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Author Topic: Unloaded current?  (Read 1937 times)

Offline Mark Mc

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Unloaded current?
« on: May 10, 2017, 11:21:27 PM »
Last night I was setting up my electric Flight Streak.  With the 9x4.5 APC prop, the motor was pulling 28.5 amps static.  Is there any thumb rule about how much the current will drop when the prop unloads in the air?

Mark
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Offline Paul Smith

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Re: Unloaded current?
« Reply #1 on: May 11, 2017, 01:15:09 PM »
Wow !!  28.5 amps in a model airplane.

I lived in a house with 60 amps for 35 years.  I had a stove, air conditioning, lights, TV, computer, washer & dryer and plenty of power tools. 
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Offline Tim Wescott

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Re: Unloaded current?
« Reply #2 on: May 11, 2017, 01:36:45 PM »
Quote from: Paul Smith on May 11, 2017, 01:15:09 PM
Wow !!  28.5 amps in a model airplane.

I lived in a house with 60 amps for 35 years.  I had a stove, air conditioning, lights, TV, computer, washer & dryer and plenty of power tools. 

But his is 28.5 amps at 11.1 volts (probably) -- so a bit over 300 watts.

Your service was 60A at 220V, for a maximum of 13kW or so -- or about 40 times more.  Voltage matters.
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The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Tim Wescott

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Re: Unloaded current?
« Reply #3 on: May 11, 2017, 01:39:10 PM »
Mark, I dimly remember someone commenting that the power draw on the ground is close to what you'll see at maximum in the air.  I know that the maximum power draw should be around 11W/ounce of aircraft weight, and that the average (with a steady-speed timer like a Hubin) is 7W/ounce.  So, as a wild-ass guess, 18A.

If you have a 3-cell pack, that power draw kind of indicates that you've got enough peak power for a plane that's 30 amps all up.  But -- I don't know if I'm cracked or not, so don't go stomping anything into the trash on my say-so.
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The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Dennis Adamisin

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Re: Unloaded current?
« Reply #4 on: May 11, 2017, 03:16:58 PM »
Quote from: Mark Mc on May 10, 2017, 11:21:27 PM
Last night I was setting up my electric Flight Streak.  With the 9x4.5 APC prop, the motor was pulling 28.5 amps static.  Is there any thumb rule about how much the current will drop when the prop unloads in the air?

Mark

Hi Mark

Current draw on the ground is somewhere around 30% higher than airborne - and the fraction climbs or wanes as the rpm goes up or down.  How much - don't know & don't care.  Current draw on the ground is not a terribly useful parameter.  A poster on the RC Groups site suggested clipping 2" off the prop to simulate in-flight loads.  Again I'm not sure the result means anything.  Current draw on the ground is not a terribly useful parameter.

I do like using clipped or undersized props when setting RPM on the ground, it helps prevent releasing the "magic smoke" caused by overloading the motor or ESC.  I have a couple 11x4.5's clipped to 8" that have accumulated a couple hours use 20 seconds at a time to set-up new airplanes. 

Wander through the "List your Set-up" sticky thread and you will see a few folks have posted data-log info - pull up one of those curves sift through till you find the CURRENT draw line.  Nice and flat in level flight with spikes whenever the handle gets moved.  I would expect a full size Streak with the 9x4.5 turning around 11k (useful RPM for that prop) should be drawing around 15-18A in level flight, spiking up as you do tricks...
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Denny Adamisin
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Offline Crist Rigotti

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Re: Unloaded current?
« Reply #5 on: May 11, 2017, 07:08:54 PM »
+1 what Dennis said.
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Offline Mark Scarborough

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Re: Unloaded current?
« Reply #6 on: May 11, 2017, 07:34:28 PM »
+1 what they said,,
I just loaded my last flight log, the static amps on the ground were 39.1, it shows average current at 21.8. but the peaks in flight are more like 29. it was a windy day so the results are somewhat harder to interpret. Peaks in the reverse wingover are to the tune of 32 amps. I am using 6S packs an eflight 32 motor and the apc 13x4.5 EPF2B prop. in a BIG heavy airplane, read that as over 700 square inches and 75 ounces
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Offline Mark Mc

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Re: Unloaded current?
« Reply #7 on: May 12, 2017, 08:13:00 PM »
Wow! I did not expect the drop to be that much.  I was thinking around 10% maybe.  I'm using a 20C 2200 mAh pack and a 40A speed controller, as that's what I had on hand, so if I stay well below 40A I'm not gonna smoke the battery or speed controller.  Granted, a 35A speed controller would be more efficient, but I don't want to run out and buy one.

If the current drop is approximately 30%, then I can get right about 4 minutes and 45 seconds of run time with a 30% reserve.  I was aiming at 3 1/2 to 4 minutes of flight time for now, as I only want to do the Beginner pattern.  Even if it's not 30%, I should still have plenty of juice to practice the Beginner pattern.

Y'all have given me a lot to think about.  I don't have enough props that I can go out and just lop off the tips for static testing.  I guess I'll just run it and tweak it until I get comfortable lap times, and then see how much it takes to fill the batteries back up on charge after a 3 1/2 minute run.


Paul, It's been 30 years since I used the conversion factors (for some reason, the 3/4/5/12 rule escapes me), but 28.5 amps at 11.1 volts DC is closer to 2.7 amps at 120VAC.  Now I'm going to have to sit down in quiet room and try to remember my old thumb rules...

Thanks, Mark
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