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Speed 600 with 2.8:1 gearbox. 2s or 3s lipos?

Old 06-27-2014, 07:54 AM
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Default Speed 600 with 2.8:1 gearbox. 2s or 3s lipos?

Please don’t laugh. I’m coming back into electric rc gliding after a break of many years and have some stupid questions…

I have a venerable 100S thermal soarer (1.02kg without batteries) which is fitted with a Graupner speed 600 8.4v with a 2.8:1 gearbox and a 15x8 folding CFK prop. Originally fitted with 7 cell nicads and a Jeti 350 controller.

I’ll be delighted to change to lipos, and to swap the Jeti for a brushed ESC with LVC, but I’d like to keep to the original motor and prop. So, should I be looking at 2s or 3s with this motor / prop combo? Not burning the poor old Graupner tin can into oblivion is more important than all out performance, but some sort of positive climb rate would be nice…

There are probably a few who reckon that as I’m buying a new ESC anyway just get a brushless one and a cheapie inrunner to go with it. If so, any inexpensive recommendations from the Hobbyking site that will be a similar / smaller size and might even still be OK with the 15x8 prop?


PS Given the retro theme of this post, I’d like to reassure you that I’m not still on 27mHz as well

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Old 06-27-2014, 08:46 AM
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Well technically, a 2s lipo is 7.4V, but much more light weight and far less voltage sag, meaning that the 7.4V stays throughout the charge. Now a 2s will charge to 8.4V however (1s up to 4.2) so from my understanding, the output voltage is more than 7.2 and as such 2s is usually considered greater than a 6 cell nimh, and more it less like a 7 cell, and this can be seem, even running in a brushed setup. Thus, 2s would be the best bet. 3s would go up to 11.1V, a lot more voltage that your battery, and this could cause issues. The other benefits are less weight with lipo, either allowing less weight for the plane, or a higher mah, meaning more run time. As for brushless, that is the best option realistically, as it is more efficient and has virtually no wear. I too came from brushed only a few years ago with rc cars. I started with 2s and a small brushless setup. This alone increase? run time with a similar size and weight of battery (but more mah), but too speed improved, going from around 20mph to 30-35mph. In terms of planes, this means a similar weight of a setup can deliver more airspeed, or a lighter setup can give similar power but increased run times as well as higher airspeed, as the plane will be lighter.

Also with brushless, you can remove the gearbox, something that is used to deliver rpm without losing torque. With brushless, torque is generally better and is not lost for the most part, with different prop sizes.

This is my understanding at least.
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Old 06-27-2014, 11:20 AM
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If keeping the Speed 600 ...

Then it has to be 2S. Suggest weigh the original pack and also note it's dimensions. Find similar in 2S pack ... you can stay with a lower C rating than many .. a 20C would likely be fine (higher C rating increases weight !).

So as other guy says - you get increased run-time and performance ... BUT you have a factor that needs to be carefully monitored.

Brushed ESC's do not generally cater for LVC on LiPo .. that is LOW VOLTAGE CUTOFF ... so it means that you could if not carefully monitoring flight time - you could run your LiPo down past the 20% charged level.

It may be worth tacho'g the original set-up ... that is knowing the RPM the set-up runs at will give you good idea of what replacement brushless you could use - saving weight .. removing gearbox ... having ESC that has LVC to save your LiPo ... in other words maximising the efficiency of your set-up.

A Speed 600 is a moderate sized can motor ... and you had a 15x8 prop - no wonder it was geared ..

What to use ? Well - if we replace the can motor with brushless and want to stay with the folder 15x8 ... then it has to be low KV with 3S ... or higher KV with 2S. My thoughts are in the realm of 500 - 800kv on 3S ... 800 - 1400kv on 2S. (Kv is the technical RPM per 1Volt of an electric motor .. in reality - it can be around the 70 - 85% depending on loads).

You could in fact reduce your prop size and go up in KV ... my ASK 21 2m has a 10x6 folder on 3S :

Wing Span: 2000mm
Length: 1000mm
Airfoil: RG-15
Dry Weight: 760g
Motor: DST-1200kv Brushless Outrunner Motor
ESC: 20A w/BEC
Servo: 9g x 4
4 Channel (aileron, elevator, rudder, throttle
With 3S 2100 this makes her just under 1.0kg flight weight.

She climbs out beautifully ... in fact it's best not to use full throttle till she's gone a few metres from hand. Launches on 75% throttle.


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Old 06-27-2014, 02:53 PM
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This type of setup has worked many times with gliders that were given to me. The geared motor could handle some more voltage than printed on the side. Often with a 2000Mah 3 cell pack I could get the plane to balance and reduce the weight by alot, This would shorten the required motor runs to altitude and make the plane a thermal master.
A watt meter kept me from smoking the motor. I think the gear ratio was 3:1 and the prop was 12 X 8 pulling 20 amps with a fresh battery.
Your prop is bigger and the gearing lower so a 2 cell may be the best choice. If you want better thermal flying use a lighter pack. If you want to keep the flight parameters the same follow what Nigel said and find a pack of the same weight and size.

Geared motors are still used but they are now brushless.
Once the former owner saw a plane hooked up and climbing in a thermal and said they had never been able to do that before. Rarely would I run out of battery as I would get tired first.

Good luck and have fun
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Old 06-27-2014, 04:16 PM
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Originally Posted by Flubber View Post
...............Once the former owner saw a plane hooked up and climbing in a thermal and said they had never been able to do that before. Rarely would I run out of battery as I would get tired first.

Good luck and have fun
That's true ... I get my Ask 21 up there and flights 18 .. 20 mins etc. - I actually get tired looking up and land ... Still with more than half a batt pack left.

But I have to say - there is something about a Glider up there - it's Majestic, Graceful ..

For me - it makes a nice peaceful change from screaming around with EDF and Pushers ..

I used to do Thermal back in 70 - 80's but power jobs then were terrible - overweight NiCD packs .. brushed car motors stuffed in to do the job ... horrible !!
But now .. wonderful.

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