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Nov 08, 2017, 07:59 PM
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Suggestions, discussion on power system

I need help/direction/discussion on a power system for the following:
10' wingspan biplane flying boat (NC4) weighing a guesstimated 12 to 14 lbs. Sport Scale. The original airplane had two centerline engines (one pusher/one tractor) and one tractor each side of those. Scale propeller diameter is 10". The airplane's nacelles are relatively small in frontal area, so I'm leaning towards in-runners. The airplane will have a lot of drag in the form of struts & rigging, not to mention general arrangement, so a slower flying speed is desired. The hull/fuselage is ample so hardware will be inside. The hull will be permanently fastened to the center section wing segments and fuselage structure to and including the vertical and horizontal stabilizers.

Thanks all

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Nov 08, 2017, 09:08 PM
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scirocco's Avatar
I'd be looking at about 300W in to each motor, with headroom for up to 400W per motor to cater for the drag and rise off water case.

I'd be looking first at whether 35mm outrunners will fit in the nacelles - inrunners without gearboxes will be a more difficult solution; with gearboxes expensive.

If you don't have an existing stock of batteries earmarked, then I'd suggest considering a pair of 4S 4000mah - 5000mAh each, each feeding a pair of ESCs. Plan on extending the motor wires to the ESCs in the fuselage near the batteries, rather than ESCs close to the motors and long battery to ESC wires.
On 4s an example of a motor that would work well with 10" props is the Scorpion SII 3014-830 (129g) with estimated power from 300W with 10x5 to 410W with 10x8. Other brand motors of similar Kv and weight will achieve about the same outcome.
Nov 09, 2017, 12:09 AM
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vollrathd's Avatar
I'd suspect that you'd be looking at around 100 Watts per pound, or 1400 Watts total for reasonable performance. Especially in getting a flying boat out of the water. IMHO, you don't want to build a model that does not have enough power to get out of the water.

Running perhaps 150 Watts per pound, or 2100 watts, or 500 Watts for each motor should work. Problem is, with this type of power, you'd be looking at a propeller on the order of 12 -13 inches in diameter.

I've got three 1200 Watt electric models, one 2200 Watt, one 2800 Watt, and one 4500 Watt electric model. They are running 15 inch, 19, 19 and 22 inch props respectively. One model is a twin running a pair of 450 Watt motors with 10-6 props.

One of those computer programs such as will help. This program is free for 30 days, then $39.00.
Last edited by vollrathd; Nov 09, 2017 at 12:15 AM.
Nov 09, 2017, 11:02 AM
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manuel v's Avatar
10 feet of wing is not very clear.
much better if you give us the surface data in square inches or dm2.
Wing loading, better!
Nov 09, 2017, 11:54 AM
Registered User
Thank you all-
Manuel V....14.58 sq. ft. (not including 2 large lifting stabilizers)
.... 14.27 oz./sq. ft. (@13 lbs)

Scirocco/Vollrathd........You're confirming my fidgeting. The original airplane suffered a similar dilemma. Originally a three engined (Liberty 400 HP V-12)airplane, they added the pusher to overcome lack of power available at the time. I was hoping for a magic sized in-runner that has worked for others. Options seem to be heading towards out-runners placed in a slightly fattened nacelle, nacelle spacing altered to accommodate larger propellers, upper/lower wing spacing increased for the same reason. Batteries and ESC's in the hull.

Thank you all again

Nov 09, 2017, 02:12 PM
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scirocco's Avatar
If the nacelles are that skinny, you could use the Tower Hobbies Ammo 28mm diameter 28-45-3600 inrunner with the Tower 4.3 planetary - about 190g each. At $100 each, more expensive than a cheap outrunner, but way cheaper than the Neu/Hacker options.

Effective Kv is thus 837 rpm/V; on 4S expect about 37A/470W per motor with 10x8 and about 5lb static thrust per prop. And ample pitch speed for very good part throttle cruise thrust. I use one of these in a glider at about the same power level.

Even allowing for as much as 50% losses in the push pull arrangement, that should provide a thrust:weight well over 1:1, probably at least 5 times greater than the original and should get it up and going very smartly.

IMO you could readily stay true to scale this way. And if you wanted to cheat the nacelles a bit, still easily doable with 10" props and outrunners so you can keep at least the scale spacing
Nov 09, 2017, 04:58 PM
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manuel v's Avatar
stall speed is around 15.11 MPH, 24.18 KPH, 6.75 m/seg.

to 2 stall speed is necessary only 700 watts out. with 3 propellers APC 13x6 to 7092 rpm. speed model aprox. 48.32 KPH.
considerate 25% drag model vs propeller speed.
Nov 14, 2017, 01:46 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Originally Posted by scirocco
... Plan on extending the motor wires to the ESCs in the fuselage near the batteries, rather than ESCs close to the motors and long battery to ESC wires. ...
More info
too long battery wires will kill ESC over time: precautions, solutions & workarounds - RCG

Vriendelijke groeten Ron

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