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PostPosted: Mon Jun 16, 2014 4:49 pm 
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So I was talking to Steve about motors and batteries for this airplane, so I thought I would post how I would go about selecting an electric setup for this airplane.

First we need to know the total overall power. To do that we figure flying weight x 200 watts (yea, it's a lot, but it will make this airplane really scream)

Extreme flight says this airplane ends up being 5-6lbs for good flying weight so lighter is better so lets go with 5lbs x 200 watts for 1000 total watts.

In order to get 1000 watts we need a motor that weights about 300g. This is because a good motor can do 3-3.5 watts per gram or even a little better in Alaska due to cooler temps.

Now that we know that we need to know the KV. We have a number of choices:

6s (22.2v) x 500kv = 11100 rpm
5s (18.5v) x 500kv = 9250 rpm

(Torque 4016T-500 280g)

We can also do:

6s (22.2v) x 540kv = 11988 rpm
5s (18.5v) x 540kv = 9990rpm

(Cobra 4120/18 290g)

Or

6s (22.2v) x 430kv = 9546 rpm

(Cobra 4120/22 290g)

So what is the difference? Well, it's what rpm (read what size prop) you want to run.

Take a look at this chart for the 540kv motor:

http://innov8tivedesigns.com/Cobra/Cobr ... _Specs.htm

Notice that on 6s your pretty much stuck running a 14" prop, or perhaps at 15/4. On 5s you can run a 15/8 which will make this airplane work much better. So motor is really the last thing you want to pick, you decide the prop first, then the batteries (preferably something you have on hand), then the motor.

So for me I have a stack of 4s4000 packs, so I would probably run the cobra 4120/41 710kv motor which gives me around 10500 rpm and I would prop in the 15/8 range. According to the chart (http://innov8tivedesigns.com/images/spe ... _Specs.htm) this is 64 amps which is 16c for my 4000 packs (64 amps / 4ah batteries).

Lets talk about battery weight for a second. The battery in my example is about 400g (4s4000). If we wanted a 6s setup we could use the 4120/22 motor with a 15/8 prop at 35amps (though a little less power, a 15/9 would be close if there is such a thing). At 35 amps that is still 16c for a 2200mah batter (35 / 2.2ah batteries). These batteries are about 190g each. So it's basically a wash if we use 6s or 4s as long as we get the battery size right and the right kv motor.

If you wanted long flight times you could use a 6s setup with 4000mah batteries, but that will add another 250ish grams of weight which is 1/2lb of weight or 10% more weight.

Since we are talking about weight, if one wanted to push things a little and baffle the area around the motor and not use continuous full throttle you could get a cobra 3525/18 430kv on a light 6s2200 battery (350g). That would keep the motor and battery down to 600g total. It wouldn't be a speed plane, but I bet it would hover at half throttle and do nice floaty acro.

schu


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PostPosted: Fri Jul 11, 2014 6:18 pm 
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Schu,

This is great straight forward information. I greatly appreciate all the info. NOW, time to dial the plane in!


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PostPosted: Fri Jul 11, 2014 9:41 pm 
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No worries Steve.

It might be time for another electric clinic, perhaps ask about interest at the next club meeting. We could also do a 3d clinic and we can talk about setup and how to get the airplane to fly the best it can.

schu


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PostPosted: Sat Jul 12, 2014 7:03 am 
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Location: Wasilla, Alaska
I would be interested in a clinic on setting up a 3d plane.

Daryl

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PostPosted: Sat Jul 12, 2014 7:47 am 
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Rough syllabus:

Control surface hinging for max throws.
Making control horns completely linear and adjusting throws/leverage.
The importance of using another aileron channel and not using a y-cable.
Setting up for equal distance throws in all directions for all surfaces (adjusting endpoints to ensure that the left aileron doesn't move more or less than the right in any direction).
Expo and the pros and cons.
CG and how it effects flight, how to check it very accurately.
Weight and why you really want to get rid of as much as you can.
Prop selection and why that matters.
Basics of 3d flight (start with slow rolls)

Does that sound interesting? Not nearly as technical as the battery/charging/motor/esc/kv stuff but perhaps just as helpful?

schu


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PostPosted: Sat Jul 12, 2014 8:43 am 
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Location: Wasilla, Alaska
I will volunteer a couple of my planes for demonstrations. You can point out what is wrong and help me fine tune them. :D :D

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