Interesting thread.....

Generally the smaller the prop diameter, the higher the idle RPM. That is why the 12 x 6 slows the plane the best. The bigger prop will act as a bigger flywheel with higher rotational inertia. It will idle slower than the 11 x 6 or 10 x 6. The 12 x 6 will have a lower "pitch speed" at idle, pitch multiplied by RPM, and will slow the plane.

I fly at 4200 feet and have flown at Evanston with a density altitude of 10,000 feet. The 12 x 6 is too much prop for these altitudes. I run prop sizes of 10 x 6 to 11 x 6 on the .46's.

If you really want to slow is down, try the APC 12.25 x 3.75 or 12 x 4 prop. It will keep the plane slow as well as act as a good air brake. I have tried the prop on a .46 powered plane and have to land with a little power on.

One of the big problems with high altitude flying is over 'prop'ing the motors. There is a difference in power up here and sometimes the recommended props on the high end are too much for the engine. The engine will run, but not very well. It will also overheat in some cases.

The LT-40 is a floater. I've flown one once and it reminds me of a powered glider. I had to throttle down early to land it compared to other planes.