As a starting point I went for the Electronic Model Twister 60 outrunner and 40 Amp speed controller. On paper this should put a 13 x 6 at close to 10,000 rpm on 6s1p 3,100 cells -- ample for a 50 sized model.

As with others, to run a 13 inch prop and fly off grass, I dremeled out the wheel spats and put in 2.75" wheels, and mounted them as far down the gear legs as possible.

Up the front I did some reinforcing of the nose plate and the motor bolted in neatly. After experimenting with balance points I built a frame below the motor to hold the 2 x 3s 3,100 packs. This all came together neatly and fitted well within the cowl.