Motor/battery analysis

[bearcat]

Okay, I ordered the following motor for my new 2-meter sailplane:


This is a 230 Watt motor. So I have a 3-cell Lipo battery and I want to find out how it will do. It's a small battery though, 1320 maH. I know, I need a bigger battery and I'll get one. But for the sake of analysis,
230 W/ 11.1 V= 20.7 Amps. Round off to 21 Amps.
1.320 Amp-hours/21 Amps= 0.063 hours
.063 hours x 60 min/hr= 3.77 Minutes of flight time at full throttle.

So that's not too bad. On this little battery, I've got almost 4 minutes of power. That's enough to get it up into the thermals with a few minutes remaining. But a bigger battery would do better. Or more voltage. But this motor is rated for 3 to 4 cell lipos. So more voltage means less need for current capacity.
For example, RC Hot Deals has a 14.8 V 4-cell Lipo with 1700 maH for $60. They also have a 3300 maH 11.1 V 3-cell Lipo for $90.
If I wanted to compare them:
1) 230 W/14.8 V= 15.54 Amps
1.7 Amp-hrs/15.5 Amps= 0.109 Hrs
0.109 hrs x 60 min/hr= 6.6 minutes

2) 230 W/11.1 V= 20.7 Amps
3.3 Amp-Hrs/ 20.7 Amps= 0.16 Hrs
0.16 hrs x 60 min/hr= 9.6 minutes

So either battery would be good. Let's say the 1320 maH lipo might be good for one or two trips up to altitude, with some reserve for landing. The 1700 maH 4-cell would give me twice as much flight time, and the huge 3300 maH 3-cell would be triple as long. Okay that gives me what I needed to know.

The motor is recommended to turn a 9x5 prop. That's 9" diameter, and 5 inches forward progress per revolution. So how fast will it go at full throttle ideally, drag notwithstanding?
This motor is rated at 1500 rpm/volt, and is 80% efficient.
On the 11.1 volt lipo, that's:
1500 rpm/volt x 11.1 volts x 0.80= 13,320 rpm
Multiply by the pitch and Divide that by a constant of 1056 to convert to mph (to simplify the math) and you get:
13,320 rpm x 5 in/ 1056= 63 mph

Clearly that's probably pushing the sailplane's speed redline, so let's say you only need it about half throttle for normal flight. So that's about 30 mph or a little over, and the battery drain is cut in half, meaning that little 1320 maH lipo could last me for 6 minutes of continuous power at half throttle, or longer when the motor is shut off. I could probably get 10 to 15 minutes of flight out of a small battery if my hangtime is good. Or double for the 4 cell 1700 maH battery and triple for the 3-cell 3300 maH battery.

So I'll try out my 1320 lipo for now, but will upgrade to one of the bigger batteries in time.
My top speed for the 4 cell is even higher.
1500 rpm/V x 14.8 V x 0.8 efficiency = 17,760 rpm at full throttle
17760 rpm x 5 in/ 1056= 84 mph.

So the only time I'll need full throttle on that power cell is if I need to pull hard out of a situation. The 3 cell lipo has some safety built in, so that I don't tend to overspeed.
I think the 3 cell, 3300 maH lipo would be the ultimate battery for this airplane. But the 4-cell 1700 maH battery would work, IF I'm careful with the throttle. I think it would be worth the extra $30 bucks for the built-in insurance that I wouldn't exceed the airframe's redline.

[Lt Swiss]

WOW! You sir are into it... No comment...

[bearcat]

Okay I now find that in order to fly my foamy J3 Cub in this thin air, I need to really soup up the motor. Problem is, I don't want to soup it up too much. I am learning though, that more power is not a bad thing. Not enough power is.

So I have a 230 watt outrunner and the next size up is a 430 watt outrunner. The 230 watt motor can theoretically fly a 3 pound airplane at 70 watts per pound. My sailplane is supposed to weigh in at about 3 pounds. The J3 Cub foamy is probably a pound and a half. It might not be a very appropriate motor for the little J3 Cub, but it sure as hell won't be underpowered. Chances are, it would have to be throttled back quite a bit for normal flight.

I'm not sure if I should try it. I really only need the motor size Terry uses in his trainers from R&RRG. Maybe I should buy a smaller motor. Or do you think I should just try it once and see how it does?

And then what about the sailplane? The 430 watt motor I have will power a 6 pound plane, twice as heavy as the sailplane. Too much motor for the sailplane? Or would it make that plane one sweet flyer with real wind penetrating capability? I saw how Mitch's Fox did with twice the power and it really turned that plane on.

[bearcat]

I did some research online and the 230 watt motor will make the sailplane mildly aerobatic and the Cub will be fully aerobatic...not quite 3D but fully aerobatic.

It's a nice motor for the sailplane but would tend to make the J3 Cub apeshit. I might want to search for a weaker motor. I found this one, it's 120 watts, which will be a great motor for the Cub. But I don't want to have to wait 14 days for this thing to be shipped from China.
cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=250532799763&ssPageName=STRK:

[bearcat]

I found a 125 watt outrunner that I am going to buy from Hobbies N Stuff. It might or might not be here this week, so whether I have a plane to fly next weekend or not, I'm not sure yet. I'll still go out there, but we'll see.

There is so much modifying to do just to make the Cub flyable that I wouldn't call it a proper ARF. I guess there, you get what you pay for. The wing-loading for one thing is so damn high that you HAVE to fly it very fast just to be able to fly. Plus that will tend to make it unforgiving. It weighs a pound and a half on just a 37 inch wing with a thin airfoil.
Best thing I can do with the Cub is just to fly it until it breaks, and then use the spare parts to build something else.

So hey, why am I the only one posting here???

[Lt Swiss]

That thing will dial in I'm sure. And you will get a lot of hours out of it.

Wishing to crash usually leads to crashing...

[bearcat]

Yeah I know, self-fulfilling prophecy.