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Old Dec 10, 2014, 04:23 PM
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United Kingdom, Northern Ireland, Belfast
Joined Jun 2011
362 Posts
Motor/prop/model size advice please.

Hey guys. I've recently acquired an extra 300 of 74" wingspan and I reckon the all up weight to be I. And around 10 to eleven lbs. my question is this. I have a scorpion 4035 460kv motor that coupled with a 15x10 prop and a 6s lipo should produce 85amps and @ 1700-1880 watts which is more than enough to fly a model of this weight although the recommended prop for this model is @ 18x8 and up. Do you think my setup will be good enough?

If I can get the model to within 11 lbs then it will meet 154 watts per lb. which is suppose to be good enough for aerobatic performance. All oppinions are welcome thanks.
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Old Dec 11, 2014, 03:41 PM
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Letchworth, Great Britain (UK)
Joined Jul 2004
12,343 Posts
I have a 71" Xtra Wot (a Chris Foss low-wing sport model that looks a bit like an Extra) that now weighs 10lb 10oz with battery, and used to fly on an OS120AX. It now flies on 12S with a Hyperion ZS-4045/12 motor (Kv=247) and a 16x10 prop, giving me 2028 watts, or 190 watts per pound. That's plenty for me, and in fact I normally cruise around at such low throttle that my amps draw is only around 15A

If you can get 1880 watts out of the motor, then that should be enough for normal "sport" flying.
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Old Dec 11, 2014, 09:48 PM
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Canada, ON, Owen Sound
Joined Oct 2008
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I believe the motor would be fine, but I'd be tempted to run a 16/8 with about the same draw/load. The Scorpion's can be run right on the max edge with adequate cooling, no problem. 150w/lb should be plenty for acrobatic flight. jm2cw Doug B
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Old Dec 12, 2014, 12:00 AM
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Australia, ACT, Kambah
Joined Feb 2001
3,459 Posts
You can probably get the power you need with what is a relatively small motor for that size model, and but you'll be pushing everything hard. And with such a light motor, you'll probably need ballast anyway. I would seriously consider 10s or 12s and pick a much larger motor with Kv to suit the prop you want to use.
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Old Dec 14, 2014, 07:28 AM
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Leeds/Bradford Leeds, Great Britain (UK)
Joined Sep 2004
18 Posts
Got to agree with 10~12S li-po, motor no more than 250 Kv, rated at 60 Amps, will fly just fine. 3000mah battery will give about 6 mins of sport flying, so bigger battery if you want longer flight times.
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Old Dec 14, 2014, 09:14 AM
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Oakland township, Michigan
Joined Jun 2005
802 Posts
Hi -

I fly a Vanquish 2M (74" wingspan, about 11 lbs RTF) on a 10S lipo pack (2 5S in series) and a Plettenberg motor with a KV of 220. The prop is a 20x13 and the plane has plenty of power to get through the AMA Advanced sequence.

I believe that the Kv of the motor you have is mismatched for the size of the airplane. Although the watts are there, you would be better off putting that power into a larger prop (16 or 17). If you want to stay with a 6S pack, the motor Kv should be in the 380-400 range. That would allow you to spin a 16" or 17" prop. If you have a lot of 6S packs sitting around, then a motor with a Kv of 180 or so would work if you want to go 12S. Keep in mind that you want to spin a largish prop (17-20" depending on set-up).

Good luck,

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Old Dec 14, 2014, 09:39 AM
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Moorpark, Ca.
Joined Apr 2004
1,138 Posts
I wouldn't discount an intermediate solution, such as an 8s system. Power 90 , (325kv), on 8s with a 16 - 18" prop will give you the numbers you're looking for without stressing the components. Probably a slightly lighter system with say, 2X 4s 3300-4000mah batts. As mentioned, if you're going to need balast, might as well be in the form of power/battery. If you can achieve balance with a lighter system, that's the road I'd follow. Good luck. Blue skies.

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