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Old Jun 28, 2012, 08:19 PM
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Originally Posted by rezzey View Post
If there are other combinations you can find on the Hobby King website that would suit my plane better let me know!
Jake, I think you can do better. Did you notice that WebOCalc told you that you'd need about 2700 grams of thrust ("Desired Thrust"), but the power system you came up with only provides about 2050 grams? That's a full 650 grams shy of the initial suggestion - nearly 25% low. For combat, I don't think you're going to be happy - I think you're going to want more power.

It's not usual to need a huge 5S lipo pack and 500 watts in such a small model - the reason is that it is extremely heavy. 2 kg for a small 1.2 m wingspan is bordering on a flying brick. Is there any way to take some weight out of the model?

Edit: I just realised it's made of Coroplast. Ahh. Now I understand why it's so very heavy for such a small airframe!

Generally speaking, extremely heavy airframes aren't the best choice for electric conversions. Especially with cheap (but heavy) motors, an electric conversion will add even more weight to an already heavy airframe. But prices have fallen to the point where you can get away with it nowadays, so if you prefer to go ahead with an electric powertrain, it's certainly possible.

If you're set on an electric powertrain and nothing can be done to reduce the weight of the airframe, then you would get the biggest improvement by using a larger propeller and correspondingly lower Kv motor than your initial choices.

A bigger propeller makes an enormous difference - you can get much better performance without flogging the batteries and motor any harder. The motor you chose will only drive a tiny little 9" propeller (APC 9x4.5 prop at nearly 1.00 ratio). That's way too small for a very heavy 46" electric model. Ideally something around 12 inches would be a good match to your model, if you can find a motor with a low enough Kv. But if that is not possible, even a 10" or 11" prop would be a big, big improvement over that 9x4.5 prop.

You're also going to need a bigger capacity battery unless you're happy with pretty short flight times. I think 3500 mAh is closer to the mark than 2500 mAh for this model.

Given all that, here are my suggestions:
I like the Turnigy Plush ESC's, and higher capacity ones come with a switching BEC (UBEC). This is good, you NEED a UBEC if you're running on a 5S lipo pack as you propose to do. I agree with you about oversizing, your model might require 45A current, so I'd suggest using an ESC rated at 60A or more. The Turnigy Plush 80A seems like a safe choice:

1) ESC Link (Europe):

2) ESC Link 2 (China?):

(The ESC you chose may work fine, too. Just make sure it has a switching BEC, or use a separate external UBEC with it.)

You need a motor with a low Kv, and 700 rpm/V is the best I could find at Hobby King. I found two possible candidates, one marginal, one very good.

First the marginal one - this motor is really a little too small to handle the power your model needs, but Hobby King carries very few low Kv motors, so I'll list it here just in case:

3) Motor #1, marginally sized (Turnigy Aerodrive SK3 3548-700kV):

If you use the above motor, be kind with the throttle, and you should be fine. Fly it like a glow plane with the throttle all the way up for extended periods, and the motor may cook itself. (It's gonna be hard to manage throttle during combat flying, no?)

Second, the good motor. This motor is much higher quality and certainly up to the job, but costs a lot more (Scorpion SII-3026-710 ):

4) Motor #2:

With either motor, use an APC 12x8 Thin Electric, and 5S lipo pack. 3000 mAh should get you 6 - 10 minutes, and is probably the smallest capacity worth using. 3300 mAh or 3500 mAh would be better. (20C or higher in every case - these days it's hard to find lipo packs rated below 20 C, so no worries on that score.)

5) Here's one suitable battery: 5S 3700 mAh 25C Zippy.

6) Propeller: APC 12x8 Thin Electric.

I've attached a WebOCalc screenshot of the predicted performance below.

Hope that helps,

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