View Full Version : Rule of Thumb
09-08-2005, 12:15 PM
Hope you get to see this.
We have all become accustomed to refering to model airplanes as .40, .60 or .80 etc and all know more or less what size of plane that is.
Is there no such rule of thumb as to what power set up would be needed for planes in these catorgories. i.e. 700 watts/lb for planes weighing 10-12 lbs or 900 watts/lb for planes weighing 12-14 lbs?
Due to accidents and the like I have ended up with a 25 Amp Castle Creations ESC and would like to use it in a new project. I know that an Axi 2808/16 fits the current of the ESC and that a 3S 2000 mah batt will do the trick, what I don't know is what size plane will this set up pull.
09-08-2005, 03:33 PM
The rule of thumb that I use is 85 watts/lb for ROG, 100 watts/lb for good sport/acrobatics, 115-125 watts/lb for good 3D and 150 watts/lb + for insane performance. These numbers will remain the same regardless. However it is the starting point for me to figure out what my watts need to be (or I desire them to be).
A lot depends on the plane as well and how it is meant to fly. I wouldnt need 150 watts/lb on a top wing trainer when 80-90 watts would suite it just fine. Even on a conversion like the H9 Jug, I would consider 100-110 watts/lb, more than adequate for some serious straffing runs :).
The 2808/16 is a bit of a hotter wind and will require a smallish prop. However I have seen guys use this motor on a 22oz foamy. Attached is a great little chart (in PDF format) that was copied from www.flyingmodels.org. They have done a lot of the hardwork for us in testing props and battery combo's so that we dont need to. Your results wont be exact, but they should be darn close and by the chart, it looks like this motor can push around 150-200 watts on 8 cells. That is 19-20 amps and leaves you with some headroom on your 25a ESC.
09-08-2005, 10:00 PM
Another source of AXi information is the manufacturer's website, www.modelmotors.cz (http://www.modelmotors.cz). You can see that the flyingmodels results are very close.
09-09-2005, 06:42 AM
Thanks for the reply.
Mike - please check my calcs. I figure that with 3S (11,1 v) and 2000 mah (2 amp) I will have 22,2 Watts and discharge at 15C will give me 333 watts. Using 100 watts/lb for sport I should be able to power a plane weighing not more than 3,33 lbs (1,4 kgs)
09-09-2005, 11:54 AM
The problem with 11.1 volts is that your prop will have to be in a low pitch 8" or 7" range to keep the amps within limits. The motor is a 15a nominal, 25a burst to 30 seconds. Ill run it through motocalc this morning to see what it says. I think 3S may be a bit much, but again, I am going off of what others are using on this motor as well as the charts state.
09-09-2005, 05:26 PM
Geoff, for a 3S pack, the 2808/24 would be a better choice. That way you could spin a useful-size prop on the motor. As Mike says, the prop would have to be quite small to keep the Amps within reason on the 2808/16.
It's true that you've got a maximum of 333 Watts of input power from the battery, but don't forget to take into account the limitations of the motor and ESC. That 333 Watts is at 30 Amps, which as Mike says, is WAAAAAAY too much for the little AXi motor.
09-09-2005, 06:51 PM
Here are a couple of tables that motocalc spit out. One is the AXI on a 3S and the other is an AXI on a 2S. The 3S and 9" prop would be good for a go fast, keep the speed up kind of plane. The 2S and 12X3.8 would be good for a 18-22oz 3D'ish plane with about 24 oz of thrust or a 30oz sport flyer.
09-12-2005, 07:52 AM
Thanks to all that answered - Have taken your advice on the motor and I selected a true 25 Amp motor which will meet my needs.
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