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Old Sep 06, 2007, 03:33 AM
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Truglodite's Avatar
Carmichael, CA
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nice flying kendall.

so she weighs 42oz, or 2.6lb (big motor & big bird)
at roughly ~25A on 2s she burned ~175W.
this would give her a 70W/lb ratio.

that's ok, but after glancing at the motor review it looks like it shouldn't hit 175W with 2s. what esc are you using? settings? i think that 3s would be the ticket.

kev
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Old Sep 06, 2007, 06:59 AM
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Tony65x55's Avatar
Oshawa, Canada
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Kendall, I have to second Kev's recommendation. Put a big (2000mah+) 3s pack in her.

Tony
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Old Sep 06, 2007, 08:56 AM
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kev,

OEMRC (Hobbywing) 40A set for lipo (of course), 2.85V/cell LVC, soft start, brake off, "medium" timing (recommended for 6+ poles).

Quote:
Originally Posted by Tony65x55
Kendall, I have to second Kev's recommendation. Put a big (2000mah+) 3s pack in her.
I think I need to figure out wants going on with the motor first. The Eagle Tree logger should be great for that (hopfully delivered tomorrow). I just don't see how I could have drawn so much juice at 2s. Based on other folks testing I was thinking I'd be drawing around 13A to get 70W/lb. Even at full-time WOT I was expecting 10-12 minutes from 2600mAh. I'm hesitant to put 3s on there and increase the amp draw.
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Old Sep 06, 2007, 09:09 AM
I'm not flying backwards!
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Oshawa, Canada
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Quote:
Originally Posted by tuppertn
kev,

OEMRC (Hobbywing) 40A set for lipo (of course), 2.85V/cell LVC, soft start, brake off, "medium" timing (recommended for 6+ poles).


I'm hesitant to put 3s on there and increase the amp draw.
A 3s pack will decrease amp draw. Volts X Watts = Amps

To get the power out in watts, the juice has to come from somewhere. If the motor can't pull it out in volts, it pulls it out in amps.

Tony (definitely NOT a motor guru)
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Old Sep 06, 2007, 10:24 AM
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Ipoh, Perak Malaysia
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Quote:
Originally Posted by Tony65x55
A 3s pack will decrease amp draw. Volts X Watts = Amps

To get the power out in watts, the juice has to come from somewhere. If the motor can't pull it out in volts, it pulls it out in amps.

Tony (definitely NOT a motor guru)

Doesn't work that way, Tony. If you use the same prop, then Amp drawn will increase with a corresonding increase in waste heat. If you want to go to 3S, you will have to prop down a bit.

chewy
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Old Sep 06, 2007, 10:30 AM
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Actually chewy, I knew that and made that assumption. As I said, I'm not a motor guru, I do airframes, so correct me if I'm wrong. If you are seeking a certain thrust level, you can achieve that at a lower amp draw with 3s than with 2s. Is that correct?

Tony
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Old Sep 06, 2007, 10:53 AM
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Yes, but you need to prop down. Otherwise, you will get a higher amp draw and higher thrust/speed. If you reduce the prop diameter 1 or 2 sizes, you can still achieve similar thrust with less amp drawn.

I am not a motor guru either. This is just my experience from burning out some motors and an esc for not doing what I said above...

chewy
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Old Sep 06, 2007, 11:25 AM
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It's my understanding that the throttle on our radios is a essentially a voltage regulator, basically a dimmer switch. The proportional control means that at "0" throttle you get "0" volts and at WOT you get whatever the max voltage of you pack is. And, of course, RPM is proportional to voltage. So 3s will give you the same RPM at 66% throttle that 2s will give you at WOT because around that throttle setting (66% of 11.1) you'd be delivering around 7.4V.

With the same motor and prop setup the amp draw from both batteries will be the essentially same for any given RPM (ie-same voltage: 2s@wot vs 3s@66%). Maybe what Tony's getting at is that the 3s would have a slightly lower draw in that the extra cell would help it hold it's pack voltage better than the 2s for a given amp load. That would become more significant the higher the current draw. Going over 66% throttle with a 3s is where you get the extra power above what 2s can deliver, but, by the V=IR thing again, increasing the voltage under constant resistance draws more current. And, as chewy pointed out, starts killing the efficency with a prop sized for 2s.

That's my understanding anyway.

Here's something curious. Just put the 2 bats I flew with last night on a voltmeter--both read 7V remaining. ESC was set for a 2.85V/cell LVC. Something's screwy there. Gonna check out the ESC when my wife brings my radio home (left it in the trunk of her car ).

kendall
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Old Sep 06, 2007, 11:43 AM
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No, it's not a dimmer switch. More than that, you will have to ask an expert, since I am a civil engineer, not EE.

Can any expert help out here and give us a short(!) course on esc technology? Something a lowly CE can understand instead of an EE?

Anyway, when you are drawing current, the voltage will drop. Measuring the voltage with no load is not the same as the voltage under load. Depending on the type of battery in question, the behaviour and/or voltage drop under load when the batt is near the end of its discharge is vastly different, especially under high current loading situations. That much I do know.

chewy
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Old Sep 06, 2007, 01:21 PM
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Perhaps "dimmer switch" is a little technically simplistic. But since the motor kV is constant the only way to regulate RPM is to change the voltage, no?
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Old Sep 06, 2007, 01:50 PM
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true, given the same prop, adding a cell will increase it's draw at WOT, but WOT current is not what makes a plane fly. power loading is a better indicator of climb performance. so it's already taking 70W/lb to fly as is. increasing to 3s you'll have slightly more weight, thus will need slightly more power to stay at 70W/lb.

lets calculate this out: let's say you're going from a 2s2100 to a 3s 2100. the mass difference is roughly 1.8oz, which is 4% of 42oz. now let's calculate how much current the 3s setup would use to get the 4% more power needed to fly the same power loading:

175W * 1.04 = 182W = 10 * i
i = 18.2A

remember, we assumed 25A for the same 70W/lb on 2S. clearly the 3s gets the same job done on less current. in reality, this means if you flew the exact same flight with the 3s, you're flight times could increase by about:

100*(25-18)/25= 28%

and you'd have way more reserve power if/when it's needed. of course you'd need to prop down to 18A on 3s. you'd probably also want that 18A to hit around 1/2 to 3/4 throttle so you can summon 100W/lb at WOT if needed.

fyi, i've always used a wattmeter to check my prop choices before flying. they're not very expensive & having one makes it very easy to get to the core of what's going on. add on a tachometer & you'll have all the bench test data you need (thrust stands are nice, but not necessary since thrust vs rpm data is available for every prop online). in flight data is nice, but not absolutely needed. usually the best in flight prop is one or two steps from what works on the bench. so trial & error works out quickly in the air once you're bench tuned.

btw, your esc settings sound good to me.

cheers, now it's back to work...
kev
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Old Sep 06, 2007, 08:24 PM
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If I were using an outrunner, I would set the esc to hard start instead of soft start. The soft start is only needed for gearboxes to prevent damage to the teeth as the motor will only spin up slowly : ie slow throttle response.

chewy
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Old Sep 07, 2007, 01:43 AM
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tony, a quick correction to an error i just noticed. it's:

volts * amps = watts (aka P=iV)

tuppertn, another thing i overlooked. you noticed your packs landed at 7V, after your LVC was activating at 2.85V/cell (ie 5.7V). as chewy mentioned, voltage will drop when the lipo is under load, so it is likely your pack dipped below 5.7V momentarily during the flight. this tells me you're lipo is weak. a 2s2600 lipo shouldn't drop 1.3V with a load of only 25A. that's only 9.6C, well within range most modern lipos for continuous use. i'd try a fresh 3s2100 15C pack & see how it works (of course, prop appropriately). heck, at 42oz you could toss in a 3s2600 without noticing much a difference in wingloading.

kev

[edit: note that the calcs in my previous post don't include the small changes in effeciency going from 2s to 3s. it is more often the case that with reasonable prop choices this difference is only a few %.]
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Old Sep 07, 2007, 02:16 AM
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Quote:
Originally Posted by tuppertn
Perhaps "dimmer switch" is a little technically simplistic. But since the motor kV is constant the only way to regulate RPM is to change the voltage, no?
esc's regulate motor rpm via pulse width modulation (PWM), so in theory the motor is only seeing either 0V or battery voltage at any time, no in between voltage (theory is pretty close to reality here). to describe pwm in leighman's terms, imagine cycling a light switch on & off every 1 second. if it is on for 2/3sec & off for 1/3sec of every cycle, the bulb will be brighter (higher RPM) than if it is on for 1/3sec & off for 2/3 sec. WOT is analogous to holding the light switch on. of course, increasing the voltage going to the light is going to make it brighter at WOT (higher RPM). in this context, you can also say that prop size and kV are load factors analagous to bulb wattage. more kV and/or a bigger prop is similar to using a higher wattage bulb.

hope this helps,
kev
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Old Sep 07, 2007, 07:43 AM
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Thanks for the explanation, Kev.

chewy
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