ORIGINAL: BrentL
Right, so...
If I have a 4 blade 7 x 3 prop, would this be anything like a 2 blade 14 x 3 prop, or maybe a 2 blade 7 x 6 prop?
I know that it doesn't quite work like that, due to efficiencies and things (more blades, less efficiency, or so I've read), but how then doen one work out the sort of output that one can expect from a 3 / 4 blade prop?
Nope
For the same reasons a 40' wingspan monoplane flies differently than a 20' winspan biplane. But with one huge difference. And a few minor differences.
Props are basically wings, not really much else.
The area of a circle does not come close to changing at the same rate as the diameter. Change the diameter and you get a huge difference. And the area is what the wing/prop works with.
Aspect ratio has a significant part in wing performance. The efficiency of a prop is greatly affected by it's aspect ratio. But a prop has two aspect ratios. And both matter. One is the same as a wings, the chord:span. The other is the pitch versus the radius. And it matters a lot to efficiency. And it's effect is sorta backward. High aspect ratio wings are more efficient. With the pitch:span ratio, the efficiency of a prop increases as the pitch increases up to a point and then decreases. So a 14x3 prop will have less efficiency and the 7x3 has more. If I remember correctly, about the optimum is when the pitch is about half the span.
Also, tip loss efficiency is the same as with wings. Biplanes lose efficiency because they're dragging around 4 wingtips instead of 2. That ain't all of it, but it's a big reason.
But you asked two questions..................