Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

Fast electric guys, need some advice on props.

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Old 11-25-2008, 01:06 AM
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Kmot
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Default Fast electric guys, need some advice on props.

My buddy has built a 29" swamp boat/air boat. It will be powered by a brushless motor that has a Kv of 3900. I have looked at the APC pylon props, the Master Airscrew Formula One props, and the Graupner Cam speed props. I figure something in the 7-8" diameter is about max for this motor? Probably need more like a 6" prop?

I'm concerned about how much rpm these props are safe up to.

Are there other props available to purchase besides the three listed above?

Thank you.
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Old 11-25-2008, 01:33 PM
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Default RE: Fast electric guys, need some advice on props.

at 3900 kv a 8" prop is going to draw a LOT of amps! how much is the motor rated for? that and KV will tell you prop diameter.

air boats need thrust, you may be better served with a lower kv motor swinging a bigger prop.
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Old 11-25-2008, 04:14 PM
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Default RE: Fast electric guys, need some advice on props.

He has already ordered this setup and I am just trying to help him get it sorted. Thanks!
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Old 11-25-2008, 05:50 PM
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Default RE: Fast electric guys, need some advice on props.

A 3900KV motor is going to be in the 4.75X4.75 range on 3S. A APC 5X3 would be a good choice. If you use a 2S pack you can run a 5.5-6" prop, APC has a 6X4 that would be good on 2S.

The thing is to match the prop to the motor and not draw more than about 120% of the amps the motor is rated for.

Is this the United Hobby inrunner?
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Old 11-25-2008, 06:00 PM
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Default RE: Fast electric guys, need some advice on props.

a [link=http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=2082]gearbox[/link] may be a very nice possiblility for this motor, outrunner or inrunner? with a 5 to 1 gearbox on 6s you could spin a 10-11 prop with the average 540 size inrunner


I agree with Evan, a 5" (maby 6") prop is probably max on 3s unless it can handle a few hundred amps.
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Old 11-25-2008, 06:13 PM
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Default RE: Fast electric guys, need some advice on props.

I suggested that this motor would need a gearbox but his airboat does not have the provision to mount it wiuthout some sort of surgery to the setup. I am going to suggest a 2S and 6" prop. Thanks!

Here is the motor in question:
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Old 11-25-2008, 07:07 PM
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Default RE: Fast electric guys, need some advice on props.

we are all just guessing with out the type and size of the motor. you could have a 60mm inrunner with that KV or a 12mm one we need more detail to give the best advice but the KV no matter the size is way to high for this.
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Old 11-25-2008, 07:29 PM
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Default RE: Fast electric guys, need some advice on props.

It is designed as a replacement for r/c cars so I presume it is a 540 sized motor, but in brushless terms I have no clue. Thanks for the help, we will see what we can do with it.
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Old 11-26-2008, 01:02 AM
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Default RE: Fast electric guys, need some advice on props.

i would try a apc 7x3 cut down to a 6" prop then try it on two cells if running cool then try the 3 cells you need lots of thrust to move an airboat
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Old 11-29-2008, 11:46 PM
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Default RE: Fast electric guys, need some advice on props.

kmot, any of the small pylon props are good to high RPMs. APC rates theirs modestly and most are well over 30K. Same with the CAMs. Start with a smaller prop and measure the current. So many guys run motors and never check the current. I use a Sears clamp on style meter but there are also in-line plug in style meters that will tell you watts (amps, volts). If you over prop you risk frying the motor, esc, or battery.
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Old 12-01-2008, 01:42 AM
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Default RE: Fast electric guys, need some advice on props.

Thanks for the advice Troy. I have a WattsUp meter and I will use that to check his amps.
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