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EMAX CF2805 underperforming - what are my options?

Old 11-22-2011, 02:20 AM
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mclarkson
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Default EMAX CF2805 underperforming - what are my options?

I am running this CF 2805 0.9oz outrunner from HeadsUp RC.

Jeff, whose numbers I generally trust, shows this motor delivering 15oz of thrust on a 2cell with a GWS 7035 prop at 13amps.

I have the motor on a 9oz AUW plane and, though I don't have a way to measure it exactly, I am getting nowhere near 15oz of thrust. (I'm assuming that with 15oz of thrust, a 9oz plane should go vertical like a rocket ...)

Where am I going wrong?

I am running a 600mAh 20C LiPo. 20C seems okay to me. Will more mA make things that much better? I don't really need more capacity, per se. Sadly, the plane was designed around this battery so a larger one will be very difficult to shoehorn in there.

Is there something else I'm missing?
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Old 11-22-2011, 02:47 AM
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earthsciteach
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Is the motor solid? Maybe broken winding?
Calibrated the ESC?

I'm definitely a hack, so not sure if either of those would have a real impact.
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Old 11-22-2011, 03:08 AM
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mclarkson
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The motor is solid. I think the ESC is set to 'soft', as recommended for 6-pole motors and below. (I have no idea how many 'poles' I have...)
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Old 11-22-2011, 04:31 AM
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flydiver
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I think you picked the wrong motor. My guess it it's working fine, just the wrong application. Did you read the first line at HURC carefully?

The Emax CF2805 is a 0.9 ounce, 2800KV direct drive outrunner motor that works well on small pusher jets, flying wings and a wide variety of small foam models

(color is my emphasis)

That is a very high KV motor designed for low torque and high speed. It won't 'prop hang' worth a darn. It ain't a motor for a high drag pokey plane.
You want a motor designed for high torque and low speed for a bipe like that.

You might want to try the HURC 2712-12 on 3S, or the HURC 2712-17 on 2S for more appropriate motors. Prop it for torque and thrust, not speed.
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Old 11-22-2011, 04:46 AM
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gyrocptr
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Originally Posted by mclarkson View Post
I am running this CF 2805 0.9oz outrunner from HeadsUp RC.

..... shows this motor delivering 15oz of thrust on a 2cell with a GWS 7035 prop at 13amps.......

I am running a 600mAh 20C LiPo. 20C ....
Here's something you may want to consider:
600mAh, 20C => 12amps max current that can be delivered with a healthy, freshly charged pack.
If you want to provide the needed 13A, you'd need a larger (mah) battery or a 600mah battery that's capable of 25C or more.
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Old 11-22-2011, 04:57 AM
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Flydiver, you're totally correct. This was just the motor I had lying around - I've always known it was the wrong one. This plane is built almost entirely out of parts I had lying around.

Hence the now apparently tragic choice of the 600mAh 2cell. It turns out that no one makes a battery in this size any more. (HK has a Rhino 610 and that's absolutely all I can find.)

In actuality, I'm only seeing about 8.5A from that 600. I probably need to move up to an 850, but that'll necessitate some serious chopping and gouging.

Ultimately, I need to grab a 2712-17. (Or build a new plane altogether, with modifications based on the many mistakes I've made with this one.)
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