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School me on choosing the right Lipo plz

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School me on choosing the right Lipo plz

Old 07-02-2009, 01:18 PM
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amadio1
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Default School me on choosing the right Lipo plz

Hi guys. Im making the switch from 25 years of gas and nitro flight over to the electrics. Ive gotten a ton
of help from the people on this forum, and I have a few more questions.

Ive chosen the plane I want to pick up, and found one at my LHS.
http://www.electrifly.com/parkflyers/gpma1580.html
The EP Reactor Bipe
35" wingspan
about 1000g

Ive done alot of searching and a TON of learning and I belive I found a suitable outrunner for it
Kv: 910rpm/v
Turns: 8
Resistance: 75
Idle Current: 1.12A
Shaft: 4mm
Weight: 112g
Rated Power: 310w
ESC: 30A
Ive read many reviews on the motor as it seems to be of good quality and people claim it
can pull a 1000g 3D plane without a problem.

MY QUESTION: Now that I have my plane size and weight, and the motors specs... is there a formula
for deciding how much battery I will need?
Is there a formula for choosing the Mah and C of battery to use in a given application?
Old 07-02-2009, 01:33 PM
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jmohn
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Default RE: School me on choosing the right Lipo plz

Looks like a 3S 1320 to 2300 pack would work fine. You probably want to go a little larger than 1320 so you get longer flight times.


Jeff
Old 07-02-2009, 01:40 PM
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amadio1
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Default RE: School me on choosing the right Lipo plz


ORIGINAL: jmohn

Looks like a 3S 1320 to 2300 pack would work fine.

Jeff
What im trying to figure out is how to come to the conclusion you just did, but on my own.
Did you figure that out based on some formula, or is it strictly an experience thing?
Old 07-02-2009, 01:58 PM
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bobferguson
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Default RE: School me on choosing the right Lipo plz

I find I tend to use experience with comparable set-ups is the most accurate. I will some times take a known set-up and try work up a formula but I have never come up with a accurate one. There is just to many variables.


ORIGINAL: amadio1


ORIGINAL: jmohn

Looks like a 3S 1320 to 2300 pack would work fine.

Jeff
What im trying to figure out is how to come to the conclusion you just did, but on my own.
Did you figure that out based on some formula, or is it strictly an experience thing?
Old 07-02-2009, 02:53 PM
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speedy72vega
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Default RE: School me on choosing the right Lipo plz

amadio1, here's a kinda quick rundown on how to choose. Let's say the motor/prop setup you choose will run around 25 amps constant, 30 amps burst (10-15 seconds or so), and your motor is designed to run 3S, or 11.1 volts. For a 1 kilo plane, you probably want a battery in the 1500-2200ma range, 3S. Each cell is 3.7 volts, so a 3s, or 3 cell = 11.1 volt.
With me so far? Now, the discharge rating, 20C, 30C, etc. A 10C battery means 10 times discharge rate, 20C- 20 times discharge rate, etc. With that in mind, a 2200ma battery (1000ma= 1 amp, 2200ma = 2.2 amps) at 10C can safely discharge 22 amps. A 2200ma 20C battery can safely discharge at 44 amps, 30C at 66 amps, ok?
So, a 25 amp draw from your motor/prop combo would be too much for a 2200ma 10C, a 20C would be fine, a 30C would be more than neccessary.
I hope this clears it up a little.
Old 07-02-2009, 03:05 PM
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bobferguson
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Default RE: School me on choosing the right Lipo plz

You need to confirm your motor wattage requirements. The RimFireâ„¢ 35-36-1200 is the recommended motor which is a 660 watt motor. One of my friends has that plane with I believe a stock motor and I would not say it was overpowered. I'll ask what battery he is running and confirm the motor.
Old 07-02-2009, 03:24 PM
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bobferguson
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Default RE: School me on choosing the right Lipo plz

I very good rule of thumb write up.
http://www.horizonhobby.com/Explore/...ArticleID=1563
By it a 2lb 3d plane needs 300-400 watts. That should be 80% of max so I would suggest at least a 500 watt motor. You can always prop down to reduce the wattage if to much. Also to get 500-600 watts with less than 45 amps would probably require a 4s pack. A 3s pack is about 10 voltages under load so at 500 watts that would be 50 amps which is above the motor's rating. I have been fining on my setups than 3s is not giving me the motor max (80%) wattage output. I've been changing to 4s packs and reproping to get more power without overloading the motor on plane that have room and can handle the extra weight of the battery.

I hope I have not confused you to much. Selection is a black art not a science. Or find someone with the setup your looking at since real world information is the best data.
Old 07-02-2009, 04:02 PM
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speedy72vega
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Default RE: School me on choosing the right Lipo plz

amadio1, by the way, my capacity numbers I stated were all hypothetical, just used whatever numbers came to my mind first for explanation purposes. The process for choosing your pack is still the same though. I just wanted to explain what all the numbers mean and how to use that info for your setup, it wasn't literal in regards to the motor you mentioned.
On a side note, I fly an Electrifly 41" Edge 540 that weighs in at about 32-34oz, and I'm running a Rimfire 35-30-1450 motor. It runs right at 372 watts with a 10x7E APC prop, and it pulls the plane good. It's not the most efficient motor though, I run a 3S 2650ma 20C pack, and it uses about 2200ma for a 6 or 7 min flight if the wind is more than 5 mph.
Old 07-02-2009, 04:50 PM
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DaveFlynn
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Default RE: School me on choosing the right Lipo plz

Both higher capacity an higher C ratings add weight. There are several trade-offs involved. For example a 1000mAh 30C battery would give you top performance, but wouldn't last long. A 3300mAh 10C battery would give you long 2D flights, but may be too heavy for good 3D. The most popular and therefor least expensive is the 2100mAh, 16C to 20C. At 5.5oz they're a good compromise between weight and capacity.
Old 07-02-2009, 05:29 PM
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bobferguson
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Default RE: School me on choosing the right Lipo plz

Going to 4s vs 3s adds little weight but should add 25% more run time if set-up at the same wattage though I have yet to prove this to be true for myself.
Old 07-02-2009, 06:09 PM
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DaveFlynn
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Default RE: School me on choosing the right Lipo plz

Mostly true. The max prop size depends on ground clearance and the min on the thrust required. I try to choose the lowest Kv for a given frame size then choose the cell count to get the pitch speed I want or the maximum rpm rating for the prop. Pay close attention to the numbers, too many watts will cook the motor, too many amps and the weakest link (batt, ESC or motor) will be damaged.

I recently changed the motor on one of my planes from a Kv=650RPM/Volt to a Kv=380RPM/Volt motor of the same weight and size. The prop was changed from 12x6 to 14x6 and the battery from 6S2P to 9S1P. Flight times are unchanged at around 10 min and the plane is much faster and 5.5oz lighter. I'm getting slightly more thrust for fewer amps at a lower rpm and lower wattage. This is a big slow trainer, so no 3D stuff here.
Old 07-02-2009, 06:16 PM
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bobferguson
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Default RE: School me on choosing the right Lipo plz


How did you save weight?


ORIGINAL: DaveFlynn

Mostly true. The max prop size depends on ground clearance and the min on the thrust required. I try to choose the lowest Kv for a given frame size then choose the cell count to get the pitch speed I want or the maximum rpm rating for the prop. Pay close attention to the numbers, too many watts will cook the motor, too many amps and the weakest link (batt, ESC or motor) will be damaged.

I recently changed the motor on one of my planes from a Kv=650RPM/Volt to a Kv=380RPM/Volt motor of the same weight and size. The prop was changed from 12x6 to 14x6 and the battery from 6S2P to 9S1P. Flight times are unchanged at around 10 min and the plane is much faster and 5.5oz lighter. I'm getting slightly more thrust for fewer amps at a lower rpm and lower wattage. This is a big slow trainer, so no 3D stuff here.
Old 07-02-2009, 07:14 PM
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speedy72vega
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Default RE: School me on choosing the right Lipo plz


ORIGINAL: DaveFlynn

Both higher capacity an higher C ratings add weight. There are several trade-offs involved. For example a 1000mAh 30C battery would give you top performance, but wouldn't last long. A 3300mAh 10C battery would give you long 2D flights, but may be too heavy for good 3D. The most popular and therefor least expensive is the 2100mAh, 16C to 20C. At 5.5oz they're a good compromise between weight and capacity.
Excellent post Dave, I did forget to mention weight in the equation.

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