ORIGINAL: DeviousDave
My remarks about prop size are relevant to glow engines that most people have rather than the one off diesels that a few play with. This thread is about the glow Norvel .15, which I would start out with a 7-5 and work down towards a 6-6 until I stopped seeing substantial RPM gains with each drop in size or a reduction in in-air performance.
Re: the Picco, from your vids the props you ran were too big to get to where the Picco makes it's power as a glow engine. A 6-3 prop on ANY modern 1/2A is too big, even for a Big Mig .061. I doubt you could put a small enough prop on a Picco to get it to turn up like it does in a car. A 3-3 would probably do it but not have enough flywheel mass to get the engine started.
Diesels are totally different because the prop size efficiency from a 5'' prop to an 8'' prop is probably greater than the horsepower difference. Put a gearbox on that Picco and see how fast it swings an 8-4!
Some interesting comments about props above. The Cox 8x4 was regarded as being underpitched IIRC when they were in common use. Carbon Fibre single blade props are used in Australia on converted .11 and .12 CI car motors to make them run in their power bands. They are available commercially. I can post a pic if you're interested.
A suitable standard prop could be converted to single blade for a high reving .061 with a bit of experimentation. There are some articles on the topic on the Supercool Propeller web site.
Ray