Hi Bruce, I am using the 4120/18 with a 13x8 'master' undercambered wooden (electric) prop with 16x2400 cells and it is reasonably powrfull but I would rely on this set up as I have tried it out on the 18 and 20 cells if you need more power. I find the lower cell count better because of the large props allowed up to a 14x6 (wooden).
Don't forget that you need a 12% min wing thickness to allow easier slow flight.
I had to lengthen the udercart to allow the fitting of my 14x6 prop and get clearence as well. Don't use a fully symett section as they don't lift well at low speed. e-mail me for a secret section which I use on all of my 6-7ft Scale jobs and makes heavy planes behave as if they are lightweights, cheers Robin Andrew, Birmingham, England