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Old Aug 25, 2014, 09:57 PM
tolip redilg
Joined Sep 2006
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Relative performance increase: Brushed 600 vs Brushless Power 10

I currently have a brushed 600 size motor made for use with a 7-cell 8.4V battery and it spins an 8x4 prop. The motor looks just like this:


My 2m sailplane weighs about 5 pounds, so it is pretty underpowered and can only climb at 5 degrees or so. I am going to upgrade it to brushless. I was recommended an E-Flite Power 10 motor or equivalent. It is the same diameter as my existing motor and draws 32A. I am planning on a 40A ESC accordingly.

If I use the same 8x4 prop, what kind of performance difference can I expect, roughly? Will it be worth it, or do I need something like the Power 15 or Power 25? The one advantage of the Power 10 that I see is it can be used with 6 to 10 cell NiMH so I can re-use my battery. The larger motors require a bigger ESC and LiPo.

If I could double or triple the performance that is perfectly fine. I don't need any thing too crazy, I'll save that for a different airframe




Any bit of input here would be awesome, before I plunge in!
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Old Aug 26, 2014, 05:35 AM
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Australia, ACT, Kambah
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First off, don't be misled by motor labels such as Power 10, 15, 25. Those 'equivalences' are only valid for a narrow range of prop and battery voltage (or LiPo cell count) combinations so you can't assume a motor with a bigger number will automatically provide more power. There are plenty of situations where if the cell count and/or prop size is limited, the motor with the lower glow number will use more power and deliver more prop rpm. For example, if you stuck with 3 cells and the existing 8 x 4 prop, by far the best solution would be the Power 10, because with its higher Kv ( 1100 rpm per volt) it will turn the prop at higher rpm than either the P15 (950 rpm per volt) or Power 25A (870 rpm per volt) .

Another point to note from your post is "Power 10 ... and draws 32A". This is a really common misconception, not helped by slack motor specs. It's not unreasonable for a person new to electric motors to assume that a spec that says "Continuous current: 32A" means that the motor will literally draw 32A. What the spec really means is that 32A is the current limit for continuous operation that you need to remain below by your choice of prop size and voltage. Actual current might be a lot more or a lot less than 32A. For example, an 8 x 4 prop on 3S LiPo will only draw about 12A. This is a valid, though inefficient use of the motor as 12A is less than the 32A continuous max. If you tried a 12 x8 prop on 3S, you could expect about 52A - way over the motor limit. The motor doesn't self limit - it will bravely try to cope with whatever combination you give it, even if it burns out in the process.

Also, while Eflite recommends ESC sizes and battery sizes for the larger motors based on the potential current they might use, the actual ESC required depends entirely on the prop and battery you use. If you prop a Power 15 for only 23A, eg a 10x6 on 3S, then there's no need to use a 45A ESC. Similarly, the bigger motors don't have to use LiPos - they can run just fine on NiMh if you're not trying to pull more current from the battery than it can deliver.

Sorry about all that background, let's cut to the chase. Sounds like a GP Spectra setup. 5lbs?? A Speed 600 8.4V, if that's your motor has Kv=1890 rpm per volt and weighs near 8 oz. An 8x4 on 7 cells NiMh is about as much as it can cope with, pulling about 22A/150W and maybe 11000 prop rpm for about 22oz static thrust.

You basically can't use that same prop with any of the motors suggested unless you increased the voltage a lot. They all have a much lower Kv than the S600 - they are intended to run much bigger props. On 7 cells NiMh, 8 x 4 prop the Power 10 (which would be the best in this scenario of the 3 you mentioned ) will only use about 9A/70W/8800 rpm - far worse than your current setup.

If you want to stick with your 8x4, you'd have to use 4S LiPo with the Power 10, to get about get about double the thrust of your current setup, at reduced weight. BUT your prop is for a 1/8" shaft and these motors all have 5mm shafts so if you're going to have to change props, you might as well use something more suitable for a 2m glider.

Let's start with assuming a 3S 2200 pack - under $10 at Hobby King. Let's look at the light solution using the Power 10 and the heavy solution using the Power 25, which might make sense if the model was designed to balance with an 8oz S600 in the nose and a heavy 7cell NiMh in the cockpit. A 10x6 prop with the Power 10 should draw about 30A/300W with about 2.5 times the thrust of the existing setup. A 12 x 6 prop on the Power 25A should be about 320W. Either should be plenty for good solid climbs on even a heavy 2m glider.

Note that mounting any of the Power series through a firewall requires reversing the shaft, and you also need to consider clearance for the motor wires. I'd strongly suggest for what you want to do seriously considering one of the Turnigy GliderDrive motors. They'll fit where the S600 fits and no problem with wires and the price is good.

This 5 oz 3530-960 will need an 11" prop on 3S for about 320W.
This 6oz 3858-1120 will need a 10x6 fo about 300W

Again, sorry for a complex answer to what seems like a simple question, but what you were suggesting was going to go wrong several ways so I thought it worth explaining why.
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Old Aug 26, 2014, 06:19 AM
No bounce, No play.
davidmc36's Avatar
Canada, ON, Ottawa
Joined Oct 2010
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I put up a 2M Gentle Lady with a Park 480 spinning 10 inch prop (forget the pitch but 6ish?) with 3S 2000ish. Climb like there's no tomorrow.
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Old Aug 26, 2014, 06:21 AM
No bounce, No play.
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Just noticed you said a 5 pound 2M sailplane?!?!?
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Old Aug 26, 2014, 08:40 AM
yank and bank!!
Joined May 2013
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I understand that gliders are an intense study in outfitting as light a possible, but I disagree about using an esc that is just a little more than the current draw of the motor with a particular prop.... if anything goes wrong, that esc stands a good chance of frying.

I very much advocate an esc current rating that is a good bit above the motor's maximum current rating.... at the very least I would use an esc with a continuous current rating 110% of the motor's continuous current rating.
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Old Aug 26, 2014, 11:51 AM
tolip redilg
Joined Sep 2006
46 Posts
Quote:
Originally Posted by scirocco View Post
First off, don't be misled by motor labels such as Power 10, 15, 25. Those 'equivalences' are only valid for a narrow range of prop and battery voltage (or LiPo cell count) combinations so you can't assume a motor with a bigger number will automatically provide more power. There are plenty of situations where if the cell count and/or prop size is limited, the motor with the lower glow number will use more power and deliver more prop rpm. For example, if you stuck with 3 cells and the existing 8 x 4 prop, by far the best solution would be the Power 10, because with its higher Kv ( 1100 rpm per volt) it will turn the prop at higher rpm than either the P15 (950 rpm per volt) or Power 25A (870 rpm per volt) .

Another point to note from your post is "Power 10 ... and draws 32A". This is a really common misconception, not helped by slack motor specs. It's not unreasonable for a person new to electric motors to assume that a spec that says "Continuous current: 32A" means that the motor will literally draw 32A. What the spec really means is that 32A is the current limit for continuous operation that you need to remain below by your choice of prop size and voltage. Actual current might be a lot more or a lot less than 32A. For example, an 8 x 4 prop on 3S LiPo will only draw about 12A. This is a valid, though inefficient use of the motor as 12A is less than the 32A continuous max. If you tried a 12 x8 prop on 3S, you could expect about 52A - way over the motor limit. The motor doesn't self limit - it will bravely try to cope with whatever combination you give it, even if it burns out in the process.

Also, while Eflite recommends ESC sizes and battery sizes for the larger motors based on the potential current they might use, the actual ESC required depends entirely on the prop and battery you use. If you prop a Power 15 for only 23A, eg a 10x6 on 3S, then there's no need to use a 45A ESC. Similarly, the bigger motors don't have to use LiPos - they can run just fine on NiMh if you're not trying to pull more current from the battery than it can deliver.

Sorry about all that background, let's cut to the chase. Sounds like a GP Spectra setup. 5lbs?? A Speed 600 8.4V, if that's your motor has Kv=1890 rpm per volt and weighs near 8 oz. An 8x4 on 7 cells NiMh is about as much as it can cope with, pulling about 22A/150W and maybe 11000 prop rpm for about 22oz static thrust.

You basically can't use that same prop with any of the motors suggested unless you increased the voltage a lot. They all have a much lower Kv than the S600 - they are intended to run much bigger props. On 7 cells NiMh, 8 x 4 prop the Power 10 (which would be the best in this scenario of the 3 you mentioned ) will only use about 9A/70W/8800 rpm - far worse than your current setup.

If you want to stick with your 8x4, you'd have to use 4S LiPo with the Power 10, to get about get about double the thrust of your current setup, at reduced weight. BUT your prop is for a 1/8" shaft and these motors all have 5mm shafts so if you're going to have to change props, you might as well use something more suitable for a 2m glider.

Let's start with assuming a 3S 2200 pack - under $10 at Hobby King. Let's look at the light solution using the Power 10 and the heavy solution using the Power 25, which might make sense if the model was designed to balance with an 8oz S600 in the nose and a heavy 7cell NiMh in the cockpit. A 10x6 prop with the Power 10 should draw about 30A/300W with about 2.5 times the thrust of the existing setup. A 12 x 6 prop on the Power 25A should be about 320W. Either should be plenty for good solid climbs on even a heavy 2m glider.

Note that mounting any of the Power series through a firewall requires reversing the shaft, and you also need to consider clearance for the motor wires. I'd strongly suggest for what you want to do seriously considering one of the Turnigy GliderDrive motors. They'll fit where the S600 fits and no problem with wires and the price is good.

This 5 oz 3530-960 will need an 11" prop on 3S for about 320W.
This 6oz 3858-1120 will need a 10x6 fo about 300W

Again, sorry for a complex answer to what seems like a simple question, but what you were suggesting was going to go wrong several ways so I thought it worth explaining why.

Wow, I definitely did not expect that I actually really appreciate it, because I lot of times I find people saying "use this or that" without giving an explanation. It makes more sense now.

A little backstory - the motor/esc are actually from the discontinued GP Electro-streak back in 2005. I bought it as my first RC airplane. Yeah, that was a terrible mistake. It was probably the worst plane for a beginner because of its twitchy handling and high price. This glider I'm flying now is a much easier flying plane, but still not very forgiving. Let's just say I've learned how to repair balsa and use monokote a couple times. The plane is an old school Thunder Tiger, but is very similar to the GP Spirit 2M sailplane. I added the motor mount and battery in a custom way.

About the weight - yes, it's pretty heavy but 5lbs is just a guess since I haven't weighed it. The 7-cell NiMH added a lot of weight directly beneath the wing. The servos are full size and the fuselage is solid wood paneling. The wings are large, so it can thermal if the conditions are right. I've done it a couple times.

Okay, so it looks like my original idea is out. It looks like ultimately it doesn't matter what power my motor is, if it only spins a certain RPM. I'm wondering - how did you figure the current draw based on the prop size? Is there a calculator?

Also - it seems that my 7-cell NiMH can't carry over in any of these scenarios if I want to upgrade, correct? I probably shouldn't compromise the entire power setup due to the battery, however the battery seems to work well for me and it's brand new. It fits just right and is definitely safer than LiPos.

So you would recommend:
-New motor http://www.hobbyking.com/hobbyking/s..._6_1120kv.html
-3S LiPO battery (recommendation on a brand? The form factor needs to be no thicker or wider than a 7-cell sub-c pack.)
-ESC with 110% rating (40A?)
-11x? Folding prop (recommendation?)

If I'm upgrading everything, might as well make it a worthwhile improvement right?

Thanks again for your input
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Old Aug 26, 2014, 01:06 PM
tolip redilg
Joined Sep 2006
46 Posts
Wow - been researching since making this last post.

There are no *correct* answers. I could do this 1000 different ways. Fun!
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Old Aug 26, 2014, 01:07 PM
jrb
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Edina, MN, USA
Joined Oct 1999
11,430 Posts
Using the same prop mean a 1/8” shaft is needed

If you used the Great Planes Ammo 28-56-1530 In-Runner Brushless Motor on 3S; it would pull about 30amps.

And produce about 24oz of thrust, as compared to 14 from the Goldfire 550 on 7xNiXx @ 20amps; 12k RPN vs. 9k.


However, I don’t that prop should be operated that fast.

On 2S you’d have slightly lower performance compared to the 550

If you Google GP Electro-streak you can find some brushless upgrade thread and articles that might be of interest.

For maximum happiness and success you should shop/look for a folding prop & motor combo that used these – Radian maybe?
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Old Aug 26, 2014, 01:30 PM
tolip redilg
Joined Sep 2006
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Quote:
Originally Posted by jrb View Post
Using the same prop mean a 1/8” shaft is needed

If you used the Great Planes Ammo 28-56-1530 In-Runner Brushless Motor on 3S; it would pull about 30amps.

And produce about 24oz of thrust, as compared to 14 from the Goldfire 550 on 7xNiXx @ 20amps; 12k RPN vs. 9k.


However, I don’t that prop should be operated that fast.

On 2S you’d have slightly lower performance compared to the 550

If you Google GP Electro-streak you can find some brushless upgrade thread and articles that might be of interest.

For maximum happiness and success you should shop/look for a folding prop & motor combo that used these – Radian maybe?
Yeah - I think the prop isn't going to work anymore. I will need something 10 to 12" in diameter.
That's a great idea. I was looking at what other electric sailplanes use as a comparison too.
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Old Aug 26, 2014, 10:33 PM
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Australia, ACT, Kambah
Joined Feb 2001
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The existing 7 cell nimh could be used by choosing kv and prop size to suit - the pack voltage will be similar to 2s lipo. There is a glider drive motor with kv=1400 that could do the job on 7 NiMh. Let us know what capacity the cells in your pack are. To get decent power for this heavy beast on 7 cells you'll need to be pulling over 30A

With Kv=1400, 10x6 prop and motor around 140g, you could expect up to 40A/320W, depending on the capability of the cells
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Last edited by scirocco; Aug 27, 2014 at 03:10 AM. Reason: added some power info
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Old Aug 27, 2014, 11:14 AM
tolip redilg
Joined Sep 2006
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Quote:
Originally Posted by scirocco View Post
The existing 7 cell nimh could be used by choosing kv and prop size to suit - the pack voltage will be similar to 2s lipo. There is a glider drive motor with kv=1400 that could do the job on 7 NiMh. Let us know what capacity the cells in your pack are. To get decent power for this heavy beast on 7 cells you'll need to be pulling over 30A

With Kv=1400, 10x6 prop and motor around 140g, you could expect up to 40A/320W, depending on the capability of the cells
This is the pack: http://traxxas.com/gallery/batteries...ll-84V-4200mAh

Let me make sure I understand this correctly. Assuming the same motor is used, in this case the 1400kV version. By using the 7-cell with lower voltage, the motor will be spinning more slowly than a 3S or 4S lipo. So, I can use a larger prop and it will draw more current, ultimately being similar to a smaller prop that spins faster, and the overall power will be the same.

These glider drive motors are listed with a voltage rating 3S-4S. But it sounds like you can run whatever voltage you want (2S or 5S for example) as long as the current stays within range. Is 3S-4S just the sweet spot as far as prop size and staying at a reasonable motor speed?

Thanks for everyone's input. It's very helpful!
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Old Aug 27, 2014, 11:31 AM
jrb
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OK we gave up on the old prop; what about the pack?

Why not go LiPo?

Checkout the E-Filte Allusive's power system too.

Most motor can run over a 3 to 4 LiPo cell range, by using smaller props as the count goes up to keep the amps in check.

You'll also have to make sure the ESC can handle the cell count.
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Old Aug 27, 2014, 01:44 PM
tolip redilg
Joined Sep 2006
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Quote:
Originally Posted by jrb View Post
OK we gave up on the old prop; what about the pack?

Why not go LiPo?

Checkout the E-Filte Allusive's power system too.

Most motor can run over a 3 to 4 LiPo cell range, by using smaller props as the count goes up to keep the amps in check.

You'll also have to make sure the ESC can handle the cell count.
Yeah, gave up on everything basically. I will retrofit it with all new parts, and hopefully something I can use in another airframe down the line, like the Mystique or a warmliner.
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Old Aug 27, 2014, 04:39 PM
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Quote:
Originally Posted by kuksul08 View Post
This is the pack: http://traxxas.com/gallery/batteries...ll-84V-4200mAh

Let me make sure I understand this correctly. Assuming the same motor is used, in this case the 1400kV version. By using the 7-cell with lower voltage, the motor will be spinning more slowly than a 3S or 4S lipo. So, I can use a larger prop and it will draw more current, ultimately being similar to a smaller prop that spins faster, and the overall power will be the same.

These glider drive motors are listed with a voltage rating 3S-4S. But it sounds like you can run whatever voltage you want (2S or 5S for example) as long as the current stays within range. Is 3S-4S just the sweet spot as far as prop size and staying at a reasonable motor speed?

Thanks for everyone's input. It's very helpful!
Not quite ...

The smaller glider drives come in 2 versions, Kv=960 rpm/volt and Kv=1400 rpm/volt. Assume a 3S pack is 10V under load. Assume a 7 cell Ni Mh is 7V under load.

960 rpm per volt x 10 V = 9600 rpm 1400 prm per volt x 7 V = 9800 rpm. The two versions of the motor driven by different voltage batteries "want" to turn at about the same rpm, and thus for the same prop need about the same amount of power, which for a 10 x 6 is around 320 watts (from similar motors in eCalc). The difference is that for say 320W, the 3S (~10V) setup draws about 32A, while the 7 NiMh (~7V) draws about 45A.
The pack you linked should be able to deliver 45A and the motor is rated for that as well.

Re "spinning more slowly than a 3S or 4S lipo. " : the rpm is not just about the voltage-it's voltage x Kv. To use the lower voltage 7 NiMh pack, we choose a motor with higher Kv. The net result is about the same rpm and about the same power.

So you could keep using your existing pack/s, and if you later wanted to save some weight, just change out the 7 cell 4500 NiMh for a 2S 4500 LiPo.

The downside is that for the 7 NiMh / 2S setup, at around 320W you are pushing up close to the current limit of the smaller glider drive motor so there isn't any headroom to increase power by running a bigger prop. If you want more than 320W, you are going to have to change packs anyway and I'd use a bigger motor
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Old Aug 27, 2014, 11:48 PM
tolip redilg
Joined Sep 2006
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Good to know!
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