The 18.5 12 is used primarily for noise issues.
Of the glow models at the worlds,I think Team Japans engines were the quietest.
12 pitch seems to be the magic number for that.
In my testing, However, the midrange on the 12 pitch wpn has a step feel like getting on the pipe with a two stroke and it can get fast on you quick if your not on top of it.
The 11-11.5 pitch props in the 19-19.5 range can be made as quiet and work the best overall IMHO most constant curve and better brake and speed control
particularly if you Carbon the prop and stiffen it up ( a little trick I learned from Lockhart
Your better off extending the blade for noise reduction and sticking with the 11-11.5 pitch range.
I`m not a fan of the WPN series of blades except for the 10 pitch when I was running the YS1.6
Bryan