Sorry, Rock, you're beatin around it but not followin ya. You're close tho.
There is another reason for multi-bladed props for full-scale. Matching the blade area to engine power. It is also a clearance consideration sometimes. However, it's primary reason is needing more blade area. The 7 blades on some turbo-props is a perfect example. Imagine the diameter of a 2 bladed prop with the same blade area. And of course, as you mentioned, imagine the scream from those 2 tips.
Blade area and clearance:
I agree on matching blade area to HP. But my point is the designer has to do it (1) with the blade tips below Mach 1 and (2) with as few blades as possible.
The Corsair was not conceived as a gull-wing originally, but the design was altered to allow for the large 13'4" prop diameter needed to absorb 2,800 HP put out by the P&W 18-cyl. Double Wasp. They could have used 4 (or more) blades at a smaller diameter turning higher RPM and left the wing alone, but they would lose (1) speed and (2) efficiency. The tip speed is limiting (Mach), so the bigger diameter, using fewer blades turning slower, is more efficient. Those tips are near or at Mach 1 at takeoff power on the F4U, as they are on many full-size prop planes.
As for the 5- & 7- blade modern turboprops, the reason for that is each blade can be thinner (and weigh less) and each absorbs a smaller percent of horsepower. Advances in blade aerodynamics and metallurgy allow for those thin scimitar or deeply swept blades, but the designer will tell you they give up some efficiency to blade count, as each blade that's added is increasingly operating in more disturbed air. The newer, thin blades are more efficient than the older, fat, flat blades, but the net gain is still just a few percent. The 90-series King Air has a 5-blade aftermarket blade available called the Blackhawk.
Re-read what I posted. It's not that more blades are more efficient, the opposite is true. Fewer blades, turning slowly, are more efficient. And yes, those speed guys are still onto something as single-blade props are still used at CL speed events. They are also used by free-flighters, as I saw indoor here last year. In full-scale, some self-launching gliders use 1 blade, and 1-blade props were common for early low-powered planes (Aeroncas) and for engine break-in at engine shops.
Model speed flyers have flown 1 bladed props off and on. They read that fewer tips were more efficient.
Them durned fellers at MIT an sech been spreadin' them rumors agin! Durn their hides!
http://www.flyinggiants.com/forums/s...ad.php?t=10595
One blade may be odd & rare, but it is most efficient.
(Zziiiipp) OK. Suit's on.
[8D]