RCU Forums - View Single Post - Need help for prop size on electric motor
Old 06-02-2010 | 12:35 PM
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opjose
 
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From: Poolesville, MD
Default RE: Need help for prop size on electric motor

Gorish

Unfortunately you are approaching this BACKWARD.

With electrics the prop size determines what the draw, thrust, power requirements, etc. will be.

In turn the prop size used is determined by the plane, weight, and expected levels of performance.

You already have a motor that has a rather high KV rating for props.

The HK/HC web site states that for a 3S LiPo the maximum prop you can use is a 7x4, and an 18A ESC is required.

It also puts the power output ( at max ) of the motor @ 140 watts

Knowing that we can work some things out....

140w / 11.1v = 12.7A max draw

1650kv x 11.1v = 18,315 RPM

18,315 RPM * .8 (to emulate static thrust values ) = 14652 RPM

14652 RPM on a 7x4 prop gives a pitch speed of about 56 MPH and a static thrust around 1.5lbs.

That is quite fast for a small 3D foamy.

A larger prop would overload the motor causing it to overheat and burn out.

This motor is really better suited to a small fast pusher foam jet than it is for your intended plane.

What you want is a higher thrust, lower pitch speed motor for 3D instead.