Not sure if this i the proper place for this , but I'm gonna give it a shot.
I am finishing building a Multiplex Magister w/brushless outrunner. Hobby Lobby sells an outrunner kit, but I already had an E-Flight Power 25 outrunner,, so I decided to use it. I'm running a 45 amp ESC and 11.1V 4000ma lipo......Hobby Lobby specs a 12X6 prop for their conversion, but the AXI motor they use has a higher KV rating. I installed the 12X6, but it really looks tiny. E-Flight says that the motor is good to a 14X7. I'm actually thinking this may be a good size for my set up.....There's plenty of room for the 14, but I'm not sure the motor can swing it.....How the heck do you select the proper prop, without buying half a dozen to experiment with???......Pat

Also.....I found that the slow flyer props will pull more watts, for the same pitch and diameter, but a slow flyer will not fit on the prop adaptor. It requires a thin electric. So with a thin electric, I'm thinking the 14 inch would be ok as the tips are very narrow....VS. the wide paddles of a slow flyer.....

First, do you have a wattmeter? If you are going to play around with set-ups, you need a wattmeter to see the effect of your changes and to insure you are not over drawing on your battery, your ESC or overloading your motor.

Props are cheap. If you are going to experiment with your own set-ups, you have to expect to spend a few dollars on things to test. So buy a couple of props.

You have a 12X6 - What wattage does it draw, full throttle, on the bench? How does that compare to the motor's rating, the ESC's rating and the battery's rating?

Try it with another prop and see what happens.

Depending on what kind of flying you are trying to do you will need a watt/pound target to tune to. But if you are getting at least 50 watts/pound, And at least 4000 RPMs, it should fly, so give it a try. then try other props.

50 watts/pound - Caual flying

75 watts/pound - sport aerobatics

100 watts/pound - Agressive aerobatics - sport pattern

150+ = Competition pattern, 3D

So, what is your weight and what is your target?

Sizing Electric Power Systems -
Starts at post 2
http://www.rcuniverse.com/forum/m_43...tm.htm#4310056

I will finish the build, tomorrow. Then I'll be able to weight it, decals and all......The MFG says the electric version should come in around
5 1/2 pounds....but the weight listed is with a brushed motor, gearbox, and nimh batteries....so I'm hoping for a little less than that.....Even so, E-Flight lists the Power 25 as being capable of 550 watts. If I can get close to these numbers, I'm sure I'll be happy.

A question about watt meters......Mine reads AMPS, VOLTS, WATTS, and cumulative ma......I have it wired with Dean's Ultra's, so it is plug n play with all my stuff...... However, I do not understand how powering up the plane, on the bench, can give a realistic reading. I would think that a prop moving through the air would draw far less power than one that is being restrained. Either way, I will get some readings, tomorrow....

Also........How does one measure prop RPMS????.......and how important would that be on a static bench test.....
As you can see I'm having a hard time relating motor performance, in the air, versus a static bench test....The motor may not be able to properly spin a 14 X 7 on a static bench test, but could be quite capable of spinning it in an airborne craft traveling 25-35 mph....

Your bench watts will be higher than your air watts but it is a reference number. If you wattmeter will fit in the plane and will maintain max watts, then you will see what your take-off watts are. I would expect this to be the highest number.

RPMs are done with a tachometer which is an RPM meter.

Assuming you have not over propped the motor, you should be able to estiamte based on KV and Volts sustained by the pack on nder load.

If you are running 11 V and this is a 1000 KV motor you should be turning 11,000 RPM on the motor. Assuming there is no gearbox, then that is what the prop should turn.

On the other hand if this is a 550 KV motor, then you will get 5500 RPMs

Based on RPMs and the pitch of the prop you can calculate MPH, assuming no slippage.

11000 X 6" = 66000 Inches per minute/12 = 5500 feet per minute or a little over 60 mph pitch speed. I think 45-60 mph would be a reasonable top end target for this plane.

at 5500 RPM X 6" - half of that or 30 MPH. I think that 30 mph might be a bit slow as a top speed for this plane, but with a large enough prop, it might have a terrific climb, or do well as a tow plane for a glider.

That's how we play with props.