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prop and esc question

Old 08-01-2009, 03:37 AM
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wbuttry
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Default prop and esc question

im running a 2700kv motor inrunner on my slinger an a tower pro esc 25 this is what the motor is calling for i want to find a happy medium in prop choices im thinking a 5x5 or 6x4 i have no watt meter im useing a rpm meter to come as close to the 2700 kv that i can im useing a 1800mah 3s lipo and im wondering im not getting near 2700 rpms im getting like 18 or 19 im wondering if i could up my esc size it calls for a 25 esc i wonder if i could up it to say 30 or 35 and still be safe to the motor and battery does anybody have any ideas im open to feedback
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Old 08-01-2009, 12:02 PM
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Sky Sharkster
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Default Prop Size vs Kv?

Hello WButtry, Welcome to Wattflyer!
I believe the motor Kv is too high to support those props. You would be better off with a 6x3 or even smaller, a 5x4. According to this Calc;
http://brantuas.com/ezcalc/dma1.asp
A similar motor, the Himax HA2025-2700 will be pulling 21.899 amps with a 6x4 APC prop and the Thunder Power 1950 3S battery. The efficiency is way down too, 60.9% out of a max efficiency of 83%. RPM [email protected] 130.92 watts out. Static thrust is 27.1 oz, prop pitchspeed 67.8 mph.The amp draw is dangerously close to the ESC rating.
By comparison, the 6x3 uses 18.009 amps, efficiency is up to 67.6, RPM is 19349, with watts out 124.32. Thrust 23.8 oz, pitchspeed 55 mph.
Last, a 5x4 prop; 14.376 amps, efficiency is 73.3, RPM 21488 and Watts are 109.53. Thrust 22.6 oz, pitchspeed 81.4 mph.
Nothing's free. The bigger, high-pitch prop (6x4) has the best static thrust and high theoretical speed, but is very inefficient and the amp draw is high.
The 5x4, on the other hand, has the best efficiency, lowest amp draw and high RPM, but static thrust/Watts is lower. Notice the pitchspeed is much faster. If you can live with the lower thrust, this is the way to go.
Only a wattmeter can tell you if these figures are accurate for your application, but they do show a definite advantage to using a smaller, lower-pitch prop. Unless your model is very heavy, that is. In that case, you would be better served with a larger motor or lower Kv motor.
The only way you might get 27000 RPM is to use a tiny (like 3" diameter) prop, which likely wouldn't even fly the plane.
Try the calc yourself, just get a listing of available props and plug' em in!
Good Luck!
Ron
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Old 08-01-2009, 04:14 PM
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pd1
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Originally Posted by wbuttry View Post
im running a 2700kv motor inrunner on my slinger an a tower pro esc 25 this is what the motor is calling for i want to find a happy medium in prop choices im thinking a 5x5 or 6x4 i have no watt meter im useing a rpm meter to come as close to the 2700 kv that i can im useing a 1800mah 3s lipo and im wondering im not getting near 2700 rpms im getting like 18 or 19 im wondering if i could up my esc size it calls for a 25 esc i wonder if i could up it to say 30 or 35 and still be safe to the motor and battery does anybody have any ideas im open to feedback
Changing the ESC won't affect the RPM.
RPM is determined by the Kv of the motor. The Kv rating is how many RPM's the motor will try to spin per volt applied.
How are you measuring the RPM you're already getting?
Reducing the prop diameter as Ron suggested will increase RPM.
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Old 08-01-2009, 05:59 PM
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i was useing a optical tach globee puts out a good one give or take a 100 rpms on the esc issue i was trying to figure if i used a bigger esc i would be able to run a little bigger prop without burning it out i would take a loss in rpms and add more thrust my wing weighs about 27 ozs im not trying to get real fast i just want it to alot better than speed 400 i blew my brushed esc15 on brushed motor running a stock prop and 1800 3s lipob it flew good no climbing power it took full throtle to get it to climb then it was still slow on climb
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Old 08-01-2009, 06:47 PM
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You can increase the size of the ESC so it can handle any amperage you can draw through it.
Loading the motor up with a prop that is too large will smoke the motor.

A motor with a 2700RPM Kv is not a motor I would want to load up.

Try a larger motor, or one with a lower Kv if you want to run a larger prop.

You might even be able to go faster on a smaller prop that turns faster.
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Old 08-02-2009, 04:12 AM
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Sky Sharkster's advice is good. use one of the free motor calculator programs.

A motor's Kv is its theoretical no-load speed per volt. You will never get that speed with a prop on it. Even a prop that is well matched to the motor will not let it turn at its full Kv. So don't set that as your goal or you will burn up the motor trying to get there.

So use one of the calculator programs. There's also the very rough-and-ready test method: Put the prop on, hold the plane firmly, run the motor at full speed for 30 seconds (staying well out of the way of the prop), then shut down and touch the motor and see how hot it got. If it's anything more than just warm, you have too big a prop on it.
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Old 08-02-2009, 04:26 AM
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Who makes the motor your referring to?
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Old 08-02-2009, 04:34 AM
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hxt a china brand motor from hobby king
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Old 08-02-2009, 04:46 AM
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im just getting the hang of the calc i never used one before. on a 5x4.3 i got somewhere around 17 or 18 amps on a 25 esc so im well below the 25 amp max so i need to see what size prop to use i can use it as long as it stays under my max by at least 5 rite give or take a amp or so like im running a 25esc i need to get around 20 correct that way if my bat dont shut off like it is suppose to oh bec my amp draw wont burn esc or motor am i getting warm im new to electric i've been a nitro head for 14 yrs i need all the help i can get badly and i wanna say thank you very much i love wattflyer it is such a cool forum.
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Old 08-02-2009, 05:18 AM
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Default Now you've Got It!

Hi WButtry,
That's the way to go, now you're becoming a watthead!
Think of it this way; A chain will break at the weakest link.
The power train (our chain) consists of the battery, the ESC and the motor/prop.
The only one with "brains" is the ESC.
The motor will take all the amps you feed it, until it burns up. This is much more likely to happen if the prop is too large or too small.
The battery will provide all the power it can, until it melts or cooks. It has a "C" rating, a capacity it's capable of delivering (discharge rate) and if a demand is put on it for more, it will try to produce it. Result; Meltdown.
So, the ESC is in the middle. That's what it does. Control. First, it's a throttle. Simple function (not really!)
Next, it has a max amp overload rating. If it's rated for 25 amps and your motor asks for 30, it shuts down the motor. You still have radio control and can glide in for a landing. you will learn a lesson and live to fly again.
Most ESC's also have a thermal overload circuit. If the battery gets too hot (too much demand from the motor, again) it will shut down the motor.
But both of these circuits and the Low Voltage Circuitry (LVC) depend on readings that are changing quickly. The servos and receiver need power too. If something changes too fast, or the readings are off or it's not an accurate reading, the result is usually a melted (smoking) battery. The ESC may not catch the problem in time.
Which means, in simple terms, a crash. You not only lose motor power, you lose radio signal reception and servo/receiver power.
So, before we fly, we run a calc, based on the closest approximation of our power train. We try to find props (the easiest thing to change) that keep the amp draw about 20%-25% below the rated continuous max.
At the field (or on the bench at home) we use a Wattmeter to confirm our calc figures. Sometimes they're right on, sometimes they're off. These are real-life figures, not theoretical numbers.
So, what you're doing is correct. Keep your amp draw within the limits of your ESC and try to borrow or buy a Wattmeter to confirm the numbers.
It's safer, cheaper and makes more sense!
Ron

Last edited by Sky Sharkster; 08-02-2009 at 09:22 PM.
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Old 08-02-2009, 05:45 AM
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im trying to do the best i can i need to invest in a watt meter badly so much for the tach told you i was a nitro head gone clean thank you very much if you find one very reasonable holler at me ive spent to much money already but thats something i need
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Old 08-02-2009, 08:17 PM
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Ron's, 100% correct with the information he's provided you. All electronic components should not be run at max continuous ratings as they all +/- a few percent. I personally always run as Ron stated 20% + over what I need.

A Watt meter will change your life. Check the for sale threads as I see them all the time at nearly half price.
Jeremy
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Old 08-03-2009, 12:24 AM
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thank you guys for all your help needless to say i been needing it badly thank alot
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