Hi vmsguy,

OK - I think we all have a clear understanding of your frustration. []

I don't use a simulator program when trying to calculate these things. Instead I take an educated guess (based on the motor I have bought, and what other motors of the same kv, or rpmv, say) and then check the static amp draw on my actual test.

So, the things you need to know about your motor are the kv (or rpmv it is sometimes referred to - revs per minute per volt) and it's maximum continous and burst amps.

When you look at your system (any motor + any esc + any battery) you need to determine the current limit for the system - this will simply be the lowest of the currents that each of the devices can handle. For example if the motor can handle 20A, the esc can handle 25A and the battery is a 20C 1500mAh (So 20 x 1500mA = 30A) then your motor is the limitation on the system (20A). If the motor could handle 35A then the limitation would be your esc (25A), and if the esc could handle 40A it would be your battery and so on.

Picking the prop is how you control the number of amps your system draws. Now some rules of thumb - as you increase prop diameter the current drawn increases on the square of the diameter. As you increase the prop pitch the current increases linearly. So let's say you started with a 10x5 and it drew 20A. An 11x5 would draw approximately 24A (10x10=100, 11x11=121 ratio is 1.2) and with a 12x5 you would draw 29A, with 13x5 33.8 etc.

Now looking at your particular setup - a 2pack in parrallel configuration is going to give you something like 30+A per pack, so 60+A when connected in parrallel. So far your ESC is the lowest current device. I'm guessing your motor will probably have a limit around the 30A mark, but you need to check this.

So - buy yourself a wattsup meter - this thing plugs inline between your battery and esc and will give you the readings on current and voltage when you actually spin the plane up for a static test. Put a prop on the plane and with the wattsup meter in place bring the throttle up (with the plane tethered!) and watch the amp draw. Provide you don't exceed the maximum amps of the system bring it to full power, read the amps of, and figure out whether you are close enough, need a bigger, or smaller prop (and remember you can increase pitch to increase amps as well).

Calculating your input watts - once you know the current you draw with the right prop, multiply this by your volts (so 11v for a 3s setup) and there is your input watts. In terms of whether your setup can ever deliver the necessary power that's easy to figure out too - find your lowest current device, and multiply that by your volts, and that gives you the most your system can ever safely produce, provided you can find a prop that draws exactly that much. Now - the one caveat on this is whether or not you can fit a large enough prop to the aircraft (due to structural issues as you mentioned).

So, that's the theory - now - your setup - at 1200rpmv I would probably start with a 10x5 to 10x7 prop. I would expect that to pull around 20-25A. However, many motor makers give a list of recommended prop sizes with their documentation - use my suggestion only if such a list doesn't exist with yours. If you want more than 20-25A (you have a 40A esc after all) then maybe start with an 11x5-11x7 (somewhere in the order of 24A-30A. You can also look at increasing pitch size to draw more amps out of your motor.

The important thing is - if there is no documentation from the motor manufacturer on recommended setups then I think you should measure the amp draw to be sure. A wattsup meter is a once off purchase that you will find invaluable. Also, if you happen to have access to one, a clamp meter (that can measure large current) is also good, although it can't measure volts under load.

I wrote a primer on electric flight here - you have probably read these same things already, but you might find it useful:

http://www.oz********.com/2007/01/el...ic-flight.html

Cheers,
oz.