


Joined Feb 2011
21 Posts

ok so i'm also a noob only im using one arm now but i need to replace my motor and esc now which is a brushed 370 size greatplanes piece of crap spinning an 11x4.7 piece of pain with an 11.1v 3 cell 1520c parkzone battery and a 25a deep fried esc with a gross weight of 15 oz. what would i need to go brushless at a budget.




Joined Jul 2006
22,990 Posts

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Malaysia, Selangor, Seri Kembangan
Joined Mar 2010
26 Posts

hai all ....
i need help, how can i determine prop saiz if the vendor not say what suggestion prop can used for certain motor... 


Joined Jul 2006
22,990 Posts

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http://www.rcgroups.com/forums/showthread.php?t=1136470 





LI, New York, USA
Joined Mar 2003
25,423 Posts

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One that I use is Webocalc. Not very comprehensive but it is free. http://flbeagle.rchomepage.com/ If you have a specific plane in mind, put in its numbers. If not, then just approximate something. Put in the max prop size, (based on ground clearence deisred and a starting pitch speed of 45 mph and any thrust number. Put in the max amps and kV rating of the motor as well as the the proper battery pack cell count. Some props will be suggested. Vary the amperage or pitch speed to see various prop suggestions. 




After reading a lot of this stuff, I have come to the following "recipe",
It is an attempt to simplify things, by an example. could you please verify it is correct ? [1] We begin by knowing (or guessing) the weight and speed required for the model. [2] From the weight we determine the Watts and from the speed we determine rpm/prop combination. [3] We try to find a proper motor/prop combination that conforms with the above. It is difficult to determine the optimal power system for our model, but it is relatively easy to determine a decent one. For example, for a 16 oz, 1m Cessna, we could assume 120 Watts power and 50 mph speed. Suppose some motor works at 12 Amps at 3 cells, revolving a 9x7 prop at 8000 rpm. This yields power = 12 x 11.1 = 133.2 Watts, and speed = rpm x pitch/1056 = 53 mph. Ok this sounds good enough ! Suppose another motor works at 12 Amps, revolving a 10 x 4.7 prop at 6000 rpm. This yields 133.2 Watts which is nice, but speed = rpm x pitch / 1056 = 6000 x 4.7 / 1056 = 26 mph. Oh gee, this is too slow fly, even at full throttle it will not go faster than 26 mph, so this is not a good power system for our airplane. Even if it manages to take off, it will remain dangerously close to stall speed, and therefore will not provide a fun experience. Could you please verify that the above reasoning is correct ? 



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Weight and speed is important but ** plane style and piloting is also important If you have a hand launching or launching from grass .. If you have a trainer or a glider or a racer .. If you have a solar plane .. Louis 



LI, New York, USA
Joined Mar 2003
25,423 Posts

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If you are going to select your own power systems, as opposed to buying planes that have motors already installed, then it is probably a good idea to invest in once of the calc programs that help you "model" a variety of combinations till you find one you really like. 




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Pilot and plane style is of course greatly important, but don't you think that it can be expressed by "max speed" ? 



Joined Jul 2006
22,990 Posts

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Take the case of a plane like your example Cessna propped to only go 20 mph top speed. For one pilot, looking to fly 3D with it, it may be ideal for his style. But for another, looking to fly scale like, it would not be what he was after. 






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In my opinion .. yes but The two major "parameters" are a) Static thrust b) Pitch speed (or speed for prop transparency) The product is proportional to watts out If you count on the motorgear box efficiency (could be 80%) you have all. So I prefer a) and b) parameters instead of a) OR b) + the product a)b) For take off from grass you need static thrust For hovering you need static thrust For a racer you need pitch speed ... Louis 

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