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PostPosted: Mon Mar 25, 2013 10:18 am 
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Here is an alternate method:

First you need to figure out what you want to end up with:

1. What prop rpm do you want to run? A small prop at 12k rpm or a large prop at 10k rpm?

I'm going to pick 10k rpm because they work better for 3d flying.

2. How much performance do you want? 125 watts/lb to 225 watts/lb going from mild to ridiculous.

I'm going to pick 200 watts/lb.

3. Now that we know what we want performance wise we need to figure out how much total power we want:

I'm going with 200 watts/lb and I'm looking at a 40 size airplane at 5lbs so I'm looking for 1000 watts.

4. Now we need to pick what size battery to run. Here are a couple of different options:

volts x amps = watts.

3s battery = 11.1 volts
4s battery = 14.8 volts
5s battery = 18.5 volts
6s battery = 22.2 volts

So with my 1000 watt target I can do any of the following:

1000 watts / 11.1v = 90 amps
1000 watts / 14.8v = 64.57 amps
1000 watts / 18.5v = 54 amps
1000 watts / 22.2v = 45 amps

So now that I know how much current (amps) I need from each battery, I can figure out the minimum pack size based on which battery I want to run. So if I go for a 20c battery (I never want to discharge faster than 20 times the battery size) I get:

(3s) 90 amps / 20c = 4.5 amp/hr or 4500mah
(4s) 64.57 amps / 20c = 3.2 amp/hr or 3200mah
(5s) 54 amps / 20c = 2.7amp/hr or 2700mah
(6s) 45amps / 20c = 2.2 amp/hr or 2200mah

Now discharging at 20C is reasonable for a light 3d airplane, but will give short run times. You can double this for a sport airplane and get really long run times:

So for a 10c discharge you get:

(3s) 90 amps / 10c = 9 amp/hr or 9000mah
(4s) 64.57 amps / 10c = 6.4 amp/hr or 6400mah
(5s) 54 amps / 10c = 5.4amp/hr or 5400mah
(6s) 45amps / 10c = 4.5 amp/hr or 4500mah

As you can see, weight and run times are a direct trade off.

I have some 4s3900 packs on hand for another airplane so lets use those.

5. Now that we know what battery we are going to use now we pick out the ESC.

This is really simple, we need something that can handle 70-75 amps on 4s.

6. Now we can pick out the motor:

To get the kv we just divide the target rpm by the volts:

10k rpm / 14.8v = 675kv motor

To get the size we figure a good motor can do about 3 watts per gram:

1000watts / 3watts = 333g motor.

7. Now that we know the battery, motor, watts we just need to find the prop to use. I like to use a calculator for this:

http://www.ecalc.ch/motorcalc.htm?ecalc&lang=en

Set the field elevation to 0 and OAT to 65F
Put in a 4s4000 30c battery
Choose a motor that is around 675kv (I used a scorpion SII-3032-690)

Then start playing with props until you get something around 1000watts

I found the APCe 14x7 to come out at 818W and the APCe 15x7 to come out at 967 watts.

That is your starting point. I would probably pick up a 14x7 a 15x6 and a 15x7 and use the watt meter to fine tune.

Hope that helps,
schu


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PostPosted: Mon Mar 25, 2013 2:11 pm 
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Good stuff Matt.

Mike B.


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PostPosted: Tue Mar 26, 2013 10:58 pm 
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I should point out that this method largely ignores the figures that the motor vendor provides with thier motor, but that is ok because they all lie anyway, and give different specs. For example, some vendors give as burst current and others don't. Others will flat underrate thier motors (hacker) while others will give example setups that don't make sense (eflight).

In this example pretty much any quality motor (consistent kv/windings, good materials, quality wire/magnets) will work just fine.

For this application I would probably use this motor:

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

Notice that the propellor data for the 15" props comes out to be very close to my original post?

Schu


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PostPosted: Wed Mar 27, 2013 7:30 am 
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Here is another motor that would work fine, but looking at the specs you wouldn't think it:

http://www.hobbypartz.com/86mc450-4120-660kv.html

The specs show 1850 watts, 22.2 volts, 180amp burst.

That doesn't make a lick of sense. 22.2 volts x 180amp = 3996 watts. Out of a 300g motor..... right...

All I know is that the KV is right and the motor is heavy enough to handle the power I need from it.

All this said, there is another important concept I forgot to mention. The more volts generally the more efficient the motor is. Also, some motors claim an absolute amp rating which has some truth to it as pumping 100 amps at 1 volt (100 watts) through a 33g motor (3 watts/g) is going to cook it.

My example airplane would be more efficient on 5s and some would argue it should be 6s, but hey, I already have 4s3900 packs floating around, and I know I can get away with it, so I'll spend a few more bucks on an ESC and live with the 5% less efficiency if I can use packs I already have on hand.

So for me I have 3s1000 3s2200 4s3900 and 6s5000 packs. Every heli (larger than my 130x) and airplane I have will work on some combination of one or two of those packs.

If I was to build a new airplane, it would either be another combination of the above or I would thing long and hard before adding another battery size to my list.

Or I would just skip the electric thing and go gas since I really do want a 100cc airplane.

schu


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PostPosted: Fri Mar 29, 2013 5:45 pm 
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Extremely helpful Shu thanks. Keep the analysis coming.


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PostPosted: Sun Mar 31, 2013 1:46 pm 
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Location: Anchorage
/quote]
Or I would just skip the electric thing and go gas since I really do want a 100cc airplane.

schu


Schu,

Your welcome to try the sticks on one of my 100cc gas birds.

You can also get great deals on used 100cc birds. Several hit the AK market last year.

Very good deals.
Dean


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PostPosted: Sat Apr 06, 2013 10:17 pm 
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Matt,

Just as a check that I understand what you have posted, I'm looking to replace a brushed motor in an old plane of mine.

Here is what I came up with using what you posted above.

So the plane would perform reasonably I choose 150 watts - per pound

The plane is 2 lbs with electronics.

So this makes the motor need to put out about 300 watts.

Since I want to use a 4s (cause that is what I have) that would mean I'd need something that is rated at about 20 amps (300/14.8) as a minimum.

Since we want to overrate the motor, it should probably be more like 30 amps.

Going off of Matt's rule of 3 watts per gram I get about a 100g+ motor (300/3).

The other thing that I needed to figure out is what sort of motor did I need in order to turn a prop that is about nine inches long.

I found this table below for some help.

900-1000KV with 9-10 Inch propeller
1200-1400KV with 8-9 Inch propeller
1600-1800KV with 6-7 Inch propeller
2200-2800KV with 5 Inch propeller
3000-3500KV with 4.5 Inch propeller

As a final issue, the motor needed to be 36mm inrunner.....this is an extra requirement that made finding a good motor a little harder.

Here is what I located.

http://www.hobbyking.com/hobbyking/stor ... oduct=9823

From the specs it looks like it should put out 421W (28.5 x 14.8 using a 9 inch prop)

This will be more than enough for the plane it is intended to power.

Additionally this motor is 179g, which doesn't quite match the 3 watts/gram, but that is just a rule of thumb.

I may be able to go to a 3s battery, but it would be very docile.

Matt, I would love to hear your feedback on this thought process.

Mike B.


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PostPosted: Sun Apr 07, 2013 7:20 am 
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Sure Mike, I would be happy to help.

So we need 300 watts on 4s.

So that means that we will pull right at 20 amps (300w / 14.8v = 20.27amps)

So now lets figure minimum battery size. 20 amps / 20c = 1amphour or 1000 mah.

So anything above 4s1000mah will work, but the more you go over that the more weight.

Ok, so now onto the motor. Lets figure you want to turn 12k rpm (assuming this is a sport plane) so you are looking for 810kv since 120000/14.8v = 810.

Motor weight needs to be about 100g to be 3 watts a gram. Now take this as a general rule, if you have a really high quality motor you can get away with more depending on how it's cooled.

This motor seems to be the most appropriate:

http://www.hobbyking.com/hobbyking/stor ... 810kv.html

It has enough weight and the kv is right. They show max current at 23 amps in the chart, but in the data text field they show that they are pulling much more depending on prop.

The calculator at http://www.ecalc.ch/motorcalc.htm?ecalc&lang=en using several different ~800kv motors shows you are going to be using an 11" prop which sounds about right for this power.

I would try a 11x4.7 slow flyer and a 11x5.5e prop. I suspect both will work well, but you might be able to get away with a little more prop if have enough cooling.

This would be consistent with what we find in the reviews:

Code:
V    A    W    Rpm    Pull, gr   Prop
13.49   27.4   370   8145   1980    13X4
13.66   28.6   390   8310   1990    13X4
13.58    28.6   388   8250   2000    13X4
13.14   33.7   443   14430   1540    12X8
12.69   36.3   461   6435   2200    14X7
13.55   40.6   551   6750   2300    14X7
Battery Type: 4S Li-Po. ESC: 50A   - Hiperion May be these data will be useful for somebody. Fine motor


All of these numbers are running the motor too hard, as well as way to much voltage drop for a 4s pack (I suspect the battery was dead or junk), so we don't want to trust them too much, but we can see with a little more voltage (14.8v) we are going to need much less prop than a 13x4 to stay in the 300 watt range.

Hope that helps,
schu


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PostPosted: Sun Apr 07, 2013 7:37 am 
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I should note that hk says it can handle 50A in the specs above that table that says it can handle 23A. I assure you that 50amps will cook this motor in a hurry.

Also, why do you need an inrunner? This an odd size for a prop motor.


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PostPosted: Mon Apr 08, 2013 7:15 am 
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It looks like I forgot to mention that this motor is replacing a current brushed motor that is mounted inside....which is why I chose to go with an inrunner.

What wasn't clear to me is how you decided we needed 12k rpm?

I assume this is just experience with motors leading you to that conclusion.

Also, there is a physical limit of a 9 inch prop, unless I modify the landing gear a bit.

Thanks for running through the thought process, it was very helpful.


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PostPosted: Sun May 12, 2013 11:57 am 
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As a follow-up to this discussion, the 1024kv motor that I picked out worked well for the plane.

I was even able to use some 30c 2200 3s batteries. They had to be moved all the way to the front of the plane for it to balance though.

This exercise brought a lot of confidence to me about being able to pick a motor for my next plane.

The basic steps I took to do this were:
- identify all known parameters (especially limitations, like prop height)
- determine motor size (see posts above)
- install motor and fly!

Thanks again Matt for taking your time on all this.

The stats I got out of this motor using my watt meter pretty closely matched the ones from the web link above.

If you would like me to post actuals, just let me know.

Mike B.


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