What am I doing wrong? 2-stage prop spitfires.

[justme262]

I just did a test in Spit Ia and got IAS of 255mph @ 17300ft @ 2750rpm on coarse pitch setting.
I put that into a TAS calculator and got 342mph.
I was running level till the speed stabilized after a shallow dive at about 270mph IAS. But it was holding pretty steady for 20 seconds or so.

The course prop setting gives much higher RPM at altitude where the air is thinner. I have come to like de Havilland two speed props. I think they actually historically had a better top speed than the Rotol prop fitted versions. But the Rotol had better climb and acceleration. You just gotta be at high alt to get the most out of em...




http://www.csgnetwork.com/tasinfocalc.html