Rob,
Not that I am an authority on the suject, but I do fly model aircraft and am doing an aero eng degree. From what I understand in the past, the aim is to prevent the blade tips from touchin supersonic speeds. At these speed, the blade section becomes very inefficient as the normal shockwaves developing over the blade promote seperation of the flow - essentially, when the blades hit sonic speeds, they stall.
A lot of work was carried out, as mentioned by Mark 1, in the late eoghties/early nineties on large scale propfans. These utalised blades that were swept rearwards to delay the onset of shock development on the section - the same reason as civil jets have swept wings. The aim was to allow propeller engined aircraft to travel at higher speed, as propellers are very efficient! Noithing really came of this as far as i am aware (if anyone knows anythinf, please correct me!).
In terms of your model (what class are you - F3A?), then the idea is usually to prevent the blade from touching sonic speeds. As you will know setting a higher pitch will allow you to go faster at the detrement of static thrust - a fixed pitch propeller can only be designed to be most efficient at one single pitch value, hence vaiiable pitch props on most full scale A/C.
In my experince, gearing has often been used to allow the size of the propeller to increase whilst still lowering the blade tip speed. Fixed pitch propellers are a bit of a black art, but what I would personally suggest is avioding sonic tip speeds - it doesn't work!!
Hope this is of some help,
Cuban