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Old Oct 30, 2006, 10:35 PM
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Provo, Utah
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Differences in Power output with 4s and 3s lipos with high RPMs

Hi everyone, i own a Mega 16/15/3 and am currently running 3s, 4.75x4.75 and getting right around 30A WOT. I'm wondering whats going to happen at 4s lipo. Obviously to keep the same wattage i can reduce the amps, and then I can get higher RPMS as well. But what will happen if I stick on 4s and hit it WOT? I'm just wondering what the difference in prop size will need to be to keep the wattage down so as not to overheat the motor....I'm kind of wondering also basically what I need to do once i go 4s with the mega...smaller prop? Same size prop just more RPMS? What?
thanks!
fflyer
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Old Oct 31, 2006, 01:39 AM
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Airdrie, AB Canada
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Don't think you'll find many that have done this. On 4s you need a VERY small prop to be anywhere near spec.

Same prop. 4.75x4.75 you are going to jump to over 45A assuming your batts can maintain somewhere around 13V. That's a whopping 585watts. You'll fry it VERY fast.

Even a 4" prop is gonna be over 40A. You'll need some serious throttle control assuming your other components can handle it. I'd be extremely careful definately run it on a wattmeter to start with. Lots would say you're over the edge.

There are better calcs out there, but this will show you the basics. http://brantuas.com/ezcalc/dma1.asp

On 3s you are pretty close to the limit for this motor. You can get more speed by cutting down a 5x5 to something like 4x5 or a 4x5.5 so you're pitch speed goes up. Depending on airframe you may or may not have excess thrust with the slightly oversquare prop.

Good Luck,

Mike
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Old Oct 31, 2006, 04:00 AM
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going from 2s to 3s is about a 20% reduction in prop size to keep the same CURRENT. so 10-15" would be about right - 4.1x4.1 mebbe..

BUT with a 16/15/3 its actually less efficient below 300W to use 4s..3s is the sweet spot.

And 400W+ is a LOT to put through..the EDF boys do it but they have a really good blast of air past the motor.

I'd say don't do it.
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Old Oct 31, 2006, 05:21 AM
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The loss in thrust efficiency by going to a tiny prop that runs a lot faster will offset any efficiency gains in motor efficiiency. right on 4" diameter props don't work very well with heavier models.

If you want to keep the same diameter can and go 4S with more power have a serious look at the HET2W20 they will handle 4S and 700W without a problem and make a good training motor for F5D pylon and the best bit is they will set you back under a $100 USD
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Old Oct 31, 2006, 05:42 AM
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I believe i've seen people using a APC 4.1x4.1 with that setup. Makes a small pylon type plane like the Sunracer really move out. Obviously good cooling and time to cool between WOT runs are important.
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Old Oct 31, 2006, 06:02 AM
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On a plane like a Sunracer it will work but you need a really good arm to launch them straight quick and level. I have seen plenty roll in to the ground due to the reduced thrust You are still better off changing motor kV and going up in motor size and keep the props around the 4.5-5" range in diameter.

Tim
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Old Oct 31, 2006, 06:24 AM
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Quote:
Originally Posted by tim_mellor
On a plane like a Sunracer it will work but you need a really good arm to launch them straight quick and level. I have seen plenty roll in to the ground due to the reduced thrust You are still better off changing motor kV and going up in motor size and keep the props around the 4.5-5" range in diameter.

Tim
Actually I have a 36" plane on a fairly small prop..these days its not straight and level, its a 45 degree upwards javelin throw, because its nearly got unlimited vertical..yes it rolls, but it soon gets up to a speed with enough control authority. Airspeed OR altitude..its easier to get altitude from a hand launch with a plane that needs 20mph plus to stay in the air..
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Old Oct 31, 2006, 06:42 AM
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As the model was not mentioned in the first post it is speculation but if it is a Speed 400 Racer they are about 700mm and already running over 20oz wing loading stall speeds are a lot higher I am a big fan of getting your speed by pitch on larger props not by more revs on smaller props. None of the current high end pylon planes have seen run 1:1 thrust.

Tim
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Old Oct 31, 2006, 07:10 AM
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esc's bec may get toasty
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Old Nov 01, 2006, 09:13 AM
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Ok guys. Thanks for the comments. I'll stay 3s until I have to get a new motor. I'm running it on a Speedwing 400 (coming today!) so It should have pleennnntyy of power.
Even though I'm only runing a 4.75x4.75. I wanted a 5 incher, but it was drawing wayyyy to much current.
Maybe I'll just go with a sweet 2000mah 20C 3s pack...that would fit the bill
fflyer
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Old Nov 01, 2006, 01:50 PM
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Do yourself a favour and try the 5.5x4.5 APC and carefully trim it to 5" in diameter or until the current comes back to where you are happy. You might find you like the extra thrust at the expense of a little speed works for you.

Tim
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Old Nov 01, 2006, 01:58 PM
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Heck if you want to throw out the HET2W motor then just go for 1105/2.5 /3 Neu
motors if you want real speed!!!

Then lay it on thick!!!

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Old Nov 01, 2006, 03:43 PM
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Even though it will be out of spec. You probably won't find the motor to be unhappy on the 5x5. I run mine there all the time with fair cooling in a pylon racer. Pushed harder if it's in the wind. Controller and batts will have to be up to it. Races are under 2 minutes WOT. General flying is a non issue.

My airframe was slick enough to get away with cutting down the 5x5.5 and picking up the extra speed. Trimming down a 6x4 or the 5.5x4.5 will get you the thrust you're after. Unfortunately there's no free lunch without more watts so you have to trade thrust for speed or vice versa.

Haven't done the 4.1x4.1 on 4s. But doesn't suprise me to hear it's been tried in something like a sunracer. Your wing would probably benefit by a slightly larger prop as they are no where near as slick as a mouldie.

Sounds like you're right at the limits of both motor and batt... so without swapping components you pretty much have to pick thrust or speed and custom fit props till you get what you want. Then chuck it all and slam in better batts and motor

Mike
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