[Bryan Hewitt]

i have 4 Como rocket sp 400 mtrs 3-7.2 volts $9.00 each
3 Promax 400's 7.2-8.4 v $9.00 each
2 graupner spd 480 RACE for $20.00 each
7 graupner spd 400's for $9.00 each
these motors are all new, i just have a large surpluss that i'd like to move to fund a new project.

[foamball]

Hi Bryan
OK, got a question for ya.... My Feather Merchant, 78" wingspan model that I used to fly in SAM competitions, mit a noisey four stroke....
Have rebuilt the motor mount area and am switching to electric... I have an older Astro 05 motor, brushed, laying around, with a 4-1 Master Air Screw gear box on it... Was wonderin if this would fly this model, weight is, sans equipment, 3.5lbs.. If I remember correctly, the total weight back then, was 6lbs but I expect it to be lighter with electric in it....
Naturally I need a brushed ESC and am told a 40-5 amp one would be a good start... Now, battery.... I plan on using a Lipo, 3C/1500mah pack.. I also was told that I could "tie" in a 2C lipo, of the same amp rating, giving an increase in voltage to operate the Astro 05....
Now, is this feasable or am I just dreaming???? Would like to get this SAM ship in the air again as it was a good flyer, back a few years now....
Let me know, huh?
Art.

[Mike Anderson]

Let's make an assumption that you would want the Astro 05 to not exceed 25000 rpm, because it is a brushed motor. That means you would not want to use more than 4 LiPos. Even with 3 LiPos your "load speed" would still be around 18000 rpm. (Load speed is the speed under load)

With 3 cells, you could prop the motor to draw up to 45-ish amps and still have good efficiency ( ~77%). The current for peak efficiency is 24 A. This setup could give you about 350 watts to the prop which will be tons of power for a 4 lb SAM-type model. Maybe even too much power. I suspect you could probably even get away with 2 cells.

3 cell prop @ 4:1 ratio = 17x9 @ 4700 rpm
2 cell prop @ 4:1 ratio = 18x14 @ 2900 rpm

[foamball]

Originally Posted by Mike Anderson

Let's make an assumption that you would want the Astro 05 to not exceed 25000 rpm, because it is a brushed motor. That means you would not want to use more than 4 LiPos. Even with 3 LiPos your "load speed" would still be around 18000 rpm. (Load speed is the speed under load)

With 3 cells, you could prop the motor to draw up to 45-ish amps and still have good efficiency ( ~77%). The current for peak efficiency is 24 A. This setup could give you about 350 watts to the prop which will be tons of power for a 4 lb SAM-type model. Maybe even too much power. I suspect you could probably even get away with 2 cells.

3 cell prop @ 4:1 ratio = 17x9 @ 4700 rpm
2 cell prop @ 4:1 ratio = 18x14 @ 2900 rpm

Sound advice, Mike... Now, if I was to go the 2C route, what mAh size would I go with?? Or, if I chose the 3C, what would you suggest as to mAh size on this??
Looked at higher amp 3C and they are expensive, unless you have a source better than I do....
Have a Thunder power 3C, 1320mAh pack and was thinking of getting another, same size, and tying these together... Will that be enough??

[Mike Anderson]

Originally Posted by foamball

Now, if I was to go the 2C route, what mAh size would I go with?? Or, if I chose the 3C, what would you suggest as to mAh size on this??

How much wide-open-throttle time do you want to have on a typical flight? Will you be doing climb and glide style flying or mostly putting around a t half throttle? Or some other regime?

[foamball]

Originally Posted by Mike Anderson

How much wide-open-throttle time do you want to have on a typical flight? Will you be doing climb and glide style flying or mostly putting around a t half throttle? Or some other regime?

Mornin Mike..
Well, as all SAM models do, they climb and glide, but, since this is just for enjoyement of the hobby now, I only plan on climbing, gliding for a while, then climb again, and then, the dreadful landing....
I am told I could use a pair of matched 3C tied together to give double amperage, still 11.1V... Am wondering if this is the way to go or do I get a 3C of around the 4000mAh, and go with that...
Unfortunately, it's getting expensive now... Dern batteries are still pricy, for these larger ones...
I'm open for your further thoughts...
Art.

[hifly2104]

Hey Brian you might want to move these motors over to items for sale . Just a thought

[Mike Anderson]

Here's how to calculate the capacity you need:

OK, let's assume you will do 4 x 15 second climbs in a typical flight. That's one minute of full throttle pulling 45 amps. That's 45000 milliamps for 1/60 of an hour or 750 mAh.

BUT - Let's also assume that a battery rated for 25C burst needs to deliver 45A. That's 45000 milliamps divided by 25 = 1800 mAh which would be the smallest capacity pack you would want.

BUT - you will want some reserve power PLUS you don't want to drain more than about 75% of the capacity in a given flight, so add on about 1/3 to the 1800 mAh for a final selection of 2700 mAh.

NOW - with 1800 mAh useable capacity you could do 1800 / 45000 = 1/25th of an hour or 2.4 minutes @ full throttle, which could be used in about 10 x 15 second climbs.

Or you could put-put around at 1/3rd throttle for about 7 minutes.

Are you following the logic here? So your idea of running two 1320 mAh packs in parallel to get ~2700 mAh total capacity is a good solution. (Assuming they are rated for 20C to 25C)

Don't forget that this assumes you will use a sufficiently large propeller to draw the full 45A at WOT. If you prop down for milder performance, you can redo the calcs above to get a smaller capacity pack.

As for sources, it's getting pretty hard to ignore Hobby King in Hong Kong. Their battery prices are amazingly low and their reputation for reliable performance seems to be getting better & better.