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Old Oct 20, 2008, 06:06 PM
All under control, Grommit!
leccyflyer's Avatar
United Kingdom, Aberdeen
Joined Sep 2000
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Twin Sp400 7.2V on 3s1p 2500 lipos?

Hokay, I'm just putting the finishing touches to a refurb on a twin Sp400 model- looks like a twin Mini Flying Machine. I wanted to keep the 7.2V Graupner Speed 400 motors, with Graupner 6x3 folders, but replace the eight cell pack the model had previously with a 3s1p 2500mah Maxpoly 20c lipo. I'm expecting current draw to be in the region of ca 25 amps but haven't used these motors with 3 cell lipos.

There was a thread the other day where someone was asking about using 4s1p lipos with these motors and the concensus was that they'd burn out which is what I'd have expected - question is does anyone use 7.2v sp400s on 3 lipos these days, say in something like a Sportwin or even a Twinstar?
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Old Oct 20, 2008, 06:27 PM
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Dr Kiwi's Avatar
Chattanooga, Tennessee, United States
Joined May 2003
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You'll be pushing them very hard [128W, or perhaps closer to 150W with a decent pack - into 80-90W motors], but maybe they'll survive: here's what I got from a test a very long time ago (thrust should probably be higher than stated here).

7.2v Graupner Speed 400: 6x3 Graupner folder:

3s TP 2100: 10.2v 12.6A 128W 12510rpm 326g 11.48oz 2.55g/W
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Old Oct 21, 2008, 04:26 AM
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East Anglia, UK
Joined Sep 2002
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Hullo Brian!

I've run a 7.2v with a 6x3 folder. On 3s1100.

The long and the short of it, it does work, but with a GOOD pack that holds voltage, you are up around 10-11A WOT static on a fresh pack

Which is pretty much the edge of the comfort zone. So lots of WOT at slow airspeeds is probably inadvisable. Mine is in a glider, so its up to altitude and then let the motor cool.

Long can 400's are better, but current is up around 15A, and the props tend to disintegrate. DAHMIKT.


If you are prepared to be conservative about throttle management, and the motors are well run in you should have no problems.


But flat out aerobatics? the motors wont last that long.

It's a pretty decent power to weight combo tho.
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Old Oct 21, 2008, 11:37 AM
All under control, Grommit!
leccyflyer's Avatar
United Kingdom, Aberdeen
Joined Sep 2000
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I was estimating something like 13 amps per motor on those 6x3 folders, which, as you chaps have said, leaves a lot to be lost as heat. I could come down to 4.7x4.7 Camspeed props I suppose since it's supposed to be a racer.

I really, really, really don't want to have to change the motors, since I've just recovered the model and the wiring loom is well buried in the foam wing. Swapping the motors for something small and brushless would be disruptive. Maybe three A123s might be a better option if they could be made to fit somehow.
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Old Oct 21, 2008, 01:22 PM
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peterangus's Avatar
Blackpool, Great Britain (UK)
Joined Dec 2003
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Brian

Most of my Sp400/7.2 models were upgraded to 3s Lipo [from 8 NMH] a few years ago. I used 5 x 4 props, to draw 8-9A peak. Full throttle used sparingly.

And I'm now involved in further upgrades to brushless. Definitely worth doing.
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Old Oct 21, 2008, 01:23 PM
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East Anglia, UK
Joined Sep 2002
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If you are prepared to prop down to not much more than 9A per motor, you should be OK..I think 4.75x4.75 should be pretty fast for twin if its light enough.

Not a DH comet is it?
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Old Oct 21, 2008, 01:31 PM
All under control, Grommit!
leccyflyer's Avatar
United Kingdom, Aberdeen
Joined Sep 2000
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Hiya Peter - Haven't see you this year, with the weather being so bad and all. Thanks for the info - I'm in the middle of a few upgrades to the fleet myself, but for some models am going the brushed can and lipo rather than lipo and brushless route.

She's not a DH Comet as such Vinnie, but there's more than a passing resemblance to the Grosvenor House machine. The model should be light enough not to need full throttle and the colour scheme that my boy has decided on is one that would have me running screaming. It couldn't be more black. Even the canopy is black. The covering is done now, so I'll post pictures when I've put the elevator and props on.

I'll also run some tests to compare the 6x3's and some smaller Graupner camspeed props. They have the advantage of being black as well
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Old Oct 21, 2008, 02:26 PM
All under control, Grommit!
leccyflyer's Avatar
United Kingdom, Aberdeen
Joined Sep 2000
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Here's a couple of pictures. The boy is at band so you'll have to make do with my efforts, rather than his artistic renderings. The elevator is just taped on and hanging loose.
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Old Oct 21, 2008, 09:29 PM
Motors beat engines!
Milwaukee Wisconsin, United States
Joined Feb 2001
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Quote:
Originally Posted by leccyflyer
There was a thread the other day where someone was asking about using 4s1p lipos with these motors and the concensus was that they'd burn out which is what I'd have expected - question is does anyone use 7.2v sp400s on 3 lipos these days, say in something like a Sportwin or even a Twinstar?

3 cells work great on sp400's, and I found out recently, so does 4 cells as I found out on my twinstar2 experiment. No overheating or unusal wear has shown up with quite a few flights. The motors are nearly ambient temp on landing.

http://www.rcgroups.com/forums/showthread.php?t=921899

The only real trick is limiting amps, in my case by gearing, to about 8 amps.
The success of this has me looking anew at using sp400's for all sorts of projects.

Dean in Milwaukee
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Old Oct 21, 2008, 09:34 PM
Motors beat engines!
Milwaukee Wisconsin, United States
Joined Feb 2001
4,564 Posts
If theres room for gws eps 400's in there, motocalc came up with this for 4 cells with the 3.0 ratio and your 6x3's:

Batt Amps = 7.9
Motor Volts = 14.1
Input (W) = 111.1
Loss (W) = 28.0
MGbOut (W) = 83.0
MotGb Ef(%) = 74.8
Motor RPM = 40275
Prop RPM = 13425
Thrust (oz) = 19.0
PSpd (mph) = 38.1
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Old Dec 13, 2008, 09:54 PM
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United States, MI, Roseville
Joined Dec 2000
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OK, I run 2x 7.2 volt 400's on 3s2100 all the time. It is different than testing one motor on the same pack and multiplying it x 2. There is an added draw on the battery with two motors.
I recently began building a new twin with 7.2's and decided to run the wattmeter on it. I found about a 20% drop on amp draw with a very good pack going from a single motor to a twin.Wiring losses as well as added draw most likely cause this.
I tried 6 volt motors with a 3 cell and 5/5's and got 24 amps. Surprised? I was. Figure a ten to fifteen percent unload in the air and you are at about 21 1/2 amps for two motors, decent range for $3.00 motors, and plenty of pull for the $.
Mark
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