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Gear/prop ratios

Old 11-13-2006, 11:49 AM
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JVD
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Default Gear/prop ratios

I've got a Wattage Impress electric powered plane and I'm replacing the 370 brushed motor with a 6 pole, 2500 rpm/volt brushless motor. I'm using a 3 cell lipo and an 8x6 APC "E" prop.
The question is I don't know which gear ratio to use as the new gearbox has 3 pinions with ratio's of 2.1:1, 2.5:1 and 3:1. I want to use an 8" prop or maybe a 9" but the ground clearance is limited above that.
The old gearbox had a 1.85:1 ratio with the 370 motor.
I'm not after loads of power or torque as the model is only a high wing aileron trainer. I just want the right gear/prop ratio as to not draw excess current and give an efficient run.

Hope anyone can help?

JVD
Old 11-13-2006, 09:31 PM
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Matt Kirsch
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Default RE: Gear/prop ratios

In order to know how many Amps to prop for, you'll need to know how many Amps your gear is capable of.

What are the specifications on the battery? ESC? Motor?
Old 11-14-2006, 04:55 PM
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JVD
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Default RE: Gear/prop ratios

Hi Matt,

The battery is a FlightPower 20C 11.1v 1200mAh, the ESC is a Ripmax Xtra brushless 25A. The only spec on the motor is that it's made by SF, 6 pole 2500rmp/volt, same can size as a Speed 400. Not a lot to go on but thats all the info I have.

Thanks,

JVD
Old 11-18-2006, 11:57 PM
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Dr Kiwi
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Default RE: Gear/prop ratios

Contact the manufacturer of "SF?" and find out what the max allowable current draw/watts are for the motor.

Set it up with the 3:1, fit the 8x6 or smaller, connect your Whattmeter to it, throttle up slowly and see what it draws as you head to WOT. If it doesn't exceed the specs, try the next higher ratio and so on.

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