
Mar 03, 2011, 04:47 PM  
Joined Jul 2006
22,991 Posts

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

Mar 04, 2011, 03:29 AM  
LI, New York, USA
Joined Mar 2003
24,347 Posts

Quote:
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. 

Mar 07, 2011, 05:46 AM  

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 ? 
Mar 07, 2011, 06:04 AM  

Quote:
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 

Mar 07, 2011, 06:31 AM  
LI, New York, USA
Joined Mar 2003
24,347 Posts

Quote:
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. 

Mar 07, 2011, 06:44 AM  

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

Mar 07, 2011, 06:46 AM  
Joined Jul 2006
22,991 Posts

Quote:
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. 

Mar 07, 2011, 06:49 AM  

Quote:


Mar 07, 2011, 06:54 AM  

Quote:
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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