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Dec 12, 2017, 08:38 PM
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T_Hartigan's Avatar
Discussion

Motor recommendation Great Planes J3


Won a Great Planes J3 cub kit for 0.40-0.60. 76.5Ē wingspan.

Any recommendations on motor setup? Nothing in kit recommendation for electric. I need to modify for battery hatch so need to get a handle if Iím looking at 4s or 6s setup. Box says 6.5-7.5 lbs.

Looking for sporty performance.

Thanks
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Dec 13, 2017, 03:49 AM
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scirocco's Avatar
For a model that size that's probably going to need .60 glow engine weight in the nose, you're going to have a lot more options if you choose 6S.

4S works just fine on the ~6lb 68" Eflite Supercub 25E, but your J3 is a whole size class bigger and heavier. It's similar in size and weight to the 80" Hangar 9 Pawnee that most electric versions including mine run 6S - I wouldn't want to try to do a 4S setup on mine.

6S 4000-5000mAh, a motor around 350-400g, Kv around 500 rpm/V and a 14 or 15" prop will give you nice sporty performance for over 6 minutes.

The Eflite Power 60B @470 rpm/V, Turnigy G60-500, Cobra C4130/12 or /14 @ 540 or 450 rpm/V come to mind
Dec 13, 2017, 07:30 AM
Jack
jackerbes's Avatar
Quote:
Originally Posted by T_Hartigan
Won a Great Planes J3 cub kit for 0.40-0.60. 76.5” wingspan.

Any recommendations on motor setup? Nothing in kit recommendation for electric. I need to modify for battery hatch so need to get a handle if I’m looking at 4s or 6s setup. Box says 6.5-7.5 lbs.

Looking for sporty performance.

Thanks
Here are some rules of thumb for electric flight that will help you get started on working out the details of an electric power system.

SPECIFICATIONS - Piper J-3 Cub 40
Wingspan - Standard Wing: 76.5 in (1945 mm) Clipped Wing: 61.5 in (1560 mm)
Wing Area - Standard Wing: 820 in≤ (52.9 dm≤) Clipped Wing: 653 in≤ (42.1 dm≤)
Weight - 6.5-7.5 lb (2.95-3.40 kg)
Length - 49 in (1245 mm)
Requires - 2-stroke .40-.60 cu in or 4-stroke .48-.70 cu in;

One example using the Fuel to Electric Conversion rule:

"Two stroke fuel motor to electric motor conversion
displacement in cu. in. x 2000 = electric power in watts"

To replace a .60 fuelie 1200 Watts of electric power would be about right

To use another rule for motor weight to continuous power capability of a motor:

"Motor Input Power - Typical quality outrunners can handle an input power (in Watts, and Volts x Amps = Watts) of 3W per gram of motor weight when running at their continuous rating"

To get a 1200W motor using the 3W per gram rule would be a 400g motor. But that is more power than your need for basic flight, 75% of that would work fine so a 300g motor would be OK.

I have done fuel to electric conversions using those rules and they outperformed the glow motors that they replaced. Even glow fuel addicts who hated electric planes admitted that the electric motor out performed the original .40 glow fuel power system on a SIG Rascal 40.

Jack
Last edited by jackerbes; Dec 15, 2017 at 07:32 AM.
Dec 13, 2017, 10:04 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
Vriendelijke groeten Ron
Dec 15, 2017, 06:54 AM
Registered User
T_Hartigan's Avatar
Thanks everyone. I have 6 cell packs, so I’ll go this route. Planning on copying H9 way of installing the packs.
Dec 15, 2017, 07:39 AM
Jack
jackerbes's Avatar
When I converted a SIG Rascal 40 I put the battery (5S A123 1100 mAh) as far forward in the cabin and under the engine cowl as I could get it. As a fuelie the motor and tank of fuel were pretty far forward and I wanted to try to match that.

With the battery in place I put a 8 oz. or so outrunner from the firewall and found that I had a tail heavy airplane and was going to have to add at least a couple of ounces of dead weight behind the motor to get the CG right.

At the time I had a nice Torque Revolution 4014-570 motor (10 oz., 570 Kv) and realized that it would add the needed weight. So I mounted that directly to the same firewall that the fuel engine had used and with the 11 oz. 5S A123 (11 oz.) battery pack in the same location where the fuel tank had been. And that worked out to be a wonderful thing. The Rascal was off the ground and flying with a 10 foot or so take off roll and would cruise the pattern in a lovely fashion with less than half throttle.

The Rascal ended up flying at 6-1/2 pounds and with up to 625 Watts of power turning a APC 14 x 10 TE prop. And the fuelie guys that were skeptical about the conversion all said it was amazing and that it flew as well as or even better than the Rascal did as a fuelie.

A radio glitch or failure took the Rascal's life a few months later but she is still a treasured memory.

Jack
Dec 23, 2017, 07:33 AM
Registered User
T_Hartigan's Avatar
Quote:
Originally Posted by jackerbes
When I converted a SIG Rascal 40 I put the battery (5S A123 1100 mAh) as far forward in the cabin and under the engine cowl as I could get it. As a fuelie the motor and tank of fuel were pretty far forward and I wanted to try to match that.

With the battery in place I put a 8 oz. or so outrunner from the firewall and found that I had a tail heavy airplane and was going to have to add at least a couple of ounces of dead weight behind the motor to get the CG right.

At the time I had a nice Torque Revolution 4014-570 motor (10 oz., 570 Kv) and realized that it would add the needed weight. So I mounted that directly to the same firewall that the fuel engine had used and with the 11 oz. 5S A123 (11 oz.) battery pack in the same location where the fuel tank had been. And that worked out to be a wonderful thing. The Rascal was off the ground and flying with a 10 foot or so take off roll and would cruise the pattern in a lovely fashion with less than half throttle.

The Rascal ended up flying at 6-1/2 pounds and with up to 625 Watts of power turning a APC 14 x 10 TE prop. And the fuelie guys that were skeptical about the conversion all said it was amazing and that it flew as well as or even better than the Rascal did as a fuelie.

A radio glitch or failure took the Rascal's life a few months later but she is still a treasured memory.

Jack
Jack,

Were the batteries removable or did you charge in the plane?
Dec 23, 2017, 09:11 AM
Jack
jackerbes's Avatar
Quote:
Originally Posted by T_Hartigan
Jack,

Were the batteries removable or did you charge in the plane?
I usually charged them in the plane. The pack was held in place with a square of Velcro on the pack and a matching square glued in place in the plane. And there was a Velcro strap around the pack too. So I could remove the pack if I wanted to use more than one pack in a flying session.

Jack
Dec 25, 2017, 06:33 PM
I am a nice guy! Really!
The A123 battery uses LiFe chemistry, which is much more stable than the LiPo chemistry. Therefore it is okay to charge them while in the plane. It is a really good idea to remove LiPo batteries from the plane for charging. That way a fire will only kill the battery instead of destroying the entire plane.
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Dec 25, 2017, 08:35 PM
Registered User
T_Hartigan's Avatar
Decided to go power 60 @ 470 kv. 6s 3300-3700 packs that I already have. With a Talon 90.

The power 60 should be here this week and I should get an idea of balancing. I don’t like bottom hatches but that looks like the best option right now. I’ve halted fuselage construction till the motor gets mounted.


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