Max. Cells for a Speed 700 - RC Groups
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Jul 12, 2001, 10:30 AM
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Max. Cells for a Speed 700

A real quick question - what is the maximum number of cells that can safetly (without smoking the motor) be used with a 9.6V speed 700 motor?

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Jul 12, 2001, 11:19 AM
Registered User

It's not so much a matter of the number of cells as it is a matter of how much current the motor can safely handle under load. The Speed 700 should be good for 30 amps or so. If you load it with a 10X6 prop and run it on 10 cells, then the peak current should be right around 30 amps.

If you use more cells, then consider using a smaller prop, either in diameter or pitch, to keep the amps around 30. Over 30, travel at your own risk! The life of the motor can be seriously compromised after a few hot runs.

Jim Ruggiero
Jul 12, 2001, 11:50 AM
Registered User
Thanks Jim...

With the propping/gearbox I'm thinking of using I should be only drawing about 22-23 amps continuous (says motocalc). In order for the performance I'm looking for Motocalc is recommending 22 cells, and I just wanted to make sure I wasn't going to fry my speed 700 with that many cells.


Jul 12, 2001, 12:12 PM
Registered User

Without knowing the particulars of your gearbox ratio, another factor in motor longevity is the motor RPM. If you are using a box with a high ratio, so that motor RPM is high, like 20,000 and up, then you could run into a problem where either the motor bushings wear prematurely (assuming your Speed 700 does not have ball bearings), or the armature throws a winding from the high centrifugal force.

There are so many variables that go into the operation of a "simple" electric motor, it's not possible to forecast the useful life. If you sift carefully through posts on these forums, you come across all kinds of ways to abuse a motor. Some guys claim they fried a motor on its first run; some claim they have hundreds of hours on a cheapy "can" motor. You have to try and fry!

Jim Ruggiero
Jul 12, 2001, 12:18 PM
Registered User
fw-190's Avatar
Hi Jim,
I have Graupner speed 700 Turbo Ball bearing 9.6V with Neodyme Magnets. I think the number is 6306. What do you think of that motor. I want to use two of these with 10x7 prop and 16 cells with motors in series. So considering 8 cells and 10x7 props what do you think this motor will draw? and how many watts can I expect from it? Thanks.
Jul 12, 2001, 04:45 PM
Andy McCowan's Avatar
I have run a SPEED700 TURBO NEODYM with 2.7/1
gearbox on 14 cells and 12/8 12/7 props no
problems so far. The motor has done about 7
hrs flight time up until now,current draw is
about 30-32 amps,hauls a 100 oz WOT 4 around
quite nicely.

Jul 13, 2001, 12:11 AM
Registered User

22 cells is way to much (?). Try 12 to 14 and a bigger prop on Mcalc.

Jul 13, 2001, 01:52 AM
Registered User

I know nothing about the S700 Neo BB motor, other than what I've seen in Hobby Lobby's catalog. In fact, I've never come across anyone on these forums mentioning it. You are in a position to educate the rest of us!

Jim Ruggiero
Jul 13, 2001, 02:26 AM
ChrisP's Avatar
Originally posted by JRuggiero:

If you load it with a 10X6 prop and run it on 10 cells, then the peak current should be right around 30 amps.

Jim Ruggiero
I've got 11 cells in a 'soft hotliner'.
Jul 13, 2001, 10:01 AM
Registered User

I'm using 14 cells right now and as big a prop as I can fit on it (11x7) and it's not cutting it. The trick is, the aircraft is an amphibian with the motor mounted in a pod above the wing/deck and 11" props *just* clear the deck. However, on the take-off run, any water that washes over the bow/deck gets sucked into the prop due to it's proximity to the deck surface.

Additionally, I don't have enought power with the current SP700/2:1 belt drive/14 cell setup to even get the AC on step.
SO, it's my plan to increase the cell count and go with a 3-blade 9" vario prop (all per motocalc experimentation).

If anyone has any better suggestions please feel free to let me know!!

Jul 13, 2001, 10:15 AM
310mph Kolibri T25 Swist
henke's Avatar

if you look in the Graupner "bible" youŽll se a CUB using SP700 on 12cells 1:2 with 12x7....if i remember right!

I flew a SIG 1:6 Cub with my Sp700 BB 9,6v using the GR 1:2 grer and a 16x6prop.... (14cells)

Jul 13, 2001, 10:41 AM
ChrisP's Avatar

So at least we now know what you're flying.

I'm sure your problem is the gearing rather than power unless you've got a lead sled. You are cutting down the velocity of the air from the prop too much.

A 7'' pitch prop on 2:1 gearing is crudely equivalent to a straight drive with 3.5'' pitch.

What does your model weigh ? My soft hotliner is at 1950 gms which is a good match for the power of a 700. I set it up with 11 cells on a straight drive 10x6 so I would get a prop airspeed of 20 metres/sec.

I don't know the motocalc program at all, but if it shows calculated airspeed you should see what it is giving. You may be better off by going to straight drive.

[This message has been edited by ChrisP (edited 07-13-2001).]
Jul 13, 2001, 12:58 PM
Registered User
fw-190's Avatar
Did some testing with the 700 Neodyme magnets, 9.6V motor direct drive and Astro Watt meter.

8-cell pack and 10x7 APC prop
34 Amps and 8.2 Volts = 278 Watts

10 cell pack and 9x6 prop
32 Amps and 9.3 volts = 297 watts

Jul 13, 2001, 06:03 PM
Registered User

Hmmm...the lead sled analogy might not be too far off - it's about 2300 gms (a bit over 5 lbs) all up (that's with 14 1700mah cells).

Is there any hope of getting this beast off the H2O?
Jul 14, 2001, 02:08 AM
Registered User
Hi Steve,

At 5 lbs * 50 watts per lb (minimum power to fly most planes) you need 250 watts to fly.

Better yet, 5lbs * 75 watts (per lb) = 375 watts.

(Refer to Chris and FW post above)

Getting "unstuck" from the water may be an issue.

Sounds like Direct drive Astro 25 / 40 time to me and 14 to 16 cells.