Ok i'm trying to work out a few things with propellers but I am struggling to work out if I have got this right. If I have an 1800 shp engine running at about 2000 rpm and I want my aircraft to have a maximum speed of approximately 350 mph what is the maximum diameter I can have the propeller. I have read in a few books that it is best to keep propeller speed between 0.7 and 0.8 Mach. Working back from this I have calculated the diameter to be 78.5 inches however I know the spitfire used to run at 3500 rpm with at prop diameter of 10ft so I am assuming that I have done something wrong in my calculations. Any help would be appreciated.