motor size, prop size/pitch, battery size etc

[eagle4]

Hey guys, Just wondering is there a certain formula or something that can be applied when working on scratch builds to help determine the size of your motor, prop, esc, battery etc?

Some way that you could put in the weight and size of your aircraft and then have it spit out all the details?

Apart from experience, when you scratch build, how do you know what motor, battery, esc and prop to put on your plane?

 

[earthsciteach]

Here is a great resource!

http://www.wattflyer.com/forums/showthread.php?t=24238

Also, the app featured on Flite Test a couple of weeks ago has some calculators on it to help. Its called RC Flight Source by 2BFLY.com. It was free when I downloaded it.

Things can get a bit fuzzy at times, depending on the specs listed for a given motor. Some are less reliable than others, for whatever reason. I like to find published data for a given motor/prop combo. Sometimes its easy, sometimes, not.

 

[pgerts]

There are no good formulas.
The size and type of plane is the first to determine.
Then you can find out the need of power (watts).
The problem might be that on the exact same plane you can possibly use a WIDE range of power from the lowest possible to get the plane flying up to extreme power that will “rocket” the plane vertical to the sky.
There are some "rules of thumb" for the selection of power/weight normally classified for different types of planes from "slowflyers" to "3D" or competition racers.
When you know your need of power then you have to know the size of propeller, normally depending on the size of the plane (free space from ground or parts of the plane). The power and the prop size get you the data to select a proper KV/voltage for your motor.
The esc is easy - a reasonable margin above the power needed.
Battery can be hard to select - most important is to get a battery that can deliver well above the highest power you have selected for your plane. My choise is normally a smaller battery that i can land and replace rather than getting a heavy battery that vill last more than one flight without charging.
All planes are to heavy. Making the planes less heavy makes them fly better ;-)