General Electric Discussions Talk about topics related to e-powered RC flying

prop size question

Old 07-15-2015, 03:24 PM
  #1  
Flysohigh
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Default prop size question

I currently have a 12x8 prop on my electrifly mr mulligan and want a bit more torque/power, i was thinking going up to a 13x7. What changes in amps/power should i expect?
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Old 07-15-2015, 03:40 PM
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tobydogs
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flysohigh,

i sit here on vacation at the shore and figure there will be plenty of answers to this question. i think you need to purchase a watt meter and hook it up while the 12"prop is installed. then purchase the 13" prop to see the difference for yourself. a great experiment first hand. lots of variables involved where we need to ask motor,esc size,battery size,ect...ect.
also seek out past threads on choosing prop sizes that are already written.

one thing you and i do share is a plane that whistles when flying.i built the great planes extra 300s kit with eflite power 60 motor,100amp castle esc,using 14"prop and 7cells 4000mah 40c pack. she has awesome power and unlimited vertical climb. the normal slow flying has minimal whistling noise,but increase the throttle and speed and she wrrrrr's thru the air freaky loud which i attribute to a slightly gapped canopy catching the air just right to cause the sound. i actually don't know for sure where the sound is from. will be interested in other response to that thread you started.
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Old 07-15-2015, 04:31 PM
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Wattmeter ... try it and see what readings you get.

Nigel
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Old 07-15-2015, 06:01 PM
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The change of 12X8 to 13X7 should be slightly higher watt demand.

Higher RPM will amplify the difference since power is related to tip speed squared (as one factor out of many)
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Old 07-15-2015, 07:34 PM
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JetPlaneFlyer
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A wattmeter is the only way to play with props safely. You would expect a 10-20% increase in power with that change, if that is ok or not depends on how close to the limit you were in the first place.
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Old 07-15-2015, 10:11 PM
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mclarkson
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Personally, I think you'd probably be fine.

As you probably know, in very rough terms, an inch of diameter is about equal to an inch of pitch and you can usually trade them. So the math suggests that moving from a 12x8 to a 13x7 would result in a 3% increase in the 'work' your motor would have to do.

But, as others have pointed out, there are a number of real world factors that come into play. It's always safest (and more accurate) with a watt meter.
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Old 07-16-2015, 12:45 AM
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JetPlaneFlyer
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Originally Posted by mclarkson View Post
As you probably know, in very rough terms, an inch of diameter is about equal to an inch of pitch and you can usually trade them. So the math suggests that moving from a 12x8 to a 13x7 would result in a 3% increase in the 'work' your motor would have to do.
FWIW It doesn't work that way. Power increases to the fourth power of prop diameter but only linearly with pitch, so diameter has a FAR bigger effect than pitch.

If you want the math the prop power formula is:
Power = Prop Constant x prop pitch x (RPMs/1,000)^3 x prop diameter^4
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Old 07-16-2015, 07:12 AM
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The error comes from the age old "rule of thumb" of 1 inch pitch for 1 inch diameter.... which applies for a 10X6 as the start prop and 10k rpm and glow engines....

And it works reasonably well moving from 10x6 to 9X7 or 11X5... at 10k rpm when using a glow engine.
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Old 07-16-2015, 07:25 AM
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kyleservicetech
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Or

Horsepower = Prop Pitch times (Prop Diameter)^4 times (Prop RPM)^3/1.4E17

Multiply horsepower by 746 watts/hp to get watts output on the prop shaft.

This formula is in the ballpark for accuracy, even with a prop used on the full scale airplane engines.

Here is a very interesting web page from APC, one of the propeller suppliers:
https://www.apcprop.com/v/PERFILES_W...tDatafiles.asp
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Old 07-16-2015, 11:58 PM
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Flysohigh
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Right now im at 700.4w with the 12x8 prop and between 38-45a on the esc. I have a castle edge 50amp. Going up on the diameter im hoping to get a bit more pull on the up line. Now my plane weighing 6.2 lbs seems to feel like a cow when pulled into a vertical position. I know it has a fat fuselage and lot of weight/drag but the rimfire .32 is capable of 850 constant watts. Im just trying to get more torque when demanded not worried about losing a bit of speed
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Old 07-17-2015, 12:23 AM
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As many have stated, the Wattmeter is your best bet (you should use one everytime you build or change anything with a plane's PP).

As for your whistling, have you tried taping the gaps in the control surfaces?
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Old 07-17-2015, 12:36 AM
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i just read the reviews at tower and watched a few videos there. the mulligan is a scale flyer and really not some super aerobatic bird. she looks heavy by design and handles as such,a really beautiful plane.
aside from being able to do aerobatics ,the reviewer states landing can be difficult and if a go around is needed there is a possibility of torque roll and crash tip stalls...in fact he says it's almost a sure bet.

the cowel is very large and round and the prop in the video looked to small for the size of the cowel. i would think the 13"prop would work fine in the mulligan and might help with the needed extra power to help for that extra pull up power in the event of a go around approach during landing. other wise i wouldn't expect unlimited vertical from the rimfire 32. maybe a 46power eflite with the 13"prop and a 5cell pack. 6.5lbs is not a feather weight 55"wingspan plane. glad you got the watt meter,now nothing will be left to chance.
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Old 07-17-2015, 01:02 AM
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Flysohigh
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I actually dont have a wattmeter im going by what the previous owner measured and shared with the current setup
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Old 07-17-2015, 07:53 PM
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Originally Posted by Flysohigh View Post
I actually dont have a wattmeter im going by what the previous owner measured and shared with the current setup
You need to get one. Don't trust what others report, small differences such as battery performance and prop brand can make very significant differences to the current that the motor pulls. Even variables such as altitude and temperature can make a noticeable difference.

If you fly e-power you need a wattmeter.
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Old 07-17-2015, 08:13 PM
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I would actually go further and suggest that trying to 'improve' a high power set up without using a Watt meter was a receipt for trouble!
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Old 07-17-2015, 08:41 PM
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Originally Posted by Flysohigh View Post
Right now im at 700.4w with the 12x8 prop and between 38-45a on the esc. I have a castle edge 50amp. Going up on the diameter im hoping to get a bit more pull on the up line. Now my plane weighing 6.2 lbs seems to feel like a cow when pulled into a vertical position. I know it has a fat fuselage and lot of weight/drag but the rimfire .32 is capable of 850 constant watts. Im just trying to get more torque when demanded not worried about losing a bit of speed
Flying at 700 Watts with a 6.2 pound airplane is around 113 Watts per pound of airplane. Somewhere around 150 Watts per pound of airplane will fly acrobatics very nicely.

Assuming that level of power doesn't tear the nose off of your model. And, that level of power doesn't let the magic smoke out of your Rimfire motor. The Rimfire motor is rated at 850 Watts, but be a little careful of actually running it at that power level.

How many cells does your LiPo battery pack have? IMHO, a six pound model would have a four or even five cell LiPo battery around 3300 Mah or so. And, a motor to match.

As the others in this thread have indicated, you need to get a wattmeter! Flying electric models without checking power levels is similar to driving a car without a speedometer. Sooner or later, it's going to cost you.
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Old 07-17-2015, 08:44 PM
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Flysohigh
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Wait....i have had over a dozen planes and i can CONFIRM that a wattmeter is nice to have to know what your drawing but not an issue if you dont have one as long as your within the motor / esc range thats recommended . All im asking are the opinions and what to expect doing a change from a simple 10x8 to 11x7 which will practically be minimal.....
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Old 07-17-2015, 09:01 PM
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kyleservicetech
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Originally Posted by Flysohigh View Post
Wait....i have had over a dozen planes and i can CONFIRM that a wattmeter is nice to have to know what your drawing but not an issue if you dont have one as long as your within the motor / esc range thats recommended . All im asking are the opinions and what to expect doing a change from a simple 10x8 to 11x7 which will practically be minimal.....
That all depends on the flying speed of your model. The 11X7 has a lower pitch, and would result in a lower flying speed. Could be a real issue if you're flying a fast airplane. Or really good if you're flying a Piper Cub.

Using a program such as www.motocalc.com (Free for 30 days, then $39) will give clues on what to expect. Running the numbers in motocalc, expect around 20% more watts input with the 11X7 prop. These motors are kind of a constant RPM versus a glow engine at kind of constant torque.

On my 17 pound Giant Scale Bit Stick model with its Hacker A60-16M motor, during a vertical climb out at 6700 RPM with full power, the motor is pulling near 3000 Watts. And in a dive at full power and 7200 RPM, that motor drops down to around 1000 Watts, per the Castle Creations on board data recorder.

(Note that your original posting indicated 12X8?)
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Old 07-17-2015, 09:04 PM
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Flysohigh
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Im flying the mr mulligan from electrifly thats a racer style plane but has flaps and doesnt need to be flown at full throttle , its a gentle flyer as well
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Old 07-17-2015, 09:45 PM
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Adding a larger prop does not increase torque as that is a function of the motor/controller current as with any electric motor.

The real question is does you motor/controller combo have any more power to spare.
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Old 07-17-2015, 10:17 PM
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Originally Posted by dereckbc View Post
Adding a larger prop does not increase torque as that is a function of the motor/controller current as with any electric motor.

The real question is does you motor/controller combo have any more power to spare.
Actually it does increase torque. The increased load causes an increase in current which increases output torque.

That's why it's dangerous to 'play' with props on electric motors. Electric motors are dumb, they don't know their own limits. If you load them up with a big prop they will just keep increasing their output torque until the amps burn them up.
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Old 07-17-2015, 10:18 PM
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Flysohigh
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700.1w and 43 amps, motor can output 850w and esc is a castle 50a....
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Old 07-17-2015, 10:21 PM
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Originally Posted by Flysohigh View Post
All im asking are the opinions and what to expect doing a change from a simple 10x8 to 11x7 which will practically be minimal.....
If the prop is of the same type it could be as much as a 20% increase, if it's a different type it could easily be 40%... These are not 'minimal' increases. They are easily enough to smoke a motor or ESC that was already close to the limit, and without a wattmeter you really don't know if you were close to the limit.

If you fly 'ready to fly' type models and dont change anything from stock then fine, you don't really need a wattmeter. But as soon as you get into the game of going to bigger props you need one. Sure you might get away with it several times but that doesn't prove that's it's ok, it just proves you were lucky.
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Old 07-17-2015, 10:22 PM
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Originally Posted by Flysohigh View Post
700.1w and 43 amps...
And how do you know if you havent measured it?

FWIW 700.1W and 43 amps doesnt make much sense. If that was on a 4s battery then it would indicate about 4.1V per cell under load, which is too high to be realistic. If on the other hand it was a 5s battery then it would indicate 3.3V under load, which is too low... So I'd seriously question the validity of the numbers. If it was genuinely pulling 700W and if you are running 4s then you in reality close to 50A, so already at or very close to ESC limit.
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Old 07-17-2015, 11:14 PM
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These are tested by previous owner at full throttle.
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