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Oct 27, 2020, 07:21 AM
raythomas's Avatar
Thread OP

Best place to get SPEED 280 size motors?

Hi All

I'm in the process of building a blucore foamie DC-3 from the plans I downloaded here a couple days ago.
Am trying to stay close to original posters build recommendations to keep plane as light as possible and close to his flight performance.
So I'm looking for a good "cheap" place to get two speed 280 size brushed motors.
Yeah, I know, why brushed when you could go brushless. But I was trying to keep the motor wiring simple just like the original design.
And its easier to wire two brushed motors in series or parallel than it is brushless stuff.
Anyone got any suggestions?

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Oct 28, 2020, 02:59 AM
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Skylar's Avatar
Let's have a look at what a Speed 280 motor really is.

Kv: .............. 2330
Io: ............... 0.275
Rm: ............ 0.882
Weight: ....... 41g (1.5oz approx.)
Max. continuous current: 2.5A (my guesstimate)

The motor constants above is given in case you want to model it in your favorite motor simulator, i.e. eCalc, MotoCalc or Scorpion Calc. If you do that, you'll notice that motor efficiency is about 67%, which means 33% goes to waste as heat. And that is for a new motor with new brushes and new bushes. No ball bearings in those old brushed motors. Expect rapid degradation of the brushes, which will lower efficiency further.

I you want to keep the setup 100% original and period-true, where will you get Ni-Cad batteries from?

Be careful if you use LiPo. 3S LiPo (11.1V) is quite a bit higher voltage than the usual and 7-cell (8.4V) or 8-cell NiCad (9.6V) that were commonly used with that motor. A Speed 280 won’t run for long at LiPo voltages. And 2S (7.4V) is too low again and you won’t get much thrust. That is if you use the same props. Using smaller props is an option, though, but you'll have to experiment to get it just right.

Add to that the extremely low efficiency of those old brushed motors. I’ve used Graupner Speed 280 motors extensively at one stage and remember that anything over 3A would cut their lives very short. 24W (input) was the safe norm. That is 2.5A @ 9.6V. To get usable thrust, we used gearboxes with bigger props. I remember using a gear ratio of 1:2.6 in a DIY gearbox with 8x4 Graupner Slim prop. Graupner also made a 1:3 gearbox. Direct drive was just not practical.

Then the old brushed ESC’s were enormous by today’s standards. 30g for a 10A ESC was not uncommon. To sum it all up, with a weight of 82g (41g motor + 21g gearbox + 20g ESC) excluding battery, you’d be lucky to get 180g thrust. 150g is more realistic.

I’d rather opt for a nice, modern outrunner with lower Kv for direct driving a larger prop and then make up the difference in weight with a bigger LiPo battery for longer flights. Modern brushless ESC’s are so light and small, it’s no problem using one for each motor.
Last edited by Skylar; Oct 28, 2020 at 03:59 AM.
Oct 28, 2020, 09:49 AM
raythomas's Avatar
Thread OP
I wanted to try and stay as close to the designers plans but you do make a very good point.

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