanks for the replies.
Is the motocalc suggesting that the axi motor prop and 20x13 and 10s will run hot? If so this can't be the case as this combo has been used a lot for f3a aeobatics with no problems. Or is it that this combo would run hot in this type of airframe? I was thinking that I have the combo on the shelf and so would save me buying new gear and the weight of the combo would be useful. However, if it might not be good set up then I guess l'd rather get it right first time and get a new set up. What do you think?
The current pulled by these motors depends on the prop, motor, and flying speed. What worked well with your 20X13 prop on a fast airplane might not work well with the same setup on a slow draggy Bipe.

As an example, if the prop blade airspeed is say 80 MPH at full RPM, and the models airspeed is 60 MPH, there is a considerable reduction of how much power is required to turn that prop. Or, do the same thing with a model that flies at 35 MPH. There will be much more load applied to the motor shaft, even though it's the same motor, same battery pack, same prop.

So, do you need a new motor/prop/battery? Probably not. If you hang to small of a prop on a glow/gas engine, it will over-rev, and possibly damage it. Hang to small of a prop on an electric motor, and it may not get off the ground. It simply won't work if you put a 6 inch diameter prop on your motor. Won't hurt, just won't work. (Of course, hanging to BIG of a prop on an electric motor is a real no no. These motors are DUMB. Overload them, and they will put out fantastic power. Until you see the smoke trail from the motors windings burning up)

So, somewhere between that 6 inch prop and your 20X13 prop will be a propeller that will work well with your model. Running the numbers through Motocalc, with your AXI motor and 10S LiPo, a prop on the order of an 18X8 would be in the ball park. With the 18X8 prop, motor efficiency is around 85%, a decent number.

With the AXI, 18X8 prop and 10S LiPo, motocalc predicts 7200 RPM, with a blade pitch speed of 52 MPH. (Your model won't fly faster than 50 MPH!) Static current will be around 50 Amps, Watts is around 1800, and the important Watts per pound of airplane will be around 120 Watts/pound. Anything over 100 Watts per pound of airplane for a slow flying model is very good.

Predicted climb rate will be around 1400 feet per minute at 38 degrees.

Motocalc is only as accurate as the specifications provided with the motor. I've got a number of Hacker motors, and motocalc is always within 5 or 10% of the predicted Amps and Watts.

As far as larger motors go, again, motocalc is close with my Hacker A60-5S, A60-16M, and Rimfire 50 cc electric. If you should find that with the AXI and 18X8 prop is off by perhaps 20%, just adjust the motors KV up or down til the number match. Then, use motocalc with the new KV number to select a better prop.

This is where it's about mandatory to use a Wattmeter, to make certain you are not overloading your motor.

Last edited by kyleservicetech; 03-28-2016 at 04:17 AM.
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