Sig 1/4 scale Cub and Astro 40G - RC Groups
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Sep 04, 2003, 08:15 PM
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Matt Dyer's Avatar

Sig 1/4 scale Cub and Astro 40G

Last year, I came into possession of an Astro 40G via Ebay. I also have under construction a Sig 1/4 scale J-3 Cub built, which I had set up for an OS .91 Surpass. My interest in glow power has diminished, and I am wondering if the Astro 40G might be the motive power to use. I also have a Jeti JES 60 ESC that will take up to 30 cells.

What do you think? Can I power this model with the Astro 40G, JES 60 and up to 30 cells?

Any thoughts and observations would be much appreciated.
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Sep 04, 2003, 09:11 PM
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Arthur Knowles's Avatar
I flew a 60 size glow trainer on an Astro 40 geared 2.3:1 with a 13 x 8 prop and 21 cells. You can check out the specs here ( I used 2000 mah cells and could fly for about 8 minutes.

I would think a 1/4 scale cub could do as well. I would not recommend more than 21 cells unless you have a Astro Superbox on it (higher gear ratio).

FWIW, the Astro 40 is good for about 35 amps static.
Sep 04, 2003, 11:41 PM
We come in peace.
Synth's Avatar
I have a 20 year old 1/4 scale sig cub that I built very heavy and is powered by a os gemini 120 twin that puts out a whopping 1 horsepower and it still flies at 1/2 throttle. I don't think you'll have any problem flying it with the astro. Good luck and keep us posted eh. You will love this cub. Cheers Ron.
Sep 05, 2003, 12:49 AM
Our Daddy and Heli Junkie
Fred Bronk's Avatar
The AF 40 is good to well over 700 watts and with a gearbox will throw a nice big prop!

Motocalc says;

NC = 20
Gear Ratio = 2.38
Diam (in) = 18.0
Pitch (in) = 10.0
Weight (oz) = 235.9
Batt Amps = 31.3
Motor Amps = 31.3
Motor Volts = 23.3
Input (W) = 731.4
InPLd (W/lb) = 49.6
Loss (W) = 310.1
MGbOut (W) = 421.3
OutPLd (W/lb) = 28.6
MotGb Ef(%) = 57.6
Shaft Ef(%) = 56.0
Motor RPM = 10307
Prop RPM = 4331
Thrust (oz) = 114.7
PSpd (mph) = 41.0
RofC (fpm) = 250
Time (m:s) = 19:55

Looks like some scale flying.
Sep 05, 2003, 09:37 AM
Build more, websurf less
FlyingW's Avatar
I highly encourage setting up the Cub with the Astro 40. I have a Sig 1/6th Cub and it's the best plane I ever flew. Lots of side slips and touch and goes.

The smaller Cub is equipped with an Astro 15, 2.4:1 belt drive, 9-1950 FAUP cells, and 12x8 APC-E. I get 6-10 minute flights depending on how rough I get.

I also recommend a belt drive from Modelairtech - They're wonderfully quiet and smooth for scale Cub flying.

The H-1000 or H-1000HD (heavy-duty) version ought to do it - and give you a higher ratio than the Astro 1.63:1 gearbox that may be on that 40 right now.
Sep 05, 2003, 02:43 PM
Registered User
Sabrejock's Avatar
I flew mine very succesfully with the AF 40 but with the Astro Super Box gears. (3.33:1). This combined with 24 cells and 18x8 or 18x10 prop gave good perfpormance and 10 min flights. This is the combo that Kirk Massey told me about, but that was years ago.

The standard gearing won't be as effective as the SB, as the 18" prop was good at the slow speeds (and was scale diameter and looked great).

As this is a one-time thing and probably a keeper, I'd advise using a belt drive as mentioned above and going with a high ratio to allow the big prop. I fly the belt drive also and prefer it over the gears. Tex.
Sep 05, 2003, 06:49 PM
Visitor from Reality
I fly a Sig 1/5th Cub on a geared MaxCim and 20 cells - at 8lb 2 oz, she's off the deck before 3/4 throttle and flies on a sniff of power. Used 20 as I had them and they fitted - they are 2000 NiMh and I've flown for nearly 8mins with plenty left.

See no reason your AF40 / 1/4 scale Sig shouldn't be a good flier. Have never seen a Sig that didn't go great as an electric, in fact! Would echo the others in suggesting the Superbox to allow for bigger props - I fly my 1/5th on a near scale diameter 15 x 8.

DOn't know about your 1/4 but fitting and accessing the battery is a snip in the 1/5th, thanks to its functioning side door / window. I didn't even bother making up the battery fittings until I had the model built, as the door offered so much interior access.

What I did find was that my guesstimation of the pack going in the centre of the cabin was way off. I had to move it forward to hard against the back of the front cabin frame former to achieve balance.

If I could carry one around, would be looking at a Sig 1/4 Cub - my 1/5th is a lovely sight in the air and is a dream to fly. Good luck with your project. Can you figure what the model is in my avatar?


Sep 06, 2003, 05:28 PM
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Matt Dyer's Avatar
This is why I use Ezonemag as my homepage. There is always good advice available. I really appreciate the replies to my question abut the J-3. I think I should defintiely look into a different gearbox or belt drive. But now I have a lot of confidence in this project. Thanks to everyone for all the advice!

Matt Dyer