I have a balsa products 2s1p 1500 mAh LiPoly battery for my dandy sport (eps 350c-c, 10x8 prop). The guy at the LHS where I bought the battery told me that I could expect 45 minute flight times with this battery. Well, I tested the battery on the ground in my plane with the throttle holding at about 75%. The motor cut off at 13 min 50 sec. Isn't this quite a lot shorter than it should be? Also, the pack was HOT when I pulled it out of the plane.

Complicating things is that my charger, a Hobbico Quick Field Mk II, is apparently incapable of charging LiPolys all the way up (didn't know this when I bought it). At full charge, my pack produces 8.25 volts.

So do I have a defective pack? Do I need a different charger? What's going on?

Thanks,
Matt

Nothing is wrong.
Your 10-8 prop is too big. It is not even listed on the chart:
http://www.balsapr.com/catalog/motor...ductId=T433964
Try a 9-6 or 10-4.7
Extrapolating from the chart, your 350 with 10-8 prop will draw over 8amps at full power, 6 amps at 75%.
The battery has 1500mah which gives 1.5amps for 1 hour, 3amps for 30 minutes or 6 amps for 15 minutes. Add a minute for reserve to keep the battery from discharging completely and destroying itself. So your time of ~14 minutes is correct.
About your battery. The writeup on the Balsa Products page about your battery:
http://www.balsapr.com/catalog/battN...ductId=T231537
is written by the same crackhead that told you it would run the motor for 45 minutes. This LiPoly cannot discharge at 15 amps contineously, maybe 8 amps but even that is pushing it. That's the reason yours is hot. It is running at max discharge. Also, if you follow the note on the battery page about charging at 2C expect to call the fire department because the battery will explode.
In short, nothing is wrong with your runtime.

Fritz

I'm not sure I would recommend running your motor for 13 mins. on the ground. There is little cooling air flow as would be the case in flight.

Best to use the stop watch during a flight and only do very short "test" runs on the ground or workbench.

Sierra Gold

How about a 10x6 prop. According to the chart, this will draw 6.9A at full throttle and produce 345g of thrust. Projected run times are 12 min at 100% throttle, 16 min at 75% throttle, and 26 min at 50% throttle. I like this prop because it also produces reasonable values when I use a 720mah NiMH battery -- so I don't have to change props when I change batteries.

Is thrust the only consideration there is when determining how a prop will perform?

Matt

One other factor. Your prop will unload in the air and will draw less current, which will also extend battery time. Thrust is not the only consideration. Compare the 1047 with the 1080. The 1047 has a lot more thrust than the 1080. But you couldn't fly a lot of planes with the 1047, because you couldn't get enough speed out of it. Most measurement you see are static thrust. The thrust you would see from the plane standing still. Once the plane is in forward motion, the pitch starts to become more important. One thing I would like to see, but I know it would be a pain to measure, is the static thrust and then the thrust against a 20mph wind.

There is absolutely nothing wrong with your battery, and nothing wrong with the charger. At least, there wasn't before you got the battery HOT... Now, there might be a little loss of capacity... Running the plane on the ground for extended periods of time is BAD for the battery and motor. Keep static runs to 30 seconds, or preferably LESS

You simply fell for the myth that just because the battery is a LiPoly, it will magically make you plane fly for some ungodly length of time. Time for a little myth busting:

The mAh rating of the battery, not its chemistry, is what determines how long the plane will fly. mAh is mAh, no matter what kind of battery it is. If you get 13 minutes from a 1500mAh NiMH pack, you will get 13 minutes from a 1500mAh LiPoly pack of the same voltage.

LiPolys have an advantage in that you can pack five times as much capacity into the same space and weight as a NiCd, or about three times as much as a NiMH. On a plane like the Dandy Sport, you can expect about 5-6 minutes of full-throttle flying from a typical 650-720mAh NiMH pack. 13+ minutes, running static on the ground tells me that your battery is working fine. The heat buildup will make the motor draw more current, and make the battery less efficient. If you were flying, you can easily get 13+ minutes at full throttle. Using some throttle management, that is, only going to full throttle when you really need to, you could probably fly for 20+. Flying at the lowest possible setting, you could probably fly for 30 minutes or more.

Hobbico's MkII charger is conservative, and that is not a bad thing. You do not want to overcharge these cells. The pack was probably at 85-90% full, so your flying times would be much longer if it were fully charged.

Thanks everyone. I've learned a LOT in the last few days. I've been playing with MotorCalc to aid in prop selection.

Performance seems to be good with the 10x4.7 prop. I flew for about 15 min yesterday and about 20 min today and had a great time. I've been flying on soccer fields at my university.

It's nerve-wracking to fly with a LiPoly battery and a GWS speed control because it won't cut the motor until the battery is below 6v unloaded. (Guess how I know this?) I hope my experimentation has not harmed my battery. I flew for a long time today and could have flown more but I was getting nervous about the voltage. So I brought it home and put it on the multimeter -- battery was still reading 7.2 volts. Cool.

Incidentally, the reason I did all this was because I read somewhere that it is a good idea to test your setup on the ground to see how much flight time you can expect. Thanks to everyone here, I now know that to be bad advice.

As soon as I can get my hands on the proper adapter, I'm going to try flying with a 9x6 prop. I expect to lose some thrust but gain some speed. It's a HD prop and has a 4mm hole rather than a 3mm hole so I'm going to have to get an adapter for it.

Oh, according to MotorCalc, the 10x8 prop will draw 10.5 amps, so I probably won't be using it until the KAN 650's I ordered come in. MotorCalc says that the prop will be stalled. What does this mean as far as flight characteristics are concerned? Less efficiency?

MotorCalc predicts that the 9x6 prop will perform much better in flight. I definitely need to get that adapter.

Matt

Actually it looks like 9x7 will give me much more airspeed with a little less thrust and keep the amperage at about the same level that it is now. The prop will be stalled until the plane hits about 5 mph or so, but I don't think this will be a big deal at all. Plus, the 9x7 will fit right on my gearbox without an adaptor.

Hi ho, hi ho, off the the LHS I go...

Matt

Don't worry about the gws esc and voltage cut off.
Most planes won't have enough power to remain airborne at 6.5 to 6 volts under load. So when the battery gets that low, you will be landing whether you want to or not...
I think the gws cut off is around 4.5-5 volts, and I think even that is probably a half volt off to the plus side...
The main thing you don't want to do is fly til the batteries are low, let them "rest" on the ground, then try and fly again before recharging. That will kill the Lipos by pulling them below 5 volts under load. Under load voltage is mainly what we're concerned about with lipos and r/c airplanes anyway. I mean, who sits around and attempts to draw these batteries below 6 volts no load anyway?