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Relationship between cell and prop ?

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Relationship between cell and prop ?

Old 08-02-2009, 07:32 PM
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SebM
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Default Relationship between cell and prop ?

With Lipo batterie, as the number of cell increase the voltage increase and thus the motor get more power. So my understanding is that the higher the number of cell, the bigger the prop should be but when I look at review on Hobby City web site I see the opposite ?? Can someone explain me why a 4 cell Lipo should take a smaller diameter prop than a 3 cell ? Doesn't make sense to me and I'm really confuse now...

Thanks,
Séb
Old 08-02-2009, 08:59 PM
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Dorsal
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Default RE: Relationship between cell and prop ?

The limitation of prop size is related to the MOTOR / ESC combination, and the amperage they can handle.
Amperage is the Voltage divided by the Resistance (I = E/R). That means, if you have the same motor/esc combo that has 0.74 Ohms, at 7.4 Volts (a 2-cell) then they will run at 10 Amps. BUT if you connect that same combination to 11.1 Volts (a 3-cell) you will be running at (11.1 / 0.74 = ) 15 Amps! That's going to let the magic blue smoke out of your 10-amp speed controller!
So, they ONLY way to allow your ESC to survive the increased Amperage is to decrease the prop size enough to reduce the load on the system. Oh, and stop guessing and buy a WattMeter. It will be the best investment you ever made. http://tinyurl.com/6anu4j

Yeah, yeah, I know it's an oversimplification, but the results stay the same - if you're near the limits of your ESC or the battery's "C-rating", then you've got to reduce the prop size or pitch or both.
Old 08-02-2009, 11:10 PM
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Default RE: Relationship between cell and prop ?

Switching from 3 cells to 4 cells without reducing the prop size can be a motor killer, too.

Let's say you have a motor rated for 250 watts.

If the motor/prop combination pulls 20A and you are using 3 cells, that's:
11.1V x 20A = 222 watts.

You are safely under the power rating of the motor.

Now switch to a 4 cell battery without changing the prop. You are still pulling 20A, so:
14.8v x 20A = 296 watts

Now you have exceeded the power rating of your motor, which means it will be running hot, perhaps hot enough to melt the insulation on the windings.

- Jeff
Old 08-03-2009, 05:47 AM
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cyborg1
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Default RE: Relationship between cell and prop ?

This is interesting. I've flown glow for over 10 years, and I'm just starting out on my first ep. In this case, I have a 30 amp esc and a 1600kv motor running on 3s 2200 milliamp 20C lipo. I've propped my plane with a 8x6 propeller, which is so so in acceleration and speed, and since it's a jet style foamie (reaction 54 ep), I'd like to prop it with a 7x6 or 6x8 propeller. What I'd like to know is, will this combo kill/fry my motor and esc? I don't have any other motor or esc specs, sorry about that. Any advice/suggestions will be greatly appreciated. Thanks!
Old 08-03-2009, 10:24 AM
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Default RE: Relationship between cell and prop ?

Ok, I understand it better now. So that mean the RPM could be much higher is you increase the cell and reduce the prop. It reduce the load ( amp. draw ) but increase the RPM so better response. The things is how do you know which prop size will manage to get normal amp. draw and not too high RPM ? Testing only with a watt-meter and tach. while trying many prop. ?

Can you check my numbers please ?

I have this motor : http://www.hobbycity.com/hobbycity/s...8_900Kv_/_770W

I will be running it with a 60A ESC and a 4S 5000mah Lipo. So, I guess this motor is 0,35 ohm ? So it will give 14,8/0,35 = 42,28 A

Now I have a 12X8 folding prop in order, I hope it will be ok. The way I understand this is there is no way to tell how much amp this motor/prop combinaison will draw just from the specs, right ? Is the only way with a watt-meter ? I wanted to buy a watt-meter,what do you think of the Watts-up : http://www.hobbycity.com/hobbycity/s...idProduct=6380

If using a watt-meter is the only way to tell if the system is ok what do you do when you start up and don't have many propellers ? My problem was ordering the right prop !
Old 08-03-2009, 12:23 PM
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Default RE: Relationship between cell and prop ?

If you are going to get into electrics, you need to buy a Wattmeter. The WattsUp meter is great, that's what I have.

If you do some research on your motor, you can usually find some data on the current draw for various props. HobbyCity usually has recommended prop sizes and/or user posts with prop size & current draw data.

Props for small electrics are cheap ($2-$3), so get at least 2 or 3 different sizes and try them out.

Different prop types and brands will cause the amp draw to vary, even among props of the same size. In general, APC e-props draw less than MAS e-props of the same size (APC props are more efficient, less drag). APC e-props will also have less current draw than APC slow-fly props of the same size. So if someone quotes prop sizes and current draw for various props on specific motor, make sure to note the brand/type of prop they used as well. For example, I found an APC 10x7e prop draws about the same as an MAS 10x6 electric prop on one of my motors.
Old 08-05-2009, 03:28 AM
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Default RE: Relationship between cell and prop ?

ORIGINAL: jdetray

Switching from 3 cells to 4 cells without reducing the prop size can be a motor killer, too.

Let's say you have a motor rated for 250 watts.

If the motor/prop combination pulls 20A and you are using 3 cells, that's:
11.1V x 20A = 222 watts.

You are safely under the power rating of the motor.

Now switch to a 4 cell battery without changing the prop. You are still pulling 20A, so:
14.8v x 20A = 296 watts

Now you have exceeded the power rating of your motor, which means it will be running hot, perhaps hot enough to melt the insulation on the windings.

- Jeff
Had to point out this jdetray: If you switch to 4S without changing propeller it won`t for sure pull 20A, I guess 25-30A. Just making the numbers even worse
Old 08-05-2009, 10:24 AM
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Default RE: Relationship between cell and prop ?

jooNorway -

Thanks! Yes, I should have said it will pull "at least 20A" or "more than 20A."

This is a good example for why a wattmeter is so important if you wish to experiment with different props and different cell counts.

- Jeff
Old 08-05-2009, 12:17 PM
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Default RE: Relationship between cell and prop ?

Thanks to all for all the info, I will order a Watt-meter and few props to do some testing. I am thinking of maybe selling all my glow equipments and going to electric only...

Séb
Old 08-15-2009, 06:11 PM
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Default RE: Relationship between cell and prop ?

You guys need to start reading here:

> EVERYTHING YOU WANTED TO KNOW ABOUT ELECTRIC FLIGHT
> http://www.rcuniverse.com/forum/m_7100376/tm.htm

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