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Yet another Prop Size Question

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Old 01-22-2014, 12:45 PM
  #1
W7APD Alan
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Default Yet another Prop Size Question

Greetings to all...

I'm just getting back into RC after a somewhat lengthy absence. Though I have been at this hobby off and on for over 40 years, I know from nuthin' about electric power. I decided a worthwhile first project would be to electrify an old "Easy Breezy" 2m sailplane, which I previously flew with a Black Widow .049 on a pod.

To this point, I've installed a Park 450 outrunner, Eflight 20A ESC, 1300mAH 3S 20C Lipo. I anticipate short motor runs in keeping with normal sailplane tactics, maybe 30 to 60 seconds. Airplane balances nicely, looks ready. But....

The guy at the store talked me into a 9 X 7.5 folding prop (I insisted on a folder, because they're cooler on sailplanes) and that was the only one he had in the store. The datasheet for the Park 450 motor calls for "9x6 to 11x3.8 Slow Flyer or 10x5 to 12x6 Electric" props.

Pertinent data for the Park 450 motor: rated 14A continuous, 18A surge for 15 seconds.

Now for the question: I just measured the static current draw at 15.5A using the 9x7.5 prop, at full throttle. So... Is this setup dangerously overpropped? Will the current draw increase or decrease when the prop unloads in flight? Should I change the prop before I burn up my glider, or should I be OK with limited motor runs?

Inquiring minds want to know.

Last edited by W7APD Alan; 01-22-2014 at 01:22 PM.
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Old 01-22-2014, 08:53 PM
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Bozarth
 
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Your prop is perfect. It pulls 15.5 amps at full throttle - a nice margin below the 20amp ESC. You should be able to run full throttle all day and not harm the ESC. Your battery should be able to provide 26amps (20c means .02 times the 1300mAH rating = 26 amps) and not burn itself up. The amps decrease inflight - opposite to what we expect from nitro motors (which unload and see an increase in rpms in flight).

You are nowhere close to a "dangerously overpropped" configuration.

Kurt
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Old 01-23-2014, 06:42 AM
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You should be okay with limited motor runs... I wouldn't run WOT all day!
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Old 01-24-2014, 10:33 AM
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Please tell us why not? What restrictions are you applying?

Kurt
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Old 01-24-2014, 01:46 PM
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My mistake... I was using the lower weight of a 56g Park 400... at 72g and 3W/g the 450 should be good for ~216W... if you are drawing 15.5A max (and probably slightly lower in-flight) you won't be exceeding 170W... good to go!

The ESC and battery capability are in this case irrelevant, since neither will be exceeded.. it was the motor I was concerned with.......and I needlessly messed that up!

Last edited by Dr Kiwi; 01-24-2014 at 01:50 PM.
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Old 02-04-2014, 11:05 AM
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W7APD Alan
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Thanks to all for the input. It's always nice to bounce new information off people with a bit more experience, to confirm or repair what you think you know.

I finally received my new battery for my old Futaba 72MHz transmitter yesterday. The old battery still took a charge after sitting idle for 20 years, but I'm not about to trust it with anything that isn't on the ground! Now I'm waiting on weather, and a club instructor since I'm technically a "new" member after so long, and club rules require a check flight before one is allowed to fly alone at the club field. No problem with that so wait I shall.

The glider weighs 730 grams all up. With around 150 to 170 watts, I'm guessing it will be a bit of a rocket ship at full throttle? Can't wait...
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Old 02-05-2014, 05:28 PM
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It should climb very steeply. I used to fly a Gentle Lady with a .049 years ago. That's about 100 watts using an inefficient prop (@6x3). It would climb fairly well if I could get a complete engine run. Your power system should perform much, much better.

I like WeboCalc to "what if" power systems for different planes. Click "Help" for an extensive tutorial. The latest version is here:
http://flbeagle.rchomepage.com/softw..._imperial.html
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Old 02-08-2014, 08:34 AM
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W7APD Alan
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Wouldn't ya know it? I gets da plane all ready to fly, and now dere's 8 inches of snow on the ground, more in the air, and freezing rain a-coming.

My abject apology to everyone who knows to blame me for this crummy weather...
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Old 04-07-2014, 10:59 PM
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Thought I'd come back to this thread and share the maiden flight...

http://youtu.be/khKaw1ioL5c
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