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Prop size for 1800kv, 400W motor with 3S battery???

Old 04-07-2020, 03:57 PM
  #1  
GregG7
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Default Prop size for 1800kv, 400W motor with 3S battery???

I'm looking for recommendations for a prop size for the following:

Motor: Turnigy LD2840A - 1800kv Brushless Motor - 400 watt maximum power
Battery: 3S Lipo
ESC: 40 Amp
Plane: Robinhood 25 - 3.5 lbs, high wing trainer type plane.

Thanks in advance.
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Old 04-07-2020, 06:58 PM
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solentlife
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Dunno .... the motor is not listed in ecalc for examples.

This model ?

http://www.oldschoolmodels.com/rh25.htm

Talks about a 25 glow equivalent E-motor ...

Given the KV .... I would be looking at 7x6 ... 8x5 maybe ...
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Old 04-08-2020, 02:41 PM
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It's a little hard to guestimate for this setup. It looks like an 8x5 might overload the motor at full power. I'd suggest starting with an 8x4 and get some measurements.
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Old 04-08-2020, 03:10 PM
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To be honest on a trainer .. I would have been looking for a less KV motor and swing a larger prop ..... 1800kv is a bit of a fast revver for a trainer.
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Old 04-08-2020, 04:09 PM
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Indeed, my calculations indicate > 16,000 RPM at full power. That will make it a screamer, noise wise.

A much lower Kv motor with a larger prop would be much more pleasing to fly.
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Old 04-08-2020, 04:17 PM
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GregG7
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Thanks for your suggestions. I now know where to start for a prop size/pitch. If the 1800kv motor doesn't work out, I'll purchase something with less kv.
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Old 04-08-2020, 05:00 PM
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solentlife
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Originally Posted by GregG7 View Post
Thanks for your suggestions. I now know where to start for a prop size/pitch. If the 1800kv motor doesn't work out, I'll purchase something with less kv.
A trainer like that ... if its the 25 size as I linked to ... then about 1000 - 1100kv is better with a prop in the 10" arena ... the G25 motor from HK ??
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Old 04-08-2020, 05:50 PM
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The G25 is physically a much bigger motor weighing more than twice as much (193g) as the LD2840A (79g). WIth a 5mm shaft it's almost certainly much sturdier as well.

If the OP needs the extra weight in the nose to balance the model then having the bigger motor with lower Kv (870) would support a much bigger, slower turning, propeller with higher efficiency and quieter flight. Assuming he has ground clearance to swing a bigger prop.
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Old 04-08-2020, 07:35 PM
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I have a 3542 with 10x6 and 3S pulls my Mig3 around ... not a bad combo at all.
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Old 04-08-2020, 08:04 PM
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GregG7
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Unfortunately the Turnigy G25 is out of stock. I've had good luck with Himax motors, although they are more expensive. Maybe I'll look at the Himax HC3516-0840. It's only 840Kv, so that should be better suited to the Old School Modelworks Robinhood 25. I do have enough ground clearance for a fairly large prop.
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Old 04-08-2020, 10:08 PM
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Are you far enough along in your build to know how much weight in the motor area you're going to need? I'm assuming you have a good idea about the battery size and location you plan to use.
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Old 04-09-2020, 01:31 PM
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The plane is not covered yet, but it balances nicely with the lighter Turnigy LD2840A motor. I think I can mount a little heavier motor and still get the plane to balance just by moving the battery back a little.
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Old 04-09-2020, 07:35 PM
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Here's a nice motor without being too heavy :

http://www.rctimer.com/?product-142.html

Model: BC3530-14
Motor size: Ф35*30mm
Shaft size: Ф5.0*47.5mm
Weight: 74g
KV(rpm/v): 1100
Max Power: 315W
Battery: 2-4Li-Po
Prop: 10X4.7/9x5
Ri(MΩ): 0.077
ESC: 30A

RC Timer is an excellent source of motors ...
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Old 04-09-2020, 09:01 PM
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Thanks for the suggestion. Much better price than a Himax motor.
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Old 04-11-2020, 09:16 AM
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Originally Posted by GregG7 View Post
The plane is not covered yet, but it balances nicely with the lighter Turnigy LD2840A motor. I think I can mount a little heavier motor and still get the plane to balance just by moving the battery back a little.
use a 7x6 APC prop for your 1800kv motor on 3 cells, I use 1800kv motors all the time with a 7x6 prop, my friend kosh is using a 1800kv motor in his x41 scratch built plane with a 7x6 prop on 3 cells in the video

https://wattflyer.com/forums/showthread.php?t=71505&highlight=kosh+1800+kv+motor

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Old 04-11-2020, 09:36 AM
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Originally Posted by GregG7 View Post
Thanks for the suggestion. Much better price than a Himax motor.
RCTimer .... Banggood ...... Hobby King are mo goto's for motors ....

RCTimer and Hobby King are near identical motors with RCTimer having a greater range and usually give more info on what it can do.

Banggood have such as Sunnysky and Racerstar ... both very good ...
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