If you are flying with more than one motor, determine the number of watts required based upon the weight of the model and divide by the number of motors to get the power output required of each motor.

Also pick the right motor based up on the prop diameter the motor can turn. There are cases where the motor has the right amount of power but turns a much larger prop that won't fit on the model. Example the DC-3 motors are very close to the fuselage so it's possible to pick a motor for the DC-3, the prop the motor would like run would hit the fuselage.

I purchased a watt meter to try out different propellers (including 3 blade).  Keep trying different diameters and pitches until you reach the motor rating and/or ESC rating.

This blog discusses how to size the right electric motor for your model. They discuss Watts per Lb, I have used this same idea to pick the right electric motor for my models.

https://innov8tivedesigns.com/resources/lucien-s-blog.html

Prop performance charts are a thing that is sorely lacking in the aeromodeling hobby in general.  Almost all motor manufacturers out there provide just the basic info about their motors, and then time and again I see modelers struggling to figure out which battery and prop to use with which motor, usually wasting time and money in the process.  When a battery/motor/prop combo is found to work well for one scenario by one pilot, everyone else just copies that setup without giving it another thought.  Time and money saved....right?

Someone correct me if I'm wrong, but as far as I know, Innov8tive Designs may be the only entity that has performed an extensive amount of performance testing on all their motor products with a variety of props and voltages.  It's a really comprehensive treasure trove of information that is useful for any airplane with an electric motor and conventional props, be it R/C or C/L.

So, based on that article and all the motor performance data that Innov8tive Designs collected and published over the years, I recently built an app that lets me select a suitable motor/prop combo when given any relevant criteria.  For example, if one needed to power a hypothetical CL stunter such as a Vector 40 with around 590 Watts of power on 4 cells, targeting a pitch speed near 53 mph with a 10"-11" prop and limiting motor weight to 180 grams max, one would have this grab bag of motor/prop combos to choose from:



Adjust any variable above, immediately get new results below.  No more sifting through even a single prop chart, and more importantly -- no more wondering whether the stuff you selected is the only/optimal/cheapest/most ridiculous combo that's actually available.

I also threw in a capacity calculation based on throttle usage and flight time.  This is useful for selecting the actual battery pack for each combo as well.

The app exists as a PC desktop application for my own use for now, but someone with web dev chops could --and should-- turn it into a web app for a much wider audience.

 
-Andrey