Hi, the size of a prop is given as 2 numbers diameter/pitch. In terms of maximum speed, the pitch is the important number a one inch pitch would give 2.84 mph, so a 49in pitch prop would give a max speed of 139 mph at 3000 revs and 64in pitch gives 181 mph at 3000 revs.
VWs are normally limited to around 3400revs( in UK by PFA edict ), so a 49in prop would give me a max of 138mph. this is regardless of how big the prop is - that's the max speed. At this speed the blades angle of attack becomes zero, so it cannot provide any more thrust. The only way I can get to go any faster is by increasing revs. So given that I want ground clearance, I am limited to say 60 inch diam. Now if you imagine a prop that is about the same width as a tape measure, it will obviously need less energy to turn it than big fat wide blades, so the designer has to come up with a prop that will have:- 1 have a theoretical max speed somewhat higher than the required max speed. 2 shovels sufficient air to overcome the aircraft drag at the required max speed. 3 is not stalled at speed below the aircraft take off speed. 4 does not produce so much drag that the engine cannot produce full power at take off speed. I am puzzled why prop manufactures only give the diam/pitch figures for a prop, when as explained above the prop width/cord is vitally important when choosing a prop I suspect that it is not that Senns & Sturba have different ways of specing a prop, rather one makes fatter props. Pete Diffey St Albans, UK
