we are going to use two astroflight motor 603g (http://www.astroflight.com/sport610.html) for our counter rotating helicopter setup. the helicopter is just designed to fly, hover and land. no acrobatic moves.

the typical prop stated in the info sheet of the motor says 11x7. But there are only 14x5, 14x6, 14x8,10x4 and 10x8 in the list of props available.

Which prop would be the best substitute for the recommended prop? What will be the effect if we choose a bigger or a smaller prop for the motor?

thanks!

10x8 will put about the same load on the motor. However since it's going to hover you probably want a prop that will give you lots of thrust but not necessarily much speed; something with less pitch like a 12x6 would be better. If you just increase diameter or pitch the motor will have to work harder and draw more current; if you decrease them it draws less. If one goes up a bit and the other goes down a bit then current will stay about the same.

Download a copy of Motocalc from www.motocalc.com and see how much current the 11x7 draws (it may not be exactly what the manufacturer says since (a) motocalc isn't perfect and (b) the manufacturer may not be giving you *exact* numbers). Then find a bigger prop with less pitch that will draw about the same amount of current.

BTW APC does make an 11x7 electric prop (two actually, one is a slowflyer version - not sure if that would work or not).