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Author Topic: Why do 4S batteries for "40 size" planes seem to be the popular choice ?  (Read 422 times)

Offline redout

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Going through many examples of electric specs. in this forum, I have formed the impression that people who fly "40 size" planes, e.g. Thunderbirds and Vectors, are using motors of around 800 to 900 kV's, and hence powering them with 4S batteries. 

And also, that people who fly larger aircraft ( 650 to 700 sq. inches ) are using motors with lower kV's ( say around 700 ) and hence are using 5S ( and even 6S ) systems.

Are my impressions correct and if so, why are the advantages of using  5S ( and 6S ) systems not being utilised for the smaller "40 size" models ?

Is it somehow related to a "sport flyer" thing for the 40 size planes vs. a "competition" aspect for the larger aircraft ?

Offline Dennis Toth

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Re: Why do 4S batteries for "40 size" planes seem to be the popular choice ?
« Reply #1 on: September 20, 2022, 06:36:13 AM »
Red,
The main reason is weight vs. power. There are 5S setups that work in Ringmaster size ships (HobbyShack 5S   1600mah, with a BadAss 2320   860Kv motor) but for the full PA pattern the capacity is marginal. With electric you are always trying to get the most power for the least amount of dead weight (excess battery capacity) that gets you through the pattern with at least 15% left. For the "35 - 40" size ships the 4S   2200mah Thunder Power packs are about the min you can do and still turn a 10" diameter prop. As you get to the larger ships there is more tolerance for weight and more choices for pack size that fit the available space.

Best,    DennisT 
« Last Edit: September 21, 2022, 06:43:36 AM by Dennis Toth »

Offline redout

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Re: Why do 4S batteries for "40 size" planes seem to be the popular choice ?
« Reply #2 on: September 21, 2022, 12:46:01 AM »
Thanks for your answer Dennis.

Perhaps my thinking is over-simplified but it is :

Higher voltage ( e.g. 5S battery instead of 4S ) --> Lower current at same power level --> Lower capacity battery needed, hence lighter battery.

It was mainly a for interest question, since I am totally committed now ( financially ) to 4S set-ups.

Offline Dennis Toth

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Re: Why do 4S batteries for "40 size" planes seem to be the popular choice ?
« Reply #3 on: September 21, 2022, 07:21:51 AM »
Red,
In general terms you are correct, but the limiting factor is the availability of the pack with the correct capacity. You can get a rough idea of how different packs compare by taking the number of cells times the capacity then divide by the number of cells in the 5 or 6 cell pack. This gives a good starting point for looking at what is reasonably available. I played this game when trying to save some weight on an OTS S1 Ringmaster. Many fliers use the 4S 1800mah pack on this ship with a 9x6 prop set for around 9900 ish rpm. This setup usually winds up with the battery tucked into a pocket in the wing and is a challenge to get it balanced correctly (I fought this on my ship). I did the quick math 4 x 1800 / 5 = 1440 other option is 4 x 2200 / 5 = 1760. Looking at the available 5S packs there is a Thunder Power 1350mah and a HobbyShack 1600mah. The 2200 pack weighs 7 1/2 oz. the TP 1350 is 5 1/2, the HobbyShack 1600 is 7 1/4 oz. I worked on a prop/rpm setup that allowed the 1350 pack (have to make sure the amp draw is around 13 - 15 static (cut down APCE 9x6 to 8 1/2 then pitch up to 7). This worked for the OTS pattern, but the ship was tail heavy. Switching to the 1600mah pack got the balance right (both the 5S 1350 & 1600 packs are short only 2 5/8" long, they fit where the tank would go in the nose) and allowed loading the motor to ~17 amps, a 10x7 prop at 8650 rpm. The bigger prop will pull a bit more weight, I am still evaluating if it is hurting the turn rate (vertical eight is a challenge) and might drop to a 9 1/2" or 9. Also might try the 9x6 at a higher rpm (the 8650 rpm is the lowest governable ESC rpm and gives a 4.6 ish lap time, 60' C to C lines) and see if the lower pitch works better.

All this shows you can play the game of size/weight/load to see if you can save some weight, but the standard setups do work.

Best,   DennisT

Online Brent Williams

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Re: Why do 4S batteries for "40 size" planes seem to be the popular choice ?
« Reply #4 on: September 21, 2022, 09:11:36 AM »
Redout, do you have a specific application that you're looking for recommendations?
Laser-cut, "Ted Fancher Precision-Pro" Hard Point Handle Kits are available again.  PM for info.
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Offline redout

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Re: Why do 4S batteries for "40 size" planes seem to be the popular choice ?
« Reply #5 on: September 21, 2022, 06:10:24 PM »
Thanks Dennis. Good explanation.

Brent, I did not have a specific application in mind when I asked the question.

There were some Turnigy motors on sale here in Australia and as I have a Thunderbird, Nobler and Vector in kits, I thought I would purchase some motors for them. This then had me thinking whether it's best to use
840 kV motors with 4S batteries or 700 kV motors with 5S batteries.

My choice ended up being the 840 kV / 4S set-up as I already had a good supply of 4S batteries, so the question was really out of interest. It seems the 5S set-up is technically superior but at my level, any advantage would not be apparent I am sure.

Based on much previous looking at specs. etc. in this forum over the past few years, I decided on the 3548-840 kV motors. These seem to be a good match to the "35-40" ships.

Offline TDM

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Re: Why do 4S batteries for "40 size" planes seem to be the popular choice ?
« Reply #6 on: September 21, 2022, 06:42:53 PM »
12000RPM is my magic number.
12000/14.8=810 KV for 4S
12000/18.5=650 KV for 5S

redout your choice is great for the application specially if you go with a 10-11x6 prop. The lighter the prop the better you are. Joker at Lindinger Germany has some great motors relatively cheap.

https://www.lindinger.at/en/AIRPLANES/ELECTRIC-POWERPLANTS/Motors/PLANET-HOBBY-JOKER-3548-4-5-V3-850-KV-BRUSHLESS-MOTOR/9741922


My choice ended up being the 840 kV / 4S set-up as I already had a good supply of 4S batteries, so the question was really out of interest. It seems the 5S set-up is technically superior but at my level, any advantage would not be apparent I am sure.

Based on much previous looking at specs. etc. in this forum over the past few years, I decided on the 3548-840 kV motors. These seem to be a good match to the "35-40" ships.
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Online Ken Culbertson

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Re: Why do 4S batteries for "40 size" planes seem to be the popular choice ?
« Reply #7 on: September 21, 2022, 11:13:24 PM »
I decided on the 3548-840 kV motors. These seem to be a good match to the "35-40" ships.
Why not a 2826 or for that matter a 2820?  A 3548 is 4oz heavier.  Both of those sizes thrive on a 4s in the 2200-2600 range.
A 3515 is about 1/2 the weight of a 3548.  I have put a 3520/12 on a .35 size ship and I won't do it again.  The 28xx are more than enough.

Ken

Just read Brent's post.  So, Never Mind....
« Last Edit: September 22, 2022, 06:42:33 AM by Ken Culbertson »
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Online Brent Williams

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Re: Why do 4S batteries for "40 size" planes seem to be the popular choice ?
« Reply #8 on: September 21, 2022, 11:23:23 PM »
Why not a 2826 or for that matter a 2820?  A 3548 is 4oz heavier.  Both of those sizes thrive on a 4s in the 2200-2600 range.
A 3515 is about 1/2 the weight of a 3548.  I have put a 3520/12 on a .35 size ship and I won't do it again.  The 28xx are more than enough.

Ken

It's a different measuring point or nomenclature for the same thing.  Internal stator measurements vs the external rotating "Can" measurements. 
You just need to look twice at this stuff to see which measuring convention a given brand is using. Most motors from brand to brand will fall within a general weight range, and an outside "can" diamter for any given size, so if something seems strange, check the weight and compare against similar motors.

Here's some examples of cross reference sizing using common motors:

35mm rotating "Can" O.D. = 28mm internal Stator.
-  Cobra 2808 = Turnigy 3530 = Park 480
-  Cobra 2814 = Turnigy 3536 = Power 10
-  Cobra 2820 = Turnigy 3542 = Power 15
-  Cobra 2826 = Turnigy 3548 = Power 25

42mm rotating "Can" O.D. = 35mm internal Stator.
-  Cobra 3520 = Turnigy 4250 = Power 32 = 35mm Stator

« Last Edit: September 22, 2022, 01:33:06 PM by Brent Williams »
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Offline Dennis Toth

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Re: Why do 4S batteries for "40 size" planes seem to be the popular choice ?
« Reply #9 on: September 22, 2022, 04:20:22 PM »
Red,
Using the 840Kv motor with the 4S pack is going to have very little headroom from start rpm to end of flight rpm. Normal setup is to use something like a 900Kv or even 1000Kv range. This gets the motor in about the 75 - 85% rpm for Kv range and allows the ESC to add power to maintain rpm as the battery pack voltage starts to drop through the flight.

You might do a 5S pack with the 840 Kv motor, just need to watch the prop load to match the pack capacity. This is where it gets tricky in electric. You need to check the amp draw against the run time against the pack mah capacity.

One easy thing to do for a starting point is check the List Your Setup section of this forum for setups that others have used in similar size ships.

Best,      DennisT

Offline redout

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Re: Why do 4S batteries for "40 size" planes seem to be the popular choice ?
« Reply #10 on: September 23, 2022, 05:19:20 PM »
TDM : Yes those Joker motors are quite economically priced. If ever I use up my Turnigy stock, I will definitely have a closer look at those.

Dennis : The Turnigy 3548 1050 kV motors ( the next model offered after the 840 kV ) were not on sale nor even in stock. I did actually study as many relevant power set-ups on stunthangar as I could and seemed to get the idea that I was pretty spot-on with 840 kV and 4S power, but this electric thing seems to get more and more involved the harder you look. Maybe also things have evolved a bit since the specs. I viewed. Anyway, I'm going to have to use what I got now :).



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