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Old Dec 12, 2012, 07:51 PM
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Picking your motor:

Originally Posted by BpPilot View Post
Soooo, what MEGA for 6s-8s?

I'm pretty good with throttle management so it can be pretty high powered. Im planning on doing the build with the angled battery setup so probably no 7s configuration. I would like to be able to run 6s 5000mah and 8s 4000mah if possible. I think that is about what Peter M did on his GT40 build (correct me if im wrong...?)
You could still do the build to fit 8S, but run a 3S pack in place of one of the 4S to get 7S.
Gives you more options that way

For a setup that will be running with good throttle management, we can shoot for a couple different prop setups.

I've seen the highest top speed (171mph level, no-wind) on my Demons with a 6x6 prop at 37,500 RPM.
It's a tough hand launch, though, and takes a lot of room to get up to speed. But once it does, it squeezes the most speed out of the 16/35 and 16/40 motors that I ran it on.

For larger motors, you could shoot for turning that 6x6 prop at higher RPM and see what speed you get. It may be on the verge of becoming inefficient. Its tips will be transsonic over 38K.

My goal would be to run the higher pitched 6.5" pylon props at around 33K RPM.
They'll ensure you're not dealing with transsonic inefficiency. A 16/40 or 16/45 motor can safely turn these props at 33K.

So... start figurin'...
You need 33K RPM
Motor runs at about 85% of no-load KV. To get unloaded KV use the inverse (1/X) of 85% which is 117.6%
So 33K target x 117.6% = 38,808 no-load RPM.

You can then use this to figure out your target KV on any cell count you want. I'm using 3.5V/cell. If you have in-flight data, you can plug in your actual loaded voltage for more accurate predictions.

6S: 38,808 / 21V = 1848kv
7S: 38,808 / 24.5V = 1584kv
8S: 38,808 / 28V = 1386kv

Those are your ideal KVs for 6.5x5.5 prop at 33,000 RPM in-flight.

Then it's just down to looking at the 16/40 or 16/45 motor options to see which fits your cell count and target KV the closest.

Look! 16/45/1.5SCE is 1594kv!
That's stupid close to your ideal 7S motor.

16/45/2C is 1404kv. That's pretty close to ideal KV for 8S.
16/45/F5DE is 1360kv. Little lower than ideal 8S KV, but could turn a little larger prop, or account for batteries that hold better than 3.5V/cell under load.

If you
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Last edited by murdnunoc; Dec 12, 2012 at 07:58 PM.
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