40 Size Piper Cub (Old) Need Some Help Sizing Lipo - RC Groups
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Jul 01, 2012, 12:04 PM
Registered User

40 Size Piper Cub (Old) Need Some Help Sizing Lipo

New to the Forum apologies if I am not on the correct topic board.

I searched forum did not find much help.

The cub was given to me and trying to get it running. It had NiCad Batts in it that were removed before I received the model. The plane has been flown many times with NiCads.

I tried running this drive system on MotoCalc 8 and I can't find enough info on this 40 size motor to get good results from the calcs. I don't want to exceed the 30 fuse. Perhaps the motor is not capable of running above 30 amps but the ESC is rated for 50 amps.

Question - What size Lipo can I use with this setup?

Astro Cobalt 40 size Motor - Has 8P stamped on the motor.

Astro 204 ESC for Brushed Motor.

13 x 8 electric prop

2.5:1 belt drive reduction

30 amo DC fuse between Battery and ESC.

See Pics.

Help much appreciated........

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Jul 01, 2012, 12:12 PM
Registered User
E-Challenged's Avatar
I plan to connect two 2200mah 3S lipos in series for 22-24 volts in my old Sig 1/6 scale Cub with geared AF 25 in aluminum tubular Joe Pasquito mount I'm hoping cg comes out about right with both packs on front of cabin near instrument panel. If not, may have to add lead to front of cowl. Mount servos after you have some idea if nose weight will be needed. Heavy Cubs fly too fast for scale-like. You AF forty may be overkill but heaviness may be good for CG.
Jul 01, 2012, 05:52 PM
Pro Hoarder
turbonut's Avatar
I would contact Astro and get specs for the motor..you need to know for sure which wind it is..if it has the FAI wind then run it as e challenged has said with a 6s lipo.just keep the throttle down a bit at first so not to over heat the motor
Jul 01, 2012, 06:38 PM
Registered User
Originally Posted by turbonut
I would contact Astro and get specs for the motor..you need to know for sure which wind it is..if it has the FAI wind then run it as e challenged has said with a 6s lipo.just keep the throttle down a bit at first so not to over heat the motor
I will plan on calling Astro. I think that 8P is the model but not sure.

Any idea if this type of ESC has BEC included?
Jul 01, 2012, 08:45 PM
Registered User
E-Challenged's Avatar
The old AF ESC that I have , has no BEC, yours probably doesn't either because of the high voltage required. Your cheapest course may be to use a 4 cell Nimh flight pack same as in an IC powered model. Otherwise you will need an add-on BEC rated for 6S lipos and four or more servos.

Confirmed on AF website, called a 204D, has no BEC. You can watch for reduction in thrust then land, no real need for cutoff feature if you don't continue flying and discharge your lipo pack(s) too far
Last edited by E-Challenged; Jul 01, 2012 at 09:02 PM.
Jul 01, 2012, 08:49 PM
Registered User


A couple of comments:

(1) this speed control does NOT have a cutoff in it that will work with lithium batteries. You'll need to use an aftermarket one. Dimension Engineering makes them and sells them.

(2) I have a factory geared Astro .40 motor, marked '8T6' and the instruction sheet that came with it said to use 18 NiCd's and a 13x8 propeller, which the instructions say will turn the prop 6600 rpm. Your rpm will vary, as the belt drive might not have the same gear ratio as Astro's gearbox. The current draw at 18 NiCd's and a 13x8 propeller was calculated to be around 25 amps.

(3) Close to the 18 NiCd cell battery pack is a 5S lithium battery. My guess would be to use somewhere around 3000mah cells.

(4) It would be a very good idea to purchase an Astro Flight Whatt-meter to measure voltages and current flows so that you don't hurt any of the electronics.

Good luck with this revival of the model. If any of my comments aren't crystal clear, then please ask around among modelers local to you and find someone who can tutor you through the process. He doesn't need to have done this very sort of project, but he does need to be very thorough and careful.
Jul 02, 2012, 11:24 AM
Registered User
E-Challenged's Avatar
More research: My Astro 25 G is rated for 35 amps and 12-20 volts max. I'll test run the motor on 6S lipo with amp meter with smaller/lower pitch props at wide open and 3/4 throttle to see if amps stay below 35. If edgy, I'll go to 5S lipo 3S +2S in series, for around 19 volts.

The AF prices for these motors are still very high, $425 for 25 G. Im honored to have any motor worth that much in my stash. For some reason, AF specs for cobalt motors still talk mostly about nicad round cells
Jul 02, 2012, 04:59 PM
Visitor from Reality
Hi Will

Had the similar Sig 1/6th clipwing until a couple of years back. It used a somewhat odd drivetrain - a European inrunner driving direct. Not a combo that came cheap nor easy to find nowadays. In terms of battery power, I flew it on 3S and 4S LiPo, around 3600mA and a 4S A123 pack - 2300mA. Model power into around 12" props, give or take a little, was around 450 - 550W - 'very adequate' for the model to behave like a clipwing.

Okay, with 4S, a clipwing with a turbo and retro firing rockets at times!

Was a big geared Astro fan before sales and marketing discovered the outrunner. Apart from some horrendous competition versions, 30A was a smart figure to stop at - brushes do not like huge amounts of current. I suspect you have what was Astro's heftiest ESC - it would work with LiPos, but you'd have no safety cut-off at low voltage. Suchlike hadn't been invented in the days of nicads and nimh - you had it really pointedly pointed out to you when the battery had run out!

Most of then flew with a timer of some sort, set to let us know when it was time to head for the barn before it went embarasingly quiet on the motor front...

That motor is a huge, heavy lump for this small scale model. Suggest you look up the cheap Chinese outrunner copies - motor and ESC, and matched LiPos if proven good quality gear is not in the running here.

Good luck with the model - Sig make great electrics, shame they keep printing the glow conversion on their plans In the meantime, have a couple of inspirationals of my 1/6th clipwing - the colour scheme is scale, I just can't abide having a model that might be like someone else's, so all my Cubs tend to be any colour but yellow!

Good luck with your project

Jul 03, 2012, 05:31 PM
Registered User
Thanks for all the good input guys.

Was on the phone with AstroFlight and received this info for their Cobalt 40 size motor. They did not know what a model "8P" motor was but had this info to share.

"Hi Will,

It was nice speaking to you today. Attached is the documentation for a cobalt 40, 8 turn motor. I am not familiar with and 8P motor. Hope the information for the 8T motor will be useful for you.


Astro Flight

Based on what I am finding I am leaning towards a 5s or 6s Lipo. If I use the 6S Lipo I can take (2) 3s in series as E-C mentioned. I will run the calcs with this motor info to size the lipo. I am going to eliminate the DC fuse and buy a Whatt Meter to measure current/voltage during testing. Go with no BEC. I can test the drive system including servos by running a duty cycle with a timer and whatt meter. Fly like the old days where you have to get the plane down before battery gets to low.

I would love to buy a new brushless and esc but why spend the money if this works.

If I am missing anything let me know.

I will let you all know how it goes.

Jul 03, 2012, 09:18 PM
Senile Member
Lnagel's Avatar
Originally Posted by WillFly1966
If I am missing anything let me know.

I will let you all know how it goes.

One thing you didn't mention is the 2.5:1 belt drive. If you use that you will have to use a larger propeller to obtain the same output power.


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