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Old Jul 07, 2009, 03:15 PM
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Choosing the right propeller

Hey guys I have a question about propeller diameter/pitch. How do we calculate the maximum propeller diameter and pitch for a given motor/esc/battery combination? Thanks

Keat
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Old Jul 07, 2009, 04:03 PM
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M Ashmore's Avatar
Bakersfield, California
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Lots of us use these type of calculaters,
http://brantuas.com/ezcalc/dma1.asp
http://www.motocalc.com/
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Old Jul 07, 2009, 04:32 PM
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Thanks...One more question. How do I determine the Power(watt) of a NimH Pack? Lets say I have a 7 cell NiMh Pack(8.4V),rated at 2300maH, because LiPos
often have discharge rates of like 1C or 2C but I cant find them on NimH packs.

I need to produce about 100W of power with a NimH pack. How do I choose a right pack? I have read other sections and there are lots of info for LiPOs but I am limited to NimH packs so any help would be appreciated.
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Old Jul 07, 2009, 04:33 PM
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The sticky at the top of the forum gives you a whole load of calculators to play with http://www.rcgroups.com/forums/showthread.php?t=606703

For the NiMH batteries you simply have to look up the specification (or ask about SPECIFIC cells).

Steve
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Old Jul 07, 2009, 09:27 PM
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Ok this is what I learned so far, correct me if I am wrong...

A 100W motor means it can take a maximum of 100W of power...it could be less depends on battery,propeller etc

A NiMh battery of 9.6V, 10A means it can supply 96W of power to that 100W motor to drive the propeller.

ESC Amp has to be more than that which the motor can carry.

Thanks for all the help
Keat
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Old Jul 07, 2009, 11:37 PM
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http://www.all-battery.com/84v4200ma...forrccars.aspx

This battery is rated a 8.4v 4200mAh 40A drain rate...does this mean that it can deliver maximum of 8.4X40=336W of power?
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Old Jul 08, 2009, 04:44 AM
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Basically yes.

It doesn't HAVE to though.
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Old Jul 08, 2009, 04:45 AM
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If you accidentally short the battery out, it can supply a hell of a lot of current, enough to cook the battery, (and anything else nearby).

Nimh batteries will supply more amps than you safely need, they don't normally have a 'C' rating other than for charging.

The propeller is the load, it's the thing that determines what current will be pulled from the battery.
The ESC need to be rated at 25-50% over the max current you are likely to pull for safety.

A very good tool, is the Wattmeter. It tells you voltage, current, and watts when the motor is under load. If you use one it also can stop the 'magic smoke' escaping from the ESC and motor, well worth buying.

The prop will depend on what you want from the plane, e.g. speed = smaller diameter coarser pitch; thrust, but not top speed for aerobatics = larger diameter and finer pitch. What ever you choose, check with the calculators or get a wattmeter to protect the ESC and motor.

Watts isn't a guarantee of power or speed, it's how you transfer those watts into the correct propeller for the model in question.
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Old Jul 08, 2009, 04:33 PM
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First of all, thanks for the help, however I am doing this for a school project which is only a conceptual design of an airplane, so I dont think buying a Wattmeter is worth it. Here is what I have now:
92W power required, so I chose a 100W motor which can handle 8A max. I am thinking of a 12V NimH 2600maH batt, so it can deliver about 12VX8A=96W which is enough power.

A 10A ESC to handle the 8A max current of motor. How do I determine how much current a propeller of particular diameter pulls from the battery? Is there a formula for it? Because I consulted my professor and he does not approve using online calculator tools. So I really need help here.

Thanks
Keat
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Old Jul 08, 2009, 10:25 PM
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Dickinson, Tx.
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Quote:
Originally Posted by Keat
A 10A ESC to handle the 8A max current of motor. How do I determine how much current a propeller of particular diameter pulls from the battery? Is there a formula for it? Because I consulted my professor and he does not approve using online calculator tools. So I really need help here.
Thanks
Keat
Using an online motor calculator is smart for several reasons. It gives you a ball park of amps or watts with the motor/prop/voltage. It lets you see how much thrust or pitch speed you can get with the combo. It saves you the hassles associated with buying what others recommend and you are not satisfied with the results.

I use one of the free motor calcs. every time I am looking to buy a new motor.

P-calc has always been real close to what it shows online versus what I see on my amp probe or watt meter. So use a calc. to give you an idea of how many amps a motor/prop/voltage would pull, but always check it with a amp probe or watt meter to ensure you aren't over amping anything...

Challenger413
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Old Jul 09, 2009, 04:33 AM
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Staffs, UK
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Quote:
Originally Posted by Keat
How do I determine how much current a propeller of particular diameter pulls from the battery? Is there a formula for it? Because I consulted my professor and he does not approve using online calculator tools. So I really need help here.
You're into fairly complex territory here. It might be worth asking your professor what help he can give you. Profs are supposed to teach you stuff aren't they, not just tell you how not to do things ?

Alternatively if you want to do some work yourself there is an excellent series of articles on The EZone that will give you all the information you need to do the calculations yourself. The articles are over 10 years old but still about the best presentation of the subject I've seen. Although written about brushed motors the basics still work for modern brushless motors. The calculations are not too difficult but are painful enough that these days everyone uses the calc programs that already have them programmed in . Try a search for "Understanding Electric Power Systems" in the Ezonemag Archive or http://www.rcgroups.com/forums/showthread.php?t=333326 will get you to the first of the series.

Steve
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