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sig cub conversion


Old 07-23-2004, 09:09 PM
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Default sig cub conversion

I'm considering getting a sig 71" cub that normally calls for a .24-.40 2S. I was wondering how does one do the math to convert to electric. I'm seeing there are many considerations i.e. type of plane, but is there a formula that one could plug-in the specs and get at predicted performance level.

New to the hobby in general but having a blast.
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Old 08-03-2004, 07:22 AM
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Default RE: sig cub conversion

Wow. Sorry this sat so long. New job, and I need to feel out some time for RCU. I'm surprised nobody else chimed in on this one.

Anyway, I've gone through the process many many times in this forum, so you might get some additional ideas from skimming this forum. The basic rule of thumb is "Watts per pound." A plane needs a certain amount of power to fly in a certain way, regardless of what motor you choose. That's physics, Newton's Laws, and whatnot

Cubs have that magical ability to fly on a surprisingly small amount of power, but I would personally recommend a minimum of 75 Watts per pound. You'll need to estimate what the plane will weigh ready to fly, which can be tough, but the high end of the recommended weight range is generally a good starting point. Other sources of final weight are accounts of similar planes.

I'm estimating the plane will probably weigh around 5 pounds, which means you want at least 375 Watts at the battery. Let's do 400 Watts to make it easy and give you a little "reserve" power. 400 Watts is what the famous Gary Wright E3D power system puts out, and it's inexpensive:

10-cell CP2400 NiCd or GP3300 NiMH pack
Any brushed ESC rated for at least 40 Amps
Kyosho Endoplasma motor
Great Planes GD600 gearbox
10 tooth pinion from Model Electronics Corp.

Total power system weight of around 2lbs.
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Old 08-03-2004, 12:20 PM
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Default RE: sig cub conversion

A few years ago I did the Anniversary Cub in Clipped wing with Astro 25G on 18 cells. The plane flew "heavy" but was still fun to fly. I don't remember the weight or current but it looked very realistic in takeoff, landing and flight. Sometimes I don't want a plane that will jump off the ground in 10'.

If I were to do that again I wouldn't do the clipped wing to help with the wing loading. Then again, if I were to do that again, I would go with brushless for efficiency and probably LiPo's for weight.

I used to try for 50 watts/lb minimum into the motor but now I go for 50 watts/lb to the prop. That seems to be a little more reasonable and you can always go up from there.

I used to go with the area:cell ratio, i.e. 700 sqin.=14 cells and 500 sqin.= 10 cells, but that doesn't seem to cut it anymore with the new cells and motors available.

The bottom line seems to be to keep the wing loading reasonably in line with the size/type of the plane. A war bird has a higher loading than a trainer. My 3.2M XL3200 sailplane has 2x the loading of my 2M S400 sailplane but still likes to float like a butterfly. I think there is something about 2x the area and 4x the weight or some other highly tekky stuff that someone else can explain.

One more thing, if it says .25 - .40 go with something to give the power of the .40. There's nothing worse than getting to the field and saying that you wish it had just a bit more "umph" for the wind or what ever.

Keep 'em hummin'

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